UBC Theses and Dissertations

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UBC Theses and Dissertations

Inner shell and valence shell electron excitation of gaseous molecules studied by electron energy loss… Sodhi, Rana N. S. 1984

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INNER SHELL AND VALENCE SHELL ELECTRON EXCITATION OF GASEOUS MOLECULES STUDIED BY ELECTRON ENERGY LOSS SPECTROSCOPY  by  RANA N.S. SODHI B.Sc.  (Hon.), U n i v e r s i t y  M.Sc, University  of Reading, 1975  of A l b e r t a ,  1980  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY  in  THE  FACULTY OF GRADUATE STUDIES Department of Chemistry  We accept t h i s  t h e s i s as conforming  to the r e q u i r e d  standard  The U n i v e r s i t y of B r i t i s h  Columbia  August, 1984  ©  Rana N.S. Sodhi, 1984  E-6  In p r e s e n t i n g t h i s t h e s i s requirements  i n p a r t i a l f u l f i l m e n t of  f o r an a d v a n c e d d e g r e e a t  the  the  University  o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l make it  freely available  for reference  and s t u d y .  agree t h a t permission f o r extensive  I  further  copying of t h i s  thesis  f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t h e h e a d o f my d e p a r t m e n t o r by h i s o r h e r r e p r e s e n t a t i v e s . understood that copying or p u b l i c a t i o n of t h i s for  financial  CHtniSTRV  The U n i v e r s i t y o f B r i t i s h C o l u m b i a 1956 Main M a l l V a n c o u v e r , Canada V6T 1Y3 Date  (3/81)  \%  is thesis  g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n  permission.  Department of  It  Oc-To$£R.  H'Sii-  Abstract  E l e c t r o n energy l o s s spectroscopy  has  been used to o b t a i n  i n n e r s h e l l e l e c t r o n e x c i t a t i o n s p e c t r a of s e v e r a l d i f f e r e n t gaseous molecules.  The  transfer conditions  ( u s u a l l y 2.5  s c a t t e r i n g ) and  s p e c t r a were a l l recorded  (<1000 eV)  NH  compounds (PX ,  X = H,  3  3  3  and  the methyl amines) and F, C l and  CH ; 3  PF , 5  the s p e c t r a of S i C C H j ) ^ have been obtained s p e c t r a of r e l a t e d S i c o n t a i n i n g compounds.  OPF and  (~1°)  ligand)  have been measured.  s e r i e s of molecules i n v e s t i g a t e d i n c l u d e n i t r o g e n  molecules ( N F ,  s m a l l angle  (both c e n t r a l atom and  instrumentation  s e r i e s of  under s m a l l momentum  impact energy and  a l l s p e c t r a l regions  a c c e s s i b l e by the present The  keV  the  containing  s e v e r a l phosphorus and  3  0PC1 ). 3  compared with  In a d d i t i o n published  A l l of the i n n e r  s p e c t r a show continuum s t r u c t u r e s which i n many cases can be  shell reasonably  * assigned  to a  shape-resonances.  e l e c t r o n e x c i t a t i o n s p e c t r a of NF  However, comparison of the i n n e r 3  with  the X-ray p h o t o e l e c t r o n  ( a l s o r e p o r t e d here) show that continuum s t r u c t u r e can a l s o be i n some cases,  to onsets of "shake-up" c o n t i n u a .  shape-resonance p o s i t i o n and  The  shell  spectra ascribed,  r e l a t i o n s h i p of  bond l e n g t h i s a l s o examined i n the  systems  s t u d i e d here.  The  valence  s h e l l e l e c t r o n energy l o s s s p e c t r a of many of  above molecules are a l s o r e p o r t e d .  The  assignment of these s p e c t r a i s  shown to be g r e a t l y f a c i l i t a t e d by a comparison w i t h spectra.  the  the i n n e r  shell  - iii  Finally, spectra  the  inner  s h e l l and  of t r a n s - 1 , 3 - b u t a d i e n e and  assigned.  In p a r t i c u l a r , the  -  v a l e n c e s h e l l e l e c t r o n energy l o s s a l l e n e are  also reported  s p e c t r a l assignment of the  inner  spectrum of a l l e n e a l l o w s c l a r i f i c a t i o n of i t s complex and valence s h e l l  spectrum.  and shell  controversial  -  iv  -  T a b l e of Contents Page Chapter  1  Introduction  1  A.  General I n t r o d u c t i o n  1  B.  D e s c r i p t i o n of V a r i o u s Processes i n E l e c t r o n Spectroscopy I) Electron Excitation II) Photoionisation I I I ) "Shake-up" and " s h a k e - o f f " IV) X-ray F l u o r e s c e n c e , Auger Decay and Autoionisation  C.  D.  E.  F.  Chapter 2  4 4 7 12 14  Fundamental concepts i n E l e c t r o n Impact and R e l a t i o n s h i p to P h o t o a b s o r p t i o n  the  The R e l a t i v e M e r i t s of E l e c t r o n Energy Loss Photoabsorption Spectroscopies  and  Inner I) II) III)  18  S h e l l E l e c t r o n E x c i t a t i o n Spectra Discrete Portion Continuum F e a t u r e s Comparisons of Inner S h e l l E x c i t a t i o n S p e c t r a w i t h Valence S h e l l E x c i t a t i o n Spectra  P o t e n t i a l B a r r i e r and Shape-Resonance E f f e c t s I) Potential Barriers II) Shape-Resonances I I I ) R e l a t i o n s h i p of Shape-Resonance P o s i t i o n w i t h Bond-lengths  27 33 33 38  39  41 44 52  Experimental  55  A.  55 55 61  B.  E x p e r i m e n t a l Methods I) The Spectrometer II) Sample H a n d l i n g III) Spectral Acquisition, Calibration Spectrometer Performance IV) Other Measurements Reference E n e r g i e s f o r Inner S h e l l Energy Loss Spectroscopy  and 64 67  Electron 69  - V -  Page Chapter 3  Inner S h e l l E x c i t a t i o n , Valence E x c i t a t i o n and Core I o n i s a t i o n i n NF s t u d i e d by E l e c t r o n Energy Loss and X-ray P h o t o e l e c t r o n S p e c t r o s c o p i e s 3  Chapter 4  Chapter 5  E l e c t r o n Energy Loss S p e c t r a of the S i l i c o n 2p, 2s, Carbon Is and Valence S h e l l s of Tetramethylsilane  120  E l e c t r o n i c E x c i t a t i o n s i n Phosphorus containing Molecules. I . Inner S h e l l E l e c t r o n Energy Loss S p e c t r a of PH , P ( C H ) , P F and P C 1  150  E l e c t r o n i c E x c i t a t i o n s i n Phosphorus containing Molecules. I I . Inner S h e l l E l e c t r o n Energy Loss S p e c t r a of P F , 0 P F and 0PC1  192  3  Chapter 6  5  Chapter 7  83  3  3  3  3  3  3  E l e c t r o n i c E x c i t a t i o n s i n Phosphorus containing Molecules. I I I . Valence S h e l l E l e c t r o n Energy Loss S p e c t r a of P ( C H ) , PC1 , P F , 0PC1 and P F 3  3  3  3  3  5  225  * Chapter 8  Chapter 9  Chapter  Inner S h e l l E l e c t r o n Energy Loss S p e c t r a of the M e t h y l Amines and Ammonia  255  High R e s o l u t i o n Carbon Is and Valence S h e l l E l e c t r o n i c E x c i t a t i o n S p e c t r a of A l l e n e and Trans-1,3-Butadiene s t u d i e d by E l e c t r o n Energy Loss Spectroscopy  275  10 C o n c l u d i n g Remarks  References  304  306  - vi -  - vii -  Table 5.1  5.2  Description Transitions symmetry  Page  from the *A^ ground s t a t e f o r C^  v  154  E n e r g i e s , term v a l u e s and p o s s i b l e f o r the P 2p, 2s s p e c t r a of PH  assignments 159  3  5.3  E n e r g i e s , term values and p o s s i b l e assignments f o r the P 2p, 2s s p e c t r a of P ( C H )  162  E n e r g i e s , term v a l u e s and p o s s i b l e f o r the P 2p, 2s s p e c t r a of P F  assignments 165  E n e r g i e s , term v a l u e s and p o s s i b l e f o r the P 2p, 2s s p e c t r a of PC1  assignments  3  5.4  3  3  5.5  169  3  5.6  T r a n s i t i o n s from the *Aj^ ground s t a t e f o r D-j^ symmetry  176  5.7  Resonance term v a l u e s and (P-X) bond lengths  181  5.8  E n e r g i e s , term v a l u e s and p o s s i b l e assignments f o r the F Is spectrum of P F  184  E n e r g i e s , term v a l u e s and p o s s i b l e assignments f o r the C Is spectrum of P ( C H )  187  E n e r g i e s , term v a l u e s and p o s s i b l e assignments f o r the C l 2p,2s s p e c t r a of PC1  190  Transitions symmetry  196  3  5.9  3  5.10  3  3  6.1  6.2  from the ^A^ ground s t a t e i n D^  n  E n e r g i e s , term v a l u e s and p o s s i b l e assignments f o r the P 2p,2s s p e c t r a of P F  199  E n e r g i e s , term v a l u e s and p o s s i b l e assignments f o r the P 2p,2s s p e c t r a of 0 P F  203  E n e r g i e s , term v a l u e s and p o s s i b l e assignments f o r the P 2p.2s s p e c t r a of 0PC1  206  Resonance p o s i t i o n s above the mean i o n i s a t i o n edge and bond l e n g t h s  209  5  6.3  3  6.4  3  6.5  - viii  Table 6.6  -  Description  Page  E n e r g i e s , term v a l u e s and p o s s i b l e assignments f o r the F Is r e g i o n of P F  212  E n e r g i e s , term v a l u e s and p o s s i b l e assignments f o r the 0 I s , F Is r e g i o n s of OPF  217  E n e r g i e s , term v a l u e s and p o s s i b l e assignments f o r the 0 Is r e g i o n of 0PC1  219  E n e r g i e s , term v a l u e s and p o s s i b l e assignments f o r the C l 2p,2s r e g i o n s of 0PC1  222  Term v a l u e s f o r phosphorus L - s h e l l s p e c t r a and the c a l c u l a t e d s o r b i t a l quantum d e f e c t  228  M o l e c u l a r o r b i t a l s and e x p e r i m e n t a l i o n i s a t i o n p o t e n t i a l s f o r the v a l e n c e o r b i t a l s of PH , P F 3 , P C 1 , P ( C H ) , 0PC1 and P F  230  E n e r g i e s , term v a l u e s and p o s s i b l e assignments f o r the VSEELS spectrum of P ( C H )  232  E n e r g i e s and p o s s i b l e assignments f o r the VSEELS spectrum of P C 1  236  E n e r g i e s , term v a l u e s and p o s s i b l e assignments f o r the VSEELS spectrum of P F  240  E n e r g i e s and term v a l u e s f o r the VSEELS spectrum of 0PC1  246  5  6.7  3  6.8  3  6.9  3  7.1  7.2  3  3  7.3  3  3  3  3  3  7.4  3  3  7.5  3  7.6  3  7.7  Dipole allowed/forbidden  transitions for PF  5  In  ^3h - y - - - y n  7.8  e  -48  r  E n e r g i e s and p o s s i b l e assignments f o r the VSEELS spectrum of P F  250  Term v a l u e s from ISEELS and VSEELS f o r PH , P ( C H ) , P C 1 , P F , P F and 0PC1  252  E n e r g i e s , term v a l u e s and p o s s i b l e assignments f o r the C Is r e g i o n of the methyl amines  260  E n e r g i e s , term v a l u e s and assignment f o r the N Is energy l o s s spectrum of NH  265  5  7.9  3  3  8.1  8.2  3  3  3  5  3  3  - ix -  Description E n e r g i e s , term v a l u e s and p o s s i b l e assignments f o r the N Is r e g i o n of the methyl amines Resonance energy p o s i t i o n s from edge and C-N l e n g t h s f o r the methyl amines  bond  E n e r g i e s , term v a l u e s and p o s s i b l e assignments f o r the C Is energy l o s s spectrum of butadiene E n e r g i e s , term v a l u e s and p o s s i b l e assignments f o r the C Is energy l o s s spectrum of a l l e n e E n e r g i e s of the f e a t u r e s i n the v a l e n c e s h e l l e l e c t r o n energy l o s s spectrum of butadiene E s t i m a t e d t r a n s i t i o n e n e r g i e s from the v a l e n c e o r b i t a l s of butadiene assuming constant term v a l u e s f o r the v a l e n c e s h e l l E n e r g i e s , term v a l u e s and v a r i o u s assignments f o r the v a l e n c e e l e c t r o n e x c i t a t i o n spectrum of a l l e n e up to the 1st IP E n e r g i e s and term v a l u e s f o r the f e a t u r e s above the 1st i o n i s a t i o n p o t e n t i a l i n the v a l e n c e s h e l l e l e c t r o n energy l o s s spectrum of a l l e n e  -  X  -  - xi -  Figure 4.1  Description  Page  Wide range i n n e r s h e l l e l e c t r o n energy l o s s spectrum of Si(CH ),,  123  4.2  S i 2p e l e c t r o n energy l o s s s p e c t r a o f S K C H ^ ^  125  4.3  S i 2p e x c i t a t i o n s p e c t r a of v a r i o u s s i l i c o n c o n t a i n i n g compounds w i t h SI i n a t e t r a h e d r a l environment  129  3  4.4  R e l a t i o n s h i p of bond l e n g t h to shape-resonance term v a l u e s f o r s i l i c o n c o n t a i n i n g compounds  137  4.5  C Is e l e c t r o n energy l o s s spectrum of S K C H ^ i ^  139  4.6  Valence s h e l l e l e c t r o n energy l o s s spectrum o f SiCCHg)^  143  5.1  P 2p,2s wide range e l e c t r o n energy l o s s s p e c t r a of P F , P C I , PH 3  3  3  and P ( C H ) 3  152  3  5.2  P 2p and 2s e l e c t r o n energy l o s s s p e c t r a of PH  5.3  P 2p and 2s e l e c t r o n energy l o s s s p e c t r a of P ( C H i )  5.4  P 2p and 2s e l e c t r o n energy l o s s s p e c t r a of P F  5.5  P 2p and 2s e l e c t r o n energy l o s s s p e c t r a of P C 1  5.6  158  3  161 164  3  168  3  Inner s h e l l e l e c t r o n energy l o s s s p e c t r a of P C 1 and P F a t v a r i o u s s c a t t e r i n g angles - P l o t of the r a t i o (peak h e i g h t of f e a t u r e X)/(peak h e i g h t of f e a t u r e 2) f o r v a r i o u s t r a n s i t i o n s (X) i n the P C 1 s p e c t r a o f F i g . 5.6 as a f u n c t i o n o f (momentum t r a n s f e r ) 3  3  5.7  3  172  3  2  5.8  5.9  173  Expanded p l o t of the PH P 2p ISEELS continuum structure. The 2p s a t e l l i t e s t r u c t u r e from XPS measurements [175] i s shown below p l o t t e d on the same r e l a t i v e energy s c a l e r e f e r e n c e s to the 2p (mean) edge  178  Long-range and d e t a i l e d i n n e r s h e l l e l e c t r o n energy l o s s s p e c t r a of the F Is r e g i o n of P F  183  3  3  - xii  Figure 5.10  -  Description  Page  Long-range and d e t a i l e d i n n e r s h e l l e l e c t r o n energy l o s s s p e c t r a of the C Is r e g i o n of P ( C H ) 3  5.11  3  Long-range and d e t a i l e d e l e c t r o n energy l o s s s p e c t r a of the C l 2p and 2s r e g i o n s of PC1  189  3  6.1  P 2p,2s wide range e l e c t r o n of P F , 0 P F 3  3  and 0PC1  energy l o s s  spectra 194  3  6.2  P 2p and 2s e l e c t r o n energy l o s s s p e c t r a  of P F  6.3  P 2p and 2s e l e c t r o n  of 0 P F  6.4  P 2p and 2s e l e c t r o n energy l o s s s p e c t r a  6.5  F Is e l e c t r o n energy l o s s spectrum of P F  6.6  Wide range e l e c t r o n energy l o s s s p e c t r a 0 Is and F Is r e g i o n s of 0 P F  6.7  D e t a i l e d e l e c t r o n energy l o s s s p e c t r a 0 Is and F Is r e g i o n s of 0 P F  energy l o s s s p e c t r a  198  5  of 0PC1 5  221  Valence s h e l l e l e c t r o n energy l o s s spectrum of P(CH )  231  Valence s h e l l e l e c t r o n energy l o s s spectrum of PCI  235  Valence s h e l l e l e c t r o n PF  energy l o s s spectrum of 239  Valence s h e l l e l e c t r o n 0PC1  energy l o s s spectrum of  3  3  7.4  3  7.5  216  218  3  7.3  215  E l e c t r o n energy l o s s spectrum of the C l 2p,2s r e g i o n of 0PC1  3  7.2  211  energy l o s s spectrum of the 0 Is r e g i o n  3  7.1  205  of the  3  6.9  3  of the  3  Electron of 0PC1  201  3  3  6.8  186  243  Valence s h e l l e l e c t r o n energy l o s s spectrum of  PFc  247  - xiii  Figure 8.1  8.2  -  Description Long range e l e c t r o n energy l o s s s p e c t r a C Is r e g i o n of the methyl amines  Page of the 258  Short range, h i g h r e s o l u t i o n e l e c t r o n energy l o s s s p e c t r a of the C Is r e g i o n of the methyl amines  259  Long range e l e c t r o n energy l o s s s p e c t r a of the N Is r e g i o n of the methyl amines  262  Short range e l e c t r o n energy l o s s s p e c t r a of the N Is r e g i o n of ammonia and the methyl amines  263  9.1  Inner s h e l l e l e c t r o n energy l o s s s p e c t r a of butadiene  278  9.2  Inner s h e l l e l e c t r o n energy l o s s s p e c t r a of a l l e n e  284  9.3  Valence s h e l l e l e c t r o n energy l o s s s p e c t r a butadiene  290  8.3  8.4  9.4  of  Valence s h e l l e l e c t r o n energy l o s s s p e c t r a of a l l e n e  295  - xiv-  Acknowledgement s  I wish t o express my s i n c e r e thanks to my r e s e a r c h Dr.  C.E. B r i o n .  support,  I t has been a p l e a s u r e  to have worked w i t h him and h i s  d i r e c t i o n and a s s i s t a n c e w i l l always be g r e a t l y  Thanks are a l s o due to the v a r i o u s members of Dr. B r i o n ' s f o r c o n t r i b u t i n g t o an e n j o y a b l e  working environment.  t o Dr. Suzannah D a v i e l f o r r e c o r d i n g the v a l e n c e and  supervisor,  appreciated. research  group  S p e c i a l thanks go  s h e l l spectra of NF  3  SiCCHg)^ on the new spectrometer and t o Tong Leung f o r some  assistance with Dr.  computing.  D.P. Chong, Dr. M.C.L. Gerry and Dr. A . J . Merer of t h i s  department a r e thanked f o r h e l p f u l d i s c u s s i o n s , as a r e Dr. A.P. Hitchcock and  (McMaster) and Dr. P.L. Langhoff ( I n d i a n a ) .  Anna-Marie V e n e z i a - F l o r i a n o  Dr. R.G. C a v e l l  ( A l b e r t a ) are thanked f o r s u p p l y i n g a  number o f i o n i s a t i o n p o t e n t i a l s of the molecules presented Much a p p r e c i a t i o n i s due to T i l l y this  Schreinders  here.  f o r the t y p i n g o f  thesis. The  s t a f f In the departmental workshops must a l s o be thanked f o r  t h e i r capable a s s i s t a n c e i n the maintenance o f the spectrometer. Finally Daniels  I wish t o express my great a p p r e c i a t i o n t o M a r i l y n  f o r a l l her patience  dedicated  to her.  and encouragement.  This thesis i s  - 1 -  CHAPTER 1  INTRODUCTION  A.  General I n t r o d u c t i o n In the l a s t  twenty years t h e r e has been a growing  the use of e l e c t r o n impact  interest i n  s p e c t r o s c o p y to probe e l e c t r o n i c  excita-  t i o n s i n atomic and m o l e c u l a r systems.  E l e c t r o n energy  copy (EELS) [1-3] I s a w e l l e s t a b l i s h e d  technique f o r the study o f  e l e c t r o n i c t r a n s i t i o n s I n the v a l e n c e r e g i o n . technique has been extended electron transitions.  More r e c e n t l y  t o the study o f i n n e r s h e l l  [4] the  (core)  I n the past decade i n n e r s h e l l e l e c t r o n  l o s s spectroscopy (ISEELS) has produced s p e c t r o s c o p i c data  loss spectros-  energy  much new and I n t e r e s t i n g  [5-9] a t r e s o l u t i o n s comparable t o , o r i n some cases  b e t t e r than, t h a t a c h i e v a b l e w i t h p h o t o a b s o r p t i o n techniques i n the s o f t X-ray  range.  Much of the work performed  on i n n e r s h e l l  electron  e x c i t a t i o n (both ISEELS and p h o t o a b s o r p t i o n ) up t o 1982 has been summarised i n a r e c e n t l y p u b l i s h e d b i b l i o g r a p h y [ 1 0 ] . In EELS, a monoenergetic  beam o f e l e c t r o n s i s used  v a r i o u s e l e c t r o n i c t r a n s i t i o n s i n the sample. such a t r a n s i t i o n can be found by measuring  The energy  the energy  s c a t t e r e d e l e c t r o n s ( i . e . the e l e c t r o n s which caused The  system may be d e s c r i b e d a s : -  to e x c i t e required f o r  l o s s of the  the t r a n s i t i o n s ) .  -  e(E ) + M o  where E  q  2  -  • M* + e ( E -E') o ' v  i s the energy of the i n c i d e n t beam and ( E - E ' ) i s the energy Q  of the s c a t t e r e d e l e c t r o n .  Thus E', the energy l o s s  photon e n e r g y ) , i s the energy r e q u i r e d ground s t a t e to the M  excited  (analogous t o  f o r the sample to go from the M  state.  I t can be seen that i n f o r m a t i o n a k i n to p h o t o a b s o r p t i o n i s o b t a i n e d , however, the use of e l e c t r o n s has some d i f f e r e n c e s which i n some cases l e a d 1)  to d i s t i n c t  advantages.  P h o t o a b s o r p t i o n i s a resonant p r o c e s s , the photon energy must e x a c t l y match the t r a n s i t i o n energy, whereas w i t h e l e c t r o n the process i s non-resonant by the s c a t t e r e d  2)  Different  impact  s i n c e the excess energy i s c a r r i e d o f f  electron.  sources and hence techniques are r e q u i r e d  s o r p t i o n i n o r d e r to cover a wide range of e n e r g i e s .  i n photoabElectron  impact, however, a l l o w s coverage of a range e x t e n d i n g from zero energy l o s s to the X-ray r e g i o n w i t h a s i n g l e s p e c t r o m e t e r . 3)  In the s o f t X-ray r e g i o n (~200 eV - 1000 e V ) , the r e s o l u t i o n of e l e c t r o n impact e x c i t a t i o n i s comparable or b e t t e r than t h a t a c h i e v e d so f a r w i t h p h o t o a b s o r p t i o n [ 8 ] .  4)  With low momentum t r a n s f e r  ( h i g h i n c i d e n t energy and s m a l l  s c a t t e r i n g a n g l e ) a f a s t e l e c t r o n beam p r o v i d e s a d i p o l e  excita-  t i o n mechanism which i s an e f f e c t i v e a l t e r n a t i v e to the use of a tuneable photon s o u r c e .  A v i r t u a l photon f i e l d  i s induced i n the  -  t a r g e t by the p a s s i n g e l e c t r o n .  3  -  The  a s s o c i a t e d with the beam as a sharp h i g h enough energy,  t a r g e t sees the e l e c t r i c impulse  i n time, which, w i t h  approaches a d e l t a f u n c t i o n .  Fourier  t r a n s f o r m i n g t h i s i n t o a frequency domain y i e l d s a range of f r e q u e n c i e s (assuming intensity.  continuous  an i d e a l d e l t a f u n c t i o n ) of  Thus s p e c t r a o b t a i n e d i n t h i s manner w i l l  resemble o p t i c a l  s p e c t r a and  s e l e c t i o n rules w i l l apply.  i n essence On  field  uniform  closely  the o p t i c a l ( d i p o l e )  l o w e r i n g the impact  energy  and/or  i n c r e a s i n g the s c a t t e r i n g a n g l e , the momentum t r a n s f e r i s i n c r e a s e d and o p t i c a l l y f o r b i d d e n t r a n s i t i o n s become more In  t h i s way  the use of e l e c t r o n s not o n l y compliments  important.  the  i n f o r m a t i o n o b t a i n e d by p h o t o a b s o r p t i o n but i s a l s o a b l e to  extend  it. The primary concern of the work d e s c r i b e d i n t h i s t h e s i s i s the study of d i p o l e allowed  t r a n s i t i o n s i n a v a r i e t y of m o l e c u l a r  as o b t a i n e d by u s i n g h i g h energy angle.  The major focus w i l l  e l e c t r o n impact  and  small  be on i n n e r s h e l l e l e c t r o n  systems  scattering  excitation,  however, v a l e n c e s h e l l e l e c t r o n e x c i t a t i o n f o r many of the systems w i l l a l s o be p r e s e n t e d . s i m p l e r and spectrum  i n n e r s h e l l e x c i t a t i o n spectrum  l e s s ambiguous to a s s i g n than the v a l e n c e s h e l l  s i n c e the o r i g i n a t i n g o r b i t a l  the other o r b i t a l s and of  The  i s often excitation  i s u s u a l l y w e l l separated  can be p o s i t i v e l y i d e n t i f i e d .  Thus knowledge  the term v a l u e s ( d i f f e r e n c e of the e x c i t a t i o n f e a t u r e from  i o n i s a t i o n l i m i t ) from  the i n n e r s h e l l spectrum  p r e t a t i o n of the v a l e n c e s h e l l spectrum.  from  can a i d i n the  the inter-  Before d i s c u s s i n g some of  - 4 -  the  t h e o r e t i c a l considerations  relevant  r e l a t i o n s h i p w i t h p h o t o a b s o r p t i o n and e x c i t a t i o n spectra of  the a s p e c t s of e l e c t r o n  to e l e c t r o n  B.  2s,  Figure 2p,  the  features  i t i s useful  impact,  the  observed i n e l e c t r o n  to b r i e f l y review some  s p e c t r o s c o p y i n g e n e r a l and  how  they r e l a t e  excitation.  Description The  in  i n more d e t a i l ,  to e l e c t r o n  of v a r i o u s  various 1.1.  3s,  3p,  processes i n e l e c t r o n  types of processes which can  Note that both X-ray (K, L, M, ....)  notation  spectroscopy occur are  ...)  and  illustrated  orbital  (Is,  f o r e l e c t r o n energy l e v e l s w i l l be  used  i n t e r c h a n g e a b l y throughout t h i s t h e s i s .  I)  Electron As  Excitation.  t h i s i s the main t o p i c of t h i s work, t h i s i s  introduced  f i r s t , however, o n l y a b r i e f d e s c r i p t i o n i s presented h e r e .  A more  d e t a i l e d and  sections.  complete d e s c r i p t i o n f o l l o w s  The  process i s i s i l l u s t r a t e d  one  electron picture.  i n Figures  Interaction  i n the 1.1B  of the  and  subsequent 1.1C  i n i t i a l ground s t a t e of  molecule or atom with e i t h e r a photon or e l e c t r o n can electrons Use  of the  target  species  of a photon r e q u i r e s  to the  the  e l e c t r o n while Figure  Figure 1.1C  t r a n s i t i o n e n e r g i e s f o r the  1.1B  cause one  to be  incoming e l e c t r o n can shows the  the  level.  exactly  equal  impart whatever  promotion of an  inner  shows that of a v a l e n c e e l e c t r o n . former (1.1B) l i e  a a  of  promoted to an unoccupied  t r a n s i t i o n energy (E')  photon energy whereas an  energy i s r e q u i r e d .  to be  In terms of  The  shell  -  5  -  t  11 I  t J, t  11  11  IT k  4  1—  1  E  J t  Ci)  (il)  D  photon  unocc. 3 2 L,  2s  K  1s  L  1t i  t  i t  Figure  1.1  Processes involved  .  L  in electron  spectroscopy.  A. I n i t i a l s t a t e B,C. E l e c t r o n e x c i t a t i o n by e l e c t r o n impact or p h o t o a b s o r p t i o n ( s e c t i o n B.I) D. P h o t o i o n i s a t i o n ( i ) XPS ( i i ) UPS ( s e c t i o n B.II) E. P h o t o i o n i s a t i o n r e s u l t i n g i n e x c i t e d i o n s t a t e s "shake-up" - " s h a k e - o f f " ( s e c t i o n B . I I I ) F. Core v a c a n c y r e s u l t i n g from i o n f o r m a t i o n a l l o w s G and H ( s e c t i o n B.IV) G. X-ray f l u o r e s c e n c e H. Auger p r o c e s s I . Core v a c a n c y r e s u l t i n g from f o r m a t i o n of an e x c i t e d n e u t r a l s p e c i e s a l l o w s J ( s e c t i o n B.IV) J. Autoionisation.  -  6  -  ( t y p i c a l l y ) i n the XUV or s o f t X-ray r e g i o n s of the e l e c t r o m a g n e t i c spectrum w h i l e f o r the l a t t e r the  vacuum or f a r UV.  Thus i n p h o t o a b s o r p t i o n s e v e r a l  chromators a r e u s u a l l y Transitions  (1.1C) the t r a n s i t i o n e n e r g i e s a r e i n  needed  t o cover the UV and X-ray  spectrum c o n s i s t i n g  l i n e s c o n v e r g i n g upon the p a r t i c u l a r i o n i s a t i o n l i m i t . following  Rydberg  T  =  E  I P -  E  '  = 7 ^  where E' Is the t r a n s i t i o n energy; E _ Rydberg  c o n s t a n t (13.605eV);  and i s o b t a i n e d by c o r r e c t i n g quantum d e f e c t ,  6^,  n  = 7-7-2-  p  n  i s the i o n i s a t i o n l i m i t ;  t o an s, p, d e t c . type o f  atom 6^ = 0, thus the quantum d e f e c t can  3  of the Rydberg  I n essence i t can be thought of as  b i n d i n g energy of the newly promoted  f a l l s as n ~  from simple h y d r o g e n - l i k e  The term v a l u e , T, i s the d i f f e r e n c e  orbital.  Ris  the p r i n c i p a l quantum number, n, by a  f e a t u r e from the i o n i s a t i o n l i m i t .  Rydberg  (1-B.l.)  ( ~^$)  be thought of as a measure of the d e v i a t i o n  the  fits  i s the e f f e c t i v e quantum number  i s promoted  F o r the hydrogen  behaviour.  The s e r i e s  which i s c h a r a c t e r i s t i c of the A quantum number  ( i . e . whether the e l e c t r o n orbital).  of a s e r i e s of  formula.  (n )  the  region.  t o the unoccupied l e v e l s from a p a r t i c u l a r l e v e l i n  atoms w i l l r e s u l t i n an e x c i t a t i o n  the  d i f f e r e n t mono-  electron  The i n t e n s i t y o f t r a n s i t i o n s  occupying the  to the Rydberg  [11,12], thus i t becomes i n c r e a s i n g l y  the h i g h e r l y i n g Rydberg  states.  difficult  orbitals to observe  -  The cules.  concept  7  -  of the Rydberg o r b i t a l can be extended  to mole-  The Rydberg o r b i t a l s are l a r g e and d i f f u s e and hence w i l l  i n c r e a s i n g l y see the molecule  as one  to Rydberg o r b i t a l s , molecules  large  core.  However, i n a d d i t i o n  w i l l a l s o have unoccupied  o r b i t a l s which a r i s e out of the MO  scheme.  the a n t i b o n d i n g c o u n t e r p a r t s of the bonding  virtual  These o r b i t a l s are MOs.  valence  usually  Thus they are  d e l o c a l i s e d and of s i m i l a r s i z e to the occupied outer v a l e n c e o r b i t a l s of the m o l e c u l e .  Depending on the p a r t i c u l a r molecule,  the  virtual  v a l e n c e o r b i t a l s can be low l y i n g , i n which case t r a n s i t i o n s to them w i l l be seen i n the d i s c r e t e p o r t i o n however, they may The  be h i g h l y i n g and  thus occur i n the continuum.  above a s p e c t s of Rydberg and v a l e n c e o r b i t a l s w i l l  expanded upon In s e c t i o n E. molecular  of the e x c i t a t i o n spectrum,  A good review of the o r b i t a l concept  in  s p e c t r o s c o p y has been g i v e n by W i t t e l and McGlynn [ 1 3 ] .  [12] a l s o g i v e s a d e t a i l e d d i s c u s s i o n  II)  be  of Rydberg and v a l e n c e  Robin  states.  Photoionisation. This  is illustrated  of a molecule energy,  i n F i g u r e 1.1D.  In t h i s the i n i t i a l  state  or atom i s bombarded with photons of a c h a r a c t e r i s t i c  hv, a t r a n s i t i o n occurs i n which the f i n a l  a free electron.  C o n s e r v a t i o n of energy  hv = E  R  +  E_  requires  p  s t a t e i s an i o n p l u s that  (I.B.2)  -  8  -  where E ^ i s the k i n e t i c energy of the e j e c t e d energy r e q u i r e d  to form the i o n .  e l e c t r o n and  i s the  Since the mass of the i o n i s s e v e r a l  thousand times that of the e l e c t r o n , c o n s e r v a t i o n  of momentum d i c t a t e s  t h a t e s s e n t i a l l y a l l the k i n e t i c energy i s taken up by the e j e c t e d electron.  Thus by measuring  and knowing hv, E.^, which i n the one  e l e c t r o n example shown i n F i g . 1.1D i s the i o n i s a t i o n p o t e n t i a l o r binding  energy of an e l e c t r o n from a p a r t i c u l a r o r b i t a l , can be  obtained. in  The i o n i s a t i o n p o t e n t i a l can be equated to the d i f f e r e n c e  the t o t a l e n e r g i e s of the i o n s t a t e and the ground  s t a t e of the  species:  E  IP  =  E  f ^ S  N  1  )  "  E  (  N  (1.B.3)  )  G  g  where E _ i s the i o n i s a t i o n p o t e n t i a l of e l e c t r o n s; E (N) i s the IP d T  t o t a l energy of the ground the  s t a t e and E ^ ( N - l ) i s the t o t a l energy o f  i o n which i s formed when e l e c t r o n s i s removed.  Thus the i o n i s a -  t i o n e n e r g i e s can be o b t a i n e d t h e o r e t i c a l l y by r i g o r o u s of the a p p r o p r i a t e  total  calculations  energies.  Quantum m e c h a n i c a l l y the p r o b a b i l i t y of a t r a n s i t i o n from the i n i t i a l ground is  proportional  s t a t e (<|/') to the f i n a l s t a t e (<\>' = i o n + f r e e  electron)  to the square of the t r a n s i t i o n moment i n t e g r a l  M -<«|>" | Sp| c|>'>  (1.B.4)  -  9  -  where p Is the d i p o l e moment o p e r a t o r . Oppenheimer approximation of e l e c t r o n i c  separates  (4^) and n u c l e a r  A p p l i c a t i o n of the Born-  the wave f u n c t i o n i n t o a  (<l^) f u n c t i o n s .  Thus equation  product (I.B.4)  can be w r i t t e n as  M-  /4>* (RH '(R)dR/c|> v  v  e  (r,R)|2 P j c | / ( r , R ) d r  (I.B.5)  where the n u c l e a r f u n c t i o n (^M) bas been f u r t h e r p a r t i t i o n e d t i o n a l (<1>) and r o t a t i o n a l components. V  vibra-  I n the m a j o r i t y of cases r o t a -  t i o n a l s t r u c t u r e cannot be r e s o l v e d and i s ignored a p h o t o e l e c t r o n t r a n s i t i o n to be allowed, e q u a t i o n must be non-zero.  into  (as i n ( I . B . 5 ) ) .  For  the i n t e g r a l s i n the above  Since the f i n a l s t a t e i n c l u d e s a f r e e  e l e c t r o n there i s always a non-zero v a l u e and as a consequence a l l one e l e c t r o n t r a n s i t i o n s are a l l o w e d . The  v i b r a t i o n a l p a r t of equation  (I.B.5) i s termed the Franck-  Condon f a c t o r and g i v e s the i n t e n s i t i e s and shape of the v i b r a t i o n a l envelope.  T h i s shape can be i n d i c a t i v e of the type of e l e c t r o n being  ionised.  F o r example, i o n i s a t i o n of a non-bonding e l e c t r o n would l e a d  to very l i t t l e  change i n the n u c l e a r c o o r d i n a t e s of the i o n from that of  the ground s t a t e molecule and thus  the v" = 0 •> v' = 0 (where v denotes  the v i b r a t i o n a l quantum number) t r a n s i t i o n would have the s t r o n g e s t o v e r l a p and there would o n l y be a s h o r t p r o g r e s s i o n .  I n other words the  a d i a b a t i c and v e r t i c a l i o n i s a t i o n e n e r g i e s c o i n c i d e .  An a d i a b a t i c  t r a n s i t i o n i s d e f i n e d as the l e a s t energy r e q u i r e d to e j e c t a p a r t i c u l a r  - 10  e l e c t r o n from a molecule v" = 0 •* v*  •  i n i t s ground  0 transition.  -  s t a t e , i . e . i t corresponds to the  A v e r t i c a l t r a n s i t i o n i s d e f i n e d as the  most i n t e n s e , i . e . a v" = 0 •*• v  f  = n t r a n s i t i o n , where n i s the  v i b r a t i o n a l f u n c t i o n w i t h the l a r g e s t o v e r l a p w i t h the ground  state.  T r a d i t i o n a l l y p h o t o e l e c t r o n s p e c t r o s c o p y has been d i v i d e d two s e c t i o n s depending  upon the photon  source b e i n g used.  One  branch i s P h o t o e l e c t r o n Spectroscopy (PES) or U l t r a v i o l e t e l e c t r o n (UPS).  In t h i s case u l t r a v i o l e t  (21.22 eV) p r o v i d e s the photon o r b i t a l s are a c c e s s i b l e .  Use  into  major  Photo-  r a d i a t i o n , t y p i c a l l y He  I  source and hence o n l y the v a l e n c e of the He(II) resonance  l i n e at 40.81  eV  i n p r i n c i p l e a l l o w s the i n n e r v a l e n c e o r b i t a l s to be probed but i n p r a c t i c e t h i s i s p r e c l u d e d by the He I "shadow".  An e x c e l l e n t book by  Rabalais  The o t h e r ^branch i s  [15] covers the v a r i o u s a s p e c t s of UPS.  E l e c t r o n Spectroscopy f o r Chemical A n a l y s i s e l e c t r o n Spectroscopy (XPS). X-ray.  (ESCA) or X-ray  In t h i s case the photon  The most common sources used are the Mg  Ka (1486.58 eV). much work on XPS  Ka (1253.64 eV) and A l There has been  ever s i n c e the p i o n e e r i n g work of Siegbahn  e l e c t r o n shows a c h e m i c a l s h i f t  o b t a i n e d from Siegbahn's t h i s Swedish group  [16,17],  that the i n n e r s h e l l  i n i t s b i n d i n g energy which r e f l e c t s  change i n the m o l e c u l a r environment.  XPS  source i s an  Thus the core l e v e l s can be a c c e s s e d .  The primary a p p l i c a t i o n a r i s e s from the f a c t  Photo-  A p e r s p e c t i v e of i t s impact  Nobel l a u r e a t e address  has culminated i n a new  [18].  a  can be  The work from  h i g h - r e s o l u t i o n multipurpose  spectrometer which f e a t u r e s X-ray monochromation [19].  - 11  Ionisation are that  invaluable  -  e n e r g i e s o b t a i n e d from p h o t o e l e c t r o n  to the  an a c c u r a t e and  interpretation  of e l e c t r o n  spectroscopy  excitation  unambiguous v a l u e i s obtained f o r the  energy (or i o n i s a t i o n l i m i t ) of a p a r t i c u l a r e l e c t r o n . analysis  of a Rydberg s e r i e s  (l.B.l)).  should y i e l d the  However, o n l y a few  v a l e n c e s h e l l spectrum due  l e v e l s can  IP  usually  to such f a c t o r s  as  [20].  interactions  s e r i e s due  to p o s s i b l e  These f a c t o r s ,  the  be  the n ~  i n t e n s i t y , other o v e r l a p p i n g t r a n s i t i o n s as w e l l lower Rydbergs from the  (see  spectra  in  binding  In  theory,  equation identified in  the  drop i n  3  as d e v i a t i o n s  of  valence-Rydberg  l a c k of r e s o l u t i o n  and  the  orbital lifetime  c o n s i d e r a t i o n s make matters even worse f o r i n n e r s h e l l  transitions.  Thus i t i s of f a r more use  spectrum  knowing the  IP a c c u r a t e l y  e s t a b l i s h the v a l u e s are, section  to i n t e r p r e t from UPS  the  or XPS  excitation  and  hence being a b l e  term v a l u e s w i t h c o n s i d e r a b l e a c c u r a c y .  to  These term  i n g e n e r a l , c h a r a c t e r i s t i c of p a r t i c u l a r t r a n s i t i o n s  (see  E). A further  come from the o n l y the  aid in interpreting  spectral  nature of the  shape of the initial  i n p a r t i c u l a r the  spectra In  s u g g e s t i v e of one  to a Rydberg l e v e l .  non-bonding nature of  excitation  the  i s v e r y s i m i l a r to the  c o n s i d e r e d , however,  Thus i f the  spectra,  This arises  the  spectral resolution)  transition is  from the  Rydberg o r b i t a l and ion state.  can  ionisation  v i b r a t i o n a l envelope at h i g h  i o n i s a t i o n and  (Rydberg) s t a t e  to be  considered.  i s s i m i l a r i n the  d i f f u s e and  excitation  p h o t o i o n i s e d band.  o r b i t a l has  i n e x c i t a t i o n both l e v e l s must be shape (and  electron  so  large, the  However, c a r e  upper has  - 12 -  to be e x e r c i s e d anti-bonding  Ill)  s i n c e some of the lower Rydberg s t a t e s may have some  character  due to Rydberg-valence mixing [ 2 0 ] .  "Shake-up" and "Shake-off". Upon p h o t o i o n i s a t i o n  which the p h o t o e l e c t r o n e x c i t a t i o n of an outer i n t o the continuum. is  an e x c i t e d i o n s t a t e may be formed i n  has been emitted  along  w i t h the simultaneous  e l e c t r o n e i t h e r to an e x c i t e d bound s t a t e or  The former Is termed "shake-up" w h i l e the l a t t e r  termed " s h a k e - o f f " .  The process i s i l l u s t r a t e d  i n F i g . 1.1E.  Upon  i o n i s a t i o n the r e s u l t a n t i o n can be i n one of a number of s t a t e s w i t h the i o n i s a t i o n energy of the p h o t o e l e c t r o n  being  g i v e n by a m o d i f i e d  form of e q u a t i o n (1.B.3)  E  S I  ,  t  p  = E  S f  ,  t  - E (N) G  when t = 0, the i o n i s i n i t s ground s t a t e and t h i s g i v e s major p h o t o i o n i s a t i o n  peak.  The s a t e l l i t e  (1.B.6)  r i s e to the  l i n e s are defined  by t =  1, 2 e t c . and s i n c e they denote e x c i t e d i o n s t a t e s the s a t e l l i t e s  will  appear on the low k i n e t i c energy s i d e o f the major peak. E q u a t i o n (1.B.6) i m p l i e s  t h a t there  i s no fundamental  differ-  ence between the major peak and the s a t e l l i t e s and that each s t a t e i s reached by a one-step, " o n e - e l e c t r o n " photoionisation  d i p o l e t r a n s i t i o n [18] (note:  f o l l o w s d i p o l e s e l e c t i o n r u l e s - see e q u a t i o n  (1.B.5)).  On a s i m p l i s t i c model, "shake-up" can be thought of as the e m i s s i o n o f  - 13  a photoelectron  -  w i t h the attendant e x c i t a t i o n of a valence  Since the a l l o w e d , e x c i t e d  electron.  i o n s t a t e s must have the same symmetry as  the  i o n i n i t s ground s t a t e , the v a l e n c e e l e c t r o n t r a n s i t i o n must i n essence f o l l o w monopole s e l e c t i o n r u l e s .  Thus i t might be  a d i r e c t r e l a t i o n s h i p between XPS  satellite  thought t h a t there  s t r u c t u r e and  e l e c t r o n e x c i t a t i o n as observed by p h o t o a b s o r p t i o n EELS.  However, care must be  taken i n any  p h o t o a b s o r p t i o n s p e c t r o s c o p y leads e s s e n t i a l l y be d e s c r i b e d the XPS al.  s a t e l l i t e structure  leads  valence  spectroscopy  comparison s i n c e EELS or  describe  the  included  to an e x c i t e d i o n s t a t e .  i n both h o l e and  An  Martin  configuration to  photoionisation  a l t e r n a t i v e approach which a l s o accounts f o r c o r r e l a t i o n  e f f e c t s i s the many-body Green f u n c t i o n method used by Cederbaum co-workers  et  ground s t a t e i n order  s a t e l l i t e s t r u c t u r e accompanying  adequately.  can  e l e c t r o n d e s c r i p t i o n whereas  [21,22] have shown t h a t many e l e c t r o n theory i n c l u d i n g  i n t e r a c t i o n must be  or  to an e x c i t e d n e u t r a l s t a t e which  by a simple one  [23,24] to d e s c r i b e  and  inner valence s h e l l i o n i s a t i o n  processes. I n a d d i t i o n to p r o v i d i n g  information  e x c i t a t i o n , knowledge of the XPS  s t a t e s and processes.  The  XPS  i n ISEELS and  spectrum p r o v i d e s i n f o r m a t i o n  i n p a r t i c u l a r gives The  on v a l e n c e  s a t e l l i t e spectrum can  i n t e r p r e t a t i o n of continuum s t r u c t u r e spectra.  is  production  f e s t e d i n ISEELS by  electron assist in  the  photoabsorption  on the e x c i t e d  ion  the v e r t i c a l energy f o r the i o n i s a t i o n  of these e x c i t e d i o n s t a t e s would be  ( a d i a b a t i c ) onsets of new  should be p o s s i b l e to i d e n t i f y which f e a t u r e s  continua. i n the  mani-  Thus i t  ISEELS spectrum  -  14  above the i o n i s a t i o n edge ( t h e major  -  i o n i s a t i o n peak) a r i s e from the  onsets of e x c i t e d i o n s t a t e s and by i n f e r e n c e which f e a t u r e s a r e due to o t h e r types of phenomena such as resonances or double Continuum  excitations.  s t r u c t u r e i n ISEELS or p h o t o a b s o r p t i o n s p e c t r o s c o p y w i l l be  d i s c u s s e d i n s e c t i o n E.  IV.  X-ray F l u o r e s c e n c e ,  Auger Decay and A u t o i o n i s a t i o n .  F o l l o w i n g the c r e a t i o n of a h o l e s t a t e , e i t h e r as an i o n ( F i g . 1.1F), o r as an e x c i t e d n e u t r a l s t a t e ( F i g u r e 1.11), secondary p r o c e s s e s can occur which a l l o w the system to a c h i e v e a lower energy s t a t e . the i o n s t a t e i n which t h e r e i s a core vacancy, the two major modes are X-ray f l u o r e s c e n c e and the Auger  process.  For  decay  I n the case of  X-ray f l u o r e s c e n c e the excess energy i s r e l e a s e d i n the form o f a photon whereas i n the Auger electron.  p r o c e s s the excess energy i s g i v e n to an e m i t t e d  The two p r o c e s s e s a r e i l l u s t r a t e d  i n F i g u r e s 1.1G and 1.1H  respectively. The r e s p e c t i v e y i e l d s f o r atomic K s h e l l f l u o r e s c e n c e and Auger decay a r e g i v e n by  P  where W  R  f  + P A  and  a.  P  + P A  f  i s the K s h e l l f l u o r e s c e n c e y i e l d , a  R  i s the Auger y i e l d ;  P^  and P^ a r e the t r a n s i t i o n p r o b a b i l i t i e s f o r X-ray f l u o r e s c e n c e and the Auger  process r e s p e c t i v e l y .  F o r the l i g h t e r elements the Auger  process  dominates  [ 2 5 ] . F o r example, the f l u o r e s c e n t y i e l d  s h e l l of the n i t r o g e n atom i s 5.2 x 1 0 ~ [25].  F o r the  3.9 x 1 0 ~  5  f o r the K  and i t s Auger y i e l d  3  i s 0.995  ( 2 s ) l e v e l of phosphorus, the f l u o r e s c e n t y i e l d i s  and f o r the L  (2p) l e v e l i t i s 6.2 x I0~ [ 2 5 ] . h  I n s p i t e of the low y i e l d  f o r l i g h t elements v a r i o u s X-ray  f l u o r e s c e n c e s t u d i e s have been r e p o r t e d u t i l i s i n g h i g h r e s o l u t i o n spectrometers  [26-28] .  X-ray t r a n s i t i o n s a r e governed by d i p o l e  s e l e c t i o n r u l e s and so the L^ ^ (-P) "* K ( I s ) t r a n s i t i o n (as shown i n F i g . 1.1) i s allowed  but the L^ ( 2 s ) -*• K ( I s ) t r a n s i t i o n i s f o r b i d d e n  T h i s f a c t o r can a i d i n the i d e n t i f i c a t i o n of v a l e n c e provide  s t a t e s and  complimentary i n f o r m a t i o n to UPS [ 2 6 ] . In other  Nordgren e t a l . [27] have r e p o r t e d s p e c t r a of C 0  2  the C K - s h e l l X-ray  and from a Franck-Condon f i t of harmonic  studies,  fluorescence oscillators  have determined the n a t u r a l width of the C Is s t a t e to be 0.07 ± 0.02 eV.  A f u r t h e r a p p l i c a t i o n stems from the combination of X-ray  f l u o r e s c e n c e e n e r g i e s w i t h UPS i o n i s a t i o n p o t e n t i a l s to estimate  core-  e l e c t r o n binding energies [28]. The vacancies  Auger process of l i g h t atoms.  i s the dominant decay mode f o r i n n e r  The energy of the Auger e l e c t r o n i s g i v e n  by the d i f f e r e n c e between the t o t a l energy of the i n i t i a l hole and  shell  state  that of the two hole s t a t e :  E (XYZ) = A  E  w  f  (  x  )  -  E  m  +  +  (  y  z  )  where E.(XYZ) i s the k i n e t i c energy of the emitted  (1.B.7)  XYZ Auger e l e c t r o n  - 16 -  E^+^j  i s the t o t a l energy  of the i n i t i a l  -•jy-H-^yz)  s p e c i e s with a h o l e i n l e v e l X; final  two-hole s t a t e , a doubly  and Z.  ionised  hole s t a t e , a s i n g l y -  s  t  n  e  t  o  t  a  -  energy  ionised  of the  s p e c i e s with h o l e s i n l e v e l s Y  Thus i n F i g . 1.1H the emitted Auger e l e c t r o n would be d e s i g n a t e d  as the K L ^ L  3  Auger t r a n s i t i o n .  U n l i k e X-ray  f l u o r e s e n c e , the Auger  process i n v o l v i n g an i n i t i a l K h o l e s t a t e can i n v o l v e the  shell.  Its  s e l e c t i o n r u l e s a r e AL = AS = AJ = 0 and p a r i t y unchanged.  Thus the  Auger process i s not a d i p o l e t r a n s i t i o n f o l l o w e d by e j e c t i o n o f an e l e c t r o n but r a t h e r i t a r i s e s from a coulombic electrons involved [29]. l i g h t elements ( i t i s ~ 1 0 fluorescence  [11]).  C l e a r l y i t i s the much f a s t e r process f o r 3  times f a s t e r f o r C and 0 K - s h e l l s than  A s p e c i a l type of Auger decay i s termed a  Coster-Kronig t r a n s i t i o n .  T h i s i s a very r a p i d process and i n v o l v e s an  Auger t r a n s i t i o n i n which the primary vacancy  t r a n s i t i o n would i n v o l v e the f i l l i n g 2  is filled  by a h i g h e r  Thus a L ^ L j 3 M C o s t e r - K r o n i g  l y i n g e l e c t r o n w i t h i n the same s h e l l .  the L  rearrangement of the two  of a L^ h o l e by an e l e c t r o n  from  3 s u b - s h e l l with the r e s u l t i n g e m i s s i o n of a M - s h e l l e l e c t r o n . Those Auger s p e c t r a which o n l y i n v o l v e i n n e r s h e l l e l e c t r o n s  are a t o m i c - l i k e i n n a t u r e .  As i n the case of XPS s p e c t r a , i n n e r s h e l l  Auger l i n e s a l s o show a chemical  shift.  E x t e n s i v e s e r i e s of K L 2 L  3  Auger  e l e c t r o n chemical s h i f t s have been r e p o r t e d f o r S i [30], P [31] and S [32] c o n t a i n i n g compounds.  A combination  of Auger and XPS chemical  s h i f t s a l l o w s an estimate of the e x t r a - a t o m i c r e l a x a t i o n ( i . e . a t t r a c t i o n of the h i g h e r l e v e l s  towards a h o l e ) upon c r e a t i o n of  - 17 -  a core-hole.  With t h i s i t i s p o s s i b l e  p o l a r i s a b i l i t y of the v a r i o u s involve  ligands  t o estimate the r e l a t i v e [31,33].  Auger s p e c t r a  the valence s h e l l are very complex due t o the many  transitions.  However, v a r i o u s  overlapping  gas phase s t u d i e s as w e l l as c a l c u l a -  t i o n s have been performed on Auger s p e c t r a  [34-36].  S i m i l a r types o f decay modes w i l l occur when the h o l e a r i s e s from e l e c t r o n e x c i t a t i o n g i v i n g r i s e s t a t e ( F i g . 1.11).  The n o n - r a d i a t i v e  termed a u t o i o n i s a t i o n involves electron.  the f i l l i n g  which  ( F i g . 1.1J).  to an e x c i t e d  state  neutral  process a k i n t o Auger decay i s  As i n Auger decay, the process  o f the h o l e w i t h the concomitant e j e c t i o n of an  The emitted e l e c t r o n may be the i n i t i a l l y  excited  electron  or some other e l e c t r o n . These decay modes are important to p h o t o i o n i s a t i o n  and e l e c t r o n  e x c i t a t i o n as they govern the l i f e t i m e of the h o l e s t a t e and hence the n a t u r a l l i n e width of the f e a t u r e .  The n a t u r a l l i n e width (AE) i s  r e l a t e d t o the l i f e t i m e (At) v i a the Heisenberg u n c e r t a i n t y p r i n c i p l e :  AE • AT ~ ft = 6.582 x 1 0 ~  For  1 6  eV.sec  (1.B.8)  example l e v e l s which can undergo r a p i d C o s t e r - K r o n i g  have a s h o r t  l i f e t i m e and hence are wide.  l i n e w i d t h f o r the  the n a t u r a l  ( 2 s ) l e v e l i n P i s 1.26eV whereas i t i s o n l y  0.033 eV and 0.032 eV f o r the L  3  ( 2 p , ) and L 3/  v e l y and 0.53 eV f o r the K ( I s ) l e v e l linewidth  For i n s t a n c e  transitions  (energy) f o r a g i v e n  2  [37].  2  ( 2 p ^ ) levels respectix  In g e n e r a l ,  subshell increases  the n a t u r a l  with increase  i n Z,  - 18 -  a l o n g the p e r i o d i c T a b l e [ 3 7 ] . The cies.  above d i s c u s s i o n s have d e a l t mainly w i t h i n n e r s h e l l  O b v i o u s l y s i m i l a r processes can occur f o r v a l e n c e h o l e  below the f i r s t  IP.  state  Another form of mechanism which can occur  r e d u c i n g l i f e t i m e s of a p a r t i c u l a r  C.  vacan-  state i s p r e - d i s s o c i a t i o n .  Fundamental Concepts i n E l e c t r o n Impact and the R e l a t i o n s h i p t o Photoabsorption To understand  e l e c t r o n energy  the processes which produce the f e a t u r e s i n an  l o s s spectrum  and to r e l a t e them to the o p t i c a l  ( p h o t o a b s o r p t i o n ) spectrum, i t i s necessary fundamental concepts behind  to d i s c u s s some of the  the c o l l i s i o n p r o c e s s .  T h i s can o n l y come  from a quantum mechanical  d e s c r i p t i o n such as was f i r s t  in  review of Bethe's treatment  1930 [ 3 8 ] .  Inokuti  [39].  A thorough Wight  g i v e n by Bethe  has been g i v e n by  [ 5 ] , u s i n g the i d e a s d i s c u s s e d by L a s s e t t r e [ 4 0 ] ,  has a l s o d i s c u s s e d i n d e t a i l the Bethe theory f o r e l e c t r o n s c a t t e r i n g by the hydrogen atom i n c l u d i n g a g e n e r a l i s a t i o n to more complex s p e c i e s . T h i s approach [5,40] w i l l be f o l l o w e d i n the present work. the e a r l i e r  r e f e r e n c e d works, a d e t a i l e d  treatment  In view o f  w i l l not be g i v e n and  o n l y the p e r t i n e n t p o i n t s and d e f i n i t i o n s w i l l be d i s c u s s e d . When an i n c i d e n t beam of e l e c t r o n s i n t e r a c t s with a t a r g e t molecule,  the l a t t e r may be e x c i t e d from  excited state.  its initial  s t a t e to some  The p r o b a b i l i t y of such a t r a n s i t i o n i s known as the  - 19 -  Differential electrons  Cross-Section  scattered  (DCS).  one  second.  1  state, divided  i n the i n c i d e n t beam which crossed  by the  u n i t area i n  s c a t t e r i n g o f f an H atom i s g i v e n by [ 5 ] :  |g  o  to i t s n** e x c i t e d  angle  I f the i n c i d e n t beam i s approximated by a plane wave the  DCS f o r i n e l a s t i c  where k  i s the number of i n c i d e n t  per second through an angle 9 i n t o a s o l i d  dQ a f t e r e x c i t i n g the t a r g e t number of e l e c t r o n s  This  and k  n  (9) dQ - ^ _ o  |f (9)| n  2  dQ  (l.C.l)  a r e the wave numbers f o r the i n c i d e n t and s c a t t e r e d  beams r e s p e c t i v e l y and f ( ® ) -  s  t  n  n  e  s c a t t e r i n g amplitude.  The square  2 of the s c a t t e r i n g amplitude, volume a t u n i t d i s t a n c e  |f (9)| n  , i s the number of e l e c t r o n s / u n i t  and angle 9 which have e x c i t e d  the atom to i t s  th n  state.  Since the wave number i s p r o p o r t i o n a l  to momentum,  2 k |f (9)| n  i s proportional  n  crossing  to the number of s c a t t e r e d  u n i t area i n one second a t angle 9, and k  the  t o t a l number o f e l e c t r o n s  per  second.  these  Q  electrons  i s p r o p o r t i o n a l to  i n the i n c i d e n t beam c r o s s i n g  unit  area  Thus the DCS can be e x p e r i m e n t a l l y determined by measuring  quantities. In order to c a l c u l a t e the DCS knowledge of If  (9)1 i s r e q u i r e d . n  T h i s w i l l come out of the s o l u t i o n to the Schrodinger e q u a t i o n f o r the c o l l i s i o n , which f o r the p r o t o t y p e case of e l e c t r o n - H atom s c a t t e r ing, gives  an i n f i n i t e s e t of coupled d i f f e r e n t i a l equations of the  20  -  -  type [41]  [V v  2  r, b  + k  2  n  -  ^1. n  V  ) F (r, ) = ^ nn' n b  L Tl  [-+/] V F (r. ) , mn m b m+n  (1.C.2)  where the m a t r i x element, V , i s d e f i n e d as mn * _  2  V m n = Ju ( r ) (--= mn  and  r  &  n  a  v  r,ba  2  f - ) U ( r )dr  r,b  m  and r ^ a r e the c o - o r d i n a t e s  a  (1.C.3)  a  of the t a r g e t and i n c i d e n t  t i l e ) electrons r e s p e c t i v e l y ; F (r, ) represents m b  (projec-  the wave f u n c t i o n of the  i n c i d e n t e l e c t r o n s and U ( F ) are the complete s e t of e i g e n f u n c t i o n s of the unperturbed H atom. As  e q u a t i o n (1.C.2) stands i t cannot be s o l v e d and v a r i o u s  approximations have to be made. i n c o l l i s i o n theory  One o f the most common ones a p p l i e d  i s the Born approximation  [42],  assumption behind t h i s approximation i s t h a t there a c t i o n between the p r o j e c t i l e and the t a r g e t . is undistorted  Q n  »  Thus the i n c i d e n t wave by an u n d i s -  terms are zero  except  the p o t e n t i a l energy of the i n t e r a c t i o n between the  s c a t t e r e d e l e c t r o n and the atom i n i t s f i n a l  s t a t e i s s m a l l and so the  therefore V =0. nn f o r h i g h i n c i d e n t e l e c t r o n energy  d i s t o r t i o n o f the s c a t t e r e d wave can be n e g l e c t e d , The  inter-  the e x c i t a t i o n i s due t o a d i r e c t t r a n s i t i o n from  the ground t o the e x c i t e d s t a t e and so a l l v  is little  by the i n t e r a c t i o n and can be represented  t o r t e d plane wave;  for  The b a s i c  Born approximation I s only v a l i d  - 21 -  (> 5-7 times  the e x c i t a t i o n energy) and hence w i l l be v a l i d  m a j o r i t y of the cases Application  i n the  r e p o r t e d i n the present work.  of the Born approximation  simplifies  equation  (l.C.2) to  (V  2  r, b r  + k  2  n  ) F (7 ) = ^ n D _z  V  •fi  on  e  i k  o' b  (l.C.4)  r  which can be s o l v e d by the method of Green's f u n c t i o n s to y i e l d an expression f o r f ( 9 ) . n  dc< 9)dQ =  Thus the DCS can be w r i t t e n as  m  2  k  n  , i ( k -k ) »F  7X4iTl  dQ  - ,2,_  f  °  J e  n  V  on  d r  IdQ  ( 1  'C-5)  (note r = r, ) F i n a l l y by d e f i n i n g a momentum t r a n s f e r v a r i a b l e , K, where: IK| 1  generalising r  1  2  = I k -k ' o n  1  |  2  = k + k - 2 k k cos9 o n o n 2  (l.C.6) '  to a N e l e c t r o n system (whose c o o r d i n a t e s a r e g i v e n by  ) and u s i n g the i n t e g r a t i o n formula  namely:  2  d e r i v e d by Bethe  [38,39],  - 22 -  c e  iK. r  J  I 1  the  following  r-r  I  s  . ,rr ,— 4it iK.r dr = —-r e s K  (  . -, (l.C.7) N  1  e x p r e s s i o n f o r the DCS i s o b t a i n e d :  2 4k dc( 9) = 4m e n dQ  e (K) | on '  K  (l.C.8)  2  where the m a t r i x element e (K) i s on  * e (K) = /U on n J  N E , s=l  e  s U o  dx„ N  (l.C.9)  where dx^ i n d i c a t e s i n t e g r a t i o n over a l l c o o r d i n a t e s of the N e l e c t r o n system. A u s e f u l concept which predates quantum mechanics i s t h a t of the  oscillator  strength,  strength.  