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K-shell excitation of molecules by fast electron impact Wight, Gordon Robert 1974

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K-SHELL  EXCITATION  FAST  OF  ELECTRON  MOLECULES  BY  IMPACT  by GORDON ROBERT WIGHT  B.Sc.  Hons., Memorial  University  o f Newfoundland, 1970.  A T H E S I S SUBMITTED I N PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF  DOCTOR OF PHILOSOPHY  in  t h e Department o f CHEMISTRY  We a c c e p t t h i s  t h e s i s as c o n f o r m i n g t o t h e  required standard  THE UNIVERSITY OF B R I T I S H A u g u s t , 1974.  COLUMBIA  In  presenting  this  an a d v a n c e d d e g r e e the I  Library  further  for  shall  agree  scholarly  by  his  of  this  thesis  in  at  University  the  make  that  it  freely  representatives. for  of  be g r a n t e d  It  is  financial  J^LAAQ  iCo'L,  of  Columbia,  British for  by  the  gain  Columbia  shall  not  the  requirements  reference copying of  Head o f  understood that  /CJ^^uJx^yj  The U n i v e r s i t y o f B r i t i s h V a n c o u v e r 8, Canada  Date  of  available  written permission.  Department  fulfilment  permission for extensive  p u r p o s e s may  thesis  partial  I  agree  and  be a l l o w e d  that  study.  this  thesis  my D e p a r t m e n t  copying or  for  or  publication  without  my  -ii-  ABSTRACT  Energy l o s s s p e c t r a targets  through small  o f 2.5  angles,  respective  carbon, nitrogen,  s u l f u r LJJ  j j j edges.  t r i a t o m i c and ions  electrons, scattered  have b e e n s t u d i e d o x y g e n and  E l e c t r o n energy l o s s s p e c t r a  Most s p e c t r a  s i m u l t a n e o u s t r a n s i t i o n s o f a K - s h e l l and  (i.e.  s h a k e - u p and  hole).  In the  c o r e m o d e l was  shake-off  shown t o p r o v i d e  energies  On  the  of molecules.  i n the  species  energy l o s s spectra  existence  o f an  structure  valence shell  represents  electrons  c r e a t i o n o f an  inner  carbon monoxide, a  d e s c r i p t i o n f o r the  simple K-shell  ionization  K - s h e l l energy l o s s spectra  of a  number  agreement between the  a r e s u l t o f an  respective  the  estimated  (core  for larger molecules  analogy)  e l e c t i o n promotion.  of carbon d i s u l f i d e ,  Finally, carbonyl  the  geometry carbon  sulfide  possibly associated  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 i n these  and  i s less  l a r g e changes i n m o l e c u l a r  c a r b o n t e t r a f 1 u o r i d e show f e a t u r e s w h i c h a r e the  Rydberg  from  s a t i s f a c t o r y , p o s s i b l y because o f the  K-shell  and  have been p r e d i c t e d  observed K-shell e x c i t a t i o n energies  w h i c h o c c u r as  respective  b a s i s o f t h i s m o d e l , e x c i t a t i o n and  observed  The  and  an a c c u r a t e  f o r some e x o t i c c h e m i c a l  r e l a t i v e energies  This  events f o l l o w i n g the  case of molecular nitrogen  excited molecule.  Discrete excitat-  s t r u c t u r e above t h e  K-continuum.  the  diatomic,  valence molecular o r b i t a l s  normal  of the  promotion of the  show c o n s i d e r a b l e  K-edge, i n a d d i t i o n t o the  molecular  regions  for  p o l y a t o m i c m o l e c u l e s have been s t u d i e d .  electron to u n f i l l e d  orbitals.  i n the  by  f l u o r i n e K-edges and  have b e e n i n t e r p r e t e d i n t e r m s o f t h e  K-shell  the  keV  and with  molecules.  -i i i-  TABLE  OF  CONTENTS  Page CHAPTER ONE Introduction  1  CHAPTER TWO Theory  o f F a s t E l e c t r o n Impact Energy  5  2.1.  Electron  Loss Spectroscopy  5  2.2.  The V i r t u a l  2.3.  The F i r s t  2.4.  E l e c t r o n - H y d r o g e n Atom S c a t t e r i n g  10  2.5.  Generalization  14  2.6.  Generalized O s c i l l a t o r Strengths  P h o t o n Model  6  Born A p p r o x i m a t i o n  to Scattering  9  by C o m p l e x Atoms  17  CHAPTER THREE Experimental  Methods f o r I n n e r - s h e l l  Excitation Studies  23  CHAPTER FOUR Experimental  30  4.1.  180° E l e c t r o s t a t i c A n a l y s e r  30  4.2.  The E l e c t r o n  34  4.3.  The S p e c t r o m e t e r  37  4.3.1.  Spectrometer Construction  37  4.3.2.  Spectrometer Operation  40  4.3.3.  Energy C a l i b r a t i o n  42  4.3.4.  Vacuum S y s t e m  42  4.4.  Sample P u r i t y  Source  4  5  - i v-  Page CHAPTER  FIVE  Diatomic 5.1.  Molecules  46  N i t r o g e n and Carbon Monoxide  46  5.1.1.  46  Nitrogen a. V a l e n c e S h e l l  Spectrum  b. N i t r o g e n K - s h e l l 5.1.2.  Carbon Monoxide a. V a l e n c e S h e l l  5.2.  Nitric 5.2.1.  49 60  Spectrum  60  b. C a r b o n K - s h e l l  Excitation  60  c. Oxygen K - s h e l l  Excitation  65  O x i d e and Oxygen Nitric  69  Oxide  a. N i t r o g e n  69 K-shell  b. Oxygen K - s h e l l 5.2.2.  Excitation  46  Excitation  Excitation  Oxygen a. V a l e n c e S h e l l b. Oxygen K - s h e l l  71 80 84  Spectrum Spectrum  84 86  CHAPTER S I X Triatomic Molecules 6.1.  90  Carbon D i o x i d e and N i t r o u s O x i d e  90  6.1.1.  90  Carbon D i o x i d e a. V a l e n c e S h e l l  6.1.2.  Spectrum  90  b. C a r b o n K - s h e l l  Excitation  92  c. Oxygen K - s h e l l  Excitation  102  N i t r o u s Oxide a. V a l e n c e S h e l l  105 Spectrum  b. N i t r o g e n K - s h e l l c . Oxygen K - s h e l l  Excitation  Excitation  105 106 112  -V-  Page 6.2.  Carbon D i s u l f i d e 6.2.1.  and C a r b o n y l  116  Carbon D i s u l f i d e a. V a l e n c e S h e l l  6.2.2.  Sulfide  116 Spectrum  b. C a r b o n K - s h e l l  Excitation  c. S u l f u r L J J  (2p)-shell  .  116 118  Excitation  Carbonyl S u l f i d e a. V a l e n c e S h e l l  123 127  Spectrum  127  b. O x y g e n K - s h e l l  Excitation  129  c. Carbon K - s h e l l  Excitation  129  d. S u l f u r L  (2p)-shell  n  n  i  133  CHAPTER SEVEN Polyatomic Molecules  136  7.1.  Introduction  136  7.2.  M e t h a n e , Ammonia, W a t e r , M e t h a n o l , D i m e t h y l  Ether  and M o n o m e t h y l a m i n e  137  7.2.1.  137  Methane a. V a l e n c e S h e l l b. C a r b o n K - s h e l l  7.2.2.  Spectrum Excitation  Ammonia a. V a l e n c e S h e l l  Spectrum  b. Oxygen K - s h e l l 7.2.4.  Excitation  Water a. V a l e n c e S h e l l  145 145 150  Spectrum Excitation  Methanol a. V a l e n c e S h e l l  139 144  b. N i t r o g e n K - s h e l l 7.2.3.  137  150 150 155  Spectrum  155  b. C a r b o n K - s h e l l  Excitation  155  c. O x y g e n K - s h e l l  Excitation  159  -vi-  Page 7.2.5.  Dimethyl Ether a. V a l e n c e S h e l l  7.2.6.  161 Spectrum  b. C a r b o n K - s h e l l  Excitation  162  c. Oxygen K - s h e l l  Excitation  166  Monomethylamine a. V a l e n c e S h e l l b. C a r b o n K - s h e l l  166 Spectrum Excitation  c. N i t r o g e n K - s h e l l 7.2.7. 7.3.  162  Excitation  Term V a l u e s  166 169 169 173  Carbon T e t r a f l u o r i d e  173  a. V a l e n c e S h e l l  175  b. C a r b o n K - s h e l l c. F l u o r i n e  Spectrum Excitation  K-shell  Excitation  7.4.  Carbon K - s h e l l  Energy Loss Spectrum o f Acetone  7.5.  E s t i m a t i o n o f t h e E x c i t a t i o n and I o n i z a t i o n E n e r g i e s o f NH^, H3O a n d HoF R a d i c a l s u s i n g C o r e A n a l o g i e s a p p l i e d to K-shelT E l e c t r o n Energy Loss S p e c t r a  178 182 185  189  CHAPTER EIGHT Conclusion  REFERENCES  196  197  - vi i -  LIST  OF  FIGURES  Figure 1  Page E l e c t r i c f i e l d , E ( t ) , and c o r r e s p o n d i n g f r e q u e n c y spectrum, I(v), associated with a distant c o l l i s i o n of a fast e l e c t r o n and m o l e c u l a r t a r g e t , a. C o l l i s i o n p a r a m e t e r s ; v, e l e c t r o n v e l o c i t y and b, impact parameters, c. and d, r e a l i s t i c p i c t u r e  7  o  2  R e s o l u t i o n , A X (A),  plotted  a g a i n s t energy f o r f i x e d  v a l u e s o f r e s o l u t i o n , AE (0.01 3  t o 0.05)  Schematic diagram o f a hemispherical  electron  29 energy  analyser  31  R e s o l u t i o n , AE (FWHM), V S . e l e c t r o n e n e r g y f o r t h e 180° e l e c t r o n e n e r g y a n a l y s e r : • o b s e r v e d ( c o n v o l u t i o n o f gun and a n a l y s e r s p r e a d s ) , • a n a l y s e r o n l y ( g u n s p r e a d subtracted)  35  5  E l e c t r o n gun power s u p p l y  36  6  Schematic diagram o f t h e apparatus  39  7  Energy c a l i b r a t i o n o f K - s h e l l s p e c t r a ; a. ammonia c a l i b r a t e d u s i n g m o l e c u l a r n i t r o g e n (400.93 eV p e a k ) , b. m e t h a n e c a l i b r a t e d u s i n g c a r b o n d i o x i d e (290.7 eV p e a k ) . .  43  8  Valence s h e l l  47  9.  K-shell  4  10  11  energy l o s s  energy l o s s  spectrum o f m o l e c u l a r nitrogen  ...  spectrum o f m o l e c u l a r n i t r o g e n  51  Comparison o f t h e r e l a t i v e e n e r g i e s o f v a l e n c e e x c i t e d s t a t e s o f n i t r i c o x i d e and K - s h e l l e x c i t e d s t a t e s o f n i t r o g e n a n d c a r b o n m o n o x i d e ( c a r b o n K)  53  Comparison o f t h e K - s h e l l energy nitrogen obtained using electron radiation  55  loss spectra o f molecular impact and s y n c h r o t r o n  12  Valence shell  13  Carbon K - s h e l l  14  Oxygen K - s h e l l e n e r g y l o s s s p e c t r u m o f c a r b o n m o n o x i d e . I n s e r t a ( t a k e n f r o m a s e p a r a t e d a t a r u n ) shows t h e t h r e e h i g h e r d i s c r e t e p e a k s on an e x p a n d e d s c a l e  energy l o s s energy l o s s  spectrum o f carbon monoxide spectrum o f carbon monoxide  61 62  67  -vi i i -  Figure  Page  15  Nitrogen  K-shell  energy l o s s spectrum o f n i t r i c  oxide  72  16  Comparison o f the r e l a t i v e e n e r g i e s o f : (a) v a l e n c e 0 s t a t e s ( e x p e r i m e n t a l ) and N * s t a t e s ( t h e o r e t i c a l ) ; ( b ) v a l e n c e NF s t a t e s ( e x p e r i m e n t a l ) and NO^* states (theoretical). N^ 0 and N 0 ^ s p l i t t i n g s are from X-ray PES d a t a  76  17  Oxygen K - s h e l l  81  18  Valence s h e l l  19  K-shell  20  Valence s h e l l  21  Q u a l i t a t i v e representation (not to s c a l e ) of the p o t e n t i a l e n e r g y s u r f a c e s , as a f u n c t i o n o f t h e b e n d i n g c o o r d i n a t e , o f some s t a t e s o f n i t r o g e n d i o x i d e and K - s h e l l e x c i t e d carbon d i o x i d e . Note: These i n d i c a t e t h e n a t u r e o f the e n e r g y c o r r e c t i o n s w h i c h w o u l d h a v e t o be a p p l i e d i n o r d e r t o c o m p a r e d a t a f r o m t h e two m o l e c u l e s on t h e b a s i s o f t h e c o r e a n a l o g y model  ?  K  +  +  energy l o s s spectrum of n i t r i c  oxide  energy l o s s spectrum o f m o l e c u l a r oxygen  85  energy l o s s spectrum o f m o l e c u l a r oxygen  87  energy l o s s spectrum o f carbon d i o x i d e  carbon K-shell  91  22  The  ..  95  23  C o r r e l a t i o n o f the o b s e r v e d peaks i n t h e K - s h e l l energy l o s s s p e c t r a o f c a r b o n d i o x i d e and n i t r o u s o x i d e ( b o t h c a r b o n and o x y g e n K - s h e l l s ) . The d a s h e d l i n e s r e p r e s e n t t h e e x p e c t e d p o s i t i o n s o f u n r e s o l v e d peaks (see the t e x t ) . The r e l a t i v e e n e r g i e s ( c o r r e c t e d ) o f a p p r o p r i a t e s t a t e s from the v a l e n c e s h e l l s p e c t r u m o f n i t r o g e n d i o x i d e have a l s o been i n c l u d e d for comparison  98  oxygen K - s h e l l  energy l o s s spectrum of carbon d i o x i d e  94  24  The  energy l o s s spectrum o f carbon d i o x i d e  25  Valence shell  26  The  nitrogen  27  The  oxygen K - s h e l l  28  Valence s h e l l  29  Carbon  30  S u l f u r L j j j j j ( 2 p ) energy l o s s spectrum o f carbon d i s u l f i d e  energy l o s s spectrum o f n i t r o u s K-shell  K-shell  oxide  energy  103 107  energy l o s s spectrum o f n i t r o u s  energy l o s s spectrum of n i t r o u s  energy  ..  oxide .  o x i d e ...  loss spectrum of carbon d i s u l f i d e l o s s spectrum o f carbon d i s u l f i d e  108 113 117 119 124  - i x-  Figure  Page  31  Valence shell  energy l o s s  32  Oxygen K - s h e l l  energy l o s s  spectrum o f carbonyl  sulfide  130  33  Carbon K - s h e l l  energy  spectrum o f carbonyl  sulfide  131  34  Sulfur  LJJ  m(2p)  35  Sulfur  LJJ  m^p)  e  n  e  loss r  9  spectrum o f carbonyl  l°  v  L  I 1 , 111  e  d  g  e  spectrum o f carbonyl  s s  energy l o s s  w i t h an e x p a n d e d e n e r g y s c a l e  sulfide  spectrum of carbonyl  sulfide  128  134  sulfide  i n the region of the  s  1  energy l o s s  5  36  Valence s h e l l  37  Carbon K - s h e l l  38  Valence s h e l l  39  Nitrogen  40  Valence s h e l l  41  Oxygen K - s h e l l  42  Valence s h e l l  43  Carbon K - s h e l l  energy l o s s spectrum o f methanol  157  44  Oxygen K - s h e l l  energy l o s s  160  45  Valence s h e l l  46  Carbon K - s h e l l  energy l o s s  spectrum o f dimethyl  ether  164  47  Oxygen K - s h e l l  energy l o s s  spectrum o f dimethyl  ether  167  48  Valence s h e l l  spectrum o f monomethylamine  168  49  Carbon K - s h e l l  50  Nitrogen  51  Valence s h e l l  energy l o s s energy l o s s  K-shell  energy l o s s energy l o s s  energy l o s s  energy l o s s energy l o s s energy  electron  138  s p e c t r u m o f methane  140  s p e c t r u m o f ammonia  energy loss  energy loss  K-shell  s p e c t r u m o f methane  3  146  s p e c t r u m o f ammonia  147  spectrum o f water  151  spectrum o f water  152  spectrum o f methanol  156  spectrum o f methanol spectrum of dimethyl  ether  spectrum o f monomethylamine  loss  s p e c t r u m o f m o n o m e t h y l a m i n e ...  energy loss  163  170 172  spectrum o f carbon  tetrafluoride  176  52  Carbon K - s h e l l  energy l o s s  53  Fluorine K-shell tetrafluoride  spectrum o f carbon t e t r a f l u o r i d e  energy l o s s  179  spectrum o f carbon 183  -X-  Figure  Page  54  Carbon K - s h e l l  energy l o s s spectrum o f acetone  186  55  The c a r b o n K - s h e l l e l e c t r o n e n e r g y l o s s s p e c t r u m o f methane and c a l c u l a t e d e n e r g y l e v e l s o f t h e ammonium r a d i c a l ( N H ) . A  194  -xi-  LIST  OF  Plate  PLATES  Page  1  The  Spectrometer  2  Complete Experimental Arrangement  38 44  -xi i-  LIST  OF  TABLES  Table 1  2  3  4  5  6  7  8  9  10  11  Page A b s o l u t e e n e r g i e s ( e V ) , r e l a t i v e e n e r g i e s and assignments of peaks observed i n Region I o f the K - s h e l l s p e c t r a o f m o l e c u l a r n i t r o g e n and c a r b o n m o n o x i d e ( c a r b o n K - s h e l l )  52  A b s o l u t e e n e r g i e s ( e V ) , r e l a t i v e e n e r g i e s and p o s s i b l e assignments o f peaks o b s e r v e d i n Region I o f t h e oxygen K - s h e l l spectrum o f carbon monoxide  68  E l e c t r o n c o n f i g u r a t i o n s and e l e c t r o n i c s t a t e s o f e x c i t e d n i t r i c o x i d e and m o l e c u l a r o x y g e n  70  K-shell  A b s o l u t e e n e r g i e s ( e V ) , r e l a t i v e e n e r g i e s and p o s s i b l e a s s i g n m e n t s o f p e a k s o b s e r v e d i n t h e n i t r o g e n and o x y g e n K-shell spectra of n i t r i c oxide  73  A b s o l u t e e n e r g i e s ( e V ) , r e l a t i v e e n e r g i e s and p o s s i b l e assignments o f peaks observed i n the K - s h e l l spectrum o f m o l e c u l a r oxygen  88  A b s o l u t e e n e r g i e s ( e V ) , r e l a t i v e e n e r g i e s and p o s s i b l e a s s i g n m e n t s o f p e a k s o b s e r v e d i n t h e c a r b o n and o x y g e n K - s h e l l s p e c t r a of carbon d i o x i d e  96  A b s o l u t e e n e r g i e s ( e V ) , r e l a t i v e e n e r g i e s and p o s s i b l e assignments o f peaks observed i n the n i t r o g e n K - s h e l l spectrum of n i t r o u s oxide  109  A b s o l u t e e n e r g i e s ( e V ) , r e l a t i v e e n e r g i e s and p o s s i b l e assignments o f peaks observed i n the oxygen K - s h e l l spectrum o f n i t r o u s oxide  114  A b s o l u t e e n e r g i e s ( e V ) , r e l a t i v e e n e r g i e s and p o s s i b l e assignments o f peaks observed i n the carbon K - s h e l l spectrum o f c a r b o n d i s u l f i d e and t h e c a r b o n and o x y g e n K - s h e l l spectra of carbonyl s u l f i d e  120  A b s o l u t e e n e r g i e s ( e V ) , r e l a t i v e e n e r g i e s and p o s s i b l e a s s i g n m e n t s o f p e a k s o b s e r v e d i n t h e s u l f u r 2p ( L J T J T J s h e l l ) s p e c t r a o f c a r b o n d i s u l f i d e and c a r b o n y l s u l f i d e  125  A b s o l u t e e n e r g i e s ( e V ) , r e l a t i v e e n e r g i e s and p o s s i b l e assignments o f peaks observed i n the carbon K - s h e l l s p e c t r u m o f methane  141  -xiii-  Table 12  13  14  15  16  17  18  19  20  21  Page A b s o l u t e e n e r g i e s ( e V ) , r e l a t i v e e n e r g i e s and p o s s i b l e assignments o f the peaks o b s e r v e d i n the n i t r o g e n K - s h e l l s p e c t r u m o f ammonia  148  A b s o l u t e e n e r g i e s ( e V ) , r e l a t i v e e n e r g i e s and p o s s i b l e assignments o f peaks o b s e r v e d i n t h e oxygen K - s h e l l spectrum of water  153  A b s o l u t e e n e r g i e s ( e V ) , r e l a t i v e e n e r g i e s and p o s s i b l e a s s i g n m e n t s o f p e a k s o b s e r v e d i n t h e c a r b o n and o x y g e n K - s h e l l s p e c t r a o f methanol  158  A b s o l u t e e n e r g i e s ( e V ) , r e l a t i v e e n e r g i e s and p o s s i b l e a s s i g n m e n t s o f p e a k s o b s e r v e d i n t h e c a r b o n and o x y g e n K-shell spectra of dimethyl ether  165  A b s o l u t e e n e r g i e s ( e V ) , r e l a t i v e e n e r g i e s and p o s s i b l e a s s i g n m e n t s o f p e a k s o b s e r v e d i n t h e c a r b o n and n i t r o g e n K - s h e l l s p e c t r a o f monomethylamine  171  The 3s and 3p R y d b e r g t e r m v a l u e s o b s e r v e d f o r K - s h e l l e x c i t 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 ( o u t e r m o s t e l e c t r o n ) i n m e t h a n e , ammonia, w a t e r , m e t h a n o l , d i m e t h y l e t h e r and m o n o m e t h y l a m i n e  174  A b s o l u t e e n e r g i e s (eV) o f peaks o b s e r v e d i n t h e v a l e n c e s h e l l spectrum of carbon t e t r a f l u o r i d e  177  A b s o l u t e e n e r g i e s ( e V ) , r e l a t i v e e n e r g i e s and p o s s i b l e a s s i g n m e n t s o f p e a k s o b s e r v e d i n t h e c a r b o n and f l u o r i n e K-shell spectra of carbon t e t r a f l u o r i d e  180  A b s o l u t e e n e r g i e s ( e V ) , r e l a t i v e e n e r g i e s and p o s s i b l e assignments o f peaks observed i n t h e carbon K - s h e l l spectrum of acetone  188  Estimated energy H F 9  radicals  levels  ( e V ) o f t h e NH., 7...T  H.,0  and  hypothetical 193  - x i v-  ACKNOWLEDGEMENTS  I would  like  t o t h a n k s i n c e r e l y D r . C. E. B r i o n  f o rh i s interest,  e n c o u r a g e m e n t a n d a s s i s t a n c e a n d D r . M. J . v a n d e r W i e l he  i n j e c t e d i n t o t h e work. I a l s o a c k n o w l e d g e many h e l p f u l d i s c u s s i o n s w i t h  thank him f o r h i s i n t e r e s t . Mr.  