f , to be the number of e l e c t r o n s  p a r t i c u l a r frequency. electrons  The c l a s s i c a l p i c t u r e d e f i n e d  been r e t a i n e d  i n f r e e o s c i l l a t i o n at a  The t o t a l o s c i l l a t o r s t r e n g t h  i n the t a r g e t .  the o s c i l l a t o r  was the number of  The concept of t o t a l o s c i l l a t o r s t r e n g t h  has  i n quantum theory as a u s e f u l means of d e f i n i n g t r a n s i -  tion probability.  I n e f f e c t the t r a n s i t i o n p r o b a b i l i t i e s a r e being  n o r m a l i s e d to the t o t a l number of e l e c t r o n s  i n the system.  known as the Thomas-Reiche-Kuhn (TRK) sum r u l e [39].  This i s  23  -  -  In the d i p o l e approximation (which covers  optical selection  r u l e s ) the g e n e r a l form f o r the o p t i c a l ( d i p o l e ) o s c i l l a t o r  strength  is  f  Bethe [38,39] has  on  2E ( 0 )  =  J rJ <(|  E  IV  s s=l r  (LC.10)  defined a generalised o s c i l l a t o r strength (GOS)  particle collision f^^K)  given  for  by  2E  N  K  -  -  s=l (l.C.11)  2E =  s u b s t i t u t i n g (l.C.11)  _  ^ r l  E  o n W l  i n t o ( l . C . 8 ) and  s w i t c h i n g to atomic u n i t s  Bethe-Born r e l a t i o n s h i p [38,39] i s o b t a i n e d ,  the  namely:  n  T h i s form of o s c i l l a t o r  s t r e n g t h i s only v a l i d  t i o n h o l d s , however, Bethe has f '(K,E ), N  Q  where E  q  i f the Born approxima-  a l s o d e f i n e d the apparent  i s the impact energy.  GOS,  T h i s i s c a l c u l a t e d from  the  -  experimental The the GOS.  24 -  parameters.  optical Expanding  (dipole) o s c i l l a t o r  s t r e n g t h i s a s p e c i a l case of  the e x p o n e n t i a l f u n c t i o n i n e q u a t i o n ( l . C . 1 1 ) as a  power s e r i e s i n K, i . e .  e  iK.r  _  x  +  1  R  >  r +  ( l K > r )  2  >  >  (iK.r)  #  2!  e  on  m  M!  becomes  e  Q n  = 0 + e (iK) + e ( i K )  where e = — - <<b E r m m! n' s s  x  1  2  2  + ... ^ ( i K ) *  1  (l.C.13)  4> > and o r t h o g o n a l i t y d i c t a t e s that e o o J  = <<J> 6 >=0. n o 1  For m = 1, t h i s m a t r i x element becomes  e, - <(|» |E r U > n' s• o 1  (l.C.14)  which i s n o t h i n g more than the t r a n s i t i o n moment term i n the d i p o l e approximation.  Substituting  (l.C.13) back i n t o ( l . C . 1 1 ) , the expanded  form of the GOS i s o b t a i n e d , namely:  f(K) = 2 E  R  {c\ + ( e 2 - 2 e e ) K 1  + 0(K )}  2  4  3  = f(0) + f ( l ) K  2  + f(2)K  4  +  (l.C.15)  - 25 -  Thus as the momentum t r a n s f e r , K, approaches z e r o , approaches f ( 0 ) which i s the o p t i c a l o s c i l l a t o r Limit f(K) = f ( 0 ) ) .  The h i g h e r  terms, f ( l ) ,  quadrupole, o c t o p o l e  and higher  transitions.  strength ( i . e .  f(2) etc.,  represent  L a s s e t t r e et a l . [43] have  shown that the GOS approaches the o p t i c a l l i m i t regardless  the GOS, f ( K ) ,  as K approaches  zero  of whether the Born approximation a p p l i e s or n o t .  From the above d i s c u s s i o n i t can be seen that i f the e x p e r i mental c o n d i t i o n s approaches zero  a r e s e l e c t e d such that  [44] then there  the momentum t r a n s f e r  e x i s t s a d i r e c t r e l a t i o n s h i p between  the DCS and the o p t i c a l o s c i l l a t o r  strength.  Bethe-Born r e l a t i o n s h i p ( l . C . 1 2 ) .  To achieve low momentum t r a n s f e r ,  high  Q  i n c i d e n t e l e c t r o n energies  ( E ) and s m a l l angle (9) s c a t t e r i n g  ( i d e a l l y zero degrees) a r e r e q u i r e d . much g r e a t e r  T h i s i s governed by the  I f the i n c i d e n t energy i s v e r y  than the energy t r a n s f e r ( i . e . t r a n s i t i o n energy, E ) and n  9 i s s m a l l , s u b s t i t u t i o n of these q u a n t i t i e s i n t o e q u a t i o n (note:  k o  2  = 2E and k o n  2  K  = 2(E -E ) i n atomic u n i t s ) o n  2  = 2E  [\{- ) o IL  Q  E  2  + 9 ) 2  (l.C.6)  gives:  (l.C.16)  thus I f 9 = 0 , the momentum t r a n s f e r i s p r o p o r t i o n a l to the energy t r a n s f e r and hence e q u a t i o n l.C.12 i n d i c a t e s  35 " „ " '<°> E  3  that:  (l.C.17)  - 26 -  Thus d i p o l e s p e c t r a produced i n t h i s manner by e l e c t r o n impact be q u a l i t a t i v e l y  s i m i l a r to t h e i r p h o t o a b s o r p t i o n  should  c o u n t e r p a r t s and  o n l y d i f f e r by a r e l a t i v e decrease i n i n t e n s i t y which i s p r o p o r t i o n a l to the i n v e r s e of the energy l o s s cubed. problem f o r a simple  T h i s f a c t o r presents  little  q u a l i t a t i v e comparison between o p t i c a l and  e l e c t r o n energy l o s s s p e c t r a , e s p e c i a l l y over a short range.  However,  the above c o r r e c t i o n ( l . C . 1 7 ) has to be a p p l i e d i f a q u a n t i t a t i v e comparison i s r e q u i r e d strengths  as obtained  [45].  A review of continuum  oscillator  by e l e c t r o n impact spectroscopy  has r e c e n t l y  been g i v e n by B r i o n and Hamnett [ 4 5 ] . Alternatively dipole o s c i l l a t o r  strengths  can be o b t a i n e d by  measuring a s e r i e s of e l e c t r o n energy l o s s s p e c t r a a t v a r i o u s momentum t r a n s f e r s and e x t r a p o l a t i n g back to zero momentum t r a n s f e r . done i n two ways: e i t h e r by f i x i n g s c a t t e r i n g angle, varying  the impact energy.  Lassettre et a l . and  or by f i x i n g  Ross [46,47].  [2] w h i l e  the impact energy and v a r y i n g the  the s c a t t e r i n g angle ( u s u a l l y a t 0°) and  The former method has been u t i l i s e d by the l a t t e r method has been used by H e r t e l  One aspect  of these methods i s that  t r a n s i t i o n s can be s t u d i e d and i d e n t i f i e d w i l l extrapolate allowed  T h i s can be  back to z e r o .  non-dipole  s i n c e the o s c i l l a t o r  strengths  However, f o r the study o f o p t i c a l l y  t r a n s i t i o n s by e l e c t r o n impact i t i s f a r s u p e r i o r and much l e s s 2  tedious  to work d i r e c t l y  by a p p r o p r i a t e  ( i . e . as c l o s e as p o s s i b l e ) to the K  s e l e c t i o n of experimental  conditions rather  employing the above mentioned e x t r a p o l a t i o n  techniques.  = 0 limit  than  27  -  D.  -  The R e l a t i v e M e r i t s o f E l e c t r o n Energy Loss and P h o t o a b s o r p t i o n Spectroscopies In the p r e v i o u s s e c t i o n the r e l a t i o n s h i p between the o s c i l l a t o r  s t r e n g t h s o b t a i n e d by EELS and p h o t o a b s o r p t i o n established.  s p e c t r o s c o p i e s was  I t was shown t h a t the two techniques  c o u l d produce the  same i n f o r m a t i o n w i t h regard to d i p o l e ( o p t i c a l ) t r a n s i t i o n s .  In t h i s  s e c t i o n the r e l a t i v e m e r i t s of u s i n g e l e c t r o n s or photons t o produce "optical"  ( i . e . d i p o l e ) s p e c t r a w i l l be d i s c u s s e d .  s i o n w i l l be g i v e n of the a b i l i t y dipole forbidden t r a n s i t i o n s The  obvious  No f u r t h e r d i s c u s -  of e l e c t r o n impact  techniques  t o probe  [1-3].  q u e s t i o n t o ask i s why would anyone wish t o s i m u l a -  te o p t i c a l s p e c t r a u s i n g e l e c t r o n s when presumably the s p e c t r a c o u l d be o b t a i n e d d i r e c t l y w i t h a photon source?  C l e a r l y each  must have i t s own advantages and d i s a d v a n t a g e s . the experimental  These w i l l  arise  To o b t a i n a p h o t o a b s o r p t i o n source i s r e q u i r e d .  techniques.  spectrum a s u i t a b l e continuum  U n t i l the advent o f s y n c h r o t r o n  was no e f f e c t i v e means of o b t a i n i n g tuneable  radiation  Conventional  light  sources  light  there  r a d i a t i o n i n the f a r UV  X-ray r e g i o n s which would g i v e a continuum source o f h i g h  beyond 20 eV.  from  methods r e q u i r e d to o b t a i n the s p e c t r a and hence the  v a r i o u s l i m i t a t i o n s i n h e r e n t i n these  and  technique  flux  u t i l i s i n g hydrogen and noble  gas c o n t i n u a have p r o v i d e d a u s e f u l , a l b e i t weak, and o f t e n s t r u c t u r e d source extending  up t o ~20 eV [48] w h i l e the X-ray r e g i o n was l i m i t e d to  the use of weak bremmstrahlung c o n t i n u a s o u r c e s .  However, even  - 2b  -  w i t h the a v a i l a b i l i t y and use of s y n c h r o t r o n sources there s t i l l various experimental  exist  l i m i t a t i o n s to the use of photons i n c e r t a i n  r e g i o n s of the e l e c t r o m a g n e t i c  spectrum.  In order to use a continuum l i g h t  source one must s e l e c t  the  r e g i o n of r a d i a t i o n t h a t i s r e q u i r e d by s u i t a b l e o p t i c a l monochromat i o n and sitates  then t r a n s m i t t h i s s e l e c t e d l i g h t  to the sample.  the use of windowless ( f o r e n e r g i e s >10  spectrometers. l i g h t absorbed  This  neces-  eV) g r a t i n g vacuum  S i n c e t h i s i s a d i s p e r s i v e technique and  the amount of  i s measured as a f u n c t i o n of wavelength (X,), d i f f e r e n t  g r a t i n g and monochromator designs are r e q u i r e d to o p t i m i s e the  running  c o n d i t i o n s f o r the v a r i o u s p a r t s of the e l e c t r o m a g n e t i c spectrum. [49]  Brown  shows i l l u s t r a t i o n s of three types of monochromator i n use. U n l i k e p h o t o a b s o r p t i o n , EELS i s a non-resonant  p a s s i n g through  technique.  the sample, some of the primary beam i s s c a t t e r e d and  the amount of energy  required for a particular  t r a n s f e r r e d to the system.  t r a n s i t i o n i s simply  T h i s amount can e a s i l y be o b t a i n e d  by  adding an e q u i v a l e n t v o l t a g e back to the a n a l y s e r system  thereby  a l l o w i n g the s c a t t e r e d e l e c t r o n s to reach the d e t e c t o r .  Thus a  spectrometer through  i s a b l e to cover a wide s p e c t r a l range (from the  to the X - r a y ) .  so the energy  whole s p e c t r a l It  i n a "constant a n a l y s e r pass  IR  energy  r e s o l u t i o n can be kept constant throughout  range.  i s this f i n a l  single  By use of s u i t a b l e r e t a r d i n g v o l t a g e s i t i s  p o s s i b l e to run the spectrometer mode" and  On  f e a t u r e which p r e s e n t l y g i v e s a major  the  - 29 -  advantage .region.  to EELS over p h o t o a b s o r p t i o n f o r the 200 - 1000 eV energy  loss  T h i s a r i s e s from the i n v e r s e r e l a t i o n s h i p of energy and  wavelength.  The l a r g e r the t r a n s i t i o n energy, the s h o r t e r the wave-  l e n g t h and hence  the worse the energy r e s o l u t i o n .  EELS becomes i n c r e a s i n g l y advantageous.  Thus above ~200  For example,  eV  the "grass-hopper"  monochromator shown by Brown [49] i s quoted as having a r e l a t i v e bandwidth  (AX/X) of b e t t e r than 1 0 "  a t 40 A ( i . e . a r e s o l u t i o n ,  3  0.04  A ) . T h i s corresponds to a FWHM of 0.3  eV.  R e c e n t l y Shaw et a l . [50] have r e p o r t e d  the C Is  it t r a n s i t i o n  AX, of  eV f o r a t r a n s i t i o n at the ISEELS  spectrum  310  for  (287.40 eV) of CO w i t h a r e s o l u t i o n of 0.055eV.  I t should be noted, however, t h a t the same wavelength r e s o l u t i o n (AX = 0.04  A ) would be e q u i v a l e n t to 0.03  eV at 100 eV.  example remains the ISEELS N Is •> n resolution  [51-53]  t r a n s i t i o n r e c o r d e d a t 0.075eV  i n which s i x v i b r a t i o n a l l e v e l s  eV) can be seen (see F i g . 2.5).  Perhaps the best  ( c e n t r e d at 401.10  T h i s s t r u c t u r e has not thus f a r been  r e s o l v e d i n any p u b l i s h e d o p t i c a l experiment. r e q u i r e a wavelength r e s o l u t i o n of 0.006 A.  T h i s spectrum would F i g u r e 1.2  ( t a k e n from r e f .  [6]) shows the e q u i v a l e n t v a l u e of A \ ( A ) f o r f i x e d EELS r e s o l u t i o n s (0.01 eV eV  {state of the a r t w i t h e l e c t r o n monochromation [50,53]} to  {unmonochromated e l e c t r o n beam from an oxide cathode}) as a f u n c t i o n  of energy l o s s .  Thus w i t h regard to energy r e s o l u t i o n , o p t i c a l methods  are s u p e r i o r below ~200 eV.  0.5  eV whereas EELS i s p r e s e n t l y s u p e r i o r above  The r e s o l u t i o n c r i t e r i a  i s not p a r t i c u l a r l y important above  eV due to the n a t u r a l l i n e w i d t h s of the t r a n s i t i o n s . advantageous  a g a i n to use s y n c h r o t r o n  I t becomes  ~200  ~1000  Figure  1.2  Wavelength r e s o l u t i o n p l o t t e d a g a i n s t e x c i t a t i o n energy f o r f i x e d values of energy r e s o l u t i o n (taken from r e f . [ 6 ] ) .  - 31  methods, c e r t a i n l y beyond 2.0 a g a i n more e f f i c i e n t .  - 2.5  -  keV  where o p t i c a l monochromators are  T h i s r e s u l t s from a combination  of the h i g h  flux  c a p a b i l i t i e s of the s y n c h r o t r o n a g a i n s t the i n c r e a s i n g i n t e n s i t y problems experienced (see e q u a t i o n  w i t h EELS a r i s i n g from the E ~  3  n  l.C.17).  g r a t i n g s and m i r r o r s at UV  by the low r e f l e c t i v i t y of  these can o f t e n r e s u l t  or i n c o r r e c t o s c i l l a t o r  s t r e n g t h s being observed.  and  gratings.  and  contamination  These i n c l u d e the  to order o v e r l a p p i n g ,  from the decomposition  t o r r ) c o n d i t i o n s [54]. indeed  s e r i o u s i n the C Is  w i l l a r i s e from carbon d e p o s i t s formed  of d i f f u s i o n pump o i l s  ( f o r example) and  a v a r i a t i o n i n the i n t e n s i t y of the l i g h t beam due Surface contamination  the  of the monochromator m i r r o r s  T h i s l a t t e r problem i s p a r t i c u l a r l y  r e g i o n s i n c e the c o n t a m i n a t i o n  synchrotron  i n additional "spectral features"  p o s s i b i l i t y of h i g h e r energy r a d i a t i o n due of s t r a y l i g h t  the  and X-ray e n e r g i e s .  V a r i o u s problems occur w i t h the u t i l i z a t i o n of  presence  factor  However, i n a l l o p t i c a l monochromators the h i g h  synchrotron f l u x i s severely attenuated  r a d i a t i o n and  intensity  by carbon has  cause  to a b s o r p t i o n .  even been seen under UHV  (<10  - 9  These same problems do not a r i s e i n ISEELS and  the t r u e s p e c t r a l shape ( a f t e r a p p l y i n g the Bethe-Born c o r r e c -  tion i . e . E ~  3  f a c t o r ) i s obtained.  A s t r i k i n g example of  incorrect  n s p e c t r a l i n t e n s i t y d i s t r i b u t i o n i s the v a l e n c e spectrum of N  2  r e p o r t e d by G u r t l e r et a l . [ 5 5 ] .  shell The  photoabsorption EELS spectrum,  which shows the c o r r e c t r e l a t i v e i n t e n s i t y , i s shown i n F i g . 2.4. d i f f e r e n c e a r i s e s from l i n e [55] caused  s a t u r a t i o n e f f e c t s i n the o p t i c a l work  by the band width of the l i g h t being l a r g e r than  the  The  -  natural l i n e width.  32  -  Since EELS i s a non-resonant process  cannot a r i s e i n e l e c t r o n impact s t u d i e s . d i s c u s s e d by I n o k u t i techniques  may  oscillator  strengths  [39].  T h i s aspect  problem  a l s o been  Thus, somewhat i r o n i c a l l y , e l e c t r o n impact  p r o v i d e a more a c c u r a t e  means of o b t a i n i n g  than o p t i c a l methods!  method i n EELS should  has  this  Indeed the  be i n t r i n s i c a l l y more a c c u r a t e  optical  experimental  than photo-  a b s o r p t i o n s i n c e the number of e l e c t r o n s which have been s c a t t e r e d i s d i r e c t l y measured whereas i n p h o t o a b s o r p t i o n  a d i f f e r e n c e i s taken  between the i n c i d e n t and  Methods which use  to  transmitted  e x c i t e the sample but use  "absorption"  spectrum  grossly distort processes  e l e c t r o n y i e l d methods to o b t a i n  the t r u e s p e c t r a l shape.  secondary processes  The  l a r g e and  i n the d i s c r e t e and  the i n n e r s h e l l e x c i t a t i o n s p e c t r a has  Since obtained,  experiments  lising  forward provided  TRK  f a s h i o n due  sum  variable effect  strengths  can be  determined.  e i t h e r by norma-  i n t e n s i t y o p t i c a l spectrum or  sum  r u l e (see the  previous  r u l e n o r m a l i s a t i o n can be a p p l i e d i n a  to the f l a t  of  continuum p o r t i o n s of  s t r e n g t h s c a l e can be obtained  more r e a d i l y by a p p l i c a t i o n of the TRK The  one  been c l e a r l y demonstrated i n  to a known f e a t u r e i n an a b s o l u t e  section).  so more than  the t r u e s p e c t r a l shape to be  relative oscillator  An a b s o l u t e o s c i l l a t o r  they  [57-59].  the EELS method a l l o w s  accurate  the  T h i s a r i s e s from secondary  (Auger or m u l t i p l e Auger e f f e c t s ) and  dipole coincidence  light  [56] a v o i d the " d i f f e r e n c e " e r r o r , however,  e l e c t r o n per photon i s produced. these  light.  straight  (equal i n t e n s i t y ) v i r t u a l photon  by a f a s t e l e c t r o n (see above d i s c u s s i o n ) .  field  -  33  -  A major advantage o p t i c a l methods have over EELS i s the a b i l i t y obtain  spectra  of s o l i d s or condensed phases v i a t h i n f i l m  to  transmission  techniques.  E.  Inner S h e l l E l e c t r o n E x c i t a t i o n In t h i s s e c t i o n the  features  spectra o r i g i n a l l y introduced detail.  They w i l l a l s o be  parts.  i n the  contrasted  appropriate  to s p e c t r a  Discrete  the d i s c r e t e r e g i o n  considered,  the  of the  other words, as the  forbidden  Since  only  following discussion  will fast  w i t h s m a l l angle s c a t t e r i n g .  s h e l l e l e c t r o n e x c i t a t i o n , features  (semi-quantitatively)  vacant l e v e l  counter-  spectrum,  discussed.  i n the  ( i . e . below the i o n i s a t i o n l i m i t ) can u s u a l l y be  described  B.I,  with t h e i r valence s h e l l  Portion  In i n n e r portion  i n more  produced by e i t h e r o p t i c a l methods or by  e l e c t r o n impact i n c o n j u n c t i o n  I.  shell electron excitation  continuum spectrum w i l l a l s o be  the d i p o l e t r a n s i t i o n s w i l l be be  of i n n e r  i n s e c t i o n B w i l l be d i s c u s s e d  As w e l l as c o n s i d e r i n g  features  Spectra  discrete  adequately  i n terms of a o n e - e l e c t r o n  picture.  In  promotion of an e l e c t r o n from a c o r e - l e v e l to a  (see F i g . 1.1B).  T r a n s i t i o n s w i l l e i t h e r be  on the b a s i s of d i p o l e s e l e c t i o n r u l e s .  As  allowed  stated  or  in section  the vacant o r b i t a l s are e i t h e r Rydberg or v i r t u a l v a l e n c e i n  origin. The  Rydberg o r b i t a l s are  large, d i f f u s e , atomic-like  and  extend  - 34 -  well of  beyond the  bounds of  the m o l e c u l a r s t r u c t u r e  Rydberg e l e c t r o n  similar  p h o t o e j e c t i o n of the  excitation  shape.  The  exhibit  which are  same e l e c t r o n  ionisation  l e s s important w i t h charged core  ionised  state  hence the  the  As  i n the  By  atomic case,  the  quantum d e f e c t ( 6 ^ ) which i s i t s e l f a measure of  of  the  A quantum number and  particular  decreases as  so does the  atoms t y p i c a l v a l u e s f o r s, p and respectively and  not  spectra  be  [12].  applied  [12]  and  (equation  i t i s possible the  has  r e f l e c t s the the  amount of  core.  to 0.7  moving to the  t h i r d row  e f f e c t i v e and  so  to  estimate from  For  d quantum d e f e c t s are  Robin has  f o r p o r b i t a l s and elements, the  l i m i t s of 0.7 -0.2  to 0.2  shielding  quantum d e f e c t s of  of  penetration  second ~1,  surveyed many  0.6  row and  0.1  used as a guide excitation  to 1.3  for  s  for d o r b i t a l s . the  of  penetration  Since the  quantum d e f e c t .  estimated e m p i r i c a l  o r b i t a l s , 0.5  the  strictly.  l.B.l)  deviation  However, these v a l u e s should only be too  transitions  quantum d e f e c t i s c h a r a c t e r i s t i c  type of o r b i t a l i n t o  s>p>d e t c ,  thus  term v a l u e s ( i . e .  b i n d i n g energy of  ,The  in  spectral  [20,60,61] and  knowing the  Rydberg e l e c t r o n ) ,  from  show some  the  particular  resulting  f i t a Rydberg s e r i e s  limit.  simple h y d r o g e n - l i k e b e h a v i o u r .  The  respective features  to valence-Rydberg mixing  ionisation  the  [13].  s p e c t r a should have a s i m i l a r  v i b r a t i o n a l broadening.  converging to the  details  e s s e n t i a l l y non-bonding, w i l l  and  to m o l e c u l a r Rydberg o r b i t a l s w i l l  the  Thus the  l y i n g Rydberg o r b i t a l s , however, may  a n t i b o n d i n g c h a r a c t e r due may  become l e s s and  i n geometry to the  the  and  low  ground s t a t e m o l e c u l e .  seeing e f f e c t i v e l y a s i n g l e  h i g h l y i n g Rydberg s t a t e s , t h e r e f o r e be  the  core i s not  the more p e n e t r a t i n g  levels  On as  - 35 -  should  i n c r e a s e markedly.  T y p i c a l values  Rydberg l e v e l s of the t h i r d  row  Since the t r a n s i t i o n s  are ~2,  suggested f o r the s, p and  1.6  and  d  0.0 r e s p e c t i v e l y .  s t u d i e d i n the present  work are i n g e n e r a l  governed by d i p o l e s e l e c t i o n r u l e s the f o l l o w i n g t r a n s i t i o n s : s •* s, p •*• p, d •* d and case. one  s «-> d would be  formally forbidden  i n the p u r e l y  However, i n the molecule each Rydberg o r b i t a l w i l l  of the i r r e d u c i b l e r e p r e s e n t a t i o n s of the m o l e c u l a r  hence one  or more of the above t r a n s i t i o n s may  T r a n s i t i o n s to l e v e l s which would be atomic case,  atomic  transform  p o i n t group  be d i p o l e  f o r m a l l y d i p o l e f o r b i d d e n i n the  of the atomic c o n t r i b u t i o n to the o v e r l a p .  T h i s has  because  been d i s c u s s e d  Schwarz [60,61] i n r e l a t i o n to the 2p a b s o r p t i o n  s p e c t r a of Ar and  i s o - e l e c t r o n i c t h i r d row  the 2p •* 4p  as assigned and  3d  and  allowed.  though, c o u l d be expected to e x h i b i t l e s s i n t e n s i t y  hydrides.  as  In a l l cases  by Schwarz [60,61] i s weaker than the t r a n s i t i o n s  by the  transition, to the  4s  levels. Thus a combination of the above c o n s i d e r a t i o n s  r u l e s , term v a l u e s  and  ( i . e .selection  i n t e n s i t i e s ) can prove to be most h e l p f u l i n the  i n t e r p r e t a t i o n of the Rydberg p o r t i o n of i n n e r s h e l l e x c i t a t i o n spectra. The  v i r t u a l valence  u s u a l l y the a n t i b o n d i n g  o r b i t a l s a r i s e out  counterparts  l e v e l s may  occur  scheme and  of the bonding o r b i t a l s .  are s t r o n g l y c h a r a c t e r i s t i c of the m o l e c u l e . valence  of the MO  the d i s c r e t e r e g i o n of the e x c i t a t i o n spectrum. v i r t u a l s t a t e s are expected to be r e l a t i v e l y  Thus  T r a n s i t i o n s to the  r e s u l t i n g i n s t a t e s which may  or may  are  not  they  virtual l i e in  T r a n s i t i o n s to bound  s t r o n g s i n c e the  virtual  - 36 -  v a l e n c e o r b i t a l s are d e l o c a l i s e d around  the m o l e c u l a r framework and have  a comparable s p a t i a l extent to that of the ground  s t a t e molecule  [13].  These f e a t u r e s are a l s o expected to be somewhat broader than those to Rydberg t r a n s i t i o n s because result  due  of t h e i r a n t i b o n d i n g nature which can  i n a l a r g e geometry change i n the upper  s t a t e and hence r e s u l t i n  c o n s i d e r a b l e v i b r a t i o n a l e x c i t a t i o n of the f i n a l  state.  Depending on the m o l e c u l e , i n t e n s e t r a n s i t i o n s to the v i r t u a l v a l e n c e l e v e l s may  precede  the Rydberg t r a n s i t i o n s and  so have l a r g e r  term v a l u e s than those c h a r a c t e r i s t i c f o r Rydberg t r a n s i t i o n s . example the 2p ISEELS spectrum  of C l  2  [62] shows two broad  For  transitions  * to the a  l e v e l w i t h term v a l u e s of 9.50  and  edges.  2  p^^)  (2p^/2  These are w e l l separated from the Rydberg f e a t u r e s  which are c h a r a c t e r i s t i c a l l a y t r a n s i t i o n s are 3.56 0.19  eV from t h e i r r e s p e c t i v e  sharp.  eV and 1.72  The  term v a l u e s f o r the 4s and  eV g i v i n g quantum d e f e c t s of 2.04  3d  and  r e s p e c t i v e l y , i n good a c c o r d w i t h the "expected" v a l u e s d i s c u s s e d  above.  The C Is ISEELS s p e c t r a of the methyl h a l i d e s  [63,64] a l s o  * p r o v i d e examples where broad  f e a t u r e s can be a s c r i b e d to C Is +  a  * t r a n s i t i o n s p r e c e d i n g the Rydberg t r a n s i t i o n s . o r b i t a l s are c l o s e r i n energy same symmetry. mixing  [20,65].  to the lowest Rydberg l e v e l s and of the  Thus there e x i s t s the p o s s i b i l i t y of T h i s i s most l i k e l y  [53,64], however, f o r the remainder Rydberg l e v e l s  a  In t h i s case the  Rydberg-valence  i n CH F where the f e a t u r e s o v e r l a p 3  of the molecules the a  and  ns  (n = 3 f o r F, 4 f o r C l e t c ) are c l e a r l y s e p a r a t e d .  term v a l u e s f o r the a  l e v e l v a r y from 4.65  the ns l e v e l go from 4.05  eV to 3.65  eV to 5.7  eV w h i l e those f o r  eV f o r CH F through to C H I 3  The  3  -  respectively.  In c o n t r a s t  f e a t u r e i n the  discrete  the  f e a t u r e s are  to the  portion  ascribed  37  -  methyl h a l i d e s ,  attributable  methane shows  to a  to Rydberg t r a n s i t i o n s  a  no  t r a n s i t i o n and  all  [66,67].  * above examples have a l l been f o r systems w i t h a  The  valence o r b i t a l s .  V i r t u a l v a l e n c e o r b i t a l s of  l y i n g features i n t h e i r electron  to i t  ISEELS spectrum of C H  shows an  2  The  exist  6.3  eV  lt  excitation  s p e c t r a which can  low  be  antibonding o r b i t a l s .  *  intense C  whereas the  Is •+ i t  spectrum of C H 2  For  example,  the  t r a n s i t i o n with  a  o n l y shows Rydberg  6  [68]. difference  between v i r t u a l v a l e n c e and  Rydberg o r b i t a l s  and  t h e i r extent i s r e a l l y emphasized i n molecules i n which " i n n e r - w e l l " "outer-well"  states  can  e x i s t e n c e of some s o r t can  lead  to the  expense of  the  exist.  S 2p  These " w e l l "  of p o t e n t i a l b a r r i e r  enhancement of  transitions  p r o v i d e d by  to  "outer-well"  spectrum of  SF  g  states (see  "inner-well"  the and  ( v a l e n c e ) t r a n s i t i o n s at  the  [69,70]. F.  An  example i s  T h i s aspect w i l l  The  extent of  the  Rydberg  i l l u s t r a t e d by  condensed phases  [12].  be  The  comparing virtual  v a l e n c e o r b i t a l s , being l o c a l i s e d around the  m o l e c u l a r framework  o n l y be  gas  s l i g h t l y p e r t u r b e d on  However, the  going from the  Rydberg l e v e l s w i l l  spectrum w i l l not  be  be  visible.  and  subsequent s e c t i o n )  l e v e l s compared to v a l e n c e l e v e l s i s a l s o w e l l a b s o r p t i o n s p e c t r a of gaseous and  a r i s e from  (Rydberg) s t a t e s .  discussed i n greater d e t a i l i n section  i n the  in  *  a s s i g n e d to t r a n s i t i o n s  transitions  type a l s o  M o l e c u l e s which c o n t a i n it-bonds a l l show s t r o n g ,  many m o l e c u l e s .  term v a l u e of  it  the  type v i r t u a l  to a s o l i d  v e r y much a f f e c t e d  and  F r i e d r i c h et a l . [65]  will  state. the  features  have used  38  -  -  t h i s method to i n v e s t i g a t e the valence-Rydberg c h a r a c t e r absorption  spectrum of S i H ^ amd  PH .  f e a t u r e s may  II)  be  Continuum  T h i s technique p r o v i d e s  3  of a s c e r t a i n i n g Rydberg or valence  of the  character  i n s p e c t r a where  one  overlapped.  Features  seen above the i o n i s a t i o n edge i n the continuum. from v a r i o u s processes and  means  the  In i n n e r s h e l l e l e c t r o n e x c i t a t i o n s p e c t r a , f e a t u r e s can  continua  2p  The  i n c l u d i n g double e x c i t a t i o n , onsets of  can g i v e r i s e to prominent, l o c a l i s e d  "shake-up"  continuum. structure(s)  which can be observed as much as ~20eV above the i o n i s a t i o n edge and be d e s c r i b e d  i n terms of the out-going  q u a s i - s t a t i o n a r y s t a t e by  or p h o t o a b s o r p t i o n  T h i s shows two  can  in a  Such a  l e v e l from the i o n i s a t i o n c o n t i n u a  thereby g i v i n g the s t a t e an i n c r e a s e d  SFg.  trapped  some form of p o t e n t i a l b a r r i e r .  b a r r i e r would i s o l a t e the v i r t u a l  the ISEELS [69]  e l e c t r o n being  be  f e a t u r e s can a r i s e  t r a n s i t i o n s to q u a s i - s t a t i o n a r y s t a t e i n the  T h i s f i n a l process  often  lifetime.  [70]  A good example i s i n  spectrum of the S 2p r e g i o n i n  prominent f e a t u r e s above the i o n i s a t i o n l i m i t  which  * can be a s s i g n e d  i n t„ and 2g  shape-resonances. i n s e c t i o n F, no  As  a  e g  l e v e l s or a l t e r n a t i v e l y to t , and 2g  t h i s t o p i c i s going  to be d i s c u s s e d  e x c i t a t i o n and/or i o n i s a t i o n .  result  i n more d e t a i l  from m u l t i - e l e c t r o n  Double e x c i t a t i o n i n v o l v e s the  neous e x c i t a t i o n of the c o r e - e l e c t r o n along w i t h a v a l e n c e i t i s the  g  f u r t h e r mention w i l l be made of i t here.  Other f e a t u r e s i n the continuum w i l l  effect  e  ISEELS or p h o t o a b s o r p t i o n  (see s e c t i o n B . I I I ) .  These f e a t u r e s can  simulta-  electron.  In  analogue of "shake-up" i n  XPS  be q u i t e prominent and  have  -  39  -  been observed, f o r example, i n the i n n e r s h e l l e l e c t r o n e x c i t a t i o n s p e c t r a of it-bonding and  N  [4,71].  2  2  2  g  analogue of XPS " s h a k e - o f f " )  will  [68], CO  result  same c o n f i g u r a t i o n as an XPS "shake-up" s t a t e .  f e a t u r e s appearing i n XPS s a t e l l i t e transitions  6  [4,45,71]  Simultaneous e x c i t a t i o n and i o n i s a t i o n (ISEELS or  photoabsorption the  systems such as 0 2 ^ , C H , C H  Thus the ( v e r t i c a l )  spectra assigned  ( i . e . i o n i s a t i o n and e x c i t a t i o n ) w i l l  i n an i o n with  to "shake-up"  be m a n i f e s t e d i n  I S E E L S / p h o t o a b s o r p t i o n s p e c t r a as ( a d i a b a t i c ) onsets of "shake-up" continua.  Ill)  Comparison of Inner S h e l l E x c i t a t i o n Spectra Excitation  w i t h Valence S h e l l  Spectra  Inner s h e l l e l e c t r o n e x c i t a t i o n s p e c t r a a r e g e n e r a l l y simple  to a s s i g n due to the energy i s o l a t i o n of the i n i t i a l  which unambiguously d e f i n e s tion arises. that w i l l closely  the i n i t i a l  This i s i n contrast  usually exist  spaced v a l e n c e  y i e l d more d e f i n i t e i n f o r m a t i o n  Obviously two  shell  spectra. valence  orbital.  s p e c t r a can o f t e n  on the p r e v i o u s l y unoccupied l e v e l s and clarify  the assignments i n the more  spectra.  ( i . e . core  and v a l e n c e ) i n a d d i t i o n to the o v e r -  t r a n s i t i o n s o f t e n present First  situation  there w i l l be d i f f e r e n c e s between the s p e c t r a o f the  s p e c t r a l regions  lapping  hole  s p e c t r a due to the numerous  Thus Inner s h e l l  t h i s i n t u r n can be used t o h e l p complex valence  shell  core  from which the t r a n s i -  to the much more complex  i n valence orbitals.  orbital  relatively  consider  i n valence  electron excitation  the promotion of an e l e c t r o n to a v i r t u a l  On the promotion of an i n n e r  s h e l l e l e c t r o n the f i n a l  - 40 -  state w i l l  have a l o c a l i s e d  electron w i l l  result  core h o l e whereas the promotion  i n a more d e l o c a l i s e d v a l e n c e h o l e .  newly promoted e l e c t r o n w i l l  see almost  of a v a l e n c e  Therefore  the  a whole e x t r a u n i t of charge  in  the i n n e r s h e l l case s i n c e the l o s s of s h i e l d i n g of the nucleus  should  be g r e a t e r than t h a t from a d e l o c a l i s e d v a l e n c e h o l e .  term  value  ( i . e . b i n d i n g energy) from  l a r g e r than the term v a l u e from one  the i n n e r s h e l l  Thus the  spectrum  should  the v a l e n c e s h e l l spectrum.  In  i s s a y i n g t h a t upon the c r e a t i o n a core h o l e , the occupied  levels  ( i n c l u d i n g the newly occupied l e v e l ) w i l l  case of the c r e a t i o n of a v a l e n c e h o l e . orbitals,  see one  essence valence  r e l a x more than i n the  In c o n t r a s t to the v a l e n c e  the Rydberg o r b i t a l s are l a r g e and d i f f u s e .  Rydberg e l e c t r o n w i l l  be  Therefore a  l a r g e core w i t h u n i t p o s i t i v e charge.  w i l l be l e s s i n f l u e n c e d by whether the h o l e i s i n the v a l e n c e s h e l l at  the c o r e .  Consequently  on going from  the v a l e n c e s h e l l  the Rydberg t r a n s i t i o n s . upto 0.5 excess  there w i l l be l i t t l e spectrum  Wight and  i n c r e a s e i n term  or  value  to the i n n e r s h e l l spectrum  B r i o n [72] have noted  It  for  d i f f e r e n c e s of  eV o n l y i n Rydberg term v a l u e s whereas d i f f e r e n c e s w e l l i n  of 2 eV can occur w i t h v i r t u a l v a l e n c e l e v e l s , as i s a l s o  seen i n the present work. spectra w i l l  clearly  Knowledge of the term v a l u e s from i n n e r  shell  thus p r o v i d e upper bounds f o r term v a l u e s f o r v a l e n c e  shell  s p e c t r a and a l s o h e l p i d e n t i f y Rydberg and v a l e n c e t r a n s i t i o n s  respecti-  vely. C l e a r l y the above d i s c u s s i o n assumes that a one  electron  picture  i s adequate i n both cases to d e s c r i b e the t r a n s i t i o n s between the levels.  While  t h i s i s probably s a t i s f a c t o r y  for inner s h e l l  excitations  - 41  i t might not  be  for valence e l e c t r o n  valence-valence t r a n s i t i o n s  [13].  excitations  For  v a l e n c e - v a l e n c e t r a n s i t i o n s the  different.  For  i n s t a n c e the  it -»• it  6.2  and  F.  Potential  Barrier  I)  Potential  Barriers  For  and  discrete spectra.  localised  ligands on  the  [73].  ligands  r e g i o n and The  an  seen above the  p o s t u l a t e d that  E.  lu  with  inner s h e l l  near the  excitation  electron  i n an enhanced p r o b a b i l i t y expense of t r a n s i t i o n s  to  f o r c e would  ligands  b a r r i e r would e x i s t i n the field  These  electronegative  a strong repulsive  electronegative  of  somewhat broad  i o n i s a t i o n edge.  b a r r i e r would separate the  example of  very  "anomolous" i n t e n s i t y d i s t r i b u t i o n i s  and  so  an  v i c i n i t y of  Into an  act  the  "inner-well"  region.  e f f e c t s of such a b a r r i e r are w e l l  clearest  1  noted f o r molecules c o n t a i n i n g h i g h l y  "outer-well"  shell electron the  often  (charge) p o t e n t i a l The  be  [13].  Furthermore o t h e r s t r o n g and  I t was  [73].  , ^B, and lu  respectively  t r a n s i t i o n s at the  escaping e l e c t r o n  effective  degenerate  e n e r g i e s may  and/or continuum r e g i o n of the  f e a t u r e s are  e f f e c t s were f i r s t  2u  This manifests i t s e l f  Rydberg l e v e l s .  for  Shape-Resonance E f f e c t s .  core to v a l e n c e d i s c r e t e the  eV  c e r t a i n molecules an  seen i n the excitation  6.7  so  t r a n s i t i o n i n benzene, which  i s e, -*• e„ , r e s u l t s i n three s t a t e , lg 2u  e n e r g i e s of 4.8,  especially  degenerate w i l l r e s u l t more than  state.  first  and  A t r a n s i t i o n from one  l e v e l to another l e v e l which i s a l s o one  -  spectra  of S F  such e f f e c t s .  g  The  i l l u s t r a t e d by  the  inner  [69,70] which p r o v i d e , perhaps, ISEELS s p e c t r a  for  the  - 42  F  I s , S 2s and  2p r e g i o n s are shown i n F i g . 1.3  W i t h i n a minimal  and  symmetries would be expected.  S 3d o r b i t a l s would extend  s  (taken from r e f . [ 6 ] ) .  b a s i s set (s and p o r b i t a l s ) t r a n s i t i o n s to two  v a l e n c e l e v e l s of a ^  and e  -  virtual  A d d i t i o n of  these to i n c l u d e two  symmetry r e s p e c t i v e l y .  The  f u r t h e r l e v e l s of t„ -g S 2p spectrum shows a s t r o n g  a b s o r p t i o n i n the d i s c r e t e r e g i o n which can be a s s i g n e d to a t r a n s i t i o n to  the  v i r t u a l valence l e v e l .  s t r u c t u r e a s s i g n e d to a mixture  T h i s i s f o l l o w e d by v e r y weak  of o v e r l a p p i n g Rydberg t r a n s i t i o n s and  ( d i p o l e f o r b i d d e n ) t r a n s i t i o n to the and  (note t h a t both the F Is  S 2s s p e c t r a show a s t r o n g f e a t u r e c o r r e s p o n d i n g to an  t r a n s i t i o n to t h i s l e v e l ) . the edge.  spectrum  Two  allowed  v e r y prominent f e a t u r e s are seen above  These can be a s c r i b e d to t r a n s i t i o n s to the d - l i k e s t a t e s of  t£g and e^ symmetries.  for  level  a  I t should be noted  of the L s h e l l of S F  6  t h a t the  i s identical  photoabsorption  (within experimental e r r o r )  both the gas and s o l i d phase [70] thereby p r o v i d i n g evidence t h a t  the t r a n s i t i o n s d e s c r i b e d above are to f i n a l s t a t e l e v e l s w i t h i n the m o l e c u l a r core i . e . i n n e r - w e l l ( v a l e n c e ) s t a t e s and not Rydberg Thus the e f f e c t First  of the p o t e n t i a l b a r r i e r i s seen to be  Secondly,  from the continuum by the  barrier.  the v i r t u a l s t a t e s w i t h i n the i n n e r - w e l l w i l l have a s t r o n g  o v e r l a p w i t h the i n i t i a l  s t a t e wave-function  f e a t u r e s w i t h g r e a t l y enhanced i n t e n s i t i e s .  lar  two-fold.  i t can support q u a s i - s t a t i o n a r y s t a t e s above the i o n i s a t i o n edge  which are e f f e c t i v e l y decoupled  states  states.  resulting i n spectral C o n v e r s e l y , the o u t e r - w e l l  ( i . e . d i f f u s e Rydberg o r b i t a l s ) , being i s o l a t e d  from  the molecu-  core by the b a r r i e r , w i l l have l i t t l e o v e r l a p w i t h the i n i t i a l  state  - 43  -  0  -10  I  1  1-oH  +20 i  •10  +30 eV 1  '  r  A ii  F  .' 1  IS  /  \  •' \  1  1S  \  0-51g  •2g  Hu  (0  •  a  m  o  o o  Lit I< CC  H  z  0-  30-  1  1  r  700  680  2g  e  Liu  20-  -i  r  1  r  1  720  eV  S2s g  10-  D  o o  —i  120H  r  1  —i  250  230  270  / a  170 Figure  1.3  eV  S2p  80H  40H  r  1  \ (R .) yd  1Q  •2g  i  190  r  1  Energy l o s s s p e c t r a of S F i n the F I s , S 2s e x c i t a t i o n r e g i o n s (taken from r e f . [6]). g  r  210 eV and S  2p  - 44  wave f u n c t i o n and  this w i l l  ponding t r a n s i t i o n Features described such as N  and  l e a d to a d r a s t i c r e d u c t i o n i n the  i n i n n e r s h e l l e l e c t r o n e x c i t a t i o n s p e c t r a such as  CO  6  are a l s o present  [4,71],  Obviously  for SF ) 6  model i s r e q u i r e d .  i s not  those  i n the s p e c t r a of molecules  a b a r r i e r formed by a r e p u l s i v e  i n t e r a c t i o n of the e l e c t r o n e g a t i v e l i g a n d s on the e s c a p i n g postulated  corres-  probabilities.  above f o r S F 2  -  a p p l i c a b l e i n t h i s case and  Such a model has  electron  (as  an a l t e r n a t i v e  been proposed by Dehmer et a l .  [74-77] i n v o l v i n g a shape-resonance d e s c r i p t i o n .  II)  Shape Resonances The  N Is ISEELS spectrum of N  2  [4,71] i s dominated by a  intense  t r a n s i t i o n at 401.10 eV which has  respect  to the N Is i o n i s a t i o n edge (IP = 409.9 eV)  assigned  to a Is -*• n  transition.  A few,  a term v a l u e  maximum i s seen spectrum of N intensity  2  [4,71].  eV  i t may  v e r y weak f e a t u r e s  able to Rydberg t r a n s i t i o n s are a l s o present about 9 eV above the edge ( i . e . at ~419  and  of 8.8  very with  be attribut-  i n the d i s c r e t e r e g i o n .  eV) a broad r e l a t i v e l y  intense  Thus e f f e c t s are o c c u r r i n g i n the i n n e r  s i m i l a r to those observed f o r SFg,  f o r t r a n s i t i o n s to v i r t u a l valence  those to the Rydberg l e v e l s , and  i.e.  At  shell  enhancement of  l e v e l s at the expense of  a l s o prominent f e a t u r e s i n the  conti-  nuum. Dehmer and  Dill  [74,75] have d e s c r i b e d  these o b s e r v a t i o n s  terms of a c e n t r i f u g a l b a r r i e r which a g a i n s e p a r a t e s i n t o i n n e r - w e l l and and  outer-well regions.  The  the continuum f e a t u r e are then a s s i g n e d  strong  in  the m o l e c u l a r  field  f e a t u r e at 401.10 eV  to shape-resonances caused  - 45 -  by  the  s c a t t e r i n g of the p h o t o e l e c t r o n  the a n i s o t r o p i c m o l e c u l a r f i e l d . stationary  s t a t e which can  be  ( i . e . the e x c i t e d  A shape-resonance i s a  supported i n an i n n e r - w e l l  p h o t o e l e c t r o n ( i n t h i s case) can  be  temporarily  escaping.  the  the the  - 1  stationary  s t a t e s which are  c o r r e c t dimensions t h i s w e l l can  the m o l e c u l a r  U s i n g an X^ m u l t i p l e have c a l c u l a t e d  the  eigenfunctions  On  2  p a r t i a l cross-sections  t h i s basis  the  p a r t i a l wave expansion of the  n  quasi-  p o t e n t i a l and  loca-  of the  Dill  four dipole  [74,75]  allowed  o ) f o r the N Is e x c i t a t i o n / i o n i s a u'  n  and  channels r e s p e c t i v e l y .  wave f u n c t i o n  amounts of p-wave (A=l)  k i n e t i c energies ( i . e .  w h i l e the and  o"  indicates  A  that i t  has  channel i s seen to have  f-wave (A=3)  character  p h o t o e l e c t r o n energy above the  at  low  ionisation  E x c i t a t i o n of the N Is e l e c t r o n w i l l produce a p-wave  which i s then s c a t t e r e d h i g h e r A components.  by  the  shape resonances observed at 401.10 eV  s i g n i f i c a n t d-wave (Jl=2) c h a r a c t e r  energy) [75].  of the  support  s c a t t e r i n g approach, Dehmer and  eV are a s s i g n e d to the  appreciable  [77,78].  core.  e x c i t a t i o n channels (it , it , a and g u' g tion in N .  total effective potential  r e s u l t i n a c e n t r i f u g a l b a r r i e r at the p e r i p h e r y of  With the  l i s e d within  this  a t t r a c t i v e coulomb term ( ~ r , where r i s  molecule.  419  In  by a competing i n t e r a c t i o n between  measured from outer n u c l e u s ) i n the balance can  The  (centrifugal)  2  m o l e c u l a r c e n t r e ) and  and  potential.  c e n t r i f u g a l terms (~Jl( A + l ) / r , where r i s measured from  repulsive  This  quasi-  Thus a b a r r i e r concept i s s t i l l needed.  case the b a r r i e r i s s u p p l i e d  by  trapped at a p a r t i c u l a r  resonance energy b e f o r e t u n n e l l i n g through the p o t e n t i a l b a r r i e r and  electron)  the a n i s o t r o p i c m o l e c u l a r f i e l d i n t o  These w i l l c o n t r i b u t e  to the allowed  the  a and it  -  46  -  i o n i s a t i o n channels thereby making the i n t e n s i t y of the t r a p p i n g of the The to the  now  be  p h o t o e l e c t r o n by  the  appropriate c e n t r i f u g a l  i n t e n s i t y of  formed by  noted that ~2.3  t r a n s i t i o n s can  the  eV  A=2  t h i s state  i n e-N  lations  an  [80]  incident  experiments the  e-N  attaches i t s e l f  c u l a r o r b i t a l ) it  valence o r b i t a l .  a  barrier.  be  attributed  Dill  [75]  have  shape-resonance observed Molecular o r b i t a l to the  which p e n e t r a t e s the  to the  resonant  centrifugal  Dehmer and  resonance i s due  2  at  calcu-  resonant A=2  centri-  LUMO (lowest unoccupied mole-  S i n c e a c o r e - h o l e would e x i s t i n  excitation  s h i f t e d to a lower energy and,  the  the  shape-resonance would  i n t h i s case, appear In the  be  discrete  [81]. The  continuum shape-resonance i s e x p l a i n e d  t r a p p i n g of the channel. stationary  excited  electron  In t h i s case the state  at ~9  p a r t i a l f-wave (A=3)  r i s e to the  eV  an  above the  A=3  [74,75] by  c e n t r i f u g a l b a r r i e r i n the  potential  can  i o n i s a t i o n edge.  the  the  support a  In other words  0*^ wavefunction can  t r a n s i t i o n at ~419  eV  I t should be noted that  the  to a h i g h A A=l  u  the  overcome i t s  [74,75].  attributed  0  quasi-  r a p i d l y p e n e t r a t e the m o l e c u l a r core t h e r e b y  Thus these resonances are barrier effect.  by  inner-well  component of  c e n t r i f u g a l b a r r i e r and giving  can  The  g  case of i n n e r s h e l l e l e c t r o n  region  n  [79].  d-wave e l e c t r o n  f u g a l b a r r i e r and  *  state  l o c a l i s e d state within  Is analogous to the  have shown that  t r a p p i n g of an  u  effective potential.  scattering  2  e x p l a i n e d i n terms of  t r a n s i t i o n to the  l o c a t i o n of t h i s h i g h l y  barrier  above t r a n s i t i o n s p o s s i b l e .  centrifugal  component of the  0"^  - 47 -  channel does not show resonant not s u f f i c i e n t  behaviour  [82], i . e . the J!=l p o t e n t i a l i s  to form an e f f e c t i v e b a r r i e r .  w e l l known f o r c e r t a i n atoms and  Centrifugal barriers  can l e a d to prominent f e a t u r e s .  example, cerium has a c e n t r i f u g a l b a r r i e r f o r the A=3 p o t e n t i a l which s e p a r a t e s  the 4f wavefunction  (which  i s t h e r e f o r e an  the h i g h e r f l e v e l s are suppressed  N)  the wavefunction  penetrate molecule  [11,73].  For l i g h t e r atoms ( e g .  i n t o the c o r e - r e g i o n of atoms, however, i n the the added m o l e c u l a r  dimension  ( i . e . making the w e l l w i d e r )  f-waves i n t o the m o l e c u l a r  aspect i s emphasized i n t h a t the u  p e r p e n d i c u l a r to the m o l e c u l a r a resonance  [75].  diatomic  these q u a s i - s t a t i o n a r y s t a t e s , i . e . i t  a l l o w s p e n e t r a t i o n of the d- and dimensional  transitions  f o r the h i g h e r JI components are not a b l e to  a l l o w s the p o t e n t i a l to support  support  (outer-well  Thus a s t r o n g 3d •*• 4f t r a n s i t i o n i s seen whereas  to  For  effective  i n n e r - w e l l s t a t e ) from the 5f and h i g h e r wavefunctions states).  are  core.  This  channel, which a c t s  a x i s , has an Jl=3 component but does not  Furthermore, s i n c e the a b i l i t y of a system to  support a resonance depends on the dimensions of the a n i s o t r o p i c f i e l d , the s p e c t r a l p o s i t i o n of the resonance should p r o v i d e a s e n s i t i v e of  inter-nuclear separation.  detail  later.  probe  T h i s aspect w i l l be d i s c u s s e d i n more  - 48  Absolute o s c i l l a t o r e l e c t r o n e x c i t a t i o n and [71] u s i n g ISEELS.  -  s t r e n g t h measurements f o r the K - s h e l l  i o n i s a t i o n of N  An e l e c t r o n impact  2  have been made by Kay  energy of 8 keV was  generate the spectrum w i t h the i n e l a s t i c a l l y samples a t zero degree  s c a t t e r i n g angle.  Kay  used  to  scattered electrons being et a l . [71] have compared  t h e i r r e s u l t s w i t h the c a l c u l a t i o n s of Dehmer and D i l l  [74,75].  s e m i - q u a n t i t a t i v e agreement i s found between the experiment theory.  et a l .  Only a  and  the  However, a l l the major f e a t u r e s are e x p l a i n e d except f o r those  a r i s i n g from d o u b l e - e x c i t a t i o n processes which are not accounted the o n e - e l e c t r o n s c a t t e r i n g model of Dehmer and D i l l  [74,75].  for i n  The  c a l c u l a t i o n s o v e r e s t i m a t e the peak areas of the resonances w i t h the continuum  shape-resonance  i n p a r t i c u l a r disagreement.  a l s o d i s p l a c e d upwards by ~3 eV i n the  The  positions  calculations.  A b e t t e r agreement i s o b t a i n e d w i t h the techniques employed Langhoff and co-workers more i n terms of a MO  [83-88].  The  terminology.  Hartree-Fock c a l c u l a t i o n s u t i l i s i n g the ground  s t a t e of the m o l e c u l e .  resonance  In t h i s approach, c o n v e n t i o n a l g a u s s i a n b a s i s s e t s are performed  which w i l l be a p p r o p r i a t e to d e s c r i b e the motion  of the e x c i t e d  form:  on  From t h i s c a l c u l a t i o n a n o n - l o c a l can be c o n s t r u c t e d  the f o l l o w i n g  by  phenomena i s d e s c r i b e d  p o t e n t i a l f o r the i o n i s a t i o n channel of i n t e r e s t  i n the f r o z e n f i e l d  are  of the remaining N - l e l e c t r o n s .  The  electron  potential  has  -  (N-l)  -  49  N  =  (l.F.l)  G  where J and K are the u s u a l coulomb and  exchange o p e r a t o r s .  s u b s c r i p t j denotes the d o u b l y - o c c u p i e d  o r b i t a l s w h i l e G r e f e r s to the  o r b i t a l from which the e l e c t r o n has been e x c i t e d . c o n j u n c t i o n w i t h the k i n e t i c energy  operator  The  This p o t e n t i a l , i n  (T) and  the n u c l e a r frame-  work p o t e n t i a l (V) form the n e c e s s a r y H a m i l t o n i a n r e q u i r e d to d e s c r i b e the p a r t i c u l a r e x c i t a t i o n c h a n n e l . e l e c t r o n SchrOdinger  the f o l l o w i n g one-  equation  ((T + V +  can be s o l v e d .  Consequently  V  (N-l) G  )  -  e)  T h i s r e s u l t s i n the so c a l l e d  -  (1.F.2)  0  "improved  virtual  o r b i t a l s " which p r o v i d e v a r i a t i o n a l l y c o r r e c t approximations e x c i t e d s t a t e o r b i t a l s w i t h i n the approximation [84,89].  The  f o r the  of a f r o z e n core  bound f u n c t i o n s w i l l p r o v i d e adequate r e p r e s e n t a t i o n s of  the d i s c r e t e e x c i t a t i o n s w h i l e the unbound f u n c t i o n s p r o v i d e a pseudospectrum  of t r a n s i t i o n s which c o n t a i n s a l l the n e c e s s a r y  i n f o r m a t i o n to d e s c r i b e the i o n i s a t i o n continuum. workers then use  physical  Langhoff  S t i e l t j e s - T c h e b y c h e f f moment theory  and  co-  [83,88] to c o n v e r t  the pseudo-spectrum i n t o a c o r r e c t r e p r e s e n t a t i o n of the  oscillator  strength. U s i n g t h i s approach,  Rescigno  and Langhoff  [84] have c a l c u l a t e d  - 50  the  p a r t i a l photoionisation  dipole-allowed  -  cross-sections  f o r the  accessible  e x c i t a t i o n channels i n the K - s h e l l e l e c t r o n  spectrum of N . 2  They a s s i g n  the  a p p r o x i m a t e l y 4 eV  too  This  [84]  is attributed  low  and  the  c a l c u l a t e d energy i s  strength  l e s s s e n s i t i v e to  eV w i t h experiment.  s e c t i o n f o r the continuum i s i n b e t t e r o b t a i n e d from the m u l t i p l e  i s overestimated.  of c o r e - r e l a x a t i o n .  t i o n s to the Rydberg o r b i t a l s , which are 0.5  The  oscillator  to the n e g l e c t  e f f e c t s , agree to w i t h i n  excitation  d i s c r e t e shape-resonance at 401.10 eV  to a l a •+ l u core to v a l e n c e t r a n s i t i o n . u g  that  four  The  The  transi-  relaxation  calculated  cross-  agreement w i t h experiment  than  s c a t t e r i n g c a l c u l a t i o n [74,75].  The  to a 1 a -*• 3 a core to v a l e n c e g u resonances are both a t t r i b u t e d to t r a n s i t i o n s to  continuum shape-resonance i s a s c r i b e d transition.  Thus the  *  v i r t u a l v a l e n c e o r b i t a l s which can  be  equated with the  *  it and  a  anti-  * bonding o r b i t a l s [84] .  a  In t h i s p a r t i c u l a r case the  orbital is in  the  continuum. The  MO  p i c t u r e and  ary ways of l o o k i n g The  the  s c a t t e r i n g model present two  at e l e c t r o n e x c i t a t i o n and  success of both models i n d e s c r i b i n g  q u a n t i t a t i v e l y , should not  be  a l o c a l i s e d , quasi-stationary  within  the  t i o n should a l s o be between the MO  of  able  to d e s c r i b e  c a l c u l a t i o n and the  the  multiple  the  scattering  (resembling a bound  the m o l e c u l a r p o t e n t i a l .  f u r t h e r emphasised i n t h a t 3dit and  state  resonance phenomena.  phenomena, at l e a s t semi-  s u r p r i s i n g i n that  places  confines  the  the  compliment-  As  situation.  such a MO The  picture state)  descrip-  relationship  scattering picture for N  2  is  lit and 3o o r b i t a l s c o r r e l a t e w i t h the g u 4 f a atomic o r b i t a l s w i t h i n the u n i t e d atom l i m i t [84], c o n s i s -  - 51  tent w i t h the d-wave and  -  f-wave n a t u r e s found i n the x  a  calculation  [75]. The  above d i s c u s s i o n has f o c u s s e d on N . 2  has been made f o r CO SFg and B F  3  [71,86].  A similar  comparison  C o n s i d e r a t i o n of other s p e c i e s , such as  l e n d support to the p a r a l l e l i n t e r p r e t a t i o n of  nance phenomena u s i n g e i t h e r a MO  shape-reso-  or m u l t i p l e s c a t t e r i n g p e r s p e c t i v e .  For example, the i n n e r s h e l l e x c i t a t i o n s p e c t r a of SFg have been p r e t e d w i t h LCAO-MO c a l c u l a t i o n s  [90] as have those f o r B F  M u l t i p l e s c a t t e r i n g c a l c u l a t i o n s have a l s o been performed t i o n s p e c t r a of B F [94].  3  [93] and on the analogous  e-SF  6  3  inter-  [91,92].  on the e x c i t a -  scattering  The r e s u l t s of the m u l t i p l e s c a t t e r i n g c a l c u l a t i o n on B F  system [93]  3  should be p a r t i c u l a r l y noted w i t h r e g a r d to the nature of the t r a p p i n g mechanism i n these m o l e c u l e s .  While  the angular momenta i s found to be  l a r g e enough (Jl>2) i n the d i a t o m i c s to form a c e n t r i f u g a l b a r r i e r cient of B F  suffi-  to support q u a s i - s t a t i o n a r y s t a t e s , t h i s i s not found i n the case 3  [93].  