W.-C. The  D r . A. J . M e r e r a n d  I n a d d i t i o n , t h e many d i s c u s s i o n s  with  Tam a n d Mr. S. Tong L e e w e r e m o s t h e l p f u l a n d a r e a p p r e c i a t e d . capable s t a f f o f t h emechanical  a tremendous a s s e t Mr.  f o rthe stimulus  during  and e l e c t r o n i c s workshops were  a l l phases o f t h i s work;  i n particular,  E. Gomm a n d Mr. J . S h i m . Financial  Scholarship  i s also  Finally, contributed  support i n t h e form o f a N a t i o n a l  Research Council  Science  acknowledged.  f r o m my w i f e , d a u g h t e r a n d m y s e l f ,  t o an e n j o y a b l e Ron  stay  i n Vancouver, e s p e c i a l l y ;  a n d Bea Thompson  C h r i s and E l i z a b e t h Bill  t h a n k y o u t o a l l who h a v e  and M a r i l y n  Frank and J o y c e  Brion  Henderson Roberts.  -1-  CHAPTER  ONE  INTRODUCTION  Electron  i m p a c t e x c i t a t i o n has b e e n u s e d a s a  technique since experimental  the beginning o f this  demonstration o f the quantization  s y s t e m s was p r o v i d e d inelastically experiment are  quite  1  .  scattered  by m e r c u r y a t o m s , i n t h e c l a s s i c  with  et a l .  processing  1960's t h e r e  At that  and q u a l i t y o f d a t a ,  by two g r o u p s ;  Lassettre  and s i g n a l  the early  impact spectroscopy.  in both the quantity provided  Franck-Hertz 2  i n 1930  i n view o f t h e l i m i t e d development o f e l e c t r o n  However, u n t i l  in electron  o f a t o m i c and m o l e c u l a r  The e l e c t r o n e n e r g y l o s s m e a s u r e m e n t s o f R u d b e r g  impressive  time.  Indeed, the f i r s t  by an e n e r g y l o s s m e a s u r e m e n t o f e l e c t r o n s ,  o p t i c s , e l e c t r o n energy analysers that  century.  spectroscopic  electronics at  were few e x p e r i m e n t s  time, there  was a r a p i d  growth  l a r g e l y due t o t h e s t i m u l u s  t h a t o f B o e r s c h , G e i g e r e t a l . and t h a t o f  I n f a c t , a s e a r l y a s 1 9 6 6 , an e l e c t r o n  a r e s o l u t i o n ^ 0.010 eV was i n o p e r a t i o n .  This  spectrometer  was s u f f i c i e n t t o 3  resolve  rotational structure  m o l e c u l a r hydrogen.  i n the e l e c t r o n energy l o s s spectrum  Since these measurements, t h e r e s o l u t i o n o f e l e c t r o n  e n e r g y l o s s s p e c t r o m e t e r s has n o t been i m p r o v e d . q u a n t i t a t i v e measurements, there determinations of generalized Bethe s u r f a c e . impact energy  for  Recently,  However, i n terms o f  has b e e n a c o n t i n u a l  oscillator  strengths  improvement i n  and p o r t i o n s  of the  t h e dependence o f e x c i t a t i o n c r o s s - s e c t i o n s  (the "excitation function")  on  a n d s c a t t e r i n g a n g l e has b e e n  -2-  used t o i d e n t i f y the nature o f atomic and m o l e c u l a r t r a n s i t i o n s .  In  p a r t i c u l a r many e l e c t r i c 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 have been i d e n t i f i e d i n t h i s manner u s i n g low impact e n e r g i e s .  The developments and c u r r e n t 4-9  s t a t u s o f experiments are d i s c u s s e d i n a number o f reviews  .  In e l e c t r o n energy l o s s s p e c t r o s c o p y a " m o n o e n e r g e t i c " beam o f rons i s used t o e x c i t e the t a r g e t s p e c i e s .  elect-  E x c i t a t i o n s are d e t e c t e d  energy l o s s e s i n the s c a t t e r e d e l e c t r o n beam.  as  The process may be r e p r e s -  ented as f o l l o w s ; X + e -> X* + e  *  where X i s the t a r g e t s p e c i e s , e i s the " c o l l i d i n g " e l e c t r o n and X  is  the d i s c r e t e s t a t e o f the t a r g e t which i s e x c i t e d by the c o l l i s i o n .  Discrete  e l e c t r o n energy l o s s e s o c c u r f o r e v e r y a c c e s s i b l e s t a t e o f the  target.  T h e r e f o r e , e l e c t r o n energy l o s s s p e c t r o s c o p y i s an a l t e r n a t i v e  t o the use of  p h o t o a b s o r p t i o n f o r i n v e s t i g a t i n g the e x c i t e d s t a t e s o f atoms and m o l e c u l e s . In a d d i t i o n , the s c a t t e r e d i n t e n s i t i e s observed f o r f a s t e l e c t r o n and small angle s c a t t e r i n g may be q u a n t i t a t i v e l y oscillator strengths^'^. simulates a v i r t u a l dominant.  impact  r e l a t e d to o p t i c a l  Under t h e s e c o n d i t i o n s the i m p i n g i n g e l e c t r o n  photon f i e l d and e l e c t r i c d i p o l e t r a n s i t i o n s  are  D i p o l e s e l e c t i o n r u l e s do not a p p l y f o r low impact e n e r g i e s ,  p a r t i c u l a r l y a t l a r g e s c a t t e r i n g a n g l e s , and magnetic d i p o l e , quadrupole and s p i n f o r b i d d e n p r o c e s s e s may be o b s e r v e d .  electric  Electron  impact  e x c i t a t i o n i s p a r t i c u l a r l y u s e f u l a t s h o r t wavelengths ( h i g h e x c i t a t i o n energies)  where u s e f u l photon c o n t i n u u a are d i f f i c u l t t o produce.  T h i s t h e s i s d e s c r i b e s the a p p l i c a t i o n o f f a s t e l e c t r o n impact t o a study o f the high energy d i s c r e t e s t a t e s i n the r e g i o n s o f i n n e r s h e l l e x c i t a t i o n s o f small m o l e c u l e s .  These high energy s t a t e s r e s u l t from the  -3-  p r o m o t i o n o f an (which to  inner shell  f o r most m o l e c u l e s  u n f i l l e d molecular  Little  e l e c t r o n , f o r example a Is  i s n o n b o n d i n g and  orbitals  and  e x c i t e a molecule  atomic  Rydberg o r b i t a l s  i n f o r m a t i o n i s a v a i l a b l e about these  To  mainly  of the  sources  exist:  because they  continua  continuum, but K-shell  and  i t i s a very  An  strahlung continuua  expensive  are mainly  edges"^"^.  '  .  i n the  Although  have b e e n  and  light radiation. studies  produces a u s e f u l  of limited  f o r molecular  trans-  availability.  nitrogen using  Brems-  13 Since then,  regions  Bremsstrahlung absorption  continuua  have been  s p e c t r a , although  fluorine  radiation  b e e n a v a i l a b l e f o r some 27  has  spectra for molecular  inner  the  o f s u l f u r and  synchrotron  time, only K-shell absorption  nitrogen  shell  and  2  methane  reported.  R e c e n t l y , e l e c t r o n i m p a c t has shell  facility  the  continuum  f o r photoabsorption  electron synchrotron  u s e d t o o b t a i n a number o f i n n e r s h e l l results  sources  a b s o r p t i o n s were f i r s t observed 12  two  ( i i ) electron synchrotron  are d i f f i c u l t  a r e u s u a l l y weak.  molecule.  to the energy r e q u i r e d f o r the  In the energy r e g i o n f o r K - s h e l l e x c i t a t i o n ,  Bremsstrahlung  character)  photoabsorption,  ition.  ( i ) Bremsstrahlung  in  high energy s t a t e s .  t o a d i s c r e t e s t a t e by  p h o t o n e n e r g y m u s t be e x a c t l y e q u a l  (K) e l e c t r o n  electrons.  n i t r o g e n and  Low  been u s e d f o r t h e e x c i t a t i o n  of  inner  r e s o l u t i o n , K - 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  c a r b o n m o n o x i d e h a v e been m e a s u r e d a t an  impact energy  of  of  29 10  keV  and  f u r t h e r m e a s u r e m e n t s h a v e been made i n c o i n c i d e n c e w i t h 30 s p e c i f i c ion products . A l s o , t h e K - s h e l l e n e r g y l o s s s p e c t r a o f some n u c l e i c a c i d b a s e s h a v e b e e n o b s e r v e d by p a s s i n g 25 keV e l e c t r o n s t h r o u g h 31 thin  solid  samples  .  Further evidence has  been p r o v i d e d  by  on  the d i s c r e t e e x c i t a t i o n  the occurrence  of inner shell  electrons  of high energy a u t o i o n i z a t i o n l i n e s  in  -4-  Auger e l e c t r o n s p e c t r a carefully  e x c i t e d by  s e l e c t e d X-ray l i n e  .  ( i ) e l e c t r o n impact  , and, ( i i ) a  High energy c o n t r i b u t i o n s t o the K a  37-39  emission  spectra  of nitrogen  and  (as p o i n t e d  o u t by S i e g b a h n  ) in  40 carbon monoxide states. The d a t a electrons  , have been a t t r i b u t e d t o r e s o n a n c e e m i s s i o n  from  neutral  p r e s e n t l y a v a i l a b l e on t h e d i s c r e t e e x c i t a t i o n o f i n n e r 27  i s extremely  limited  and i n c o m p l e t e .  Molecular  are the only  common g a s e s f o r w h i c h r e a s o n a b l e  nitrogen  shell and  28 methane spectra  have been o b t a i n e d .  interpret using  inner shell  fast electron  The o b j e c t o f t h i s  "absorption" impact.  spectra  research  K-shell  absorption  i s to obtain  f o r a v a r i e t y o f gaseous  and  molecules  -5-  CHAPTER  THEORY  2.1.  Electron  Energy  In e l e c t r o n electrons  beam.  FAST  ELECTRON  IMPACT.  Loss Spectroscopy.  energy  l o s s s p e c t r o s c o p y a beam o f " m o n o e n e r g e t i c "  i s used t o e x c i t e  Excitations  OF  TWO  discrete  states  o f an a t o m i c o r m o l e c u l a r t a r g e t .  a r e d e t e c t e d as e l e c t r o n  energy  losses  i n the scattered  The p r o c e s s may be r e p r e s e n t e d as f o l l o w s ; X + e ( E ) -+  X*(E ) +  Q  n  where X i s t h e t a r g e t  e(E e) r  atom o r m o l e c u l e i n i t s g r o u n d e l e c t r o n i c s t a t e , *  is  the k i n e t i c  state and  energy o f the i n c i d e n t e l e c t r o n  of the target  w h i c h has e n e r g y E  E-| i s t h e k i n e t i c  through angle e the  electron  n  with  e n e r g y o f an e l e c t r o n  (with  respect  beam, X  1  excited state  is inelastically  scattered i n which  The e n e r g y e q u a t i o n i s g i v e n b y ;  w h e r e E^ i s t h e k i n e t i c  energy o f the p r o j e c t i l e  ferred  energy o f the t a r g e t  to translational  i s the n  t o t h e i n c i d e n t beam) i n a c o l l i s i o n  t a r g e t i s e x c i t e d t o t h e n^' s t a t e . E = E, + E + E. o 1 n t  Q  th  r e s p e c t t o t h e ground which  E  electron  during  which  i s trans-  the c o l l i s i o n .  Using  41 the  c o n s e r v a t i o n o f e n e r g y a n d momentum, E  ^  t  (2mE /M) o  |^1 - ( E / 2 E ) n  0  i t has b e e n shown  - { l - (E /E )}  where m and M a r e t h e masses o f t h e i n c i d e n t respectively. target,  this  Because term  n  electron  o f t h e l a r g e mass d i s p a r i t y  i s very small  1 /  Q  2  that  cosej  (2.1.1)  and t a r g e t  molecule  between t h e e l e c t r o n and  a n d may be n e g l e c t e d .  For example,  _3  E  f  °» 10  eV f o r t h e p r o m o t i o n o f a n i t r o g e n  K-shell  electron  (E  n  ^ 400 eV)  -6-  with  experimental  T h e r e f o r e , the required of  the  used f o r t h i s  e l e c t r o n energy l o s s , E  to e x c i t e  the  n  excited  d i s c r e t e energy l o s s e s  electron the  conditions  study  - E-|,  of the  current  Experimentally,  measured a t a g i v e n  scattered  is proportional  to the  the  differential  to the  target,  E^.  electrons  and  A measurement  produces  scattered  - E-j = E  p  and  6 ^ 0 ) .  energy  energy absorption  the magnitude of the energy l o s s , E  = 2 5 0 0 eV  i s equal  s t a t e of the  energy l o s s spectrum which gives  target.  (E  the  spectrum  of  electron  s c a t t e r i n g angle  cross-section,  da da °" f o r the  e x c i t a t i o n of the  stated  that  f o r high  n  v  (E„,e) o'  s t a t e of the  i m p a c t e n e r g i e s and  target.  small  a q u a n t i t a t i v e r e l a t i o n s h i p between e l e c t r o n optical  data^'^.  relationship  2.2.  The  The  Virtual  distant  with  The  E(t)  electric  field  =  there  impact c r o s s - s e c t i o n s t r e a t m e n t shows why  p a r t i c l e impact, e x c i t a t i o n s are collisions  dimensions of  i n t i m e and  impulsive  been is  and a  Photon Model  field  a fast electron  pulsed  scattering angles,  following semiclassical  (or glancing)  l a r g e r than the 1(a)].  already  should e x i s t .  In f a s t charged by  I t has  ( o r any  be  / l ( ) e w  i  ,  t  du,;  target  [see  i n space.  The  from the  I (a,)  =  f r e q u e n c y components o f f o u r i e r transform  /E(t)e  _  1  u  t  dt  is  Figure  in a distant  s t r u c t u r e l e s s charged p a r t i c l e ) i s  obtained  a  the  produced  i m p a c t p a r a m e t e r , b,  atomic or molecular target  e x p e r i e n c e d by  uniform  may  the  i n which the  generally  collision  sharply this  relationship:  e,  FIGURE 1.  E l e c t r i c f i e l d , E ( t ) , and c o r r e s p o n d i n g f r e q u e n c y s p e c t r u m , I ( v ) , a s s o c i a t e d w i t h d i s t a n t c o l l i s i o n o f a f a s t e l e c t r o n and m o l e c u l a r t a r g e t . a. C o l l i s i o n p a r a m e t e r s ; v , e l e c t r o n v e l o c i t y and b , i m p a c t p a r a m e t e r s . b. I d e a l i z e d c a s e f o r a v e r y f a s t e l e c t r o n . c. a n d d. realistic picture.  a  -8-  where E ( t ) i s t h e t i m e dependence o f t h e e l e c t r i c co = 2TTV, i s t h e i n t e n s i t y electric  field.  a delta  field,  1(b)].  o f the frequency  The f a s t e r t h e i n c i d e n t p r o j e c t i l e ,  f u n c t i o n and i n t h i s  electric Figure  distribution  hypothetical limit  I(co), h a s e q u a l Therefore  coefficients  the e l e c t r i c  field  a distant c o l l i s i o n with a fast electron a s s o c i a t e d w i t h a beam o f w h i t e t a r g e t may be v i e w e d In  field  practice,  light.  field  t h e more E ( t )  resembles  transform o fthe  a t a l l frequencies [see experienced  i s similar  by t h e t a r g e t i n  to the electric  The i n t e r a c t i o n  field  o f t h e e l e c t r o n and  photon  p u l s e has a f i n i t e  I (co) i s n o t c o n s t a n t o v e r t h e e n t i r e  components o f t h e  the fourier  as t h e c r e a t i o n o f a v i r t u a l  the electric  a n d I(co), w h e r e  field. width  range o f f r e q u e n c i e s .  and t h e r e f o r e  This  problem  42 has  been c o n s i d e r e d by C h r i s t o p h o r o u  .  The r e s u l t s  are i l l u s t r a t e d i n  F i g u r e 1 ( c ) and 1 ( d ) w h e r e t h e c o m p o n e n t s o f t h e e l e c t r i c as a f u n c t i o n been p l o t t e d . direction  ( v ) i s much l e s s  light  It  N CO  v/b.  analogy  const,  a l a r g e range o f  F i g u r e 1 ( d ) shows t h a t t h e p a r a l l e l  constant)  intensity  constant over  component,  distribution. t h e range o f  i n v o l v e d i n t h e e x c i t a t i o n o f atoms a n d m o l e c u l e s . isstill 42  (l/fico)  maintained.  t h a t t h e number o f v i r t u a l  co, i s a p p r o x i m a t e l y ^  perpendicular tothe  d e c l i n e s s h a r p l y as v approaches t h e  i n t e n s e and has a peaked  has b e e n shown  frequency  and then  intensities  s p e c t r a I ( v ) have  = hv w h e r e h i s P l a n c k ' s  I ( v ) ^ I ^ ( v ) and i s e s s e n t i a l l y  frequencies normally white  i s a constant over  ( o r e n e r g i e s s i n c e energy  frequency,  Therefore,  o f b/v a n d t h e f r e q u e n c y  F i g u r e 1 ( c ) shows t h a t t h e i n t e n s i t y  a t zero frequency  "cut-off" I|l  i n units  of incidence, I^(v),  frequencies starting  o f time  field  .  inversely  photons  (Nco) a t  p r o p o r t i o n a l t o the energy;  The  -9-  The is  number o f e l e c t r o n i c t r a n s i t i o n s , N ,  i n d u c e d by  n  proportional  energy, E ,  to the  and  n  the  Therefore i n this N For  *  n  N -f u  (i.e.  electric  optical oscillator  "optical *  n  fast electron  number o f p h o t o n s w i t h  these v i r t u a l  energy equal  strength,  f  photons  to the t r a n s i t i o n  , of the t r a n s i t i o n .  approximation";  const.  i m p a c t and  (f /E ) n  (2.2.1)  n  large o s c i l l a t o r  dipole allowed),  the  strength  transitions  o p t i c a l approximation gives  a  reasonable  42 estimate f o r the equation inner  number o f p r i m a r y e x c i t a t i o n s  (2.2.1) i s t h a t  shell  electron  valence shell  excitation since  .  One  and  E  i s much l a r g e r f o r t h e  is  required.  impact c r o s s - s e c t i o n  In the  approximate agreement w i t h 2.3.  The  First  Born  data,  latter.  moreover t h a t  the  strengths  a quantum m e c h a n i c a l  d e r i v a t i o n s which f o l l o w , i t w i l l  a r e l a t i o n s h i p d o e s e x i s t and  over  3  a q u a n t i t a t i v e r e l a t i o n s h i p between o p t i c a l o s c i l l a t o r  fast electron  of  e l e c t r o n e x c i t a t i o n predominates  n For  implication  the  be  treatment  shown t h a t  such  o p t i c a l approximation  is in  quantum r e s u l t s .  