I t has been concluded that the b a r r i e r i n B F  3  results  from  a combination of c e n t r i f u g a l f o r c e s and s t r o n g e l e c t r o n r e p u l s i o n i n the neighbourhood suggested  of the l i g a n d s  f o r GeCl^  [95].  [93].  S i m i l a r c o n c l u s i o n have been  R e g a r d l e s s of how  the p o t e n t i a l  a r i s e s , f e a t u r e s a s s o c i a t e d w i t h shape-resonances many systems, present work.  barrier  can be r e c o g n i s e d i n  as w i l l be seen i n many of molecules s t u d i e s i n the In l i g h t  of the above d i s c u s s i o n these  can be o f t e n i d e n t i f i e d w i t h knowledge of the MO  shape-resonances  scheme of the  system.  - 52 -  III)  R e l a t i o n s h i p Between Shape-Resonance P o s i t i o n and As  should stood  stated e a r l i e r ,  provide  the s p e c t r a l p o s i t i o n of the  a probe f o r i n t e r - n u c l e a r s e p a r a t i o n .  shape-resonance  T h i s can be  under-  i n t h a t the b a r r i e r , which w i l l d e f i n e the i n n e r - w e l l p o t e n t i a l ,  i s l o c a t e d on the p e r i p h e r y c u l a r dimensions. the e n e r g i e s with  Bond Length  1/R ,  of the molecule and  If a simplistic  of the  " p a r t i c l e i n a box"  analogy i s made,  s t a t i o n a r y s t a t e s i n such a p o t e n t i a l should  where R r e p r e s e n t s  2  hence r e f l e c t s the mole-  vary  the dimension of the w e l l ( i . e . bond  length). A more q u a n t i t a t i v e way m u l t i p l e s c a t t e r i n g treatment. pointed  out  of c o n s i d e r i n g  t h i s a r i s e s from a  Gustafsson  Levinson  and  [96]  have  S i a t w i t h i n a m u l t i p l e s c a t t e r i n g p i c t u r e , the wave v e c t o r  of the p h o t o e l e c t r o n to the d i s t a n c e  on resonance (k_)  should  be  inversely proportional  (R) between the s c a t t e r i n g c e n t r e s , i . e .  k R = constant r  Thus i f the p h a s e - f a c t o r s  can be n e g l e c t e d ,  k i n e t i c energy of the p h o t o e l e c t r o n to 1/R . 2  Gustafsson  and  Levinson  resonance energy from t h r e s h o l d and  have o b t a i n e d Recently,  (1.F.3)  i t then f o l l o w s t h a t  on resonance should  be  the  proportional  [96] have p l o t t e d the d i f f e r e n c e of  ( 6 ) against  1/R  2  for several  diatomics  a reasonable c o r r e l a t i o n . N a t o l i [97]  has  demonstrated t h e o r e t i c a l l y , w i t h i n  a  complete m u l t i p l e s c a t t e r i n g p i c t u r e , that f o r c e r t a i n c o n d i t i o n s equation  (1.F.3) i s v a l i d .  g i v e n by  [97,98]  More s p e c i f i c a l l y ,  the wave v e c t o r  (k_) i s  - 53 -  k  where V  r  = /(6-V  i s a mean i n t r a m o l e c u l a r  q  [98,99] and  o  )  (l.F.A)  p o t e n t i a l r e l a t i v e to the vacuum  the constant depends on atomic phase s h i f t s  phase s h i f t s are energy dependent, however, B i a n c o n i pointed  out  t h a t i f the p h a s e - s h i f t  then (1.F.3) i s v a l i d . generally  be  Bianconi  The .term V  o  located  et a l . [98]  though not  have simply i n v e s t i g a t e d length  empirically.  the  suggest that  resonances  [97-99].  [98-100].  Thus H i t c h c o c k et a l .  [100]  r e l a t i o n s h i p of resonance p o s i t i o n and  bond  They have shown [100]  that  f o r a s e r i e s of type  6 = aR + b  i s adequate to r e l a t e the known C-C Sette  et a l . [99]  (1.F.5) to i n v e s t i g a t e  a  (1.F.5)  distance  to the  have a l s o a p p l i e d  c  shape-resonance  this linear relationship  shape-resonance p o s i t i o n s and  a l a r g e v a r i e t y of m o l e c u l e s .  o  system under i n v e s t i g a t i o n -  hydrocarbons a simple l i n e a r r e l a t i o n s h i p of the  position.  have  t h i s should  where the continuum  for n  i s p a r t i c u l a r to the  bond order as w e l l as atomic p a i r  et a l . [98]  The  dependence w i t h energy i s smooth  the case f o r the energy r e g i o n s  shape-resonances are  [97-99].  level  bond l e n g t h s  Good l i n e a r c o r r e l a t i o n s are  c l a s s e s of m o l e c u l e s c h a r a c t e r i s e d  by  the  sum  in  seen f o r  of the atomic numbers of  -  54  -  the atom p a i r i n v o l v e d i n the s c a t t e r i n g process i n the V will  q  term.  As Z i n c r e a s e s  the Z dependence a r i s e s  the a t t r a c t i v e p a r t of the p o t e n t i a l  i n c r e a s e and t h i s r e s u l t s i n the d i f f e r e n t  linear  dependence  observed f o r shape-resonance p o s i t i o n w i t h bond l e n g t h f o r each s e t [78,99].  The f i n d i n g s of S e t t e et a l . [99] a l s o i n d i c a t e t h a t the  p h a s e - s h i f t dependence can be assumed c o n s t a n t , 18, f o r the a  at l e a s t  f o r Z = 12 -  resonances.  From the above d i s c u s s i o n i t i s seen that a c o n v i n c i n g f o r shape-resonance p o s i t i o n and bond l e n g t h s  can be made.  argument  This  allows  a f u r t h e r means of p o s i t i v e l y i d e n t i f y i n g shape-resonance f e a t u r e s i n the continuum i n a s e r i e s o f r e l a t e d m o l e c u l e s . considerations  of the l i m i t a t i o n s i t should  procedure t o o b t a i n bond l e n g t h s  [98-100],  Indeed w i t h c a r e f u l  be p o s s i b l e to r e v e r s e the  - 55 -  CHAPTER 2  EXPERIMENTAL  In t h i s chapter the e x p e r i m e n t a l methods used e l e c t r o n energy During  l o s s s p e c t r a presented  to o b t a i n the  i n t h i s work w i l l be d e s c r i b e d .  the course of t h i s work, a d i g i t a l v o l t m e t e r (DVM) of h i g h s t a t e d  accuracy  (Datron model 1071  [101]) was o b t a i n e d .  This permitted  measurement of a s e t of a c c u r a t e r e f e r e n c e e n e r g i e s f o r i n n e r s h e l l e l e c t r o n e x c i t a t i o n spectroscopy. this  A.  T h i s l a t t e r work i s a l s o presented i n  chapter.  E x p e r i m e n t a l Methods  I)  The  Spectrometer  An e x i s t i n g h i g h r e s o l u t i o n ISEELS spectrometer f o r a l l the e l e c t r o n energy  [6,63] was  l o s s measurements r e p o r t e d i n the present  work with the e x c e p t i o n of the v a l e n c e s h e l l s p e c t r a of N F (see l a t e r i n t h i s s e c t i o n ) . instrument  3  and S i ( C H ) 3  The c o n s t r u c t i o n and o p e r a t i o n of t h i s  has a l r e a d y been d e s c r i b e d i n d e t a i l and so o n l y a b r i e f  d e s c r i p t i o n i s presented h e r e . referred  used  For a f u l l  d e s c r i p t i o n the reader i s  to the Ph.D. t h e s i s of A.P. H i t c h c o c k  [6] or the d e s c r i p t i o n  given i n r e f . [63]. A schematic i n F i g . 2.1. thoriated energy  of the spectrometer  E l e c t r o n s produced  (taken from r e f . [6]) i s shown  i n the gun r e g i o n (G) by a heated  tungsten r i b b o n or wire are a c c e l e r a t e d to the d e s i r e d impact  ( t y p i c a l l y 2.5 keV) to form a focussed e l e c t r o n beam.  The beam  1 +  F i g u r e 2.1  Schematic of the Inner S h e l l E l e c t r o n Energy Spectrometer  (taken from r e f . [ 6 ] ) .  Loss  -  is  57  then r e t a r d e d by a two-element  of a h e m i s p h e r i c a l mator (M).  -  l e n s ( L l ) to the r e q u i r e d  e l e c t r o s t a t i c a n a l y s e r which a c t s as the monochro-  Following  this  the beam i s a c c e l e r a t e d back to the impact  energy by a second two-element  l e n s (L2) to pass through the i n t e r a c t i o n  r e g i o n (CC) a f t e r which i t i s r e t a r d e d second e l e c t r o s t a t i c a n a l y s e r transmitted  to the d e t e c t o r  to the s e l e c t e d pass-energy of a  (A) by a three-element l e n s (L3) and  (CEM).  Various  d e f l e c t i o n p l a t e s (D1-D4)  a l l o w minor c o r r e c t i o n s to the beam path to be made. monitored throughout the spectrometer by measuring various apertures Initially  pass-energy  (P1-P3) with an  The beam can be  the c u r r e n t  on  electrometer.  the instrument i s set up on the primary  beam ( z e r o energy l o s s ) at zero degree s c a t t e r i n g a n g l e .  unscattered T h i s i s done  by c a r e f u l l y a d j u s t i n g the d e f l e c t o r p l a t e v o l t a g e s and m o n i t o r i n g beam c u r r e n t on p l a t e s P l to P3.  The c u r r e n t i s maximised  minimised on each p l a t e i n t u r n and then f i n a l l y  maximised  d e t e c t o r cone which, i n t h i s case, a c t s as a Faraday cup.  and  the  then  on the The middle  element of the three-element lens i s s e t at the v o l t a g e which  maximises  the t r a n s m i s s i o n of the s c a t t e r e d c u r r e n t f o r the energy l o s s r e g i o n about to be s t u d i e d .  F i g u r e 2.2  determined optimum focus v o l t a g e s  shows a p l o t of the (with respect  experimentally  to ground) f o r the  middle element of L3 as a f u n c t i o n of energy l o s s f o r d i f f e r e n t pass e n e r g i e s  at 2.5  keV  impact energy.  F i g u r e 2.3  shows the  analyser  focus  v o l t a g e as a f u n c t i o n of a n a l y s e r pass energy f o r the energy l o s s approp r i a t e to the C Is r e g i o n (~ 290 eV), the N Is r e g i o n (~ 400 eV) and the 0 Is r e g i o n (~ 540  eV).  -  58 -  Pass Energy  300  400  500  600  ENERGY LOSS (eV) Figure 2.2  Optimum focussing voltages for the middle element of the three-element lens (L3) (see Figure 2.1) as a function of energy loss for different analyser pass energies at 2.5 keV Incident energy.  - 59 -  N1s i 10  '  i  1  20  r  3 0  PASS ENERGY (eV)  Figure  2.3  Optimum f o c u s s i n g v o l t a g e s f o r the middle element three-element l e n s (L3) (see F i g u r e  of t h e  2.1) as a f u n c t i o n of  a n a l y s e r pass energy f o r s e l e c t e d energy l o s s e s a t 2.5 keV incident  energy.  -  The  60  -  r e s o l u t i o n of the spectrometer depends on the pass  of the two a n a l y s e r s .  The t h e o r e t i c a l r e s o l u t i o n of t h i s  energies  spectrometer  i s g i v e n by [ 6 ]  ^FWHM  =  °'  01  ^M+V  (2  -Aa)  where V_, and V. are the monochromator and a n a l y s e r pass e n e r g i e s M A tively.  The a c t u a l r e s o l u t i o n can be o b t a i n e d  respec-  by measuring the p r o f i l e  of the primary beam. When running elastically  a sample, p a r t of the primary beam i s s c a t t e r e d  or i n e l a s t i c a l l y by the gas i n the i n t e r a c t i o n r e g i o n .  o b t a i n a spectrum a v o l t a g e , e q u i v a l e n t to the i n e l a s t i c  to the energy l o s s  s c a t t e r i n g , i s added on top of the s m a l l  a l r e a d y a p p l i e d to the complete a n a l y s e r  system.  By u s i n g a s u i t a b l e o f f s e t v o l t a g e and scanning  l o s s r e g i o n of i n t e r e s t a f u l l  degree s c a t t e r i n g a n g l e .  voltage  through the the energy  spectrum can be o b t a i n e d .  A h i g h count r a t e i s a t t a i n a b l e with valence energy l o s s spectroscopy  corresponding  Thus the s c a t t e r e d  e l e c t r o n s can r e g a i n t h e i r energy l o s s and be t r a n s m i t t e d analyser.  To  shell electron  and so the s p e c t r a can be measured a t zero However, i t i s not p o s s i b l e to o b t a i n  s h e l l s p e c t r a a t zero degree s c a t t e r i n g angle  inner  on t h i s spectrometer due  to the r e l a t i v e l y low count r a t e r e l a t i v e to the s m a l l but not i n s i g n i f i c a n t background produced by the b a c k s c a t t e r i n g of the i n t e n s e main beam on the a n a l y s e r  surface.  To a v o i d t h i s ,  the main beam i s passed  - 61 -  through the c e n t r e o f the gas c e l l a t a s m a l l angle by use o f a d e f l e c t i o n " system (DD).  T h i s system c o n s i s t s o f two s e t s of d e f l e c t i n g  p l a t e s , o p e r a t i n g i n the energy d i s p e r s i n g planes whose f i e l d s a c t i n o p p o s i t e d i r e c t i o n .  cell.  first  set  s e t and are p l a c e d  s e t of p l a t e s and the c e n t r e of the gas  The f i e l d s a r e generated  beam i s d e f l e c t e d by the f i r s t angle  of the a n a l y s e r s ,  The p l a t e s i n the second s e t  a r e twice the l e n g t h of the p l a t e s i n the f i r s t e q u i d i s t a n t from the  "double-  by a s i n g l e v o l t a g e source. s e t of p l a t e s i n one d i r e c t i o n  Thus the through  0 and then d e f l e c t e d back i n the o p p o s i t e d i r e c t i o n by the second  o f p l a t e s through angle  28.  The net r e s u l t i s that the beam passes  through the c e n t r e o f the gas, where the c o n c e n t r a t i o n o f gas i s the g r e a t e s t , at ^ n angle obtained  0.  Table  2.1 shows the d e f l e c t i o n angle (9)  a t i n c i d e n t beam e n e r g i e s of 1.5 keV and 2.5 keV f o r v a r i o u s  v o l t a g e s a p p l i e d t o the " d o u b l e - d e f l e c t i o n " system.  Typical operating  c o n d i t i o n s employed i n the present work a r e 2.5 keV impact energy and a s c a t t e r i n g angle o f ~ 1 ° . dipole-dominated  T h i s a n g l e , w h i l e s m a l l enough t o ensure  s p e c t r a , a l l o w s i n t e r c e p t i o n of the main beam by p l a t e  P3 b e f o r e i t can reach the a n a l y s e r . for  During  the set-up  procedure and  v a l e n c e - s h e l l s p e c t r a the long p l a t e s are grounded out and the s h o r t  p l a t e s ( a l o n g w i t h t h e i r p e r p e n d i c u l a r c o u n t e r p a r t s ) a c t i n the same manner as the other  II)  (x,y) d e f l e c t o r p l a t e s .  Sample-Handling A second sample i n l e t  system, completely  l e s s s t e e l , was added to the o r i g i n a l brass i n l e t  c o n s t r u c t e d of s t a i n system  [6] which was  -  TABLE 2.1:  62  -  D e f l e c t i o n angle (9) for various voltages (V ) applied to the d o u b l e - d e f l e c t i o n system f o r impact e n e r g i e s ( E ) of 1.5 keV and 2.5 keV' y  Q  Voltage across plates (V )  D e f l e c t i o n angle (9) f o r impact energies ( E )  y  Q  1.5  2.5  keV  52  0.90  0.54  106  1.84  1.10  161  2.79  1.68  218  3.78  2.27  277  4.80  2.88  D e f l e c t i o n angle g i v e n 9 = tan  keV  - 1  where x = 1.0 y = 1.1  by  V «x y  cm, cm,  l e n g t h of s h o r t p l a t e gap between p a r a l l e l p l a t e s  (see any i n t r o d u c t o r y p h y s i c s t e x t eg. F.W. Sears, M.W. Zemansky and H.D. Young, " U n i v e r s i t y P h y s i c s " , Addison Wesley (1980) p. 450-451).  -  retained  -  63  f o r the r e f e r e n c e gas l i n e .  The gas p r e s s u r e  i n the new  was c o n t r o l l e d by means of a G r a n v i l l e - P h i l l i p s s e r i e s 203 l e a k while  t h a t from the r e f e r e n c e  valve  (model 951-5100).  connected on the low p r e s s u r e i n t o the gas c e l l .  these  of 4 x l 0 ~  7  s i d e of the l e a k v a l v e s p r i o r  The ambient p r e s s u r e  t o r r to 5 x l 0 ~  to r i s e from a base  are observed  commercially  system w i t h  "swagelock" f i t t i n g s .  (copper  [102].  from the c y l i n d e r  r e g u l a t o r and a l l the connections  1/4" t u b i n g  Under  and were of h i g h  The gas samples were taken d i r e c t l y  u s i n g the a p p r o p r i a t e  which was  t o r r on sample i n t r o d u c t i o n .  the samples were o b t a i n e d  stated purity.  to b e i n g f e d  of the spectrometer,  c o n d i t i o n s o n l y s i n g l e s c a t t e r i n g processes All  inlet  5  leak  (sample and r e f e r e n c e ) were  monitored by an i o n i s a t i o n gauge, was allowed pressure  valve  system was c o n t r o l l e d w i t h a V a r i a n  The two l i n e s  system  made to the  or s t a i n l e s s s t e e l ) and  L i q u i d samples were t r a n s f e r r e d from  their  c o n t a i n e r to an evacuated g l a s s v i a l equipped w i t h a t e f l o n v a l v e t o which was a l s o a t t a c h e d thereby  a s h o r t p i e c e of 1/4" diameter g l a s s  a l l o w i n g the sample to be connected d i r e c t l y  f i t t i n g s of the i n l e t  to the "swagelock"  system by u s i n g t e f l o n f e r r u l e s .  samples were degassed by repeated  freeze-thaw c y c l e s .  tubing  The l i q u i d Valence-shell  s p e c t r a were run to check t h a t the samples were f r e e of any obvious volatile  i m p u r i t i e s and a l s o to ensure that the system was a i r t i g h t , the  l a t t e r b e i n g an e s p e c i a l l y important  consideration f o r inner  s p e c t r a i n the N and 0 K - s h e l l r e g i o n s . by o b s e r v i n g eV [103].  the i n t e n s e N  A l e a k could e a s i l y be d e t e c t e d  (X •* b !! ) valence 1  2  shell  s h e l l f e a t u r e a t 12.93  From i t s v a l e n c e - s h e l l spectrum, the sample of P C 1  3  was  seen  -  64  -  to c o n t a i n a s m a l l amount of HCl i m p u r i t y which was continuous mixture.  pumping on the sample cooled down w i t h a A s i m i l a r process was  f u r t h e r p u r i f i c a t i o n was  dry-ice/methanol  a l s o performed w i t h  p r e c a u t i o n a r y measure, even though no HCl was No  removed by  performed on any  OPCI3  as a  apparent i n the  spectrum.  of the samples s i n c e the  s p e c t r a i n d i c a t e d t h a t they were e s s e n t i a l l y f r e e of i m p u r i t i e s .  Ill)  S p e c t r a l A c q u i s i t i o n , C a l i b r a t i o n and The  s p e c t r a were o b t a i n e d  i n the f o l l o w i n g manner.  sample (or sample p l u s c a l i b r a n t ) was was  Spectrometer Performance The  f e d i n t o the spectrometer  gaseous which  set up i n the manner d e s c r i b e d above f o r the r e q u i r e d s p e c t r a l  lution.  I t was  introduction.  always necessary  to retune  In p r a c t i c e , i t was  found  the spectrometer  > 5 eV,  r e s o l u t i o n was 21.218 eV  however, f o r h i g h r e s o l u t i o n valence  best o b t a i n e d by measuring the He(I)  [104],  The  to the r e q u i r e d i n i t i a l  v o l t a g e a l r e a d y a p p l i e d to the a n a l y s e r system.  (2.A.1))  s h e l l s t u d i e s the  s e l e c t e d by  energy, on top of The  r e g i o n was  s u c c e s s i v e l y scanned by v o l t a g e programming a power supply which was  i n s e r i e s w i t h the e n e r g y - l o s s / a n a l y s e r  410B), u s i n g the ramp output 1064).  the ramp v o l t a g e was readout  from the m u l t i c h a n n e l  However, f o r the continuous  was  f o r pass  resonance l i n e at  s p e c t r a l r e g i o n of i n t e r e s t was  a voltage, corresponding  upon sample  that the a c t u a l r e s o l u t i o n was  very c l o s e to the t h e o r e t i c a l r e s o l u t i o n (see equation energies  reso-  the  then  (Kepco  power supply  adding  PX100)  (Fluke  analyser (Fabritek  wide range scan shown i n Chapter  monitored by a D i g i t e c v o l t m e t e r  connected to the s h a f t of a potentiometer  and  4,  i t s mechanical  which  -  65  -  could r e s i s t a n c e programme the e n e r g y - l o s s / a n a l y s e r ref. was  [6] f o r f u l l  details).  standard  p u l s e s from the d e t e c t o r  s t o r e d i n the m u l t i c h a n n e l v i a a ratemeter.  The  ramp v o l t a g e output  Any  channel  and  s p e c t r a l range was number up  a program c o n t r o l u n i t  analyser. address  3  drop-off  u n i t s and  The  the output  signal  determined by channel to 1000  step s i z e and  with  the  (Ortec 4610).  (see equation  the energy r e g i o n s t u d i e d  (l.C.17)).  h i g h e r energy l o s s e s (N I s , 0 Is and 48 hours.  the a i d of  S p e c t r a l a c q u i s i t i o n time depended  hours f o r the  i n n e r - s h e l l s p e c t r a v a r i e d between 12 and  long  times  f o r the h i g h r e s o l u t i o n  48 hours whereas h i g h r e s o l u -  s h e l l s p e c t r a c o u l d be obtained  i n a matter of minutes  though the s p e c t r a were t y p i c a l l y run f o r 1 to 2 hours to o p t i m i s e  The (Hewlett  eV)  those f o r  F Is) required c o l l e c t i o n  A c q u i s i t i o n times  (due  A low r e s o l u t i o n (0.35  eV energy l o s s r e g i o n s w h i l e  ~ 350  the number of  p o i n t s c o u l d be s e l e c t e d with  range s p e c t r a i n the 100  signal/noise  was  so a complete spectrum can be s i g n a l averaged.  took between 6 - 1 2  t i o n valence  by  s i g n a l c o u l d a l s o be monitored  i n n e r s h e l l spectrum t y p i c a l l y  between 24 and  however,  a l s o used.  advances synchronously  on the number of p o i n t s , r e s o l u t i o n and to the E ~  was  DVM,  system  ( c h a n n e l t r o n ) were processed  pre-amp/amplifier/discriminator  points.  the Datron 1071  of the s p e c t r a a Data P r e c i s i o n 3500 DVM  The  (see  v o l t a g e a p p l i e d to the a n a l y s e r  measured i n the m a j o r i t y of cases with  f o r a few  The  The  power supply  the  ratio. d a t a , once o b t a i n e d , was  Packard 7004B).  For data m a n i p u l a t i o n  p l o t t e d on a X-Y  A l l measurements were taken  point  plotter  from t h i s  (background s u b t r a c t i o n , s c a l e expansion  plot. etc.)  the  -  data was  t r a n s f e r r e d to f i l e s  66  -  on the UBC  computer by use of a  digi-  tiser. In p r i n c i p l e , once the spectrometer  i s set-up  on the  primary  beam f o r zero energy l o s s , a l l that i s r e q u i r e d to put a spectrum on a b s o l u t e s c a l e i s an a c c u r a t e r e a d i n g of the v o l t a g e added onto a n a l y s e r system (see next set-up  section).  the  However, s i n c e the spectrometer  u s i n g the cone as a Faraday cup  and  observing  the  was  (analogue)  c u r r e n t on an e l e c t r o m e t e r whereas the s p e c t r a were run i n a p u l s e mode, i n p r a c t i c e , the s p e c t r a o b t a i n e d  an  i n t h i s work were put  count  on  a b s o l u t e s c a l e s by comparison w i t h a known f e a t u r e run under the same operating conditions.  The  process  i n v o l v e d c a l i b r a t i n g a f e a t u r e i n the  sample spectrum by u s i n g an e x t e r n a l r e f e r e n c e and e s t a b l i s h e d standard  to i n t e r n a l l y c a l i b r a t e  s p e c t r a l f e a t u r e s i n other energy ranges.  then u s i n g the newly  the r e s t of the  To ensure the same o p e r a t i n g  c o n d i t i o n s when u s i n g an e x t e r n a l r e f e r e n c e , r e f e r e n c e and run as a m i x t u r e . f e a t u r e s and  The  r e f e r e n c e was  being c a l i b r a t e d  repeated  approximately  as the inverse-cube  s e v e r a l times.  of the three element l e n s (L3) was  chapters. He(I)  The  valence  resonance l i n e  calibrating a f t e r the This  Since the c o u n t - r a t e  drops  of the energy l o s s , the middle element set f o r the f e a t u r e with Specific  s p e c t r a are presented  i n the  the  largest  calibration appropriate  s h e l l s p e c t r a were a l l c a l i b r a t e d a g a i n s t  (21.218 eV  feature  no v o l t a g e d r i f t .  energy l o s s so as to maximise i t s count r a t e . d e t a i l s f o r the i n n e r s h e l l  The  run both b e f o r e and  to ensure that there was  procedure was  sample were  chosen so that i t s s p e c t r a l  those of the sample d i d not o v e r l a p .  f e a t u r e ( e x t e r n a l or i n t e r n a l ) was  sample  the  -  [104]) except  f o r PC1  b r a t e the spectrum  3  67  -  i n which case the HC1  b e f o r e i t was  i m p u r i t y was  used  to  cali-  pumped away.  Most of the compounds presented  i n t h i s work q u i c k l y degraded  the h i g h r e s o l u t i o n performance c a p a b i l i t i e s of the spectrometer. spectrometer  c o n s i s t s of o n l y one  chamber pumped by a s i n g l e  The  diffusion  pump w i t h no d i f f e r e n t i a l pumping systems to i s o l a t e the gas c e l l the r e s t of the spectrometer gases with the gun  or the e l e c t r o n gun.  f i l a m e n t produced  t i o n which a f f e c t e d  optimum performance. v a l e n c e s h e l l * and bilities  R e a c t i o n of. r e a c t i v e  a c o n s i d e r a b l e amount of contamina-  the c h a r a c t e r i s t i c s of the e l e c t r o n beam and  frequent c l e a n i n g of the spectrometer The  was  necessary  to b r i n g i t back t o  i n n e r s h e l l s p e c t r a of N F i g u r e 2.4  2  which i l l u s t r a t e  shows the N  2  the  eV which compares v e r y f a v o u r a b l y w i t h the spectrum h i g h e r r e s o l u t i o n by Geiger et a l . [103b].  (Is  IV)  o b t a i n e d at  0.017 slightly  This feature provides a  f o r the c a p a b i l i t i e s of the spectrometer  as does the  N  2  •*• TI ) c o r e - v a l e n c e t r a n s i t i o n shown i n the next s e c t i o n ( F i g . 2.5).  Other Measurements The  v a l e n c e s h e l l s p e c t r a of N F  3  and  S i ( C H ) ^ presented  work were o b t a i n e d by Dr. Suzannah D a v i e l on a new which has instrument used  capa-  (X •*• b^II )  v a l e n c e - v a l e n c e t r a n s i t o n o b t a i n e d w i t h a s p e c t r a l r e s o l u t i o n of  test  thus  e f f e c t i v e n e s s of t h i s can be seen i n the  of the spectrometer.  stringent  from  3  ISEELS  spectrometer  r e c e n t l y come i n t o o p e r a t i o n i n t h i s l a b o r a t o r y . [53], though s i m i l a r i n p r i n c i p l e to the ISEELS  i n the present work employs a number of s i g n i f i c a n t new  including  in this  This  new  spectrometer features  N  2  valence  AE=0.017eV ON 00  /  0 12.4  12.6  12.8  13.2  13.0  ENERGY LOSS(eV) F i g u r e 2.4  High r e s o l u t i o n  electron  r e g i o n of the X + b  1  energy l o s s spectrum o f N  ^ transition.  2  i n the  - 69 -  1) separate and  analyser  d i f f e r e n t i a l pumping of the gun, monochromator, i n t e r a c t i o n regions  2) l a r g e r a d i u s (20 cm) h e m i s p h e r i c a l a n a l y s e r s and 3) c a r e f u l l y designed  and h i g h l y e f f i c i e n t  f e a t u r e s have produced s i g n i f i c a n t and 0°.  electron optics.  These  improvements i n r e s o l u t i o n , i n t e n s i t y  s t a b i l i t y as w e l l as p e r m i t t i n g a background f r e e o p e r a t i o n a t 6 = T h i s l a t t e r p o i n t i s not o n l y s i g n i f i c a n t  but a l s o f o r v a l e n c e  f o r inner s h e l l  s h e l l s p e c t r a as i t sometimes proved very  spectra difficult  to tune out " g h o s t i n g " e f f e c t s produced by the back s c a t t e r i n g of the main beam i n v a l e n c e  s h e l l spectra obtained  on the o l d instrument ( s e e  Chapter 7 ) . A f u l l d e s c r i p t i o n of the c o n s t r u c t i o n and performance of the new instrument The  i s given i n r e f .  XPS "shake-up" s p e c t r a of N F  Chapter 3 were recorded  B.  The e x p e r i m e n t a l  Reference E n e r g i e s  3  presented  and d i s c u s s e d i n  s e v e r a l years ago by the author  ESCA 36 p h o t o e l e c t r o n spectrometer Alberta.  [53].  on a MacPherson  s i t u a t e d at the U n i v e r s i t y of  d e t a i l s are g i v e n i n Chapter 3.  f o r Inner  S h e l l E l e c t r o n Energy L o s s  Spectroscopy  S p e c t r a l measurements of atomic and molecular  energy  levels  g e n e r a l l y r e l y on s u f f i c i e n t l y a c c u r a t e l y known r e f e r e n c e v a l u e s  against  which measurements can be c a l i b r a t e d and i n t h i s regard ISEELS i s no exception.  I n t h i s s e c t i o n a s e t of r e f e r e n c e e n e r g i e s determined by  ISEELS f o r the energy range 100 - 1000 eV are p r e s e n t e d .  I t should be  -  noted  t h a t e l e c t r o n energy  70  -  loss spectroscopy  (EELS) p r o v i d e s  an  e x c e l l e n t d i r e c t means of measuring the e n e r g i e s of i n n e r s h e l l t i o n s s i n c e , due  to the i n h e r e n t nature of the technique  transi-  itself,  i t only  i n v o l v e s the d i r e c t measurement of a v o l t a g e or a v o l t a g e d i f f e r e n c e . T h i s i s i n c o n t r a s t to o p t i c a l methods which are i n d i r e c t  s i n c e they  r e l y on a g r a t i n g e q u a t i o n which i s l i a b l e to l e a d to g r e a t e r e r r o r s at s h o r t e r wavelengths ( h i g h e r e n e r g i e s ) .  energy  This i s well  t r a t e d i n the case of the c h l o r i n e L - s h e l l e x c i t a t i o n spectrum where two  s e t s of independent  illusof  HC1  ISEELS measurements have r e c e n t l y c o n c l u -  s i v e l y shown [105,106] t h a t the energy  s c a l e s of e x i s t i n g  a b s o r p t i o n s p e c t r a are i n e r r o r by ~ 0.5  optical  eV.  A major source of e r r o r i n d e t e r m i n i n g h i g h e r energy  levels i n  e l e c t r o n s p e c t r o s c o p y p a r t i c u l a r l y EELS i s the o f t e n l i m i t e d a c c u r a c y of commonly a v a i l a b l e d i g i t a l v o l t m e t e r s (DVM). o n l y 4-1/2 1000  eV,  digit  c a p a b i l i t y was  used  For example, i f a DVM  to measure a v o l t a g e , up to say  the r e a d i n g alone c o u l d o n l y be determined  t y p i c a l manufacturer's  quoted  of  t o ±0.1  eV.  With a  accuracy of ±0.1% the r e a d i n g a t 1000  would be no more a c c u r a t e than ±1 v o l t !  eV  As a r e s u l t even though  somewhat more a c c u r a t e DVM's are u s u a l l y used many p u b l i s h e d e x c i t e d s t a t e and  i o n i s a t i o n e n e r g i e s maybe of somewhat l i m i t e d a c c u r a c y .  examination  of the l i t e r a t u r e  values p a r t i c u l a r l y  i n XPS  t h a t i n the case of XPS  non  r e v e a l s i n c o n s i s t e n c i e s i n some p u b l i s h e d  measurements. linearities  are the p r i n c i p a l d e t e r m i n i n g  An  factor.  However, Lee i n spectrometer  [107] has energy  However, i n ISEELS  shown  response  spectrometers  run a t constant pass e n e r g i e s the c o n s i d e r a t i o n s are d i f f e r e n t and  DVM  - 71 -  accuracy can be a major  factor.  I n o r d e r to make s u f f i c i e n t l y a c c u r a t e measurements, up t o 1000 eV, a t l e a s t  6-1/2 d i g i t  c a p a b i l i t y i s d e s i r a b l e w i t h an a c c u r a c y o f a t  l e a s t ±0.001% or ±0.01 eV.  Such instruments a r e not r o u t i n e l y  on most e l e c t r o n s p e c t r o m e t e r s .  employed  I f h i g h l y accurate c a l i b r a t i o n  values  are a v a i l a b l e then new i n n e r s h e l l s p e c t r a may be put on a c c u r a t e  energy  s c a l e s by measuring w i t h r e f e r e n c e t o these c a l i b r a t e d l e v e l s u s i n g mixed sample gases.  P r o v i d e d t h a t the c a l i b r a n t  then a DVM of lower accuracy than t h a t used determination w i l l scale.  i s c l o s e by i n energy  f o r the o r i g i n a l  s u f f i c e to obtain a s u f f i c i e n t l y accurate  calibration energy  I t i s the purpose of the work presented i n t h i s s e c t i o n t o  p r o v i d e such a range o f r e f e r e n c e c a l i b r a t i o n v a l u e s f o r t h i s Of the r e l a t i v e l y  few commercially  purpose.  a v a i l a b l e DVM which come near t o the  s p e c i f i c a t i o n s needed a DATRON model 1071 DVM was s e l e c t e d and the v a l u e s r e p o r t e d here were measured w i t h t h i s i n s t r u m e n t . [101] model 1071 6-1/2 d i g i t  The DATRON  DVM has a s t a t e d accuracy o f ±0.001% (90  day) o r ±0.002% (1 y e a r ) i n normal o p e r a t i o n . mode i s s e l e c t a b l e which y i e l d s 7-1/2 d i g i t  I n a d d i t i o n an a v e r a g i n g  c a p a b i l i t y and then the  a c c u r a c y i s claimed t o be f u r t h e r i n c r e a s e d by a f a c t o r of two. also features a u t o - r e c a l i b r a t i o n v i a a b u i l t - i n microprocessor compensates f o r s h o r t - t e r m a g i n g . were completed  The DVM  which  The measurements i n the present work  w i t h i n 90 days o f a c e r t i f i e d  f a c t o r y c a l i b r a t i o n to the  above s p e c i f i c a t i o n s . The  energy  l o s s e s c o r r e s p o n d i n g t o s e v e r a l known i n n e r s h e l l  e l e c t r o n i c t r a n s i t i o n s have been redetermined  u s i n g the ISEELS s p e c t r o -  -  72  -  meter d e s c r i b e d i n the p r e v i o u s s e c t i o n i n c o n j u n c t i o n w i t h the DATRON model 1071  DVM.  Each of the t r a n s i t i o n s was measured at s e v e r a l  diffe-  r e n t energy r e s o l u t i o n s to check f o r any change i n peak envelope or position.  T h i s i s an e f f e c t i v e t e s t s i n c e change of r e s o l u t i o m  changes  the pass energy of the a n a l y s e r and the v o l t a g e s on the three-element l e n s i n the energy l o s s p a r t of the spectrometer.  The f a c t  e n e r g i e s are independent of r e s o l u t i o n  shows that  changes  (Table 2.3)  i n t r a j e c t o r y i n the a n a l y s e r do not a f f e c t  w i t h i n the s t a t e d  t h a t peak  the energy  any scale  uncertainties.  In choosing s u i t a b l e t r a n s i t i o n s the f o l l o w i n g requirements were considered. (1)  P o s s e s s i o n of a s u f f i c i e n t l y  i n t e n s e , sharp and d i s t i n c t  the energy r e g i o n of i n t e r e s t .  feature i n  The sharpness of s p e c t r a l l i n e s i s  i n c r e a s i n g l y l i m i t e d by n a t u r a l width c o n s i d e r a t i o n s as the atomic number i n c r e a s e s . (2)  Ready a v a i l a b i l i t y of the c a l i b r a t i n g atom or molecule.  (3)  Ease of sample h a n d l i n g and  (4)  I n e r t n e s s of the substance w i t h r e s p e c t to decomposition on the hot  introduction.  cathode or on the spectrometer and i n l e t (5)  surfaces.  A range of s u i t a b l y spaced t r a n s i t i o n s to span the 100 - 1000  eV  region. I t s h o u l d be noted t h a t i n EELS the i n t e n s i t y i n a spectrum v a r i e s a p p r o x i m a t e l y as the i n v e r s e cube of the energy l o s s tion  (see equa-  l.C.17) and hence t r a n s i t i o n s at h i g h e r energy l o s s e s have an  intrinsically  lower count r a t e .  T h i s and the f a c t  that higher r e s o l u -  73  -  -  t i o n i s o b t a i n e d at the expense of decreased compromise between count  r a t e and  However, f o r ISEELS a s a c r i f i c e i s o f t e n unnecessary h i g h e r energy t i o n s (AE*At  and  count  r a t e means t h a t a  r e s o l u t i o n must o f t e n be made.  i n count  r a t e to o b t a i n h i g h  resolution  indeed u n d e s i r a b l e s i n c e the t r a n s i t i o n s a t  l o s s e s are s i g n i f i c a n t l y broadened by l i f e t i m e c o n s i d e r a = 7 x 10  - 1 6  eV s e c ) .  Thus n o t h i n g i s gained i n such  by t a k i n g the s p e c t r a at u n n e c e s s a r i l y h i g h  resolution.  Once the sample has been i n t r o d u c e d and f o r zero energy  cases  the spectrometer  l o s s a l l t h a t i s r e q u i r e d f o r determining  the  set up  energy  s c a l e i s a s u f f i c i e n t l y a c c u r a t e r e a d i n g of the a p p l i e d v o l t a g e to the e l e c t r o n a n a l y s e r system.  In order to ensure  a c c u r a t e l y s e t up f o r zero-energy was  that the spectrometer  was  l o s s the e l a s t i c a l l y s c a t t e r e d peak  measured i n the case of the h i g h e s t r e s o l u t i o n (0.070 eV FWHM).  It  i s p o s s i b l e to do t h i s because the primary u n s c a t t e r e d beam does not e n t e r the a n a l y s e r system.  However, at lower  r e s o l u t i o n s the s i g n a l at  the c h a n n e l t r o n f o r normal o p e r a t i n g c o n d i t i o n s was d i r e c t measurement of the e l a s t i c peak and  too l a r g e to a l l o w  so under these c o n d i t i o n s the  * v a l u e o b t a i n e d f o r the C0(C  Is -»• it (v=0)) t r a n s i t i o n by d i r e c t measure-  ment at h i g h r e s o l u t i o n was  used  these measurements CO was sample gas.  as an a l t e r n a t i v e r e f e r e n c e p o i n t .  i n t r o d u c e d s i m u l t a n e o u s l y w i t h the o t h e r  A l t e r n a t i v e l y , a w e l l known lower  t r a n s i t i o n i n the sample i t s e l f was can be seen from Table 2.3 independent,  In  used  energy  v a l e n c e or core  as an i n t e r n a l c a l i b r a n t .  As  (see below) the measured t r a n s i t i o n energy i s  w i t h i n e x p e r i m e n t a l e r r o r , of the r e s o l u t i o n , and whether  an e l a s t i c or an i n e l a s t i c r e f e r e n c e was  used.  Thus over  the range of  - 74  energy  -  r e s o l u t i o n s used, which corresponds  be concluded  t h a t e n e r g i e s are independent  to normal o p e r a t i o n s , i t can of any  instrumental  response  function.  * F i g u r e 2.5  shows the v i b r a t i o n a l l y r e s o l v e d N  t r a n s i t i o n measured at h i g h r e s o l u t i o n .  The  2  (N Is  n )  s p e c t r a l f e a t u r e s are i n  e x c e l l e n t agreement w i t h e a r l i e r p u b l i s h e d works by H i t c h c o c k and [52] and a l s o by King e t a l . [ 5 1 ] . stringent  This t r a n s i t i o n provides a very  t e s t of the performance of the spectrometer  as w e l l as being a  most u s e f u l c a l i b r a t i o n p o i n t i n the median of the energy 1000  eV.  time  t h a t the n a t u r a l l i n e width  range up  N i t r o g e n K s h e l l e x c i t e d s t a t e s have a s u f f i c i e n t l y (~ 0.1  m e r i t s the use of h i g h r e s o l u t i o n and  eV,  The  long  see r e f . [51] and  the energy  to life-  [52])  values obtained i n t h i s  study as w e l l as those from e a r l i e r work [51,52] are l i s t e d 2.2.  Brion  i n Table  r e s o l u t i o n achieved here i s comparable to that i n the  earlier  work w h i l e the s i g n a l to background r a t i o i s somewhat s u p e r i o r .  The  p r e s e n t l y o b t a i n e d v a l u e s are seen to be i n e x c e l l e n t agreement w i t h those r e p o r t e d by King et a l . [51] which were a l s o obtained u s i n g a h i g h l y a c c u r a t e v o l t a g e measuring system.  The  present v a l u e s are  c o n s i s t e n t l y h i g h e r than the e a r l i e r v a l u e s r e p o r t e d on the same i n s t r u ment by H i t c h c o c k and  Brion [52].  T h i s , as noted  l i m i t e d a c c u r a c y of the l e s s s o p h i s t i c a t e d DVM  below, i s due  used  i n the  to the  earlier  work. Carbon monoxide i s a very s u i t a b l e secondary C Is -*• u  t r a n s i t i o n i s dominated by the v=0  there i s no reason  to expect  any  significant  standard s i n c e ,  transiton shift  [50,52].  i n the energy  the  Thus of  the  400  401  402  ENERGY F i g u r e 2.5  L O S S  403  (EV)  High r e s o l u t i o n e l e c t r o n energy l o s s spectrum of N  *  r e g i o n of N Is + i t  excitation.  data p o i n t s r e p r e s e n t s  the sum  The s o l i d  line  of s i x L o r e n t z i a n  2  i n the  through the line  shapes.  - 76 -  TABLE 2.2:  T r a n s i t i o n Energies (eV) i n the high r e s o l u t i o n N Is -*• n ) e l e c t r o n energy l o s s spectrum  F i n a l State Vibrational Level  This Work  Hitchcock et a l  0  400.88(2)  400.70(5)  400.86(3)  1  401.10(2)  400.93(1)  401.09(1)  2  401.33(2)  401.16(1)  401.31(1)  3  401.56(5)  401.39(1)  401 .54(1)  4  401 .77(7)  401 .60(2)  401.76(1)  5  401.98(10)  401.82(3)  401.98(2)  6  2  (N  T r a n s i t i o n energy (eV)  -  -  a  King et a l  b  401.19(2)  a - reference [52] b - reference [51] ^ - In the present work a l l estimated e r r o r s are absolute whereas the errors f o r v=l and above i n references the energy f o r the v=0 peak.  [51] and [52] are r e l a t i v e to  -  77  -  peak p o s i t i o n with changes i n r e s o l u t i o n . t r a n s i t i o n has been found t i o n used.  The peak v a l u e f o r the v=0  to be 287.40(2) eV r e g a r d l e s s of the r e s o l u -  T h i s was e s t a b l i s h e d by running a mixture  the He(I) t r a n s i t i o n (21.218 eV) [104] being used energy  scale.  to s e t the a b s o l u t e  The v a l u e o b t a i n e d f o r the C0(C Is  at both h i g h and low r e s o l u t i o n s was found  of He and CO w i t h  u (v=0)) t r a n s i t i o n  to be the same w i t h i n e x p e r i -  mental e r r o r . Measurements have been made f o r s e l e c t e d t r a n s i t i o n s i n a s e r i e s of atoms and molecules  whose s p e c t r o s c o p y had p r e v i o u s l y been s t u d i e d  u s i n g ISEELS.  S p e c t r a l f e a t u r e s observed  those observed  i n the e a r l i e r r e p o r t e d s t u d i e s of S F  [50,52], N  [51,52] and Ne  2  o b t a i n e d f o r the s e l e c t e d might be expected  [109] .  were i n a l l cases I d e n t i c a l to 6  [ 6 9 ] , Ar [108], CO  Table 2.3 summarises the data  t r a n s i t i o n s at v a r i o u s energy  t h a t o n l y the S F  6  (S 2 p ^ ^  2  •* t  2 g  resolutions.  ) and N  2  It  ( I s ->•  *  a ( v = l ) ) p o s i t i o n s would vary with r e s o l u t i o n as these alone  posses  sufficiently  ( i n the  case of N ,  nents  transitions  the other v i b r a t i o n a l components and f o r S F , the S 2 p ^ y  2  spin-orbit  i n t e n s e and r e s o l v a b l e neighbouring  6  component).  i n SF  6  In f a c t  the s e p a r a t i o n of the s p i n - o r b i t  2  compo-  i s l a r g e enough t h a t the measured p o s i t i o n of the (S 2 p ^ ^  2  •»• g ) peak maximum does not vary w i t h i n e x p e r i m e n t a l e r r o r over the c  2  range of r e s o l u t i o n s employed.  For N  shows t h a t there i s no s i g n i f i c a n t  2  the curve f i t t i n g  i n F i g u r e 2.5  s h i f t of peak maxima f o r v=0, 1 or 2  at h i g h r e s o l u t i o n due to o v e r l a p from n e i g h b o u r i n g peaks. possible shift  i n peak maximum observed  with decrease  The s m a l l  i n r e s o l u t i o n (see  -  78  -  TABLE 2.3: Measured energy levels of the reference l i n e s as a function of resolution  Inner-shell Transition  Energy lot;s (eV) at quoted resolutions (eV) t r a n s i t i o n 0.070  SF  6  S2p  1 / 2  *t  2p2/2***  Ar  2 g  s  b  -  CO C ls-»n*(v-0)  287.40(2)  N  401.10(2)  ls*n*(v-l)  2  CO 0 ls-m*  SF  6  F ls*a  Ne ls-»3p  534.21(9) l g  -  -  0.105  0.140  184.54(5) 244.37(2)  -  c  0.210  287.40(2)  -  d  3  0.350  184.51(5)  -  401.10(4)  401.08(4)  401.05(5)  534.19(10)  534.20(10)  534.12(10)  -  -  688.27(15)  -  867.13(8)  a  A l l values quoted with the energy loss scale established from the C0(C Is •*• it (v»=0)) t r a n s i t i o n except where indicated otherwise. The gases were run as mixtures.  b  Zero-energy loss determined from the e l a s t i c peak.  c  Internal c a l i b r a t i o n against Ar(I) (11,828(5) eV) - reference [104].  d  Measured with respect to He(l) (21.218(1) eV) - reference [104].  e  Internally calibrated against S F  ( S 2 p ^ + 2g^* t  6  1  e  - 79 -  T a b l e 2.3) i s c o n s i s t e n t w i t h the shape of the i n t e n s i t y d i s t r i b u t i o n i n the broad quoted  v i b r a t i o n a l envelope.  However  i t can be seen t h a t the v a l u e s  f o r each of the v a r i o u s t r a n s i t i o n s a t the d i f f e r i n g  are the same w i t h i n e x p e r i m e n t a l e r r o r .  resolutions  Thus i t i s p o s s i b l e , w i t h the  s e l e c t e d t r a n s i t i o n s , to use the c a l i b r a t i o n v a l u e s f o r the peak p o s i t i o n s i n a g i v e n spectrum 0.35 eV FWHM)  independent  of r e s o l u t i o n ( a t l e a s t below ~  A summary of the best v a l u e s o b t a i n e d here t o g e t h e r w i t h  l i t e r a t u r e v a l u e s from both ISEELS [50-52,69,108,109] and tion  photoabsorp-  [110-114] are shown i n T a b l e 2.4 t o g e t h e r w i t h the t o t a l  estimated  uncertainties. The major p o s s i b l e sources of s y s t e m a t i c e r r o r i n t h i s work i n c l u d e (a) DVM inelastically  accuracy  s c a t t e r e d e l e c t r o n s due to e l e c t r o n o p t i c a l e f f e c t s i n the  l e n s system ( c ) d i f f e r e n t and  (b) d i f f e r e n c e s i n t r a j e c t o r i e s f o r e l a s t i c and  inelastically  i n s t r u m e n t a l response  functions f o r e l a s t i c  s c a t t e r e d e l e c t r o n s (d) spectrometer  spectral linewidth.  stability  (e)  The e r r o r s due t o (a) should be l e s s than 0.01  a c c o r d i n g to the manufacturers'  specifications.  problem due to the f a c t t h a t the DVM  had 7-1/2  eV  R e s e t a b i l i t y was not a digits.  E r r o r s due to  (b) and ( c ) a r e c o n s i d e r e d to be n e g l i g i b l e compared to the s t a t e d u n c e r t a i n t i e s i n T a b l e 2.4 s i n c e the measurements at v a r y i n g r e s o l u t i o n ( T a b l e 2.3) show no s i g n i f i c a n t r e g a r d i t s h o u l d a l s o be noted observed peaks.  s h i f t s i n measured e n e r g i e s .  In t h i s  t h a t peak p o s i t i o n s h i f t s were not  between measurements u s i n g e l a s t i c and i n e l a s t i c r e f e r e n c e Change i n r e s o l u t i o n of the spectrometer  i n v o l v e s change o f pass  e n e r g i e s i n the a n a l y s e r s , and thus changes i n the d e c e l e r a t i n g t h r e e  - 80 -  TABLE 2.4:  Reference energies for inner s h e l l electron energy loss spectroscopy  Transition energy ( e V )  Inner-shell Transition  Optical  ISEELS This work  SF  6  S 2p  Ar 2p3/  2  1 / 2  +t  2  g  ••• 4s  CO C Is + n*(v=0)  N  Is * it*(v=l)  2  CO 0 Is + n* SF  6  F Is - a  Ne Is •+ 3p  l g  b  Literature  Ref  Literature  Ref  184.54(5)  184.27(10)  [69]  184.55  [110]  244.37(2)  244.39(1)  [108]  287.40(2)  287.31(5) 287.40(2)  [52] [50]  401.10(2)  400.93(6) 401.09(4)  [52] [51]  534.21(9)  534.11(8)  [52]  534.2(3)  [111]  688.27(15)  688.0(2)  [69]  687.5 687.8  [112] [113]  867.13(7)  867.05(8)  [109]  867.13(5) [114]  3  a  Average values taking into account a l l the data from the d i f f e r e n t resolutions (see Table 2.3).  b  Errors are shown i n brackets e.g. 184.52(6) means 184.52 + 0.06 eV.  - 81  element energy l o s s l e n s r a t i o s .  -  T h i s would r e v e a l i f any  significant  energy s c a l e s h i f t s a r i s i n g from changes i n t r a j e c t o r i e s were o c c u r r i n g . Spectrometer s t a b i l i t y  (item  ( d ) ) i s of n e c e s s i t y very  h i g h by  design  s i n c e long s i g n a l averaged scans are n e c e s s a r y f o r h i g h r e s o l u t i o n operation  [52].  Any  drift  or i n s t a b i l i t y more than ~ 0.01  immediately apparent as a l o s s of r e s o l u t i o n and position. being  b e t t e r than 0.01  eV.  The  voltage  a b l u r r i n g of peak  the s t a b i l i t y c o r r e s p o n d i n g  ramp f o r the energy l o s s scans  from the F a b r i t e k M u l t i c h a n n e l  ramp over the range employed was was  Analyser.  w i t h i n the  The  measurement o f f the s p e c t r a .  The  (uncertainty p r i n c i p l e ) considerations. u n c e r t a i n t i e s f o r the r e s p e c t i v e While the present reported  values  data (see T a b l e 2.4)  and  ( i i ) direct  to n a t u r a l  T h i s i s r e f l e c t e d i n the  agree w e l l w i t h some of the  there  are some n o t a b l e  energy v a l u e s  produced u s i n g  consistently slightly  low  discrepancies  which  The  t h i s ISEELS s p e c t r o m e t e r  compared to the present  However t h i s i s d i r e c t l y a t t r i b u t a b l e to the DVM  previously  discrepancies  e a r l i e r obtained  digit  larger  peaks.  the boundaries of s t a t e d e x p e r i m e n t a l e r r o r .  l e s s s o p h i s t i c a t e d 5-1/2  higher  linewidth  l i e well outside  [52,69,109] are  This  ( i ) point  s p e c t r a l l i n e w i d t h (e) f o r  energy t r a n s i t i o n s i s o f t e n q u i t e l a r g e due  this  stated uncertainties.  over the number of channels i n q u e s t i o n  to  was  l i n e a r i t y of  e s t a b l i s h e d by double c h e c k i n g the energy s c a l e by  counting  be  S i m i l a r l y a l l power s u p p l i e s were s e l e c t e d w i t h the r i p p l e  l e s s than 0.002 eV peak to peak and  derived  eV would  l i m i t e d a c c u r a c y of  used i n the e a r l i e r work s i n c e  work. the the  are w e l l w i t h i n the manufacturer's s t a t e d e r r o r l i m i t s  for  - 82  the instruments  used.  Agreement of the present work f o r N ,  excitation  i n SF  g  (> 0.5  CO  and  Ar  [50,51,108] i s e x c e l l e n t .  The  most  2  w i t h the o t h e r p u b l i s h e d ISEELS data serious discrepancy  -  eV) i s w i t h the o p t i c a l v a l u e s f o r the F Is  [112,113].  In t h i s regard i t should be noted  s e p a r a t i o n s between the ISEELS v a l u e s f o r S F Is •*• a, ) are i d e n t i c a l i n the p r e s e n t and lg The  6  (S 2 p ^  t^)  2  and  SF (F 6  than the energy  p a r t i c u l a r l y at s h o r t wavelengths.  T h i s together w i t h the g e n e r a l l y e x c e l l e n t s e l f - c o n s i s t e n c y of the measurements lends c o n f i d e n c e to the p r e s e n t SFg.  t h e r e f o r e peak  c o u l d be d r a s t i c a l l y a f f e c t e d by l i n e s a t u r a t i o n e f f e c t s  can occur i n o p t i c a l s p e c t r a due transition  other  ISEELS measurements f o r  I t i s p o s s i b l e t h a t the o p t i c a l peak shape and  position  the  e a r l i e r [69] ISEELS work.  e l e c t r o n energy l o s s measurement i s much more d i r e c t  c a l i b r a t i o n of o p t i c a l instruments  that  to the resonant  nature  which  of the  [39,115,116].  Thus a c o n s i s t e n t set of c a l i b r a t i o n i n the energy l o s s p r o v i d e s convenient  range below 900  eV.  The  e n e r g i e s has been o b t a i n e d set of v a l u e s  (Table  r e f e r e n c e p o i n t s f o r c a l i b r a t i o n purposes and  v a l u e s are used throughout  the p r e s e n t work.  2.4) these  - 83  -  CHAPTER 3  INNER SHELL EXCITATION, VALENCE EXCITATION AND IONISATION IN NF, AND  3  different  the ISEELS and  are presented  VSEELS spectrum.  STUDIED BY ELECTRON ENERGY LOSS  X-RAY PHOTOELECTRON SPECTROSCOPIES  In t h i s chapter r e g i o n s of NF  The  and  XPS  s p e c t r a of the N Is and F I s  examined i n d e t a i l t o g e t h e r w i t h  i n f o r m a t i o n o b t a i n e d by each technique  yet complimentary and  (VSEELS, ISEELS and  The  XPS)  the  is  the i n t e r p r e t a t i o n of the r e s u l t s  be f a c i l i t a t e d by a j o i n t c o n s i d e r a t i o n of a l l three  p h o t o e l e c t r o n data  CORE  should  spectroscopies  together with p r e v i o u s l y published  [117-119].  VSEELS spectrum of a molecule i s o f t e n complex and  ambiguous  i n i t s assignment because of the p o s s i b i l i t y of many o v e r l a p p i n g transitions orbitals  arising  from the c l o s e p r o x i m i t y and number of  as w e l l as the numerous m a n i f o l d s  of Rydberg l e v e l s .  ISEELS s p e c t r a , however, are g e n e r a l l y r e l a t i v e l y simple forward  to a s s i g n s i n c e the i n i t i a l  b e i n g w e l l separated g i v e more d e f i n i t e this  i n t u r n may  spectrum.  t h i s Chapter. The  and  The straight-  core l e v e l i s u s u a l l y unambiguous,  i n energy from other l e v e l s .  Thus ISEELS can o f t e n  i n f o r m a t i o n on the p r e v i o u s l y unoccupied  orbital  and  be u s e f u l i n c l a r i f y i n g the assignments i n the VSEELS  T h i s depends on the extent  t r a n s f e r a b l e and  valence  to which term v a l u e s  an attempt to address XPS  spectrum p r o v i d e s  are  t h i s c o n s i d e r a t i o n i s made i n i n f o r m a t i o n on the e x c i t e d i o n  - 84  states  and  and  i n p a r t i c u l a r g i v e s the  i o n i s a t i o n processes. be new  The  continua.  due  well"  Thus the  f e a t u r e s i n the  from the  p r o d u c t i o n of  XPS  these e x c i t e d  p y r a m i d a l w i t h C^  and  v  ligands,  since  symmetry.  by  Rydberg l e v e l s . also  fluorinated  highly  excitation  often  often  spectra.  much work has excitation  anisotropic  nature of  [73,91-93,120-122], CF^  F  frequently  valence  the  repulsive  e x p e r i m e n t a l l y on  6  an forces  barrier  [75,77].  compounds, such as SF  be  f o r m a t i o n of  electron  to  i o n i s a t i o n edge  These e f f e c t s can  the m o l e c u l a r f i e l d  [66,92,123,124] and  of  nature of the  [73,93] or a c e n t r i f u g a l  of other f l u o r i n a t e d  is  expense of t r a n s i t i o n s  been done both t h e o r e t i c a l l y and  spectra  and  of core to v i r t u a l  [73,77] e i t h e r by ligands  3  In p a r t i c u l a r f o r core  shape-resonances caused by  barrier  "inner  compounds are  electronegative  been observed at the  neighbourhood of the the  analogue of NH  observed i n such m o l e c u l e s .  effective potential  caused by  shape-resonances, or  Other d i s t i n c t i v e f e a t u r e s beyond the  e x p l a i n e d i n terms of  which  [77],  a very high r e l a t i v e p r o b a b i l i t y  t r a n s i t i o n s has  e n e r g i e s ) of  i n f e r e n c e which f e a t u r e s  S t u d i e s of f l u o r i n a t e d  to the  electron  would  i o n i z a t i o n edge a r i s e  "anomolous" i n t e n s i t y d i s t r i b u t i o n s are  observed i n the spectra,  continuum  i s the  3  p a r t i c u l a r i n t e r e s t due  i n the  ion states  trapped i n the  molecule NF  ion states  s a t e l l i t e d a t a p r o v i d e s i n f o r m a t i o n on  to other types of phenomena such as  states  the  onsets ( a d i a b a t i c  ISEELS spectrum above the  onsets of e x c i t e d  The  are  v e r t i c a l energy f o r  expected to be m a n i f e s t e d i n ISEELS by  of the  are  -  BF  While the  3  [69,70,73,77,90] v e r y  - 85 -  l i t t l e work has been done on N F . 3  To date no ISEELS s p e c t r a have been  r e p o r t e d f o r e i t h e r the N or F Is r e g i o n s of N F . 3  There has,  however,  been l i m i t e d d i s c u s s i o n of these r e g i o n s i n e a r l i e r r e p o r t e d  photo-  a b s o r p t i o n s t u d i e s [120,121,125] o b t a i n e d a t lower r e s o l u t i o n than t h a t used  i n the present work.  Some apparent  inconsistencies i n this  work a r e i n v e s t i g a t e d i n the present more d e t a i l e d The  o n l y VSEELS spectrum  ISEELS measurements.  i s t h a t of u n p u b l i s h e d work r e f e r r e d t o i n the  book by Robin  [12], w h i l e the o n l y UV p h o t o a b s o r p t i o n  valence s h e l l  [126] does not extend  s p e c t r a of t h e  beyond 10 eV and i s f e a t u r e l e s s .  To f u r t h e r a i d i n the assignment of the VSEELS spectrum of the occupied l e v e l s a r e r e q u i r e d .  Only  the outermost  (below 21.2 eV) a r e a c c e s s i b l e w i t h H e ( I )  r a d i a t i o n and such s p e c t r a hae been r e p o r t e d f o r N F  3  by P o t t s e t a l .  [117] as w e l l as B a s s e t t and L l o y d [118] who have a l s o used radiation.  