Approximation.  A t h e o r e t i c a l d e s c r i p t i o n of  fast electron  impact e x c i t a t i o n  was  43 initially  derived  more p h y s i c a l The that  basis the  of  by  Bethe  i n the  i n s i g h t i n t o the the  1930's.  A recent  B e t h e t h e o r y has  Bethe theory i s the  i n t e r a c t i o n between the  first  electron  and  i n c i d e n t wave i s n e g l i g i b l y d i s t o r t e d by  for  the  the  first  the  incident electron  of  this  approximation  the the  by  Inokuti^.  target  i s weak and  interaction.  The  i s somewhat a r b i t r a r y , b u t  B o r n a p p r o x i m a t i o n i s a s s u m e d t o be i s some 5-7  been w r i t t e n  gives  Born a p p r o x i m a t i o n which  the  validity  review which  times the  valid  i f the  assumes therefore criterion  generally  k i n e t i c energy  of  e x c i t a t i o n energy of a p a r t i c u l a r  -10-  transition which  and.the s c a t t e r i n g angle i s s m a l l .  i s transferred  also s a t i s f i e d to  by f a s t e l e c t r o n  illustrate  scattering  i n the c o l l i s i o n  the f i r s t  problem w i l l  A l t e r n a t i v e l y , t h e momentum  s h o u l d be s m a l l .  impact and small  electron-atom  be c o n s i d e r e d .  t h e n be  The r e s u l t s w i l l  2.4.  E l e c t r o n - H y d r o g e n Atom i s a three  body p r o b l e m a n d a p p r o x i m a t i o n s a r e n e c e s s a r y .  f o l l o w i n g d e r i v a t i o n i s b a s e d on t h a t  The  t r e a t m e n t by M o i s e i w i t s c h  v  l  v  +  r  where s u b s c r i p t s electron E  Q  and Smith  given  i s also  r  informative  1 and 2 a r e a s s o c i a t e d  respectively, E = E  Q  r  r ) 2  =  44  .  .  0  (2.4.1)  r  + E  with  t h e i n c i d e n t and a t o m i c  i s the total  t  energy o f t h e s y s t e m , where  i s t h e e n e r g y o f t h e g r o u n d s t a t e o f t h e h y d r o g e n atom a n d E^ i s t h e  k i n e t i c e n e r g y o f t h e i n c i d e n t e l e c t r o n , r-j a n d position  vectors  nucleus and  by Massey and Burhop 45  Schrbdinger equation f o r t h e system i s 2 2 2 E + — + — +v 2 + ^ * ( r 2 ^ 2 l 2 12  2  general-  Scattering.  The  v  In order  Born a p p r o x i m a t i o n , t h e s i m p l e s t  t o more c o m p l e x atom a n d m o l e c u l e s c a t t e r i n g .  The  condition i s  angle s c a t t e r i n g .  ized  Even t h i s  This  r^)  The  total  Hr  v  r) 2  respect  e ^ o ' H . ^ )  +  to the distance  t h e two e l e c t r o n s .  w a v e f u n c t i o n 'F(r-j, rv,) may be e x p r e s s e d  -  with  o f mass), r - ^ i s the i n t e r e l e c t r o n  i s a wavefunction describing  where t h e f i r s t function  o f t h e i n c i d e n t and a t o m i c e l e c t r o n s  ( e s s e n t i a l l y the centre  ^(r-j,  are respectively the  i n the form,  ^  t e r m on t h e RHS o f e q u a t i o n  (2.4.2) r e p r e s e n t s  t h e wave-  i n t h e a b s e n c e o f any i n t e r a c t i o n ( i . e . a n a s y m p t o t i c s o l u t i o n o f  -n-  equation  ( 2 . 4 . 1 ) , r-j -* °°) a n d c o n s i s t s o f t h e p r o d u c t o f a n i n c i d e n t p l a n e  wave d e s c r i b i n g the  t h e i n c i d e n t e l e c t r o n and t h e e l e c t r o n i c w a v e f u n c t i o n o f  ground s t a t e o f t h e hydrogen atom, ^ ( r ) . 0  2  ^he  w  a  v  e  n u m  ber k  Q  is  g i v e n by  The  2mE /^ . 2  t  s e c o n d t e r m on t h e RHS o f e q u a t i o n  (2.4.2) r e p r e s e n t s terms  introduced  by t h e i n t e r a c t i o n . The w a v e f u n c t i o n <f>(r-|, r ) y ma  2  of  states  o f t h e hydrogen  be e x p a n d e d  i n terms o f a complete s e t  atom,  (r )  (2.4.3)  2  w h e r e t h e sum a n d i n t e g r a l a r e o v e r t h e d i s c r e t e a n d c o n t i n u u m respectively. fact  Substitution  into  (2.4.2), then i n t o  states  (2.4.1) and u s i n g t h e  that 2  v  2m ^2  +  2  imp!ies:  2  >  n  (•• * 0 n  " l  /  0  * (r )  2  +  k  2  n > n<V 2  (2.4.4)  *„< )  F  ?  2  J  2meJ *o  fi  <fy  +  (2.4.5)  where k and E  n  n  (2m/f^) ( E  t  -  E  n  +  E ) Q  .th i s the energy o f t h e n atomic state.  (2.4.6)  -12-  Multiplying and  s i d e s o f (2.4.5)  both  by ^ * ( ^ 2 ^ '  integrating  n  over r  2  using the o r t h o g a n a l i t y o f the atomic f u n c t i o n s ,  fi> * ( r n  2  )^ - ( f )  dr  2  2  =  0  ,  (n  f  i )  gives;  <*1  2  +  n  k  2  )  W  =  +  (?•  +  /) nm^l) m(V U  F  (2.4.7) where  U  2meJ ^2  nm  (2.4.7)  Equation equations  and a p p r o x i m a t i o n s  the i n t e r a c t i o n  0, s i n c e they a r e small  'F > m f  i n v o l v e s t h e i n c i d e n t wave.  Born a p p r o x i m a t i o n t h e weak  In the f i r s t  T h e r e f o r e , on t h e RHS o f e q u a t i o n  m  terms i n U  m u s t be u s e d .  s e t of coupled  differential  Born  approximation,  i s a s s u m e d t o be weak a n d t h e r e f o r e t h e s c a t t e r i n g  F (r.j), are small.  term which  r e p r e s e n t s an i n f i n i t e  amplitudes,  ( 2 . 4 . 7 ) we n e g l e c t t h e  i n comparison  Hence t h e e s s e n c e  i s t h a t t h e i n c i d e n t wave i s n e g l i g i b l y  with the f i r s t of the f i r s t d i s t o r t e d by  interaction.  We t h e n m u s t s o l v e  ",  2  •"„>'„ff,> 2  and we r e q u i r e an a s y m p t o t i c  F  nV (  =  r _ 1  V '*) 8  (2  -' 4  9)  s o l u t i o n o f t h e form  (2.4.10)  -13-  which  i s an o u t g o i n g  f (8,<|>) i s t h e s c a t t e r i n g  s p h e r i c a l wave,  corresponding to the e x c i t a t i o n o f the n where t h e e l e c t r o n the  i s scattered  d i r e c t i o n of incidence.  excitation  i s given  ^~ (e,cO)  =  1  s t a t e o f t h e h y d r o g e n atom  a t p o l a r a n g l e s e and $ w i t h  The d i f f e r e n t i a l  by t h e r a t i o  I (e,4>)  =  on  cross-section  of the scattered  I n  r  f  ( 8  amplitude  »*  )  respect  to  f o r the  to the incident  I  flux;  (2A.U)  2  o where t h e f a c t o r k / k n  velocity  i s i n the ratio of the scattered  Q  (V = -hk/m).  To d e t e r m i n e f (e,<|>) a s o l u t i o n o f ( 2 . 4 . 9 ) n  method o f Green's F u n c t i o n f (e.+ )  =  n  -(47T)-  and g i v e s  j  1  U  S u b s t i t u t i o n o f (2.4.12) for the differential  o n  (e,*)  =  e  Q n  i s required  by ( 2 . 4 . 1 0 ) .  asymptotic form o f F ( r - j ) i s given  I  to incident  i (  may b e d o n e u s i n g t h e  45 46 '  *o " *n)-ri  into  This  such t h a t t h e  d  +  (2.4.11) g i v e s  (2.4.12) the following  expression  cross-section,  ( ^ ) "  Z  ^  /  U  on^l)  e  H  t  °  '  t n )  '"  ]  d  ^l  (2.4.13)  2  ->  It  i s convenient to introduce -tfK  -  =  ^ k  n  t h e momentum t r a n s f e r v a r i a b l e - U K w h e r e ,  ,  t h e momentum t r a n s f e r i n t h e c o l l i s i o n the  i n c i d e n t and s c a t t e r e d  given  electron  a n d "fik a n d f i k a r e t h e momenta o f o n  respectively.  The m a g n i t u d e o f K i s  by, K  2  =  k  ^o  2  +  k  „n 2  "  where e i s t h e s c a t t e r i n g  2ko k n cose angle.  ( 2 . 4 . 1 4' )  x  -14-  The  differential  o n  Q  2.5  ^r M  =  I (e,4.) The  c r o s s - s e c t i o n (2.4.13) i s then  total  2  on  o  J  ( f  cross-section, Q  = ff  orS*'^  l  o n  U  Q n  l  '  ) e l i  l d  I  ^l  , may be o b t a i n e d  2  ( - 2  by i n t e g r a t i n g  4  15  ^  (2.4.15).  sined0d<i),  G e n e r a l i z a t i o n t o S c a t t e r i n g by Complex Atoms. I f t h e i n t e r a c t i o n b e t w e e n t h e p r o j e c t i l e a n d t h e atom i s C o u l o m b i c ,  then  the interaction  potential i s  V  ( r - ^ r  where r  =  g  -e  2  ^  +  1  ^  (2.5.1)  i s t h ep o s i t i o n v e c t o r o f the  s*' a t o m i c e l e c t r o n , r i s t h e 1  position vector of theincident electron with ( e s s e n t i a l l y the centre o f mass), extends over  i s t h e n u c l e a r c h a r g e a n d t h e sum  a l l N atomic e l e c t r o n s .  t h e n g i v e n by ( 2 . 4 . 1 5 ) w i t h U  The d i f f e r e n t i a l  cross-sectioni s  ( r ^ ) g i v e n by  +  "on  t ^i  +  h r  ^ The  respect t o t h e nucleus  expression  d f  f o r the d i f f e r e n t i a l  i n t e g r a t i n g over  the coordinates  (2.5.2)  N  c r o s s - s e c t i o n may be s i m p l i f i e d b y  o f the  incident electron (r) using the  43 relation  (Bethe s  integral),  1  f  -  r  s  I"  1  e  i K  ' dr ?  =  4rr K "  2  e  i K  ^s  (2.5.3)  -15-  If  the atomic wavefunctions are orthogonal,  contribute therefore are  to the d i f f e r e n t i a l be o m i t t e d  i tis a trivial  The d i f f e r e n t i a l  cross-section  where d x ^ i n d i c a t e s  t o add t h i s  then  becomes  J  K  where t h e m a t r i x  W  elements,  K  e o n  /v  *  / * „ * X  "  (K)>  . i K -Sf  1  =  (2.5.4)  c  K/V I  4  are given  K  dx  function of e [ i . e . I Q  spatially  C©»<t>)  l  e o n  (K) |  From an e x p e r i m e n t a l differential (2.4.14)  d(K )  Since and  I  Q  and k  = 2k k o n  2  '  (e,<t>) =  finally  I  ( ) l e  o  n  obtain  n  i s only  4)  o f the N atomic  i n the following  form,  (2-5.5)  2  by  cross-section  because  1 1  ;  i s only  a  either the i n i t i a l  state  atoms a r e r a n d o m l y o r i e n t e d .  a function  as a f u n c t i o n  are constant,  sinede  of  Under  |K|.  =  t h a n e.  Differentiating  k k — o n TT  (e,cf>) we r e p l a c e (2.5.5)  of K rather  gives;  da  from  5  v i e w , i t i s more c o n v e n i e n t t o e x p r e s s t h e  cross-section  where k  =  ,2 <2  (2.5.6)  symmetric o r the t a r g e t  these conditions  dT  N  For most e x c i t a t i o n s t h e d i f f e r e n t i a l  ty i s  s  5=1  I t i s convenient to express  )  will  term.  i n t e g r a t i o n over a l l the coordinates  e  K  term  and i f t h e w a v e f u n c t i o n s  ei  0  'on < -*> '  - n (  The n u c l e a r  task  1  4TT TI  E  term does n o t  rvfe £/"•»* i, "" *° " - -  i (9, > =  electrons.  cross-section.  i n the following discussion  not orthogonal,  - *  the nuclear  V  dn  by 2ir..sinede = -rrd(K  (2.5.7) '  0  )/k k Q  n  -16-  °onM  =  d  It  4  Consider  case the d i f f e r e n t i a l  d(K )  2  having  electronic, vibrational  Q n  wavefunctions  and r o t a t i o n a l  d  quantum n u m b e r s , R nucleus  t h  *  e v r  where v  e  v  =  ^ (r,Q )-^ e  o  designates  r  the total  vibrational  (v). a n d r o t a t i o n a l  electrons,  Q the coordinates  uration.  intensities  coordinates  i s simply  given  t  h  g  are  molecular  a l l nuclear  the molecular  coordinates. wavefunctions  d e p e n d i n g o n l y on and a  Therefore  (Q)  (2.5.10)  wavefunction  describing the electronic ( e ) ,  (r) motions, r are the coordinates of the nuclei  and Q  Q  vanish  a fixed  excitation  of the  nuclear config-  upon i n t e g r a t i o n o v e r t h e  i fthe e l e c t r o n i c wavefunctions  of vibrational  and s  nuclear separation)  The n u c l e a r t e r m s i n ( 2 . 5 . 9 ) w i l l  electronic  transition  v r  and r  i s the nuclear charge o f  d e p e n d i n g on t h e n u c l e a r m o t i o n .  (r,Q)  9  m  o f an e l e c t r o n i c w a v e f u n c t i o n  the p o s i t i o n s o f t h e e l e c t r o n s ( a t a f i x e d wavefunction  h  which a r e f u n c t i o n s o f  and d r ^ i n d i c a t e s i n t e g r a t i o n over  as t h e p r o d u c t  t  <"- >  0  In terms o f t h e Born-Oppenheimer a p p r o x i m a t i o n , are expressed  of the n  *V*N  ^  the p o s i t i o n vectors of the m  nucleus  1  and N e l e c t r o n s .  ( K ) i s defined by;  e l e c t r o n w i t h r e s p e c t t o t h e c e n t r e o f mass, t  M nuclei  cross-section f o r the e x c i t a t i o n  w h e r e t h e <y's d e n o t e m o l e c u l a r  the m'  (2.5.8)  2  \  "  respectively  (K)|  a molecule  i s g i v e n by ( 2 . 5 . 8 ) w h e r e e  •on™  o n  easy t o g e n e r a l i z e (2.5.8) t o t h e case o f e l e c t r o n -  scattering.  In t h i s  K- |e  2  0  i s relatively  molecule  state  k "  are orthogonal.  accompanying a g i v e n  The  electronic  by t h e F r a n c k - C o n d o n f a c t o r s w h i c h a r e p r o p o r t i o n a l  t o t h e o v e r l a p between t h e i n i t i a l  and f i n a l  vibrational  wavefunctions  -17-  (see  R e f e r e n c e s 11 a n d 4 7 ) .  I t s h o u l d be n o t e d t h a t t h e f i r s t  a p p r o x i m a t i o n has b e e n a s s u m e d t o be v a l i d . 48 has b e e n f o u n d  t h a t the Franck-Condon  i s such t h a t the f i r s t  a d d i t i o n , the r e l a t i v e  transition  T h e s e f a c t s may  be u s e d  levels  2.6.  discussing  generalized Bethe  peaks  to advantage  i n electron  In  belonging to the angle  49  .  i m p a c t s p e c t r o s c o p y and  o f t h e "C"  s t a t e o f ammonia  treatment of the e x c i t a t i o n o f  50  4 3  51  .  =  (E /Q)  impact e x c i t a t i o n  = ti /me 2  13.606 eV, f (K)  2  n  and  (2.6.1)  |  Qn  first  introduced  the  by  (2.6.1)  2  has t h e u n i t s o f e n e r g y .  = 0.52918 x 1 0 "  =  n  8  cm and  Using  t h e Rydberg  11  t h e Bohr  radius,  energy, R = me /2ti 4  2  =  becomes;  (E /R  (Ka )"  n  o  2  |c  i s then a g e n e r a l i z a t i o n  n  i t i s c o n v e n i e n t t o use  s t r e n g t h , f ( K ) , w h i c h was  \e (K)  n  2 2 w h e r e Q = fi K /2m  f (K)  electron  .  n  o n  (K)|  (2.6.2)  2  of the o p t i c a l  oscillator  strength  defined  by f where  .  vibrational  i m p a c t has b e e n g i v e n by Bonham and G e i g e r  oscillator  f (K)  Q  excitation  Generalized O s c i l l a t o r Strengths. In  a  electron  are almost independent of s c a t t e r i n g  theoretical  by e l e c t r o n  derived from  v a l u e s e v e n when t h e  of v i b r a t i o n a l  s p e c i f i c example i s the i d e n t i f i c a t i o n  A comprehensive,  factors  B o r n a p p r o x i m a t i o n no l o n g e r a p p l i e s .  intensities  same e l e c t r o n i c  one  However, e x p e r i m e n t a l l y , i t  49 '  impact d a t a a r e i n agreement w i t h o p t i c a l energy  Born  n  -  (E /POM n  2 0  n  (2.6.3)  -18-  =  2  on  a ~ o  I L  2 M  *V  J "  2  4^1  i s the dipole matrix  cross-section (dipole  *  ,„  H T  !  (2-6.4)  2  e l e m e n t s q u a r e d and f  f o r the e x c i t a t i o n of the n ^  i s proportional  n  to the  s t a t e by p h o t o a b s o r p t i o n  approximation). -y  C o n s i d e r t h e c a s e when K i s d i r e c t e d a l o n g t h e z a x i s a n d l e t z be th -> z coordinate of the s a t o m i c e l e c t r o n , K«r = K z i n ( 2 . 5 . 6 ) . Equation g  the  s  (2.6.2)  $  becomes  f (K)  =  n  (E /R) ( K a ) " n  |  2  Q  e  l  K  Z  *o  s  N  d r  ^  ( 2  ' ' 6  5 )  ->  For in  small  K, t h e e x p o n e n t i a l  i n ( 2 . 6 . 5 ) may  be e x p a n d e d i n a p o w e r s e r i e s  K, e  i K z  s  * 1 + (TKz ) + | ( i K z ) s  + ... + ^ . ( 1 K z )  2  s  (2.6.6)  n  s  4  Assuming t h a t e  o n  a n d <j; a r e o r t h o g o n a l we o b t a i n  n  (K) *  £ ]  ,  Q  (iK)  e (iK)  +  2  2  +  E  (iK)  s  ...  +  3  (2.6.7)  and f (K)  =  n  (E /R)a " n  +  2  0  (e  2  2  -  2  E l  e ) 3  K  2  +  ...  +  0(K ) 4  (2.6.8) where  e  *  -  i: / v E  In e x p r e s s i n g  (2.6.7)  s *o  d T  N  f ( K ) as a f u n c t i o n p  been assumed t h a t e*e,  Z  s  ( - -9) 2  o f even powers o f K i n ( 2 . 6 . 8 )  t h e w a v e f u n c t i o n s i|> a n d ^  substituted  n  into  o  are real  ( 2 . 6 . 5 ) , are imaginary.  6  i t has  (odd powers o f K i n  -19-  For  very  s m a l l momentum t r a n s f e r s , as K ^ 0, t h e r i g h t s i d e o f  (2.6.8) i s d o m i n a t e d by t h e f i r s t f (K)  = (E /R) a  n  lim  -  n  =  2 e  ]  f  (2.6.10)  n  K + 0  where f  i s the optical  52 et a l .  h a v e shown t h a t  approximation.  oscillator  strength  (2.6.10) a p p l i e s r e g a r d l e s s  2  Lassettre  of the f i r s t  Born large  2 = 0 , may be s u b j e c t  m i n i m a may o c c u r i n t h e g e n e r a l i z e d of K  i n (2.6.3).  n  of K , to K  values  defined  However, e x t r a p o l a t i o n s o f f" (K) from r e l a t i v e l y  2 values  2  o  term and  as i l l u s t r a t e d  to considerable  oscillator  by t h e X  strength  -> B  error.  F o r example,  f u n c t i o n a t small  transition  o f carbon  monoxide^ . On t h e b a s i s o f ( 2 . 6 . 1 0 ) i t i s e a s y t o d i s t i n g u i s h b e t w e e n 3  dipole allowed  and f o r b i d d e n  transitions:  f  ( K ) -v f > 0 -> e l e c t r i c n n 1 im K -»• 0  f (K)  f  1im  0  n  K  dipole  - 0 -> e l e c t r i c d i p o l e  a s one w h i c h  forbidden  i t i s conventional  i s r i g o r o u s l y allowed  r u l e s even a t low e n e r g i e s approximation  allowed  r  In e l e c t r o n impact s p e c t r o s c o p y transition  to define  by e l e c t r i c  does n o t h o l d .  T r a n s i t i o n s f o r which  " q u a d r u p o l e moment", £r>, i s n o t i d e n t i c a l i n optical  an  allowed  dipole selection  and l a r g e s c a t t e r i n g a n g l e s where t h e f i r s t = 0 and  (2.6.8) a r e termed " e l e c t r i c q u a d r u p o l e " t r a n s i t i o n s .  which occurs  electric  spectroscopy  5  .  z  2  0in  However, t h e  to the e l e c t r i c  Expressing  Born  q u a d r u p o l e moment  2 2 2 a s r /3 + ( z - r / 3 ) ,  (=2 becomes  e  2  =  1  /  3  r  l  *o  d T  N  +  < s " s / > *o z  r  3  d T  N  (2.6.11)  -20-  In  optical  s p e c t r o s c o p y o n l y the second  t e r m on t h e RHS o f  (2.6.11) 5  contributes is  to the intensity of e l e c t r i c  no a n a l o g u e  to the f i r s t  term.  quadrupole t r a n s i t i o n s  ;  there  The L y m a n - B i r g e - H o p f i e l d b a n d s o f  m o l e c u l a r n i t r o g e n p r o v i d e an e x a m p l e w h e r e o n l y t h e s e c o n d t e r m o f ( 2 . 6 . 1 1 ) i s n o n z e r o w h i l e f o r t h e l ^ S -»- 2 S t r a n s i t i o n i n h e l i u m o n l y t h e 1  5 first  term  i s nonzero  54 .  Recently,  have b e e n d e r i v e d w h i c h a r e v a l i d The optical electron 1.  some g r o u p  forall  theoretical  impact  strength  (2.6.10)  s t r e n g t h and t h e  has i m p o r t a n t i m p l i c a t i o n s f o r  impact s p e c t r o s c o p y ; f o r small  momentum t r a n s f e r s , e l e c t r i c  dipole selection  a p p l y t o t h e e x c i t a t i o n o f a t o m s a n d m o l e c u l e s by e l e c t r o n 2.  rules  energies.  r e l a t i o n s h i p between t h e g e n e r a l i z e d o s c i l l a t o r oscillator  selection  optical  oscillator  s t r e n g t h s may be d e d u c e d  optical  oscillator  s t r e n g t h s may be u s e d  rules  impact  from e l e c t r o n  impact  d a t a and 3. electron  impact data.  Three optical 1. (recall  to normalize experimental  classes of electron  oscillator  i m p a c t e x p e r i m e n t s have b e e n u s e d  to derive  strengths:  F i x t h e i n c i d e n t e n e r g y , E , v a r y e and e x t r a p o l a t e f ( K ) t o K Q  that K  2  = k  2  + k  o  2  n  -> 0  n  - 2 k k cose from o n  (2.4.14).  been u s e d e x t e n s i v e l y by L a s s e t t r e a n d c o - w o r k e r s  T h i s method has  ( f o r examples  see  R e f e r e n c e s 53,55 a n d 5 6 ) . 2 2.  F i x the scattering  a n g l e e, v a r y E  Q  and e x t r a p o l a t e f" (K) t o K n  -> 0.  T h i s m e t h o d h a s b e e n u s e d by H e r t e l a n d R o s s ^ ' ^ . 3. Use h i g h i n c i d e n t e n e r g i e s (k ^ k ) a n d s m a l l s c a t t e r i n g a n g l e s 2 s u c h t h a t K = 0, t h e g e n e r a l i z e d o s c i l l a t o r s t r e n g t h i s e q u a l t o t h e o p t i c a l n  -21-  oscillator et a l .  strength.  This  m e t h o d h a s b e e n u s e d e x t e n s i v e l y by G e i g e r  ( s e e R e f e r e n c e s 3, 5 9 - 6 2 ) a n d v a n d e r W i e l  When t h e f i r s t strength  Born a p p r o x i m a t i o n  c a n be d i r e c t l y  (2.6.2) and  o  an e f f e c t i v e g e n e r a l i z e d  1 1  of the v a l i d i t y  of the f i r s t E.  When t h e f i r s t  Born a p p r o x i m a t i o n  Born e x p r e s s i o n f (K,E ) Q  first  f (K) , n  although  -f^(K,E ) Q  should  different  impact e n e r g i e s ,  invalid.  On t h i s  the  64  Therefore,  to apply.  ( l a r g e E ) we c a n u s e t h e ( 2 . 5 . 5 ) t o show, (2.6.13) strength  condition  1 1  defined  by ( 2 . 6 . 2 ) .  Q  E , the f i r s t Q  and carbon monoxide are high  53  oscillator incident  function of K at  Born a p p r o x i m a t i o n  i s clearly  and L a s s e t t r e have f o u n d t r a n s i t i o n s t n w h e r e d e v i a t i o n s a r e a p p a r e n t e v e n when  enough t o e x p e c t t h e f i r s t  Born  approximation  On t h e b a s i s o f a s u r v e y o f a number o f a t o m i c a n d m o l e c u l a r  transitions,  i t has been f o u n d  53  A  for the v a l i d i t y ofthe  i f f (K,E ) i s a different  b a s i s , Skerbele  incident energies  da  Q  oscillator  n  strength  measurements  have t h e same K d e p e n d e n c e a t d i f f e r e n t  E .  nitrogen  E  i s that the effective generalized  energies,  Q  o  cross-section  not s u f f i c i e n t ,  Born a p p r o x i m a t i o n  strength  Using  approximation.  /i  i s valid  large E  i s the generalized  p  necessary,  +  Born  A  f o r the d i f f e r e n t i a l  oscillator  from experimental  Q  where f ( K )  cross-section.  da  f ^ ( K , E ) w h i c h c a n be c a l c u l a t e d e n t i r e l y  n  oscillator  (2.5.5),  i s convenient t o i n t r o d u c e  regardless  the generalized  related to the differential  Jl  It  i svalid,  (seeReference 63).  that deviations  from t h e f i r s t  Born  -22-  a p p r o x i m a t i o n a r e o b s e r v e d when t h e t e r m s y m b o l s  of the i n i t i a l  states  a r e dependent  are i d e n t i c a l .  operator which  is totally  Experimentally, ential  the  first  o n  dQ  Born a p p r o x i m a t i o n a p p l i e s  =  4a 4 a  2  V  1 -  o  A t e = 0° s u c h t h a t  energy  l o s s measurement, the  f o r the t r a n s i t i o n i s measured a t a f i x e d  and s c a t t e r i n g a n g l e 9.  Q  on  final  an  symmetric.  i n an e l e c t r o n  cross-section  energy, E ,  T h e r e f o r e , the d e v i a t i o n s  and  2  Using  (2.6.12)  such t h a t  (Ka r Q  2  Ka„ i s a minimum and o  incident  and a s s u m i n g  that  f^(K,E ) = f ( K ) , Q  (R/E ) f ( K ) n  E  differ-  n  n  (2.6.14)  << E , i t has b e e n shown n o  11  that, da  on  dQ  where f  1  p  6  a  o  2  r 2  E  o  E  n"  3  f  (2.6.15)  n  i s the o p t i c a l o s c i l l a t o r strength  energy o r e q u i v a l e n t l y  the e l e c t r o n  energy  and loss.  E  n  i s the e x c i t a t i o n  -23-  CHAPTER  EXPERIMENTAL METHODS FOR  For m o l e c u l e s composed o f (or core) e l e c t r o n s mainly atomic  are  the  INNER-SHELL E X C I T A T I O N STUDIES  s e c o n d row  Is  in character.  THREE  elements, the  (K) e l e c t r o n s  Transitions  inner  which are  shell  nonbonding  i n v o l v i n g the  and  discrete excitat-  ion of a K - s h e l l e l e c t r o n occur i n the  approximate energy  regions;  200  f o r c a r b o n , 400  (31  eV  (62 A )  f o r b o r o n , 300  eV  (41  A)  o  nitrogen, The  2s  550  eV  ( L j ) and  (22.5  A)  f o r o x y g e n and electrons  c o r e e l e c t r o n s when t h e s e e l e m e n t s a r e environment.  For  690  o  s u l f u r L j and  for sulfur L^  eV  A bibliography i n Chapter I.  (77.5 of  A)  inner shell  Information  four types of experiments;  X-ray emission  certain  D i s c r e t e e x c i t a t i o n by energy e x a c t l y equal  required  to the  of producing  f o r s u l f u r K,  for  elements are in a  220  also  molecular  shell  excitation studies  on  for fluorine.  excitations o  eV  (56 A)  for  JJJ. has  previously  d i s c r e t e e x c i t a t i o n s has  photoabsorption,  s p e c t r o s c o p y and  t h e s e t e c h n i q u e s has  difficulty  row  sulfur-containing molecules, inner  o  by  (18 A)  incorporated  ( 5 A)  given  eV  of t h i r d  r e q u i r e a p p r o x i m a t e l y 2475 eV 160  A)  o  ( L j j JJJ)  2p  eV  been  Auger e l e c t r o n  been  provided  spectroscopy,  e l e c t r o n energy l o s s spectroscopy.  Each  of  limitations. photoabsorption requires energy r e q u i r e d  a useful  f o r the  transition.  photon continuum i n the  f o r K - s h e l l e x c i t a t i o n s ( s o f t X - r a y ) has  B r e m s s t r a h l u n g c o n t i n u u a a r e weak  a photon with  (particularly  energy  previously  b e l o w 1000  an The  region  been m e n t i o n e d ;  eV)  and  electron  -24-  synchrotron  facilities  electron synchrotron higher  are not r e a d i l y  available.  produces a l a r g e i n t e n s i t y o f photons having  than t h a t r e q u i r e d f o r K - s h e l l e x c i t a t i o n  the spectrograph  I n a d d i t i o n , an  seems t o be a p r o b l e m . 27  and o r d e r  energies  overlapping i n  In the case o f the K - s h e l l  absorption  spectrum o f n i t r o g e n  absorption  b e l o w 20 A) had t o be i n c l u d e d i n o r d e r  The d e s i g n  a n d c o n s t r u c t i o n o f monochroma.tors f o r t h e s o f t X - r a y r e g i o n i s  o  also d i f f i c u l t .  Surface  wavelengths and g r a z i n g resolution  has s t r o n g  to suppress t h i s  are extremely  poor a t such  i n c i d e n c e m o n o c h r o m a t o r s must be u s e d .  effect.  short  Also,  since  difficult  energy r e s o l u t i o n i n t h e s h o r t wavelength r e g i o n o f t h e  energy spectrum.  Since  i s a practical  s o f t X-ray  reflectivities  (which  i s on a w a v e l e n g t h s c a l e , i t becomes p r o g r e s s i v e l y m o r e  to obtain high  there  , an e x c e s s o f o x y g e n  r e s o l u t i o n i s gained  a t t h e expense o f i n t e n s i t y ,  l i m i t t o t h e r e s o l u t i o n w h i c h c a n be o b t a i n e d  i n the  region.  In o r d e r  t o d i s c u s s t h e a p p l i c a t i o n o f Auger e l e c t r o n  to inner shell  e x c i t a t i o n studies i t i s convenient  ion to the technique.  to give a brief introduct-  T h e e j e c t i o n o f an i n n e r s h e l l  a b s o r p t i o n , e l e c t r o n impact o r o t h e r methods, r e s u l t s a highly unstable  species.  spectroscopy  e l e c t r o n by X - r a y i n the production  The d o m i n a n t r e l a x a t i o n p r o c e s s ,  of  f o r molecules  65 c o m p o s e d o f s e c o n d row e l e m e n t s , i s by A u g e r e l e c t r o n e j e c t i o n process  may  be r e p r e s e n t e d  i.  X  n-—> o r hv  X  n.  X  (E  +  )  where ( i ) r e p r e s e n t s represents  The  as f o l l o w s ; +  X  .  e  (E  the i n i t i a l  Initial  )  +  Ionization  e ( E ^  i o n i z a t i o n o f a K - s h e l l e l e c t r o n and ( i i )  the Auger r e l a x a t i o n process  i n which the inner s h e l l  "hole" i s  -25-  filled  by  a valence shell  a p p e a r s as  electron  k i n e t i c energy  -  E ]  E  i s given -  K +  E  the  energy l i b e r a t e d i n the  (E-|) o f a s e c o n d v a l e n c e s h e l l  Auger e l e c t r o n ) which i s e j e c t e d Auger e l e c t r o n  and  i n the  process.  energy of the  energy of  a doubly i o n i z e d s t a t e of the molecule I f the  initial  initial  iii.  (E *)  where X the  +  X * K  represents  valence s h e l l s .  equal  to the  s t a t e and E  Referring  K  -  to processes  ion  ( i i ) and  the  doubly charged  electron. by  the  (E )  +  be  higher  +  +  e  (both  state  E  is  the  vacancies i n  the  in ( i i ) is neutral, singly  2  i n energy between the  the  v a c a n c y i n one  ejected initial  electron, E ,  is  2  K-shell  of  excited  state;  +  ( i i ) and  (iii),  ion states  identified  the  energies of  ( i i i ) usually differ i n ( i i ) are  s i n g l y charged states since  the  by  typically  in ( i i i ) .  initial  l e s s t h a n 10 20  - 30  eV  be  K-shell  in conjunction  t a k e n up  eV  while  higher  ejected by  ion states with  K-shell  in  autoionization  k i n e t i c energies of the  I t is also possible for "excited" valence electrons  the  Therefore,  t h a n t h e maximum e n e r g y w h i c h c a n  shake-up of  the  (E )  k i n e t i c energy of the  discrete states  energy than the  rons are  The  E  energy of  i s a u t o i o n i z a t i o n which produces a  s i n g l y charged ion  E *  p r o c e s s e s can  X  "hole"  i o n s t a t e and  a s i n g l y charged ion s t a t e with  difference  the =  2  -  K  K-shell  K-shell  then the main r e l a x a t i o n process state:  kinetic  (the  + +  i s the  charged f i n a l  electron  by;  where E  valence s h e l l ) .  The  process  K-shell  an  elect-  Auger  (produced ionization)  -26-  to give r i s e that  t o h i g h energy Auger peaks.  the i n i t i a l  by c o m p a r i n g  state  i s neutral  H o w e v e r , i t c a n be  a n d n o t an " e x c i t e d "  t h e r e s u l t s o f e x c i t a t i o n by e l e c t r o n  established  K-shell  ion state  i m p a c t and X - r a y  33- 36 absorption  "  .  The d i s c r e t e s t a t e s  are not excited  by an X - r a y  line  w i t h e n e r g y f a r i n e x c e s s o f t h e t r a n s i t i o n e n e r g i e s and t h e c o r r e s p o n d i n g autoionization  l i n e s a r e absent from t h e Auger spectrum.  e n e r g y o f an a u t o i o n i z a t i o n  line  the  state  initial  neutral  charged s p e c i e s .  excited  However, t h e  i s equal t o t h e energy d i f f e r e n c e a n d some f i n a l  state of the singly  T h e r e f o r e , an a m b i g u i t y i n p e a k a s s i g n m e n t may  u n l e s s one o f t h e s t a t e s  involved  between  arise  i n t h e p r o c e s s c a n be p o s i t i v e l y i d e n t -  ified. A c o m p e t i t i v e r e l a x a t i o n p r o c e s s f o r a m o l e c u l e w i t h an i n n e r vacancy, i s X-ray e m i s s i o n , i n which t h e " h o l e " shell  electron  escence). photon  is filled  by a v a l e n c e  and t h e l i b e r a t e d energy appears as a photon  I f the i n i t i a l  state  (X-ray f l u o r -  i s n e u t r a l , then t h e energy o f t h e e m i t t e d  i s equal t o t h e e x c i t a t i o n energy o f t h e d i s c r e t e s t a t e op on c c  emission).  shell  R e c e n t l y , Siegbahn e t a l .  '  '  (resonance  , have c o n s t r u c t e d a h i g h  r e s o l u t i o n X - r a y e m i s s i o n s p e c t r o m e t e r ( A E (FWHM) - 0.1 e V ) w h i c h i s capable o f r e s o l v i n g The  some o f t h e v i b r a t i o n a l s t r u c t u r e  resonance e m i s s i o n from t h e lowest K - s h e l l 38 39  nitrogen  has been c l e a r l y o b s e r v e d  higher energy  K-shell  excited  states  '  .  excited  o f e m i s s i o n bands. state of molecular  However, e m i s s i o n bands f r o m t h e  ( o b s e r v e d i n R e f e r e n c e 27) have n o t  been r e p o r t e d . In a d d i t i o n , h i g h energy s a t e l l i t e l i n e s i n t h e K - s h e l l X-ray e m i s s i o n s p e c t r u m o f c a r b o n monoxide have v e r y low i n t e n s i t i e s and 66 e n e r g i e s and a s s i g n m e n t s o f t h e s e l i n e s have n o t been g i v e n for  .  However,  l o w a t o m i c n u m b e r s , e m i s s i o n i n t e n s i t i e s a r e e x p e c t e d t o be s m a l l  since  -27-  the  competition  by  the  b e t w e e n A u g e r e m i s s i o n and  nonradiative  process  c a s e o f a u t o i o n i z a t i o n , an the  final  state involved  electronic the  s t a t e o r any  (see  X-ray fluorescence  Reference 65).  I n a d d i t i o n , as  a m b i g u i t y i n p e a k a s s i g n m e n t may  i n the o f the  i s dominated in  the  arise,  e m i s s i o n p r o c e s s may  e i t h e r be  n e u t r a l , valence shell  excited  since  the  ground  states  of  molecule. The  only  condition  impact i s t h a t the than the  collision  encountered lifetimes natural states  energy.  the  region,  e x c i t a t i o n of a neutral  k i n e t i c energy of the  transition  i s d e t e r m i n e d by  f o r the  S i n c e the  p r o b l e m s o f an  k i n e t i c energy of the between the  energy s t a t e s  l i n e widths are i n the  regions  large.  of the  The  not  electron  are  relatively  uncertainty  respective  exist.  greater  incident  and  and  the  therefore  broadening of  carbon, nitrogen  and  excitations  In a d d i t i o n , short  electron  source  energy source f o r K - s h e l l  i n p h o t o a b s o r p t i o n s t u d i e s , do  of these high  electron  i n c i d e n t e l e c t r o n m u s t be  potential difference  the  s t a t e by  the  the  excited  o x y g e n K-edges  39 is  probably  resolution  a r o u n d 0.1  eV.  This  estimate i s supported  of v i b r a t i o n a l structure  (0.26  eV  by  the  s p a c i n g s ) i n the  clear  high  energy  32 autoionization obtainable  the  natural  electron  information with  In p r i n c i p l e t h e  use  l i n e widths  (see  about these s t a t e s  as  optical  of a s u i t a b l e r e t a r d i n g and  the  e n t i r e energy l o s s spectrum. i n the  keV  r a n g e , and  i s an  resolution  order of  magnitude  f o r e x a m p l e R e f e r e n c e 3)  i m p a c t s p e c t r o s c o p y can  at constant energy, E,  energies  .  i n e l e c t r o n energy l o s s spectroscopy  lower than the therefore,  band o f c a r b o n m o n o x i d e  p o t e n t i a l l y provide  absorption  lens, electrons  resolution  and as  much  spectroscopy. can  be  energy  (AE/E) i s constant over  Also, analysed the  T h e r e f o r e , even f o r i n c i d e n t e l e c t r o n s energy l o s s e s  i n the  300  - 700  eV  with  energy  -28-  region,  "high"  electrons  resolution  and e n e r g y a n a l y s i n g  shows t h e p a r a m e t e r s and e n e r g y b a s i s and e l e c t r o n corresponding a function following  c a n b e o b t a i n e d by p r e r e t a r d i n g  relevant  at a sufficiently  the scattered  low energy.  t o a comparison o f r e s o l u t i o n  and i s h e l p f u l  i n comparing  impact spectroscopy.  Figure  2  on a w a v e l e n g t h  and c o n t r a s t i n g  photoabsorption  I n F i g u r e 2 t h e r e s o l u t i o n , AA ( A ) ,  t o f i x e d v a l u e s o f r e s o l u t i o n , A E ( e V ) , has been p l o t t e d  o f energy. examples;  as  To i l l u s t r a t e t h e u s e o f F i g u r e 2, c o n s i d e r t h e f o r carbon K-shell  e x c i t a t i o n , ^ 3 0 0 eV (41 A ) , a  r e s o l u t i o n , A E , o f 0.5 eV c o r r e s p o n d s t o a r e s o l u t i o n  AA o f ^ 0.1 A , w h i l e  o  for  f l u o r i n e K-shell  e x c i t a t i o n , ^ 700 eV ( 1 8 A ) , a r e s o l u t i o n  o f 0.5 eV  o  c o r r e s p o n d s t o a r e s o l u t i o n , A A , o f 0.02 A. spectrum o f nitrogen  27  was o b t a i n e d w i t h  The K - s h e l l p h o t o a b s o r p t i o n  a resolution  ° < 0.03 A w h i c h  corres-  o  ponds t o a r e s o l u t i o n impact, a r e s o l u t i o n  < 0.4 eV a t 4 0 0 eV (31 A ) .  o f ^ 0.1 eV c a n be o b t a i n e d w i t h  chromation" o f the incident difficulty  i n obtaining  suggests that  In the case o f e l e c t r o n  beam.  an energy  t h e r e a r e advantages  p h o t o a b s o r p t i o n methods  This,  o n l y modest  and t h e f a c t t h a t  source i n electron  impact  t o t h e use o f e l e c t r o n  i n t h e s o f t X-ray and X-ray  there  i s no  spectroscopy,  impact  regions.  "mono-  over  o  FIGURE 2.  Resolution, resolution,  AX AE  ( A ) , p l o t t e d a g a i n s t energy f o r f i x e d values (0.01 t o 0.05 eV).  of  -30-  CHAPTER  FOUR  EXPERIMENTAL  4.1.  180° E l e c t r o s t a t i c One  Analyser.  o f t h e most i m p o r t a n t  c o m p o n e n t s o f an e l e c t r o n s p e c t r o m e t e r i s  t h e e l e c t r o n e n e r g y , o r momentum a n a l y s e r .  