knowledge  The i o n i z a t i o n p o t e n t i a l s o f these  can be o b t a i n e d from p h o t o e l e c t r o n s p e c t r o s c o p y . valence s h e l l o r b i t a l s  earlier  He(II)  The v a l e n c e - s h e l l p h o t o e l e c t r o n s p e c t r a u t i l i z i n g  both Zr  (151.4 eV) and A l Ka (1486.58 eV) have a l s o r e c e n t l y been r e c o r d e d [119] and  these s p e c t r a p r o v i d e the i o n i z a t i o n p o t e n t i a l s f o r the i n n e r  valence e l e c t r o n s .  Some e a r l i e r measurements o f the Is core  i o n i z a t i o n p o t e n t i a l s of NF [127,128]. NF . 3  al.  3  electron  have been r e p o r t e d f o r both F and N  A v a r i e t y of MO c a l c u l a t i o n s have a l s o been r e p o r t e d f o r  The o n l y a b - i n i t i o c a l c u l a t i o n so f a r has been t h a t of Unland e t [129].  Other  l e s s s o p h i s t i c a t e d c a l c u l a t i o n s have been used  i n c l u d i n g CNDO [118,130,131], MNDO [132] and the Xa method  [133,134].  - 86 -  Experimental  1)  Details  EELS measurements The  i n n e r s h e l l s p e c t r a were recorded on the ISEELS  d e s c r i b e d i n the p r e v i o u s c h a p t e r .  An impact  and  scattering angle.  the s p e c t r a were sampled a t ~1°  energy  spectrometer  of 2.5  keV was  The N I s  spectrum  * was  c a l i b r a t e d a g a i n s t both the CO  f e a t u r e s (see T a b l e 2.4). used  spectrum energy  was  degrees.  was  The He(I)  (C I s •*• it ,v=0)  the F Is spectrum.  recorded on the new  of 3 keV  * and N 2  (N I s ^ it v=l.)  The newly e s t a b l i s h e d N Is v a l u e of N F 3  to i n t e r n a l l y c a l i b r a t e  used  The  valence  ISEELS spectrometer  [53].  was  shell An  impact  used w i t h the s c a t t e r e d e l e c t r o n s sampled at zero line  (21.218 eV  [104]) was  used  to c a l i b r a t e  the  spectrum. 2)  XPS  Measurements  The  Is core l e v e l s and  a s s o c i a t e d s a t e l l i t e s t r u c t u r e were  recorded u s i n g a McPherson ESCA 36 p h o t o e l e c t r o n spectrometer at  the U n i v e r s i t y of A l b e r t a .  The  s p e c t r a were o b t a i n e d by  situated  irradiating  the sample w i t h A l K a (hv = 1486.58 eV) X-rays which entered the sample cell was  through  an aluminum f o i l window of 0.0001" t h i c k n e s s .  fed i n t o the c e l l v i a s t a i n l e s s s t e e l feed l i n e s .  The  The  sample •  pressure  c o n t r o l l e d by a G r a n v i l l e - P h i l l i p s s e r i e s 203 v a r i a b l e l e a k v a l v e was  monitored  u s i n g a MKS  B a r a t r o n p r e s s u r e meter.  Typical  was and  pressures  - 87 -  used were between 150-200 microns. The major ( I s ) l i n e the gas and  f o r each r e g i o n was  calibrated  c a l i b r a n t being mixed on the low p r e s s u r e s i d e of the  system p r i o r to i n t r o d u c t i o n i n t o the sample c e l l . r e f e r e n c e l i n e was Sample and  The  between r e f e r e n c e and Is l i n e was  sample peaks due  to s l i g h t  c a l i b r a t e d a g a i n s t a mixture  KLL Auger l i n e  Repeated r a p i d c y c l e s  The F Is l i n e was  energy) were used  any  pressure d r i f t s .  the Ar 2s core  c a l i b r a t e d a g a i n s t the Ne KLL Auger l i n e and  the Ne  s p e c t r a are due  energy) s i d e of the Ne  the low k i n e t i c energy  Is peak was  Is spectrum  f e a t u r e between t h i s and l o s s processes  Is  the major Ne  of the major Is component. eV  3  at  ( i . e . , high binding  Since the f i r s t eV  "shake-up"  [17], any to  scattered photoelectrons.  would be w i t h i n 25 eV  l o s s f e a t u r e at 16.85  inelastic  Is component would be due  involving i n e l a s t i c a l l y 3  satellite  of Ne and NF  does not occur u n t i l 37.3  Energy l o s s c o n t r i b u t i o n s from NF  prominent energy  recorded.  of the to  s c a t t e r i n g of the o u t g o i n g p h o t o e l e c t r o n , a mixture run and  Ne  energy) [135].  s t r u c t u r e s i n the N Is and F Is XPS  equal p r e s s u r e s was  N  level [135].  order to t r y t o d i s t i n g u i s h which, i f any,  s p e c t r a , f i g . 3.5)  The  as the r e f e r e n c e v a l u e s  (870.31(2)eV-binding  f e a t u r e i n the Ne  possible error  of Ne and Ar from which the  (804.56(2)eV- k i n e t i c energy) and  (326.37(5)eV-binding  energy  leak valve.  r e f e r e n c e gases were of approximate equal p r e s s u r e s w i t h a  were run ( c a l i b r a n t - s a m p l e - c a l i b r a n t ) to minimize  In  inlet  p r e s s u r e of the  a l s o c o n t r o l l e d by a G r a n v i l l e P h i l l i p s  t o t a l p r e s s u r e somewhere between 150-200 microns.  level  separately with  (see VSEELS  There would a l s o be  [136], due  to the i n t e n s e  a  - 88 -  2p-*3s t r a n s i t i o n i n Ne.  The N F v a l e n c e f e a t u r e s w i l l of course be 3  present i n the N Is and F Is XPS s p e c t r a i f the c o r r e s p o n d i n g  energy  l o s s processes a r e o c c u r r i n g . R e s u l t s and D i s c u s s i o n NFj possess  symmetry and i t s e l e c t r o n c o n f i g u r a t i o n i s :  ( l a p 2 (le) 4  (2a )  F Is  (3e)  4  (5  3 l  )  2  Oa^2  2  L  N Is  4  (4a >  4  2  (Sa^  N 2s  2  ( 7 ) ° (6e)° a i  virtual  The  experimental and c a l c u l a t e d  2  x  F 2s  ( 4 e ) ( 5 e ) (la,,) 4  (2e)  valence  i o n i s a t i o n p o t e n t i a l s a r e presented i n  T a b l e 3.1. The o r d e r i n g of the occupied l e v e l s i s that o b t a i n e d the X^ c a l c u l a t i o n s  [133,134].  o r b i t a l s i s not c l e a r . o r b i t a l i s a t the lower  However, the o r d e r i n g of the v i r t u a l  A MNDO c a l c u l a t i o n suggests energy,  c a l c u l a t i o n s g i v e the unoccupied  from  t h a t the 7 a  1  whereas HAM/3 and Hartree-Fock [137] o r b i t a l s v i r t u a l l y the same energy  with  the e o r b i t a l being s t a b i l i z e d more r e l a t i v e to the a^ o r b i t a l upon c r e a t i o n of a N Is h o l e .  S i n c e a l l of these methods a r e not expected  - 89 -  Table 3.1  Experimental and c a l c u l a t e d  Orbital  Expt (a) (eV)  ionization potentials  f o r NF  C a l c u l a t e d IP (ev)  l p  Unoccupied Valence ••  •• •• •• ••  N 2s F 2s „  N Is F Is  (a)  6a j la 5e Ae 2  l 3e a  S 2e l l le 3  a  2  a  l  a  (DV)  ( b )  X (MS) a  ( c )  MNDO  HF< >  14.55 16.77* 16.61* 17.16 19.A3 21.17 27.86 A3.69 A9.71  14.74 18.23* 17.96* 19.63 22.63* 22.57* 30. A2 45.49 A9.45  d  CND0-M0^  CND0  (f}  -  V  5  X  a  13.73 16.15 16.55 17.52 19.71 21.14 26.A9 39.62 A3.06 A14.36 693.2A  13.82 15.91 16.08 17.19 19.45 21.30 25.87  13.97 15.45 15.63 16.71 18.74 20.62 25.89  13.95 16.94* 16.18* 17.84 21.14 22.47 27.47 41.76 A8.63  14.18 18.05* 16.34* 18.89 22.71* 22.42*  l  (a) Experimental values from r e f . 118, H e l , H e l l UPS ( v a l e n c e ) ; r e f . 119, Zr M XPS ( 4 8 ^ ; r e f . 119, A l Ka XPS (2e, 3 a j ) ; t h i s work (N Is and F I s ) see Table 3.3. (b) r e f . 133. (c) r e f . 134. (d) r e f . 129. (e) r e f . 131. ( f ) r e f . 118. * order d i f f e r e n t from l i s t e d . +  order i s u n c e r t a i n ; MNDO suggests o^* i s 7a^, and o^* i s 6e, but t h i s i s not considered c o n c l u s i v e — s e e d i s c u s s i o n  i n text.  - 90 -  to  g i v e a r e l i a b l e or meaningful  v a l u e to the unoccupied  c o n c l u s i o n about t h e i r o r d e r i n g w i l l designated to  as  a  and  1  o* •  t h i s d e s i g n a t i o n and  i n T a b l e s 3.1,  1.  3.2  and  A l l spectra w i l l  2  to the e x p e r i m e n t a l  and  and  3.3b.  v a l u e (414.2 eV)  The  be  be d i s c u s s e d w i t h r e s p e c t  IP's and  t r a n s i t i o n s shown  3.2  XPS  show the XPS  and F Is r e g i o n s r e s p e c t i v e l y . 3.3a  they w i l l  The  s p e c t r a o b t a i n e d f o r the N Is  r e s u l t s are summarized i n Tables  N Is IP (414.36 eV)  i s i n f a i r agreement w i t h  p r e v i o u s l y r e p o r t e d by F i n n et a l . [127].  work f o r F Is (693.24 eV) and  t h a t g i v e n by Davis  the present work c a l i b r a t i o n was  section).  I t should a l s o be noted  In  the peak i n previous  g i v e s a check on the l i n e a r i t y  the F Is e l e c t r o n produced by A l Ka X-rays  t  present  [128].  c a l i b r a t i o n v a l u e s (as o u t l i n e d i n the  T h i s c a l i b r a t i o n procedure  the spectrometer.t  (694.45 eV)  achieved by sandwiching  the  However,  there i s s e r i o u s disagreement between the v a l u e r e p o r t e d i n the  q u e s t i o n between the two  no  3.3.  Inner S h e l l I o n i z a t i o n by F i g u r e s 3.1  be drawn and  levels,  of  t h a t the k i n e t i c energy of i s 793.34 eV which i s o n l y  The k i n e t i c energy of the p h o t o e l e c t r o n i s determined by K.E. = kAV+C where AV i s the p o t e n t i a l d i f f e r e n c e between the a n a l y s e r p l a t e s , k i s an e x p e r i m e n t a l l y determined machine constant and C accounts f o r contact p o t e n t i a l s . Use of two c a l i b r a t i o n v a l u e s allows an e x p e r i m e n t a l d e t e r m i n a t i o n of k f o r each run.  Table  Transitions  3.2  f o r C_  Symmetry  Dipole Transition  a  a  l  ~  l  a  2  l  a  are  allowed  A  State  E  ~  A  a  2  from ground s t a t e  yes  no  2  E  •«-> e  A  x  + A  Allowed*  yes  l  «-* e  e <-> e  * Transitions  Final  yes  2  + E  (A^) to f i n a l  yes  s t a t e s of A^ and E symmetry  - 92  -  RELATIVE E N E R G Y (eV) -10  10  0  20  30  40  50  60  0 410  420  430  440  450  460  470  BINDING E N E R G Y (eV)  F i g u r e 3.1: The X-ray p h o t o e l e c t r o n spectrum f o r the n i t r o g e n Is l e v e l and a s s o c i a t e d s a t e l l i t e s t r u c t u r e of NF-j obtained w i t h AA K o j (1486.58 eV) radiation. The f e a t u r e s have been f i t t e d u s i n g a g a u s s i a n ' l i n e shape. The shaded p a r t s are the c o n t r i b u t i o n s from h i g h e r X-ray (Kcu and Ko^) components. A l s o shown i s the spectrum obtained of the low ( k i n e t i c ) energy s i d e of the neon Is spectrum from a mixture of Ne and NF-j (see t e x t f o r details). The p o s i t i o n of the prominent VSEELS t r a n s i t i o n s are a l s o i n d i c a t e d f o r Ne and N F 3 . 2  -  93  -  RELATIVE 10  0  20  E N E R G Y (eV) 30  40  50  60  i— —i— —i— —i— —i—•—i—"—i— —r 1  50  1  700  1  710  1  720  1  730  740  750  7€  BINDING E N E R G Y (eV)  F i g u r e 3.2: The X-ray p h o t o e l e c t r o n spectrum f o r the f l u o r i n e Is l e v e l and a s s o c i a t e d s a t e l l i t e s t r u c t u r e of N F obtained w i t h AJl Ka^ (1486.58 eV) radiation. The f e a t u r e s have been f i t t e d u s i n g a g a u s s i a n ' l i n e shape. The shaded p a r t s are the c o n t r i b u t i o n s from h i g h e r X-ray (Kaq and Ka^) components. A l s o shown i s the spectrum obtained of the low ( k i n e t i c ) energy s i d e of the neon Is spectrum from a mixture of Ne and NF-j (see t e x t f o r details). The p o s i t i o n of the prominent VSEELS t r a n s i t i o n s a r e a l s o i n d i c a t e d f o r Ne and N F 3 . 3  2  -  94 -  Table 3.3(a) Peak Energies i n the N Is XPS Spectrum of NF Energy (eV)  Feature  Literature X  414.2  Assignment  D i f f e r e n c e energy (eV) from main l i n e  414.36  0  A  421.47  7.11  B  427.13  12.77  Sh  430.39  ~16.03  C  433.26  18.90  D  437.20  22.66  a  445-475  BAND E a  This work  3  Is hole  shake up & shake o f f  Ref. 127. Table 3.3(b) Peak Energies i n the F Is XPS Spectrum of NF  Feature  Energy (eV) Literature  This work  D i f f e r e n c e energy (eV)  Assignment  from main l i n e  a  693.24  0  P  698.45  5.21  Q  702.64  9.40  R  709.77  16.53  Y  3  694.45  S  -719.7  -24.5  T  -722.3  -29.1  U  -737.2  -44.0  Ref. 128.  Is hole  -  11.22  95  -  eV below the Ne KLL Auger peak used f o r c a l i b r a t i o n .  p r e s e n t v a l u e i s thought  Thus the  correct.  Both the N Is and F Is r e g i o n s e x h i b i t r i c h s a t e l l i t e s t r u c t u r e extending  to b i n d i n g e n e r g i e s at l e a s t 60 eV above ( i . e . , at  k i n e t i c e n e r g i e s than) the major Is component.  The  lower  peaks are broad  t h i s i n p a r t r e f l e c t s the use of unmonochromated A l Ka X-rays the Ne  Is under the c o n d i t i o n s used here was  c o n t a i n s a v a r i e t y of lower ( u n r e s o l v e d ) components. (mainly K a procedure  3  and Ka^)  ~ 1 eV).  f o r i n the  t h a t used a m o d i f i e d v e r s i o n of SUNDERE [138].  higher binding energies and  3.2  i n t e n s i t y , unresolved  3  mixture.  and  The K a 3.2.  3  and  Only  The  Is peak o b t a i n e d form the  f e a t u r e s are i n d i c a t i v e of i n e l a s t i c  N, and F Is r e g i o n s s i n c e the impact  energies. energy-loss  and The  to  A l s o shown i n each of  by N F  3  scattering and Ne.  It  assumed t h a t the energy l o s s f e a t u r e s would be q u i t e s i m i l a r i n the  1072,  the  (uppermost t r a c e ) i s the spectrum of the h i g h b i n d i n g  ( e n e r g y - l o s s ) of the e m i t t e d p h o t o e l e c t r o n caused  (616,  ^  fitting  s t r u c t u r e extending  (lower k i n e t i c e n e r g i e s ) .  energy (low k i n e t i c energy) s i d e of the Ne Ne/NF  source  s t r u c t u r e has been f i t t e d w i t h no attempt made  f i t the e x t e n s i v e lower  F i g s . 3.1  T h i s X-ray  C o n t r i b u t i o n s from the h i g h e r X-ray components  Ka^ X-ray c o n t r i b u t i o n s are shown shaded i n F i g s . 3.1  to  (FWHM of  i n t e n s i t y l i n e s as w e l l as the main Ka^  have been l a r g e l y accounted  most prominent s a t e l l i t e  and  e n e r g i e s of the  excitation  experiment should g i v e an estimate of the extent spectra.  components i n the (low r e s o l u t i o n ) Ne/NF  3  The  Ne,  photoelectrons  793 eV r e s p e c t i v e l y ) are a l l w e l l above the  c o n t r i b u t i o n s to the XPS  was  major energy  spectrum i n the Ne  of loss  Is r e g i o n  were found to be at 13.4  eV and  - 96  -  16.8  eV  (see F i g s . 3.1  which of course are the same spectrum). to energy l o s s c o n t r i b u t i o n s from NF  The  and 3.2,  inserts,  former peak i s s o l e l y  and agrees w e l l w i t h f e a t u r e s d i -  3  r e c t l y observed i n the h i g h r e s o l u t i o n v a l e n c e s h e l l spectrum of o b t a i n e d on the EELS spectrometer (see peaks 5-6 F i g . 3.5).  A second NF  3  NF  3  (12.81-13.75 eV) o f  v a l e n c e f e a t u r e of approximately e q u a l i n t e n -  s i t y would be expected a t ~ 16.2 3.5).  due  eV ( c o r r e s p o n d i n g to peak 8 of F i g .  However, what i s observed i s a peak at 16.8  double the expected i n t e n s i t y from NF ,  alone.  equal contributions a r i s i n g  3  3  from both NF  eV of a p p r o x i m a t e l y  This i s attributed  and Ne energy l o s s  (the Ne 2p->3s energy l o s s f e a t u r e o c c u r s at 16.85  eV  [136].  features Structure  at ~ 16.0 eV appears i n both the N Is ( s h o u l d e r (sh) i n F i g . 3.1) the F Is (peak R i n F i g . 3.2)  XPS  spectra.  eV i s e s s e n t i a l l y absent i n the F Is XPS  to  and  The energy l o s s peak at  13.4  spectrum, w h i l e some c o n t r i b u -  t i o n i s p r o b a b l y o c c u r r i n g i n the N Is spectrum.  However, a  of the r e l a t i v e i n t e n s i t i e s of the s p e c t r a i n each of f i g u r e s  comparison 1 and  2  leads to the c o n c l u s i o n that a l a r g e percentage of the XPS  "satellite  s t r u c t u r e " i s due  inelastic  to "shake-up" p r o c e s s e s r a t h e r than from  scattering. I t can be seen that the s a t e l l i t e r e g i o n s are q u i t e d i f f e r e n t . above the major Is component.  s t r u c t u r e s i n the N Is and F Is  The N Is r e g i o n has a peak A a t 7.11  T h i s i s f o l l o w e d by a peak B at 12.77  above the main l i n e .  This along with a p a r t i a l l y  at  to be a t l e a s t p a r t i a l l y due  16.0  eV i s thought  features.  eV eV  r e s o l v e d s h o u l d e r (sh) to the e n e r g y - l o s s  However the r e l a t i v e i n t e n s i t y of the 12.77  eV  feature  -  97 -  suggests t h a t some c o n t r i b u t i o n from genuine s a t e l l i t e present. lie  The most i n t e n s e f e a t u r e s  i n the  18-25  bands extending The  eV r e g i o n . out  (C,D)  s t r u c t u r e may be  i n the N Is s a t e l l i t e  T h i s i s then f o l l o w e d by l a r g e l y  spectra  unresolved  t o ~ 60 eV b i n d i n g energy above the main Is l i n e .  F Is r e g i o n shows a doublet  the major Is component.  (P,Q)  i n the r e g i o n 5-10 eV above  T h i s i s f o l l o w e d by a l a r g e peak (R) a t 16.6 eV  which c o i n c i d e s w i t h the second e n e r g y - l o s s  feature.  The e s s e n t i a l  absence i n the XPS spectrum o f any i n t e n s e f e a t u r e a t ~ 13 eV (the lower energy-loss  f e a t u r e ) l e a d s t o the c o n c l u s i o n t h a t the peak a t 16.6  l a r g e l y due t o "shake-up" r a t h e r than e n e r g y - l o s s k i n e t i c energies  processes.  t h e r e f o l l o w s a broad r e g i o n o f l a r g e l y  s t r u c t u r e s t r e t c h i n g out  At higher  unresolved  t o ~ 65 eV above the main Is l i n e .  s t r u c t u r e s a t S, T, and U are a l s o apparent. the XPS s p e c t r a w i l l be f u r t h e r c o n s i d e r e d  eV i s  Other  The v a r i o u s f e a t u r e s o f  i n the i n t e r p r e t a t i o n o f the  e l e c t r o n energy l o s s s p e c t r a ( i e : the ISEELS and VSEELS measurements) i n the f o l l o w i n g s e c t i o n s .  Inner s h e l l e x c i t a t i o n by ISEELS The NF  3  3.3b  e l e c t r o n energy-loss  s p e c t r a f o r the N Is and F Is r e g i o n s o f  are shown r e s p e c t i v e l y i n f i g u r e s 3.3 and 3.4. show d e t a i l s of major N Is t r a n s i t i o n s recorded  0.14-0.28 eV FWHM w h i l e F i g . 3.3c r e s o l u t i o n s o f 0.36 eV FWHM.  Figures  3.3a and  a t r e s o l u t i o n s of  shows a long range scan recorded a t  I n a s i m i l a r way F i g . 3.4a shows the  range spectrum o f the F Is spectrum and s p e c t r a i n F i g . 3.4 were recorded  F i g . 3.4b the d e t a i l .  Both  a t a r e s o l u t i o n of 0.36 eV FWHM.  long  - 98 -  a)  i  rrr 234  A  NF,  b)  N K - S H E L L  AE=028eV ! 1  I  I  tz  2  3  4  i  CO  -z. LU  N  LU >  2  IS-TT*  I  < _l LJ  11  CC  NF N "AE=0.28eV +  3  N IS-7T* J Impurity i 2  'I  f  2  ,NF AE=O.I4eV  /  3  404  4 0 2 4 0 6 410 414 4 0 0  408  412  C) I  NF,  23 4  10-  N K-SHELL Sh C  B  NK-EDGE  XPS  fc to UJ  AE = 0.36eV  I-  LU  N  i  v.  LU N IS—7T* I Impurity / 2  —i 400  >  1 410  '  1 420  •  i 430  •  i 440  •  r 450  ENERGY LOSS (eV)  F i g u r e 3.3: N i t r o g e n Is e l e c t r o n energy l o s s s p e c t r a of NF-j. 3.3b) a l s o shows the spectrum o f a ^/NF-j m i x t u r e . The p o s i t i o n s of the n i t r o g e n K-edge and t h e XPS s t r u c t u r e i n F i g u r e 3.3c) a r e taken from F i g u r e 3.1 and Table 3.3(a).  -  lo-r  a)  99  -  A  NF 3 FK-SHELL  \  AE = 0.36 eV 5  4  >-  I  FK-EDGE  700  680  CO  740  720  —I  b)  3  2  ^10-  A  /  XPS  FK-EDGE  r  760  P  R  Q  LU  cr  5  H  685  695  705  715  E N E R G Y L O S S (eV)  F i g u r e 3.4: F l u o r i n e Is e l e c t r o n energy l o s s s p e c t r a of NF^. The p o s i t i o n s o f the f l u o r i n e K-edge and the XPS s a t e l l i t e s t r u c t u r e a r e taken from F i g u r e 3.2 and Table 3 ( b ) .  - 100  The  -  e n e r g i e s of the s p e c t r a l f e a t u r e s and p o s s i b l e assignments f o r the  s p e c t r a shown i n F i g s . 3.3  and  3.4  are g i v e n i n Tables 3.4  and  3.5  respectively. Both e x c i t a t i o n s p e c t r a have been s t u d i e d e a r l i e r by et a l [125] u s i n g s o f t X-ray  photoabsorption.  While  the present  work i s i n g e n e r a l l y good agreement w i t h the F Is spectrum the p h o t o a b s o r p t i o n work [125] case of the N Is spectrum. a prominent unassigned  r e p o r t e d here.  and  i s thus almost  The  of N  2  and NF  should be noted  3  ( I s •> i t ) t r a n s i t i o n  r e g a r d i t should be noted  absence of N ,  which has  2  12.93  eV  [103].  N Is spectrum  to an N  N  2  impurity.  2  have o b t a i n e d f o r a (upper  trace).  It  t r a c e of  N  2  however the magnitude of the  [4] i s at most v e r y s m a l l .  t h a t there i s no sharp  the v a l e n c e s h e l l spectrum  absent  of t h i s peak  t h a t there does appear to be a v e r y s l i g h t 3  2  we  which i s shown i n f i g u r e 3.3b  i m p u r i t y i n a l l the N Is s p e c t r a of NF ; dominant N  energy  c e r t a i n l y due  T h i s view i s confirmed by the ISEELS spectrum mixture  et a l . [125] r e p o r t  i t * t r a n s i t i o n i n molecular  c l o s e l y to t h a t of the N Is  ( [ 4 ] and T a b l e 2.4)  reported i n  f e a t u r e at 400.9 eV which i s e s s e n t i a l l y 3  ISEELS  there i s a s e r i o u s d i s c r e p a n c y i n the  In p a r t i c u l a r Vinogradov  i n the ISEELS s p e c t r a of NF corresponds  Vinogradov  ( F i g . 3.5), and  In  this  f e a t u r e at 12.93  eV i n  t h i s i s i n d i c a t i v e of the  i t s most i n t e n s e v a l e n c e s h e l l t r a n s i t i o n a t  Thus i t i s r e a s o n a b l e to conclude  i s e f f e c t i v e l y f r e e from any  t h a t the p r e s e n t  c o n t r i b u t i o n s above 402  NF  3  eV  a r i s i n g from n i t r o g e n i m p u r i t i e s . The  N Is ISEELS spectrum  i n t e n s e Is •*• a  of NF  3  t r a n s i t i o n at 407.10 eV  ( F i g u r e 3.3) (peak 1).  i s dominated by  T h i s can be  inter-  an  - 101  -  Table 3.4  E n e r g i e s , term values and p o s s i b l e assignments i n the N K - s h e l l energy l o s s spectrum of NF,  Feature  Energy Loss (eV) Term Value  P o s s i b l e Assignment  Photoabsorption(eV) 400.9  1  407.10  2  411.02  3  411.99  4  413.24  (c)  7.26  Is + 0*  3.34  Is •+ 3s  2.37  Is -* 3p  411.9  1.12  Is > 4p  413.0  0  Is +  v  '  ( b )  406.6  413.4 K-edge 5  414.36  (d)  414.1  425  (a) Ref. 125. (b) This feature was not explained.  I t i s thought to be due to impurity N  2  ( I s -* it*).  See t e x t . (c) This f e a t u r e c a l i b r a t e d against CO ( C i s -* it*) 401.10 eV. (d) XPS t h i s work, see Table 3.3.  287.40 eV and N  2  ( N l s •* it*)  -  102  -  Table 3.5  E n e r g i e s , term values and p o s s i b l e assignments i n the F K - s h e l l spectrum of NF  Feature  1 K-edge  Energy Loss(eV)  Term Value  3  e l e c t r o n energy l o s s  P o s s i b l e Assignment  687.42  (b)  5.82  Is  693.24  (c)  0  Is •+ »  2  697  3  -709  (o*)  Photoabsorption(eV)  686.4  (a) Ref. 125. (b) I n t e r n a l l y c a l i b r a t e d against feature 3 of NF (c) XPS - see Table 3.3.  3  (N I s ) energy l o s s spectrum.  (a)  -  preted  103  -  as an enhanced i n n e r - w e l l f i n a l  p o t e n t i a l b a r r i e r created  by  On  s t a t e trapped by  the h i g h l y e l e c t r o n e g a t i v e  p o t e n t i a l b a r r i e r model concept has [73,77].  (valence)  been d i s c u s s e d  the  F ligands.  This  by Dehmer et a l .  the b a s i s of t h i s model the p r o b a b i l i t y of t r a n s i t i o n s *  to unoccupied  a  type v a l e n c e o r b i t a l s  (Inner-well)  the expense of e x c i t a t i o n s to Rydberg o r b i t a l s with t h i s view the and  (outer-well).  spectrum does show some Rydberg s t r u c u r e  4) of much lower r e l a t i v e i n t e n s i t y l e a d i n g up  Significant, The  would be enhanced a t  f o r the S 2p  spectrum of S F  [69]  6  i n n e r w e l l v a l e n c e e x c i t a t i o n s and  3  to the K-edge. continuum.  s i m i l a r to that observed  which was  accord  (peaks 2,  lower i n t e n s i t y , s t r u c t u r e a l s o appears i n the  pre-edge spectrum i s q u a l i t a t i v e l y  In  a l s o a t t r i b u t e d to  earlier intense  weak outer w e l l Rydberg s t r u c t u r e .  I t i s of i n t e r e s t to compare the N Is energy l o s s spectrum of NF  3  w i t h that of the  8.4).  The  i s o e l e c t r o n i c molecule N ( C H ) 3  s p e c t r a are very  d i f f e r e n t and  3  (see F i g s . 8.3  t h i s i s a t t r i b u t a b l e to  behaviour of the e l e c t r o n e g a t i v e F l i g a n d as compared to the donating CH  3  ligand.  T h i s aspect  i s discussed  i n the N Is ISEELS spectrum of NF , 3  i n Chapter 8.  and the  electron Finally,  a broad maximum i s observed i n  the  continuum at '>425 eV. The Is -»• a  F Is ISEELS spectrum of NF  band l o c a t e d at 687.42 eV.  s t r u c t u r e , but  t h i s may  be masked by  3  ( F i g . 3.4)  a l s o shows a  There appears to be no  strong  Rydberg  the expected l a r g e n a t u r a l width of  * the  a  band and  i t s proximity  Is energy l o s s spectrum, there  to the F Is edge.  As  i n the case of the  e x i s t s d e f i n i t e s t r u c t u r e i n the c o n t i -  N  - 104 -  nuum w i t h maxima (peaks 2 and 3) a t ~ 697 and ~ 709 eV r e s p e c t i v e l y . However, i n the case of the F Is spectrum t h i s s t r u c t u r e i s on a r e l a t i v e l y more i n t e n s e background compared to the s i t u a t i o n i n the N Is region.  T h i s i s t o be expected,  however, s i n c e the vacancy i s on an F  atom which i s on the p e r i p h e r y of the m o l e c u l e .  The e l e c t r o n s o r i g i n a -  t i n g from here would l i k e l y have much l e s s of a p o t e n t i a l b a r r i e r t o overcome, and thus would have s i g n i f i c a n t p r o b a b i l i t y of going t o o u t e r w e l l s t a t e s as w e l l . of  Thus there would l i k e l y  be l e s s enhancement  * the a band r e l a t i v e t o the continuum.  * The term v a l u e f o r the N Is •+• a t r a n s i t i o n i s 7.26 eV, whereas t h a t f o r the F Is -»• a t r a n s i t i o n i s 5.82 eV ( T a b l e s 3.4 and 3.5). Thus i t would appear t h a t the term v a l u e s are not t r a n s f e r a b l e between the two  core-hole centres.  T h i s r a i s e s two q u e s t i o n s ; (a) what i s the make  * up of the a  envelope  i n each case, and (b) what i s the e f f e c t of the  c o r e - h o l e being on the p e r i p h e r y of the molecule the  as opposed to b e i n g a t  centre?  * In N F  3  the unoccupied  a  o r b i t a l s a r e of a^ and e symmetry, and  s i n c e the N Is and F Is o r b i t a l s t r a n s f o r m as a^ and a ^ e r e s p e c t i v e l y , t r a n s i t i o n s t o both Table 3.2). unresolved expected,  a  l e v e l s a r e d i p o l e allowed  Both peaks a r e broad components.  (see  and c o u l d be composed o f v a r i o u s  In an attempt t o see what c o n t r i b u t i o n s might be  MO c a l c u l a t i o n s u s i n g HAM/3 [139] were performed.  i s p r i m a r i l y parametrised rily  from each c e n t r e  f o r u- systems i t i s not expected  g i v e good r e s u l t s f o r a type systems.  Since HAM/3 to necessa-  However, whereas the e i g e n -  v a l u e s o b t a i n e d might n o t be s a t i s f a c t o r y , the e i g e n v e c t o r s should g i v e  - 105  a r e a s o n a b l e i n d i c a t i o n [140] Calculations  of the make up  were performed on  e l e c t r o n v a c a n c i e s and  -  of the v a l e n c e o r b i t a l s .  the molecule with r e s p e c t i v e  w i t h h a l f an e l e c t r o n  core  ( T r a n s i t i o n State  [141]) d i f f u s e l y added to the v i r t u a l o r b i t a l s .  The  Formalism  e i g e n v e c t o r s of  the  v i r t u a l o r b i t a l s i n the molecule with an N Is h o l e i n d i c a t e the c o n t r i bution  from the N atom to the  (implying  a strong  6e o r b i t a l to be predominantly 2 p  s •*• p d i p o l e allowed t r a n s i t i o n ) w h i l e the  t a l i s comprised of a p p r o x i m a t e l y equal 2s and  basis  the  On  6e o r b i t a l s but  from C„ 3v  to C  s  since  tively.  The  the a" o r b i t a l s , l e a v i n g remaining a* reflect  the  contributions from  l o c a l i s e d on  a^ o r b i t a l s become a',  the one  F  a",  a'  and  e i g e n v e c t o r s of these o r b i t a l s f o r the F atom with  vacancy show v i r t u a l l y no  p orbital contribution the  only  this  the  symmetry of the molecule i s reduced  the vacancy w i l l be  In t h i s case the v i r t u a l e and  2p^  orbi-  On  with a larger c o n t r i b u t i o n  c r e a t i n g an F Is h o l e the  7a^  character.  * a envelope i n the N Is spectrum would have  from both 7a ^ and 6e.  2p^  and  x  to one  s •*• p c o n t r i b u t i o n  centre. respecthe  of the a'  coming from  orbital.  Thus the d i f f e r e n c e  i n term v a l u e s may  difference  i n the c o n t r i b u t i o n  from the v a r i o u s  and the  simply possible  t r a n s i t i o n s to the broad envelope. As w e l l as p o s s i b l e electrons electron  e f f e c t s on  symmetry the  removal of  core  ( i . e . , N Is or F Is) w i l l determine the p o t e n t i a l i n which i n the newly occupied o r b i t a l f i n d s i t s e l f .  the p o t e n t i a l b a r r i e r model are  the  o  expected to be mainly w i t h i n  w e l l are hence l o c a l i s e d around the N Is e l e c t r o n i n c r e a s e s  The  ( c e n t r a l ) N atom.  core charge by one  and  The  so the  orbitals in the  inner  removal of a  the  orbital  an  - 106  energies w i l l The  effect  be determined  -  by a Z + 1 ( i . e . , an 0 atom) c e n t r a l  core.  of c r e a t i n g a p e r i p h e r a l h o l e would not have as much e f f e c t  s i n c e the v i r t u a l o r b i t a l s are l o c a l i s e d around the c e n t r a l atom.  Hence  an e l e c t r o n i n a v i r t u a l o r b i t a l would be harder to remove ( i . e . , i t would have an i n c r e a s e d b i n d i n g energy c e n t r a l atom vacancy  (term v a l u e ) ) when t h e r e i s a  as opposed to a l i g a n d vacancy.  e a r l i e r noted i n ISEELS core s p e c t r a S F  6  This effect  [69], i n which the  o r b i t a l s are w e l l s e p a r a t e d , thereby removing any ambiguity to  the f i n a l  states.  w i t h a S 2p h o l e and  The  was  virtual w i t h regard  d i f f e r e n c e i n term v a l u e s between the s p e c i e s  t h a t w i t h an F Is hole was  ~ 1.8  eV.  Therefore  no  d e f i n i t e c o n c l u s i o n s can be drawn about the r e l a t i v e s e p a r a t i o n of the 7a^ and eV  6e o r b i t a l s i n . N F  (i.e.,  3  except  the d i f f e r e n c e i n the two  The  N Is spectrum  1.12  eV r e s p e c t i v e l y .  term v a l u e s ) of one  c l e a r l y shows sharp Rydberg  F e a t u r e s 3 and 4 (411.99 eV and and  t h a t they are probably w i t h i n another.  structure.  413.24 eV) have term v a l u e s of 2.37  T h i s i s i n e x c e l l e n t agreement w i t h  c a l c u l a t e d term v a l u e s (2.36  eV and  1.18  for a p series.  4p l e v e l s .  Feature  a s s i g n e d as the N Is -»• 3s ( a ^ Rydberg t r a n s i t i o n . 3.34  4  0.6)  A c c o r d i n g l y f e a t u r e s 3 and 4 have been  a s s i g n e d as t r a n s i t i o n s to the 3p and  of  =  eV  the  eV) o b t a i n e d f o r n = 3 and  r e s p e c t i v e l y when u s i n g the approximate quantum d e f e c t (6 expected  1-1.5  2 has been  I t has a term  eV which g i v e s an estimated quantum d e f e c t of 0.98,  w i t h the expected magnitude of the quantum d e f e c t (6  value  i n agreement  = 1.0) a s s o c i a t e d  w i t h an s s e r i e s . The  p o s i t i o n s of the most dominant s a t e l l i t e  s t r u c t u r e s from  the  -  XPS data  107  -  ( F i g u r e s 3.1 and 3.2) have a l s o been i n d i c a t e d on the ISEELS  s p e c t r a ( F i g u r e 3.3, A-D and f i g u r e 3.4, P-S). F e a t u r e s a t t r i b u t a b l e t o i o n i z a t i o n and e x c i t a t i o n  ("shake-up") i n XPS should appear as new  c o n t i n u a beyond the i o n i z a t i o n edge i n ISEELS.  I t should be noted  that  (a) the v a l u e s r e p o r t e d from the XPS data are the v e r t i c a l p o s i t i o n s of the broad  envelopes  the onset  (ie:  whereas the new c o n t i n u a i n ISEELS w i l l appear  adiabatic);  from  (b) the r e s o l u t i o n obtained i n the ISEELS  s p e c t r a i s about 3 times b e t t e r than that of the XPS d a t a , thus more onsets might be apparent  i n ISEELS; ( c ) no d e f i n i t e i n f e r e n c e can be  drawn from comparison of the observed  c r o s s - s e c t i o n i n the one p r o c e s s  compared w i t h the other s i n c e XPS d e a l s with a p h o t o e l e c t r o n w i t h k i n e t i c energy will reflect  s e v e r a l hundreds of eV above the t h r e s h o l d whereas ISEELS  threshold behaviour.  t i o n s i t should be noted ISEELS s p e c t r a suggests  Even with the f o r e g o i n g c o n s i d e r a -  t h a t a comparison of the data from XPS and t h a t much of the ISEELS continuum s t r u c t u r e can  l i k e l y be i n t e r p r e t e d as being due to the onset of "shake-up" c o n t i n u a (ie:  I s i o n i z a t i o n and v a l e n c e s h e l l e x c i t a t i o n ) .  Possible general  forms of such c o n t i n u a , s t a r t i n g a t apparent  d i s c o n t i n u i t i e s are i n d i -  c a t e d by dashed l i n e s on F i g s . 3.3 and 3.4.  Between the N Is edge and  p o s i t i o n A i n F i g u r e 3.3 there i s an i n d i c a t i o n of c o n s i d e r a b l e complex structure.  T h i s can be a t t r i b u t e d  (simultaneous  to v a r i o u s e l e c t r o n  Is and v a l e n c e s h e l l e x c i t a t i o n ) .  t u r e i s a l s o present i n the F I s spectrum.  excitations  S i m i l a r type of s t r u c -  I n the N I s ISEELS  spectrum  ( F i g . 3.3) there i s a broad maximum i n the continuum ( f e a t u r e 5) a t ~ 425  eV.  S i n c e no continuum resonances  are expected  for NF  3  this  struc-  -  108  -  t u r e i s presumably due to i o n i z a t i o n p l u s e x c i t a t i o n on top of the d i r e c t i o n i z a t i o n continuum. tinuum due to "shake-up" f e a t u r e 3 ( F i g . 3.4)  T h i s f e a t u r e i s a s s o c i a t e d w i t h a con-  A i n the XPS  is likely  spectrum ( F i g . 3.1).  the apparent maximum of a  Similarly,  "shake-up"  continuum c o r r e s p o n d i n g to the broad peak R (see F i g . 3.2).  Valence S h e l l E x c i t a t i o n by VSEELS The v a l e n c e s h e l l e l e c t r o n e n e r g y - l o s s spectrum f o r NF i n F i g u r e 3.5 and summarized, 3.6.  3  i s shown  a l o n g w i t h t e n t a t i v e assignments, i n t a b l e  The spectrum i s q u a l i t a t i v e l y  s i m i l a r to t h a t r e p o r t e d by Robin  [12], however there a r e some v a r i a t i o n s i n r e l a t i v e i n t e n s i t y .  For  i n s t a n c e f e a t u r e s 7 and 8 are much l e s s i n t e n s e than i n the spectrum r e p o r t e d i n the p r e s e n t work. different  T h i s d i f f e r e n c e i s c o n s i s t e n t w i t h the  impact e n e r g i e s used.  The spectrum shown by Robin  [12] was  e x c i t e d by 100 eV e l e c t r o n s whereas the impact energy used here was eV.  Both s p e c t r a were o b t a i n e d at 0° s c a t t e r i n g a n g l e .  The  3000  spectrum  shown by Robin a l s o has an u n f o r t u n a t e break i n the data j u s t where f e a t u r e 9 appears i n f i g u r e Robin  3.5.  [12] has made a l i m i t e d assignment i n which most  t r a n s i t i o n s are a t t r i b u t e d to Rydberg  final  levels.  However i t would  expected t h a t the v a l e n c e o r b i t a l s would have a s t r o n g e r  interaction  w i t h the v i r t u a l v a l e n c e o r b i t a l s than w i t h the more d i f f u s e l e v e l s e s p e c i a l l y g i v e n the p o s s i b l e e x i s t e n c e of a p o t e n t i a l which may  also affect  the v a l e n c e s h e l l spectrum.  Rydberg barrier  In t h i s r e g a r d the  broadness of some of the observed bands e.g., 1, 2, and 8) i s more  be  - 109 -  3s  RYDBERG  3s  3  \ P  i  3p  "T 3p  4e  NF, VALENCE  rrrf  P J? 6a,  A E = 0.035 eV  la,  3  -  n  m y, 5 e  —I 3s  r  3s  .I *\  rrrT/  J 3 P  3s  5  • ,1 I  i  -6a, • 3  2  r  , Q  45  I  i  10  1 — 5e  3e  •3e  —I  2  6  1  P  4e 5a, 8  7  I I I  1  «  rn  n 3  a  I I  r  15  1  1  9  10  I I  1  r  ~i 20  1  1  r 25  E N E R G Y L O S S (eV)  F i g u r e 3.5: NF-j v a l e n c e s h e l l e l e c t r o n energy l o s s spectrum. The top m a n i f o l d shows the p o s i t i o n s of Rydberg s e r i e s e s t i m a t e d from term v a l u e s and the Rydberg formula. The i o n i z a t i o n l i m i t s a r e taken from p h o t o e l e c t r o n s p e c t r o s c o p y . The bottom m a n i f o l d shows the e s t i m a t e d p o s i t i o n s f o r v a l e n c e - v a l e n c e t r a n s i t i o n s (see t e x t f o r d e t a i l s ) .  Table  P o s s i b l e Assignments  3.6  i n the Valence S h e l l  Rydberg T r a n s i t i o n s Major Feature  Observed Energy (eV)  Valence-Valence Assignment  1 2  8.64 9.45  3  11.21  4  12.37  5  12.81  6 7  13.75 15.01  8  16.20  3e 3e  9  18.12  4a ^ ->• o^*  10  -19  la~ -> 7a, , i s d i p o l e  6a^  6a j  Spectrum  of NF  (eV)  la2  5e  4e  5  a  3e  l  Oj*  a2*  6a ^  -*•  Ia2 5e la2 5e 4e 4e  • Oj* > Oj* + o2* * 02* > o^* -> © 2 *  4.  5aj > Oj* 5a j + o 2 *  10.39(3s) 11.36(3p)  12.61(4p) 13.03(5p) 13.26(6p)  13.78(3p) 15.03(4p) 15.45(5p) 15.68(6p)  + Oj* + ©2*  13.21(3s) 14.18(3p) 15.43(4p) 15.85(5p) 16.08(6p)  14.13(3s) 15.15(3p)  16.40(4p) 16.85(5p) 17.05(6p)  4a j + © 2 *  forbidden.  Since  It i s not known which of  a * or o * i s  16.37(3s) 17.34(3p)  17.80(3s)  18.59(4p) 19.01(5p) 19.24(6p)  18.77(3p)  the 7a , both are  20.02(4p) 20.44(5p) 20.67(6p) given.  -  Ill  -  s u g g e s t i v e of v a l e n c e - v a l e n c e r a t h e r than Rydberg t r a n s i t i o n s . reason the spectrum  spectrum  (6a±);  ( i i )transitions  from the F l o n e - p a i r s (la » 5e and  4e) and  predominantly  (5a^, 3e).  2  (see Table 3.1)  bonding  orbitals  arising  t r a n s i t i o n s a r i s i n g from Since a l l the MO  g i v e the 6a ^ as the h i g h e s t occupied m o l e c u l a r  the most l i k e l y assignment of f e a t u r e s 1 and 0*2  superimposed.  can be d i v i d e d i n t o three s e c t i o n s ; ( i ) t r a n s i t i o n s  a r i s i n g from the N l o n e - p a i r o r b i t a l  N-F  this  has been a s s i g n e d as being composed of  v a l e n c e - v a l e n c e t r a n s i t i o n s w i t h some Rydberg t r a n s i t i o n s The  For  2 i s to the 6a  1  the  schemes orbital  -*• o"^  and  v a l e n c e - v a l e n c e t r a n s i t i o n s s i n c e t r a n s i t i o n s to both the 6e and  o r b i t a l s are a l l o w e d .  I t should be noted  f e a t u r e s 1 axtd 2 i s broad.  t h a t the band  eV)  In order to p r e d i c t  However, u s i n g  from the ISEELS data d i s c u s s e d above, the  p r e d i c t e d p o s i t i o n of t h i s f e a t u r e O s a ^ ) r e g i o n between peaks 2 and  comprising  A f u r t h e r c o n t r i b u t i o n to the width c o u l d  p o s s i b l y come from the 6aj^ •*• 3 s ( a ^ ) Rydberg t r a n s i t i o n . the 3s term v a l u e (3.34  would be a t 10.4  eV i n the  3. the p o s i t i o n s of other p o s s i b l e v a l e n c e -  v a l e n c e t r a n s i t i o n s the e x p e r i m e n t a l s e p a r a t i o n s of the remaining t a l s from the ba^,  7a^  as d e r i v e d from p h o t o e l e c t r o n s p e c t r o s c o p y  have been added to the a s s i g n e d p o s i t i o n s of the suggested  orbit-  [118],  6a ^ -*• o^  and  * o"  2  valence-valence t r a n s i t i o n s .  p a r t of F i g . 3.5. is  The  p o s i t i o n s are shown i n the  I t should be r e c a l l e d  f o r b i d d e n (see T a b l e 3.2);  t h a t the l a  2  -*• 7a^  lower  transition  however, s i n c e there i s doubt as to the  exact o r d e r i n g of the unoccupied  cr  o r b i t a l s , both t r a n s i t i o n s  are  - 112 -  indicated. X  ff  The o r d e r i n g of the occupied o r b i t a l s i s t h a t g i v e n by the  calculations  reversed  [133,134].  Other  calculations  the order of the l a ? and 5e o r b i t a l s .  (see T a b l e 3.1) have The X  c a l c u l a t i o n s use a  1  t r a n s i t i o n s t a t e formalism and t h e r e f o r e take i n t o account accompanying i o n i s a t i o n .  relaxation  T h i s o r d e r i n g a l s o agrees w i t h t h a t o b t a i n e d  by K e l l e r e r e t a l . [130] i n which they c a l c u l a t e a Koopman's theorem d e f e c t and combine i t w i t h v a l u e s p r e d i c t e d by CND0/2 f o r m a l i s a t i o n . With the e x c e p t i o n of the X^ c a l c u l a t i o n a l l c a l c u l a t i o n s presented i n T a b l e 3.1 apply Koopman's The  theorem.  agreement o b t a i n e d between the above p r e d i c t i o n s of the ener-  g i e s of a d d i t i o n a l v a l e n c e - v a l e n c e spectrum  t r a n s i t i o n s and major f e a t u r e s i n the  shown i n F i g . 3.5 i s q u i t e good.  are v i r t u a l l y non-bonding and l o c a l i s e d  The l a , 5e and 4e o r b i t a l s 2  on the F atoms [134].  Features  4 and 5 along w i t h much o f the i n t e n s i t y under f e a t u r e 6, which the second  s e c t i o n , a r e a t t r i b u t e d mainly  to t r a n s i t i o n s from  comprise  these  * o r b i t a l s to the a  orbitals.  The r e l a t i v e narrowness of the bands  comprising f e a t u r e s 4-6 i s c o n s i s t e n t w i t h t r a n s i t i o n s coming from nonbonding  orbitals.  T h i s i s i n c o n t r a s t to the h i g h e r energy  c o m p r i s i n g f e a t u r e s 7-9.  section  The width of t h i s s e c t i o n i s q u i t e broad and  the b u l k of the i n t e n s i t y i s a s s i g n e d to t r a n s i t i o n s to the a a r i s i n g from bonding.  the 5a^ and 3e o r b i t a l s , which are predominantly  The s t r u c t u r e to h i g h e r energy  t r a n s i t i o n s from  orbitals N-F  (>18 eV) probably a r i s e s  from  the 4a^ o r b i t a l .  The v a l e n c e s h e l l spectrum  shown i n F i g . 3.5 i s thus l i k e l y to  c o n t a i n c o n t r i b u t i o n s from a number of v a l e n c e - v a l e n c e  transitions.  - 113 -  However, t r a n s i t i o n s t o Rydberg l e v e l s w i l l a l s o be p r e s e n t . r e g a r d c l o s e examination  of the spectrum  f i n e s t r u c t u r e on top of the broader valence t r a n s i t i o n s . seen on d i f f e r e n t  In t h i s  ( F i g . 3.5) shows evidence f o r  l e v e l s a t t r i b u t e d to valence-  These p a r t i a l l y r e s o l v e d s h o u l d e r s were r e p e a t e d l y  scans of the same spectrum.  In a d d i t i o n the somewhat  narrower bands ( 3 , 6, and 7) have e n e r g i e s c o r r e s p o n d i n g to expected Rydberg t r a n s i t i o n s  (see below).  In order t o a s s i g n p o s s i b l e Rydberg  s t r u c t u r e i t i s assumed t h a t the term v a l u e s f o r Rydberg l e v e l s a r e t r a n s f e r a b l e between the ISEELS and the VSEELS s p e c t r a .  Before  t h i s , however, i t i s a p p r o p r i a t e t o c o n s i d e r why Rydberg term  doing  values  might be t r a n s f e r a b l e whereas v i r t u a l v a l e n c e o r b i t a l term v a l u e s a r e not n e c e s s a r i l y so. orbital  From the VSEELS data the term value f o r the LUMO  ( o ^ - F i g . 3.5, f e a t u r e 1) i s 5.09 eV, whereas the term  c o r r e s p o n d i n g to the peak of the broad  envelope  value  encompassing the N Is •+•  o* t r a n s i t i o n s i n the N ISEELS data ( F i g . 3.3, f e a t u r e 1) i s 7.26 eV. * T h e r e f o r e the o^  term v a l u e i s l i k e l y t o be even h i g h e r .  the term v a l u e g i v e s the " b i n d i n g energy" p r e v i o u s l y unoccupied  orbital.  In essence  of the e x c i t e d e l e c t r o n i n the  Thus i n the former  case the term  value  * g i v e s the b i n d i n g energy  of the e l e c t r o n i n the o^  h o l e e x i s t s i n the v a l e n c e s h e l l and i n the l a t t e r energy  o r b i t a l when t h e case the b i n d i n g  of the e l e c t r o n when an N Is core h o l e e x i s t s .  The l o s s o f  s h i e l d i n g by the removal of the v a l e n c e e l e c t r o n to the v i r t u a l o r b i t a l should be v e r y much l e s s than t h a t caused localised  core e l e c t r o n .  valence  by the removal of the  Hence, i n the ISEELS case, the e l e c t r o n i n the  0* o r b i t a l should see almost  a whole e x t r a u n i t of charge  and so i t  - 114  should be harder to remove. energy  (term v a l u e ) .  It will  -  t h e r e f o r e have an i n c r e a s e d b i n d i n g  In c o n t r a s t to the v a l e n c e o r b i t a l s the Rydberg  o r b i t a l s are l a r g e and d i f f u s e and hence w i l l  see the molecule  as  one  large core.  Thus they should be l e s s a f f e c t e d by where the  o c c u r r e d and  so have t r a n s f e r a b l e term v a l u e s , whereas the v a l e n c e  o r b i t a l s , being much more l o c a l i s e d , w i l l  vacancy  be more s u s c e p t i b l e to  local  variations i n shielding. The  expected  p o s i t i o n s of the valence-Rydberg  t r a n s i t i o n s were  c a l c u l a t e d f o r n = 3 and 4 u s i n g the term v a l u e s o b t a i n e d f o r these l e v e l s from the N ISEELS spectrum 3s, 3p, and  (i.e.,  4p o r b i t a l s r e s p e c t i v e l y ) .  3.34, The  2.37,  i n T a b l e 3.6 6a±,  l a , and 2  4p l e v e l s ) .  and F i g . 3.5  1.12  eV f o r the  p o s i t i o n s of the 5p and  Rydbergs were e s t i m a t e d u s i n g a quantum d e f e c t of 0.6 a l s o to the 3p and  and  (which a p p l i e s  These assignments and e n e r g i e s are shown  (upper  portion).  4e •*• 3p t r a n s i t i o n s  The  p r e d i c t e d v a l u e s of the  (11.36, 13.78, and  15.15  eV  r e s p e c t i v e l y ) are i n agreement w i t h the narrow f e a t u r e s 3, 6, and which are at 11.21, 13.75, and been a s s i g n e d a c c o r d i n g l y .  The  15.01  eV r e s p e c t i v e l y , and  to these.  The  spectrum  t r a n s i t i o n s w i t h valence-Rydberg The  can  the  valence-valence  t r a n s i t i o n s on  top.  assignment of f e a t u r e 9 i s not c l e a r s i n c e i t s width  and  p o s i t i o n i s not c o n s i s t e n t w i t h an assignment to a Rydberg l e v e l . may  levels  be  as a whole i s c o n s i s t e n t w i t h  i n t e r p r e t a t i o n g i v e n as t h a t of predominantly  7,  these have  t r a n s i t i o n s to the h i g h e r Rydberg  w i l l not be as i n t e n s e and weak f e a t u r e s i n the spectrum attributed  6p  a l s o a r i s e , a l o n g w i t h the i n t e n s i t y at ~ 19 eV,  It  from t r a n s i t i o n s  to  - 115  -  the o* l e v e l s o r i g i n a t i n g from the 4a^ o r b i t a l .  T h i s would g i v e i t a  h i g h e r term v a l u e than the other v a l e n c e - v a l e n c e t r a n s i t i o n s but  one  t h a t i s more c o n s i s t e n t w i t h t h a t from the N Is ISEELS spectrum. i s not unexpected  i n view of i d e a s d i s c u s s e d e a r l i e r  o r b i t a l i s e s s e n t i a l l y due localised  to the N 2s o r b i t a l and  than the outer valence  Comparison of VSEELS and  s i n c e the  This  4a±  i s thus more  levels.  ISEELS S p e c t r a w i t h the XPS  Satellite  Structure The  process  t h a t occurs i n VSEELS i s the e x c i t a t i o n of an  ( i . e . , v a l e n c e s h e l l ) e l e c t r o n to an e x c i t e d bound s t a t e v a l e n c e or Rydberg l e v e l ) .  The  outer  (virtual  s a t e l l i t e s t r u c t u r e i n XPS  i s due  to the  f o r m a t i o n of e x c i t e d i o n s t a t e s i n which the p r o c e s s , at l e a s t i n a simple model, can be thought  of as the e m i s s i o n of a p h o t o e l e c t r o n w i t h  the attendant e x c i t a t i o n of an outer e l e c t r o n to e i t h e r an e x c i t e d bound s t a t e ("shake-up") or to the continuum ( " s h a k e - o f f " ) . the r e s u l t s of the two v a l e n c e m a n i f o l d and ionisation.  s p e c t r o s c o p i e s may  y i e l d i n f o r m a t i o n on  the types of p r o c e s s e s  Care has  to be taken i n any  i s at l e a s t a two  [21] have shown t h a t many-electron a c t i o n has  occur upon  photo-  e l e c t r o n p r o c e s s , whereas  e l e c t r o n process.  M a r t i n et a l .  theory i n c l u d i n g c o n f i g u r a t i o n i n t e r -  to be i n c l u d e d i n both the h o l e and ground s t a t e s i n o r d e r t o  d e s c r i b e adequately tion.  t h a t may  the  such comparison s i n c e VSEELS  can e s s e n t i a l l y be d e s c r i b e d i n terms of one "shake-up/shake-off"  A comparison of  the s a t e l l i t e  s t r u c t u r e accompanying p h o t o i o n i s a -  Using t h i s approach they have s u c c e s s f u l l y a n a l y s e d the  s t r u c t u r e of HF  [22].  satellite  An i n s p e c t i o n of the d a t a , however, r e v e a l s (as  - 116 -  they have noted) t h a t the f o u r most i n t e n s e peaks can be i n t e r p r e t e d , a t least  to a f i r s t  Thus an attempt  approximation  i n terms of o n e - e l e c t r o n e x c i t a t i o n s .  has been made t o a n a l y s e the N F  3  XPS s p e c t r a i n terms o f  o n e - e l e c t r o n e x c i t a t i o n s i n order t o see whether any meaningful t i o n can be o b t a i n e d without  informa-  r e s o r t i n g to complex c a l c u l a t i o n s .  This i s  a i d e d by comparison w i t h the VSEELS and ISEELS s p e c t r a a l o n g w i t h a c o n s i d e r a t i o n o f the p o t e n t i a l b a r r i e r phenomenon. The N Is XPS s a t e l l i t e  spectrum  has been a s s i g n e d (see T a b l e 3.3)  as being dominated by t r a n s i t i o n s t o Rydberg l e v e l s 3.1)  w i t h the lower  intensity  s t r u c t u r e (A, B, E, F, e t c . ) a r i s i n g  p r i m a r i l y from t r a n s i t i o n s t o the unoccupied loss features.  (peaks C and D, F i g .  v a l e n c e l e v e l s and energy  These t e n t a t i v e assignments have been made on the b a s i s  of two c o n s i d e r a t i o n s . F i r s t l y , Creber s p e c t r a of the second w i t h Ne have suggested  et a l . [142] i n a study of the XPS s a t e l l i t e row h y d r i d e s (CH^, NH , H 0, HF) i s o e l e c t r o n i c 3  2  that Rydberg-like o r b i t a l s r e l a x l e s s  than  v a l e n c e - t y p e o r b i t a l s when a core h o l e i s c r e a t e d i n XPS ( t h i s ally  incident-  i s c o n s i s t e n t w i t h the p r e v i o u s a s s e r t i o n t h a t term v a l u e s are  t r a n s f e r a b l e from ISEELS t o VSEELS f o r Rydberg t r a n s i t i o n s , but n o t n e c e s s a r i l y f o r t r a n s i t i o n s t o unoccupied  valence l e v e l s ) .  of peaks A, B, and sh r e l a t i v e t o the main Is l i n e  The e n e r g i e s  ( F i g . 3.1, T a b l e 3.3)  are c l o s e to the v a l e n c e s h e l l e x c i t a t i o n e n e r g i e s i n the VSEELS spectrum  ( F i g . 3.5, T a b l e 3.6).  would correspond  F u r t h e r peaks E and F ( o f F i g . 3.1)  to v a l e n c e t r a n s i t i o n s from  v a l e n c e o r b i t a l s t o the o* l e v e l s .  the 3 a a n d  2e i n n e r  However, peaks C and D a r e a t  - 117  r e l a t i v e e n e r g i e s of 18.87  eV and  -  23.36 eV r e s p e c t i v e l y , which are much  g r e a t e r than the e s t i m a t e d e n e r g i e s of any outer v a l e n c e o r b i t a l s o b s e r v a t i o n s , and  transitions arising  to the v i r t u a l v a l e n c e l e v e l s .  t a k i n g i n t o account  In view of  t o Rydberg l e v e l s from o c c u p i e d v a l e n c e l e v e l s unoccupied  valence o r b i t a l s  but both r e l a x more than Rydberg Secondly,  (i.e.,  to  transitions  t h i s assumes t h a t  r e l a x to s i m i l a r e x t e n t s ,  levels).  assuming the above assignment of Rydberg and  t r a n s i t i o n s i n the XPS  spectrum  these  the c o n c l u s i o n s of Creber et a l .  [142], peaks C and D are a s s i g n e d as being p r i m a r i l y due  both occupied and  from  ( F i g . 3.1,  comparison of i n t e n s i t i e s between XPS a l s o a p p a r e n t l y VSEELS ( F i g . 3.5)  Table 3.3)  ( F i g . 3.1),  valence  i s correct, a  ISEELS. ( F i g . 3.3),  and  shows an i n t e r e s t i n g r e v e r s a l i n the  r e l a t i v e i n t e n s i t i e s of t r a n s i t i o n s to Rydberg and v a l e n c e l e v e l s .  In  ISEELS, t r a n s i t i o n s to v a l e n c e l e v e l s predominate over those t o Rydberg l e v e l s , whereas the r e v e r s e s i t u a t i o n seems to occur i n XPS. behaviour  i s not e n t i r e l y unexpected  a t t r i b u t e d to the t h r e e F l i g a n d s i n In NF s p e c t r a and  3  the p o t e n t i a l b a r r i e r  This  c o n s i d e r i n g the p o t e n t i a l NF . 3  (as can be seen from  the ISEELS  to some e x t e n t the VSEELS s p e c t r a here) s e p a r a t e s  the  Rydberg and v a l e n c e o r b i t a l s , c a u s i n g the ISEELS (and p o s s i b l y VSEELS) to be v a l e n c e dominated.  However, i n the XPS  of NF  3  t h i s would s u r e l y e x t e n s i v e l y reduce  s i p h o n i n g of e l e c t r o n i c charge circumstances we  might expect  the  t h e r e i s an  a d d i t i o n a l h o l e (compared to the ISEELS and VSEELS), i . e . , one l e s s , and  barrier  electron  the p o t e n t i a l b a r r i e r  from the s u r r o u n d i n g F atoms.  by  In these  to see a r e l a t i v e i n c r e a s e i n the r a t i o  of  - 118  -  Rydberg to valence e x c i t a t i o n In the XPS The F Is "shake-up" spectrum t h a t of the N I s .  of N F  3  i s rather d i f f e r e n t  T h i s d i f f e r e n c e can be immediately  r e d u c t i o n of symmetry from C ^ electrons.  "shake-up" spectrum.  to C  v  In t h i s case the a^ o r b i t a l s become a", a  the e symmetry o r b i t a l s reduce -»• a' and a"  attributed  upon removal of one  g  to a' and a".  2  probably due  to the  of the F Is  becomes a",  i n the case of N I s , f e a t u r e s P, Q,  In keeping  with  S, and T are  to t r a n s i t i o n s to v a l e n c e l e v e l s , whereas R i s due  t r a n s i t i o n s to Rydberg  and  Thus a l a r g e number of a'  a" v a l e n c e t r a n s i t i o n s become p o s s i b l e .  the arguments used  from  to  levels.  F i n a l l y , a c o n s i d e r a t i o n of the v a r i o u s types of s p e c t r a and t h e i r r e l a t i v e e n e r g i e s f o r NH preceeding c o n c l u s i o n s . ([51] and Chapter  and N F  3  8) i s dominated by t r a n s i t i o n s 3  3  [142]  lends some support  appears  to the  In p a r t i c u l a r the N Is ISEELS spectrum  s i n c e there i s no b a r r i e r i n NH . of NH  3  of  to Rydberg-type  S i m i l a r l y , the XPS  shake-up  spectrum  to be R y d b e r g - l i k e , w i t h the major f e a t u r e s b e i n g  In the l i g h t of the present work i t would be of i n t e r e s t v a r i o u s s p e c t r a of the molecules 3  and  3  levels  at s i m i l a r r e l a t i v e e n e r g i e s to the main Is l i n e as i n the case of  of NH  NH  NHF  2  to study  and NH F i n comparison w i t h 2  NF . 3  the those  NF « 3  Conclusions I t has been shown i n t h i s chapter t h a t w h i l e each (ISEELS, VSEELS, and XPS) s t r u c t u r e of m o l e c u l e s ,  spectroscopy  y i e l d s separate i n f o r m a t i o n on the  electronic  a c o n s i d e r a t i o n of a l l three i n c o n j u n c t i o n w i t h  119  -  each other  can  -  l e a d to a f u r t h e r understanding of each process and  molecular e l e c t r o n i c s t r u c t u r e . t i o n on v a l e n c e - v a l e n c e and  A l l three  spectroscopies  give  the  informa-  valence-Rydberg l e v e l e l e c t r o n i c t r a n s i -  tions. The  ISEELS s p e c t r a were found to be  with highly electronegative enhanced t r a n s i t i o n to features  t y p i c a l examples of molecules  l i g a n d s i n that they showed a  strongly  d* l e v e l s , l o w - i n t e n s i t y Rydberg s t r u c t u r e  i n the continuum.  A comparison w i t h the XPS  and  satellite  s t r u c t u r e i n d i c a t e s that the continuum s t r u c t u r e can be a s s o c i a t e d "shake-up" phenomena.  The  VSEELS spectrum show much s t r u c t u r e that  been a t t r i b u t e d to both v a l e n c e - v a l e n c e and Comparing the term v a l u e s i n d i c a t e s that  obtained  those a s s o c i a t e d  has  valence-Rydberg t r a n s i t i o n s .  from the ISEELS and VSEELS  w i t h Rydberg l e v e l s are  spectra  transferable,  whereas those a s s o c i a t e d w i t h the v i r t u a l valence l e v e l s are not  with  generally  transferable. While more s p e c i f i c c o n c l u s i o n s  ticated  t h e o r e t i c a l treatment of NF , 3  indicated provide  that the combined use  must await a d e t a i l e d and the present  of v a r i o u s  s t u d i e s have c l e a r l y  electron  spectroscopies  more d e t a i l e d i n s i g h t i n t o fundamental i o n i s a t i o n and  processes.  excitation  In a d d i t i o n , f u r t h e r i n s i g h t s have been gained i n t o  p o t e n t i a l b a r r i e r model.  sophis-  the  - 120 -  CHAPTER 4  ELECTRON ENERGY LOSS SPECTRA OF THE SILICON 2p,2s, CARBON I s and VALENCE SHELLS OF  In the p r e v i o u s  TETRAMETHYLSILANE  chapter  the ISEELS and VSEELS s p e c t r a of a  compound w i t h h i g h l y e l e c t r o n e g a t i v e l i g a n d s were presented and discussed.  I t would be i n s t r u c t i v e t o now c o n s i d e r the s p e c t r a o f a  compound which does not have e l e c t r o n e g a t i v e l i g a n d s . the ISEELS s p e c t r a of t e t r a m e t h y l s i l a n e  In t h i s  chapter  (TMS), ( C H ) S i , i n the C I s , 3  4  S i 2s and 2p r e g i o n s as w e l l as the VSEELS spectrum are presented. S i 2p s p e c t r a a r e compared and c o n t r a s t e d w i t h p u b l i s h e d s p e c t r a of S i F ^ , S i H ^ and o t h e r r e l a t e d  The  photoabsorption  S i c o n t a i n i n g molecules  with  v a r y i n g l i g a n d s t o f u r t h e r examine the e f f e c t s of the l i g a n d on i n t e n s i t y d i s t r i b u t i o n w i t h i n the s p e c t r a . TMS i s a substance  o f fundamental and p r a c t i c a l importance and i s  used as a c a l l b r a n t i n NMR s p e c t r o s c o p y . to date only very l i m i t e d e x c i t a t i o n processes al.  s t u d i e s have been made of e l e c t r o n i c  of TMS i n the gas phase.  [143] have measured the valence  some r e l a t e d molecules  F o r example Roberge e t  s h e l l photoabsorption  up to 85,000 c m  eV) which i s c l o s e t o the upper l i m i t and  TMS i s very s t a b l e , however,  - 1  ( i . e . up to an energy of 10.5  of l i g h t  t r a n s m i s s i o n by windows  l e n s e s i n c o n v e n t i o n a l o p t i c a l spectrometers.  are few sources  of TMS and  of continuum r a d i a t i o n s u f f i c i e n t l y  Above ~10 eV t h e r e intense f o r obtain  - 121 -  ing d e t a i l e d photoabsorption spectra, s i l i c o n core e x c i t a t i o n r e g i o n provides a suitable  light  (>100 eV).  [73]  has f u r t h e r  Synchrotron  radiation  source, but to date no such study o f TMS has  been made, a l t h o u g h s i m i l a r s t u d i e s [144,145] and a l s o  p a r t i c u l a r l y i n the carbon and  s i l a n e , SiH  lt  of the i s o e l e c t r o n i c m o l e c u l e ,  [65,146] have been r e p o r t e d .  SiF  Dehmer  d i s c u s s e d the S i 2p a b s o r p t i o n spectrum of S i F  4  with  r e f e r e n c e to the e f f e c t i v e p o t e n t i a l b a r r i e r model.  The S i 2p ( i . e .  LJJ.  d e r i v a t i v e s has  e l e c t r o n i c e x c i t a t i o n of TMS and i t s c h l o r o  been s t u d i e d  by Fomichev et a l . [147] u s i n g f i l t e r e d  tt  brehmsstrahlung  r a d i a t i o n from a tungsten anode i n the l i m i t e d energy range from 102 109  eV.  No p h o t o a b s o r p t i o n spectrum of e i t h e r the carbon I s or the  s i l i c o n I s r e g i o n s o f TMS has been p u b l i s h e d t o date. In c o n t r a s t  to the l i m i t e d s t u d i e s  made of e l e c t r o n e x c i t a t i o n i n  TMS much work has been r e p o r t e d on the p h o t o e l e c t r o n s p e c t r a v a l e n c e and core r e g i o n s .  These w i l l g r e a t l y a s s i s t i n the i n t e r p r e t a -  t i o n of the e l e c t r o n e x c i t a t i o n s p e c t r a . TMS have been r e p o r t e d by s e v e r a l PES  groups [148-150]. t +  several  [149],  S i 2p b i n d i n g  laboratories  Experimental The  The v a l e n c e  shell  been  e n e r g i e s of TMS have been measured i n  [30,151-154].  Details  inner  s h e l l spectra  2.5 keV w i t h the s c a t t e r e d The  H e ( I ) p h o t o e l e c t r o n s p e c t r a of  spectrum of the i s o e l e c t r o n i c m o l e c u l e , S i F , has a l s o  published  of both  were recorded u s i n g an impact energy of  electrons  energy s c a l e s were e s t a b l i s h e d  sampled a t ~1° s c a t t e r i n g  angle.  f o r both r e g i o n s ( S i L - s h e l l and C  - 122 -  K - s h e l l ) w i t h r e s p e c t t o the Ar (2p •*• 4s) t r a n s i t i o n at 244.37 eV. v a l e n c e s h e l l spectrum impact  was o b t a i n e d on the new spectrometer  [ 5 3 ] . An  energy of 3 keV was used w i t h the s c a t t e r e d e l e c t r o n s sampled a t  zero degree s c a t t e r i n g a n g l e . He(I)  The  resonance  The spectrum  l i n e a t 21.218 eV  was c a l i b r a t e d w i t h the  [104].  R e s u l t s and D i s c u s s i o n The  TMS m o l e c u l e ,  S K C H g ) ^ i s of T j symmetry and the e l e c t r o n  c o n f i g u r a t i o n and unoccupied  v a l e n c e o r b i t a l s may be w r i t t e n as [73,  148],  (la ) (2a ) (lt ) (3a )2 2t )6(4a ) (3t ) (5a ) (4t ) (le)' (lt ) (5t ) 2  2  1  6  1  Sils  2  <  2  1  Cis—'  (  2  1  S i 2 s Si2p  6  2  2  6  1  2  »  +  6  1  valence  6  2  '  (6a )0(6t )°(2e)0(7t )0 1  i  The  2  2  unoccupied  '  various spectra are u s e f u l l y discussed with respect to t h i s c o n f i g u -  ration. 1.  Inner S h e l l  Spectra  F i g u r e 4.1 shows a continuous wide range scan of the S i 2p,2s and C I s r e g i o n s of the e l e c t r o n i c e x c i t a t i o n spectrum and  380 eV.  eV FWHM.  T h i s survey  spectrum  o f TMS between 40  was o b t a i n e d a t a r e s o l u t i o n o f 0.36  The more prominent f e a t u r e s a r e numbered on the f i g u r e c o n s i s -  t e n t w i t h the d e s i g n a t i o n s on the more d e t a i l e d s p e c t r a shown i n F i g u r e s  2s,  Si  7  (CH3)4Si  8  SILICON 2p, 2s CARBON Is  3 O  c JT-T  o  235  >'v  CO  A E = 0.36eV  LU  VALENCE \ l_ SHELL \  :  :  A \  100  200  250  E N E R G Y L O S S (eV) F i g u r e 4.1: Wide range i n n e r s h e l l e l e c t r o n energy l o s s spectrum o f  tetramethylsilane.  - 124 -  4.2  and 4.5.  discussed  The assignments o f the v a r i o u s  later.  I n the r e g i o n up t o ~100  i o n i z a t i o n edges are  eV the s t e e p l y f a l l i n g  of the v a l e n c e s h e l l i o n i z a t i o n continuum i s c l e a r l y v i s i b l e .  also "tail"  The S i 2p  spectrum i s dominated by two l a r g e peaks, one ( f e a t u r e 3) below and one ( f e a t u r e 6) above the i o n i z a t i o n l i m i t . tures  ( f e a t u r e s 7,8)  S i m i l a r but  are seen below and  eV,  the onset o f carbon I s ( i . e . K - s h e l l ) e x c i t a t i o n . s t a t e s and continuum s t r u c t u r e s are a p p a r e n t l y  The  struc-  above the 2s i o n i z a t i o n l i m i t .  A sharp i n c r e a s e i n c r o s s s e c t i o n i s seen a t ~285  discussed  l e s s intense  consistent  with  A number o f d i s c r e t e  present  and these  are  i n d e t a i l below i n the s e c t i o n on C I s e x c i t a t i o n . o v e r a l l spectrum ( F i g u r e 4.1)  d i s p l a y s c l e a r evidence o f t h e  s e p a r a t e s u b s h e l l s o f S i and C a t the expected p o s i t i o n s . large s h i f t s i n o s c i l l a t o r  s t r e n g t h o r s i g n i f i c a n t d e l a y e d onsets  observed, i t i s apparent t h a t no s t r o n g e f f e c t s are o c c u r r i n g .  Since no are  inter-shell electron correlation  L i k e w i s e we may expect that the S i I s spectrum  i s even more atomic l i k e due t o i t s comparative "energy i s o l a t i o n " a t -1844  eV [155]. The-silicon  examined s e p a r a t e l y  (2p,2s) and carbon ( I s ) s p e c t r a have been f u r t h e r a t high  resolution.  Figure  4.2 (lower  section)  shows the S i 2p and 2s e x c i t a t i o n between 100 and 170 eV i n d e t a i l a t an energy r e s o l u t i o n o f 0.36 eV FWHM. i n the l a r g e peak i n the t i o n s (0.18  The d e t a i l e d s t r u c t u r e  eV i s shown a t h i g h e r  eV and 0.10 eV FWHM r e s p e c t i v e l y ) i n the  of F i g u r e 4.2. present  r e g i o n o f ~105  At l e a s t f i v e features  i n t h i s p a r t o f the  spectrum.  greater  (numbered 1-5) The 2p_,~  two upper are  resoluspectra  clearly  i o n i z a t i o n edge (105.94  125  -  P3/22P|/2  2  rrnVI 2345 2  (CKlSi  P3/2 2P|/2  r - n — r  SILICON 2p,2s  12 3 4 5  AE=O.I8eV  "c.  A E = O.IOeV  Z3  o  3  I  o|  • I . I • I • I • I •  102  >-  106  110  I i 114 100  L 102  J  i  104  L 106  J 106  i  I 110  i  L 112  2p3/22p1/2  V)  rm  z  A E = 0.36 eV  12349  UJ  ~2s  7  t I '  100  •  » 110  •  ' 120  i  I 130  i  I KO  i  I 150  i  I 160  1  L_ 170  ENERGY LOSS ( e V )  F i g u r e 4.2: S i l i c o n 2p e l e c t r o n energy l o s s s p e c t r a of t e t r a m e t h y l s i l a n e . D e t a i l s a r e g i v e n i n Table 4.1.  - 126  ev) has been a s s i g n e d u s i n g the XPS al.  [30].  Very  -  v a l u e f o r TMS  s i m i l a r v a l u e s have been recorded by P e r r y and  [151] and a l s o by Drake et a l . [152,153]. by Gray et a l .  [154]  A v a l u e of 100.7  i s clearly grossly i n error.  s u r p r i s i n g i n view of the f a c t t h a t an a r b i t r a r y of was  285.0 eV was used  [154]  a s s i g n e d to the carbon  3 / 2 1/2  s  P*  n  o r  bit  eV  reported  (and i n c o r r e c t ) v a l u e  Is b i n d i n g energy of TMS  as i s d i s c u s s e d below). splitting  Jolly  T h i s i s not  to c a l i b r a t e the s c a l e (the c o r r e c t v a l u e  should be 289.78 eV, 2 p  r e p o r t e d by K e l f v e e t  and  [152,153]  A v a l u e of 0.61  has been used i n a s s i g n i n g the  eV f o r the 2pjy  2  i o n i z a t i o n edge i n accord w i t h the f i n d i n g s of K e l f v e et a l . [30] t h e i r XPS  s t u d i e s on S i H ^ and  silicon  I t i s to be expected  [156].  S i F ^ [65,144,146] and  crystalline  t h a t the magnitude of  i s l a r g e l y independent of the molecular  type.  o b t a i n e d by V e n e z i a - F l o r i a n o and C a v e l l  of  51.4  of ~51.2  eV  eV suggested  certainly The  by Fomichev e t a l .  [147]  2s  value  T h i s g i v e s a 2p-2s  for S i .  splitting  A v a l u e of  i n S i H ^ r e p o r t e d by Hayes and  41.5  Brown [146] i s  incorrect.  e n e r g i e s , term v a l u e s and p o s s i b l e assignments of the S i ( 2 p ,  2s) spectrum are shown i n Table 4.1. o b t a i n e d by Fomichev et a l . [147] shown and are c o n s i s t e n t l y lower approximately present  SI  , i n c l o s e agreement w i t h the estimated  eV f o r the 2p-2s s p l i t t i n g almost  [157].  this  The  i o n i z a t i o n edge (157.31 eV) has been a s s i g n e d u s i n g the XPS  splitting  from  work on a v a r i e t y of s i l i c o n - c o n t a i n i n g compounds as w e l l as  photoabsorption  splitting  this  0.5  ISEELS and  eV.  The  The  energy v a l u e s of f e a t u r e s  1-5  u s i n g s o f t X-ray a b s o r p t i o n are a l s o than the present  shapes and  ISEELS v a l u e s  by  r e l a t i v e i n t e n s i t i e s of both  s o f t X-ray s p e c t r a [147]  are almost  the  i d e n t i c a l i n the  - 127  -  Table Energies  4.1  and term values of f e a t u r e s i n the Si(2p,2s) spectrum of S i ( C H ) ^ 3  Feature  Energy" (eV)  Term Value  1  103.60  2.34  2.94  2p-*a , t / 4 p -  2  104.19  1.75  2.35  2p*5s, 3d  2p-*a  3  104.73  1.21  1.81  2p-5p  2p->5s, 3d  104.15  4  105.45  0.49  1.09  2p-4d  2p-5p  104.80  5  105.97  —  0.57  -  2p+4d  105.7  —  2p-»=>  -  105.7  ^Pl/2^ *' n l  t  105.94  a  106.55  b  6  124.1  7  155.07  2s  limit  2p  3 / 2  2  0  —  P o s s i b l e Assignment 2p  ?l/2  -  0  c  103.15 1>  t£/4p  103.65  2p-.»  Resonance ( o * ( 7 t 2 ) )  2.  2s->t2  0  -  d  6  Photoabsorption^ (eV)  1 / 2  2  1  -  -157.31 -157 .6  2p  3 / 2  -173  Resonance  (o*(7t2))  a.  XPS values from Reference [30]. References 105.83 eV and 106.02 eV r e s p e c t i v e l y .  b.  The spin o r b i t s p l i t t i n g of 0.61 eV has been assigned f o l l o w i n g data i n r e f e r e n c e [30]. S i m i l a r s p l i t t i n g s are reported elsewhere [65,144,146,156],  c.  A. V e n e z i a - F l o r i a n o and R.G.  d.  Foraichev et a l . [147]  +  With respect to the 2s l i m i t  *  Estimated  suggest  u n c e r t a i n t y ±0.05  [151-153] give values  C a v e l l , reference  [157].  a S i 2p-2s s p l i t t i n g (157.31 eV, eV.  reference  o f 51.4 [157]).  eV.  of  - 128  energy range 102-108 eV. 1-5  spacing  An examination of the  to both ^?^/2  with respect  -  a n c  * ^ l/2 p  e c  i s a p p r o x i m a t e l y the same as the  i o n i z a t i o n l i m i t s (~0.6  eV  —  *S  e s  s  n  term v a l u e s o  buting  2p^^  a  s  t h a t the peak  s p i n - o r b i t s p l i t t i n g at  see p r e c e e d i n g d i s c u s s i o n ) .  t h e r e f o r e be p o s s i b l e that a double o v e r l a p p i n g i n g to the r e s p e c t i v e  w  n  f o r peaks  the  I t might  Rydberg s e r i e s c o n v e r g -  i ° i t i o n l i m i t s c o u l d be  Q  n  to the o v e r a l l 2p e x c i t a t i o n band shown i n F i g u r e  a p p l i c a t i o n of the Rydberg formula and  contri-  z a  the  2p  4.2.  However  ionization limits indica-  tes that no  simple c o n s i s t e n t assignment can be made on the b a s i s  term values  and  quantum d e f e c t s .  i s observed below the  only  broad s t r u c t u r e d band  Rydberg s t a t e s .  of the expected term value  Rydberg i s m i s s i n g  one  i o n i z a t i o n edge i t i s h i g h l y l i k e l y  encompasses (mixed) v a l e n c e and consideration  Since  i n the  ( i . e . S i ( C H ) ^ ) w i t h the  i t i s evident  s o f t X-ray a b s o r p t i o n  the  i s o e l e c t r o n l c molecule S i F ^ and  3  species  and  (solid) Si0 . 2  are c o n v e n i e n t l y  Dehmer [73].  S i 2p  same energy s c a l e and B, C, and  only two  are  S10  2  a l s o w i t h the  that the  4s  spectrum o f  spectrum  [144-146] of  s p e c t r a of the  species these  diagram i n the p u b l i c a t i o n by  spectrum of SiH^  spectrum of TMS shown i n F i g u r e [73])  S i 2p  s o f t X-ray s p e c t r a of some of  shown on one  D as i n r e f e r e n c e  S i F j , S i C l ^ and t  The  These, as w e l l as the  presently obtained  this  Si(2p,2s) spectra.  TMS  4  that  In p a r t i c u l a r from  I t i s i n s t r u c t i v e at ths p o i n t to compare the  SiH^, S i C l  of  [65]  and  the  have been reproduced on 4.3.  the  Four peaks ( l a b e l l e d  A,  occur In each of the S i 2p s p e c t r a  of  whereas I t would seem ( F i g u r e s 4.1  prominent bands e x i s t i n the  S i 2p  and  spectrum of TMS.  4.2) The  that  -  129  -  ENERGY LOSS (eV) F i g u r e 4.3: S i l i c o n 2p e x c i t a t i o n s p e c t r a of v a r i o u s S i c o n t a i n i n g compounds w i t h S i i n a t e t r a h e d r a l environment; S i C C H j ) ^ t h i s work, S i C l ^ , S i 0 and S i F as shown i n r e f . [ 7 3 ] . D e t a i l s are g i v e n i n Table 4.2. H  2  - 130  d i f f e r e n c e i n behaviour  between TMS  -  and  SiF  tt  can be a t t r i b u t e d to the  h i g h e l e c t r o n e g a t i v i t y of the F l i g a n d whereas the CH to be e l e c t r o n d o n a t i n g  [31,33].  effects  to be much g r e a t e r i n S i F  indeed  [73] are l i k e l y there may  Thus p o t e n t i a l  3  ligand  (charge) tt  barrier  than i n TMS,  be no e f f e c t i v e b a r r i e r i n S i ( C H ) . 3  is likely  1 +  and  I f t h i s i s the  case, then i n S i F ^ the band C above the S i 2p edge can be a t t r i b u t e d be an i n n e r - w e l l s t a t e trapped by the p o t e n t i a l b a r r i e r due surrounding f l u o r i n e l i g a n d s . S i C l ^ and  Si0  2  to the  Peaks of type C are a l s o seen  [73] i n  presumably because these s p e c i e s a l s o have h i g h l y  electronegative ligands.  Very b i g d i f f e r e n c e s were observed  the carbon K - s h e l l s p e c t r a of CF^  normal Rydberg type of spectrum.  earlier in  [66,92,133] and C I ^ [66,72],  being an obvious p o t e n t i a l b a r r i e r e f f e c t  i n CF^ w h i l e CH^  has a more  r e l a t e d s i l i c o n c o n t a i n i n g s p e c i e s d i s c u s s e d above.  [146,156] and  3  l +  i s v e r y s i m i l a r to those of S i H  t h i s lends f u r t h e r credence  The l+  observed  to the s u g g e s t i o n t h a t  of  poten-  TMS  the l a r g e s t r u c t u r e (103-110 eV) c o n t a i n s at l e a s t  f a i r l y e v e n l y spaced  Si  Considering  d e t a i l s of the pre-edge s t r u c t u r e i n the S i 2p spectrum  ( F i g u r e 4.2)  the other  [65,146] and  t i a l b a r r i e r e f f e c t s i n S K C H g ) ^ are e f f e c t i v e l y absent. now  to  w i t h those of S i H ^ [65,146]  and elemental S i [146,156] as w e l l as w i t h those of S i F ^ and  of S i ( C H )  there  In t h i s r e g a r d i t i s of i n t e r e s t  compare the wide range S i ( 2 p ) s p e c t r a of TMS  spectrum  to  bands at a s e p a r a t i o n of approximately  0.5  five eV.  As  has been d i s c u s s e d e a r l i e r , i t has not been p o s s i b l e to f i t the peaks t o any obvious Rydberg s t a t e s and with v i b r a t i o n a l s t r u c t u r e .  furthermore  the s p a c i n g i s not  compatible  S i m i l a r c o n c l u s i o n s were drawn by Dehmer  - 131 -  [73] f o r the c o r r e s p o n d i n g band (B) i n S i F ^ , although Hayes and Brown [146] r e p o r t having f i t t e d Rydberg s e r i e s to the same band i n S i F ^ . C o n s i d e r i n g f u r t h e r the r e g i o n of the TMS  spectrum below the S i  2p i o n i z a t i o n edge o n l y a s i n g l e broad band ( c o n t a i n i n g maxima 1-5, F i g u r e 4.2) can be seen and t h i s i s l i k e l y v a l e n c e and Rydberg s t a t e s . SiCl^  to encompass a mixture of  In c o n t r a s t f o r S i F ^ [144-146],  [73] two bands, A and B [ 7 3 ] , are c l e a r l y p r e s e n t .  and  However, a  t h e o r e t i c a l a n a l y s i s of a h i g h r e s o l u t i o n spectrum  [146] shows that the f i r s t  2  Similarly in  S i H ^ [65,146] two bands are p r e s e n t below the S i 2p edge. detailed  Si0  of S i H  4  band i s due to the o v e r l a p p i n g a (a^) and  a ( t ) v a l e n c e s t a t e s expected f o r molecules of T j symmetry (TMS, SiH^) 2  f o r which the unoccupied o r b i t a l s a r e , i n order of i n c r e a s i n g  (a )(t -p 1  2  like)(e)(t -d 2  energy;  like)  * I t i s a l s o suggested  [65] that the a (a^) has some 4s Rydberg c h a r a c t e r .  The second band i n S i H ^ i s found (sharp) Rydberg l i n e s .  [65] to c o n s i s t of s e v e r a l s e r i e s of  The nature of these assignments  i n SIH^ have  been confirmed by running the a b s o r p t i o n s p e c t r a f o r s o l i d which case the Rydberg l e v e l s are suppressed. o r b i t a l s have been found to be w e l l separated  SiH^ [65], i n  I n S i F ^ the two [73,144].  Peak A i n S i F ^  has been a s s i g n e d as 2p -*• a (a^) and peak B i s a composite a ( t ) and the v a r i o u s Rydberg l e v e l s 2  t e r i s t i c s have been confirmed  [144].  virtual  of 2p -*  A g a i n the v a l e n c e c h a r a c -  [144] by running the spectrum  of the  - 132  solid.  As noted i n r e f e r e n c e  [144]  -  the term value  (in effect  the  a  " b i n d i n g e n e r g i e s " of e l e c t r o n s i n the v i r t u a l o r b i t a l s ) f o r the l e v e l i s higher t  2  orbital.  T h i s i s d i r e c t l y a t t r i b u t a b l e to the e l e c t r o n e g a t i v i t y of t h e r e f o r e i t can be expected that as the  electronega-  of l i g a n d s decrease f o r a s e r i e s of s i l i c o n compounds bands A  (a^) and  B ( t ) should 2  S i F ^ •* S i C ^  •*• S i h ^ .  o v e r l a p and  reversed.  converge. S i n c e -CH  f a c t e l e c t r o n donating i n TMS  (a^)  i n S i F ^ than i n S i H ^ whereas the r e v e r s e i s t r u e f o r the  the F l i g a n d and tivity  *  T h i s can c l e a r l y be seen i n going i s even l e s s e l e c t r o n e g a t i v e and  3  [31,33] i t i s suggested that the a±  indeed  and  i t i s p o s s i b l e that the order may  t  from  in 2  levels  even be  A s i m i l a r trend of the merging of bands A ( a ^ l i k e  ) and  B ( t - l i k . e ) can be seen i n the S i 2p a b s o r p t i o n s p e c t r a [147] of (CH,) S i C l , , . as x goes from 0 to 4. Given a l l the c o n s i d e r a t i o n s x (4-x) 2  d  d i s c u s s e d above, i t i s c o n s i d e r e d p a r t of 2 are due  mainly to the  a±  bands.  and  t  2  valence  p o s s i b l e that the t The  2  7 with r e s p e c t  2  Y/2  c  o  m  P  i s mixed w i t h  n  e  n  the 4p Rydberg  t  to the 2s edge (see Table  4.1)  The  the  °f  s  t  n  corresponding overlapped  e  (2.35  eV)  so o n l y  term v a l u e  i s 2.24  eV.  lends  overlapped  for feature feature  as the 2s -*• a ( t ) t r a n s i t i o n w i t h p o s s i b l y some 2  the term v a l u e  f u r t h e r support  transi-  This  c o n t r i b u t i o n from the 4p Rydberg i n view of the term v a l u e . accord w i t h  i t i s also  level.  i s of aj^ symmetry and  o r b i t a l are d i p o l e a l l o w e d .  i s therefore assigned  o  In view of the term value  S i 2s o r b i t a l i n TMS  t i o n s to the t  that f e a t u r e 1, and  f o r f e a t u r e 1 of the S i 2p spectrum which  to the s u g g e s t i o n  or even r e v e r s e d  This i s i n  that the a^ and  t  2  s t a t e s are  i n the S i 2p spectrum as d i s c u s s e d  above.  - 133  The  close proximity  of the S i 2p edge would l e a d to o v e r l a p  t r a n s i t i o n s to the v a l e n c e simple  and  Rydberg l e v e l s ,  Rydberg a n a l y s i s as has  ( t ) for S i F ^ [73]. 1.6  ( p ) , and  0.0  eV)  and  4d  (3.40  Features  (0.85  1 and  the 4p Rydberg l e v e l ; is dipole forbidden  eV).  as d i s c u s s e d other  (Na-Ar) atom c o n t a i n •»• Rydberg  eV),  5s and  transi-  3d  (1.51  C l e a r l y the 4s t r a n s i t i o n i s not (but  see d i s c u s s i o n on C Is  2 c o u l d be i d e n t i f i e d w i t h a t r a n s i t i o n  i n the p u r e l y atomic c a s e . that the 4s  f e a t u r e s 1 and  above.  s i l i c o n 2p  (2.36  2.0  to  however, t h i s i n v o l v e s a p •> p t r a n s i t i o n , which  (especially considering not expected and  any  f o r the 2p  eV), 4p  observed i n the S i L - s h e l l s p e c t r a spectrum).  thus p r e c l u d i n g  (d) a p p l i c a b l e f o r t h i r d row  t i o n s would be expected: 4s 5p (1.18  the  a l s o been suggested i n the case of band B  i n g m o l e c u l e s , the f o l l o w i n g term v a l u e s  eV),  of  However, u s i n g approximate quantum d e f e c t s of  2  (s),  -  Thus a s t r o n g  t r a n s i t i o n i s seemingly absent) i s  2 are b e t t e r a s c r i b e d to the  T h i s i n t e r p r e t a t i o n i s supported by  spectra  [65,144] where no  l e v e l s have been a s s i g n e d .  In any  transition  case,  a  orbitals  those g i v e n  on  t r a n s i t i o n s to p Rydberg  the 4p Rydberg l e v e l , being  of  * t  2  symmetry, i s l i k e l y  to mix  with  the  a ( t ) orbital  to form a  2  * o" ( t ) / 4 p mixed valence-Rydberg s t a t e 2  structure c l e a r l y Indicate  [65].  The  width and  the presence of valence  l a c k of  character.  Tentative  assignments based upon a l l of the above c o n s i d e r a t i o n s are g i v e n Table  4.1.  Moving on  to the s t r u c t u r e i n the r e g i o n of and  2p edge i t would appear that the f e a t u r e C i n the S i F spectrum [6] Is absent i n TMS. a  a (2e)  As  4  and  in  beyond SiCl^  s t a t e d e a r l i e r peak C i s l i k e l y  i n n e r w e l l s t a t e , resonance enhanced [73,75,77].  the  This  to  be  - 134  i n t e r p r e t a t i o n i s supported [65,146] and  -  by the absence of such a peak i n S i H ^  c r y s t a l l i n e S i [146,156].  A r e c e n t study  asymmetry parameter, 6, f o r photoemission and  Si(CH ) 3  4  ( i . e . TMS)  [158]  of  from the S i 2p l e v e l of S i F ^  shows marked d i f f e r e n c e s i n 8 f o r the  s p e c i e s i n the r e g i o n 5-16  the  two  eV above the r e s p e c t i v e 2p i o n i z a t i o n edges -  e x a c t l y where peak C i s s i t u a t e d i n S i F ^ [144-146] but a p p a r e n t l y i n S i ( C H ) ^ (see F i g u r e 4.2).  The  3  i s expected direct  to quantum m e c h a n i c a l l y  i o n i z a t i o n continua  to peak C i n S i F ^  i n t e r f e r e w i t h the u n d e r l y i n g  thus c a u s i n g v a r i a t i o n s i n 8 compared  the simpler continuum processes A broad  s t a t e corresponding  absent  i n t h i s r e g i o n i n the case of  2p with  TMS.  continuum f e a t u r e can be seen i n the S i 2p s p e c t r a shown  * i n F i g . 4.3  ( d e s i g n a t e d as D).  T h i s can be a s s i g n e d  shape-resonance f e a t u r e [73,159].  As was  to a d - l i k e  a(7t ) 2  d i s c u s s e d i n s e c t i o n F of  Chapter 2, the p o s i t i o n of t h i s f e a t u r e should be r e l a t e d i n some way the bond d i s t a n c e between the i o n i s e d atom and  i t s neighbour(s).  Indeed, w i t h i n a simple k i n e t i c s c a t t e r i n g p i c t u r e the resonance t i o n from the i o n i s a t i o n edge ( 6 ) should vary l i n e a r l y with R~ is  the bond d i s t a n c e  placed  [96]  (see e q u a t i o n  to  (1.F.3)).  Natoli  posiwhere R  2  [97,98]  has  t h i s r e l a t i o n s h i p on f i r m e r t h e o r e t i c a l grounds but has i n d i c a t e d  that the k i n e t i c energy of the e l e c t r o n ( 6) should be r e f e r e n c e d intramolecular potential the atoms i n v o l v e d .  (V ) Q  Hitchcock  - see e q u a t i o n et a l . [100]  to an  (1.F.4), which depends on have shown that a  simple  l i n e a r r e l a t i o n s h i p ( e q u a t i o n 1.F.5) i s adequate to r e l a t e bond d i s t a n c e and  resonance p o s i t i o n i n a s e r i e s of hydrocarbons.  In view of  these  - 135  d i s c u s s i o n s , the  r e l a t i o n s h i p of the  -  resonance p o s i t i o n (D - F i g .  f o r these S i - c o n t a i n i n g compounds w i t h bond l e n g t h has T a b l e 4.2  summarises the  l i t e r a t u r e sources.  The  relevant  data from F i g . 4.3  energies  of the  c l o s e to s t r a i g h t l i n e s  (see F i g . 4.4)  6 = 38.0 6 = -14.7  I t can be species  IT  2  + 6.4  R + 45.1  been examined.  and  other  f e a t u r e s D ( i . e . the  n e n t ) as a f u n c t i o n of S i - l i g a n d bond d i s t a n c e of the  4.3)  give points  t  compo-  2  that l i e  forms  ( r = 0.973) ( r = -0.965)  seen that at l e a s t f o r t e t r a h e d r a l type s i l i c o n - c o n t a i n i n g  the i n t u i t i v e l y more reasonable  l e a s t as good a l i n e a r f i t as by H i t c h c o c k et a l . [100]  [96-98] R  - 2  v a r i a t i o n gives  at  the more e m p i r i c a l R dependence s e l e c t e d  i n the case of carbon-carbon s i t e  containing  molecules. A l s o shown i n F i g . 4.4 dependencies f o r the and  S i 0 ) and 2  C and  f o r R and  f e a t u r e s C, which are i n the continuum ( i . e . ,  R~  2  SiF^  a l s o that f o r S i C l ^ , which i s a d i s c r e t e t r a n s i t i o n below  the S i 2p edge ( 6 = -0.8 features  are the c o r r e s p o n d i n g p o i n t s  eV).  I t Is a l s o of i n t e r e s t that the p l o t s f o r  D are e s s e n t i a l l y p a r a l l e l i n each r e p r e s e n t a t i o n .  From  the l i n e a r dependencies e s t i m a t e s have been made of the expected p o s i t i o n s of type C f e a t u r e s = +1 edge. it  eV)  i s at ~107  The  i s not  eV  i n SiH^ and  i n the very  Si(CH )i 3  +  the p r e d i c t e d p o s i t i o n ( 6  broad peak ( F i g . 2) around the S i  f e a t u r e C i s g e n e r a l l y weaker than D and apparent i n S i H ^ where D i s a l r e a d y  t h i s i s probably  r a t h e r weak.  The  2p why  l a c k of  - 136  -  Shape-resonance ( f e a t u r e s C and D, F i g . 4.3) p o s i t i o n s , resonance term values ( 6 ) and bond lengths f o r r e l a t i o n s h i p s shown i n F i g . 4.4. p o s i t i o n of Molecule shape-resonance ( e V ) ^ ' a  D Sl(CH ) 3  k  Si 2 p IP (eV)  ( b )  Resonance term value 6 = E - IP (eV)  C  124.1(5)  Bond l e n g t h R ( A )  D  C  R-2  -1.0+  0.284  8.1+  107.1+  106.1(1)  18.0(6)  ( c )  R 1.875(2)  SiH\  131.9(10)  113.5+  107.3  24.6(10)  0.456  1.481(1)  S1C1„  125.8(10)  109.6  110.4(1)  15.4(11)  -0.8  0.245  2.019(4)  Si0  2  129.5(5)  115.0  108.5(10)  21.0(15)  6.5  0.386  1.61  SiF^  133.0(5)  117.2  111.9(1)  21.1(6)  5.3  0.414  1.554(4)  (d)  (a)  S i t C H j ) ^ t h i s work; S i F ^ - Ref. [144]; S i H , S i C l , S i 0  (b)  S K C H ^ , SiH , S i C l S i F , - r e f . [30]. 0.2 eV has been added to the S i 2p-jy2 values given i n r e f . [30] to give an 'average' S i 2p value f o r the IP. S i 0 - r e f . [160]. 5.0 eV has been added to the values given i n r e f . [160] to b r i n g the IP to the vacuum l e v e l see r e f . [161] f o r a d i s c u s s i o n of t h i s c o r r e c t i o n procedure.  4  4  M  4  2  - r e f . [73].  4  2  (c)  SiCCHj)^, S i H ^ , S i C l , S i F ^ - r e f . [162], S i 0  (d)  This p o s i t i o n was estimated from the spectrum of S i H  4  Estimated from F i g . 4.4 using the R~  2  2  - ref.. [163].  dependence.  4  given i n r e f . [146].  F i g u r e 4.4: R e l a t i o n s h i p of bond l e n g t h to shape resonance ( f e a t u r e s C and D on F i g u r e 4.3) term value (6) f o r s i l i c o n c o n t a i n i n g m o l e c u l e s . V a r i a t i o n s are shown of 6 with 1/R ( l e f t hand s i d e ) and R ( r i g h t hand s i d e ) ; observed values are shown as s o l i d c i r c l e s w i t h the open c i r c l e s b e i n g estimated values from the l i n e a r p l o t s as shown. 2  -  prominent i n g due  138  f e a t u r e s i n the continuum  -  of the S i H ^ spectrum i s not  to the low s c a t t e r i n g power of the ( s m a l l ) H l i g a n d s .  surprisThis  o b s e r v a t i o n lends f u r t h e r g e n e r a l support to the atom-atom s c a t t e r i n g viewpoint f o r u n d e r s t a n d i n g resonance f e a t u r e s i n m o l e c u l a r s p e c t r a . The above r e s u l t s f o r t e t r a h e d r a l l i g a n d systems  surrounding s i l i c o n  c l e a r l y support the e x i s t e n c e of a simple r e l a t i o n s h i p between resonance p o s i t i o n and the i o n i z e d  ( e x c i t e d ) atom-ligand i n t e r n u c l e a r  The carbon Is ( K - s h e l l ) spectrum of TMS an energy r e s o l u t i o n of 0.36  eV FWHM.  separation.  i s shown i n F i g u r e 4.5  The i n s e r t  at  shows a more d e t a i l e d  view of the lower energy r e g i o n at h i g h e r r e s o l u t i o n m (0.21 eV FWHM). To date no o t h e r carbon Is spectrum of TMS  has been r e p o r t e d .  g i e s , term v a l u e s and p o s s i b l e assignments are shown i n T a b l e  The 4.3  t o g e t h e r w i t h the carbon Is b i n d i n g energy as determined by XPS 153].  The spectrum i s s t r i k i n g l y s i m i l a r i n appearance  spectrum of CH^  two peaks  i n TMS  those i n  (i.e.  are 3.70  (1+2)  The  and 3) are 3.40  eV and 2.70  eV.  to the C Is  3p ( t ) symmetry r e s p e c t i v e l y . 2  The former t r a n s i t i o n  t r a n s f o r m as both a^ and t 2  eV and 2.47  eV whereas  2  [67].  (i.e.  and  and so d i p o l e allowed  o r b i t a l s can be expected.  transitions  to 3s  However i n TMS  ( a p the C  transi-  A l t e r n a t i v e l y i f the  spectrum i s a n a l y s e d i n terms of a s u b s t i t u t e d methane, the symmetry i s C,  first  l e v e l s which are of 3s (a^) and  i s allowed i n CH^ o n l y by v i b r o n i c c o u p l i n g  t i o n s to both a^ and t  term v a l u e s f o r the  consi-  The t r a n s i t i o n s i n CH^ have been  a s s i g n e d as going to the lowest Rydberg  Is o r b i t a l s  [152,  [67,72] and i n t h i s r e g a r d the a n a l y s i s c o u l d be  dered i n terms of a s u b s t i t u t e d methane.  ener-  local  from the Is (a^) o r b i t a l are d i p o l e  -  139 -  is n  23  n—i-i  r  12  5  r^1  3 4  5  (CH ) S 3 4  CARBON Is  A  A E = 0.2leV  \  i  • i i  264  -•s  286  I i i — i  2B8  290  292  1 — i — 294  A E = 0.36eV  i  290  i  I  300  I  I  310  I  I  320  I  L_  330  ENERGY LOSS(eV)  F i g u r e 4.5: Carbon Is e l e c t r o n energy l o s s spectrum of t e t r a m e t h y l s i l a n e . D e t a i l s a r e g i v e n i n Table 4.3.  - 140 -  Table 4.3 Energies, term values and possible assignments of features i n the C ( l s ) spectrum of S i ( C H ) 3  Feature  4  Energy eV  Term Value  Possible Assignment  286.26  3.52  Is + 4s  286.51  3.27  Is -»• 4s  287.31  2.47  Is + 4p Is + o * ( a , t )  (v=0) (v-1)  1  2  -289 .6 Is l i m i t  289.78  a  ~303  0  Is shape resonance (o*(7t )) 2  a  References [152,153].  *  Estimated uncertainty ±0.05 eV for peaks 1-3.  - 141  -  allowed to l e v e l s of a^ or e symmetry.  Thus peaks (1+2)  and  3 can be  a s s i g n e d as t r a n s i t i o n s from the C Is to a Rydberg 4s (a^) o r b i t a l and Rydberg 4p (a^,3) l e v e l ^ .  The  i n c r e a s e of i n t e n s i t y f o r the  a  former  t r a n s i t i o n as compared to that i n C r ^ i s a t t r i b u t a b l e to i t being d i r e c t l y allowed i n s t e a d of v i a a v i b r o n i c c o u p l i n g mechanism. A c l o s e r look a t the f i r s t of two  components (1+2)  has a l s o been observed  f e a t u r e shows that i t c l e a r l y  s e p a r a t e d by ~0.25  eV.  consists  T h i s type of phenomenon  i n halogen mono-substituted methanes [63,64]  where a s e p a r a t i o n of 0.30  eV was  observed.  These were a s s i g n e d as the  v=0  and v=l components of the c o r r e s p o n d i n g C Is •* ns (a^) t r a n s i t i o n s  and  t h e r e f o r e f e a t u r e s 1 and 2 i n the C Is spectrum of TMS  a s s i g n e d i n a s i m i l a r manner.  have been  Compared to the o t h e r s u b s t i t u t e d  methanes f e a t u r e 3 i s broader and l a c k s the v i b r a t i o n a l s t r u c t u r e which can be c l e a r l y seen i n the o t h e r s [63,64].  I t i s suggested  that f e a t u r e  3 c o n s i s t s of the Is •> 4p Rydberg t r a n s i t i o n which l i e s on top of broader C Is ->• a the S i 2p spectrum  t r a n s i t i o n ( a ^ and t ) «  The  2  (see Table 4.1)  r e l a t i v e i n t e n s i t i e s and  agrees w i t h t h i s assignment.  s p e c t r a l shapes  i s mainly due  may  The  (1+2)  l a r g e peak marked 5 on  to u n r e s o l v e d h i g h e r Rydberg l e v e l s c o n v e r g i n g  on the C Is i o n i s a t i o n edge. i o n i s a t i o n continuum  The  of f e a t u r e 3 and f e a t u r e s  a r e c o n s i s t e n t w i t h such an i n t e r p r e t a t i o n . F i g u r e 4.5  term v a l u e o b t a i n e d from  A low i n t e n s i t y broad f e a t u r e (6) i n the  be a s s i g n e d as a shape resonance  (probably  o*(7t )). 2  ^  In the case of TMS the lower Rydberg l e v e l s are d e s i g n a t e d as 4s 4p s i n c e the c e n t r a l atom Is s i l i c o n .  and  - 142 -  I t i s of i n t e r e s t  (a±)  Rydberg  to note the complete absence of a S i 2p -*• 4s  t r a n s i t i o n s i n c e no f e a t u r e s appear i n the spectrum at the  expected term v a l u e (~3  eV).  where the f i r s t  l e v e l belongs to the l i g a n d s as opposed to the  Rydberg  molecule as a whole.  Examples  t i e s of t r a n s i t i o n s to Rydberg orbitals.  T h i s appears to be an example of a case  have been seen [69] where the p r o b a b i l i l e v e l s are low compared to v a l e n c e  T h i s i s the case when there e x i s t s an e f f e c t i v e  b a r r i e r l e a d i n g to i n n e r w e l l and o u t e r w e l l s t a t e s . case f o r TMS Rydberg  potential  T h i s i s not the  as can be seen from the C Is spectrum which shows a normal  type s t r u c t u r e u n l i k e the F Is s p e c t r a of S F  6  [69] and  SiF  lt  [145], both of which e x h i b i t r e l a t i v e l y i n t e n s e i n n e r w e l l type s t a t e s .  2.  Valence S h e l l  Spectrum  The v a l e n c e s h e l l spectrum of TMS F i g u r e 4.6 4.4.  and summarized,  between 6 and 29 eV i s shown i n  a l o n g w i t h t e n t a t i v e assignments, i n T a b l e  The i o n i z a t i o n l i m i t s shown on F i g u r e 4.6  are taken from measure-  ments made by p h o t o e l e c t r o n s p e c t r o s c o p y [149,164].  Previously  p u b l i s h e d v a l e n c e s h e l l e x c i t a t i o n s p e c t r a , o b t a i n e d by UV extend o n l y as f a r as ~9.2  eV  [167], ~10.5  eV  absorption,  [143] and ~11.3  eV [168].  The spectrum r e p o r t e d here shows f o u r d i s t i n c t bands w i t h p a r t i a l l y r e s o l v e d f i n e s t u c t u r e c l e a r l y e v i d e n t i n each band.  These  a r i s e from t r a n s i t i o n s to unoccupied v i r t u a l v a l e n c e l e v e l s Rydberg  features and/or  levels. In o r d e r to i d e n t i f y which f e a t u r e s a r i s e from Rydberg  t i o n s two assumptions have been made.  Firstly  transi-  i t has been assumed  that  (CH ) S,  |10  11  8  3  I  VALENCE  12  4 -fc 'c  3 4  56  3  13  o  2  15  a be  m  nd L .3  1  P  4  3 4  a be  m  CO LU  ?  nd |_  3 4  ns I  4t,  AE = 0.035 eV  5 6 ndi  3 4  ns I •. L  J  L  5 6 J  -5t,  nd(t ) i 2  np  L  10  3 4 4 56  Y  5  a  f  i L j i i 15 ENERGY LOSS (eV) J  20  Figure 4 . 6 : Valence s h e l l e l e c t r o n energy l o s s spectrum of t e t r a m e t h y l s l l a n e . Estimated p o s i t i o n s of the f i r s t few members of the Rydberg s e r i e s a r e shown below the spectrum. D e t a i l s are g i v e n i n Table 4 . 4 .  - 144  -  Table 4.4 Energies and Possible Assignments of Valeria? Shell Transitions of SKO^  Feature Observed* Energy (eV)  Predicted Energies of Rydberg transitions (eV)  4P  4s a 1 b c a 2 b c  7.04 7.34 7.64 8.60 8.93 9.20  3d  IP ( e v )  a  4d  6.90. 7.22k 5t ) 7.5 ' 2  8.62, 8.95 K5t ) 9.2 ' 2  8.78 78 9, l l f ( 5 t ) 9.4 2  9.37, 9.70K5t ) 10.0 ' 2  10.05 10.4  10.29 (4t ) 2  10.61 11.00  10.7  10.62 (5c,) 10.9  11.0 ) (l  t l  )  11.2) 11.5 (le)  11.50 11.81  11.8 (lt ) r  12.14  12.1  12.0 (4t,)  12.4  12.3  12.3  (le)  (4t,) 12.6  12.7 12.9  12.86  I  ^lt.)  12.9 (4t ) 7  10 11  13.26 13.7  13.1 (53^  13.2 13.9  13.2 (le) 13.8 ) (4t ) 14.1)  (Sa^  2  14.4 (Sa^ 14.7 15.7  14.7  (5s ) L  ^ Originating orbitals are given In parenthesis after the estimated transition energies. a  Binding energies from ref. [149] except le [164], Order of orbitals as per ref. [164-165].  * Estimated uncertainty +0.03 eV.  15.6  (Sa^  - 145  the  term values  VSEELS and  f o r the Rydberg l e v e l s are  ISEELS s p e c t r a .  T h i s was  From the C Is spectrum of TMS value  f o r the  Rydberg l e v e l  -  discussed  ( F i g u r e 4.5,  ( l o c a l i s e d - see  t r a n s f e r a b l e between i n the  previous  T a b l e 4.3)  Rydberg l e v e l ( f e a t u r e 3) Is 2.47  the mean term  eV and  eV.  that f o r the  levels respectively. p l e g i v e the 3d the  5s and  The  S i 2p  term v a l u e .  so the v a l u e  1.21  eV  spectrum ( F i g u r e 4.2)  of the 3d eV.  Robin  has  Secondly, s i n c e TMS  i s of T^  as a^ f o r the s l e v e l s ,  t  so 1.67  4d  symmetry and  term values  resolved  the  eV) 0.92  e + t  from to  term regardeV  Rydberg  respectively.  the Rydberg o r b i t a l s  f o r the p l e v e l s , and  2  eV and  c a l c u l a t e d using  formula) have been used f o r the 3d and  levels,  (1.67  -  from the 3d quantum d e f e c t  Rydberg  in princi-  noted t h a t the  f o r the lowest d l e v e l Is c l o s e to 13,500 cm *  (obtained  gives  can o n l y be e s t i m a t e d  [170]  l e s s of the compound's c h e m i c a l nature and  5p  should  l e v e l i s not  term v a l u e  the  (l.B.l))  f o r the 5s and  However, the 3d  have an upper l i m i t of -4.8 value  eV and  ( l o c a l i s e d ) 4p  Using these to c a l c u l a t e  quantum d e f e c t by means of the Rydberg formula ( e q u a t i o n T of 1.51  chapter.  f o l l o w i n g d i s c u s s i o n of C Is spectrum) 4s  ( f e a t u r e (1+2)) i s 3.40  r i s e to term v a l u e s  the  2  f o r the  transform d  the f o l l o w i n g valence-Rydberg t r a n s i t i o n s are d i p o l e allowed  the b a s i s of symmetry  considerations:  t *1»  e  » t  a , L  ->• n s ( a )  2  c  1  l> t  l f  e,  fc  t  l  2  f  2  + np(t ) 2  •*• nd(e) t  2  -»• n d ( t ) 2  on  - 146  -  However, t r a n s i t i o n s which would be f o r m a l l y d i p o l e f o r b i d d e n i n the case of atomic systems ( i . e . s ->• s, p assumed to e x h i b i t  p, d + d ,  the case where the MO's  1, s e c t i o n E ) .  (A) type of  [164].  The HOMO o r b i t a l i s the 5 t  2  l e v e l which i s a bonding  comprised mainly of C 2p and S i 3p parentage t i o n s to the n s ( a ^ ) l e v e l s should be seen. l e v e l to be J a h n - T e l l e r s p l i t  t i o n e n e r g i e s of 10.29, 10.62 transitions  T h i s i s most l i k e l y to be  are made up of predominantly one  heavy atom atomic o r b i t a l  2  d) have been  l e s s i n t e n s i t y than those which are d i p o l e a l l o w e d i n  the atomic case (see Chapter  the 5 t  s  and  [149,164] and The UPS  orbital so  spectrum  transi[149] shows  i n t o t h r e e components w i t h  10.90  eV.  Thus the 5 t  ioniza-  -»> n s ( a )  2  1  c o u l d be expected to show t h r e e J a h n - T e l l e r components  separated by ~0.3  eV w i t h e s t i m a t e d e n e r g i e s of 6.90,  r e s p e c t i v e l y f o r the f i r s t VSEELS spectrum o f TMS separated by ~0.3  Rydberg l e v e l .  The  first  7.22  and 7.50  eV  f e a t u r e i n the  does indeed show evidence of three components  eV w i t h e n e r g i e s of ~7.0,  f o r e these have been a s s i g n e d a c c o r d i n g l y .  ~7.3 The  and ~7.6  eV and t h e r e -  c e n t r e of the  second  f e a t u r e has an energy of 8.93  eV which g i v e s an e s t i m a t e d term v a l u e of  1.69  eV).  eV ( i . e .  10.62  eV - 8.93  due predominantly to 5 t  2  F e a t u r e 2 i s thus a s s i g n e d as b e i n g  * 3d t r a n s i t i o n s .  Again the f e a t u r e i s seen t o  c o n s i s t of s e v e r a l components ( t h i s i s c l e a r e r i n the UV  spectrum  r e p o r t e d i n r e f . [167]).  splitting  and/or  T h i s may  be due  to J a h n - T e l l e r  to t r a n s i t i o n s to both the 3d(e) and 3 d ( t ) l e v e l s .  t h i s f e a t u r e (2) a l s o c o i n c i d e s w i t h t r a n s i t i o n s  2  However,  from the 5 t  9  l e v e l to  - 147 -  the 5s(aj^) l e v e l . expected  The symmetry allowed  t o occur a t ~6.1 eV.  No sharp  5t  2  •*• 4 p ( t )  t r a n s i t i o n would be  2  s t r u c t u r e i s observed  i n this  r e g i o n , c o n s i s t e n t w i t h the assumptions that the 5 t o r b i t a l i s mainly 2  of p c h a r a c t e r and hence has l i t t l e  p r o b a b i l i t y of t r a n s i t i o n s t o p  levels. The  r e s t of the spectrum has been a s s i g n e d  i n d i c a t e d i n Table 4.4 and F i g u r e 4.6. 5a ^ o r b i t a l s ,  i n a s i m i l a r manner as  With the e x c e p t i o n of the l e and  the b i n d i n g e n e r g i e s of the outer valence  e l e c t r o n s have  been taken  from the UPS spectrum r e p o r t e d by Jonas e t a l . [149].  Their  assignment  [149], based upon a CNDO/2 c a l c u l a t i o n , does not agree w i t h  the proposed assignment of the X-ray p h o t o e l e c t r o n spectrum r e p o r t e d by P e r r y and J o l l y  [164] based upon i n t e n s i t y c o n s i d e r a t i o n s and extended  HUckel c a l c u l a t i o n s as w e l l as v a r i o u s ab i n i t i o The  c a l c u l a t i o n s [165,166].  f e a t u r e a t 15.6 eV i n the UPS spectrum o r i g i n a l l y a s s i g n e d  orbital  [149] has subsequently  been a t t r i b u t e d t o the 5a ^ o r b i t a l , i n  accord w i t h the other assignments  [164-166].  i o n i z a t i o n p o t e n t i a l g i v e n by Perry and J o l l y present work.  t o the l e  The value of the l e [164] has been used i n the  F o l l o w i n g the 5 t o r b i t a l are the I t , l e and 4 t 2  o r b i t a l s which have been c o n s i d e r e d [149,164] and thus  to possess  t o have mainly  2  C-H bonding c h a r a c t e r  a l a r g e C 2p component.  The 4 t  2  orbital,  however, does have some S i 3p c h a r a c t e r a t t r i b u t e d t o i t [149,164],  The  i n t e n s e f e a t u r e c e n t r e d a t p o s i t i o n 4 i s a t t r i b u t a b l e to the 4 t •* 2  4s(ai) transition. attributed  Features  5-8, which are much l e s s i n t e n s e , can be  t o t r a n s i t i o n s from these  the 3d l e v e l s .  The symmetry allowed  three o r b i t a l s  (lt^,  l e and 4 t ) t o 2  t r a n s i t i o n s to the n p ( t ) 2  levels  - 148  have been assumed to have l i t t l e  -  or no i n t e n s i t y s i n c e they would  i n v o l v e predominantly p •*• p t r a n s i t i o n s .  The f i n a l o u t e r v a l e n c e  o r b i t a l i s the 5a! which c o n s i s t s mainly of S i 3s and C 2p orbitals  [149,164].  The  i n t e n s e s t r u c t u r e around  atomic  5aj^ -* 4 p ( t ) t r a n s i t i o n c o i n c i d e s w i t h the v e r y 2  f e a t u r e s 9 and  z a t i o n l i m i t s of the l e , l t j and 4 t  2  10 which  i s a l s o where the  o r b i t a l s occur.  the 5a^ o r b i t a l to the 3 d ( t ) l e v e l s may 2  Transitions  also contribute.  ionifrom  I t can be  seen that the t r a n s i t i o n e n e r g i e s and s t r u c t u r e s observed i n the spectrum a r i s i n g from the v a r i o u s o r b i t a l s support the assumptions at the b e g i n n i n g of t h i s s e c t i o n and i n Chapter  made  1, s e c t i o n 5 c o n c e r n i n g  the r e l a t i v e i n t e n s i t i e s of a l l o w e d and f o r b i d d e n " a t o m i c - l i k e "  transi-  t i o n s i n the l i g h t of the suggested atomic o r b i t a l compositions of the various molecular o r b i t a l s  [147,164],  G e n e r a l l y the agreement between  the observed f e a t u r e s and the p r e d i c t e d p o s i t i o n s of the t r a n s i t i o n s are q u i t e good. t r a n s f e r a b i l i t y of Rydberg  T h i s g i v e more support to the concept of term v a l u e s between i n n e r s h e l l and v a l e n c e  e l e c t r o n e x c i t a t i o n s as d i s c u s s e d i n Chapter 3. s a i d about v a l e n c e - v a l e n c e t r a n s i t i o n s . be a t t r i b u t e d  to valence-Rydberg  d e r a b l e u n d e r l y i n g i n t e n s i t y due cannot be r u l e d out.  3  So f a r n o t h i n g has been  While much of the s t r u c t u r e  can  t r a n s i t i o n s the p o s s i b i l i t y of c o n s i to broad v a l e n c e - v a l e n c e t r a n s i t i o n s  T h i s c o n c l u s i o n was  the VSEELS spectrum of NF .  valence-Rydberg  reached i n the assignment  T h i s i n t e r p r e t a t i o n would a l s o be  tent w i t h t h a t f o r the S i 2p s h e l l s p e c t r a of TMS  of  consis-  which has been  a s s i g n e d as c o n s i s t i n g of o v e r l a p p i n g t r a n s i t i o n s to both v a l e n c e and Rydberg  levels.  Thus v a l e n c e - v a l e n c e t r a n s i t i o n s a r i s i n g from MO's  with  -  149 -  Si character ( i . e . :  5 t , 5a^) could be expected.  The i n t e r p r e t a t i o n o f  Rydberg t r a n s i t i o n s  on top of v a l e n c e t r a n s i t i o n s  i s also  w i t h the s p e c t r a l  2  intensity distribution.  s i o n s can be made u n t i l TMS molecule.  good q u a l i t y  consistent  However, no d e f i n i t e  calculations  conclu-  have been made f o r the  - 150 -  CHAPTER 5  ELECTRONIC EXCITATIONS IN PHOSPHORUS CONTAINING MOLECULES» I.  INNER SHELL ELECTRON ENERGY LOSS SPECTRA OF PH,, P(CH,),, P F  I t was seen i n the p r e v i o u s  q  AND PCI,.  chapters  that the l i g a n d has a  profound e f f e c t on the i n t e n s i t y d i s t r i b u t i o n s observed i n i n n e r electron excitation spectra.  T h i s was c l e a r l y seen when c o n t r a s t i n g t h e  S i L - s h e l l spectrum of S i C C H j ) ^ w i t h o t h e r F i g . 4.3).  shell  s u b s t i t u t e d s i l a n e s (see  As a c o n t i n u a t i o n of these s t u d i e s , the energy l o s s s p e c t r a  of s e v e r a l phosphorus c o n t a i n i n g compounds are now r e p o r t e d .  To date  there have been no e l e c t r o n impact s t u d i e s on the i n n e r s h e l l e x c i t a t i o n s p e c t r a o f any phosphorus compounds and only l i m i t e d s t u d i e s on P C 1 [65,146].  3  [171-173], 0PC1  3  [173,174], SPCI3 [173] and P H  3  [60,61,65,170],  In t h i s c h a p t e r ,  s p e c t r a of the t r i v a l e n t phosphorus compounds PX f o r the P L - s h e l l r e g i o n s  coordinate  3  However, t h e r e have been s e v e r a l t h e o r e t i c a l d i s c u s s i o n s on  the P 2p spectrum of PH  Following  photoabsorption  chapters  3  the ISEELS  (X = H, F, C l and  (F and C K - s h e l l , C l L - s h e l l ) a r e  w i l l deal with  3  presented.  the ISEELS s p e c t r a of the h i g h e r  phosphorus compounds P F , 0 P F 5  3  and 0PC1  3  and w i t h  the VSEELS  s p e c t r a of some of these compounds.  Experimental D e t a i l s . The  CH )  s p e c t r a were a l l recorded  on the ISEELS spectrometer  - 151 -  d e s c r i b e d i n Chapter  2.  Unless otherwise  s p e c t r a were o b t a i n e d u s i n g an impact  s t a t e d i n the t e x t , a l l the  energy  of 2.5 keV w i t h the  s c a t t e r e d e l e c t r o n s sampled a t ~1° s c a t t e r i n g a n g l e . s p e c t r a were c a l i b r a t e d a g a i n s t the N 401.10 eV.  A l l the P 2p  (N Is -*• i t , v = l ) f e a t u r e a t  2  The o t h e r s p e c t r a were i n t e r n a l l y c a l i b r a t e d a g a i n s t  their  r e s p e c t i v e P 2p f e a t u r e s .  RESULTS AND  DISCUSSION  Phosphorus L - S h e l l (2p and 2s) S p e c t r a - General The in Fig. FWHM.  long range s p e c t r a of the P 2p,2s (L s h e l l ) r e g i o n are shown  5.1.  The s p e c t r a were recorded a t a r e s o l u t i o n of 0.36 eV  The a s s i g n e d i o n i s a t i o n edges are taken from XPS v a l u e s  Only the P 2p average  p o s i t i o n s of the Zp-^/Z  [31] due to the l i m i t e d energy  resolution.  1/2 ^  s t a t i s t i c a l w e i g h t i n g of 2:1 to p r e d i c t 3  2  (2p - 0.30 eV) and 2 p ^  procedure  2  o u  ^^  e t  w  e  r  [31,175].  reported  e  T h e r e f o r e an e s t i m a t e d  s p i n - o r b i t s p l i t t i n g of 0.90 eV has been used  2p /  Features  [ 6 1 ] , along w i t h a  the p o s i t i o n s of the r e s p e c t i v e  (2p + 0.60 eV) edges.  Using  this  the v a l u e s o b t a i n e d i n the case of PH^ agree w e l l w i t h  those  r e p o r t e d by Schwarz [ 6 1 ] . Before examining  each spectrum  i n d e t a i l , i t i s of i n t e r e s t to  note the g e n e r a l s i m i l a r i t i e s between the long range s p e c t r a p r e s e n t e d here  (Fig.  (see F i g . broad The  5.1) and the s p e c t r a of the c o r r e s p o n d i n g 4.3).  Each phosphorus L s h e l l spectrum  silicon  compounds  ( F i g . 5.1) shows a  continuum s t r u c t u r e ( a t ~150 - 160 eV i n the phosphorus  f e a t u r e was a t t r i b u t e d to a d - l i k e shape-resonance  series).  [73,77] i n the  s i l i c o n s e r i e s and presumably an i n t e r p r e t a t i o n of a s i m i l a r s t r u c t u r e  -  -i  1 T  1  n  r  3 5  10  1  1  1  152 -  1  1  1  1  1  1  i  i  i  r  P 2p edge 14  A E = 0.36eV  PHOSPHORUS 2p,2s REGION  5  PF,  10  PCI,  £ <  IO  PH,  5 H  P 2p edge  10 H  P(CH ) 3  3  5H  ~ r — i — i — ' — i — ' — i — ' — i — ' — i — • — i — • — i — 130 140 150 160 170 180 190 200  1  — n ~ 210  E N E R G Y L O S S (eV) F i g u r e 5.1:  Phosphorus 2p,2s wide range e l e c t r o n energy l o s s s p e c t r a o f PF , P C 1 , PH and P ( C H ) . A l l s p e c t r a were o b t a i n e d w i t h an impact energy of 2500 V, a s c a t t e r i n g angle ~ 1 ° , and a r e s o l u t i o n of 0.36 eV FWHM. 3  3  3  3  3  - 153 -  can be given h e r e . and  P containing  (i)  S i m i l a r i t i e s between the s p e c t r a of r e s p e c t i v e S i  compounds i n the d i s c r e t e p a r t of the spectrum a r e :  the h y d r i d e s  both show a broad f e a t u r e  followed  by Rydberg  s t r u c t u r e l e a d i n g t o the edges (ii)  the f l u o r i d e s show two w e l l separated  major bands, the second  w i t h apparent Rydberg s t r u c t u r e on top (iii)  the c h l o r i d e s both show merging bands w e l l below the edge  (iv)  a l l the d i s c r e t e s t r u c t u r e s i n the methyl compounds are v i r t u a l l y on top of one another and very  (v)  c l o s e to the edge  Both the f l u o r i d e s and c h l o r i d e s show a s t r o n g [73]  or resonance s t a t e  [77].  inner w e l l  trapped  I n the f l u o r i d e s t h i s i s j u s t  above the edge, w h i l e i n the c h l o r i d e s i t i s r i g h t a t the edge. I t can thus be seen that the l i g a n d s have a very both s e r i e s . The  These i d e a s w i l l be f u r t h e r d i s c u s s e d  assignment of the d e t a i l e d s p e c t r a  similar effect i n  i n a later  section.  f o r each molecule a r e now  considered.  Phosphorus 2p and 2s S p e c t r a The  - D i s c r e t e Regions  m o l e c u l e s , PX , a r e o f C^ 3  v  symmetry.  In a minimum b a s i s s e t  (d o r b i t a l s excluded) the empty m o l e c u l a r o r b i t a l s a r e of a^ and e symmetry ( t h e -CH lists  3  group has been c o n s i d e r e d  as one u n i t ) .  