The k i n e t i c  s c a t t e r e d e l e c t r o n s a r e u s u a l l y m e a s u r e d by d e f l e c t i n g an e l e c t r i c and  o r magnetic f i e l d .  number o f r e v i e w  articles.  magnetic d e f l e c t i o n  because uniform  t h e two d i m e n s i o n a l  The discussed  1/r  fields.  a r e more  over  difficult  In a d d i t i o n , problems  i n t h e case o f a magnetic  analyser, electro-  suited f o rcoupling to strongly decelerating  symmetry.  properties o f hemispherical by P u r c e l l  A schematic  magnetic f i e l d s  electrostatic  work  d e f l e c t i o n was c h o s e n  focusing properties o f the hemispherical  analyser are i d e a l l y  lenses of a x i a l  a n a l y s e r was s e l e c t e d f o r t h i s  (i) electrostatic  a r e more s e v e r e  ina  CQ  ' '  electrostatic  t o produce and c o n t r o l than  static  The p r o p e r t i e s a n d r e l a t i v e m e r i t s o f  o q . c~j  for the f o l l o w i n g reasons;  (ii)  of electric  o f e l e c t r o n e n e r g y a n a l y s e r s h a v e been d i s c u s s e d  A hemispherical  of fringe f i e l d s  the electrons i n  A l t e r n a t i v e l y , a combination  m a g n e t i c f i e l d s may be u s e d .  d i f f e r e n t types  energies of  70  , Simpson  71  e l e c t r o s t a t i c a n a l y s e r s have been  , Simpson and K u y a t t  d i a g r a m i s shown i n F i g u r e 3.  electrostatic  field  E l e c t r o n s a r e d e f l e c t e d by t h e  p r o d u c e d by t h e p o t e n t i a l  b e t w e e n t h e two h e m i s p h e r i c a l  surfaces with  72 73 68 ' and S a r - E l  radii  d i f f e r e n c e , V-^' > r - j . The m a g n i t u d e  -31-  FIGURE 3.  Schematic diagram of a hemispherical  e l e c t r o n energy  analyser.  -32-  of t h ee l e c t r o s t a t i c  field  i s g i v e n by  S(r) where C i s a c o n s t a n t . E  Q  =  angle  =  Consider  C/r  (4.1.1)  2  an e l e c t r o n w i t h  kinetic  mu /2 w h i c h e n t e r s t h e a n a l y s e r a t t h e p o i n t x, o  a = 0°.  radius  force, e , at r -  2  Q  0 and a t an  In order f o r t h ee l e c t r o n t o f o l l o w a c i r c u l a r path  r , the centrifugal  mu /r  =  energy  Q  f o r c e must be e q u a l  with  to the electrostatic  ;  e£(r )  =  Q  eC/r  (4.1.2)  2  2 Since the k i n e t i c  e n e r g y , mu /2 i s e q u a l  to eV  Q  Q  i nelectron volts;  from  (4.1.2), C where r  =  Q  2r V 0  Q  =  (r, +  r )V 2  (4.1.3)  Q  i s chosen as t h e m i d p o i n t . V(r)  =  £  The p o t e n t i a l +  a t point r i s given by;  B  (4.1.4)  where B i s a c o n s t a n t .  The p o t e n t i a l s o f t h e i n n e r and o u t e r  with  Q  respect to point r  a r e then  V(r^  -  V(r )=  C/r, -  C/r  Q  (4.1.5)  V(r )  -  V(r )=  C/r  C/r  2  (4.1.6)  Q  and  hemispheres  Q  2  therefore thepotential V(r ) 1  -  V ( r )= 2  = L e t x , be t h e r a d i a l  Q  -  d i f f e r e n c e a c r o s s t h e h e m i s p h e r e s i s g i v e n by  C/r, - C / r  V  Q  [(r /r ) z  }  2  (^/r^J  -  d i s t a n c e from r  Q  (4.1.7)  o f an e l e c t r o n e n t e r i n g t h e 73  analyser a t angle x , 2  a a n d e n e r g y E.  I t h a s b e e n shown  o f t h e t r a n s m i t t e d e l e c t r o n from t h e r a d i a l  that the deviation,  path, r , i s given by;  -33-  (x /r ) 2  =  Q  -  (x-,/r )  w h e r e AE = E - E . has  first  360°)^ .  Since  angular  2(AE/E ) o  there  -  i s no l i n e a r  focusing  2a  (4.1.8)  2  term i n a, t h e a n a l y s e r  (theangular  focusing i s perfect at  In a d d i t i o n , because o f t h e s p h e r i c a l symmetry, t h e a n a l y s e r  4  has  order  +  o  two d i m e n s i o n a l The  focusing properties.  e n e r g y r e s o l t u i o n ( i . e . t h e t r a n s m i s s i o n o f e l e c t r o n s as a  f u n c t i o n o f energy t a k i n g i n t o account the d i s t r i b u t i o n electrons over  space and a n g l e  c a n be a p p r o x i m a t e d by  AE(FWHM)/E w h e r e AE(FWHM) i s t h e f u l l and  S i s the s l i t  width  width  The  2  ratio  f o ra c i r c u l a r aperture.  research  =  1.5  inches  r  =  2.0  inches  r^  =  2.5  inches  S  =  0.050  inches  of the potential  dividing  2  has t h e f o l l o w i n g  r-| Q  (4.1.9)  Q  and i s o n l y a p p l i c a b l e i f a  a n a l y s e r used f o r t h i s  The  S/2r  73  a t h a l f maximum o f t h e t r a n s m i t t e d beam  or diameter  (4.1.9) n e g l e c t s a term i n a  =  of incident  resistors  Expression  <<  S/2r . Q  dimensions;  R /R-|) i s g i v e n 2  by t h e  r a t i o o f t h e p o t e n t i a l s V^/V-j a n d f o r t h e d i m e n s i o n s o f o u r a n a l y s e r (4.1.5) and R /R 2  The  (4.1.6): =  1  using  V /V 2  theoretical  =  1  3/5  (4.1.10)  resolution (neglecting the a  t e r m ) i s g i v e n by  (4.1.9), AE/E In  Q  =  order  S/2r  Q  =  0.0125  =  1.25%  (4.1.11)  t o t e s t the performance o f t h e a n a l y s e r , t h e r e s o l u t i o n ,  -34-  A E ( F W H M ) , as  (i.e.  a function of electron  energy,  E  Q  = eV )  was  Q  measured a t 0° s c a t t e r i n g w i t h h e l i u m  as a t a r g e t g a s .  shown i n F i g u r e 4.  i s a c o n v o l u t i o n of the energy  o f t h e gun  and  The  energy  to E  curve  r e p r e s e n t s the energy  Q  results  t r a n s m i s s i o n f u n c t i o n of the a n a l y s e r .  ating  obtained  = 0,  AE(obs) curve  The  t h e gun  s p r e a d was  e s t i m a t e d as ^  transmission width  by c o r r e c t e d A E ( o b s ) f o r t h e gun  0.28  By  eV.  The  (the energy  spread  extrapollower  o f t h e a n a l y s e r and  spread  are  was  distributions  73  w e r e assumed t o be  approximately  Gaussian  ).  The  experimental  resolution  o f t h e a n a l y s e r i s 1 . 2 3 % , i n good agreement w i t h t h e t h e o r e t i c a l of 1.25%  (see 4.1.11).  by p l o t t i n g electron V-|2  using  4.2.  V  eV .  c h e c k o f t h e a n a l y s e r was  experimental  the dimensions  The  E l e c t r o n Source.  The  electron  source  consisting  and  f o c u s i n g element  power s u p p l y  o f an  p l o t was  of our  (Einzel  heated  lens).  i s shown i n F i g u r e 5.  w i t h o u t u s i n g an e n e r g y  was  A circuit  cathode  a t much l o w e r t e m p e r a t u r e s 6 7  '  7 2  ,  AE(FWHM) = 2 . 5 4  ( 1 / 1 1 6 0 0 eV/°K).  This  value  calculated  The  6AW59  (BaSrO),  diagram  This  eV  , can the  means t h a t t h e y c a n  kT w h e r e k i s t h e B o l t z m a n n eV  For a  be  thermionic  constant  for a  gun  cathode  i s a r e s u l t of  than o t h e r e m i t t e r s .  i m p l i e s a AE(FWHM) o f ^ 0.25  anode  o f the e l e c t r o n  beam, A E ( F W H M ) ^ 0 . 2 5  selector.  television  grid,  a d v a n t a g e o f an o x i d e  low work f u n c t i o n o f t h e m i x e d o x i d e c a t h o d e s , w h i c h operated  line,  a Philips  oxide cathode  i s t h a t a reasonably monoenergetic  produced  a straight  of  analyser.  f o r the spectrometer  indirectly  performed  o f t h e h e m i s p h e r e s as a f u n c t i o n  , i n e x a c t agreement w i t h the t h e o r e t i c a l  gun  be  The  Q  ( 4 . 1 . 7 ) and  source  additional  t h e f o c u s p o t e n t i a l , V-^s  energy  = 1.07  An  estimate  normal  FIGURE  4.  R e s o l u t i o n , AE ( F W H M ) , V S . e l e c t r o n e n e r g y f o r t h e 180° e l e c t r o n e n e r g y a n a l y s e r : • o b s e r v e d ( c o n v o l u t i o n o f gun and a n a l y s e r s p r e a d s ) , • a n a l y s e r o n l y (gun spread subtracted).  RE 5.  E l e c t r o n gun power s u p p l y ; 1. f i l a m e n t , 2. c a t h o d e , R e s i s t o r s a r e i n Kft a n d c a p a c i t o r s i n y F .  3. a r i d ,  4. anode and 5.  focus.  -37-  operating obtained  temperature of approximately  In a d d i t i o n , t h e  television  However, a m a j o r d i s a d v a n t a g e  and  oxide.  In t h e  present  o v e r c o m e by o p e r a t i n g  t h e gun  to reduce i t s u s e f u l  lifetime.  build a differentially  4.3.  The  The  pumped  gun  experimental  by  cathode i s  (see  a higher  beam.  readily  s t r o n g o x i d i z e r s s u c h as  experiment, t h i s  with  eV  value  produces a w e l l focused  i s t h a t the oxide  by m o s t g a s e s , p a r t i c u l a r l y  nitric  °K.  e x t r a p o l a t i o n o f t h e o b s e r v e d A E ( F W H M ) i s % 0.28  by  Figure 4).  poisoned  1100  oxygen  d i f f i c u l t y was  partially  f i l a m e n t voltage which  A more s a t i s f a c t o r y  tended  s o l u t i o n w o u l d be  to  source.  Spectrometer.  P l a t e 1 i s a photograph of the schematic diagram. c a t h o d e ) and  The  spectrometer  main components a r e :  Einzel lens;  B and  A,  and  Figure  the  e l e c t r o n gun  F, q u a d r u p o l e e l e c t r i c  6 shows a (oxide  deflection plates; -4  c, gas  collision inlet;  spherical  chamber o p e r a t e d E, a n g u l a r  analyser;  selection plate;  and  4.3.1. S p e c t r o m e t e r The  spectrometer  the angular  at t y p i c a l  I , channel  and  apertures  o f 0.050".  tube c y l i n d r i c a l gap  o f 0.16  D and  o f 10  decelerating lens;  torr; H,  D,  hemi-  Construction.  was  constructed  s e l e c t i o n p l a t e , E, and  the angular  G,  pressures  electron multiplier.  from brass  the aperture  w h i c h were m a c h i n e d f r o m molybdenum. 0.6"  gas  The  s e l e c t i o n p l a t e and The  lens with  l e n s , G,  i s an  a high voltage  with  the  exception  p l a t e s of the  of  analyser,  d e f l e c t i o n p l a t e s w e r e 0.4" analyser equal  "slit"  diameter  element  plates  a l o w v o l t a g e e l e m e n t ( l e n g t h 0.84  D),  have  (D = 1.9")  ( l e n g t h 1.21  x  D),  (the lens  two a para-  PLATE 1  The  Spectrometer.  X-Y PLOT  MCA  VAR VOLTS  RAMP  DISC  RATE METER  2500 VOLTS  FIGURE 6.  Schematic diagram o f the apparatus.  REC  -40-  meters a r e e s s e n t i a l l y ation  those  u s e d by v a n d e r W i e l  spacers.  However, t h e s e  b r i t t l e , w h i c h was an i n c o n v e n i e n c e cleaning.  sapphire  additional  Initially  balls  potential.  (located i n undersized  advantage i s t h a t t h e b a l l s  a l l brass  H o w e v e r , i t was f o u n d  slot  (0.13"  slit) The  t o be v e r y  the spectrometer  h o l e s ) as  a l s o serve  of this  step.  i n t h e back hemisphere  p e r f o r m a n c e o f t h e a n a l y s e r was n o t i m p a i r e d  4.3.2. S p e c t r o m e t e r  l a y e r o f benzene  (behind  monitered  the entrance  emitted electrons.  by t h i s m o d i f i c a t i o n .  Operation.  difference.  were used t o c o n t r o l  The q u a d r u p o l e  r e g i o n by a  d e f l e c t i o n p l a t e s B and F  t h e beam d i r e c t i o n a n d t h e e l e c t r o n c u r r e n t was  by d e f l e c t i n g  t h e beam o n t o t h e a n g u l a r  selection  plate,  w h i c h was f l o a t e d , t h r o u g h  a p r e c i s i o n e l e c t r o m e t e r , by t h e h i g h  power s u p p l y .  energy  to  For K-shell  angle  because o f t h e l a r g e  (even w i t h t h e s l o t ) o f s c a t t e r e d and s e c o n d a r y primary  E,  voltage  l o s s m e a s u r e m e n t s , i t was n o t p o s s i b l e  o b t a i n s p e c t r a a t a 0° s c a t t e r i n g  by t h e f a s t  soot  In a d d i t i o n , a  e l e c t r o n beam was a c c e l e r a t e d t o w a r d s t h e c o l l i s i o n  2.5 kV p o t e n t i a l  surface  The s u r f a c e s o f t h e h e m i -  t o r e d u c e t h e number o f b a c k - s c a t t e r e d a n d s e c o n d a r y  The  locaters.  t h a t the performance o f the spectrometer  t h e number o f s u r f a c e s c a t t e r e d e l e c t r o n s . x 2.8") was m i l l e d  using  insulators.  as a c c u r a t e  and a p e r t u r e p l a t e s were c o a t e d w i t h a u n i f o r m  minimize  initially  s u r f a c e s were g o l d p l a t e d t o p r o v i d e a u n i f o r m  was n o t d e g r a d e d by t h e o m i s s i o n spheres  proved  insul-  when t h e m a c h i n e was d i s m a n t l e d f o r  T h i s p r o b l e m was o v e r c o m e by r e b u i l d i n g  precision  to  Electrical  b e t w e e n c o m p o n e n t s o p e r a t i n g a t d i f f e r e n t p o t e n t i a l s was  p r o v i d e d by b o r o n n i t r i d e  An  ).  e l e c t r o n beam c o l l i d i n g  intensity  emitted e l e c t r o n s produced  w i t h t h e back h e m i s p h e r e .  The  -41-  primary  beam was t h e r e f o r e d e f l e c t e d by t h e p l a t e s , B, s u c h t h a t i t was  i n t e r c e p t e d by t h e a n g u l a r  s e l e c t i o n p l a t e , E.  E l e c t r o n s having  an  _2 average s c a t t e r i n g angle  o f 2 x 10  selection  into  plate aperture  were o b t a i n e d ating  lens  floated  by s c a n n i n g  radians  passed through t h e angular  the decelerating lens.  the electric  energy l o s s ;  voltage  (corresponding  e.g. f o r n i t r o g e n  energy s c a l e o b t a i n e d  t h e c a t h o d e p o t e n t i a l was  t o the approximate K-shell  K - s h e l l , -425 V ) .  was ± 0.02 eV.  spectra  potential applied to the deceler-  ( u s u a l l y f r o m g r o u n d t o +40 V) w h i l e  a t a negative  Energy l o s s  The a c c u r a c y  of the  The t r a n s m i s s i o n e n e r g y o f t h e  a n a l y s e r was s e t a t 25 eV a n d t h e r e s o l u t i o n [ A E ( F W H M ) ] was ^ 0 . 5 eV. Output pulses ics  from t h e m u l t i p l i e r were p r o c e s s e d  (see Figure  by s t a n d a r d  pulse electron-  6) and s t o r e d i n a m u l t i s c a l e r , whose channel  synchronous w i t h  the scanning  a d v a n c e was  voltage applied to the decelerating lens.  The  z e r o o f t h e e n e r g y s c a l e f o r e a c h s p e c t r u m was d e t e r m i n e d by r e c o r d i n g  the  peak f r o m e l a s t i c a l l y  using  scattered electrons  (measured as a d.c. c u r r e n t  t h e e l e c t r o n m u l t i p l i e r as a F a r a d a y cup) and t h e K - s h e l l  (pulse counting)  under i d e n t i c a l  deflection  and t a r g e t gas p r e s s u r e ) .  to record  angle  i n the pulse  shell  energy  pulse  counting  mode c o u l d be u s e d .  The e l a s t i c  In order  (beam  intensity,  p e a k was t o o i n t e n s e  t o measure t h e v a l e n c e  using  primary  K - s h e l l energy l o s s s p e c t r a , were  beams i n t e n s i t i e s  o f 0.1 yA t o 1 y A .  some s t r u c t u r e was u s u a l l y a p p a r e n t a f t e r a s i n g l e s c a n ,  usually necessary in order  mode.  conditions  l o s s s p e c t r u m , t h e beam i n t e n s i t y was r e d u c e d s u c h t h a t t h e  usually obtained Although  counting  experimental  spectrum  to signal  a v e r a g e f o r some h o u r s  t o o b t a i n a spectrum w i t h  weaker i n t e n s i t y s t r u c t u r e s .  a good s i g n a l  (typically  to noise  ratio  i t was  overnight) f o r the  -42-  4.3.3. E n e r g y The as  e n e r g y s c a l e was  described  digital Since  Calibration.  in Section  voltmeter  with  usually fixed with (4.3.2).  an  accuracy  a c a l i b r a t e d voltage  continuously  source  a v a i l a b l e , the  i n the  This  0.05  C  eV,  K  periodically  provided  three  = 2 8 7 . 2 8 ± 0.05  respect  molecular  to the  - 700  and  0  K  the  i n t e n s e 4 0 0 . 9 3 eV  first  of carbon d i o x i d e are accurate  1000  343  energy standards = 534.0 ± 0.1  volt  eV)  not  K-shell  (both  carbon  power  w h i c h were Absolute  recording  the  energies  nitrogen  (see  K-shell  pressures  t h e ammonia p e a k s  peak o f n i t r o g e n  range.  (N^ = 400.93 ±  voltmeter.  calibrating  a  A calibration  containing different partial  n i t r o g e n and  Figure  of  with  7).  The  similarly calibrated  d i s c r e t e peak o b s e r v e d  i n the  K - s h e l l energy l o s s  ( 2 9 0 . 7 ± 0.2  Figure  7).  t o ± 0.2  eV  eV),  peak  v o l t r e g i o n was  c a r b o n K - s h e l l e n e r g y l o s s s p e c t r u m o f m e t h a n e was against  the  carbon monoxide  a Fluke  ammonia s p e c t r u m by  spectrum of several mixtures m e t h a n e and  200  used to c a l i b r a t e the d i g i t a l  were d e t e r m i n e d f o r the  v o l t on  n i t r o g e n and  internal eV  elastic  energy d i s c r e t e peaks i n the  oxygen K - s h e l l s ) were measured w i t h  supply.  to the  v o l t a g e s were measured u s i n g  o f ± 0.1  lowest  energy l o s s s p e c t r a of molecular and  The  respect  (see  The  f o r a l l K-shell spectra unless  absolute otherwise  spectrum  energies stated.  4.3.4. Vacuum S y s t e m . The  complete experimental  c h a m b e r c o n s i s t s o f a 16" with a l / 2 " - t h i c k wall. located the  a r r a n g e m e n t i s shown i n P l a t e 2.  o u t s i d e d i a m e t e r aluminum t u b e , The  bottom o f the  t u b e r e s t s on  i n a machined groove i n the b a s e p l a t e  chamber i s s i m i l a r l y  closed with  an  16"  The  vacuum  in  height  a viton  0-ring  (see P l a t e 1).  The  top  of  a l u m i n u m l i d ( c o n t a i n i n g an a i r  CO NH  •.v  CH,  <  4l  i  2.7 —f—  400.93 FIGURE 7.  eV  — I —  290.7  Energy c a l i b r a t i o n o f K - s h e l l s p e c t r a ; a . ammonia c a l i b r a t e d u s i n g m o l e c u l a r n i t r o g e n (400.93 eV p e a k ) , b. methane c a l i b r a t e d u s i n g carbon d i o x i d e (290.7 eV peak).  eV  -44-  PLATE 2  Complete Experimental  Arrangement.  -45-  inlet  v a l v e and i o n i z a t i o n gauge head) and 0 - r i n g  connections  a r e made v i a h i g h  feed-throughs. lower  voltage  These a r e s o l d e r e d  side of the baseplate  The shield  ceramic o c t a l - p l u g s o r s i n g l e  and s e a l e d w i t h  ready access  vacuum c h a m b e r i s s c r e e n e d  viton 0-rings.  removed to the  ( n o b o l t s a r e u s e d on t h e spectrometer.  from magnetic f i e l d s  vacuum i s p r o d u c e d by a n NRC 4" d i f f u s i o n  polyphenyl  ether) with a water b a f f l e ,  liquid  v a l v e b e t w e e n t h e pump a n d vacuum c h a m b e r . - ft  t h e s y s t e m i s ^ 1 x 10~  Sample All  and  The l i d a n d  by a mu-metal  ( n o t shown i n P l a t e 2 ) .  