the d i p o l e allowed t r a n s i t i o n s t o these l e v e l s .  orbitals  transform  T a b l e 5.1  Since  as a^ and e, and the P 2s o r b i t a l i s of a±  the P 2p symmetry,  t r a n s i t i o n s from both these o r b i t a l s t o both v i r t u a l o r b i t a l s a r e allowed.  In order  t o a s c e r t a i n the p o s i t i o n s of the t r a n s i t i o n s t o the  - 154  -  TABLE 5.1 T r a n s i t i o n s from the A 1  Final Configuration * hole  state  a  l  a  l  Symmetry  l  Yes  e  E  Yes  l  E  Yes  o* o r b i t a l  a  l  a  e  e  e  e  respectively.  3 v  Dipole Allowed from ground s t a t e  occupied  » (^p^^)"  Ground S t a t e f o r C  F i n a l State  e  The ( 2 p 3 / 2 ) ~  1  A  Aj + E A  a n <  * (2s)"~  2  Yes No  h o l e s a r e of e, a, and a^ symmetry  The o* o r b i t a l s a r e of a^ and e symmetry.  - 155 -  v i r t u a l o r b i t a l s i n the P L - s h e l l assumptions have been made. (i)  (i.e.,  2p and 2s) s p e c t r a two  Namely:  The major f e a t u r e i n the 2s energy l o s s i s assumed t o be 2s •+•  * a (e) t r a n s i t i o n (ii)  The term v a l u e s f o r f e a t u r e s i n the P 2s spectrum a r e assumed to be t r a n s f e r a b l e to the P 2p spectrum. The f i r s t  assumption i s based upon the make-up of the a ( e )  *  *  o r b i t a l as compared to the a ( a ^ . The a ( e ) o r b i t a l should have a l a r g e r p r o p o r t i o n of phosphorus 3p o r b i t a l c h a r a c t e r (mainly 3p , 3p ) x y and l i t t l e  phosphorus  3s o r b i t a l c h a r a c t e r i n comparison w i t h the a ( a j )  o r b i t a l , which i s mainly 3s, 3p *» z  T h i s i s supported by CNDO/2  c a l c u l a t i o n s we have performed, as w e l l as by Xa c a l c u l a t i o n s  [176].  Thus t r a n s i t i o n s from an s o r b i t a l t o the a ( a ^ l e v e l s h o u l d be weaker than those t o the a ( e ) l e v e l s i n c e the former would have a l a r g e r s •*• s component which i s f o r m a l l y d i p o l e f o r b i d d e n i n the case of atomic systems. The second assumption c o n c e r n i n g term v a l u e t r a n s f e r a b i l i t y has been d i s c u s s e d e a r l i e r  (Chapter 1, s e c t i o n E; Chapter 3 ) . The 2s and 2p  c o r e - h o l e v a c a n c i e s a r e on the same atom and t h e r e f o r e the e l e c t r o n s should see v i r t u a l l y  the same c e n t r a l c o r e p o t e n t i a l .  However, s m a l l  d i f f e r e n c e s i n term v a l u e may occur due to the d i f f e r e n t  shielding  Here and i n a l l cases the p r i n c i p a l a x i s of the molecule has been d e s i g n a t e d as the z a x i s .  - 156 -  c a p a b i l i t i e s of the s and p o r b i t a l s . The in  2p and 2s r e g i o n s  of the s p e c t r a shown i n F i g . 5.1 a r e shown  d e t a i l i n F i g s . 5.2-5.5, which a r e d i s c u s s e d  molecule i n the f o l l o w i n g s e c t i o n s .  The P 2p r e g i o n s  were run a t a r e s o l u t i o n o f 0.18 eV FWH w h i l e extracted  from the l o n g range s p e c t r a  t i o n o f 0.36 eV.  and presented  f o r each  i n F i g s . 5.2-5.5  the 2s r e g i o n s  have been  ( F i g . 5.1) which a r e a t a r e s o l u -  The r e l a t i v e energy s c a l e s i n F i g s . 5.2-5.5 a r e the  same f o r the r e s p e c t i v e 2p and 2s s p e c t r a and these have been a l i g n e d according  t o t h e i r r e s p e c t i v e i o n i z a t i o n edges which were determined  from XPS measurements as d e s c r i b e d obtained (Fig.  above.  The 2s s p e c t r a as shown were  by s u b t r a c t i n g a l i n e a r ramp background from the t o t a l  spectrum  5.1) so as to more c l e a r l y d i s p l a y the s p e c t r a l f e a t u r e s .  cases ( F i g s .  In a l l  5.2-5.5 i t can be seen t h a t the s p e c t r a l f e a t u r e s a r e much  broader f o r the 2s s p e c t r a than f o r the 2p case.  T h i s broadening i s f a r  beyond that a t t r i b u t a b l e t o the d i f f e r e n c e s i n energy r e s o l u t i o n (0.18 vs.  0.36 eV) which would i n any case appear n e g l i g i b l e on the energy  s c a l e of the f i g u r e s .  Furthermore, there  t r a n s i t i o n s apparent i n the 2s s p e c t r a . (assigned  seem to be few i f any Rydberg The extremely broad peak  predominantly to the a ( e ) l e v e l ) i n each of the 2s s p e c t r a i s  a t t r i b u t a b l e to the o c c u r r e n c e of a f a s t a u t o i o n i s a t i o n process gous t o an L L 1  vacancy f i l l e d electron).  2 3  M Coster-Kronig  Auger t r a n s i t i o n  analo-  ( i . e . , an i n i t i a l  from w i t h i n the same s h e l l p l u s e j e c t i o n of a v a l e n c e  In the ISEELS s p e c t r a the process  e x c i t e d s t a t e w i t h a 2s vacancy being i o n i s a t i o n of a v a l e n c e  electron.  filled  would i n v o l v e an i n i t i a l by a 2p e l e c t r o n w i t h  The r e l a t i v e l i f e t i m e  broadening  auto-  - 157  (1.2  -  eV e x t r a width) observed i n the case of argon f o r the 2s XPS  r e l a t i v e to that f o r the 2p peak (see r e f . [17], page 4) lends to t h i s argument.  Phosphine  support  (See a l s o Chapter 1, s e c t i o n B I V ) .  (PH,)  PH  i s the s i m p l e s t molecule presented  3  r e s u l t s f o r the P 2p and I t has  peak  here and  the d e t a i l e d  2s s p e c t r a are shown i n F i g . 5.2  been the s u b j e c t of s e v e r a l e a r l i e r s t u d i e s and  and  Table  discussions  [60,61,65,146,170] which have mostly f o c u s s e d  on the 2p r e g i o n  little  spectrum recorded  or no  treatment on the 2s r e g i o n .  The  5.2.  with i n the  present work i s i n good agreement w i t h the s l i g h t l y h i g h e r r e s o l u t i o n XUV  s p e c t r a r e p o r t e d by Hayes and  al.  [65].  reported  Brown [146]  and  a l s o by F r i e d r i c h et  F r i e d r i c h et a l . [65] have compared the v a r i o u s i n the l i t e r a t u r e  concur w i t h the c o n c l u s i o n s ( F i g . 5.2,  Table  5.1)  [60,61,146,170]. reached by  can be a s s i g n e d  T h e i r own  c a l c u l a t i o n s [65]  Schwarz [60,61], as 2p •*• a  assignments  Thus peaks  t r a n s i t i o n s followed  the v a r i o u s 2p •+ Rydberg t r a n s i t i o n s l e a d i n g up to the edge. of the  a  *  l e v e l s , according  f o l l o w e d by [177]  but  studies  (5.21  order  T h i s i s i n agreement w i t h e a r l i e r Xa-SW c a l c u l a t i o n s  case that both l e v e l s are very c l o s e . eV)  The  * [60,61,65] i s a (a^)  c o n t r a r y to the more r e c e n t Xoc-DV c a l c u l a t i o n  c l e a r i n any value  a (e).  to these  1-3  f o r f e a t u r e 13 which i s a s s i g n e d  2s spectrum ( F i g . 5.2)  [176].  However, the  It i s term  as 2s + a (e) i n the  i s c l o s e to that f o r peak 1 (5.05  eV)  and  f o r e i n d i c a t e s t h a t the r e c e n t Xa-DV o r d e r i n g i s l i k e l y c o r r e c t .  thereThis  by  - 158 -  T E R M V A L U E (eV) 8  -4  0 T  T  P 2 p  i—i—r  1  2 3  Aedges  «  ,  R  PH,  —  4 56 78 9 10  P2p 11  AE=O.I8eV 36  132  140 P 2 s  13  PH, P2s  edge  . .* *  AE=0.36eV "1 188  '  1 192  ENERGY F i g u r e 5.2:  ^—I  1  196  T~ 200  LOSS(eV)  Phosphorus 2p and 2s e l e c t r o n energy l o s s s p e c t r a of PH . The P 2p spectrum (upper t r a c e ) i s at h i g h r e s o l u t i o n (0.18 eV FWHM). The P 2s spectrum (lower t r a c e ) i s e x t r a c t e d from F i g . 5.1. The s p e c t r a are a l i g n e d w i t h r e s p e c t to the 2p(mean) and 2s i o n i s a t i o n edges. 3  - 159 -  TABLE 5.2 Energies, Term Values, and Possible Assignments for the P 2p,2s Spectra of PH 3  Energy Loss (a)  Feature  Term Value  (eV)  (eV) 2p  1 2 3 4 5 6 7 8 9 2p 2p  ,l i m i t ll i m i t ( > 11 12 b  3 / 2  b  1 / 2  Possible Assignments^^  132.00 132.78 133.46 134.67 135.12 135.36 136.06 136.43(12) 136.96 137.42(12) 137.05 137.95  2p  3 / 2  5.05 4.27 3.59 2.38 1.93 1.69 0.99 0.62  2p  1 / 2  5.17 4.49 3.28 2.83 2.59 1.89 1.52 0.99 0.53  3 / 2  2p  a  1 / 2  (aj)  4s 3d 5s, 4d 6s etc  4s 3d 5s, 4d 6s etc  0 0  141.4(4) 156.7(5)  -4. -19.  "shake-up" "shake-up"  2s 1  3  l(  2s l i m i t '  N.  c ;  189.67(15) 194.88  5.21 0  predominantly  2s •+ a (e)  (a) Estimated uncertainty i n energy-ljss values i s ± 0.08 eV except where stated. are calibrated against N (Is •* n , v • 1) at 401.10 eV.  Spectra  2  (b) The spin-orbit s p l i t t i n g of 0.90 eV [61] has been used to estimate the 2 p / spin-orbit components from the 2p (mean) values [31], see text for d e t a i l s . 3  (c) Ref. [175]. (d) F i n a l occupied o r b i t a l with either 2 p / 3  2  t  See Fig. 5.8.  *  With respect to the 2p (mean) edge [31].  or 2pjy  2  hole state.  2  and 2pjy  2  - 160 -  assignment cannot be c o n s i d e r e d as c o m p l e t e l y c o n c l u s i v e due to the assumptions d i s c u s s e d above.  I t should be noted that the f e a t u r e  buted to the 2s edge by Hayes and Brown i n the spectrum of PH in  fact  the d i s c r e t e pre-edge f e a t u r e  3  attri-  [146] i s  (13) observed i n the p r e s e n t ( F i g .  5.2).  T r i m e t h y l Phosphine The d e t a i l e d  (P(CH3),) 2p and 2s s p e c t r a of P ( C H ) 3  3  are shown i n F i g . 5.3  and the s p e c t r a l p o s i t i o n s and p o s s i b l e assignments summarised 5.3.  The 2p spectrum c o n s i s t s of a number of o v e r l a p p i n g  a l l w i t h i n 3.5 eV of the edge, and i s thus d i f f i c u l t unambiguously.  There are no obvious Rydberg  valence-Rydberg mixing [20,65]. transition  (feature  c l o s e l y with that  transitions,  to a s s i g n  s e r i e s , whch may  10, F i g . 5.3) i s 3.38  eV.  T h i s corresponds very  (3.30 eV) f o r the f i r s t f e a t u r e i n the 2p spectrum a  Thus the o r d e r i n g of the v i r t u a l o r b i t a l s i s i n d i c a t e d t o  *  *  be a (e) f o l l o w e d by the a (a^) as was discussion).  T h i s i s i n agreement  found f o r PH  3  (see p r e c e e d i n g  w i t h the recent Xoc c a l c u l a t i o n s  which f i n d s the lowest unoccupied o r b i t a l w i t h any s i g n i f i c a n t rus  indicate  The term v a l u e f o r the 2s •+• a (e)  which has a c c o r d i n g l y been a s s i g n e d as the 2p •> ^^2/2^~ transition.  i n Table  [176]  phospho-  c o n t r i b u t i o n to be e i n c h a r a c t e r . The remainder of the spectrum i s d i f f i c u l t  to a s s i g n .  i n many ways the s p e c t r a l shape i s i n keeping w i t h what might expected i f the 2p + a  However, be  f e a t u r e s were superimposed on the Rydberg  -  161  -  T E R M V A L U E (eV) 8  -4  0  P ( C H3'3 J P2p  P2p , 2p . edges *  i rr  • II III -,\:/ 1 2 34 5 6  7  8  AE=O.I8eV 32  136  140  P 2s edge  10  P ( C H3'3 J P2s  AE=0.36eV 188  192  ENERGY F i g u r e 5.3:  196  LOSS(eV)  Phosphorus 2p and 2s e l e c t r o n energy l o s s s p e c t r a o f P ( C H ) . The P 2p spectrum (upper t r a c e ) i s a t h i g h r e s o l u t i o n (0.18 eV FWHM). The P 2s spectrum (lower t r a c e ) i s e x t r a c t e d from F i g . 5.1. The s p e c t r a a r e a l i g n e d with r e s p e c t t o the 2p(mean) and 2s i o n i s a t i o n edges. 3  3  - 162 -  TABLE  5.3  E n e r g i e s , Term V a l u e s , and P o s s i b l e Assignments f o r the P 2p,2s S p e c t r a of P ( C H ) 3  Feature  Energy  Loss^ ^  Term Value  a  (eV)  2p  3 / 2  2 j/2 P  limit  ^  Hmit  ( b )  9  Possible  Assignments^^  (eV) 2p  1 2 3 4 5 6 7 8  3  132.65 133.09 133.79 134.05 (12) 134.67 135.03 135.9 (2) 136.8 (3) 135.95 136.85  2p  3 / 2  3.30 2.86 2.16 1.90 1.28 0.92 0  1 / 2  3.76 3.06 2.80 2.18 1.82 0.9 0  2p  3 / 2  a*(e) 4s A 3d, B 5s 4d edge  2  Pl/2  .  o-\e) 4s A 3d, B 4d edge  0 0  153.0 ( 5 )  -16. 7  Shape-resonance  t  2s  2s  limit  K c  >  190.23 (15) 193.61  3.38 0  predominantly  (a) E s t i m a t e d u n c e r t a i n t y i n e n e r g y - l g s s v a l u e s i s ± 0.08 eV except are c a l i b r a t e d a g a i n s t N ( I s •* n , v =» 1) a t 401.10 eV.  where  2s •* o*(e)  stated.  Spectra  2  (b) The s p i n - o r b i t s p l i t t i n g of 0.90 eV [61] has been used to e s t i m a t e the 2 p / s p i n - o r b i t components from the 2p (mean) v a l u e s [ 3 1 ] , see t e x t f o r d e t a i l s . 3  2  and  2p^  2  ( c ) The 2p(mean)-2s s e p a r a t i o n was.taken to be 57.36 eV based upon the average of o t h e r P 2p(mean)-2s s e p a r a t i o n s [175]. T h i s v a l u e i s w i t h i n 0.17 eV of a l l the o t h e r s e p a r a t i o n s [175]. (d) F i n a l o c c u p i e d o r b i t a l w i t h e i t h e r 2 p / or 2 p j / h o l e s t a t e . Note t h a t f o r peaks 3-6 two a l t e r n a t i v e assignments (A or B) a r e g i v e n ( s e e t e x t ) f o r the two components w i t h o ( a j ) as the f i n a l o r b i t a l . 3  With r e s p e c t t o 2p (mean) edge [ 3 1 ] .  2  2  - 163  f e a t u r e s i n the PH basis features  3  spectrum (compare F i g . 5.3  1, 3-4,  and  6 or 1, 3, and  r e l a t e d to f e a t u r e s  1, 2, and  case,  1 and  then f e a t u r e s  -  3 i n the PH  3 ( F i g . 5.3)  w i t h F i g . 5.2).  5 i n the P ( C H ) 3  3  spectrum.  On  could  3  this  be  I f t h i s i s the  can be assigned  as the  two  * s p i n - o r b i t components of the  2p -»• o (e) t r a n s i t i o n and  and  or f e a t u r e s 4 and  5 (Scheme A, Table  to the two  5.3),  components of the 2p •*• a ( a ^  s p e c t r a l i n t e n s i t y would then be due Feature  - 1  t r a n s i t i o n with  8  the concomitant 2p •*• (,2p^^)~^As defect  i s 1.34  2p •+• ( 2 p ^ 2 )  _ 1  5  s  eV,  a l s o be expected.  Table  Phosphorus T r i f l u o r i d e The  2p  5.3)  r e s t of  the  the  a c o n t r i b u t i o n from  Based upon the quantum  of f e a t u r e 2, the p r e d i c t e d  5s  f e a t u r e 5 to a l s o be a s s i g n e d  as  5.2  summarises the p o s s i b l e assignments.  (PF,)  spectrum of P F  3  ( F i g . 5.4) 3  3  There i s a c l e a r d i f f e r e n c e i n the  i s rather d i f f e r e n t  ( F i g . 5.3).  2s s p e c t r a are shown i n F i g . 5.4  and  The  data  from that f o r both  summarised i n Table  2p s p e c t r a which may  be a s c r i b e d  the e l e c t r o n e g a t i v e  l i g a n d as compared to the e l e c t r o n donating  ligand.  are c l e a r l y t r a n s i t i o n s to v i r t u a l valence The  -CH  3  Features  o r b i t a l s , as i n d i c a t e d by  second band ( f e a t u r e s 4-7)  i s probably  the  5.4.  the e f f e c t s on the valence-Rydberg s e p a r a t i o n by  l a r g e term v a l u e s .  the  T r a n s i t i o n s to the d Rydberg l e v e l s would  of the i s o e l e c t r o n i c molecule P ( C H ) 2p and  The  i n both schemes to  transition.  which allows  transition.  transition.  f e a t u r e 4 having  c a l c u l a t e d from the term v a l u e  term v a l u e  6 (Scheme B, Table  3  to v a r i o u s Rydberg t r a n s i t i o n s .  2, f o r i n s t a n c e , would be due  2p •*• ( 2 p _ j y 2 ) ^  either features  to F 1-3 the  comprised  - 164 -  T E R M V A L U E (eV) 8  4  i  i  i  0  i  i  P  I  I  3  2  P  i  , /  i i  i-i  i  4 5  67  8 9 10 11  I  1 2  ^  2  i  -4  i E  2  D  r  A  i  E  PF  S  i  I I  1213  i  14  (ARB ITRARY UNIT  -  v— .  J  v  AE=O.I8eV i  |  I  |  136  INTENSI  ,,  i*  >  *  ...»  17  18  * * *V  V 192  *•» - *  196  ENERGY F i g u r e 5.4:  |  1  144  \  P2s  P2sedge  4  .« *  l  140  ,  UJ  REl  • i  >  i  •  3  P2p  CO  ....  i  • «*-,  *•* **V  •  .»»**^*  . 4  V  V.  200  %1 «« « '  * *> t«, •  AE=0.36eV  »  204  LOSS(eV)  Phosphorus 2p and 2s e l e c t r o n energy l o s s s p e c t r a o f P F . The P 2p spectrum (upper t r a c e ) i s a t h i g h r e s o l u t i o n (0.18 eV FWHM). The P 2s spectrum (lower t r a c e ) i s e x t r a c t e d from F i g . 5.1. The s p e c t r a are a l i g n e d w i t h r e s p e c t t o the 2p(mean) and 2s i o n i s a t i o n edges. 3  - 165 -  TABLE  5.4  E n e r g i e s , Term V a l u e s , and P o s s i b l e Assignments f o r the P 2p,2s S p e c t r a of P F j  Feature  Energy  Term Value  Loss^ ^ 3  2p / 3  1 2 3 4 5 6 7 8 9 10 11 limiti ( limlt< >  135.00 . 135.61 136.52 138.0 (2) 138.64 139.45 139.83 (10) 140.61 141.30 (12) 142.03 (15) 142.7 141.77 142.67  12 13 14  143.74 (20) 144.25 (20) 145.9 ( 3 )  15  157.3  b  b  Assignments^^  U iV)  (eV)  2p,/, 2v\',\  Possible  2p  2  6.77 6.16 5.25 3.77 3.13 2.32 1.94 1.16 0.47  2  1 / 2  P3/2  2pi/  2  o*(e) 7.06 6.15 4.67 4.03 3.12 2.84 2.06 1.37 0.64 0  o  (e)  o*(e) o*(a ) 4s 1  a  3d 5s, 4d  (aj) 4s  3d 5s, 4d  edge edge  0 0 "Shake-up" -3.8  (5)  inner-well state/shaperesonance shape-resonance  f  -15.2  T  2s 16 17 2s  (a) E s t i m a t e d  limit  ( c )  192.60 (15) 196.21 (20) 197.53 (20) 199.49  6.89 3.28 1.96 0  2s -» o ^ e ) 2s •+ a ( a ^ ) 2s •»• 4p  u n c e r t a i n t y i n e n e r g y - l o s s v a l u e s i s ± 0.08 eV except  are c a l i b r a t e d  against N  2  where s t a t e d .  ( I s •* n*, v = 1) at 401.10 eV.  (b) The s p i n - o r b i t s p l i t t i n g o f 0.90 eV [61] has been used to e s t i m a t e the 2 p - y s p i n - o r b i t components from the 2p (mean) v a l u e s [ 3 1 ] , see t e x t f o r d e t a i l s . ( c ) R e f . [175]. (d) F i n a l  occupied  Spectra  o r b i t a l with e i t h e r  2p /  * w i t h r e s p e c t t o the 2p (mean edge) [ 3 1 ] .  3  2  or 2 p j /  2  hole  state.  a n t 2  *  2  Pl/2  - 166 -  of t r a n s i t i o n s to the v a r i o u s Rydberg l e v e l s .  However, the broad  shape  of the l e a d i n g edge ( f e a t u r e 4) and a l s o the width of f e a t u r e 5 suggests t h a t t h i s band may  c o n s i s t of Rydberg t r a n s i t i o n s superimposed  a valence t r a n s i t i o n .  T h i s type of s i t u a t i o n was  observed  on top of  f o r the S i 2p  spectrum of S i F ^ [144] and i n view of the s i m i l a r ways i n which the l i g a n d s seem to a f f e c t it  i s thought The  the c e n t r a l atom i n the case of both P and S i ,  to be very l i k e l y  2s spectrum  here.  shows one i n t e n s e peak ( f e a t u r e  a s s i g n e d as 2s > a (e) (as i n the case of PH  3  16) which i s  and a l s o P ( C H ) ) and 3  3  the  term value (6.89 eV) f o r t h i s corresponds approximately to t h a t f o r the first  band (peaks  1-3)  i n the 2p spectrum.  f o l l o w e d by f u r t h e r s t r u c t u r e 3.28  T h i s f e a t u r e (16) i s  ( f e a t u r e 17) which has a term v a l u e of  eV, which corresponds w i t h that f o r f e a t u r e s 4-5  spectrum.  i n the 2p  Since the 2s •> 4s Rydberg t r a n s i t i o n would be expected to be  v e r y weak ( i t would correspond to a d i p o l e f o r b i d d e n s •*• s t r a n s i t i o n i n the atomic c o r e ) , t h i s f e a t u r e (17) has been a s s i g n e d as the 2s •+• a (a^) Thus on t h i s b a s i s the a (e) - a (a^) s e p a r a t i o n would  transition. quite large al.  (~3.5  eV).  T h i s agrees w i t h the Xa c a l c u l a t i o n s of Xiao e t  [176], which p r e d i c t a ground  i s presumably The 1, 2, and  the 2s •+• 4p Rydberg  first 3).  be  s t a t e s e p a r a t i o n of 3 eV.  shows t h r e e components  (peaks  t h i s has been t e n t a t i v e l y a s s i g n e d t o  the v a r i o u s components of the 2p -»• a (e) t r a n s i t i o n . assigned to the two allowed f i n a l  18  transition.  band of the 2p spectrum  As s t a t e d e a r l i e r ,  Feature  Peaks 1 and  s t a t e s of the 2p •> ( 2 p „  / 9  )  _ i  2 are  a (e)  - 167 -  transition  ( i . e . , e <->  2p •+• ( 2 p j ^ )  e, see T a b l e 5.1) and peak 3 to the  ( ) transition.  0  e  The r e s t  f e a t u r e s i n the spectrum are summarised  Phosphorus T r i c h l o r i d e  and PH ,  3  3  i n Table  5.4.  (PCI,)  The 2p spectrum of P C 1 for PF  of the assignments of the  3  ( F i g . 5.5), as i n the analogous  spectra  shows i n t e n s e s t r u c t u r e w e l l below the edge, which must  be due to t r a n s i t i o n s  to the v i r t u a l v a l e n c e o r b i t a l s because of the  l a r g e term v a l u e s ( T a b l e 5.5).  T h i s i s f o l l o w e d by weaker Rydberg  s t r u c t u r e l y i n g on top of a broad resonance i n the r e g i o n of the 2p edge.  The e l e c t r o n impact e x c i t e d spectrum r e p o r t e d here d i f f e r s  ficantly  from a p r e v i o u s l y r e p o r t e d o p t i c a l  ( s o f t X-ray) spectrum  172] i n that a prominant e x t r a peak ( f e a t u r e 4, F i g . 5.5) (compare  w i t h the o p t i c a l spectrum as shown i n F i g . 5.6).  hood of t h i s b e i n g due to an i m p u r i t y seems remote r e g i o n s i n c e any phosphorus likely rest the  i n this  signi[171,  i s observed The  likeli-  spectral  c o n t a i n i n g i m p u r i t y a r i s i n g from PC1  3  is  to be extremely i n v o l a t i l e and a p a r t from t h i s sharp f e a t u r e the  of the spectrum i s i n agreement spectrum was  different  carefully  w i t h the o p t i c a l r e s u l t .  rechecked u s i n g a new  sample  of P C 1  However, 3  from a  source and found to be i d e n t i c a l .  Another ( n o n - s p e c t r o s c o p i c ) p o s s i b i l i t y i s that the e x t r a peak (4) a r i s e s either the  from a " g h o s t i n g " e f f e c t  due to the primary e l e c t r o n beam  h i t t i n g the anode or some o t h e r s u r f a c e .  The p o s s i b i l i t i e s o f  peak b e i n g due to energy l o s s e s caused by s c a t t e r i n g from the anode  - 168  -  T E R M V A L U E (eV) 0  4  8  P  m — r 12  3  5  4  6  7  2  -  P3/2 P|/2ed9eS  4  2  \ *  8 910  PCI. P2p  >cc < LT  m cr <  AE=O.I8eV 32  136  140  00  PCI,  JL  LU  P 2 s edge  13  P2s  UJ >  AE=0.36eV  _ J  LU  cr  188  192  ENERGY F i g u r e 5.5:  196  200  LOSS(eV)  Phosphorus 2p and 2s e l e c t r o n energy l o s s s p e c t r a of P C 1 . The P 2p spectrum (upper t r a c e ) i s at h i g h r e s o l u t i o n (0.18 eV FWHM) The P 2s spectrum (lower t r a c e ) i s e x t r a c t e d from F i g . 5.1. The s p e c t r a are a l i g n e d w i t h r e s p e c t to the 2p(mean) and 2s i o n i s a t i o n edges. 3  - 169 -  TABLE 5.5 Energies, Term Values, and Possible Assignments for the P 2p,2s Spectra of PCI3 Energy Loss  Feature  v  '  Term Value  (eV)  (eV) 2p  1 2 3 4 5 6 7 8 9 10 2p l i m i t <|»> 2pj/2 H m i t 3 / 2  ( b )  11 12  132.98 133.50 134.11 135.11 136.84 137.61 138.25 139.10 139.57 139.98 139.84 140.74  Possible A s s i g n m e n t s ^  (12)  2p  3 / 2  6.86 6.34 5.73 4.73 3.00 2.23 1.59 0.74 0.27  (10)  (12) (10) (12)  1 / 2  7.24 6.63 5.63 3.93 3.13 2.49 1.64 1.17 0.76  0  2  ?3/2  2Pi/  2  a (e) a Ce}  Symmetry For'bidden Transition 4s 4s 5s, 3d on top of 6s, 4d 5s, 3d inner-well state/shape etc 6s, 4d resonance etc.  0  149.5 (5) 152-168  9  Shape rescjnance + "Shake—iJP"  2s 13 2s  limit  ( c )  191.07 (15)  6.40  197.47  0  predominantly  2s •+ a*(e)  (a) Estimated uncertainty i n energy-lgss values i s ± 0.08 eV except where stated. are calibrated against N (Is -• 11 , v = 1) at 401.10 eV.  Spectra  2  (b) The spin-orbit s p l i t t i n g of 0.90 eV [61] has been used to estimate the 2 p ^ spin-orbit components from the 2p (mean) values [31], see text for d e t a i l s . 3  2  and 2 p j /  2  (c) Ref. [175] (d) Final occupied o r b i t a l with either 2 p / or 2pjy hole state. Assignment i n table for pyramidal excited states. See text for discussion on planar excited states. 3  *with respect to the 2p (mean) edge [31].  2  2  - 170 -  can be e l i m i n a t e d  s i n c e i t would occur  the anode v o l t a g e  (~600 V, w i t h  at an energy l o s s e q u i v a l e n t t o  respect  to the cathode).  As a f u r t h e r  check the spectrum was a l s o r e r u n u s i n g a lower impact energy of 1500 V i n s t e a d o f the o r i g i n a l 2500 V, a procedure that i n v o l v e s r e t u n i n g of the spectrometer.  Since  e n t , any e f f e c t s of " g h o s t i n g " ,  complete  the c o n d i t i o n s would now be d i f f e r -  i f present,  would be expected to change.  No change i n the number or energy of the s p e c t r a l f e a t u r e s was observed. In view of these o b s e r v a t i o n s , forbidden  f e a t u r e 4 can be a s s i g n e d  as a d i p o l e -  t r a n s i t i o n , which i s observed i n e l e c t r o n impact  where the momentum t r a n s f e r i s f i n i t e .  A t the l i m i t  spectroscopy  of zero momentum  t r a n s f e r the o s c i l l a t o r s t r e n g t h of such a t r a n s i t i o n should i . e . , the ( d i p o l e ) o p t i c a l o s c i l l a t o r limit  [2,8].  go t o z e r o ;  s t r e n g t h v a n i s h a t the o p t i c a l  Under the c o n d i t i o n s used i n the present  work the momentum  t r a n s f e r i s kept as s m a l l as p o s s i b l e so as to ensure t h a t d i p o l e processes  s t r o n g l y dominate the s p e c t r a .  non-dipole  processes  In some e a r l i e r s p e c t r a weak,  have been observed under these c o n d i t i o n s  [69,109],  In the case of P C 1 , peak 4 i s r e l a t i v e l y much more i n t e n s e  than  p r e v i o u s l y observed d i p o l e - f o r b i d d e n t r a n s i t i o n s .  to confirm  3  In order  the o p t i c a l l y f o r b i d d e n nature of t h i s t r a n s i t i o n the spectrum was r u n at 1500 V impact energy and s e v e r a l d i f f e r e n t d e f l e c t i o n ( s c a t t e r i n g ) angles system.  up t o ~ 5 ° , which i s the l i m i t of the present  beam d e f l e c t i o n  A t an impact energy of 1500 V i t was p o s s i b l e t o tune the  spectrometer t o operate a t an even s m a l l e r  s c a t t e r i n g angle (and hence  c l o s e r to zero momentum t r a n s f e r ) than a t 2500 V due t o improved electron optical focussing.  Three s p e c t r a of P C 1  3  and P F  3  obtained at  -  v a r i o u s momentum t r a n s f e r s ( i . e . , 5.6.  The o p t i c a l a b s o r p t i o n  t r a n s f e r ) of P C 1  3  -  s c a t t e r i n g a n g l e s ) a r e shown i n F i g .  spectrum ( c o r r e s p o n d i n g  t o zero momentum  [171] i s a l s o shown f o r comparison and i t c l e a r l y  i n d i c a t e s the absence o f peak 4. o p t i c a l spectra are equivalent. linear function, extrapolated i n order  171  to f a c i l i t a t e  In a l l other  the ISEELS and  In the case of the ISEELS s p e c t r a a  from the l e a d i n g edge, has been  the estimation  nature of f e a t u r e 4 i n P C 1  respects  of peak h e i g h t s .  The n o n - d i p o l e  i s c l e a r l y apparent s i n c e i t s r e l a t i v e  3  i n t e n s i t y to the r e s t of the spectrum i n c r e a s e s markedly w i t h i n s c a t t e r i n g angle.  increase  From the dimensions of the d e f l e c t o r p l a t e s and  the magnitude of the a p p l i e d v o l t a g e  i t i s p o s s i b l e t o e s t i m a t e the  d e f l e c t i o n angle (9) f o r a g i v e n e l e c t r o n energy (see Table angles  subtracted  have been estimated  2.1).  These  f o r each of the s p e c t r a shown i n F i g . 6 and  used to o b t a i n the momentum t r a n s f e r , K, i n atomic u n i t s (a.u.) (see e q u a t i o n s l.C.6  and l.C.16).  I t i s usual to consider  the v a r i a t i o n o f  r e l a t i v e i n t e n s i t i e s o f allowed  (or forbidden)  p l o t of the r a t i o s  of f e a t u r e 4)/(peak h e i g h t  versus  K  2  should  (peak h e i g h t  extrapolate  feature 4 i s dipole forbidden  t r a n s i t i o n s with K . 2  A  of f e a t u r e 2)  back t o zero a t zero momentum t r a n s f e r i f [2,9].  T h i s behaviour i s found f o r  f e a t u r e 4 as i n d i c a t e d i n F i g . 5.7, which a l s o shows s i m i l a r p l o t s f o r some of the d i p o l e - a l l o w e d  f e a t u r e s of the P C 1 P 2p spectrum 3  (features  3, 5, and 6 r e s p e c t i v e l y of F i g . 5.6) over the l i m i t e d range of momentum t r a n s f e r p o s s i b l e with for  t h i s instrument.  In c o n t r a s t t o t h e behaviour  f e a t u r e 4 i t can be seen that the r a t i o s f o r the d i p o l e - a l l o w e d  t r a n s i t i o n s remain almost c o n s t a n t .  The steep  r i s e i n i n t e n s i t y of  - 172 -  F i g u r e 5.6:  Inner s h e l l e l e c t r o n energy l o s s s p e c t r a of (a) P C 1 and (b) PF3 at v a r i o u s s c a t t e r i n g a n g l e s . The s p e c t r a were o b t a i n e d w i t h an impact energy of 1500 V. The d i p o l e - f o r b i d d e n t r a n s i t i o n i n PCI3 i s marked w i t h an a s t e r i s k (*). A l s o shown w i t h the P C 1 ISEELS spectrum i s the o p t i c a l spectrum taken from r e f . [171]. 3  3  - 173 -  Scattering Angle 1  2  3  8 (degrees) 4  5  feature 3  0.0  0.2  0.4  0.6  (Momentum Transfer)  F i g u r e 5.7:  0.8 2  1.0  K (a.u.) 2  2  P l o t of the r a t i o (peak h e i g h t of f e a t u r e X)/(peak h e i g h t of f e a t u r e 2) f o r v a r i o u s t r a n s i t i o n s (X) i n the P C I spectrum ( F i g . 6) as a f u n c t i o n of (momentum t r a n s f e r ) . S o l i d c i r c l e s ISEELS data; s o l i d t r i a n g l e s - o p t i c a l data [171]. 3  -  - 174  feature 4 with increase i n  -  i s notable  than i s u s u a l l y observed f o r f o r b i d d e n ( F i g . 5.6,  r i g h t hand s i d e ) show no  range of momentum t r a n s f e r . checked i n the same way The PC1  PF ,  P(CH )  3  3  and  3  s p e c t r a and  3  to " g h o s t i n g "  PH  and 3  provide  or o t h e r  The  Consider of PC1 .  The  PF  spectra  3  d i s c e r n a b l e changes over the same 3  and  P(CH ) 3  3  were a l s o  d i s c e r n a b l e changes were apparent.  were a l l run under the same c o n d i t i o n s as  the  f u r t h e r evidence t h a t peak 4 i s i n no way  due  instrumental  effects.  the sudden and  The  range of momentum  r a p i d emergence of peak 4 i s  without precedent i n e a r l i e r ISEELS s t u d i e s . now  the assignment of the spectrum P 2p,  Peaks 1-4  3  t r a n s i t i o n s [2],  s p e c t r a of PH  again no  t r a n s f e r i s q u i t e s m a l l and q u i t e unusual and  i n that t h i s i s much more r a p i d  are c l e a r l y due  to 2p •* a  levels.  2s  The  (Fig. term  5.5)  value  * f o r the  a (e) l e v e l from the  fact  that the term v a l u e  6.40  eV  2s spectrum ( f e a t u r e 13) i s 6.40  for feature  1 i s somewhat l a r g e r (6.86  suggests t h i s peak a r i s e s from the process  I f t h i s i s the case, a (a^) f o l l o w e d by Topol  by  2p  T h i s i s the  reverse  be given  However, s i n c e  c a l c u l a t i o n o n l y g i v e s a s e p a r a t i o n of 0.12  eV f o r the  l e v e l s the r e s u l t i s i n c o n c l u s i v e .  4 i s c l e a r l y a symmetry-  forbidden  Feature  a (a^) and  t r a n s i t i o n on the b a s i s of the evidence presented and  w i t h i n the C ^  p o i n t group of the p y r a m i d a l molecule, i s 2p(e) •*•  (2p,.  )  - 1  a  ( e ) , with  an A  The  2  only dipole-forbidden  above  a l s o F i g s . 5.6 v  5.7).  final  s t a t e (see Table  than  ( ^ p ^ ^ ) " °* (*].)•  of the assignment  based on an Xa-SW c a l c u l a t i o n .  The  eV)  the o r d e r i n g of the v i r t u a l l e v e l s would  a (e).  et a l . [172]  eV.  the  a (e)  (see  transition possible,  5.1).  The  only  other  - 175  -  p o s s i b l e e x p l a n a t i o n f o r f e a t u r e 4 would a r i s e i f there i s a geometry change i n going to some of the upper s t a t e s . most l i k e l y  be to s t a t e s of  interesting  to note  splitting  (i.e.,  I f t h i s occurs i t would  p l a n a r ) symmetry.  t h a t f e a t u r e 4 i s approximately  away from f e a t u r e 3.  context of a D_. system. Jn  It is  a spin-orbit  T h i s can be e x p l a i n e d w i t h i n the  T a b l e 5.6  shows the f i n a l c o n f i g u r a t i o n s  p o s s i b l e here f o r  symmetry and whether they would be  a c c e s s i b l e from a D^^  ground s t a t e .  optically (.^2/2^ -1  I t i s seen that o n l y the  *  a (e') ( t o an E rules.  1  final  s t a t e ) i s a c c e s s i b l e under d i p o l e s e l e c t i o n  Thus f e a t u r e 3 c o u l d be a s s i g n e d to the d i p o l e allowed i  2p •* (2P3/2)  * ( ')  0  t r a n s i t i o n w i t h f e a t u r e 4 as the  e  l  ( d i p o l e f o r b i d d e n ) 2p •*• (^P^^^ (Fig.  5.6)  and  concomitant  *  ( 'e  a  the angular v a r i a t i o n  1  transition.  ( F i g . 5.7)  f e a t u r e s (1-3) are c l e a r l y p r e s e n t .  From the s p e c t r a  three d i p o l e allowed  Hence the remaining  two d i p o l e l  allowed f e a t u r e s ( 1 , 2 )  to 2p  c o u l d be due  t r a n s i t i o n s w i t h pyramidal upper and be remembered that the i n i t i a l of  the f i n a l  (2p.j/2 1/2^  lower  states.  s t a t e i s of C^  s t a t e geometry ( i . e . ,  C^or  °  ^ l ^ a  However, i t should  symmetry and r e g a r d l e s s  v  D^),  *  t  n  e  selection  rules  apply, but w i t h the i n t e n s i t i e s of t r a n s i t i o n s being s t r o n g e s t f o r those which are allowed f o r both symmetries spectrum  (Fig.  5.6)  of the  no i n t e n s i t y at a l l i n view of the  on a l l peaks i n the spectrum.  f e a t u r e 4 i s of i n t e r e s t and more s y s t e m a t i c  Examination  optical  shows at most a weak u n r e s o l v e d shoulder at  p o s i t i o n 4 and more l i k e l y that i s apparent  [178] .  The  tailing  r a p i d emergence of  c l e a r l y t h e o r e t i c a l s t u d i e s , as w e l l as  - 17,6  -  TABLE 5.6 T r a n s i t i o n s from the *AJ Ground State f o r  Final Configuration * hole s t a t e  a*  occupied  D i p o l e Allowed from ground s t a t e  orbital AJ + A  No  e'  e'  e'  e'  E'  Yes  e'  a  2  E"  No  e'  E"  No  A'  No  a  2  a  2  (2p /2) 3  F i n a l State  Symmetry  a  an<  * ^p^/2^~  o* o r b i t a l s are of a 2  2*  holes are of e' and a and e' symmetry.  A  2  l  2  symmetry r e s p e c t i v e l y .  The  - 177 -  v a r i a b l e angle e l e c t r o n impact s p e c t r o s c o p y would b l i s h i n g the i d e n t i t y of t h i s  be h e l p f u l i n e s t a -  transition.  The remaining f e a t u r e s (5-10) can be a s s i g n e d t o the v a r i o u s 2p -*• Rydberg  transitions.  The l a t t e r few being on top o f an i n n e r w e l l  trapped s t a t e or resonance e v i d e n t by the l a r g e broad u n d e r l y i n g peak a t ~140  eV i n F i g . 5.5 (see f o l l o w i n g s e c t i o n ) .  The assignments are a l s o  summarised i n T a b l e 5.5.  Phosphorus All beyond  2p, 2s S p e c t r a - Continuum the phosphorus  Features  s p e c t r a show c o n s i d e r a b l e i n t e n s i t y a t or  the 2p edge ( F i g . 5.1).  In the case of P H  t h i s manifests i t s e l f  3  as an i n f l e c t i o n ~141.4 eV f o l l o w e d by a broad band w i t h a maximum a t ~157  eV.  T h i s has been a t t r i b u t e d t o a d e l a y e d onset caused by c e n t r i -  f u g a l b a r r i e r e f f e c t s a r i s i n g from p -»• continuum d - l i k e s t a t e tions [146].  [49,146].  This e f f e c t  i s seen i n atoms ( e . g . , S i ) as w e l l as S i H ^  A c l o s e r examination of the continuum  spectrum o f P H  s t r u c t u r e between the edge and the band maximum. to i o n i s a t i o n p l u s e x c i t a t i o n t i o n ) t h i s would PH  3  continuum  ("shake-up").  3  reveals  This i s a t t r i b u t a b l e  I n ISEELS  appear as onsets of new c o n t i n u a .  ( o r photoabsorp-  An expansion of the  spectrum i s shown i n F i g . 5.8 t o g e t h e r w i t h the s a t e l l i t e  ("shake up") p o r t i o n of the P H [175].  transi-  3  (2p) XPS spectrum measured elsewhere  Both s p e c t r a i n F i g . 5.8 a r e p l o t t e d on the same h o r i z o n t a l  r e l a t i v e energy s c a l e . the ISEELS  Much of the i n t e n s i t y i n the continuum  band o f  spectrum (upper t r a c e ) can be a s s i g n e d t o onsets of "shake-  up" c o n t i n u a , e v i d e n c e d by the peaks i n the XPS spectrum (lower t r a c e ) .  - 178 -  ENERGY LOSS (eV) — r  1  1  1  1  1  1  1  1  1  1  150  HO  |  1  1  1  1  160  1  I  I  170  * • • • >"  PH  3 /<  its*  .-.••/••  •  ••-'•>.•-•.  l,  ISEELS  "v  2p  /  XPS  ml  •V -  ]  •  1  0  1  *.  1  1  1  1  1  1  1  1  10  20  1  1  1  1  1  1  I  1  r  30  RELATIVE ENERGY (eV)  ure  5.8:  Expanded p l o t of the P H P 2p ISEELS continuum s t r u c t u r e . The 2p s a t e l l i t e s t r u c t u r e from XPS measurements [175] i s shown below p l o t t e d on the same r e l a t i v e energy s c a l e , r e f e r e n c e d t o the 2p (mean) edge. 3  - 179 -  A s i m i l a r e x p l a n a t i o n was used i n Chapter  3 to e x p l a i n continuum  struc-  t u r e s i n ISEELS s p e c t r a o f NF3. C o n s i d e r i n g now the other molecules whose s p e c t r a a r e shown i n F i g . 5.1, i t can be seen that the continuum different  to that i n P H  i n t o more l o c a l i s e d  3  s t r u c t u r e i s somewhat  w i t h the o s c i l l a t o r  channels.  s t r e n g t h being d i r e c t e d  T h i s i s d i r e c t l y a t t r i b u t a b l e t o the  e f f e c t s of the b u l k y , many e l e c t r o n l i g a n d s i n c o n t r a s t i n PH3.  Indeed,  to the s i t u a t i o n  compared to the other s p e c t r a , that f o r P H  atomic l i k e i n the continuum  The  i s v e r y s i m i l a r to those seen In  the S i s e r i e s i n F i g . 4.3 and the l o c a l i s e d s t r u c t u r e ( p a r t i c u l a r l y i n the case of P F  3  and i n t e n s e  continuum  and PCI3) p r o v i d e f u r t h e r  examples of p o t e n t i a l b a r r i e r or shape-resonance 3  i s very  (compare a l s o S i and S i H ^ [ 1 9 ] ) .  behaviour of t h i s s e r i e s of molecules  Thus both PCI3 and P F  3  phenomena  [73,77].  show an i n t e n s e , sharp f e a t u r e ( F i g . 5.1) a t  ( P C l j - f e a t u r e 10) or j u s t beyond ( P F - f e a t u r e 14) the edge which can 3  be d e s c r i b e d as a resonance d-type)  state.  enhanced t r a n s i t i o n to a trapped ( p r o b a b l y  A s m a l l peak i s a l s o seen r i g h t a t the edge i n P ( C H ) , 3  however, i t i s not as i n t e n s e as i n P F  3  or PCI3.  The methyl  ligand i s  electron-donating  [31,33] and so an e f f e c t i v e p o t e n t i a l b a r r i e r would  not be expected.  The lower i n t e n s i t y i s c o n s i s t e n t w i t h  this.  A l t e r n a t i v e l y , the f e a t u r e may simply be due to u n r e s o l v e d Rydberg l e v e l s c o n v e r g i n g onto the edge, or a combination of both  3  effects.  - 180  All  -  the t h r e e molecules show a broader s t r u c t u r e between 10 and  20 eV above the edge.  These can be a t t r i b u t e d to h i g h e r d - l i k e  shape  resonances, a l t h o u g h , as has been d i s c u s s e d above, the s t r u c t u r e i n  PH  3  can at l e a s t i n p a r t be a s s o c i a t e d w i t h shake-up p r o c e s s e s as a l s o observed i n XPS  [175].  The  shape of the PC1  i n that i t shows a sharp r i s e structure.  3  structure i s interesting  f o l l o w e d by f u r t h e r l a r g e l y u n r e s o l v e d  I t i s p o s s i b l e that t h i s shape i s due t o "shake-up"  phenomena on top of the resonance case of PH . 3  w i t h the XPS  s t r u c t u r e as suggested above i n the  I t would be i n t e r e s t i n g to compare the ISEELS satellite  spectrum,  spectrum  but as yet t h i s has not been r e p o r t e d .  In view of the r e c e n t spate of i n t e r e s t between p o s i t i o n and bond l e n g t h [95-100] and  resonance  the success of the l i m i t e d  series  of S i c o n t a i n i n g compounds shown i n the p r e v i o u s c h a p t e r , i t i s of i n t e r e s t t o examine whether such a r e l a t i o n s h i p a l s o e x i s t s f o r these compounds.  T a b l e 5.7  ces i n P F ,  PC1 ,  3  3  summarises the r e l e v a n t data f o r p o s s i b l e  and P ( C H ) . 3  The  3  lower energy  resonance  f o l l o w s the  t r e n d observed f o r s i l i c o n , however, the good l i n e a r c o r r e l a t i o n f o r the o u t e r resonance However, a d e t a i l e d  resonan-  i n the s i l i c o n s e r i e s i s not apparent  found  here.  c o n s i d e r a t i o n of t h i s phenomenan i n v o l v e s many  f a c t o r s such as d i f f e r e n t phase s h i f t s of the s c a t t e r i n g c e n t r e and v a r y i n g geometries  [99].  s i l i c o n s e r i e s may  reflect  silicon  compounds s t u d i e d .  of the PX,  s p e c i e s may  The  r e a s o n a b l e c o r r e l a t i o n observed i n the  the constant t e t r a h e d r a l geometry of a l l the The  be due  failure  of the c o r r e l a t i o n i n the case  to t h e i r v a r i a b l e geometry.  C l e a r l y more  TABLE  5.7  Resonance Term Values 6(eV) and (P-X) Bond Length  R(A)  Molecule  R (A)(a)  PF  1.563  0.4093  3.8  15.2  1.843  0.2944  0.5  16.7  2.043  0.2396  -0.1  3  P(CH ) 3  PCI3  3  R-2  (A-2)  6 (lower) ( e V ) <  b )  6 (higher) ( e V )  9.4  (a)  From Landholdt-BHrnstein (New S e r i e s ) II/7 " S t r u c t u r e Data of Free P o l y a t o m i c M o l e c u l e s " S p r i n g e r - V e r l a g , B e r l i n , 1976.  (b)  First  resonance p o s i t i o n .  Data from T a b l e s 5.3-5.5.  6 = Resonance Energy - I.P. (c)  As ( b ) , but f o r h i g h e r resonance.  ( c )  - 182 -  systematic  s t u d i e s need t o be made f o r a l a r g e group of molecules  before  more d e f i n i t e g e n e r a l c o n c l u s i o n s can be drawn.  Ligand  Spectra The  ( F I s , C I s , and C l 2p,2s)  i n n e r s h e l l s p e c t r a of the v a r i o u s l i g a n d r e g i o n s  (F I s , C  I s , and C l 2p,2s) a r e each t y p i c a l of s p e c t r a a s s o c i a t e d w i t h p a r t i c u l a r edge.  that  Each l i g a n d spectrum w i l l be d i s c u s s e d i n t u r n and  compared w i t h the r e s p e c t i v e c e n t r a l atom s p e c t r a . o r b i t a l o r d e r i n g s as d i s c u s s e d i n the p r e c e d i n g  The v i r t u a l  valence  P 2p,2s s p e c t r a have  been used i n the assignment o f the l i g a n d s p e c t r a . The Table  5.8.  by a broad  F Is spectrum of P F  i s shown i n F i g . 5.9 and summarised i n  The spectrum c o n s i s t s o f a r e l a t i v e l y i n t e n s e peak f o l l o w e d band w i t h some s t r u c t u r e both b e f o r e and a f t e r the edge.  p o s i t i o n of the edge i s taken o r b i t a l s of P F all  3  3  t r a n s f o r m as  the v i r t u a l o r b i t a l s  from XPS measurements [179].  The  The F Is  and e symmetries and so t r a n s i t i o n s t o  (a± and e) a r e p o s s i b l e .  *  Therefore,  the f i r s t  peak i s a s s i g n e d as the F Is •*• a ( e ) t r a n s i t i o n u s i n g the v i r t u a l o r b i t a l o r d e r i n g as a s s i g n e d  i n the P 2p,2s s p e c t r a .  term v a l u e o f 5.2 eV, which i s approximately the P 2s o r P 2p •*• a ( e ) t r a n s i t i o n . order as t h a t found (Chapter  1.5 eV lower than t h a t f o r  The d i f f e r e n c e i s of the same  between the N Is and F Is •*• a  3) and t h a t f o r the S 2p and F Is + o  As was p r e v i o u s l y noted,  T h i s peak has a  t r a n s i t i o n s i n NF  3  t r a n s i t i o n s i n S F [69]. 6  t h i s d i f f e r e n c e i s d i r e c t l y a t t r i b u t a b l e t o the  l o c a t i o n o f the core h o l e , s i n c e the c r e a t i o n of a core hole on the c e n t r a l atom e f f e c t i v e l y i n c r e a s e s the core charge by one, and hence the  - 183 -  AE=0.36eV  00  t-  0  FIs  cr < cr  '.Writ  FIs  CD  12  cr <  3  edge  4  5dt  i  1  1  700  1 725  1  i  1  L_ 10  cr  I 750  AE=0.36eV E F1s  LU > _1 . LU DK  I  edge  l—i—i—i—|—i—i—i—i—|—i—i—i—i—|—i—i—  690  695  700  1  1  I  '  1  1  r  705  ENERGY LOSS (eV)  F i g u r e 5.9:  Long-range and d e t a i l e d i n n e r s h e l l e l e c t r o n energy l o s s s p e c t r a of the F Is r e g i o n of P F . 3  - 184 -  TABLE 5.8  Energies, Term Values, and Possible Assignments for F Is Spectrum of PF,  (a)  Feature  Energy Loss  Term Value  (eV)  (eV)  1  688.98 (15)  5.2  o*(e)  2  691.22 (20)  3.0  o*(a )/Rydberg  3  693.64 (30)  0.6  Rydberg  694.2  0  Ionisation edge  F Is l l m l t 4  ( b )  699.6 (5)  Estimated uncertainty given i n brackets.  Ref.  Possible Assignments  [179].  -5.4  1  d-like shape resonance  - 185 -  a  o r b i t a l e n e r g i e s are determined  by a Z+l c e n t r a l c o r e , whereas t h e  c r e a t i o n of a p e r i p h e r a l core h o l e on the F l i g a n d has l e s s e f f e c t on  *  the energy  of the l o c a l i s e d  a  orbital.  Thus the e l e c t r o n i n a v i r t u a l  o r b i t a l w i l l have a h i g h e r term v a l u e when there e x i s t s a c e n t r a l h o l e as opposed to a p e r i p h e r a l h o l e .  The r e s t of the f e a t u r e s can be  a s s i g n e d i n a s t r a i g h t f o r w a r d manner.  F e a t u r e 2 i s a s s i g n e d , on the  b a s i s of i t s term v a l u e , as the P I s + fl ( a p t r a n s i t i o n , but may a l s o i n c l u d e Rydberg t r a n s i t i o n s . s e r i e s converging onto  F e a t u r e 3 i s probably due to Rydberg  the edge.  The weak continuum s t r u c t u r e (4) a t  699.6 eV i s presumably an i n n e r - w e l l  state/shape-resonance.  F i g u r e 5.10 shows the C Is spectrum summarised i n T a b l e 5.9. The  spectrum  i s similar  of P ( C H ) . 3  The C Is edge i s determined  3  The data i s from XPS [180].  to t h a t of CH^ [65] and to the C Is spectrum o f  SiCCHg)^ ( F i g . 4.5). I n a s s i g n i n g the spectrum  of S ^ C H g ) ^ i t was  u s e f u l to compare i t w i t h the C Is s p e c t r a of mono s u b s t i t u t e d methyl halides  [64].  A s i m i l a r process w i l l be used  here.  Features  1 and 2  are a s s i g n e d as the C I s * 4 s ( a ! ) and C Is + 4p(e) Rydberg t r a n s i t i o n s . In c o n t r a s t to the present case f o r P ( C H ) , the analogous 3  3  the methyl h a l i d e s p e c t r a were c l e a r l y r e s o l v e d . that the spectrum  recorded here  forP(CH ) 3  3  I t should be noted  was run at a h i g h e r  t i o n than f o r the methyl h a l i d e s p e c t r a [64]. w i t h the apparent  features i n  resolu-  With t h i s i n mind, and  r e l a t i v e c l o s e n e s s of the s and p l e v e l s ,  i t is  * suggested t h a t a C Is a t r a n s i t i o n i s also contained w i t h i n t h i s * Since the c e n t r a l atom i s from the t h i r d row ( i . e . , P ) , the f i r s t Rydberg l e v e l s a r e the 4s and 4p l e v e l s .  - 186  -P.  g Cis edge  n  2 4  r  P(CHJ 3'3  5  H  1 0  Cl s  tn  A E = 0.36eV  or < or CD  or <  5H  >CO  T—i—r  i  —I—i—i—i—i—i—i—i—i—i—i—i—i—i  r  300  290  LU  i—i—i LU >  1 2  10  310 & C1s edge  r  3  320  r  4  AE=O.I8eV  _l LU  cr 5H  286  T  288  T  T  290  ENERGY  Figure  292  294  LOSS(eV)  5.10: Long-range and d e t a i l e d i n n e r s h e l l e l e c t r o n energy l o s s s p e c t r a of the C Is r e g i o n of P ( C H ) . 3  3  - 187 -  TABLE 5.9  Energies, Term Values, and Possible Assignments for C Is Spectrum of P(CH_).  (a)  Feature  Term Value  Energy Loss  C Is  Possible Assignments  (eV)  1  (eV) 287.13  3.17  4s  2  287.83 (8)  2.47  4p(e)  3  288.31  1.99  4p(ai)  4  289.24  1.06  5p, 3d  limit(b)  * on top of a  290.30  5  293.5 (3)  -3.20  6  302.5 (5)  -12.20  (a)  Estimated uncertainty ± 0.10 eV except where stated.  (b)  Ref [180].  "shake-up" or resonance  - 188  feature. this  The term v a l u e s from  interpretation.  -  the P 2p spectrum  are c o n s i s t e n t w i t h  The core hole being l o c a l i s e d  on the C c o u l d v e r y  * e a s i l y lower  the term v a l u e f o r the  p r o x i m i t y of the  (e).  Indeed, w i t h the c l o s e  (e) and 4p(e) s t a t e s the spectrum  d e s c r i b e d i n terms of a mixed Rydberg-Valence s t a t e earlier  f o r simpler m o l e c u l e s .  might be b e t t e r [20] as d i s c u s s e d  F e a t u r e 3 Is a s s i g n e d as the  C I s + 4 p ( a ) t r a n s i t i o n and f e a t u r e 4 t o the C Is •* 5p and 3d Rydberg 1  levels.  Post-edge s t r u c t u r e can a l s o be seen and can be a t t r i b u t e d to  e i t h e r "shake-up" or resonances. hydrocarbon  s p e c t r a and l i k e l y  Finally  S t r u c t u r e 6 (~303 eV) i s seen i n many  to be a a (C-H) resonance  the C l 2p,2s spectrum  of P C 1  3  i s considered.  are shown i n F i g . 5.11 and summarised i n T a b l e 5.10. the C l 2 p ^ 2  a n t  * ^P]_/2 P i ~ 8  n  o r  bit  The p o s i t i o n s o f  [179] u s i n g s p i n - o r b i t  1.6 eV [181] along w i t h a 2:1 s p e c t r a l w e i g h t i n g . [179].  The spectrum  i s very s i m i l a r  the v a r i o u s chloromethanes  from  s e p a r a t i o n of  The C l 2s edge was  i s i n good agreement w i t h the  p r e v i o u s l y p u b l i s h e d C l 2p o p t i c a l a b s o r p t i o n spectrum T h i s spectrum  The s p e c t r a  components have been estimated  the r e p o r t e d C l 2p (mean) XPS v a l u e  taken from XPS data  [100].  of PCI [172,173] . 3  to other C l L - s h e l l s p e c t r a recorded f o r  [182].  F e a t u r e s 1 and 2 are a t t r i b u t e d  to the s p i n - o r b i t components o f  the 2p •*• a ( a ^ t r a n s i t i o n s and f e a t u r e s 2 and 3 to the 2p •+ a ( e ) . Since the a  l e v e l s a r e c l o s e , as I n d i c a t e d i n the P 2p spectrum, i t i s  p o s s i b l e t h a t 1 and 2 c o n t a i n a l l of these t r a n s i t i o n s .  The concept o f  u s i n g the C l 2s term v a l u e to estimate the p o s i t i o n s of the a components i s not as s t r a i g h t f o r w a r d here s i n c e the C l 2s o r b i t a l s  - 189 -  p  K  Mini  Cl 2p edges  I  I  PCI  AE=0.36eV  12345 7 8 9  3  Cl 2p,2s 2 s edge  1 10  WW  — I1111 1111 1111 1111 1 r 200  220  240  Cl 2p  2  2p  260  | / 2  edges  1—rr 4  280  I 8  5 6  AE=O.I8eV  -XL  —j 200  1  1 204  1  1 208  '  1 —' 212  1 216  E N E R G Y LOSS (eV)  Figure  5.11: Long-range and d e t a i l e d e l e c t r o n energy l o s s s p e c t r a of t h e C l 2p and 2s r e g i o n s of P C 1 . 3  - 190 -  TABLE 5.10 Energies, Term Values, and Possible Assignments for the Cl 2p,2s Spectra of PC1 3 Feature  Energy Loss^ a ^  Term Value  (eV)  (eV) 2p3/2  1 2 3 4 5 6 7  200.41 201.66 203.43 (8) 205.14 (12) 206.05 206.65 208.32  limit^ limit(b)  206.1 207.7  8 9  212.24 (15) 215.43 (30)  Cl 2 p 3 / 2 Cl 2 P j / 2  Possible Assignments^^  2p1/2  5.7 4.4 2.6 0.9 0 -0.6  6.0 4.2 2.5 1.6 1.0 -0.7  -5.6J  -8.8T  2  P3/2  <(al>  o * (ej  4s  2  Pl/2  a*(a,)  a*(e5 4s  etc.  or  2p -<• a  or  2p + 4s  etc  on top of resonance  "Shake-up" or resonance  2s Cl 2s  1  0  t u\;  limit  271.7 (3) 278.2  6.5 0  * 2s •* a  (a) Estimated uncertainty i n energy-loss values i s ± 0.08 eV except where stated. (b) The spin-orbit s p l i t t i n g of 1.6 eV [181] has been used to estimate the 2 p ^ 2 and 2 p j / 2 spin-orbit components from the 2p (mean) values [179]. has been used. (c) Ref.  Same procedure as for the P 2p  [179]  (d) Final occupied o r b i t a l with either 2 p 3 / 2 *with respect to 2p (mean) edge [179].  o  r  2 p j / 2 hole state.  -  191  -  t r a n s f o r m as both a^ and e combinations.  The  term v a l u e s f o r the  t r a n s i t i o n s i n the C l 2p s p e c t r a are lower than i n the P 2p  spectra,  c o n s i s t e n t w i t h the p e r i p h e r a l l o c a t i o n of the core h o l e . F e a t u r e s 3 and 4 have term v a l u e s of 2.6 ^3/2  a n C  * ^1/2  e <  *S  e s  r e s  P  e c t  i  v e  ly«  Thus f e a t u r e 3 i s e i t h e r s o l e l y due t o a Rydberg  f e a t u r e s can be a s s i g n e d to v a r i o u s Rydberg t r a n s i t i o n s . intense r i s e i n o s c i l l a t o r suggests t h a t t h i s may  The  r e s t of the  The  rapid  and  s t r e n g t h i n the r e g i o n of the i o n i s a t i o n edge  be on top of a broad 2p •*• a (a^) resonance - as  found i n * t h e P spectrum.  probably due  the  they have been a s s i g n e d  t r a n s i t i o n or to a combination of v a l e n c e and Rydberg.  was  eV from  This i s consistent with a  t r a n s i t i o n to the 4s Rydberg l e v e l and accordingly.  eV and 2.5  The o s c i l l a t o r  s t r e n g t h beyond the edge i s  to v a r i o u s d - l i k e shape-resonances  or "shake-up".  - 192  -  CHAPTER 6  ELECTRONIC EXCITATIONS IN PHOSPHORUS CONTAINING MOLECULES. II.  INNER SHELL ELECTRON ENERGY LOSS SPECTRA OF PFc;,OPF, AND  In  the p r e c e d i n g chapter, the ISEELS s p e c t r a of the  compounds PH ,  PF ,  3  PC1  3  3  and P ( C H ) 3  w i t h the s p e c t r a of the analogous  3  were examined and a l s o  silicon  series.  The  QPC1  ?  trivalent compared  l i g a n d s were  seen  to have a s i m i l a r e f f e c t on the c e n t r a l atom core s p e c t r a f o r both s e r i e s , however, u n l i k e the s i l i c o n  s e r i e s there d i d not seem to be a  s i m i l a r l i n e a r r e l a t i o n s h i p f o r shape-resonance p o s i t i o n w i t h bond l e n g t h i n the l i m i t e d pshosphorus s e r i e s . s p e c t r a i t was  found  were v e r y u s e f u l .  t h a t the term v a l u e s o b t a i n e d from  the P 2s s p e c t r a  In t h i s c h a p t e r , the study of the ISEELS s p e c t r a of  phosphorus compounds i s extended r e l a t i o n s h i p of resonance examined as w i l l  In the a n a l y s i s of the P 2p  5  OPF  3  and 0 P C 1 . 3  p o s i t i o n w i t h bond l e n g t h w i l l be  the assumption  c e n t r e are t r a n s f e r a b l e .  to i n c l u d e P F ,  t h a t term v a l u e s from  further  the same core atom  As b e f o r e , a l l i n n e r s h e l l r e g i o n s a c c e s s i b l e  w i t h the c u r r e n t i n s t r u m e n t a t i o n (P L - s h e l l , C l L - s h e l l , 0 and K - s h e l l s ) are  Experimental The for  The  F  presented.  Details  s p e c t r a were recorded and  the t r i v a l e n t  calibrated  phosphorus compounds.  i n the same manner as  - 193 I  RESULTS AND DISCUSSION Phosphorus  Spectra  The  long-range P 2p,2s s p e c t r a a r e shown i n F i g . 6.1.  A l s o shown  on the s p e c t r a are the p o s i t i o n s o f the i o n i s a t i o n edges taken from XPS [31,175].  As only the mean 2p v a l u e s were r e p o r t e d  of the 2 p ^ 3  a  n  d  s p i n - o r b i t components were estimated  using a spin-orbit s p l i t t i n g s p e c t r a l weighting eV).  [31] the p o s i t i o n s as b e f o r e ,  of 0.90 eV [61] i n c o n j u n c t i o n w i t h a  o f 2:1 ( i . e . ,  2p^  2  = 2p - 0.30 eV, 2 p ^  A comparison of the s p e c t r a r e p o r t e d here w i t h  the P F  2  = 2p + 0.60  3  and P C 1  3  s p e c t r a ( F i g . 5.1) show a number of s i m i l a r i t i e s which can be a s s o c i a t e d w i t h the e f f e c t s of the e l e c t r o n e g a t i v e l i g a n d s . show r e l a t i v e l y  The d i s c r e t e p o r t i o n s  s t r o n g t r a n s i t i o n s t o the v i r t u a l v a l e n c e  the expense of Rydberg t r a n s i t i o n s .  o r b i t a l s at  These a r e f o l l o w e d by i n t e n s e  t r a n s i t i o n s which a r e a t the edge i n the chloro-compounds, and j u s t beyond the edge i n the fluoro-compounds, and these f e a t u r e s can be associated with Following  i n n e r - w e l l trapped  states/shape-resonances  [73,77].  t h i s , the s p e c t r a show broad continuum f e a t u r e s which a r e a l s o  associated with  shape-resonances  edge f e a t u r e s w i l l be d i s c u s s e d  [ 7 7 ] . The nature of the edge and p o s t In more d e t a i l l a t e r , but f i r s t the  d i s c r e t e p o r t i o n s o f the s p e c t r a w i l l be d i s c u s s e d . The  higher  been o b t a i n e d  r e s o l u t i o n 2p s p e c t r a of these compounds have a l s o  and each spectrum i s d i s c u s s e d  w i t h the c o r r e s p o n d i n g  2s r e g i o n .  below i n t u r n i n comparison  The 2s s p e c t r a a r e p l o t t e d on the  same h o r i z o n t a l s c a l e as the 2p s p e c t r a .  In order  f e a t u r e s more apparent, a l i n e a r ramp, e x t r a p o l a t e d  t o render the from the l e a d i n g  - 194 -  i  1  r  i 1 g P 2 p edge  r  1  i r AE=0.36eV  10  PHOSPHORUS 2p,2s REGION  OPF,  OPCI.  T  ~1  130  1  1 140  1— 150  1  1 160  1  1 ' 170  1 180  1  1 ' 190  1 200  1  1— 210  E N E R G Y L O S S (eV) Figure  6.1:  Phosphorus 2p,2s wide range e l e c t r o n energy l o s s s p e c t r a of P F , 0 P F , and 0PC1 . A l l s p e c t r a were o b t a i n e d w i t h an impact energy of 2500 V, a s c a t t e r i n g angle ~ 1 ° , and a r e s o l u t i o n of 0.36 eV FWHM. 5  3  3  - 195  edge, has  been s u b t r a c t e d  s p e c t r a were e x t r a c t e d whereas the P F  5  -  from the 2s s p e c t r a .  The  OPF  3  and  2s spectrum was  rerun s e p a r a t e l y .  The  P 2p and  of the Zp-^/Z -^jl i ° * -  of the 2p s p e c t r a the mean v a l u e The  s p e c t r a due Two  3  from the long-range s p e c t r a shown i n F i g .  s p e c t r a are l i n e d up on t h e i r r e s p e c t i v e i o n i s a t i o n edges.  used.  0PC1  n  s a t  6.1,  P  2s  In the  *-  o n  case  edges  was  2s s p e c t r a are c o n s i d e r a b l y broader than those f o r the to f a s t a u t o i o n i s a t i o n processes  of a C o s t e r - K r o n i g  2p  type.  assumption used i n Chapter 5 w i l l a l s o be made here, namely:  (i)  the major i n t e n s i t y i n the 2s spectrum can be a t t r i b u t e d to t r a n s i t i o n s to o r b i t a l s w i t h  the l a r g e s t p o r b i t a l content  they w i l l have the l a r g e s t s -»• p (atomic) d i p o l e allowed  as contri-  bution. (ii)  s i n c e the c o r e - h o l e values  vacancy i s s i t u a t e d on the same atom, term  from the 2s s p e c t r a are t r a n s f e r a b l e to the 2p  Phosphorus P e n t a f l u o r i d e -  PF  R  T h i s molecule belongs to the t r a n s i t i o n s are l i s t e d  i n Table  vacant o r b i t a l s are of aJ, PF  5  and  region.  6.1.  e' and  a£  p o i n t group. Using  The  possible  a minimal b a s i s s e t ,  symmetry.  Since  i n the case of  the 2s o r b i t a l i s of a^ symmetry and  the p o r b i t a l s t r a n s f o r m  a'2,  be d i p o l e allowed  the f o l l o w i n g t r a n s i t i o n s should  phosphorus L - s h e l l s p e c t r a :  the  i n the  as  e  1  - 196 -  TABLE 6.1 Transitions from the 1 AJ Ground State i n D-j^ Symmetry  Final Configuration * hole state  (2s)  Final State  occupied o* o r b i t a l  Dipole Allowed from ground state  i  No  E'  Yes  2  Yes  i  E'  Yes  e1  e'  E'  Yes  e'  e'  A j ' + A£  No  e'  a2  E"  No  2  Yes  E"  No  i  No  a  i  a  a  i  e'  a  i  a  e'  a  a  2  a  2  a  2  , (2p-jy2)  The a* o r b i t a l s  a  i  A  2  A  i  A  e' a  <* ( 2 p j / 2 )  an  A  2  holes are of a j , e' and ajj symmetry  are of aJ, e' and a 2 symmetry.  respectively.  - 197  2s(ap  - o*(a" ),  2p(e')  >(2p  2  o*(e«)  r a* 1  3 / 2  2p(ap * ( 2 p  1 / 2  -  (ap,  C^^)"  a*(e« )  1  )-lo*(ap  i  The  t r a n s i t i o n 2p(a£) -• ( 2 p y ) 1  forbidden. 6.2, it  and  - i  2  a (e') i s f o r m a l l y d i p o l e  From a c o n s i d e r a t i o n of the spectrum which i s shown i n F i g .  the s p i n - o r b i t  seems reasonable  s p l i t t i n g s of the 2 p ^  2  ^  ionization  2  to a s s i g n f e a t u r e s 1 and 2 as the  components of the 2p -»• a (a^) t r a n s i t i o n . ( 2 p ^ ) o ' (e') t r a n s i t i o n . _ i  3  *  2  edges,  spin-orbit  Feature 3 i s the 2p(e') •+•  I t has a very s i m i l a r  term value to f e a t u r e  * 11, which i s a s s i g n e d as the 2s -*• a (e) t r a n s i t i o n .  F e a t u r e 12,  has no c o u n t e r p a r t i n the 2p spectrum, i s then the 2s •*• a ( a ) 2  tion.  The  assigned  rest  of the f e a t u r e s 4-7,  summarised i n T a b l e Two  eV.  The  are  assignments are  6.2.  p o i n t s should be emphasised i n c o n n e c t i o n w i t h the above  assignments. the  transi-  which are r e l a t i v e l y sharp,  to v a r i o u s 2p -* Rydberg t r a n s i t i o n s .  which  Firstly,  the r e s p e c t i v e term v a l u e s f o r the t r a n s i t i o n s  a (e') o r b i t a l from the 2p(e') and 2s o r b i t a l s agree to w i t h i n Secondly,  as would be expected,  the i n t e n s i t y  a s s i g n e d to the 2s e l e c t r o n to the doubly  of the  degenerate  to  0.1  transition  a (e') o r b i t a l  (mainly of P 3 p ( x , y ) ) ^ i s much g r e a t e r than that to the non-degenerate ^ The p r i n c i p a l a x i s of the molecule i n the present work i s d e s i g n a t e d as tne z a x i s .  - 198 -  TERM VALUE (eV) 4 "T  8  1  2  3  -4  0 P 2p 4  56  3l2  2pl|2edges  PF  7  5  P2p  from long-range spectrum 140  I36  I 44 P 2s edge  !L_  AE=O.I8eV  PF  5  P2s  12  AE=0.36eV I92  196  ENERGY  200  204  LOSS(eV)  F i g u r e 6.2: Phosphorus 2p and 2s e l e c t r o n energy l o s s s p e c t r a of P F . P 2p spectrum (upper t r a c e ) i s a t h i g h r e s o l u t i o n (0.18 eV FWHM). P 2s spectrum (lower t r a c e ) i s a t 0.36 eV FWHM. The s p e c t r a a r e a l i g n e d w i t h r e s p e c t to the 2p (mean) and 2s i o n i s a t i o n edges. 5  - 199 -  TABLE 6.2 Energies, Term Values, and Possible Assignments for the P 2p,2s Spectra of P F 5 Feature  Energy Loss (a)  Term Value  2p 1/2  2p3/2 2 3 4 5 6 7 b 2 P3/2 l i m i t b limit  138.22 (12)  6.16  138.97 (10) 140.73 (10) 141.67 142.51 143.10 144.00 144.38 145.28  5.41 3.65 2.71 1.87 1.28 0.38 0  8  149.1 (3)  9 10  157.5 (5) 162.2 (5)  (d)  (eV)  (eV)  1  Possible Assignments  2p3/2 (a})  0  6.31 4.55 3.61 2.77 2.18 1.28  2p1/2  a*(e') 4s 3d 5s, 4d  a  (a{)  4s 3d 5s, 4d  inner-well state/shapergsonance o^(P-F a x ) shape resonance 0 (P~^ e q) shape resonance  -4.4' -12.8 -17.5 2s  11 2s l i m i t c 12  198.12 (20) 201.87 202.60  3.75 0 -0.73  2s -» a (e') 2s •+  a'ia'p  (a) Estimated uncertainty i n energy-lgss values is ± 0.08 eV except where stated. are calibrated against N 2 (Is -»• it , v = 1) at 401.10 eV.  Spectra  (b) The spin-orbit s p l i t t i n g of 0.90 eV [61] has been used to estimate the 2p$/2 * spin-orbit components from the 2p edge (mean) values [31], see text for d e t a i l s . a n i  (c) Ref. [175]. (d) Final occupied o r b i t a l with either 2 p j / 2 *  With respect to the 2p edge (mean) [31].  o  r  n o  ^  e  state.  ^7>i/2  - 200  -  * a (a^)  o r b i t a l (mainly of P 3 p ( z ) ) .  interpretation  of  the  2s  This also  s p e c t r a of  where the maximum i n t e n s i t y of  the  the  lends support to  the  t r i v a l e n t phosphorus compounds  broad envelope was  a s c r i b e d to  the  degenerate a (e) o r b i t a l . Phosphoryl T r i f l u o r i d e - 0 P F The clear  assignment of  than that  of P F  due  5  overlapping t r a n s i t i o n s . that  f o r the  central  the  q  P 2p  to the  f a c t that  situation  atom ISEELS s p e c t r a of P F SiF^  [144].  ( F i g . 6.3)  3  i t i s a mixture  In t h i s regard the  o p t i c a l a b s o r p t i o n spectrum of consists  spectrum of 0 P F  3  less  of  is similar  ( F i g . 5.4)  Thus the  is  and  to  also  spectrum presumably  of a t r a n s i t i o n to a v i r t u a l o r b i t a l , f o l l o w e d by a  transition  to a h i g h e r v i r t u a l o r b i t a l w i t h superimposed Rydberg t r a n s i t i o n s . spectral  shape i s c e r t a i n l y c o n s i s t e n t  w i t h such an assignment.  ground s t a t e m o l e c u l e , l i k e i t s c o u n t e r p a r t P F ,  i s of C^  3  which the  dipole-allowed transitions  The e and the a^  a^  the  2p  Transitions  orbitals.  i n the  P 2s  a" o r b i t a l In P F  5  spectrum.  i n Table  indicates  i n c h a r a c t e r and The  v  The The  symmetry  of  a^,  possible  from  that  first  the  so should show l i t t l e  second aj^ o r b i t a l i s analogous  w i t h a l o t of p(z)  for  5.1.  d) b a s i s s e t , are  to a l l these l e v e l s are  A CNDO/2 c a l c u l a t i o n  o r b i t a l i s predominantly P 2s  intensity  listed  vacant o r b i t a l s , u s i n g a minimal (no  symmetry.  2s and  are  the  character.  Thus the  2s  to  spec-  - 201 -  TERM VALUE (eV) 8 1  4  1  1  1  1  1  P  (ARB ITRARY UNIT  00  11  II  12  3 4  P3/2 Pl|2 9 2  e d  e S  0PF  3  P2p  I I 5  1  6  1  "•^w"  /  *'""\  AE=O.I8eV 1  ,  ,  ,  fENSI  136  ,  140  • •*  >  ,  . ,  1  144 y P2s edge  1 10  LU  _ATI  2  1  7  >  LU  -4 1  0  OPF3 P2s 1  **  11  •. • *.  '  * • • •*• . ••».*  •  •'  •.  •*•  AE=0.36eV  •*  cr  192  .  196  200  204  ENERGY LOSS(eV) F i g u r e 6.3: Phosphorus 2p and 2s e l e c t r o n energy l o s s s p e c t r a of 0 P F . P 2p spectrum (upper t r a c e ) i s a t h i g h r e s o l u t i o n (0.18 eV FWHM). P 2s spectrum (lower t r a c e ) e x t r a c t e d from F i g u r e 6.1. The s p e c t r a are a l i g n e d w i t h r e s p e c t t o the 2p (mean) and 2s i o n i s a t i o n edges. 3  - 202  trum would be expected orbital  -  to be dominated by a t r a n s i t i o n to the  (as per p r e c e d i n g d i s c u s s i o n f o r P F ) 5  as such.  The  and  a (e)  f e a t u r e 10 i s a s s i g n e d  b r e a d t h of the peak i n the 2s spectrum and  the  intensity  on the h i g h energy s i d e of f e a t u r e 10 i s c o n s i s t e n t w i t h t h e r e being  an  * a d d i t i o n a l , l e s s i n t e n s e 2s •*• a (a^) t r a n s i t i o n underneath, which would be expected  i n the case of 0 P F  s i n c e there are two  3  o r b i t a l s , as i n d i c a t e d above.  The  a^ type  t r a n s i t i o n to the lower a^ o r b i t a l i s  l i k e l y under the low energy s i d e of peak 10, c o r r e s p o n d i n g and  2 i n the 2p spectrum d i s c u s s e d below.  summarised i n Table  virtual  to peaks 1  A l l assignments are  6.3.  A p p l y i n g these to the 2p  spectrum, f e a t u r e s 1 and  2 are a s s i g n e d  * as the s p i n - o r b i t components of the 2p to the f i r s t term v a l u e s of peaks 3 and are almost  a (a^) o r b i t a l .  4 w i t h r e s p e c t to the ^.P^/l  a n c  * ^1/2  e c  *6  the same as the term v a l u e of f e a t u r e 10 (see Table 6.3)  these t r a n s i t i o n s are a s s i g n e d to the s p i n - o r b i t components of  the  2p •*• a (e) t r a n s i t i o n s .  term  However, the i n t e n s i t y , sharpness  v a l u e of f e a t u r e 4 suggests the 2 p y 3  2  s p i n - o r b i t component of the 2p -»• 4s Rydberg t r a n s i t i o n .  5 and  w i t h o t h e r phosphorus 4s v a l u e s  6 are a s s i g n e d to  h i g h e r Rydberg t r a n s i t i o n s .  e v i d e n t l y superimposed on top of  broad  a s s i g n e d to the 2p t r a n s i t i o n t o  the second  thus  The  (see  PF ).  u n d e r l y i n g s t r u c t u r e t h a t can a ( a ^ orbital.  5  These a r e  This  i n t e r p r e t a t i o n i s c o n s i s t e n t w i t h the proposed assignment of the spectrum d i s c u s s e d above.  e s  t h a t i t a l s o c o n t a i n s a c o n t r i b u t i o n from  term v a l u e i s a l s o i n keeping Features  and  The  2s  be  -  203 -  TABLE 6.3 Energies, Term Values, and Possible Assignments for the P 2p,2s Spectra of OPF3 Feature  Energy Loss^ a ^  Term Value  (eV)  (c sV) 2  1  Possible Assignments^^  137.18  5.78  2 3 4 5 6 ^j>3/2 l i m i t b 2p^2 limltb  137.86 139.60 140.37 (8) 141.53 142.44 142.96 143.86  5.10 3.36 2.59 1.43 0.52 0  7  148.3 (3)  8 9  159.7 (5) 166.6 (5)  2p3/2  2Pi/2  P3/2  2  Pl/2  0 (aj) 6.00 4.26 3.49 2.33 1.42  o*(a 1 ) o*(e)  4s 5s, 3d  a (e) 4s Ion tgp of 5s, 3d| p+a (aj)  0 -5  inner-we11 state/shaperesonance 0 (P-F) shape resonance a (P-0) shape resonance  -16.4? -23.3 T 2s  10 2s l i m i t c 11  196.92 (20) 200.47 206.1 (5)  3.55 0 -5.6  2s •»• o*(e) inner-well state/shape resonance  (a) Estimated uncertainty i n energy-lgss values i s ± 0.12 eV except where stated. are calibrated against N 2 (Is -* it , v • 1) at 401.10 eV.  Spectra  (b) The spin-orbit s p l i t t i n g of 0.90 eV [61] has been used to estimate the 2 p 3 / 2 and 2 p ^ 2 spin-orbit components from the 2p edge (mean) values [31], see text for d e t a i l s . (c) Ref. [175] (d) Final occupied o r b i t a l with either 2 p 3 / 2 ?  With respect to the 2p edge (mean) [31].  o  r  2  Pl/2  n o  ^ - e state.  -  204  -  P h o s p h o r y l T r i c h l o r i d e - 0PC1, The d e t a i l e d phosphorus F i g . 6.4  2p and 2s s p e c t r a of OPCI3  and the s p e c t r a l i n f o r m a t i o n i s summarised  o p t i c a l a b s o r p t i o n spectrum of 0PC1 previously reported the  [173,174].  3  a  r  e  s  n  o  w  i n T a b l e 6.4.  i n the P 2p r e g i o n has  *-  n  n  The  been  However, i t i s important t o note t h a t  energy s c a l e shown i n the p r e l i m i n a r y o p t i c a l work [174] i s c l e a r l y  i n e r r o r by about 9 eV.  However, the spectrum shown i n the second paper  [173] i s c o n s i s t e n t w i t h the spectrum r e p o r t e d here. [173] a l s o shows the r e s u l t s of an X -SW a  The l a t t e r  paper  c a l c u l a t i o n , however, no  i n t e r p r e t a t i o n i s given. The molecule 0PC1  3  i s of C^  v  symmetry and so i s governed by the  ( T a b l e 5.1) as OPF3.  same s e l e c t i o n r u l e s  P r o c e e d i n g as b e f o r e , the  major f e a t u r e (15) i n the 2s spectrum i s a s s i g n e d to the 2s •*• a (e) transition.  There i s e v i d e n c e of a s h o u l d e r on t h low energy s i d e of T h i s i s presumably due t o a 2s •*• a (a^) t r a n s i t i o n .  t h i s peak.  A p p l y i n g the 2s term v a l u e s so o b t a i n e d (Table 6.4) f e a t u r e s 1 and 2 may  to the 2p  spectrum,  be a s s i g n e d to the s p i n - o r b i t components of the  2p •*• a (a^) t r a n s i t i o n and f e a t u r e s 3 and 4 to the two components of the 2p -»• a (e) t r a n s i t i o n . ^3/2 it  e (  *£  e  *  s  3*^7  eV.  The term v a l u e f o r f e a t u r e 5 w i t h r e s p e c t to the T h i s i s somewhat h i g h f o r the 4s term v a l u e when  i s compared to those observed f o r the peaks a s s i g n e d to t r a n s i t i o n s  to the 4s l e v e l i n the o t h e r phosphorus between ~2.4  and 3.1  spectra.  These v a l u e s l i e  eV f o r the assignments g i v e n here and i n Chapter 5.  Furthermore, the i n t e n s i t y of peak 4 suggests i t c o n t a i n s from more than one t r a n s i t i o n .  contributions  S i n c e the s e p a r a t i o n between  - 205 -  TERM VALUE (eV) 8  i  4  0  -4  r P  2  P3/2  2 p  i/2  rn—i—i i II I I — T T T 1  2  4  3  5  6 7  8  9  1 0 11  12  oo  >or < or  e d  9  e S  OPCI; P2p  •1  GO  AE=O.I8eV  or <  32  b  140  136  00 UJ  1  P 2s e d g e  15  OPCI. P2s  LU >  LU 188  92  196  ENERGY Figure  AE=0.36eV  •V"  200  LOSS(eV)  6.4: Phosphorus 2p and 2s e l e c t r o n energy l o s s s p e c t r a of 0 P C 1 . P 2p spectrum (upper t r a c e ) i s a t h i g h r e s o l u t i o n (0.18 eV FWHM). P 2s spectrum (lower t r a c e ) e x t r a c t e d from F i g u r e 6.1. The s p e c t r a are a l i g n e d w i t h r e s p e c t t o the 2p (mean) and 2s i o n i s a t i o n edges. 3  - 206 -  TABLE 6.4 Energies, Term Values, and Possible Assignments for the P 2p,2s Spectra of OPCI3  Feature  Term Value  Energy Loss^ ^ a  (eV)  134.06 134.85 135.81 136.67 137.49 138.00 138.27 139.03 139.61 140.59 141.09 141.73  2p limit< 2pi/2 l i m i t 13 14  b )  3 / 2  ( b )  |  (eV) 2  1 2 3 4 5 6 7 8 9 10 11 12  Possible Assignments^^  (10)  (12) (10) (10) (20)  2p  P3/2  7.00 6.21 5.25 4.39 3.57 3.08 2.99 2.03 1.45 0.47 -0.03  141.06 141.96  1 / 2  2  P3/2  2  Pl/2  a*(a ) x  7.11 6.15 5.29 4.47 3.98 3.89 2.93 2.35 1.37 0.87 0.23  o*(a ) 1  0 <aj)  a*(e) a (aj)  4s 4s + v 4s 3d, 5s etc  3d, 5s on top of an innerwell state/ shape-resonance  0 0  152.4 (5) 164.3 (5)  -11 -22 9  o*(P-Cl) sh«ipe a (P-0) sh<ipe  T  resonance resonance  2s 1 5  ( ^  2s limit -> KQ  193.67 (20) 198.86  5.19 0  2s + o*(e)  (a) Estimated uncertainty i n energy-loss values i s ± 0.08 eV except where stated. are calibrated against N  2  Spectra  (Is •* n , v = 1) at 401.10 eV.  (b) The spin-orbit s p l i t t i n g of 0.90 eV [61] has been used to estimate the 2 p j / and 2p^y spin-orbit components from the 2p edge (mean) values [31], see text for d e t a i l s . 2  (c) Ref. [175] (d) Final occupied o r b i t a l with either 2p^y or 2 p j / hole state. 2  ?With respect to P 2p edge (mean) [31].  2  2  - 207  -  f e a t u r e s 4 and 5 i s c l o s e t o the s p i n - o r b i t to the s p i n - o r b i t  s p l i t t i n g , they are a s s i g n e d  components of the 2p t r a n s i t i o n to the second  energy) a (a^) o r b i t a l .  The  (higher  r e s t of the f e a t u r e s are a s s i g n e d t o  v a r i o u s Rydberg t r a n s i t i o n s l e a d i n g up to the edge.  As w i t h P C 1 ,  the  3  l a t t e r t r a n s i t i o n s are on top of a broad peak t h a t can be a s s i g n e d to an i n n e r w e l l trapped s t a t e or  resonance.  Continuum S p e c t r a A number of broad f e a t u r e s are observed c o n t i n u a i n the wide range  s p e c t r a shown i n F i g . 6.1.  are q u i t e s i m i l a r to those of P F  3  and P C 1  e f f e c t s of the e l e c t r o n e g a t i v e l i g a n d s . f e a t u r e s on the edge i n 0PC1 0PF  3  ( f e a t u r e 7) and P F  5  3  [73,77].  3  These f e a t u r e s  and can be a t t r i b u t e d to the  The  intense, r e l a t i v e l y  sharp  ( f e a t u r e 12) and j u s t beyond the edge i n  ( f e a t u r e 8) are p r o b a b l y best d e s c r i b e d as  i n n e r - w e l l states/shape-resonances barrier  i n the phosphorus 2p  The h i g h e r energy  trapped by a p o t e n t i a l  (centrifugal)  f e a t u r e s are broader and  t e n t a t i v e l y a s s i g n e d to h i g h e r d - l i k e shape resonances  caused  can  be  by the  s c a t t e r i n g of the i o n i s e d e l e c t r o n o f f the n e i g h b o u r i n g atoms [77], d i s c u s s e d i n the p r e v i o u s two there should be an R~  2  chapters and Chapter  relationship  1, s e c t i o n  (R - d i s t a n c e between the  c e n t r e s a t a g i v e n s i t e ) f o r these f e a t u r e s .  series  similar ligands.  (Chapter 4) has  F.III, scattering  T h i s d i d not seem to be  the case i n the t r i v a l e n t phosphorus s e r i e s i n Chapter the s i l i c o n  As  5, even though  shown a reasonable c o r r e l a t i o n w i t h  I t might be i n s t r u c t i v e to compare the  p o s i t i o n s of the molecules here w i t h those i n P F  3  and PC1  resonance 3  t o see  - 208 -  whether the resonance P-F,  term v a l u e s f o r i d e n t i c a l p a i r s of atoms ( e . g . ,  e t c . ) are of s i m i l a r magnitude i n the d i f f e r e n t molecules  since  the r e s p e c t i v e bond l e n g t h s do not vary a p p r e c i a b l y (see Table  6.5).  The molecules presented here each have two d i f f e r e n t ligand position p r e v i o u s chapter position.  ( P F ) or l i g a n d 5  resonance  and  T h i s d i f f e r e n c e should be r e f l e c t e d i n the s p e c t r a . f e a t u r e s are due  c l e a r l y show two d i f f e r e n t 14, P F  and O P C I 3 ) , u n l i k e those i n the  3  where a l l l i g a n d s were i d e n t i c a l i n nature  t h a t a l l the continuum  and  (0PF  types of  5  resonance  - f e a t u r e s 9 and  10).  to resonances, OPCI3 and  contributions  The  (0PC1  spectrum of 0 P F  3  3  side.  PF  5  - f e a t u r e s 13  shows one  ( f e a t u r e 8 ) , but w i t h an i n d i c a t i o n of a second  ( f e a t u r e 9) on the h i g h energy  Assuming  resonance  The e n e r g i e s ( 6 ) of the r e s o -  nances above the r e s p e c t i v e i o n i s a t i o n edges f o r these m o l e c u l e s , as w e l l as those f o r P F  3  and P C I 3 ,  l e n g t h s (R), i n T a b l e 6.5. sites,  the lower resonance  are summarised, t o g e t h e r w i t h the bond  Where there are two  scattering (interatomic)  has been a s s i g n e d to the s i t e w i t h the l o n g e r  bond d i s t a n c e [ 9 9 , 1 0 0 ] . As can be seen from Table 6.5,  the resonance  position  (6)is  reasonably constant f o r a g i v e n type of i n t e r a t o m i c s c a t t e r i n g s i t e i n d i f f e r e n t molecules.  The  c l o s e correspondence  v a l u e s f o r the h i g h e r energy  between the e s t i m a t e d 6  f e a t u r e s i n OPF3 ( f e a t u r e 9) and  ( f e a t u r e 14) l e n d s support to t h e i r common assignment resonances.  The  to a ( P - 0 )  l a c k of a c o r r e l a t i o n found f o r P F 3 , P ( C H ) 3  can be a t t r i b u t e d to d i f f e r e n t Based upon the proposed  c i r c u m s t a n c e s i n the case of  resonance/bond  0PC1  3  3  type  and P C 1  3  P(CH ) .  l e n g t h r e l a t i o n s h i p , the  3  3  resonan-  TABLE 6.5 Resonance P o s i t i o n s ( 6 ) Above the Mean I o n i s a t i o n Edge and Bond Lengths (R)  Postulated  Interatomic S c a t t e r i n g  P -- C l Molecule  PC1  3  OPCI3 OPF •  P F  3  PF  5  3  6(eV)  R(  Site P •- 0  P •- F  A)  1  9.4  2.043  11.0  1.989  6(eV)  R(A)  1  16.4  1.524  15.2  1.563  12.8 (Ax) 17.5 (Eq)  1.577 1.534  A)  6(eV)  R(  22.1  1.455  -23  tFrora L a n d h o l d t - B B r n s t e i n (New S e r i e s ) I I / 7 , " S t r u c t u r e Data o f Free Polyatomic M o l e c u l e s , " S p r i n g e r - V e r l a g , ( B e r l i n , 1976).  t  1.436  - 210  ce i n P ( C H ) 3  PF .  The  3  3  should  feature i n question  q u i t e i n t e n s e , and the case of PH . 3  of PH  3  have o c c u r r e d  i s not  -  at a s m a l l e r value  in P(CH ) 3  (see f e a t u r e 9 i n F i g .  3  5.1)  is  i n f a c t more l i k e the broad continuum s t r u c t u r e i n The  fact  t h a t the P ( C H ) 3  3  spectrum i s more l i k e  that  e n t i r e l y unexpected s i n c e the methyl group ( l i k e H) i s  much l e s s e l e c t r o n e g a t i v e  ( i n f a c t s l i g h t l y e l e c t r o n d o n a t i n g ) than  h i g h l y e l e c t r o n e g a t i v e l i g a n d s of P F  and  3  PC1 .  "shake-up" r a t h e r than resonance-type f e a t u r e s .  3  c o u l d be a s s i g n e d It i s possible,  f o r e , that most of the continuum s t r u c t u r e i n P ( C H ) 3  i s not  i n f a c t due  to a resonance and  c o r r e l a t i o n of R w i t h  6 would not  be r e l e v a n t .  taken i n a u t o m a t i c a l l y  a s s i g n i n g any  However, i n the case of the present  the  Furthermore, i t was  3  seen that much of the continuum i n t e n s i t y i n PH  "shake-up" and  of 6 than that i n  3  as there-  i s a l s o due therefore  Obviously  to  any  care has  to  be  continuum f e a t u r e s to resonances. work the c l o s e agreement (Table  6.5)  between the p o s i t i o n s of the f e a t u r e s f o r the same i n t e r a t o m i c s c a t t e r i n g s i t e s i n d i f f e r e n t molecules lends ment f o r the f e a t u r e s i n q u e s t i o n . recent  suggestions  Ligand  Spectra  support  to the resonance a s s i g n -  T h i s c o n c l u s i o n i s i n keeping  with  made by S e t t e et a l . [99].  Having a s s i g n e d l i g a n d s p e c t r a are now  the c e n t r a l atom s p e c t r a f o r these m o l e c u l e s , analysed  with  reference  to t h e i r  the  respective  c e n t r a l atom spectrum. The  spectrum of the F Is r e g i o n of P F  summarised i n T a b l e  6.6.  There are two  5  i s shown i n F i g . 6.5  and  types of F l i g a n d , i . e . , a x i a l  - 211  -r  FIs edge (meon)  io H  12 3 4  h n !  FlS I  / i  ! V  AE=0.36eV  5 4  i—i—i—r  1 — i — i — i — | — i — r  700  i—i—i—r  750  725 Ax Eq  T  io A  FIs edge (mean)  AE=0.36eV  4  54 "j—T—i—i—i—|—i—i—i—i—|—i—i—i—i—|—i—i—i—i—|—i 685  690  695  700  i  i  7 IO  705  ENERGY L O S S ( e V )  F i g u r e 6.5: The F Is E l e c t r o n Energy Loss spectrum of P F ; wide range (upper t r a c e ) , d e t a i l e d spectrum (lower t r a c e ) . 5  r  scan  TABLE 6.6 Energies, Term Values, and Possible Assignments for the F Is Region of PF 5  Tt;rm Valu«i (eV)  Energy Loss (eV)  Feature  Ax.  Eq.  Possible Assignment  Mean^ a*  1  689.93 (20)  4.3  5.5  5.0  Is  2  692.22 (20)  2.0  3.2  2.7  Is * o* (e')  3  694.2  0  1.2  0.7  Rydberg  -2.2  -2.7  F Is (Ax) l i m i t  3  694.2  F Is (Eq) l i m i t  3  695.4  4 5 6  697.62 (30)  -3.4  •*  a* (a^)  inner-well trapped state/shape resonance  -700 711.6  Is  (a{)  -17.4  -16.2  -16.7  Ref. 1179); Ax. i s Axial, Eq. i s Equatorial. Binding energies of 694.1 eV and 695.3 eV have also been reported for the a x i a l and e q u i t o r l a l ligands respectively [183]. With respect to the mean value (694.9 eV) of the F Is edges.  shape-resonance  - 213 -  and e q u a t o r i a l .  The F Is XPS spectrum  be deconvoluted environments. i n F i g . 6.5.  [179,183] i s asymmetric and can  i n t o two components r e p r e s e n t i n g the two d i f f e r e n t Both of the edges as w e l l as the mean p o s i t i o n a r e shown  There i s no evidence  of i n d i v i d u a l f e a t u r e s l e a d i n g up t o  the d i f f e r i n g edges, and furthermore,  the s e p a r a t i o n o f the f e a t u r e s i s  c o n s i d e r a b l y g r e a t e r than the r e p o r t e d XPS s p l i t t i n g . t h a t the observed  I t i s concluded  f e a t u r e s i n the ISEELS spectrum are i n d i c a t i v e o f the  f i n a l o c c u p i e d o r b i t a l s and not due t o a d i f f e r i n g o r i g i n a t i n g a x i a l o r e q u i t o r i a l F) o r b i t a l .  Thus the term v a l u e s have been  w i t h r e s p e c t t o the mean v a l u e .  As w i t h the molecules  chapter, the term v a l u e s a r e lower f o r the c e n t r a l atom spectrum.  taken  i n the p r e v i o u s  f o r the l i g a n d spectrum than  those  T h i s d i f f e r e n c e r e f l e c t s the d i f f e r e n t  l o c a t i o n o f the core h o l e w i t h i n the molecule, the molecule  (i.e.,  i . e . , on the p e r i p h e r y o f  f o r F as opposed t o the c e n t r e f o r P.  Since the F Is  o r b i t a l s t r a n s f o r m as 2A^ + A£ + E', t r a n s i t i o n s to a l l the v i r t u a l o r b i t a l s are d i p o l e allowed.  Thus f e a t u r e s 1 and 2 ( F i g . 6.5, Table  6.6  are a s s i g n e d t o the F Is -*• a (a^) and F Is •*• a (e') t r a n s i t i o n s r e s p e c t i v e l y and f e a t u r e 4 i s a s s i g n e d t o the F Is  a (a£)  transition,  c o n s i s t e n t w i t h the a s s i g n e d o r d e r i n g i n the P 2p and 2s s p e c t r a d i s c u s s e d above. transitions.  Feature  3 i s thought  The weak, broad  t o be due t o v a r i o u s Rydberg  bands, 5 and 6, a r e a s s i g n e d t o shape  resonance t r a n s i t i o n s s i m i l a r t o those observed  i n the P 2p spectrum.  T h e i r r e l a t i v e l a c k of i n t e n s i t y can be a t t r i b u t e d t o the f a c t t h a t the e x c i t e d atom i s on a l i g a n d ;  i . e . , on the p e r i p h e r y of the molecule.  -  F i g u r e 6.6 ligands i n 0 P F , 3  214  shows the long-range w h i l e F i g . 6.7  -  0  s p e c t r a of each of the F and  shows the short-range  spectra i n  somewhat more d e t a i l .  In both f i g u r e s , the s p e c t r a are shown on a  common r e l a t i v e energy  s c a l e t h a t has been r e f e r e n c e d to the r e s p e c t i v e  i o n i s a t i o n edges.  s p e c t r a from the two  remarkably due  The  similar.  The  different  r e g i o n s are  f e a t u r e s i n the F s p e c t r a are somewhat  broader  probably to the s h o r t e r l i f e t i m e of the F core e x c i t e d s t a t e .  s p e c t r a l data are summarised i n Table 6 . 7 .  I t should be noted  The  that i n  t h i s case the term v a l u e s are i n most cases q u i t e s i m i l a r to each o t h e r and  i n the c e n t r a l atom (P 2 p , 2 s )  to those observed  f e a t u r e s 1,  2,  spectra.  Thus  and 3 are a s s i g n e d to t r a n s i t i o n s to the a ( a ^ , a ( e ) ,  * and  0 (a^) r e s p e c t i v e l y .  broad  F e a t u r e 3 cannot  shoulder on the h i g h energy  0 Is s p e c t r a shown i n F i g . 6 . 7 .  s i d e of peak 2 i n e i t h e r the F Is or Feature 4 can be a s s i g n e d to the s t r o n g  i n n e r - w e l l trapped state/shape-resonance spectrum.  As w i t h P F ,  be a c c u r a t e l y l o c a t e d i n the  f e a t u r e seen i n the P 2p  t h i s f e a t u r e i s weak and  5  t h i s a g a i n can  a t t r i b u t e d to the p e r i p h e r a l l o c a t i o n of the e x c i t e d atom. weak and broad  The  be very  s t r u c t u r e s i g n i f i e d by the b r a c k e t l a b e l l e d 5 i s  presumably a t t r i b u t a b l e to a shape-resonance. F i n a l l y the 0 P C 1  3  l i g a n d s p e c t r a are now  s p e c t r a are shown i n F i g . 6.8  and  considered.  summarised i n Table 6 . 8 .  Is very s i m i l a r i n appearance to the 0 Is spectrum  of the C l 2 p , 2 s i o n i s a t i o n c o n t i n u a .  The  The  of OPF3.  however, c o n s i d e r a b l y more background t h a t a r i s e s mainly term v a l u e s are  lower than those f o r the P 2 p , 2 s s p e c t r a ( T a b l e 6 . 4 ) ,  The 0 Is spectrum  There i s ,  from the  tail  significantly  consistent with  -  215  -  TERM VALUE (eV) 0  10  -i  -10  1  -20  r  1  1  "30  "40  -T—i—<—T  F I s edge i i r — i  123  0PFF1s  4  >cr <  AE=0.36eV  t m rr < 5  H  —|—-i  __  CO L±J  -io  LU >  1  1  1  685  nT  H  1  1  1  r  7I0  ">—r~T i  r  760  735  0 1 s edge 1__ :  0PF3  4  1 23  1  01s  AE=0.36eV LU LT  5 -J L  |  1  1  1  1  525  1 550  1  1  -i r  575  - i — i — i — i 600  ENERGY L O S S (eV) Figure  6.6:  The wide range e l e c t r o n energy l o s s s p e c t r a of the 0 Is and F Is r e g i o n s of 0 P F . The s p e c t r a a r e a l i g n e d w i t h r e f e r e n c e to the r e s p e c t i v e Is edges. 3  - 216 -  T E R M V A L U E (eV) io  CO  8  t z  1  or < cr  4  2  0  r  1  -2  -4  -6  ^ F 1s edge  OPF,  2  io H  F1s  .V.  GO  AE=0.36eV  < >CO  54  LU  694  690  698 ^ 0 1s edge  LU > <  i  706  702  10  2  —I  OPF  3  3  01s  LU  cr  AE=0.36eV • • •." -».-.v..«-<..'-*"—•"• n  -  l  -c_  532  536  ENERGY  Figure  540  544  548  LOSS(eV)  6.7:  D e t a i l e d e l e c t r o n energy l o s s s p e c t r a of the 0 Is and F Is r e g i o n s of OPF . The s p e c t r a are a l i g n e d w i t h r e f e r e n c e to the r e s p e c t i v e edges. 3  TABLE 6.7 E n e r g i e s , Term V a l u e s , and P o s s i b l e Assignments f o r the 0 Is, F Is Regions of OPF 3  Oxygen  Feature Energy Loss  Fluorine  Term Value (eV)  Energy Loss  (eV)  Is  P o s s i b l e Assignment  Term Value  (eV)  1  533.87  (20)  5.4  690.30  (30)  5.5  Is  •>•  *  2  535.81  (15)  3.5  692.71  (20)  3.1  Is  *  a*  (e)  Is  *  a*  (a^)  3 edge3  a  (eV)  Is  -694  -537  695.8  539.3  4  544.3  (5)  5  555 -  565  Ref. [179J.  (a^)  -5.0  700.1  (5)  710 - 720  -4.3  i n n e r w e l l trapped s t a t e / s h a p e resonance shape-resonance  structures  - 218 -  00  10  ZD  or < cr m  or <  >00  550  530  LU hLU > h< _J LU LT  570  AE=0.36eV  J 0 1 s edge  T  10  H  —I  530  534  1  1  538  ENERGY  Figure  1  I  542  546  550  LOSS(eV)  6.8:  D e t a i l e d e l e c t r o n energy l o s s s p e c t r a of the 0 Is r e g i o n The i n s e r t shows a wide range spectrum.  of 0PC1 . 3  - 219 -  TABLE 6.8 Energies, Term Values, and Possible Assignments for the 0 Is Region of O P C I 3  Energy Loss (eV)  Feature  Possible Assignment  1  532.60 (15)  5.5  Is •* a* (a^)  2  533.86 (15)  4.2  Is •+ a* (e)  3  537.72 (20)  0.4  538.1  0  0 Is l i m i t 4  a  Term Value (eV)  Ref. [179].  3  542 - 550  inner well trapped state/ shape-resonance  shape-resonance  - 220  -  the p e r i p h e r a l p o s i t i o n of the core h o l e . g r e a t e r than i n the case of 0 P F i n keeping w i t h t h a t observed  3  T h i s d i f f e r e n c e i s much  (compares T a b l e s 6.3  for PF  and  5  the molecules  F e a t u r e s 1 and 2 are a s s i g n e d as t r a n s i t i o n s orbitals respectively.  to the  i s i n t e n s i t y on the h i g h energy  6.7)  but more  i n Chapter  a (a^) and  There does not appear to be any  i n t e n s i t y f o r a t r a n s i t i o n to the second  such a t r a n s i t i o n  and  a (e)  significant  a (a^) o r b i t a l , however, t h e r e  s i d e of f e a t u r e 2 which may  (compare P 2p spectrum, T a b l e 6.4  be due  The weak s t r u c t u r e , l a b e l l e d 4,  a shape-resonance.  F i g u r e 6.9  The  spectrum  the p r e v i o u s l y p u b l i s h e d o p t i c a l spectrum  [173]  The  to  spectral  i s i n good agreement w i t h and  i s s i m i l a r to other  e a r l i e r r e p o r t e d C l 2p,2s s p e c t r a such as those f o r P C 1 methanes [182].  state/  i s also attributed  shows the C l 2p,2s s p e c t r a .  data i s summarised i n T a b l e 6.9.  to  and F i g . 6.4).  F e a t u r e 3 i s a s s i g n e d to a t r a n s i t i o n to an i n n e r - w e l l trapped shape-resonance.  5.  3  and  the c h l o r o -  O v e r l a p p i n g bands c o m p l i c a t e the s p e c t r a l assignment.  F e a t u r e s 1 and 2 are a s s i g n e d to the s p i n - o r b i t components of  the  2p •*• a ( a j ) t r a n s i t i o n .  orbitals  (a(e),  T r a n s i t i o n s to the other v i r t u a l  a ( a ^ ) ) are a l s o expected.  Using  * o r b i t a l s from ~2.61  The  The  *  *  ( a ( a ) - a (e) ~ 1.75 1  eV) as a guide, f e a t u r e s 3 and  transition. 6.  the P 2p spectrum  the s e p a r a t i o n of the  eV,  virtual  *  a ( a ) - a (aj) 1  5 are assigned to the 2p •*• a (a^)  4s Rydberg t r a n s i t i o n s are a s s i g n e d to f e a t u r e s 4 and  r e s t of the Rydberg t r a n s i t i o n s are on top of the r a p i d r i s e i n  oscillator  s t r e n g t h , g i v i n g r i s e to f e a t u r e s 7 and  8, which are a s s i g n e d  to a t r a n s i t i o n to the i n n e r - w e l l trapped s t a t e or shape-resonance. Feature 9 i s thought  due  to a  a ( P - C l ) shape-resonance s i n c e 6 ( t h e  - 221 -  Kfc Cl 2 p edges r—i  111HI i  I 235678  9  10  OPCI  AE=0.36eV  10 -  3  Cl 2p,2s  rr  n  n  205  —I— 225  1 — n — I 3 4  12  r  245  4\£  2P3  2  I— 265  r  fcCI 2s edge  2  I I —  5 6  285  C I 2 p edges  7  8  10  AE=O.I8eV 5 -  200  T  204  ENERGY  208  T  212  2 16  L O S S (eV)  F i g u r e 6.9: High r e s o l u t i o n (0.18 eV FWHM) e l e c t r o n energy l o s s spectrum o f the C l 2p r e g i o n o f 0PC1 (lower t r a c e ) . The upper t r a c e shows the combined C l 2p,2s r e g i o n recorded w i t h a r e s o l u t i o n o f 0.36 eV FWHM. 3  222  -  -  TABLE 6.9 E n e r g i e s , Term V a l u e s , and P o s s i b l e Assignments for  Feature  Energy  Loss^ ^  Term V a l u e  3  (eV)  3 / 2  1 2 3 4 5 6 7  200.82 202.35 203.62 204.28 205.27 205.93 207.52  8  209.22 (12)  limltfJJ limit  206.9 208.5  9 10  216.65 223  ( b )  / 2  Possible  Assignments^^  (eV) 2  2p 2pi  of OPCI3  the C l 2p,2s Regions  2pi/  P3/2  6.0 4.5 3.2 2.6 1.6 0.9 -0.7  (10)  (10) (10) (10)  2p  2  3 / 2  2P1/2  o*(a,) 6.1 4.8 4.2 3.2 2.5 0.9  a ( a ^ 4s  o*(a, ) o*(eJ o*(a ) 4s 1  etc. inner well state/shape resonance'  -0.8  inner well state/ shape-resonance  0 0 -16  o-*(Cl-P) Bht ape resonance " shaVce-up"  T  2s 11 2s  limit  ( c )  272.6 275.7 278.26  (a) E s t i m a t e d u n c e r t a i n t y i n energy (b) 2p edge R e f . [ 3 1 ] ; 2 p , 3/  splitting (c)  2  5.6 2.5 0  2s + 0 * 2s •* a  l o s s ± 0.08 eV u n l e s s o t h e r w i s e s t a t e d .  = 2p (mean) - 0.53 eV; 2 p / 1  2  = 2p (mean) + 1.07 eV; A  of 1.6 eV [18] has been assumed.  2s edge r e f . [ 3 1 ] .  (d) F i n a l o c c u p i e d o r b i t a l  with e i t h e r  2p-j/ o r 2 p ^  *With r e s p e c t t o C l 2p (mean) edge [ 3 1 ] .  2  2  hole  state.  spin-orbit  - 223  n e g a t i v e of the term v a l u e ) i s ~9.3  average  v a l u e of 10.8  resonances  eV observed  from the P 2p s p e c t r a .  -  eV, which i s reasonably c l o s e to the  *  for a (P-Cl)  (see T a b l e 6.5)  A weak f e a t u r e 10 i s p o s s i b l e due  to  "shake-up". CONCLUSIONS The  c e n t r a l atom and  l i g a n d ISEELS s p e c t r a measured f o r a s e r i e s  of t r i v a l e n t phosphorus compounds presented i n Chapter extended 0 P F , and 3  answered.  5 have been  to i n c l u d e the h i g h e r c o o r d i n a t e d phosphorus compounds OPClg.  A number of q u e s t i o n s r a i s e d i n Chapter  PF , 5  5 have been  The use of the P 2s term v a l u e s to a s s i g n the P 2p s p e c t r a i s  *  f u r t h e r supported  by the present work.  The  first  separated i n the p r e s e n t l y s t u d i e d molecules  two  5  to be w i t h i n ~0.1  l e v e l s are w e l l  and hence the assignment of  the core to v a l e n c e t r a n s i t i o n s i s l e s s ambiguous. ( o r a (e') f o r P F )  a  term v a l u e s f o r the P 2s and  The  r e s p e c t i v e a (e)  2p s p e c t r a were  found  eV of each o t h e r , which lends c o n f i d e n c e to the *  o r d e r i n g of the a  l e v e l s as g i v e n i n the p r e v i o u s c h a p t e r , and  also  i n d i c a t e s t h a t term v a l u e s are t r a n s f e r a b l e when the core h o l e i s l o c a t e d on the same atom. observed  for PF  3  and PC1  3  The  continuum s t r u c t u r e was  i n t h a t there was  s i m i l a r to t h a t  an i n t e n s e f e a t u r e at or  j u s t beyond the edge a t t r i b u t a b l e to an i n n e r - w e l l trapped resonance spectrum spectra.  f o l l o w e d by broad  shape resonances.  here showed an e x t r a resonance  state/shape-  In a d d i t i o n , each  i n comparison w i t h the  T h i s i s d i r e c t l y a t t r i b u t a b l e to the f a c t t h a t the  here each possess  two  d i f f e r e n t k i n d s of l i g a n d s .  PX  3  molecules  A comparison of the  - 224 -  resonance  p o s i t i o n s i n the P 2p s p e c t r a i n P C 1  3  and O P C I 3 confirmed  the  * a ( P - C l ) nature of the resonance s i o n was  i n these m o l e c u l e s .  reached i n the assignment  of the f e a t u r e i n OPF  * a (P-F)). The resonances i n the P 2p P F t h e r e b e i n g two types of f l u o r i n e s i n P F  of 0 P C 1  A comparison second  resonance  3  and OPF  3  5 5  The absence  T h i s lends support to the  to be due 3  3  found f o r PH . 3  of continuum  3  are due  exis-  p o s i t i o n and bond f o r the molecules  3  I t i s p o s s i b l e that  to "shake-up" p r o c e s s e s , as  O b v i o u s l y great care must be e x e r c i s e d i n the  the  was  assignment  f e a t u r e s which can be a s c r i b e d to one or more of s e v e r a l  e f f e c t s i n c l u d i n g t r a p p e d - i n n e r w e l l s t a t e s , resonances, double  (i.e. ,  to a d i f f e r e n t phenomenon  being r e s p o n s i b l e f o r the s t r u c t u r e i n P ( C H ) . features i n P(CH )  3  *  of such a r e l a t i o n s h i p  s t u d i e d i n Chapter 5 would appear  continuum  and P F  confirmed the a ( P - 0 ) nature of the  i n those m o l e c u l e s .  [96-100].  3  spectrum were c o n s i s t e n t w i t h ( i . e . , a x i a l and e q u a t o r i a l ) .  tence of some s o r t of r e l a t i o n s h i p between resonance length  A similar conclu-  excitation.  "shake-up" or  - 225 -  CHAPTER 7  ELECTRONIC EXCITATION IN PHOSPHORUS-CONTAINING MOLECULES. I I I . VALENCE SHELL ELECTRON ENERGY LOSS SPECTRA OF P(CH,),, PCI,, PF,,  Very l i t t l e  OPCI,, and  I n f o r m a t i o n on the v a l e n c e s h e l l e x c i t a t i o n  e x i s t s i n the l i t e r a t u r e on the molecules chapters. compounds  s t u d i e d i n the p r e v i o u s two  The o n l y r e p o r t e d VSEELS spectrum i s t h a t f o r PH  3  referred  processes  f o r any of t h i s s e r i e s of  to i n the book by Robin [12].  However, t h e r e have been s e v e r a l p h o t o a b s o r p t i o n s t u d i e s on some o f these molecules,  but o n l y over a r a t h e r l i m i t e d energy  range (up t o  =10 eV) d i c t a t e d by the use of c o n v e n t i o n a l o p t i c a l spectrometers light  sources.  These s t u d i e s i n c l u d e d s p e c t r a of P F  [184-186], P ( C H ) , and 0PC1 3  3  3  [185], as w e l l as PH  3  3  and P C 1  and  3  [178,184,185].  In  t h i s chapter the VSEELS s p e c t r a of P ( C H ) , P C 1 , 0PC1 , P F , and P F 3  are p r e s e n t e d up to 20 eV and beyond. the a i d of the ISEELS r e s u l t s  3  3  3  3  The s p e c t r a are i n t e r p r e t e d  from Chapters  5 and 6.  5  with  A comparison of  these two t e c h n i q u e s was u s e f u l i n t e n t a t i v e l y a s s i g n i n g the VSEELS s p e c t r a o f , NF  3  and S i ( C H ) 3  4  (see Chapters  s p e c t r a are g e n e r a l l y r e l a t i v e l y  3 and 4 ) , s i n c e the ISEELS  simple to a s s i g n due to the energy  i s o l a t i o n of the I n i t i a l core h o l e , which unambiguously d e f i n e s the i n i t i a l o r b i t a l of the t r a n s i t i o n .  EXPERIMENTAL The  DETAILS  s p e c t r a were a l l o b t a i n e d on the spectrometer  described i n  - 226 -  Chapter  2.  An impact  energy of 2.5 keV was used  to o b t a i n the s p e c t r a  with the s c a t t e r e d e l e c t r o n s sampled a t zero degree The  s c a t t e r i n g angle.  s p e c t r a were o b t a i n e d w i t h a t y p i c a l energy r e s o l u t i o n of 0.035 -  0.050 eV.  At zero degree  background-free  s c a t t e r i n g i t i s not always  o p e r a t i o n i n the v a l e n c e s h e l l r e g i o n .  e s p e c i a l l y the case f o r P C 1 indicated  p o s s i b l e to o b t a i n  3  and P F .  that the background  5  T h i s was  However, removal of the gas  s i g n a l was smoothly  v a r y i n g and possessed  no sharp f e a t u r e s due to " g h o s t i n g " e f f e c t s a r i s i n g from r e f l e c t i o n of the primary beam, br secondary the e x c e p t i o n of P C 1 (21.218 e V ) .  3  e m i s s i o n from e l e c t r o d e s u r f a c e s .  With  a l l s p e c t r a were c a l i b r a t e d w i t h the He(I) l i n e  Initially  the P C 1  HCl i m p u r i t y that was used  3  sample as s u p p l i e d c o n t a i n e d a s m a l l  to c a l i b r a t e the spectrum.  were then removed by continuous pumping on a P C 1 with a d r y - i c e / m e t h a n o l m i x t u r e .  3  A l l t r a c e s of H C l  sample t h a t was c o o l e d  A s i m i l a r procedure was used f o r  0PC1 , as a p r e c a u t i o n a r y measure even though no HCl was 3  immediately  apparent.  RESULTS AND  DISCUSSION  B e f o r e d i s c u s s i n g the VSEELS s p e c t r a i t i s u s e f u l to review some of the p e r t i n e n t p o i n t s from the d i s c r e t e p o r t i o n of the P 2p s p e c t r a r e p o r t e d f o r these compounds.  With  the e x c e p t i o n of P ( C H ) 3  3  a l l P 2p  s p e c t r a show s t r o n g core •*• v i r t u a l v a l e n c e t r a n s i t i o n s w e l l s e p a r a t e d from the core -*• Rydberg t r a n s i t i o n s . transitions  I t i s p o s s i b l e to a s s i g n these  to those going to the f i n a l o r b i t a l s expected from a minimal  b a s i s s e t . The o r d e r i n g of the p r e v i o u s l y unoccupied v a l e n c e  orbitals  - 227  was  e s t a b l i s h e d by comparison  The  remaining  -  of the P 2p s p e c t r a w i t h the P 2s s p e c t r a .  t r a n s i t i o n s can be a s s i g n e d to Rydberg t r a n s i t i o n s .  these o r i g i n a t e from a 2p l e v e l , the dominant Rydberg t r a n s i t i o n s a s s i g n e d as those going to the s and d Rydberg l e v e l s . t i o n i s f o r m a l l y d i p o l e f o r b i d d e n i n the pure atomic would be expected  to have lower  symmetric molecules  [61].  o b t a i n e d from Chapters  values.  transi-  case and hence  T a b l e 7.1  summarizes the term v a l u e s  5 and 6 f o r t r a n s i t i o n s to the v i r t u a l  (T)  valence  A l s o shown i s the  o b t a i n e d f o r the s Rydberg s e r i e s from the 4s  term  These quantum d e f e c t s a l l l i e between ~1.7-1.9, which i s o n l y  s l i g h t l y lower  than the "expected"  s e r i e s of the t h i r d  (Na-Ar) row  quantum d e f e c t of 2 f o r the s Rydberg  [12].  On moving from the core to the valence r e g i o n the term would be expected  to be lower  easily rationalised  values  f o r the v i r t u a l v a l e n c e l e v e l s .  i n terms of the l o c a t i o n of the h o l e .  This i s  The  l o s s of  s h i e l d i n g caused  by the removal of a valence e l e c t r o n should be  than t h a t caused  by the removal of a l o c a l i s e d core e l e c t r o n from  c e n t r e of the molecule  and  a whole e x t r a u n i t of charge  case of a c e n t r a l core h o l e and hence has a h i g h e r term value binding energy). 3  and  less  so the e l e c t r o n i n the newly o c c u p i e d  o r b i t a l sees something approaching  to NF  are  i n t e n s i t y , e s p e c i a l l y i n more h i g h l y  o r b i t a l s and a l s o to the a s s i g n e d 4s Rydberg l e v e l . quantum d e f e c t (6)  A p •*• p  Since  the a  i n the  (i.e.,  T h i s e f f e c t has been d i s c u s s e d p r e v i o u s l y w i t h r e g a r d  i s s i m i l a r to the e f f e c t which occurs when the core h o l e i s  l o c a t e d on a l i g a n d as opposed to the c e n t r a l atom (see the p r e v i o u s chapters).  The  e f f e c t of the i n i t i a l o r b i t a l vacancy  location  on  - 228 -  TABLE 7.1 Term values (T) for phosphorus L - s h e l l spectra and the calculated s o r b i t a l quantum defects (6)  Term Value Molecule  Virtual o*(l)  PH3  P F  5.1 6.8  3  PCI  6.9  3  (eV)  Orbitals <T*(2)  ( a )  Rydberg •4s  Quantum^ ^ Defect 6  2.48  1.66  ( b )  o*(3)  4.5 ~3.5<«> 6.4  -  3.12  1.91  3.06  1.89  -  2.86  1.82  P(CH3)3  3.3  OPCI3  7.0  6.2  4.4  3.08  1.90  PF5  6.2  3.6  -0.7^)  2.71  1.76  From Chapters  ~2.0< > d  5 and 6.  (a) Term values are with respect to the P 2p edge except where stated. (b) Symmetries of the unoccupied Chapters 5 and 6 are:  v i r t u a l o r b i t a l s (as assigned i n  o*(l) PH 3 ,  P(CH3)3>  PF3  e  o*(2)  PCI3  a  l  l e  0PC1,  a  l  e  PFc  o*(3)  a  •i  a  l  a  2  (c)D i f f i c u l t to locate accurately, mean of 2 p ^ and 2s term values used. 3  (d)Mean term value of the two possible assignments given. (e) This term value i s with respect to the P 2s edge. (f) Calculated from T = 13.605/(n-6) number.  where n = p r i n c i p a l quantum  - 229 -  Rydberg term v a l u e s should be somewhat l e s s because ( i ) the term for  Rydberg o r b i t a l s a r e s m a l l e r than those f o r the (LUMO) v a l e n c e  orbitals;  ( i i ) the Rydberg o r b i t a l s a r e l a r g e and d i f f u s e and w i l l  t h e r e f o r e tend to see the molecule In  as one l a r g e c o r e .  a s s i g n i n g the VSEELS s p e c t r a an attempt  l o c a t e the valence •* v i r t u a l v a l e n c e t r a n s i t i o n s . it  values  is initially  will first  I n order t o do t h i s  assumed t h a t the term v a l u e s a r e independent  o r i g i n a t i n g valence o r b i t a l .  A l l expected  be made t o  of the  term v a l u e s a r e c a l c u l a t e d  from the e x p e r i m e n t a l v e r t i c a l i o n i s a t i o n p o t e n t i a l s as o b t a i n e d photoelectron spectroscopy. summarised i n T a b l e 7.2.  from  The v e r t i c a l i o n i s a t i o n p o t e n t i a l s are  Once the v a l e n c e •»• v i r t u a l v a l e n c e  transitions  are l o c a t e d , the p o s s i b l e v a l e n c e ->• Rydberg t r a n s i t i o n s w i l l be assigned.  The term v a l u e s o b t a i n e d from Chapters  5 and 6 w i l l be  assumed to g i v e upper bounds f o r those i n the v a l e n c e s p e c t r a . evidence  from the l i g a n d  Other  ISEELS s p e c t r a w i l l be c o n s i d e r e d at a p p r o p r i -  a t e j u n c t i o n s i n the f o l l o w i n g d i s c u s s i o n of the v a r i o u s VSEELS spectra.  T r i m e t h y l Phosphine (P(CH-,)-,) The VSEELS spectrum  of P ( C H ) 3  3  from 4-25 eV i s shown i n F i g . 7.1.  The e n e r g i e s , term v a l u e s and p o s s i b l e assignments a r e summarised i n T a b l e 7.3. spectrum  The o n l y p r e v i o u s l y r e p o r t e d v a l e n c e s h e l l  i s the UV a b s o r p t i o n spectrum  o n l y extends  up to ~6.7 eV.  a b s o r b i n g , broad  excitation  r e p o r t e d by Halmann [185] which  T h i s UV spectrum  [185] shows a s t r o n g l y  band c e n t e r e d a t ~6.15 eV, i n very good agreement w i t h  - 230 -  TABLE 7.2 Molecular o r b i t a l s ^ ^ and experimental ionisation p o t e n t i a l s ' ^ (eV) for the 3  valence o r b i t a l s of PH-j, PF3, PCI3, P(CH ) , OPCI3, and P F 3  MO  PH3  l  10.58  2e  13.50  5  a  21.2  M0 > (c  PF  3  PCI3  P(CH ) 3  3  3  MO  OPCI3  M0  lie  11.93  2e"  15.54  12.40  6e'  16.46  12.94  5a 2  16.75  13.48  8a I  17.13  13.86  la^  17.79  15.35  5e'  18.43  8e  16.50  le"  19.1  llaj  19.53  4a 2  19.5  12.29  10.52  8.65  15.89  11.69  11.50  4e  16.31  11.99  11.25  lOe  3e  17.45  12.94  13.25  13a  l  18.57  14.23  13.70  9e  2e  19.36  15.19  14.60  l  22.60  18.81  16.65  4  3  2  a  a  & 1  2a2  12  x  3l  4e'  ( a )  5  -21  3  [189,190] for OPCI3 and [191] for P F  V  From PES, [192] for PH ; [118] for PF ; [193] for PCI3; [190] for 0PC1 3 ; 3  3  [193,194] for PFj and [195] for P ( C H ) . 3  v  PF  0rder from [176] for PH ; [133,176,187,188] for PF ; [188,189] for PCI3 3  ( b )  5  'Numbering ignoring  core-levels.  3  - 231 -  1  23 4  5  6  P(CH ) Valence 3  3  H  CO  LU h-  crV4s  (C)4s  5 s  _1_  2e  I 3a,  LJ >  3e <r'(e)  LU  i  <C)4s cr/4s <r74s  I  '3d  5s  • 1 '3d  3 d 5 s  / '  '02  (C)4S <r74s ; J  -§4e  5s '3d  J °. 4  10  n—i—i—r  i  r 20  ENERGY LOSS (eV)  i—r 25  F i g u r e 7.1:  The v a l e n c e s h e l l e l e c t r o n energy l o s s spectrum of P ( C H ) . The i o n i s a t i o n edges a r e taken from p h o t o l e c t r o n s p e c t r o s c o p y (see T a b l e 7.2). P o s i t i o n s of v a l e n c e - v a l e n c e ( t a l l b a r s ) , valence-Rydberg ( s h o r t b a r s ) t r a n s i t i o n s as e s t i m a t e d from the term v a l u e s a r e shown t o each l i m i t (see T a b l e 7.3). The dashed bar i n d i c a t e s term v a l u e e s t i m a t e d from the C Is spectrum. 3  3  - 232 -  TABLE 7.3 E n e r g i e s , term v a l u e s and p o s s i b l e  assignments  f o r the VSEELS spectrum of P ( C H ) . 3  Term Value  (eV)  3  Feature  Energy Loss (eV)  1  6.11  2.44  4aj -• a * ( e ) , a * ( a ) / 4 s  2  7.23  1.42  4a  3  7.46  1.19  4a j -»• 5s  4  8.24  3.01  4e •»• 4s  5  9.8  6  10.6  7  13.4  ( a )  Possible  Assignments  1  x  -> 3d  Term v a l u e s are c a l c u l a t e d w i t h r e s p e c t to the IP of the i n i t i a l orbital. These IP's are g i v e n i n Table 7.2.  - 233 -  feature the  1 o f the spectrum r e p o r t e d  i n t e n s i t y of t h i s f e a t u r e  the  Robin has  suggested  [12]  that  i n d i c a t e s that i t has a l a r g e HOMO  (4a )->o(a ) v a l e n c e component. 1  here.  This feature  1  i s also l i k e l y  t r a n s i t i o n from the HOMO to the 4s Rydberg l e v e l ;  t o encompass  indeed, i t i s  * likely  that the  0 ( a j ) and 4s w i l l  p a i r as d i s c u s s e d  by F r i e d r i c h e t a l . [65].  i s 2.44 eV and Robin  [12]  i n character recent 4p  X  the 4a^  orbital  (65% P 3p,  l e v e l should  not  be very  formula to the term v a l u e 3 c o u l d be the 4a^ ascribed  to the 4a^  However, ab i n i t i o  calculations  [187]  T h i s r e s u l t i s a l s o borne out be  (60% P 3p, intense  11% P 3 s ) .  [61].  Thus a t r a n s i t i o n t o  A p p l i c a t i o n o f the  o f f e a t u r e 1 (2.44  •*• 5s Rydberg t r a n s i t i o n . •*• 3d Rydberg t r a n s i t i o n .  f e a t u r e i s 1.42 eV, which i s c l o s e t o the value  of f e a t u r e 1  (the phosphorus lone p a i r ) i s l a r g e l y P 3p  14% p 3 s ) .  c a l c u l a t i o n s [176]  a  The term v a l u e  has i n d i c a t e d that t h i s would be a p p r o p r i a t e  f o r a t r a n s i t i o n to a 4p l e v e l . i n d i c a t e that  form a s t r o n g l y mixed valence-Rydberg  eV)  Rydberg  i n d i c a t e s that  feature  F e a t u r e 2 can then be The term value  for  this  1.51 eV expected f o r a 3d term  c a l c u l a t e d u s i n g a quantum d e f e c t of z e r o . The  term v a l u e s  outer-most 4a^ i n order  obtained  f o r a l l the above t r a n s i t i o n s from  o r b i t a l have been a p p l i e d w i t h r e s p e c t  t o the other  to p r e d i c t the p o s i t i o n s of the l e v e l s l e a d i n g  ive l i m i t s .  to the  the IP's  respect-  These e s t i m a t e d p o s i t i o n s are i n d i c a t e d on F i g . 7.1 and i t  can be seen t h a t the p o s i t i o n of t r a n s i t i o n s l e a d i n g  to the 4e and l a 2  l i m i t s c o n t r i b u t i n g to f e a t u r e 4 are over estimated on t h i s b a s i s . The term v a l u e  f o r f e a t u r e 4 from the next (4e)  following o r b i t a l s  o r b i t a l i s 3.01 eV. The  (4e-3a,) have s i g n i f i c a n t C 2p c h a r a c t e r  [176,187]  - 234 -  and  the term v a l u e of 3.01 eV i s much more l i k e t h a t observed f o r the  C Is •*• 4s t r a n s i t i o n .  T h i s i s s i m i l a r t o the f i n d i n g s f o r S K C H j ) ^  where i t was suggested t h a t the f i r s t  Rydberg l e v e l belonged  l i g a n d as opposed to the molecule as a whole.  t o the  Pending a d e t a i l e d  theo-  r e t i c a l treatment, n o t h i n g much can be s a i d of the r e s t of the spectrum except that i t i s b u i l t up of a number of o v e r l a p p i n g t r a n s i t i o n s t o the a  and Rydberg  levels.  Phosphorus T r i c h l o r i d e  (PCI,)  F i g u r e 7.2 shows the VSEELS spectrum o f P C 1 data i s summarised i n Table 7.4. t h a t of P ( C H ) , 3  The spectrum  3  from 4-20 eV.  i s much more complex than  having many more c l e a r l y r e s o l v e d t r a n s i t i o n s .  3  a b s o r p t i o n spectrum  reported e a r l i e r  The  The UV  [184,186] a t s l i g h t l y h i g h e r r e s o -  l u t i o n shows v i b r a t i o n a l s t r u c t u r e , but the s p e c t r a o n l y extend as f a r as 10 eV.  The term v a l u e s f o r f e a t u r e s 1 and 2 from the HOMO ( 4 a j )  o r b i t a l are 4.83 and 4.54 eV r e s p e c t i v e l y .  S i n c e these are both much  h i g h e r than the term v a l u e f o r the 4s Rydberg l e v e l 7.1) from the P 2p ISEELS spectrum,  *  (3.06 eV - T a b l e  they are a s s i g n e d as t r a n s i t i o n s to  *  the a ( a j ) and a ( e ) v i r t u a l v a l e n c e l e v e l s , r e s p e c t i v e l y . o r d e r i n g i s taken from t h a t g i v e n i n Chapter  5.  The o r b i t a l  However, these  levels  are c l o s e t o g e t h e r and the order c o u l d e a s i l y be i n f a c t r e v e r s e d . A p p l y i n g the term v a l u e s thus o b t a i n e d to the o t h e r v a l e n c e l e v e l s i t can be seen ( F i g . 7.2) that much of the spectrum r e a s o n a b l y a s s i g n e d to v a l e n c e - v a l e n c e t r a n s i t i o n s .  can be  The f e a t u r e s that  remain unassigned a r e t h e r e f o r e a t t r i b u t e d t o Rydberg t r a n s i t i o n s .  Thus  - 235 -  PCI3 v..  Valence  CO o,e  LU  4s  a,e  4s  LU > LU  cr  o,e  4s  4  4s  I I 12  P  13d  2  e  fir  '3a,  1  4e  13d' a,e  |  "3d  4s 3d 1 r ?4a, -i—1—•— 4p 13d II 34  III 567  I II I I I I 8 91011 12 13 14  n — 1 — 1 — 1 — 1 — 1 — r — n — 1 — 1 — 1 — 1 — 1 — 1 — r  10  15  20  E N E R G Y L O S S (eV)  F i g u r e 7.2: The v a l e n c e s h e l l e l e c t r o n energy l o s s spectrum of PCI 3 . The i o n i s a t i o n edges a r e taken from p h o t o e l e c t r o n s p e c t r o s c o p y (see Table 7.2). P o s i t i o n s of the v a l e n c e - v a l e n c e ( t a l l b a r s ) , valence-Rydberg ( s h o r t b a r s ) t r a n s i t i o n s e s t i m a t e d from the term v a l u e s a r e shown l e a d i n g t o each l i m i t ( s e e T a b l e 7.4).  - 236 -  TABLE  Energies  Feature  (a)  and p o s s i b l e assignments f o r  Energy Loss (eV)  1 2 3 4 5 6 7  the VSEELS spectrum of P C I ,  Valence - Valence Assignment  (a)  Valence - Rydberg Assignment  (b)  4a j -»• c^Caj)  5.68 5.97 7.03 7.43 8.25 8.63 9.05  4a  la^  4e  ^(aj)  4e °*(e) 3e 3e ->• a (e) 3a j  8  9.85  9  10.48  2e  10 11 12 13 14  10.76 11.40 11.96 12.64 13.42  2e  * o^(ai) a (e* IP's:  7.4  term v a l u e = 4.83 term value = 4.54 see T a b l e 7 . 2 .  4a j  • cr ( a , ) • a (e) °*  a  ( a  l  4s 4p  4a, 4a 1  •*•  3d,  4e *  4s  3e -»• 4s la2  )  (e)  + 3d,  3e -* 3d, 3a^ 2e + 4s, 2e  4e -»• 3d  3 a i •+ 4s * 4p 3aj •+ 3d ->• 3d  eV, eV,  (b) Only f i r s t 4s  member of  term value = 2.95  IP's:  see  Table  7.2.  series  given:  e V , 4p term value  = 2.26  eV and 3d term v a l u e = 1.46  eV.  - 237 -  the sharp f e a t u r e 7 i s d e s c r i b e d as a combination  of the 4a ^ •*• 3d and  4e •* 4s Rydberg t r a n s i t i o n s , the term v a l u e s of which a r e 1.46 eV and 2.95  eV r e s p e c t i v e l y ,  t h a t which i s expected  As w i t h P C C r l ^ g , the 3d term v a l u e Is c l o s e t o (1.51 eV) w i t h a quantum d e f e c t of z e r o .  The 4s  term v a l u e (2.95 eV) i s o n l y ~0.1 eV l e s s than t h a t i n the ISEELS spectrum.  The 4s term v a l u e from the C l 2p spectrum  i s ~2.6 eV.  These  term v a l u e s (as w e l l as those c a l c u l a t e d f o r the 5s and 4d l e v e l s ) have been a p p l i e d t o the other v a l e n c e o r b i t a l IP's and the p o s i t i o n s of the c o r r e s p o n d i n g l e v e l s a r e i n d i c a t e d on the spectrum  ( F i g . 7.2). I t can  be seen t h a t a l l the major f e a t u r e s a r e reasonably accounted  f o r by t h i s  t e n t a t i v e assignment p r o c e s s . F e a t u r e 5, which shows a c l e a r v i b r a t i o n a l p r o g r e s s i o n i n the UV spectrum  [186] ( w i t h the same v i b r a t i o n a l s p a c i n g as i n the f i r s t  i o n i s e d s t a t e i n the p h o t o e l e c t r o n spectrum by Robin (2.26  [117]) has been a t t r i b u t e d  [12] t o a 4a ^ -* 4p Rydberg t r a n s i t i o n .  The term  eV) i s i n between those of the 4s and 3d l e v e l s .  should have some P 2s c h a r a c t e r . [189] the 4 a  x  value  The a^ o r b i t a l s  A c c o r d i n g to ab i n i t i o  calculations  o r b i t a l has 14% P 3s c h a r a c t e r and 32% P 3p c h a r a c t e r .  Thus a 4a^ •*• 4p t r a n s i t i o n might be expected.  Feature 5 i s thought t o  have t h i s t r a n s i t i o n on top of the 3e •*• a (a^) t r a n s i t i o n .  The c a l c u l a -  t i o n a l s o i n d i c a t e s t h a t the 3a^ o r b i t a l has 3% P 2s c h a r a c t e r .  Apply-  i n g the term v a l u e o f 2.26 eV to the 3a± i o n i s a t i o n p o t e n t i a l y i e l d s a 3a ^ •* 4p t r a n s i t i o n energy feature  of 11.97 eV, i n e x c e l l e n t agreement w i t h  12 (11.96 e V ) . The r e s t o f the outer v a l e n c e o r b i t a l s  have no s c h a r a c t e r (as a l s o i n d i c a t e d i n the c a l c u l a t i o n  should  [189] and  - 238  hence l i t t l e  I n t e n s i t y to the p Rydberg  Phosphorus T r i f l u o r i d e The  VSEELS spectrum of P F  spectrum (up  to ~10  eV)  As has  i s shown i n F i g . 7.3,  3  i n the case of PC1 , been r e p o r t e d  to that of the  first  i s q u i t e d i f f e r e n t to t h a t of PC1  [184,186].  feature  3) f o l l o w e d  sharp f e a t u r e s i n contrast  by  overlapping  that c o u l d be o b v i o u s l y  to the  s i t u a t i o n i n PC1 .  s i m i l a r to the i n n e r - s h e l l s p e c t r a of P F ,  The  spectrum 1 +  There are  no  to Rydberg t r a n s i t i o n s ,  which are  3  a  bands ( f e a t u r e s  broad f e a t u r e s .  assigned  spectrum  has  [192].  In t h i s regard  3  l e s with h i g h l y electronegative  In the UV 3  intense  the data i s  absorption  l i k e PC1 ,  ionised state  w i t h two  3  and  the UV  3  f e a t u r e 3 shows v i b r a t i o n a l s t r u c t u r e and,  s i m i l a r spacing  2 and  levels.  (PF,)  summarised i n Table 7.5.  [186]  -  t h i s spectrum i s t y p i c a l of molecu-  l i g a n d s , i n that s t r o n g  t r a n s i t i o n s to  the v a l e n c e l e v e l s are observed at the expense of Rydberg t r a n s i t i o n s . The are 4.38  term v a l u e s  eV and  4.12  for features  eV  1 and  respectively.  2 from the  Robin  4a  [12] has  1  (HOMO) o r b i t a l  assigned  these In  * t u r n as due  to the t r a n s i t i o n s 4a ^ •*• 4s and  the P 2p ISEELS spectrum i n d i c a t e s that the therefore  the  4s term v a l u e  expected to be s m a l l e r . i n the present  work to a  a (e) l e v e l a term value  Ua-^ -*• a  (P-F).  4s term v a l u e  i s 3.13  i n the v a l e n c e s h e l l spectrum would  Therefore,  features  transitions. of 6.8  eV and  The  1 and  2 have been  features  1 and  t h a t f o r the  2 are due  eV  to the  assigned  a (a^) of ~3.5  4a^  *  and  be  ISEELS assignment gave  Assuming that t h i s assignment i s c o r r e c t , i t i s u n l i k e l y t h a t  narrowly separated  However,  the eV.  the  -•• o (e) and  4a^ •>•  - 239 -  1  1—  e  CO  a,/4s  1a2  LU  r  >  LU  e  I  T  °./4s  5s  e  5s  e  5s  ;2e  l—  —r— a,/4s  :2a,  o,/4s  IZ I  Valence  4e  5s  e  PF3  3e  -1— 5s  o,/4s  a, / 4 s  3a,  —I— 5s  1  LU  tr  i  |  10  i  I  l  I  |  15 ENERGY  Figure  i  1  i  |  I  20  i  I  I  I  |  25  LOSS(eV)  7.3:  The v a l e n c e s h e l l e l e c t r o n energy l o s s spectrum of P F . The i o n i s a t i o n edges are taken from p h o t o e l e c t r o n s p e c t r o s c o p y (see T a b l e 7.2). P o s i t i o n s of t r a n s i t i o n s from the 4a ^ o r b i t a l have been a p p l i e d to the other I P ' s . The spectrum i s summarised i n Table 7.5. 3  - 240 -  TABLE 7.5 Energies,  term v a l u e s  for  and p o s s i b l e  the VSEELS spectrum of P F 3  Feature  Energy Loss (eV)  Term Value ( e V ) ( a )  1  7.91  4.38  2  8.17  4.12  3  9.49  2.79  4  10.89  5  11.20  6 7  8  assignments  Possible  Assignments  * 4aj •*• a (e) *  4a^  -»• a ( a ^ M s  4.69  la,  -»• a (e)  11.64  4.68  4e -> a (e)  13.18  4.25  * 3e •»• o (e) 4e •*• a (a^ )/4s  (13.52)*  (2.79)  14.21  4.36  3a.  a*(e) * 3e + a ( a ^ ) / 4 s 1  (14.56)  f  (2.79)  9  14.93  4.43  2e •+ o*(e)  10  15.77  2.80  3a^ •* a ( a ^ M s  11  16.88  2.48  2e •> o * ( a 1 ) / 4 s  12  18.09  4.51  2a  ^ 'Term v a l u e s orbital.  are c a l c u l a t e d w i t h r e s p e c t  l  to the IP of the i n i t i a l  These I P ' s are g i v e n i n T a b l e 7 . 2 . ^ P o s i t i o n estimated  u s i n g term v a l u e  + a*(e)  from f e a t u r e  3.  (?)  - 241  a (aj) transitions.  -  T h e r e f o r e , f e a t u r e s 1 and  2 are assigned to Jahn-  T e l l e r components of the 4a^ •+• a (e) t r a n s i t i o n which would l e a d to a degenerate f i n a l s t a t e .  a b s o r p t i o n spectrum of P F .  The  3  f o r the v a l e n c e - s h e l l o p t i c a l  d i f f e r e n c e of ~2.5  ISEELS and VSEELS term v a l u e s f o r the  7.3).  Feature  f o r NF  3  E  T h i s c o n c l u s i o n concurs w i t h that g i v e n  p r e v i o u s l y by McAdams et a l . [186]  w i t h that found  i  (see Chapter  eV between the  a (e) LUMO o r b i t a l i s i n k e e p i n g 3) and PC1  (compare Tables 7.1  3  3 i s then a s s i g n e d as the 4a^ •* a (a^) (or very  and  likely  * the 4aj^ •+• a ( a ^ M s ) (3) i n the UV  transition.  Robin  [12] has a s s i g n e d t h i s f e a t u r e  spectrum to the 4a^ •+• 4p Rydberg t r a n s i t i o n .  value of t h i s f e a t u r e i s 2.79  eV which, as expected,  than that f o r the ISEELS 4s (3.13 Both an ab i n i t i o c a l c u l a t i o n  eV) or a ( a ) (~3.5  [187]  x  and an X  The  term  Is s l i g h t l y eV)  calculation  less  transitions. [176]  indicate  a roughly equal P 3s and P 3p c h a r a c t e r to the 4 a imply that t r a n s i t i o n s to both 4s and but the spectrum does not  1  orbital.  4p Rydberg l e v e l s might be  seem to r e f l e c t  this situation.  t h e o r e t i c a l work i s needed i n order to c l a r i f y t h i s The  r e s t of the v a l e n c e - v a l e n c e  v a l u e s of f e a t u r e s ( 1 + 2 ) i n F i g . 7.3.  On  transitions  assignments,  3  C l e a r l y more  based upon the term  and 3 w i t h r e s p e c t to the 4a ^ l i m i t are shown  can be reasonably  (Table 7.5).  seen,  situation.  t h i s b a s i s the m a j o r i t y of the remaining  the spectrum of P F  T h i s would  As  a s s i g n e d to these  features i n  valence-valence  s t a t e d above, there seems to be very  Rydberg s t r u c t u r e .  Feature 4 has been a s c r i b e d to the 4a^ •* 3d  t r a n s i t i o n by Robin  [12].  T h i s f e a t u r e has  a term value of 1.40  r e s p e c t to the 4 a i l i m i t , which i s reasonable  little  eV  with  f o r such an assignment (a  - 242 -  quantum d e f e c t of zero  Phosphoryl  would p r e d i c t a term v a l u e of 1.5 e V ) .  C h l o r i d e (OPCI,)  The VSEELS spectrum PCI3  of 0 P C 1  ( F i g . 7.4) i s s i m i l a r t o t h a t f o r  i n t h a t i t shows f e a t u r e s t h a t can be a s c r i b e d to both  v a l e n c e - v a l e n c e and valence-Rydberg r e p o r t e d spectrum ~6.7  3  eV.  transitions.  The o n l y p r e v i o u s l y  i s t h a t by Halmann [185], which extends  T h i s spectrum  as f a r as  [185] shows a weak p l a t e a u a t ~6.5 eV t h a t was •*• i t t r a n s i t i o n .  ascribed to a forbidden n  There i s evidence  o f some  o v e r y weak s t r u c t u r e i n t h i s r e g i o n i n the VSEELS spectrum. The  spectrum  has been a s s i g n e d i n a s i m i l a r manner t o those f o r  the p r e c e d i n g m o l e c u l e s .  Features  1 and 2 a r e c o n s i d e r e d to a r i s e  from  v a l e n c e - v a l e n c e t r a n s i t i o n s s i n c e i t s r e s p e c t i v e term v a l u e s (4.36 and 3.74 eV r e s p e c t i v e l y from the H e to the 4s Rydberg l e v e l  o r b i t a l ) are l a r g e r than t h a t a s c r i b e d  (3.08 eV) i n the ISEELS spectrum.  f e a t u r e s (1 and 2) a r e a s s i g n e d t o the H e transitions respectively.  Thus these  •* a (a^) and H e  The o r d e r i n g i s t h a t of the P 2p  •*> a ( e ) spectrum  (term v a l u e s o f 7 . 0 eV and 6.2 eV) and t h a t o f the 0 Is spectrum  (term  v a l u e s of 5.5 eV and 4.2 e V ) . The r e v e r s a l of i n t e n s i t y i n c o n t r a s t t o t h a t observed  i n the 0 Is ISEELS spectrum  i s c o n s i s t e n t w i t h the  o r i g i n a t i n g o r b i t a l i n the VSEELS spectrum, being of 0 2p c h a r a c t e r . The applied  term v a l u e s w i t h r e s p e c t to the H e  l i m i t have then been  t o the other IP's ( T a b l e 7.2) and the e s t i m a t e d p o s i t i o n s of the  c o r r e s p o n d i n g t r a n s i t i o n s a r e i n d i c a t e d i n F i g . 7.4.  I t s h o u l d be noted  - 243 -  4s I  i  3d  •9e  r  OPCI3  4s i  Valence  12a,  3d  4p  UL  o,  e v  L  4s  3r i  I  3d I  \ I./  r  _fl3a,  _T  \  |10e 43  1  3d I  3 Be  r  3di—r  4s  _1_  4s  _1_ 1 2  "i  r  *11e  3  4 5 6 7 8910 11—1415 1617  H  1  r  n  18  19  "T  r  10  20  21 1  1  r 20  15 ENERGY  3d  L O S S (eV)  F i g u r e 7.4: The v a l e n c e s h e l l e l e c t r o n energy l o s s spectrum o f 0PC1 . The i o n i s a t i o n edges a r e taken from p h o t o e l e c t r o n s p e c t r o s c o p y (see T a b l e 7.2). P o s i t i o n s of the v a l e n c e - v a l e n c e ( t a l l b a r s ) , valence-Rydberg ( s h o r t b a r s ) t r a n s i t i o n s as estimated from the term v a l u e s are shown l e a d i n g to each l i m i t (see T a b l e 7.6). 3  - 244  that  f e a t u r e 2 cannot  ( a ( a ^ ) ) from the 2 a under C^  v  -  be a s s i g n e d to a t r a n s i t i o n to the LUMO o r b i t a l l e v e l , as t h i s t r a n s i t i o n i s d i p o l e f o r b i d d e n  2  selection rules.  Thus the i n t e n s i t y l e a d i n g up to f e a t u r e 5  can be r e a s o n a b l y a s c r i b e d to the v a r i o u s v a l e n c e - v a l e n c e i n d i c a t e d on F i g . 7.4,  s i n c e the correspondence  transitions  of the s p e c t r a l f e a t u r e s  w i t h the p r e d i c t e d p o s i t i o n s i s good.  The molecule  a (a^) v i r t u a l o r b i t a l  t h a t so f a r has not been c o n s i -  (see T a b l e 7.1)  0PC1  3  has  a  second  * dered.  From a c o n s i d e r a t i o n of t h i s  a (a^) ISEELS term v a l u e and  of the 4s ISEELS Rydberg l e v e l , i t i s l i k e l y l e v e l to g i v e a a ( a ^ ) / 4 s conjugate Feature 5, the f i r s t  o r b i t a l and  to mix w i t h the Rydberg  [65] i n the v a l e n c e  spectrum.  f e a t u r e not f i t t e d by t h i s proposed  v a l e n c e scheme, has a term v a l u e of 1.48  valence-  eV w i t h r e s p e c t to the l i e  can be a s s i g n e d to the l i e •+• 3d Rydberg t r a n s i t i o n .  i s a l s o evidence of a s h o u l d e r on the low energy T h i s has a term v a l u e ~2.6  eV  that  There  s i d e of f e a t u r e 5.  ( s i m i l a r to the 4s term v a l u e i n the  C l 2p spectrum) from the lOe o r b i t a l , and  so i s a s s i g n e d to the lOe •*•  * 4s/a  (a^) t r a n s i t i o n .  IP's and spectrum.  These term v a l u e s have been a p p l i e d t o the o t h e r  the e s t i m a t e d p o s i t i o n s of the t r a n s i t i o n s are I n d i c a t e d on T r a n s i t i o n s to the p Rydberg s e r i e s might a l s o be  The a b - i n i t i o c a l c u l a t i o n 10% 0 2s component. have some i n t e n s i t y . r e s p e c t t o the 12a  x  expeted.  [189] i n d i c a t e s t h a t the 12a^ o r b i t a l has  Thus a 12a^ Feature  •*• 4p t r a n s i t i o n might be expected  15 has  a term v a l u e of 2.21  ionisation potential.  As w i t h PC1 , 3  a to  with  Since t h i s term v a l u e i s  c l o s e to t h a t g i v e n f o r the 4p Rydberg l e v e l i n PC1 15 i s a s s i g n e d a c c o r d i n g l y .  eV  the  3  (2.26  eV).  Feature  t h i s term v a l u e i s a p p l i e d  - 245  a l s o to the other a^ symmetry  -  orbital.  O b v i o u s l y the spectrum  i s made up of many o v e r l a p p i n g  t i o n s , and no c l e a r , unambiguous assignment can be made. data show evidence of s i g n i f i c a n t transitions.  T a b l e 7.6  c o n t r i b u t i o n s from  transi-  The  spectral  valence-valence  summarises the p o s i t i o n s of the f e a t u r e s .  Phosphorus P e n t a f l u o r i d e (PFc;) F i n a l l y , the spectrum shown i n F i g . 7.5. spectrum  of P F  5  w i l l be c o n s i d e r e d .  The molecule  From a c o n s i d e r a t i o n of a minimal  *  *  o r b i t a l s are a ( a j ) ,  spectrum  is  To date t h e r e has been no UV a b s o r p t i o n or VSEELS  r e p o r t e d f o r t h i s molecule.  symmetry.  The  PF  i s of  5  b a s i s s e t , the  virtual  *  a (e')»  and  a (a£).  However, o n l y the f i r s t  two  l e v e l s w i l l be c o n s i d e r e d s i n c e the a (a£) l e v e l i s l o c a t e d r i g h t at the i o n i s a t i o n edge i n the ISEELS s p e c t r a . The  e x p e r i m e n t a l i o n i s a t i o n p o t e n t i a l s are taken  photoelectron spectroscopy  [193,194].  ( T a b l e 7.2)  However, there i s a c o n s i d e r a b l e  l a c k of agreement between the v a r i o u s c a l c u l a t i o n s as to the ordering. al*  [194].  orbital  Some of these assignments have been summarised by Goodman et The  o r d e r i n g used  that of S t r i c h and V e i l l a r d SCF  from  i n the p r e s e n t work (see T a b l e 7.2)  [191], who  performed  c a l c u l a t i o n w i t h a medium s i z e b a s i s s e t .  is  an a b - i n i t i o LCAO T a b l e 7.7  MO  summarises  the d i p o l e a l l o w e d / f o r b i d d e n t r a n s i t i o n s f o r the outer v a l e n c e r e g i o n of this  molecule. From the ISEELS spectrum, the term v a l u e of the 4s Rydberg  i s 2.75  eV.  S i n c e f e a t u r e 1 has a term v a l u e of 3.15  level  eV w i t h r e s p e c t t o  - 246 -  TABLE 7.6 Energies and term values for the VSEELS spectrum of OPCI3  Feature  Feature  Energy Loss (eV)  Energy Loss (eV)  1  7.57  12  12.53  2  8.19  13  12.66  3  9.35  14  12.83  4  10.18  15  13.14  5  10.45  16  13.59  6  10.73  17  13.94  7  11.11  18  14.54  8  11.37  19  15.16  9  11.61  20  16.03  10  11.87  21  17.00  11  12.34  t  f  t  f  Features used to estimate term values - see text. Term values:  !!( l) o (e) a (a,)/4s 3d 4p a  a  =  A.36 eV = 3.74 eV = 2.6 eV = 1.48 eV = 2.21 eV  These have been applied to the IP's l i s t e d i n Table 7.2.  - 247 -  23  9 10  45 6 78  PF  Valence  V/  5  4e  4a  e & 5e'  <r"(a,') o-'(e') L I  L  la  I  8a 5a.  !6e' 2e"  i I0  Figure  r I5  25  20  ENERGY LOSS(eV)  7.5:  The v a l e n c e s h e l l e l e c t r o n energy l o s s spectrum of P F . The i o n i s a t i o n edges are taken from p h o t o e l e c t r o n spectroscopy (see Table 7.2). The o r d e r i n g i s from r e f . [191]. P o s i t i o n s of the v a l e n c e - v a l e n c e t r a n s i t i o n s e s t i m a t e d from the term v a l u e s are shown l e a d i n g to each l i m i t (see Table 7.8). Dotted bars i n d i c a t e f o r b i d d e n t r a n s i t i o n based on a s s i g n e d symmetry of i o n i s e d o r b i t a l . 5  - 248 -  TABLE 7.7 Dipole allowed/forbidden t r a n s i t i o n s  f o r PF,. i n D„  *  Initial Orbital  Transitions to a Dipole  Levels  Allowed/Forbidden  *  a (a ) x  *•  o Ce')  2e"  No  Yes  6e'  Yes  Yes  Yes  No  No  Yes  la'  No  Yes  5e'  Yes  Yes  le"  No  Yes  4a" ^ 2 a  Yes  No  4e'  Yes  Yes  5 a  8  2 i  a  symmetry.  - 249 -  the HOMO o r b i t a l , i t i s t h e r e f o r e l i k e l y due t o v a l e n c e - v a l e n c e tions.  transi-  Given t h a t the two h i g h e s t o c c u p i e d MOs a r e the 2e" and the 6e'  o r b i t a l s , and c o n s i d e r i n g the s e l e c t i o n r u l e s i n T a b l e 7.7, f e a t u r e 1 I s  *  *  a s c r i b e d t o the 2e" ->• a ( e ' ) and 6e' •+ a ( a j ) t r a n s i t i o n s .  Thus t h e  term v a l u e s f o r the o ( e ) and a ( a j ) a r e ~3.15 eV and 4.17 eV r e s p e c 1  tively.  T h i s compares w i t h 3.6 eV and 6.2 eV f o r the P 2p spectrum and  2.7 eV and 5.0 eV f o r the F Is spectrum  shown i n Chapter  6.  The r e d u c t -  i o n i n term v a l u e (~2 eV) f o r the LUMO o r b i t a l i n going from  ISEELS t o  VSEELS i s i n keeping w i t h those observed  with h i g h l y  f o r other molecules  e l e c t r o n e g a t i v e l i g a n d s ( e g . , N F , P F , P C 1 , and 0 P C 1 ) . 3  3  3  3  assignment o f the 2e" t o the HOMO o r b i t a l i s c l e a r l y  Thus t h e  supported by the  VSEELS spectrum. The  term v a l u e s as o b t a i n e d above f o r the 2e" and 6e l i m i t s are  now a p p l i e d t o the other I P ' s . The p o s i t i o n s o f a l l the t r a n s i t i o n s so d e r i v e d are i n d i c a t e d i n F i g . 7.5, and the assignments summarised i n T a b l e 7.8.  A dashed v e r t i c a l l i n e on the spectrum  t i o n s o f t r a n s i t i o n s t h a t are d i p o l e f o r b i d d e n .  i n d i c a t e s the p o s i -  The agreement between  the s p e c t r a and the p r e d i c t e d v a l e n c e - v a l e n c e t r a n s i t i o n s i s very good i n almost  a l l cases.  A b e t t e r agreement would be o b t a i n e d i f the o r d e r  (Table 7.2) o f the 4a*2 and 4e' l e v e l s i s r e v e r s e d . [191]  The c a l c u l a t i o n  i n d i c a t e s t h a t the s e p a r a t i o n o f these l e v e l s i s o n l y 0.16 eV, and  t h e r e f o r e not r e l i a b l e  f o r p r e d i c t i n g the o r d e r i n g .  t h i s o r d e r i n g i s supported  The r e v e r s a l o f  by the work of Cox e t a l . [193], who i n the  a n a l y s i s o f the H e ( I ) spectrum  a s s i g n e d the 21 eV f e a t u r e t o the 4a'I  - 250 -  TABLE 7.8 E n e r g i e s and p o s s i b l e for  Feature  assignments  the VSEELS spectrum of P F 5  (a)  Energy Loss (eV)  P o s s i b l e Assignments *  a ( e ' ) , 6e' •+ a ( a j ) , 5a£ * a (aj)  1  12.29  2e"  2  13.96  6e'  +  a  3  14.35  5e'  ->•  a*(ap  la'  ->• ->•  4  15.13  5e'  5  15.43  4 a  6  15.97  le"  7  16.51  t  8  16.86  4e'  9  18.65  10  19.4  4e'  -  (e*),  o*(e')  o*(e') *, o (e')  * a  (a!)  a  (e')  * a  (a )  *,  t  t  from 7aJ  Exchanging 4a? and 4e' o r d e r 5  (^Assignments  2  *  (?)  + a (a[)  7  4e' *  a*(e')  8  4a£  o * ( a p  +  based on a p p l y i n g f o l l o w i n g  term v a l u e s  a*(e')  3.15 eV  F e a t u r e 1 from 2e"  o*(a{)  4.17 eV  F e a t u r e 1 from 6 e '  to I P ' s i n Table 7.2.  - 251  level.  The  VSEELS spectrum  -  ( F i g . 7.5)  j u s t c o n s i d e r i n g predominantly  can be adequately  valence-valence  d e s c r i b e d by  transitions.  s i m i l a r to the s i t u a t i o n d i s c u s s e d above f o r P F ( a n d a l s o 3  SUMMARY AND  NF ).  have been presented i n t h i s chapter and The  compared w i t h t h e i r  compounds ISEELS  ISEELS s p e c t r a c l e a r l y i n d i c a t e the dominant presence  t r a n s i t i o n s to a  v i r t u a l v a l e n c e l e v e l s , s p e c i a l l y those w i t h  negative ligands.  are lower  (approximately  electro-  The  term v a l u e s , as  than f o r the c o r r e s p o n d i n g ISEELS t r a n s i t i o n  2-2.6  l i g a n d s , e.g. F ) .  of  T r a n s i t i o n s to v a l e n c e l e v e l s a l s o seem to dominate  i n the v a l e n c e s h e l l s p e c t r a presented h e r e . expected,  3  CONCLUSIONS  The VSEELS s p e c t r a of s e v e r a l p h o s p h o r u s - c o n t a i n i n g  spectra.  This i s  eV lower  f o r those w i t h h i g h l y e l e c t r o n e g a t i v e  The v a l e n c e - v a l e n c e nature of these t r a n s i t i o n s i s  evidenced by the term v a l u e s being even l a r g e r than those f o r the ISEELS 4s Rydberg t r a n s i t i o n s .  The  h i g h e r term v a l u e s f o r the ISEELS s p e c t r a  are e x p l a i n e d by the e f f e c t of the l o c a l i s e d , phosphorus core h o l e .  The  c e n t r a l nature of the  l i g a n d ISEELS s p e c t r a , where the core h o l e i s  s i t u a t e d on the p e r i p h e r y of the molecule,  g e n e r a l l y have term  values  t h a t l i e i n between those of the c e n t r a l atom ISEELS s p e c t r a and VSEELS s p e c t r a .  Term v a l u e s f o r the a  r e g i o n s of the molecules  slightly  lower  l e v e l s a r i s i n g from the t h r e e  are summarised i n Table 7.9.  the term v a l u e s f o r the 4s Rydberg l e v e l . than those f o r a a  the  A l s o shown are  In cases where these are o n l y  o r b i t a l of the same symmetry i n the P  2p ISEELS s p e c t r a , the c o r r e s p o n d i n g f e a t u r e i n the VSEELS s p e c t r a cannot  be s o l e l y a s s i g n e d to one or the o t h e r .  In these cases  the  - 252 -  TABLE 7 . 9 Term v a l u e s from ISEELS and VSEELS f o r P H 3 , P ( C H 3 ) 3 , P C 1 3 , P F 3 , P F 5 , and 0PC1 3  Molecule  Orbital  ISEELS  Term V a l u e s  Phosphorus (2p)  PH3  * ( e ) a (a,) 4s  5.1 4.5 2.48  o*(e) a (a,) 4s  3.3 ~2 2.86  o*(a,) o (e) 4s  a  P(CH3)3  PCI3  PF3  (eV)(a) Ligand(b)  4.83 4.54 2.95  o*(e) 0" ( a , ) 4s  6.8 -3.5 3.12  5.2 3.0  o (eT) 4s  6.2 3.6 2.71  5.0 2.7  2.6  7.0 6.2 4.4 3.08  a*(a,)  a*(e5  (a,)  ( c )  Ref.  ^With  Chapters 0 Is,  2.44  5.7  a  Is,  j.  6.9 6.4 3.06  4s  ' ^ F  3.76< e >  3.01  Cl  ^'From  }  3.17  PF5  OPCI3  VSEELS Term V a l u e s (eV)  6.0 4.5 3.2 2.6  5 and 6.  C Is,  and C l 2p as  appropiate.  [12]. reference  to F l o n e - p a i r  orbital.  J.  -4.4 (4.68)(d) 2.79  4.17 3.15  0 5.5 4.2 J.  4.36 3.74 2.6  - 253  -  f e a t u r e i s p r o b a b l y b e s t a s c r i b e d as a mixed valence-Rydberg ( a /4s) level  [65].  I t should be noted that the d i f f e r e n c e i n term v a l u e s i s  much s m a l l e r f o r the Rydberg going from the ISEELS 3d Rydberg  can be seen t h a t , e s p e c i a l l y i n the case of P F  evidence of any Rydberg  [191,194].  series.  S i m i l a r l y 0PC1  t r a n s i t i o n s are a l s o apparent. very i n t e n s e f i r s t  5  the  The VSEELS s p e c t r a of  3  and P C 1  3  5  PF  5  as b e i n g the 2e"  show many t r a n s i t i o n s however, valence-Rydberg  The spectrum of P ( C H ) 3  3  (Fig.  7.1)  has a  [12].  3  The i n t e n s i t y of the f i r s t  feature i n  c l e a r l y i n d i c a t e s a c o n t r i b u t i o n from v a l e n c e - v a l e n c e t r a n s i -  tion(s). levels.  and P F ,  f e a t u r e , and i n t h i s r e s p e c t i t i s q u i t e s i m i l a r to  t h a t observed f o r P H 3  3  by v a l e n c e - v a l e n c e t r a n s i t i o n s w i t h v e r y  a s s i g n a b l e to v a l e n c e - v a l e n c e t r a n s i t i o n s ;  3  i s consistent  levels.  support the assignment of the HOMO o r b i t a l o f P F  P(CH )  on  L e v e l s a s s i g n a b l e to the  The quantum d e f e c t o b t a i n e d (~0)  VSEELS s p e c t r a are dominated  orbital  levels  l e v e l i n the VSEELS s p e c t r a were seen to have a c o n s t a n t term  with the assignment to d  little  than f o r the lowest a  to the VSEELS s p e c t r a .  v a l u e (~1.4-1.5 eV).  It  levels  I t presumably c o n t a i n s t r a n s i t i o n s  to the a (e) and  The term v a l u e s f o r h i g h e r energy f e a t u r e s , a r i s i n g  o (a^)/4s from  o r b i t a l s l o c a t e d on the l i g a n d , are each s i m i l a r to that f o r the  first  t r a n s i t i o n i n the C Is spectrum (C Is •*• 4s t r a n s i t i o n ) and are an i n d i c a t i o n of a t r a n s i t i o n to a " l o c a l i s e d " Rydberg suggested f o r S i ( C H ) 3  In  t t  and i n the methylamines  o r b i t a l , as  was  (see Chapter 8 ) .  the p r e s e n t work i t can be seen that the ISEELS s p e c t r a can  be used as an a i d to the assignment of the more complex VSEELS s p e c t r a .  - 254  -  S p e c t r a of condensed phases t h a t r e s u l t transitions  would be  helpful  valence-Rydberg nature of the  in clarifying spectra.  h i g h l y e l e c t r o n e g a t i v e (F) l i g a n d this effect  i n t h a t the  t h e r e f o r e enhanced) by i n v o l v e d i n the  case of  i n the  on  the  the  s u p p r e s s i o n of  v a l e n c e - v a l e n c e and/or  In t h i s regard the s p e c t r a seemingly  valence-valence t r a n s i t i o n s a charge b a r r i e r inner-shell  effect  spectra.  Rydberg  of  are  the  effect  of  the  parallels  trapped  type more  (and usually  - 255  -  CHAPTER 8  INNER SHELL ELECTRON ENERGY LOSS SPECTRA OF THE  In Chapters 5 and compounds were presented  METHYL AMINES AND  6 the ISEELS s p e c t r a of s e v e r a l phosphorus and  the e f f e c t s of the l i g a n d on the  spectral i n t e n s i t i e s contrasted.  A s i m i l a r comparison was  the ISEELS s p e c t r a of SiCCHg)^ presented reported  photoabsorption  l i g a n d was  tronegative  made between earlier  s p e c t r a of r e l a t e d s i l i c o n compounds.  In c o n t r a s t to l i g a n d s such as H and  CH ,  T h i s was  a l s o seen i n the ISEELS s p e c t r a of NF  In t h i s chapter, 3  3  presented  as a c o n t i n u a t i o n of t h i s work, the  3  3  The  ISEELS s p e c t r a of NF , 3  the t h i r d row  ( C H ) N and 3  phosphorus analogues P F , 3  NH  3  P(CH )  To date the o n l y p r e v i o u s l y r e p o r t e d  3  3  3  ISEELS  2  3  2  reported  spectrum of NH valence  3  by Wight and [196].  Brion  [72] and  as  CH NH ) and 3  2  are a l s o compared and  with  PH . 3  inner s h e l l e l e c t r o n e x c i t a -  t i o n s p e c t r a of these molecules have been the ISEELS s p e c t r a of NH CH NH  in  i s presented  3  w e l l as the s p e c t r a of the other methyl amines ( ( C H ) N H and 3  transi-  l e v e l s at the expense of those to Rydberg  s p e c t r a of ( C H ) N , which i s i s o e l e c t r o n i c w i t h NF ,  NH .  The  highly elec-  3  l i g a n d s , f o r example F, enhance the p r o b a b i l i t y of  t i o n s to v i r t u a l valence  Chapter 3.  i n Chapter 4 and  relative  seen to p l a y an important r o l e i n the observed i n t e n s i t y  distributions.  levels.  AMMONIA  a recent  X-ray  3  absorption  However, there have been s e v e r a l s t u d i e s on  electron e x c i t a t i o n spectra.  and  For example, Tannenbaum et a l .  the  - 256  [197]  have r e p o r t e d  ~8 eV.  the UV  absorption  -  s p e c t r a of the methyl amines up  There have a l s o been numerous s t u d i e s on NH  e x c i t e d s t a t e s i n the v a l e n c e  and  [199]  has  molecules and  3p Rydberg c h a r a c t e r  performed s e m i - e m p i r i c a l  assigned  The  lowest  i n n e r - s h e l l s p e c t r a have been  to t r a n s i t i o n s to l e v e l s of mainly 3s and Salahub  [198].  3  the lowest v a l e n c e  to  assigned [72,197].  MO-CT c a l c u l a t i o n s on  these  e x c i t a t i o n s to HOMO (N  lone  *  a  pair) 201]  and  transitions. (CH ) N  presented  3  [201]  3  here p r o v i d e  However, ab i n i t i o  support  c a l c u l a t i o n s on NH  the Rydberg assignment.  The  [200,  3  spectra  f u r t h e r evidence f o r the assignment of the  e l e c t r o n i c t r a n s i t i o n s to Rydberg  lowest  levels.  EXPERIMENTAL DETAILS The Chapter 2.  s p e c t r a were o b t a i n e d An  impact energy of 2.5  e l e c t r o n s were sampled at ~1° amines were c a l i b r a t e d a g a i n s t SFg.  on the ISEELS spectrometer d e s c r i b e d keV  was  used and  s c a t t e r i n g angle. the S 2 p ^ ^  The  the  scattered  C Is s p e c t r a of  t2g(184.54 eV)  3  of the methyl amines.  the  f e a t u r e of  These C Is s p e c t r a were used, except i n the case of ( C H ) N ,  i n t e r n a l l y c a l i b r a t e the N Is r e g i o n s  in  3  The  N  to Is  * s p e c t r a of ( C H ) N and 3  3  = 1), 401.10 eV)  and  CO  NH  3  were c a l i b r a t e d a g a i n s t  the N  (C Is •*• n* (v = 0 ) , 287.40 eV)  2  (N  Is •> u  features  respectively.  RESULTS & DISCUSSION Carbon Is s p e c t r a The  long-range s p e c t r a of the C Is r e g i o n of the methyl amines  (v  - 257  are shown i n F i g . 8.1. 0.36  eV FWHM.  These s p e c t r a were o b t a i n e d w i t h a r e s o l u t i o n o f  More d e t a i l e d  r e s o l u t i o n (0.18  -  short-range  s p e c t r a , recorded at a h i g h e r  eV FWHM), are shown i n F i g . 8.2.  t i o n edges f o r ( C H ) N and CH NH 3  3  3  The  assigned  have been taken from XPS  2  ioniza-  [180,202].  As no v a l u e f o r the C Is IP of ( C H ) N H has been r e p o r t e d , i t s v a l u e 3  assumed to be the mean of the other two spectral  IP's.  Table 8.1  s p e c t r a are s i m i l a r to t h a t f o r CH^  [66,67,72] and  a n a l y s i s can be c o n s i d e r e d i n terms of t h a t f o r a methane. 2  The  [68].  6  P(CH ) 3  and  3  summarises  the  data.  The  C H  was  2  C Is spectrum  3  2  mono-substituted  i s v i r t u a l l y i d e n t i c a l to t h a t f o r  S i m i l a r o b s e r v a t i o n s were made i n the C Is s p e c t r a of SKCH^.  H i t c h c o c k and halides  of CH NH  the  [63,64].  B r i o n have d i s c u s s e d the C Is s p e c t r a of the methyl  I t i s i n s t r u c t i v e to compare the a n a l y s i s t h e r e w i t h  the s p e c t r a p r e s e n t e d here.  The methyl h a l i d e s a l l possess a f e a t u r e  a t t r i b u t a b l e to l o w - l y i n g a  orbital  spectrum  [64].  However, the r e s t of the  can be a s s i g n e d s o l e l y to Rydberg t r a n s i t i o n s .  s p e c t r a r e p o r t e d here t h e r e i s a complete absence of any b u t a b l e to a l o w - l y i n g a  feature.  The  In the amine feature a t t r i -  d i s c r e t e p o r t i o n s of the amine  C Is s p e c t r a r e p o r t e d here are s i m i l a r to the remainder  of the methyl  h a l i d e s p e c t r a and  can be a s s i g n e d s o l e l y to Rydberg f e a t u r e s i n an  analogous  The  manner.  shown i n Table The  assignments based  upon these arguments [64]  are  8.1  post-edge behaviour  i s different  amines and methyl h a l i d e s ) of m o l e c u l e s .  i n these two  sets ( i . e . ,  The methyl amines show a  the  broad  - 258 -  i—i—i—i— i—I—i—rr| i —Isi —edge  1  — — — — 1  rr-^f-  10-  12  1  1  1  r  n—i—i—I—I—I—I—i—i r  6  (CH ) NH . 3  C1s  X  3  X  ( x = 1 3 )  AE=0-36eV  CH NH 3  2  Is edge  >  <  i, 'i i  IOH  cc  9  "i  H CD  < b  (CH,) NH  5-  2  CO  LJJ |Is edge UJ >  3  ioH  7  UJ  cr  (CH ) N 3  —I—i—i—i—i—|—i—i—i—i—|—\—i—i—i—|—i—i—i  285  295  305  315  3  i i i i i i i  325  ENERGY LOSS (eV)  335  8.1: Long range e l e c t r o n energy l o s s s p e c t r a methyl amines.  of the C Is r e g i o n  of t h e  - 259 -  1  I I M  I 1  10 H  l i s edge  1  2  3  4 5  1 6  /  (CH ),NH .. S  A  /  3  C1s " <  ,  3 )  AE=0-18eV  CH NH 3  2  5H  1s edge  co 12 3 45 6  => 10> < cc  7 8  i'\  H  (CH ) NH  m cc <  >-  3  2  5-  CO UJ I—  1s edge  nr 12  34 5 6 A  UJ  (CH ) N 3  3  54  286  288  290  292  294  296  298  E N E R G Y L O S S (eV) F i g . 8.2: Short range, h i g h r e s o l u t i o n e l e c t r o n energy l o s s s p e c t r a o f the C Is r e g i o n of the methyl amines.  TABLE 8.1 E n e r g i e s , Term Values and P o s s i b l e Assignments f o r the C Is Region o f the Methyl Amines  3  Energy (eV)  j(a)  1  287.70  3.90  2  288.78+  2.82  Feature  Feature  (eV)  289.46 290.21 290.59 291.60 291.8  3 4 5 IP* 6  (CH ) N  (CH ) NH  CH3NH2  1 2 3 4 5 6 7  2.14 1.39 1.01 0 -0.2  Energy (eV) 287.61 287.85 288.3 288.67+ 288.95 289.46 290.03 290.35 291.43 293.2  8. IP* 9  3  2  T (eV) 3.82 3.58 3.1 2.76 2.48 1.97 1.40 1.08 0 -1.8  3  Feature  Energy (eV)  T (eV)  1 2  287.84 288.08  3.42 3.18  3 4 5 6  288.80+ 289.07 289.51 290.06  2.46 2.19 1.75 1.20  Posslble Assignment  ( b )  3s 3s+v 3p(x,y) 3p(x,y)+v 3p(z) 4p(x,y)/3d 4p(z) Is l i m i t  1.  IP* 7  291.26 294.3  0 -3.0  a  &  shaperesonance  + C a l i b r a t e d f e a t u r e , estimated u n c e r t a i n t y ± 0.08 eV f o r CH NH , ( C H ) N H ; ± 0.15 eV f o r (CH ) N. 3  3  *XPS (a) T  2  3  2  3  CH NH 3  =  I  P  2  r e f . [202], ( C H ) N 3  r e f . [180]; ( C H ) N H mean o f o t h e r two v a l u e s . 3  2  _ Energy  ( ) ( C H ) N H and CH NH : C b  3  3  2  3  2  g  symmetry p(x,y) = pa', p ( z ) = pa".  - 261 -  and f a i r l y  i n t e n s e f e a t u r e j u s t beyond  the i o n i z a t i o n edge ( F i g . 8.1).  T h i s i s not the case f o r the methyl h a l i d e s , n o r f o r that matter methane [67,72].  However, ethane  [68] shows s i m i l a r behaviour t o the m o l e c u l e s  here.  The d i f f e r e n c e between the C Is s p e c t r a o f methane [72] and  ethane  [68] i s n i c e l y i l l u s t r a t e d  i n a recent paper by H i t c h c o c k e t a l .  [100] and c l e a r l y i n d i c a t e s a peak j u s t beyond continuum f e a t u r e s can be a t t r i b u t e d t o a a a s s o c i a t e d w i t h the C-C bond i n ethane the  amines.  the edge f o r ethane. The  shape resonance [77]  [100] and w i t h the C-N bond i n  I n MO terms these can be thought o f as e x c i t a t i o n s i n t o a  * a  anti-bonding state  [87].  Thus the major d i f f e r e n c e between the  methyl h a l i d e s and the molecules here i s the l o c a t i o n o f the cr In the  orbital.  the former they are low l y i n g and i n the d i s c r e t e p o r t i o n , whereas i n The a  methyl amines they are i n the continuum.  w i l l be d i s c u s s e d f u r t h e r  shape  resonances  below.  N i t r o g e n Is s p e c t r a F i g u r e 8.3 shows the long-range N Is s p e c t r a of the methyl amines.  The s p e c t r a l r e s o l u t i o n i s 0.36 eV FWHM.  More d e t a i l e d  short-  range s p e c t r a are shown i n F i g . 8.4 a l o n g w i t h a h i g h - r e s o l u t i o n spectrum (0.14 eV FWHM) of NH . 3  XPS measurements  [203].  The Is i o n i z a t i o n edges are taken from  On going from NH  i n s p e c t r a l f e a t u r e s can be observed. t i c of a dominant  shows l i t t l e Rydberg  to (CH ) N  The NH  ( a t o m i c - l i k e ) Rydberg  t u r e l e s s continuum o f low i n t e n s i t y .  3  3  3  3  a g r a d u a l change  spectrum i s c h a r a c t e r i s -  spectrum w i t h a l a r g e l y  I n c o n t r a s t , the ( C H ) N  s t r u c t u r e and i s dominated by a broad  3  3  strucspectrum  continuum  -  i  i  i  i  i  262  -  I  i  1s edge  I I I I I I I I I I I  AE=0.36eV  12 3 4  icH  (CH ) NH . 3  X  N1s  or < or  (x=1  '  X  3)  !!!  oo 3  3  CH.NH;  5H 1s edge  m  1 2  4  CO  LU  (CH ) NH  r-  3  5  2  I 1s edge  H  LU > I- io H <  1 2  3  LU LT  (CH ) N 3  3  5H  —r—i—i—i—i—|—i—i—i—i—|—i—i—i—i—r-i—i—I  395  4 0 5  415  ENERGY Fig.  4 2 5  i  i  1  4 3 5  1  1  1  I  4 4 5  LOSS(eV)  8.3:  Long range e l e c t r o n energy l o s s s p e c t r a of the N Is r e g i o n of the methyl amines.  -  -i  1  1  1  • • • •  r-, IOH  263  12  -  r  1 tl§-edge  3j 4 5 6  (CH ) NH . 3  N1s  5  x  3  x  (x = 0-3)  H  NH  3  AE=O.I4eV 1s edge  to  1  1  0  > 10 cr < cr  2  3  4  A  H  m cr <  >-  t GO  CH NH 3  M f  2  AE=0.36eV  5  UJ  UJ  i i r  I'o-l  1  _J  (CH ) NH 3  I 1s edge  2  AE=0.28eV  2 3  LU  rr  5 10  |  H  1  1s edge  2  (CH ) N 3  3  AE=0.28eV  5  4 399  ^ 403  1  i 407  r  1  411  415  ENERGY LOSS (eV) 8.4: Short range e l e c t r o n energy l o s s s p e c t r a of the N Is r e g i o n of ammonia and the methyl amines. The ( d i f f e r i n g ) s p e c t r a l r e s o l u t i o n i s i n d i c a t e d on each spectrum.  - 264 -  f e a t u r e t h a t can be a s c r i b e d to a a (N-C) shape As s t a t e d above, the NH  3  spectrum  to the Rydberg l e v e l s and t h i s spectrum  resonance.  can be a s s i g n e d t o t r a n s i t i o n s has been p u b l i s h e d e a r l i e r [72,  * 196].  There i s no evidence  p o r t i o n , though Robin  of any d i s t i n c t  [12] has suggested  the v a l e n c e s h e l l UV spectrum  a  f e a t u r e i n the d i s c r e t e  t h a t the HOMO •»• 3s f e a t u r e i n  i s on top of a continuous v a l e n c e  t r a n s i t i o n , and Schwarz [61] suggests  an admixture  of valence  shell  anti-  bonding  c h a r a c t e r f o r the n = 3 Rydberg l e v e l s .  the NH  Is continuum (~14 eV above the edge) have been i d e n t i f i e d w i t h a  3  a (N-H) shape resonance  [99].  The NH  3  spectrum  Rather weak f e a t u r e s i n  presented here i s  summarized i n Table 8.2 a l o n g w i t h p o s s i b l e assignments. the e a r l i e r ISEELS work [72] and the X-ray are a l s o shown.  The r e s u l t s of  a b s o r p t i o n spectrum [196]  The e n e r g i e s o f the s p e c t r a l f e a t u r e s i n the d i f f e r e n t  ISEELS s p e c t r a a r e i n good agreement w i t h each other and w i t h the X-ray a b s o r p t i o n spectrum, though i n the l a t t e r ~0.1-0.2 eV h i g h e r i n energy. differ  only s l i g h t l y  the f e a t u r e s are u n i f o r m l y  The assignments In the present work  from the e a r l i e r ISEELS work [72].  from the much h i g h e r r e s o l u t i o n achieved h e r e .  Features  This results 1 and 2 a r e  a s s i g n e d to the Is -*• 3s t r a n s i t i o n p l u s v i b r a t i o n a l component, thereby c o n c u r r i n g w i t h the s u g g e s t i o n s o f Wight e t a l . [ 7 2 ] , No v i b r a t i o n a l component i s d i s c e r n a b l e a t the r e s o l u t i o n employed i n the X-ray work [196].  F e a t u r e s 3 and 4 a r e a s s i g n e d t o the 3p(e) and 3 p ( a ) Rydberg 1  l e v e l s r e s p e c t i v e l y , w i t h a s e p a r a t i o n of ~0.6 eV, which agrees w e l l w i t h s e p a r a t i o n observed spectrum  [204],  (< 0.6 eV) i n the v a l e n c e e l e c t r o n  No f e a t u r e c o r r e s p o n d i n g t o f e a t u r e 4 was  excitation observed  Table Energies,  Term Values  8.2 and Assignment  for  the N Is Energy Loss S p e c t r a of NH^  X-ray  ISEELS Wight et  Present Work Energy (eV) ± 0.08 eV  Term Value (eV)  1  400.61  4.91  3s  400.6  2  400.92  4.60  3s + v  —  3  402.29  3.23  3P(e)  4  402.85  2.67  3p(3l)  5  403.52  2.00  4s  6  404.14  1.38  IP*  405.52  0  Feature  +  ref.  *ref.  [72] [196].  *XPS r e f .  [203]  Assignment  4p/3d  Energy (eV)  402.2  al.  +  Assignment  404.1  +  Akimov et Energy (eV)  al.  Assignment  3s  400.8  3s  3p(e)  402.4  3p(e)  — 403.5  Absorption  -403.0 3p(3l) 4s/3d/4p(e)  3p(aj)  403.6  4s  404.1  4p(e)  - 266 -  i n the e a r l i e r  ISEELS work due to poorer  seen i n the X-ray spectrum [196]. re-assigned  r e s o l u t i o n [72], however, i t i s  On t h i s b a s i s f e a t u r e 5 i s now  to the I s •*• 4s t r a n s i t i o n c o n s i s t e n t w i t h the i n t e r p r e t a t i o n  g i v e n by Schwarz [61] and a l s o by Akimov et a l . [196]. I n comparing the ISEELS spectrum w i t h the valence  shell  of the three amines and ammonia, two p o i n t s can be noted. there occurs  spectrum  Firstly,  a r e v e r s a l i n the i n t e n s i t y o f the f e a t u r e s a s s o c i a t e d with  the 3s and 3p Rydberg l e v e l s .  T h i s r e f l e c t s the s - l e v e l  characteristics  of the o r i g i n a t i n g o r b i t a l i n the core s p e c t r a which favour an s •*• p t r a n s i t i o n ( d i p o l e allowed sition  i n the pure atomic case) over  the s •> s t r a n -  ( d i p o l e f o r b i d d e n i n the pure atomic c a s e ) , e x p e c i a l l y i n a mole-  c u l e as symmetrical  as NH  v a l u e s of the f e a t u r e s .  3  [61].  The second p o i n t concerns the term  The term v a l u e s a r e between 0.4 and 0.5 eV  l a r g e r f o r the 3s and 3p f e a t u r e s i n the ISEELS s p e c t r a than f o r the corresponding  f e a t u r e s i n the valence  from the l o c a l i z e d nature  s h e l l s p e c t r a [198].  of the c o r e - h o l e , and thus  e l e c t r o n sees a c e n t r e approximating  This  the newly promoted  a (Z + 1) c o r e .  Actually, this  d i f f e r e n c e i s q u i t e s m a l l and i t r e f l e c t s the Rydberg nature f i n a l o r b i t a l In both  spectra.  unoccupied  of t h e  Rydberg o r b i t a l s , being l a r g e and  d i f f u s e , a r e l e s s s e n s i t i v e to the l o c a t i o n of the h o l e . the lowest  arises  orbital i sa a  antibonding  ce i n term v a l u e s between core and valence  In N F , where 3  orbital,  the d i f f e r e n -  s p e c t r a i s 2.17 eV.  T a b l e 8.3 summarizes the s p e c t r a l f e a t u r e s of the N Is r e g i o n o f the methyl amines.  Of these o n l y the spectrum of C H N H  previously reported  [72].  3  As b e f o r e , the f i r s t  2  has been  f e a t u r e s of C H N H a r e 3  2  TABLE 8.3 Energies,  Term Values and P o s s i b l e  Assignments  f o r the N Is Region o f the Methyl Amines  CH NH 3  Energy (eV)  Feature  3  T  (a)  Feature  (eV)  3  2  Energy (eV)  T (eV)  Feature  3  Energy (eV)  T (eV)  Possible Assignment  1  400.78  4.39  1  401.04  3.89  1  (401.8)  (3.0)  3s  2*  402.03  3.14  2  402.30  2.63  2  403.0  1.8  3p  3  403.55  1.62  3  403.24  1.69  -  4  404.8  0.3  4  405.8  IP  405.17  IP  404.93  Calibrated (CH ) N. 3  T  f e a t u r e , estimated u n c e r t a i n t y  3  *XPS r e f . (a)  (CH ) N  (CH ) NH  2  =  I  P  [203]  _ Energy  -0.9  —  3  406.5  IP  404.82  —  -1.7  3d e t c . a* shaperesonance  ± 0.12 eV f o r CH NH , (CH ) NH; ± 0.2 eV f o r 3  2  3  2  - 268 -  a s s i g n e d as t r a n s i t i o n s t o the 3s and 3p l e v e l s r e s p e c t i v e l y . v a l u e s a r e lower than those i n NH i n g an H l i g a n d by an a l k y l group  which would be expected upon r e p l a c -  3  [12] and once a g a i n the term v a l u e s  are ~0.5 eV h i g h e r than f o r the c o r r e s p o n d i n g v a l e n c e s h e l l [198]. that  The term  spectra  A f u r t h e r d i f f e r e n c e between t h e s p e c t r a of CH NH2 and NH i s 3  the r e l a t i v e i n t e n s i t y of the N Is •+• 3p t r a n s i t i o n  N Is + 3s t r a n s i t i o n i s l e s s i n CH NH . 3  to t h e 3p i s s t i l l and NH  more i n t e n s e .  i s the appearance  3  p r e v i o u s l y unassigned  2  to t h a t o f the  However, the former  transition  The major d i f f e r e n c e between CH NH 3  of a broad f e a t u r e a t the edge.  f e a t u r e [72] p a r a l l e l s  spectrum and can be a s s i g n e d t o a a (N-C) spectrum  3  that observed  shape resonance  shows no evidence of any l o w - l y i n g  a  2  This i n t h e C Is [99].  The  o r b i t a l below the i o n i z a -  t i o n edge. The  spectrum o f ( C H ) N H ( F i g s . 8.3 and 8.4, Table 8.3) a l s o 3  2  shows t r a n s i t i o n s that can be a s s i g n e d to the Rydberg l e v e l s .  However,  the i n t e n s i t y o f the N Is •> 3s t r a n s i t i o n I s now l a r g e r than t h a t of the N Is •*• 3p t r a n s i t i o n .  However, the r e l a t i v e r e d u c t i o n i n the 3p i n t e n -  s i t y i s c o n s i s t e n t w i t h that observed on going from NH first  f e a t u r e i n the spectrum  the N Is -*• 11 f e a t u r e o f N f e a t u r e a r i s i n g from an N  2  2  3  t o CH NH . 3  2  The  i s a t 401.04 eV, which i s v e r y c l o s e to  (401.10 e V ) .  The p o s s i b i l i t y of t h i s  i m p u r i t y , however, can be d i s c o u n t e d s i n c e  the v a l e n c e spectrum was r u n and no t r a c e was found of the v e r y i n t e n s e N  0 z  (X  b n ) f e a t u r e a t 12.93 eV [103]. u 1  The term v a l u e s f o r the 3s and  3p l e v e l s a r e about ~0.3 eV l a r g e r than i n the v a l e n c e s p e c t r a  [198].  As w i t h CH NH , the spectrum has a broad, i n t e n s e f e a t u r e that i s 3  2  - 269 -  a s s i g n e d t o a a (N-C) resonance. j u s t above the edge. low-lying  a  The little  In t h i s case the f e a t u r e i s c e n t e r e d  Once a g a i n the spectrum  shows no evidence  f o r any  f e a t u r e below the edge.  s p e c t r a of N ( C H ) 3  Rydberg c h a r a c t e r .  3  (Figs.  8.3 and 8.4, Table 8.3) shows v e r y  There i s an i n d i c a t i o n  of a weak f e a t u r e , 1,  w i t h a term v a l u e of 3.0 eV, which i s a s s i g n e d to a t r a n s i t i o n t o the 3s Rydberg l e v e l .  The term v a l u e compares w i t h one of 3.03 eV from the  valence s h e l l spectra. transitions  case of NH . resonance,  f e a t u r e 2 presumably encompasses  to the 3p Rydberg l e v e l s and h i g h e r .  f o r the 3s t r a n s i t i o n s 3  The broad  The l a c k of i n t e n s i t y  c o n s i s t e n t w i t h the arguments made above i n the  The spectrum  i s t o t a l l y dominated by a a (N-C) shape  c e n t r e d j u s t above the edge.  GENERAL DISCUSSION The  s p e c t r a p r e s e n t e d here a l l support the c o n t e n t i o n t h a t there  are no l o w - l y i n g a  v i r t u a l o r b i t a l s , and t h a t the lowest l e v e l s a r e  Rydberg i n n a t u r e .  T h i s agrees w i t h the assignment of the v a l e n c e s h e l l  s p e c t r a (below the f i r s t  IP) as t r a n s i t i o n s  Rydberg c h a r a c t e r [12,200,201]. have been found  t o l e v e l s o f predominantly  In the ISEELS s p e c t r a the a  levels  t o be a t or o n l y j u s t above the i o n i s a t i o n edges i n the  amines ( T a b l e s 8.1 and 8.3) and seemingly valence spectra t r a n s i t i o n s  t o the a  absent  i n NH . 3  F o r the  l e v e l s would be h i g h e r i n the  continuum than f o r the core s p e c t r a s i n c e there would be no core h o l e t o cause a l a r g e r e l a x a t i o n . essentially  The C Is s p e c t r a a r e v e r y s i m i l a r and show an  c o n s t a n t 3s/3p i n t e n s i t y r a t i o .  On the other hand, the  - 270 -  Rydberg  c h a r a c t e r i s t i c s of the N Is s p e c t r a change d r a m a t i c a l l y  c e r t a i n l y do not p a r a l l e l the C Is s p e c t r a .  Indeed, the C Is s p e c t r a  c o u l d be s o l e l y i n t e r p r e t e d i n terms of a l o c a l i z e d Rydberg based upon a CH X model compound. 3  i n S K C H g ) ^ and  and  structure  T h i s phenomenon has a l s o been noted  P(CH ) . 3  3  A broad f e a t u r e at or j u s t above the edge i s p r e s e n t i n a l l of  * the methyl amine C Is and N Is s p e c t r a and i s a s s o c i a t e d w i t h the a a n t i b o n d i n g o r b i t a l formed by the C-N p i c t u r e i t i s a l s o termed as a a much d i s c u s s i o n r e c e n t l y  bond.  In a m u l t i p l e  shape-resonance  [77],  scattering  There has  been  (see Chapter 1, s e c t i o n F . I I I ) on the r e l a t i o n -  s h i p between bond l e n g t h and the shape-resonance p o s i t i o n edge (6  = E -IP). K  (E ) from the K H i t c h c o c k et a l . [100] have demonstrated that a  linear relationship i s sufficient  to d e s c r i b e the C-C  bond l e n g t h v a r i a -  t i o n and shape-resonance p o s i t i o n i n a s e r i e s of hydrocarbons. al.  Sette et  [99] have made a s y s t e m a t i c study on a l a r g e r v a r i e t y of systems.  study on some phosphorus  compounds i n Chapter 6 a l s o i n d i c a t e d  A  that  t h e r e i s a r e l a t i o n s h i p between bond l e n g t h / t y p e and resonance p o s i t i o n . Table 8.4 edge (6).  lists  the C-N  bond l e n g t h s and resonance p o s i t i o n s from the  I t i s seen t h a t as the bond l e n g t h decreases the r e l a t i v e  p o s i t i o n of the resonance w i t h r e s p e c t to the edge moves to h i g h e r energy, as has been noted b e f o r e [99,100].  The l a c k of data p o i n t s and  the e r r o r l i m i t s , however, does not a l l o w the exact r e l a t i o n s h i p to be examined. Finally, and NH  3  i t i s of v a l u e to compare the s p e c t r a of N F , 3  w i t h t h e i r t h i r d row analogues P F , P ( C H ) 3  3  3  and PH . 3  (CH ) N 3  In  3  - 271 -  TABLE Resonance Energy Lengths  Molecule  8.4  P o s i t i o n s (6) From Edge and C-N Bond (R) f o r the Methyl  R(A)  Amines  ( a )  6 (eV)  ( b )  C Is  N Is  (CH ) N  1.451  (3)  3.0  1.7  (CH ) NH  1.462  (7)  1.8  0.9  (CH )NH  1.4714 (20)  0.2  -0.3  3  3  3  3  2  2  'From microwave s p e c t r o s c o p y . Landholt - B B r n s t e i n (New S e r i e s ) I I / 7 " S t r u c t u r e Data of Free Polyatomic M o l e c u l e s , " S p r i n g e r - V e r l a g , B e r l i n (1976) (k)s = Resonance Energy  - IP = -Term  Value  - 272  -  on the P 2p s p e c t r a of some  p r e v i o u s chapters the e f f e c t s of the l i g a n d  phosphorus compounds were compared t o the e f f e c t s on the S i 2p s p e c t r a of  s i l i c o n compounds.  series.  The l i g a n d  B r i e f l y , the more h i g h l y  enhance the p r o b a b i l i t y expense of t r a n s i t i o n s v a l u e s of the lowest  a  i d e n t i t y had a s i m i l a r e f f e c t i n both e l e c t r o n e g a t i v e l i g a n d s ( e . g . , F)  of t r a n s i t i o n s  t o the a  t o the h i g h e r - l y i n g  v i r t u a l l e v e l s a t the  Rydberg l e v e l s .  o r b i t a l s were much l a r g e r  The term  i n these compounds  than those of compounds w i t h l e s s e l e c t r o n e g a t i v e l i g a n d s such as H. The  a  l e v e l s i n the t h i r d row h y d r i d e s s t i l l  levels.  precede the Rydberg  However, the spectrum i s much l e s s dominated by the a  and  r e l a t i v e l y s t r o n g Rydberg t r a n s i t i o n s  the  electron  d o n a t i n g CH  3  ligand  Rydberg-valence t r a n s i t i o n s .  are observed.  leads t o p o s s i b l e  I t i s of i n t e r e s t  levels  The e f f e c t of  o v e r l a p p i n g or mixed  t o note, however, the  complete absence o f a 2p •*• 4s Rydberg t r a n s i t i o n i n the S i 2p spectrum of  8 1 ( 0 1 3 ) ^ , i m p l y i n g that  tions  to v a l e n c e o r b i t a l s .  t h i s spectrum c o n s i s t s  e s s e n t i a l l y of t r a n s i -  By c o n t r a s t the C Is + 4s t r a n s i t i o n i s  c l e a r l y seen i n the C Is l i g a n d  spectrum of S i ( C H ) ^ . 3  F e a t u r e s i n the  continuum s p e c t r a of S ^ C H g ) ^ and phosphorus compounds a r e a l s o  present  * and  can be i d e n t i f i e d i n p a r t w i t h d - l i k e  a  shape resonances.  On comparing the ISEELS s p e c t r a of the phosphorus compounds w i t h the  s p e c t r a of the n i t r o g e n compounds i t should be remembered that i n  the  former the 2p s p e c t r a l  r e g i o n i s being c o n s i d e r e d whereas the i s  r e g i o n i s b e i n g c o n s i d e r e d f o r the l a t t e r . ces  ( o r s i m i l a r i t i e s ) can s t i l l  be noted.  However, important The e f f e c t s  would appear t o be v e r y s i m i l a r i n both s e r i e s .  differen-  o f the l i g a n d s  The major  difference  - 273 -  * a r i s e s with the p o s i t i o n of the a  a n t i b o n d i n g o r b i t a l s which are h i g h  l y i n g and i n the continuum f o r the N compounds except when i t i s a s s o c i a t e d w i t h an e l e c t r o n e g a t i v e l i g a n d .  Thus the N Is spectrum of N F  3  * has a very i n t e n s e , l o w - l y i n g N Is •+ o* t r a n s i t i o n and only weak t i o n s to the Rydberg l e v e l s .  The NH  3  transi-  spectrum can be mostly a s c r i b e d t o  Rydberg t r a n s i t i o n s w i t h the h i g h - l y i n g and weak a (N-H) shape resonance  * approximately 14 eV i n t o continuum t i o n s precede the r e l a t i v e l y  [99], whereas  i n PH  the a  3  i n t e n s e Rydberg t r a n s i t i o n s .  transi-  The N Is  * spectrum of ( C H ) N 3  3  i s dominated by a N Is + fl shape resonance  o c c u r r i n g j u s t beyond the edge and shows l i t t l e in Si(CH ) 3  4  Rydberg c h a r a c t e r .  and P ( C H ) , the l i g a n d spectrum of ( C H ) N 3  3  3  t y p i c a l C Is •*• Rydberg t r a n s i t i o n s .  shows the  3  Thus the N spectrum lends  support f o r the assignments p r e v i o u s l y g i v e n f o r P ( C H ) 3  As  3  further  and S i C C H ^ ^ ,  which i n d i c a t e t h a t the lowest Rydberg l e v e l s belong v e r y much t o the methyl group and have much C c h a r a c t e r .  The assignment of the ( C H ) N 3  v a l e n c e s h e l l spectrum t o Rydberg t r a n s i t i o n s w i t h the above d i s c u s s i o n s i n c e the a  [12,200,201]  the l a c k of i n t e n s i t y o f t r a n s i t i o n s  the N Is s p e c t r a o f N F  and ( C H ) N .  3  3  3  i s consistent  l e v e l i s i n the continuum (and a