The  4.4.  A l l electrical  i n t o flanges which a r e b o l t e d t o t h e  vacuum c h a m b e r may t h e r e f o r e be e a s i l y main chamber) t o p r o v i d e  seal.  n i t r o g e n t r a p , a n d 5" g a t e  The t y p i c a l  base pressure o f  torr.  Purity.  chemical  used w i t h o u t  degassing  pump ( u s i n g C o n v a l e x 10  samples used i n t h i s further purification.  p r o c e d u r e was f o l l o w e d .  study  were c o m m e r c i a l l y  For liquid  purchased  samples t h e normal  T h e s t a t e d minimum p u r i t y o f t h e s a m p l e s  was a s f o l l o w s : N  99.99%  CH  CO  99.5%  NH  0  99%  H 0  2  £  NO  3  99.99% 99.99% DISTILLED  98.5%  99.9%  99.99%  99.9%  98%  98%  FISHER COS  2  4  97.5%  SPECTROANALYSED  99.7% FISHER  SPECTROANALYSED  -46-  CHAPTER  FIVE  DIATOMIC  5.1.  N i t r o g e n and C a r b o n 5.1.1.  Monoxide  Nitrogen.  The g r o u n d s t a t e e l e c t r o n  (lsa )  2  g  a.  MOLECULES  (lsa ) u  Valence Shell  2  (2sa ) g  ( 2 5 a / (2p^f  2  studied  The v a l e n c e s h e l l  E  g  +  .  known a n d  I n a d d i t i o n , some i n d i c a t i o n o f t h e p o s s i b i l i t y to the K-shell  e l e c t r o n energy l o s s  i s shown i n F i g u r e 8.  Molecular nitrogen  i n t h i s e n e r g y r e g i o n by b o t h o p t i c a l  e l e c t r o n energy l o s s  1  e l e c t r o n energy l o s s spectrum i s w e l l  of forbidden transitions contributing  nitrogen  2  g  t h e s p e c t r o m e t e r p e r f o r m a n c e and a l s o t o p r o v i d e a  p u r i t y check o f t h e sample.  obtained.  (2pa ) ,  Spectrum.  The v a l e n c e s h e l l was r e c o r d e d t o t e s t  configuration of the nitrogen molecule i s  s p e c t r u m may  be  spectrum o f m o l e c u l a r has b e e n t h o r o u g h l y  ( s e e R e f e r e n c e 75) a n d  s p e c t r o s c o p y (see Reference 5).  An e l e c t r o n  energy  62 l o s s s p e c t r u m has been o b t a i n e d and a r e s o l u t i o n ,  AE(FWHM), o f 0.01  i n o u r low r e s o l u t i o n higher resolution is  associated with  This t r a n s i t i o n although  spectrum  results.  25 keV i n c i d e n t e n e r g y  eV.  The l o c a t i o n s o f t h e p e a k s  Peak A w i t h a maximum a t 9.2  t h e L y m a n - B i r g e - H o p f i e l d bands,  rise  electrons observed  (AE(FWHM) 0.5 e V ) a r e c o n s i s t e n t w i t h t h e  i s f o r b i d d e n by e l e c t r i c  i t gives  and e l e c t r i c  with  ^x  g  +  eV i n o u r s p e c t r u m -*- a ^ n  dipole selection  g  rules  (2pa (g  t o weak p h o t o a b s o r p t i o n b e c a u s e o f m a g n e t i c  quadrupole i n t e r a c t i o n s  (see Reference 76).  g  -> 2 p t r ) . Q  g), dipole  The t r a n s i t i o n i s  Intensity (arbitrary units)  -48-  also forbidden  i n our experiment s i n c e the f i r s t  s h o u l d be v a l i d  (i.e.  impact e x c i t a t i o n is  E ( 2 5 0 0 eV) » o  the i n t e r a c t i o n  Born a p p r o x i m a t i o n  and e  E n  ^ 0°).  between t h e i n c i d e n t and t a r g e t  assumed t o be p u r e l y e l e c t r o s t a t i c  and  of the i n i t i a l  equation  (2.6.11)  with e l e c t r i c scattering  final  contributes  and f a s t e l e c t r o n  has  a /o  2E  =  q  (i.e.  2  /E  2 n  t h e same m a t r i x e l e m e n t by p h o t o a b s o r p t i o n ) .  associated For forward  as where  2 £ 2  term  impact, the r a t i o o f d i p o l e to quadrupole 5  been g i v e n  E l  Since the  s t a t e s d i f f e r , o n l y the second term i n  quadrupole t r a n s i t i o n s  cross-sections  d  and  electrons  t h e r e f o r e , the t r a n s i t i o n i s  o n l y a s s o c i a t e d w i t h an e l e c t r i c q u a d r u p o l e i n t e r a c t i o n . symbol  In e l e c t r o n  aver.  E  =  E  -  Q  (E /2) n  and e-j a n d e a r e t h e d i p o l e and q u a d r u p o l e m a t r i x e l e m e n t s r e s p e c t i v e l y [see (2.6.9)]. This implies that the d i p o l e to quadrupole i n t e n s i t y r a t i o 2  increases  l i n e a r l y with  i n c i d e n t energy.  A t 48 eV  i n c i d e n t energy  and  49 6=0°,  the r a t i o  i n c i d e n t e n e r g y and  i s ^ 16. 6  a v e r  The  ^ 0-02  n o t f o r f o r w a r d s c a t t e r i n g , we  r a t i o o b s e r v e d i n o u r s p e c t r u m , 2500 eV r a d . i s ^ 18.  still  e x p e c t much l e s s  associated with a quadrupole t r a n s i t i o n . is  not c l e a r .  Although our r e s u l t s  I n f a c t , Bonharr/^, u s i n g  The  intensity  to  (see Reference 5 ) .  10 keV e l e c t r o n  i m p a c t has  the e x c i t a t i o n of the b  Reference 62).  The  (see in  eV)  h i g h e r energy  and  peaks  state (C = 14.0  (12.84 eV, eV,  v' =  D = 15.8  eV  r e s u l t f r o m t h e e x c i t a t i o n o f a number o f e l e c t r o n i c  R e f e r e n c e s 60 a n d 6 2 ) . our spectrum  also 9=0  The m a i n i n t e n s i t y o f peak B ( 1 2 . 8 eV) i s  associated with  E = 16.9  be  reason f o r t h i s discrepancy  o b s e r v e d t h e L y m a n - B i r g e - H o p f i e l d b a n d s , w h i l e G e i g e r a t 25 keV has n o t  were  The  l o c a t i o n of the f i r s t  ionization  i s b a s e d on t h e e x p e r i m e n t a l v a l u e ^ ' ^ o f 15.57  4 and  E =  states potential eV.  -49-  b.  Nitrogen The 32  studies  K-shell  l s a g and  although  Excitation.  Isa^ electrons  i n theory there  are  i n d i s t i n g u i s h a b l e i n X-ray  s h o u l d be  a small  energy  PES  difference  32 between the  two  orbitals'  essentially  n o n b o n d i n g and  t h e i r atomic character.  :  The are  The  electrons  filling  designated  removal  these o r b i t a l s are  "K-shell" electrons  of a K-shell  because  e l e c t r o n which  is  of  local-  12  i z e d on  one  "core" all  nitrogen  includes  states  the  two  nuclei  having a K-shell  approximately via  nucleus produces a n i t r i c  10~  I f the  6  molecular orbital  oxide.  intermediate  a v e r y low  K-shell  configuration  i s the  the  S i m i l a r l y , promotion of the  energy o r b i t a l s produces species Only those e x c i t e d  states  of n i t r i c  by  s i n g l e t r a n s i t i o n of a K-shell  ive,  n o n - R y d b e r g , A'  which converge to  correlated with  s t a t e and  ground i o n i c s t a t e .  produces a s t a t e analogous to the  the  energy p o s i t i o n s of the  spectrum r e l a t i v e to the first  unoccup-  resulting outer  in excited  p r o d u c e d by  These i n c l u d e  nitric  to  the  states of nitrogen  electron.  a l l o f the  ron  l e v e l s of the  the  first  low  higher states. promotion produced  the d i s s o c i a t -  +  z  the  (relaxation  in nitrogen  oxide  oxide which are  2pTr*  2  be  nitric  that  exist for  ground s t a t e of  electron  resembling  the  2 p i T g ,  that of the  K-shell  of the the  e l e c t r o n can  i s promoted to the  antibonding  same as  noted  that only  Auger e m i s s i o n  (The  p r o b a b i l i t y f o r elements of  electron  of n i t r o g e n ,  .  I t s h o u l d be  states  s e c o n d s b e f o r e t h e y d e c a y by  1 4  atomic number ^).  electronic  their K-shells.)  hole are  a r a d i a t i v e t r a n s i t i o n has  ied  and  oxide type "core"  Rydberg s t a t e s  Complete e j e c t i o n o f the  ground s t a t e of  s t r u c t u r e observed  first  of n i t r i c  i n the  N0 . +  K-elect-  Therefore,  nitrogen  K-shell  d i s c r e t e peak, s h o u l d reproduce the  Rydberg s e r i e s o f the  oxide  n i t r i c oxide molecule.  energy Nakamura  27 et a l .  have s u c c e s s f u l l y u s e d t h i s  analogy with  nitric  oxide to  interpret  -50-  the  discrete  s t r u c t u r e observed i n the o p t i c a l  n i t r o g e n , w h i c h was The and  K-shell  obtained using  e l e c t r o n energy  t h e peak p o s i t i o n s  are l i s t e d  absorption  spectrum  of  synchrotron radiation.  loss  spectrum o f N  i n T a b l e 1.  The  2  i s shown i n F i g u r e  relative  9  energies of  79 the  nitric  such t h a t  oxide molecule the f i r s t  second d i s c r e t e is  t h a t we  Rydberg  p o i n t f o r two  state  The  the second  The  spectrum  ive  energy  the  discrete  peak, which  has  discrete  this  t o two  final  as a  levels of n i t r i c s t r u c t u r e and  pppul-r  reference i t has  states  t o a Rydberg  and,  orbital  separation c l o s e to that of the  t h e r e f o r e s h o u l d have l e s s v i b r a t i o n a l  been d i v i d e d  analogy  region of the  peak i s b r o a d a n d  represents excitation internuclear  the  Therefore vibrational chosen  excitation.  i n t o t h r e e r e g i o n s on t h e b a s i s o f t h e  oxide (see Figure 9 ) . e x t e n d s up  9  to the f i r s t  Region  relat-  I includes a l l of  ionization  potential  of  o x i d e ( w h i c h c o r r e s p o n d s t o t h e K-edge o f n i t r o g e n ) , R e g i o n I I  Region  ential. with  i n the Frank-Condon  i t represents transitions  extends from the f i r s t and  ( i ) the f i r s t  s t a t e and  i n applying  s e c o n d d i s c r e t e p e a k was  i s e x p e c t e d t o h a v e an  n i t r o g e n ground  o x i d e m a t c h e s up w i t h  A difficulty  to "NO-like" states.  reasons:  been s u g g e s t e d t h a t  nitric  of n i t r i c  peak i n o u r spectrum,.  ations are uncertain.  (3sa)  level  a r e deal-jng w i t h e x c i t a t i o n  n i t r o g e n ground  (ii)  have been drawn above t h e s p e c t r u m i n F i g u r e  to the second  ionization  potential  I I I i n c l u d e s a l l the s t r u c t u r e above t h e second  The  d i s c r e t e part of the spectrum  the r e l a t i v e  nitric  oxide levels  of n i t r i c  ionization  ( R e g i o n I ) i s i n good  and a l s o w i t h  oxide pot-  agreement  the photoabsorption  27 spectrum corresponds  (see F i g u r e  10).  t o an e n e r g y 0.4  peak i n o u r s p e c t r u m  The eV  ground below  vibrational  state of n i t r i c  t h e maximum o f t h e f i r s t  oxide  discrete  (see F i g u r e 9 ) , whereas i n t h e p h o t o a b s o r p t i o n s p e c t -  I  400  I  I  4KD  1  1  420  1  ,  1 —  430  energy loss(eV) FIGURE 9.  K-shell  energy loss  spectrum of m o l e c u l a r  nitrogen.  TABLE 1 ABSOLUTE ENERGIES K-SHELL  Peak  ( e V ) , R E L A T I V E ENERGIES  SPECTRA OF N  2  AND  CO  AND ASSIGNMENTS OF PEAKS OBSERVED IN REGION I OF  (CARBON K - S H E L L ) .  Nitrogen This  CO(C -shell)  1  Optical 7  work  2 3.  400.62  2  AE  Energy  0  a  406.10  5.48  407.00  6.38  400.11 b  405.59 406.50 406.72  4.  Assignment  K  Energy  408.39  7.77  This AE  Energy 286.86  c  5.48 6.39  b  292.34 293.31  (ref. AE  d  Orbital 2pir  0 5.48  b  6 .45  6.61  407.66  7.55  407.90  7.79  a b c d e f g  1  409.9  409.5  294.77  7 .91  296.1  2  n  g  u  3 a  u  3aV  State  g  3p*  4sa  9  1  3sa  P  P e a k maximum a t 4 0 0 . 9 3 ± 0.05 eV. The s e c o n d p e a k was u s e d t o p o s i t i o n t h e r e l a t i v e n i t r i c o x i d e l e v e l s . C e n t r e o f t r u n c a t e d p e a k a t 4 0 0 . 8 4 eV. P e a k maximum a t 287.28 ± 0.05 eV. Omit t h e g and t h e u f o r c a r b o n monoxide s t a t e s . V a l u e s f r o m E S C A : N , 4 0 9 . 9 e V ; CO, c a r b o n - K 295.9 eV. O n l y t h e o u t e r o r b i t a l s i n v o l v e d i n t h e K - e x c i t a t i o n s have been i n c l u d e d . 3 2  Nitric  6  work  3dc K-edge  THE  z  n 1  +  u  9  n  Energy  AV  5.48  C n  6.49  DV  6.61  EV  7.55  H n  7.88  2  u  z  z  79)  0 u  +  1 u+ g  State  oxide  u  , 2  I  u 7.88  CJO I  NO  Relative Energy FIGURE  10.  10  eV  Comparison o f the r e l a t i v e energies o f valence e x c i t e d s t a t e s of n i t r i c and K - s h e l l e x c i t e d s t a t e s o f n i t r o g e n and c a r b o n m o n o x i d e ( c a r b o n K ) .  oxide  -54-  rum  27  the  g r o u n d v i b r a t i o n a l s t a t e was  truncated the  first  discrete  a s y m m e t r i c s h a p e we  K-shell  energy l o s s  peak. find  The  0.5  eV  below the  difference  f o r t h i s peak.  is partially  A  spectrum obtained with  centre  the  explained  comparison of  a resolution  of  the  by  nitrogen  [AE(FWHM)] of  27 0.5 is  eV  ( t h i s w o r k ) and  shown i n F i g u r e  absorption  the  11.  spectrum  photoabsorption  The  first  discrete  i s "truncated"  [ A E ( F W H M ) < 0.2  spectrum  peak o b s e r v e d  because o f  total  i n the  absorption  eV]  photoof  the  avail  30 able  radiation  a s s i g n e d by  bonding  The  discrete 27  Nakamura e t a l .  observe at electron  .  4 0 0 . 9 3 ± 0.05  to  the  eV  part  (see being  lowest u n f i l l e d  of  the  Table  s p e c t r u m has  1), with  a t t r i b u t e d to  the  the  molecular orbital  already  intense  been  peak  promotion of  of  nitrogen,  we  a  lsa  the  u  anti-  2p7Tg.  N  (lsa )  2  Osa )  2  2  (lsaj  g  (lsa^  g  (2pa ) , X ^*  2  2  -  1  g  (2pa )  1  g  2  (2pir )\  \  g  29 30 of e l e c t r o n - i o n coincidence spectra ' , and the 27 36 photoabsorption data , W u i l l e u m i e r and K r a u s e have c o n c l u d e d t h a t two From a c o n s i d e r a t i o n  excited The  ^z.. +  u  orbital  states  contribute  state  and  r e s u l t s from the  i s analogous to the  oxide molecule. these s t a t e s , of  the  to the  From t h e  i t was  one  concluded  or  two  ^z  +  state.  Our  promotion of  ^  and  a lso q 2 +  dissociative A  1  z  the  ''z  electron  state  that  a maximum o f  Therefore, i n the  two  of  to the  r e s u l t s show t h a t  to the  the  energy l o s s ''n  higher energy s i d e of the  first  states.  +  2pa,, u  nitric decay  of  vibrational levels  v i b r a t i o n a l t r a n s i t i o n s to the  broader continuum c o n t r i b u t i o n the  r  peak, the  sharp Auger peaks r e s u l t i n g from the  s t a t e were e x c i t e d .  t h e r e s h o u l d be  first  spectrum state  the  peak  d i s c r e t e p e a k has  and  a  from  a FWHM  -55-  N  2  K-shell  N  x10  2.5 kV Electron  Impact  Synchrotron Tokyo  31.0  30.2  29.5  o A  400  410  420  eV  FIGURE 11.  Comparison of the K - s h e l l energy l o s s s p e c t r a of m o l e c u l a r o b t a i n e d u s i n g e l e c t r o n i m p a c t and s y n c h r o t r o n r a d i a t i o n .  nitrogen  -56-  o f 0.8 e V , w h i c h i s s i g n i f i c a n t l y m o r e t h a n t h e 0.5 eV FWHM o f t h e peak from t h e e l a s t i c a l l y conditions.  s c a t t e r e d e l e c t r o n s under i d e n t i c a l  F u r t h e r m o r e , t h e peak i s s l i g h t l y  energy s i d e .  We a r e n o t a b l e  our  H o w e v e r , we o b s e r v e a b a s e w i d t h  results.  a s y m m e t r i c on t h e h i g h  t o make a n y d e f i n i t e  o f a b o u t 2 eV i n c o n t r a s t t o t h e 3 eV r e p o r t e d  experimental  new c o n c l u s i o n  from  ( a t 5% o f t h e p e a k  height)  by v a n d e r Wiel and  30 El-Sherbini  a t an i m p a c t e n e r g y o f 10 keV a n d a l s o , i f t h e r e  t h e y m u s t be l e s s t h a n 0.5 eV cross and  apart.  s e c t i o n s f o r t h e two p r o c e s s e s  10 keV. A value  the r e l a t i v e  a r e two s t a t e s ,  I t i s possible that the r e l a t i v e  are significantly  d i f f e r e n t a t 2.5  o f 4 0 9 . 9 eV was d e r i v e d f o r t h e K - s h e l l b i n d i n g e n e r g y  from  nitric  with  t h e X - r a y PES v a l u e  oxide 32  .  levels  and t h i s  i s i n e x c e l l e n t agreement  We h a v e d e r i v e d a n a p p r o x i m a t e c o n t i n u u m s h a p e b y  80 using (see  semiempirical  X - r a y mass a b s o r p t i o n  the hatched r e g i o n i n Figure 9 ) .  K-edge i n s t e a d o f a s m o o t h c o n t i n u o u s  electrons.  f o r nitrogen  S t r u c t u r e i s observed above t h e decrease.  a variety of multiple electron transitions o r more v a l e n c e  coefficients  +  This  structure  represents  i n v o l v i n g one K - e l e c t r o n  The f o l l o w i n g t w o - e l e c t r o n  a n d one  transitions are  The r e l a t i o n s h i p b e t w e e n h i g h i m p a c t e n e r g y . l o s s s p e c t r a a n d p h o t o a b s o r p t i o n d a t a h a s b e e n d e r i v e d i n C h a p t e r Two. For our experimental conditions the c o n v e r s i o n f a c t o r i s approximately (energy l o s s ) " (see 2.6.15), ( i f a t a momentum t r a n s f e r , K, o f a b o u t 1 a u , t h e h i g h e r t e r m s i n K i n t h e g e n e r a l i z e d o s c i l l a t o r s t r e n g t h ( 2 . 6 . 8 ) c a n be n e g l e c t e d ) . T h i s f a c t o r was u s e d t o o b t a i n t h e r e l a t i v e b e h a v i o u r o f t h e e x t r a p o l a t e d mass a b s o r p t i o n c o e f f i c i e n t s i n t h e region o f our spectrum. The a b s o r p t i o n c o e f f i c i e n t s h a v e a c o n t r i b u t i o n from shake-up and s h a k e - o f f p r o c e s s e s , b u t a t e n e r g i e s f a r a b o v e t h e K-edge we e x p e c t t h i s c o n t r i b u t i o n t o be a c o n s t a n t f r a c t i o n o f the K-continuum. The K - c o n t i n u u m was c o n s t r u c t e d b y n o r m a l i z i n g t h e d a t a t o t h e h e i g h t o f o u r s p e c t r u m a t t h e K-edge. 1  3  -57-  e x p e c t e d t o make t h e l a r g e s t c o n t r i b u t i o n s : i.  double e x c i t a t i o n ; conjunction  with  K-shell  superscript  ing  t o note that these s t a t e s  2pOg o r  denotes  2pir  u  o x i d e produced  2  ~ )  with  ionization;  •  The  i n v o l v i n g an e l e c t r o n f r o m b o t h t h e i s ejected  which  from double e x c i t a t i o n s , i . e . ( N  i s 6.4  eV^  excited ion state of n i t r i c  above t h e ground  and t h e s e c o n d  analogous t o the ( N  states.  2  " )  ionic state.  f o r shake-up  T h i s s u g g e s t s t h a t t h e 2p-n^ o r b i t a l  spectrum.  )  2  (N  2  oxide  i n t e n s i t y of the f i r s t  i s involved  )  It is interesting 79 has  been  i o n i z a t i o n p o t e n t i a l s which  events  "  oxide  are  bump a b o v e  K-edge i s a p p r o x i m a t e l y 5% o f t h a t o f t h e d i s c r e t e p e a k a t 401  which i s a reasonable r a t i o  of  The  a  associated K-1 **  oxide analogy, the lowest p o s s i b l e  observed between t h e f i r s t  second  the  the range o f the  t o n o t e t h a t a number o f a u t o i o n i z i n g s t a t e s o f n i t r i c  the  and  b r o a d b a n d o b s e r v e d i n o u r s p e c t r u m i n R e g i o n I I m u s t be  s t a t e should correspond to the f i r s t +  and  K-  d e s i g n a t e d by  simultaneous i o n i z a t i o n of a K-electron  s t a t e s , since from the n i t r i c  3  and  +*  discrete structure arising  (a z )  the Rydberg  electron,  v a l e n c e e l e c t r o n r e q u i r e d an e n e r g y o u t s i d e The  where  by t h e e x c i t a t i o n o f a  o t h e r remains behind i n a h i g h e r u n f i l l e d o r b i t a l ,  (N  " )  2  It is interest-  should correlate with  v a l e n c e s h e l l , w h e r e one o f t h e e l e c t r o n s  K— 1  (N  a hole i n the K - s h e l l .  states of n i t r i c  e x c i t a t i o n and and  K-1  of a valence electron i n  e x c i t a t i o n , d e s i g n a t e d by  the  non-Rydberg  ii.  i . e . , shake-up  eV  ( f o r example see R e f e r e n c e 8 1 ) . i n these e x c i t a t i o n s .  r i s e s t a r t i n g ^ 6 eV a b o v e t h e K-edge i s t h e n i d e n t i f i e d w i t h  The onsets  i o n i z a t i o n to a s e r i e s of (N ) s t a t e s a n a l o g o u s t o NO s t a t e s whose 79 t h r e s h o l d s (known f r o m PES) a r e t o o c l o s e t o g e t h e r t o o b s e r v e them s e p a r 2  -58-  ately.  H o w e v e r , a s shown i n F i g u r e 9, t h e p o s i t i o n o f t h e s e c o n d bump  c o r r e l a t e s with these states ( i t i s p o s s i b l e that doubly e x c i t e d s t a t e s , K-1 ** (N ) , a l s o c o n t r i b u t e to the i n t e n s i t y i n Region I I I ) . These 2  (N  " )  2  states  should give r i s e  to a s e r i e s  of s a t e l l i t e  X-ray p h o t o e l e c t r o n spectrum a t t h e low energy peak.  I t i s therefore interesting  peaks  i n an  side of the nitrogen  t o c o m p a r e t h e s t r u c t u r e we  K-shell  observe  above  81 416 eV w i t h t h e s a t e l l i t e  p e a k s o b s e r v e d by C a r l s o n e t a l .  o f d a t a p o i n t s a t the base o f the i n t e n s e not a l l o w a c o n c l u s i o n about below  the K - s h e l l  peak.  K-shell  .  The  p e a k o f R e f e r e n c e 81  the p o s s i b l e presence o f s a t e l l i t e s  In our spectrum h i g h e r onsets are not  enough t o c o m p a r e w i t h t h e s a t e l l i t e  scatter  around  6 eV  distinct  l i n e s observed i n Reference  (However, i n c a r b o n monoxide, o n s e t s a r e c l e a r l y o b s e r v e d and  does  81.  correlate  w i t h the s a t e l l i t e peaks.) I n o r d e r t o c o m p a r e t h e i n t e n s i t i e s we o b s e r v e K-1 +* 81 f o r the ( N ~ ) c o n t i n u a w i t h t h e l i n e i n t e n s i t i e s i n t h e ESCA s p e c t r u m , 2  we  note the f o l l o w i n g (i)  magnitude  The  features of our  h e i g h t o f t h e jumps i n Regions  decreased to a h e i g h t which 2  K-1 + ~ ) .  I I and  I I I a r e o f t h e same  as t h a t o f t h e K-jump.  ( i i ) At the high energy  (N  spectrum:  limit  of our spectrum the s t r u c t u r e  i s r o u g h l y 30% h i g h e r than the  81 Since Carlson et a l . observed a t o t a l  approximately  15% o f the K - s h e l l  data a t the high energy e a r l i e r work.  satellite  K-continuum, intensity  p e a k a t a p h o t o n e n e r g y o f 1487  side of the spectrum are c o n s i s t e n t w i t h  H o w e v e r , i t has  been f o u n d i n a w i d e  shell  r a t i o of double t r a n s i t i o n s  electron) r e l a t i v e to single transitions  (one  eV,  of  our  this  range o f cases  i n s t a n c e R e f e r e n c e 82) where e j e c t i o n o f a deep i n n e r e l e c t r o n t h a t the i n t e n s i t y  has  (see f o r  i s involved,  i n n e r a n d one  ( i n n e r e l e c t r o n ) as  outer a  -59-  f u n c t i o n o f photon  energy  constant.  b a s i s we w o u l d  only  On t h i s  r i s e s s t e e p l y from t h r e s h o l d expect the structures  a f e w p e r c e n t o f t h e K-jump t h r o u g h o u t R e g i o n  larger structures  are present suggests that  from a s e r i e s o f (N^ ~ ) thresholds.  there  An a l t e r n a t i v e e x p l a n a t i o n  discrete K-excitation gives  shake-up  rise  "following" K-ionization.  would  The  i s a strong  contribution  take account o f the i n d i s t a shake-off  i n i n t e n s i t y beyond  K-l  s t a t e as a  process might  transition  2  i n conjunction  ~ )  2  However, t h e f i r s t  enhance t h e "normal" i n t e n s i t y o f t h e ( N  onset o f the increase  of  I I I . The f a c t t h a t much  t o t h e same ( N  more o f t h e c h a r a c t e r i s t i c s o f a r e s o n a n c e locally  t o have h e i g h t s  s t a t e s c o n v e r g i n g t o each o f t h e i n d i c a t e d  i n g u i s h a b i l i t y o f t h e e l e c t r o n s , due t o w h i c h with  a n d t h e n becomes  have  and t h e r e f o r e  )  +*  might  continua.  t h e K-edge i n t h e p h o t o -  27 absorption  spectrum o f n i t r o g e n  Region I o f our spectrum  agrees w i t h  (see Figure  a b o v e t h e K-edge i s q u a l i t a t i v e l y energy  l o s s spectrum o f n i t r o g e n +  11).  the onset o f structure i n  A l s o , t h e s t r u c t u r e we  observe  s i m i l a r t o that observed i n the e l e c t r o n measured i n c o i n c i d e n c e  with  the N  + + 2  30  (plus N ) ions coincidence  produced  studies  by A u g e r  decay.  (From  the s i m i l a r i t y with the  o f c a r b o n m o n o x i d e w h e r e an a m b i g u i t y o f i o n i c  states  d o e s n o t e x i s t , we c a n c o n c l u d e t h a t m o s t o f t h e i n t e n s i t y i s due t o N ions.)  A normal  Auger  decay o f t h e d o u b l y e x c i t e d  s i n g l y charged i o n , o f which intensity  i n this  energy  the coincidence  range.  product i n d i c a t e s that the ( N  s p e c t r u m shows no  The i n f e r e n c e " )  states  a normal  Auger  2  s t a t e s would  that N  first  + + 2  + + 2  produce a appreciable  i s t h e predominant  autoionize  t o form  K-1 • + (N  2  fact  " ) , w h i c h t h e n undergo t h a t t h e decay  rate of the f i r s t  f a s t e r t h a n t h a t o f an A u g e r  decay.  This  a u t o i o n i z i n g step  transition.  i s s u p p o r t e d by t h e will  c e r t a i n l y be  -60-  5.1.2. C a r b o n  Monoxide.  Carbon monoxide i s i s o e l e c t r o n i c w i t h electron configuration  (lsa )  2  0  We  (lsa )  have s t u d i e d  The  s p e c t r u m has  a. V a l e n c e S h e l l The is  energy  12.  The  loss  a maximum a t 8.4  eV  is allowed,  the B 75).  The  oxygen  locations  K-shell  loss  o f peaks  48 53 55 83 ' ' ' and  peak compared w i t h Peaks  energy  loss  spectra.  spectrum o f carbon monoxide  are consistent with optical  the " f o u r t h  75  data  .  peaks, D  higher  Peak A  p o s i t i v e group"  with  of  to the forbidden Lyman-Birge-Hopfield  is reflected  states  and C  (11.3 eV)  respectively  excitations.  valence  shell  are associated  with  ( s e e R e f e r e n c e s 55  ( 1 3 . 4 e V ) , E ( 1 6 . 3 eV) a n d  a number o f o v e r l a p p i n g  transition  i n t h e much h i g h e r  peak A i n t h e n i t r o g e n  B (10.7 eV)  Rydberg  +  h i g h e r energy  associated with  V.  2  (2p<? -v TT*) a n d t h i s  and C E / E 1  (2pa) ,  4  P  However, i n the case o f c a r b o n monoxide, t h e  (Figure 8).  state  a l s o been r e c o r d e d .  i s analogous  ->- A  intensity of this  (2 TT)  e l e c t r o n energy  spectra  carbon monoxide, which  spectrum  2  i s associated with  bands o f n i t r o g e n .  has a g r o u n d  Spectrum.  valence shell  shown i n F i g u r e  resolution  (2sa*)  2  b o t h t h e c a r b o n and  valence shell  and  of  (2sa)  2  c  nitrogen  The  F  and  (17.0 eV)  location  are  of the 75  first  ionization  UV-PES  7 8  potential  v a l u e o f 14.00  b. C a r b o n The  K-shell  on  the spectrum  and  Excitation.  t h e r e f o r e we w o u l d  o f c a r b o n m o n o x i d e t o be s i m i l a r shown i n F i g u r e  on t h e o p t i c a l  eV.  p r o d u c t i o n of a carbon K-shell  o x i d e t y p e c o r e and  is  i s based  13, t h i s  "hole"  should produce  expect the carbon K-shell  to the K-shell  i s t h e c a s e , and  a  spectrum  spectrum of n i t r o g e n .  the r e l a t i v e  nitric  As  e n e r g i e s o f the  1  ELASTIC  st  LP  CO IG C  '  B-  0  10  20  Energy Loss FIGURE 12.  Valence shell  energy  loss  30  (eV) spectrum o f carbon  monoxide.  40  energy loss (eV) FIGURE 1 3 .  Carbon  K-shell  energy l o s s  spectrum o f carbon  monoxic  -63-  peaks a r e i n good agreement w i t h Figure  10).  Therefore  analogy with  those  of the n i t r i c  we have a s s i g n e d  oxide  l e v e l s (see  t h e d i s c r e t e peaks i n Region  t h e Rydberg s t a t e s o f n i t r i c  oxide  (Table  1).  The  a t i o n o f b o t h t h e d i s c r e t e and c o n t i n u u m p a r t o f t h e s p e c t r u m as not  I by  interpret-  i s t h e same  t h a t f o r n i t r o g e n and t h e r e f o r e t h e arguments f o r peak a s s i g n m e n t s be r e p e a t e d .  I n s t e a d we w i l l  discuss  will  t h e d i f f e r e n c e s b e t w e e n t h e two  spectra. The are  relative  slightly  intensities  different.  o f t h e p e a k s i n R e g i o n I o f t h e two s p e c t r a  A further difference i s that the f i r s t  discrete  peak a t 2 8 7 . 2 8 ± 0.05 eV i n t h e c a r b o n K - s h e l l s p e c t r u m h a s a FWHM o f 0.56 eV a n d i s s y m m e t r i c , i n d i c a t i n g of the results by  state are excited. 32  This  where t h e v i b r a t i o n a l  t h a t o n l y o n e o r two v i b r a t i o n a l  i s i n e x c e l l e n t agreement w i t h  spacings  of the final  CO  a u t o i o n i z a t i o n o f t h e "'n s t a t e ) have been r e s o l v e d .  5% o f t h e peak h e i g h t ) o f t h e f i r s t  peak i s o n l y  +  states  levels  t h e Auger (produced  The b a s e w i d t h ( a t  1.5 e V , r a t h e r t h a n t h e 30  3 eV r e p o r t e d  i n R e f e r e n c e 30.  (The c o i n c i d e n c e  same b r o a d a s y m m e t r i c s h a p e f o r t h e f i r s t monoxide.) for  spectra  produced the  peak i n b o t h n i t r o g e n a n d c a r b o n  This might i n d i c a t e that the r e l a t i v e e x c i t a t i o n  the states represented  by t h e f i r s t  10 keV. An e n e r g y o f 296.1 eV was d e r i v e d  cross-sections  p e a k a r e q u i t e d i f f e r e n t a t 2.5 a n d  f o r t h e K-edge, w h i c h  i s i n good  32 agreement w i t h  t h e X - r a y PES v a l u e  continuum shape (hatched joining  i n Figure  the extrapolated behaviour 84  o f methane and m e t h y l a l the  region  o f 295.9 eV.  K-edge.  (mainly  The a p p r o x i m a t e  13) was c o n s t r u c t e d  f o r t h e X-ray a b s o r p t i o n  by s m o o t h l y  coefficients  carbon-K) t o the continuum decrease  The s t r u c t u r e o b s e r v e d  near  i n Region I I i s a s s o c i a t e d w i t h d i s c r e t e  -64-  states,  (C ~'0)  (i.e.  N  carbon K - e x c i t a t i o n  and v a l e n c e s h a k e - u p ) .  Shaw  85 and Thomas  have i n v e s t i g a t e d  5.4 t o 6 eV b e l o w  t h e X - r a y PES s p e c t r u m  t h e main carbon K - s h e l l  i n t e n s i t y o f any s a t e l l i t e the  carbon K-shell  The  e n e r g i e s o f t h e NO  structures  i n the energy  region  peak a n d h a v e p u t a l i m i t  (from C O )  f o r the  s t a t e s ) as 0.4% o f  peak.  This supports o u r assignment o f the s t r u c t u r e i n K-1 +* R e g i o n I I . The s t r u c t u r e i n R e g i o n I I I r e p r e s e n t s (C 0) s t a t e s and t h e r e i s p r o b a b l y a l a r g e c o n t r i b u t i o n f r o m d o u b l y e x c i t e d s t a t e s (C ~ 0 ) . +*  s t a t e s a s g i v e n by PES  b r o a d bump i n R e g i o n I I I .  Onsets  potentials are s u f f i c i e n t l y  79  correlate with  the f i r s t  have been r e s o l v e d where t h e i o n i z a t i o n  f a rapart.  The e n e r g i e s o f t h e s a t e l l i t e  lines  81 obtained  by C a r l s o n e t a l .  e x c e l l e n t agreement w i t h  u s i n g a n X - r a y e n e r g y o f 1487 eV a r e i n  the onsets observed i n our spectrum  X - r a y PES l i n e s ) .  The l o w e s t s a t e l l i t e  is  the K-shell  a t 8.5 eV b e l o w  peak,  l i n e o b s e r v e d by C a r l s o n e t a l .  but the s c a t t e r of data points  p r o b a b l y mask a b r o a d b a n d o f s a t e l l i t e peak. the  There  are obvious d i f f e r e n c e s  lines  between t h e s t r u c t u r e s  I n Region I I t h e components making  s t r u c t u r e s do n o t h a v e t h e same r e l a t i v e carbon monoxide t h e s t r u c t u r e  reasonable t o expect d i f f e r e n t  intensities  shake-up  different unequal  nuclei,  electron  above  spectrum o f  as i n n i t r o g e n .  resolved.  In  to the I ti s  and s h a k e - o f f p r o b a b i l i t i e s  i n the  C a r b o n m o n o x i d e h a s two  and t h e r e f o r e , each m o l e c u l a r o r b i t a l  " c a r b o n and oxygen"  K-shell  up t h e d i s c r e t e  I I Iare clearly  two m o l e c u l e s , n i t r o g e n a n d c a r b o n m o n o x i d e .  would  observed  i s g e n e r a l l y more i n t e n s e r e l a t i v e  K-jump a n d t h e h i g h e r o n s e t s i n R e g i o n  of  close to the intense  K-edge i n t h e n i t r o g e n s p e c t r u m a n d t h e c a r b o n K - s h e l l  carbon monoxide.  (see F i g u r e 13,  densities.  will  generally  have  A change i n t h e s c r e e n i n g  t h e c a r b o n n u c l e u s by t h e p r o d u c t i o n o f a c a r b o n K - s h e l l  hole, should  -65-  preferentially orbitals with interaction  produce  shake-up  and s h a k e - o f f o f e l e c t r o n s  the h i g h e r "carbon" e l e c t r o n  densities.  Also  b e t w e e n t h e i n n e r and t h e v a l e n c e e l e c t r o n s  s i g n i f i c a n t e x t e n t , we w o u l d  expect t h i s  effect  from m o l e c u l a r i f a  direct  i s involved to a  t o c o n t r i b u t e more t o t h e  carbon K - s h e l l  spectrum than the n i t r o g e n spectrum since the carbon  ital  i n energy  i s closer The  continuum  the carbon K - s h e l l  to the valence  structure energy  coincidence with C 0  (C^  c. O x y g e n K - s h e l l  - 1  0)  +  by A u g e r  s t a t e s observed  c o i n c i d e n c e spectrum  a u t o i o n i z e t o form  implying  relative  As  i n Region  I I do n o t  which t h e n undergo  i n the case  a normal  decay  i n both the n i t r o g e n  spectrum o f carbon monoxide w i t h  o x i d e l e v e l s , we  e x p e c t t h e oxygen  K-shell  ( t h e p r o d u c t i o n o f an o x y g e n  K-shell  hole s h o u l d produce  s t a t e s o f the carbon monofluoride r a d i c a l  roscopic studies  86 89 ' ;  the ground  2 R e c e n t l y a D n and p o s s i b l y for details).  The  2 X n,  are well  2 + the A E and  a CF  2 the B A  o f CF  has  review). The o x y g e n  to  radical core). spect-  (see Reference in a  wide  91 '  and  92 '  .  states.  been d e r i v e d  range o f experiments w i t h e s t i m a t e s from s p e c t r o s c o p i c d a t a  values  + +  the  type  89  calculated  C0  K-shell  known f r o m  2 + a C E s t a t e have been o b s e r v e d  ionization potential  90  first to  spectrum  r e p r o d u c e t h e r e l a t i v e e n e r g y s p a c i n g s o f t h e s t a t e s o f t h e CF  Three  contribute  states  Auger  of  Excitation.  the carbon K - s h e l l  nitric  decay"^.  t h a t t h e s e (C " 0)  From t h e c l o s e a g r e e m e n t o b s e r v e d s p e c t r u m and  t h e same as t h a t o b s e r v e d i n  l o s s spectrum o f carbon monoxide measured i n  n i t r o g e n , the doubly e x c i t e d t o t h e CO  shell.  is qualitatively  ions produced  + +  K-orb-  , in fair  K-shell  energy  agreement  loss  ( s e e R e f e r e n c e 92 f o r a  complete  s p e c t r u m o f c a r b o n m o n o x i d e i s shown  90  -66-  i n F i g u r e 14.  The  outer electronic transition  r e l a t i v e e n e r g i e s o f t h o s e s t a t e s o f CF w h i c h  configurations  of a K-electron  identical  i n CO,  t h e r e f o r e not been i n c l u d e d  to those produced  a r e a l s o shown  i n the c o r r e l a t i o n ) .  by a  (the B A s t a t e The  results  t o n o r m a l i z e t h e r e l a t i v e CF e n e r g y  The  peak has  been used  included  i n F i g u r e 14 a r e t h e t h r e e h i g h e r d i s c r e t e p e a k s  on an e x p a n d e d  scale  from d i f f e r e n t  data runs.)  of the p r e s e n t l y The 2  (insert a). As  (The f u l l  are  i n t e n s e peak o b s e r v e d a t 5 3 4 . 0 ± 0 . 1  X n s t a t e o f C F ) has a FWHM o f 1.3  eV  that the r e l a t i v e  CF e n e r g y  vibrational  state  below  (see F i g u r e  14).  ionization  potential  89 spectroscopic data  eV  discrete Also  i n the  spectrum  the i n s e r t  i n our  (analogous to the  a n d t h e r e f o r e we  by t h e f a c t  The  eV  l e v e l s a r e p o p u l a t e d (5 t o 7 ) .  i s ^ 0.5  second  are  shown by F i g u r e 1 4 , t h e r e l a t i v e e n e r g i e s  known s t a t e s o f CF a g r e e w i t h t h e p e a k s  number o f v i b r a t i o n a l  listed  have been  levels.  s p e c t r u m and  single has  i n T a b l e 2 and t e n t a t i v e a s s i g n m e n t s o f t h e d i s c r e t e s t r u c t u r e s made on t h e b a s i s o f t h e c a r b o n m o n o f l u o r i d e s t a t e s .  have  scale  implies  ground  conclude that  a  This i s supported that the  the centre of the f i r s t  o f CF, e s t i m a t e d as 8.9  spectrum.  ±0.1  ground  discrete  eV  peak  from  91 '  and 9.2  ± 0.5  eV f r o m a H a r t r e e - F o c k SCF  calcul-  92 ation oxygen  l e a d s t o v a l u e s o f 542.4 eV a n d K-edge o f c a r b o n m o n o x i d e .  542.7 eV  respectively  f o r the  These v a l u e s compare f a v o u r a b l y  32 the experimental X - r a y PES e n e r g y o f 542.1 eV. uum as i n d i c a t e d by t h e h a t c h e d r e g i o n i n F i g u r e  with  An a p p r o x i m a t e K - c o n t m 14 was c o n s t r u c t e d by 80  extrapolating by an  X - r a y mass a b s o r p t i o n  (energy l o s s )  a b o v e t h e K-edge w h i c h  coefficients  f a c t o r ) t o t h e K-edge. r e p r e s e n t s shake-up  and  f o r oxygen  (corrected  Broad s t r u c t u r e  i s observed  shake-off events  associated  I.O-.  —I  530  FIGURE 14.  '  1  540  '  1  550  1  •—I—  energy loss  560  (eV)  Oxygen K - s h e l l e n e r g y l o s s s p e c t r u m o f c a r b o n monoxide. I n s e r t a (taken from a s e p a r a t e d a t a r u n ) shows t h e t h r e e h i g h e r d i s c r e t e p e a k s on an e x p a n d e d s c a l e .  TABLE  2  ABSOLUTE ENERGIES  ( e V ) , R E L A T I V E ENERGIES  OF THE OXYGEN K-SHELL  Peak  C O ( 0 - s h e l l ) t h i s work  P o s s i b l e assignments  C F  Energy  Orbital  State  State  2 p i T *  \  X n  3sa  V  AV  5.32  3piT  \  D n  6.40  3pa  V  C z ?  AE  1  533.5  2  538.8  5.3  3  539.8  6.3  a  0  4  K-edge  a. b. c. d.  P O S S I B L E ASSIGNMENTS OF PEAKS OBSERVED IN REGION I  SPECTRUM OF CARBON MONOXIDE.  K  4  AND  540.9  b  d  86-90  0  2  2  2  +  7.4  542.4  c  P e a k maximum a t 534.0 ± 0.1 eV. The s e c o n d p e a k was u s e d t o p o s i t i o n t h e CF l e v e l s . ESCA v a l u e , 542.1 eV. O n l y t h e o u t e r o r b i t a l s i n v o l v e d i n t h e K - e x c i t a t i o n s have been i n c l u d e d . 3 2  Energy  6.65  -69-  with  K - e x c i t a t i o n and  i s more i n t e n s e  5.2.  Nitric  than  K-jump, t h i s  structure  spectra.  Oxygen.  Oxide. nitric  o x i d e m o l e c u l e has  the  configuration (1sa )  (lsa )  2  Q  A valence shell  (2sa)  2  N  s p e c t r u m was  o r b i t a l s , formed from the orbitals  electronic  recorded.  an  3 results in  and  (2 TT)  (2 TT*)  4  P  ,  1  P  l s o g and  nitrogen  Iso^  K-shell  2  n.  molecular atomic  mainly atomic i n character.  The  e l e c t r o n r e s u l t s i n a number  configuration  between the  i o n i z a t i o n of  2  The  inner shell  f o r each o r b i t a l  Thus t h e  (2pa)  2  n o n b o n d i n g and  of unpaired electron spins Table 3).  not  i o n i z a t i o n of  states  (2sa*)  2  oxygen K - s h e l l  r e s p e c t i v e l y , are  e x c i t a t i o n or  3  to the  previous  ground e l e c t r o n i c s t a t e of the  electron  electron  Relative  i n e i t h e r o f the  O x i d e and  5.2.1. N i t r i c The  ionization.  c o r e and  a nitrogen  because of the valence shell  of  coupling (see  I s e l e c t r o n o r oxygen  Is  1 n and  n molecular ion states.  Using  X-ray  PES,  1 n -  n energy s p l i t t i n g s 93  have been o b s e r v e d respectively. otion of the  Is  On  1.42  eV  and  0.55  the  f o r nitrogen basis  of  the  Is e l e c t r o n  ionization limit,  Is  i o n i z a t i o n and  oxygen Is i o n i z a t i o n  c o r e a n a l o g y m o d e l we  in nitric  oxide  to produce a n i t r i c  expect the  to d i s c r e t e l e v e l s  oxide species  with  electron).  states  Ionization of  oxide species promotion of  excited  the  ( p r o d u c e d by nitrogen  the  Is e l e c t r o n  s i m i l a r to oxygen i n i t s ground an  oxygen Is e l e c t r o n  e x c i t a t i o n o f an  in nitric  should  ionic state. oxide should  prombelow  relative  energy l e v e l s s i m i l a r to those o f m o l e c u l a r oxygen i n i t s ground valence shell  eV  94 '  a nitrogen  (exchange s p l i t t i n g s ) o f  and 2pi\^  O2,  produce a  nitric  S i m i l a r l y the produce  an  TABLE  3  ELECTRON  CONFIGURATIONS  AND ELECTRONIC  ELECTRON NITRIC  1sa  3  0  OXIDE  1  S  A  N  STATES  OF  K-SHELL  CONFIGURATION  2sa  2sa*  2pa  2DTT  2p7T*  EXCITED  NITRIC  RYDBERG  ORBITALS  na  n?r  OXIDE  AND MOLECULAR  MOLECULAR  2  2  2  2  2  4  1  X n  N *0  2  1  2  2  2  4  2  V, V,  N *0  2  1  2  2  2  4  1  N *0  2  1  2  2  2  4  1  N  2  1  2  2  2  4  1  K  K  K +  0  OXYGEN  °2 K*  0  lSa  2  g  l  s  a  2  u  2  s  a  2  g  2 s  °u  2  2  P°g  2  20,  4  u  2  2  1  4  2  2  4  3  0 * 2  2  1  2  2  2  4  2  of  2  1  2  2  2  2  2  2  1  2  2  2  2  2  °2  n,  1  n,  2  V  A,  n  V(2).  2  A(2),  2  E (2] +  n  a  X E" 3  3  n,  !  1 1  5  1  n,  3  }  A  ,  g  g  (2),  n(4),  3  a T h e same m o l e c u l a r s t a t e s a r e o b t a i n e d by o x y g e n I s e x c i t a t i o n i n NO b The numbers i n b r a c k e t s r e f e r t o t h e number o f s t a t e s o f t h a t s y m m e t r y . c g , u d e s i g n a t i o n s do n o t a p p l y t o an o x y g e n m o l e c u l e w i t h a l o c a l i z e d I s h o l e .  ,  E  ,  3  A,  o,  4 - 2 - 2 E  b V  n  V, V  K  K+  2  g 2  N  n,  2  V, W .  1  2  1  U  0  P*g  2  0  STATES  CO  NO K  OXYGEN.  \(3), 2 +  A-,  V, V,  D  E  V  ]  A ,  V  -71-  "NF-like"  species.  a. N i t r o g e n K - s h e l l The  nitrogen  Excitation.  K-shell  electron  o x i d e i s shown i n F i g u r e 15 and peaks a r e l i s t e d  i n T a b l e 4.  energy  loss  t h e e n e r g i e s and  The  spectrum o f  possible  general appearance  nitric  assignments  of  of the spectrum i s  s i m i l a r t o t h a t o b s e r v e d f o r t h e d i a t o m i c m o l e c u l e s n i t r o g e n and  carbon  monoxide i n t h a t t h e spectrum  peak.  This  i n t e n s e peak, l o c a t e d  i s dominated  a t 399.1  promotion of a n i t r o g e n  Is e l e c t r o n  orbital,  resulting  4 - 2 - 2 E , E , selection  the  rules  The  2 + z electronic  A and  e n e r g i e s and ation  2p-rr*.  is valid)  .  scattering  energy f o r n i t r o g e n for  oxygen K - s h e l l  spin forbidden state  electron  as a r i s i n g  configuration  (see T a b l e 3 ) .  impact e x c i t a t i o n  angles  (i.e.  Electric  f o r high  s h o u l d n o t be o b s e r v e d .  rise  approximalways  excitation  decreases to  4.5  a r e such  that 4 E  T h e r e f o r e , the  to our spectrum.  The  to  dipole  not  i s s i x times the  promotion while the r a t i o  i s not expected t o c o n t r i b u t e  the  incident  Born  B o r n a p p r o x i m a t i o n may  promotion) the experimental c o n d i t i o n s  transitions  from  can g i v e  when t h e f i r s t  (the i n c i d e n t energy  K-shell  discrete  to the lowest a v a i l a b l e m o l e c u l a r  Although the f i r s t  apply to our experiment  i s interpreted  states  apply to e l e c t r o n  small  eV  by t h e f i r s t  2 - 2 E ,  A and  2 + E  ? states are a l l dipole with The of and  the e x c i t a t i o n first  0.4  eV  discrete  states  of these states  s h o u l d be o b s e r v e d  a A E ( F W H M ) o f 1.0  p e a k has  n state  eV  (the s l i g h t  a s y m m e t r y on  T h i s suggests t h a t the energy  i s less  t h a n 1 eV.  Since this  and  peaks  i n our  compared w i t h  o b s e r v e d f o r t h e peak a s s o c i a t e d w i t h e l a s t i c  i s symmetric  mental).  connected to the ground  the t a i l  associated  spectrum. a AE(FWHM)  scattered  electrons  o f t h e peak i s i n s t r u -  spacings of the three doublet  first  p e a k i s by f a r t h e m o s t  intense  Energy Loss FIGURE 15. N i t r o g e n  K-shell  energy l o s s  (eV) spectrum o f n i t r i c  oxide.  TABLE  4  ABSOLUTE ENERGIES I N THE  K-SHELL  ( e V ) , R E L A T I V E ENERGIES AND  ENERGY LOSS SPECTRA  N -SHELL  OF NO  ORBITAL  K  P O S S I B L E ASSIGNMENTS OF PEAKS OBSERVED  (NITROGEN  9  AND OXYGEN K - S H E L L S ) .  STATES  0 -SHELL K  ASSIGNMENT PEAK  ENERGY  AE  PEAK  2 - 2  1  399.7  0  2  404.7  5.0  2pa*  2  n,  3  406.6  6.9  3sa  2  n  4  407.6  7.9  3sa,  2  n,  5  409.0  9.3  3p  ^ K-EDGE  B  6 K-EDGE  ^ B  409.8  ^  2 p T T *  3piT  2 + A,  , 2  E  AE  1  532.7  0  2  540.2  7.5  n  7  10.1  410.3  10.6  410.4  10.7  411.8  12.1  413.1  13.4  CO  14.8  3  n  CO  K-EDGE  B  543.3  10.6  K-EDGE  B  544.0  11.3  546.3  13.6  (SHAKE-UP <  414.5  E  ENERGY  AND  (SHAKE-OFF  a Only the outer o r b i t a l s i n v o l v e d i n the K - e x c i t a t i o n s b These v a l u e s a r e from X-ray P E S . 3 2  have b e e n  included,  -74-  structure largest  i n t h e s p e c t r u m , we e x p e c t t h e s e d i s c r e t e l e v e l s t o g i v e t h e  c o n t r i b u t i o n t o the high  energy a u t o i o n i z a t i o n  lines  observed i n oc  the  Auger spectrum o f n i t r i c  oxide excited  by e l e c t r o n  impact  A t h e o r e t i c a l estimate o f t h e energy d i f f e r e n c e s 2 - 2 £ ,  and  A  2 + states  E  (arising  made on t h e b a s i s  that  identical  orbital  4 the  E  (frozen  2 and  A states  single *  2  approximation).  c a n be r e p r e s e n t e d  IlS  M A  =  | l S TT *TT *|  )  TT  TT  4  l s o ^ -> 2 p ^ * ) c a n b e  +  Omitting  states are  the f i l l e d  by t h e s i n g l e  orbitals,  determinants,  |  +  have been w r i t t e n  E E ss tt aa -t e s  between t h e £ ~ ,  wavefunctions of the four  =  where t h e o r b i t a l s T Th he e  the orbital  *( Z ) 2  BB ss pp ii nn ..  from t h e t r a n s i t i o n  .  i n complex form and t h e b a r r e p r e s e n t s  are associated  with  linear  combinations o f the three  determinants = | l s ir4" TT"*|  A  Linear  , <j>  =  | i s T7 *TT"*| a n d < f r +  combinations which a r e eigenfunctions  momentum o p e r a t o r , which s a t i s f y  | l s TT *TT"*|  =  +  ,2 of S , the total  c a n be f o u n d by t h e N e s b e t m e t h o d . 9 5  t h e above c o n d i t i o n  and have t h e c o r r e c t  spin  angular  Combinations reflection  symmetry  are M>( i')  =  1  *(V)  =  1  2  (2<|> - $ c  K-| +* = K - ] S7T  S T r  A  for the four  states  and t h e f a c t  that  * t h e energy d i f f e r e n c e s a r e  =  E( E")  +  3K  E( A)  =  E(V)  +  K  2  B  energy expressions  E( Z") 2  - * ). .  - • )  /2 From t h e t o t a l  A  4  l s 7 r +  l s i i +  *  * +  V  V  *  ....  (5.2.1)  ....  (5.2.2)  -75-  E(V)  = (E z~)  + K,  4  +* + 2K +* _ *  ISTT  where  IT  i s t h e exchange i n t e g r a l  of the core analogy  between o r b i t a l s  model, n i t r o g e n I s e x c i t a t i o n  K* production o f N 0 s t a t e s ) i s expected states.  ....  (5.2.3)  TT  i and j .  i nn i t r i c  On t h e b a s i s  oxide ( i . e .  t o produce m o l e c u l a r oxygen  T h e r e f o r e we assume t h a t t h e c h a r g e  distributions  like  o f valence  K* molecular orbitals  0 s t a t e s h a v e an e q u a l  for N  c o n t r i b u t i o n from  both  K* atomic  centres.  orbital  2 p i T *  inN  0 i s then  4>(2pir*) = 0.707PTT K* - 0 . 7 0 7 p T r  wavefunction the atomic  The  N  p-rr o r b i t a l s  0  approximated  w h e r e p^K*  by t h e  and p ^ represent  associated with the nitrogen nucleus with a I s K*  h o l e a n d t h e o x y g e n n u c l e u s r e s p e c t i v e l y i n N 0 . S i n c e p-rr^K* ^ p ^ and ISpjK* l S g , o n e - c e n t r e e x c h a n g e i n t e g r a l s f o r a t o m i c o x y g e n ^ may be u s e d t o c a l c u l a t e t h e e x c h a n g e i n t e g r a l s i n e q u a t i o n s ( 5 . 2 . 1 ) t o %  (5.2.3).  6  The t w o - c e n t r e  exchange i n t e g r a l s  are t y p i c a l l y  an o r d e r o f  94 magnitude s m a l l e r alogous  and have been n e g l e c t e d  procedure  ionization  (see Reference  94 where an a n -  has been u s e d t o e s t i m a t e t h e exchange s p l i t t i n g  i n open s h e l l  systems).  The r e s u l t s  f o r Is  a r e shown i n F i g u r e 1 6 ( a )  75 where t h e e x p e r i m e n t a l oxygen  (X z~, a A  g  energy  and b z )  levels arising  g  4 configuration core analogy  (  2pTr  u  2p7r  model t h e z  corresponding  z  z  t h e same v a l e n c e  electron  g  ) have been i n c l u d e d .  On t h e b a s i s o f t h e  s t a t e o f m o l e c u l a r oxygen g i v e s r i s e t o  2 and  from  2  3 _  4 -  o f the three states o f molecular  K* states i nN  0 w h i l e t h e energy  difference  between  the  a n d ^ z * s t a t e s i s e x p e c t e d t o be s i m i l a r t o t h e e n e r g y d i f f e r e n c e 2 2 + K* between t h e A and z s t a t e s o f N 0 . The c a l c u l a t i o n s u g g e s t s t h a t t h e 2 2 z  and  A s t a t e s s h o u l d be c l o s e i n energy  and approximately  1 eV b e l o w  2 + the  z  state.  ing experimental  This result  i s i n q u a l i t a t i v e agreement w i t h t h e c o r r e s p o n d -  oxygen energy  levels.  The e x p e r i m e n t a l  FWHM o f 1 . 0 eV  eV  •n  x n9 2  eV  x n z  1.42 3  n  I  2  b Z*  T  1  9  1  a'A 9  ~~  ?— 0,65  1.1  1  0i98  s-  aA  f  1.2  1  +  1.8  1.4 3v-  0.9  J_ 24  IM *0  NF  Theory  Exptl.  K  2  Exptl.  2  1.6 1.4  x%" _ L _ o  1.3  0.9  S  (a)  Theory  (b)  FIGURE 1 6 . C o m p a r i s o n o f t h e r e l a t i v e e n e r g i e s o f : ( a ) v a l e n c e 0 s t a t e s ( e x p e r i m e n t a l ) a n d N * (theoretical); ( b ) v a l e n c e NF s t a t e s ( e x p e r i m e n t a l ) a n d NOK* s t a t e s (theoretical). N^O and N 0 s p l i t t i n g s a r e f r o m X - r a v PES d a t a . K  2  K +  states  i CD  -77-  for if  the f i r s t  d i s c r e t e peak and t h e symmetric peak shape i n d i c a t e s  a l l three doublet states  their  are excited with  e n e r g y s p a c i n g s m u s t be s m a l l e r Our a s s i g n m e n t o f t h e f i r s t  by  the close  and  approximately equal  than those  discrete  peak  that intensity,  calculated.  (lsa  -*  N  i s supported  2p7r*)  a g r e e m e n t b e t w e e n t h e o b s e r v e d p e a k e n e r g y , 399.7 ± 0.2 e V ,  t h e v a l u e o f 3 9 9 . 8 eV e s t i m a t e d  using  the concept o f equivalent  cores  97-99 and  t h e t h e r m o c h e m i c a l method  .  The f o l l o w i n g  reaction  scheme was  used: 1.  N0(X n)  ->-  2.  N  ->  N 0(W)  3.  N 0(W) + 0  ->•  0  4.  o (w)  5.  0  6.  o (xV)  7.  0 (W) + N  8.  N  2  K +  0( n) 3  K +  6 +  +  2  2  +  (X)  + e  K +  0( n) + e 3  +  (W) + N  6 +  0(W)  AE  -+  0 (X E")  AE  o(w)  A E  +  2  3  2  ->  N *0(W) + 0  ->  N *0(w)  K  AE  =  6  4  =  -0.01  5  =  -12.07  =  0.6  6  AE  6 +  ^  N  (X)  indicates  3 9 9 . 8 + 6 - &' % 3 9 9 . 8 eV  =  s t a t e s , t h e ground b  t h e ground s t a t e  0(W) r e s p e c t i v e l y  K  1+ z  the  g  states  of 0  2  species;  N  +  0(W),0  ionic  2 ( no.  states  and t h e  72  ,  9- ?  d E ,  E ,  1 6 ( a ) , except AE^ which  t h e ground s t a t e  N  0  + 2  species ^. 1  K +  0  2 3 1 n i , ) o f 0 , t h e X E , a A and 12 2' g' g o  9+  A and n c  0 states  states inN  of N  K*  from t h e r e l a t i v e  i s b a s e d on t h e  0.  N  K*  0(w) i s  0(W), c f . Figure  2  of  2  r e p r e s e n t t h e w e i g h t e d a v e r a g e s o f t h e n and  The w e i g h t e d a v e r a g e s h a v e b e e n c a l c u l a t e d  Figure  (W), 0 ( W ) and  2  3  weighted average o f t h e doublet N  in  1 0 0  0.6  K+  where  3 2  = -6'  7  AEg  K  K  0.4  3  A E  o (x)  N 0 ( X n ) -> N * 0 ( w )  =  2  6 +  2  2  410.3 e V  AE  K +  2  =  1  A E  ->  2  2  N  energies  16(a). shown  2  n ^, 3  In the equivalent  n ^  cores  splitting approximation,  -78-  6  ^ 6'(see Reference 98). The  b r o a d band o f s t r u c t u r e w i t h  a maximum a t ^ 404.7 eV i s t o o h i g h  i n e n e r g y t o be a s s o c i a t e d w i t h  p r o m o t i o n t o t h e 2p-rr* o r b i t a l  i n e n e r g y t o be a s s o c i a t e d w i t h  the lowest  explanation  of this  band i s t h a t  Is e l e c t r o n t o the antibonding the  peak c o u l d  Table 3) r e s u l t i n g  from t h i s  bidden).  The r e s u l t i n g  character  since  results  Rydberg e x c i t a t i o n .  i trepresents  2pa* valence  be a s s o c i a t e d w i t h  orbital.  i n a d i s s o c i a t i v e A'  2  E  shell  s t a t e ^ .  +  are  This  have been a c c o u n t e d f o r , t h e h i g h e r  probably  associated with  Rydberg o r b i t a l s . derived  This  of  (see .  state i s for-  t o h a v e some d i s s o c i a t i v e excitation i n nitric  oxide  dissociative character  would c o n t r i b u t e t o t h e broadening o f t h e s t r u c t u r e . orbitals  n states 4  valence  nitrogen  The b r o a d n a t u r e  electron configuration (then  states are expected  A possible  the e x c i t a t i o n of a  e x c i t a t i o n o f t h e two  the corresponding  and t o o low  Since  a l l the valence  d i s c r e t e peaks i n t h e spectrum  the promotion o f a nitrogen  Is electron to  a s s i g n m e n t i s s u p p o r t e d by t h e m a g n i t u d e s o f t h e  quantum d e f e c t s .  o r npa Rydberg o r b i t a l s  The p r o m o t i o n o f a I s a e l e c t r o n t o e i t h e r n s a r e s u l t s i n three  2  separate  4  Rydberg  s e r i e s ( n,  n and  2  n).  Two o f t h e s e s e r i e s , t h e n a n d one o f t h e n s e r i e s w i l l c o n v e r g e 3 2 to the n K - s h e l l i o n s t a t e w h i l e the remaining n s e r i e s converges t o the 1  2 n K-shell ion state.  our two  spectrum. 2  2  The f i r s t  limit.  n s t a t e which  n states are expected  Rydberg t r a n s i t i o n ,  n s t a t e s , one c o n v e r g i n g  t o t h e ^JI i o n i c the  Only t h e  at the  The t h i r d  3 n ionic  to contribute to  l s a ^ -»- 3 s o , limit  should  result i n  and t h e o t h e r  converging  p e a k l o c a t e d a t 4 0 6 . 6 eV i s a s s i g n e d  i s associated with  the  3 n limit.  Using  the observed  3 peak e n e r g y a n d t h e X - r a y PES v a l u e  o f 4 1 0 . 3 eV f o r t h e  to  32 n limit  deduce a quantum d e f e c t o f 1.08, w h i c h compares f a v o u r a b l y  with  , we t h e quantum  -79-  d e f e c t s o b s e r v e d f o r 3s R y d b e r g e x c i t a t i o n of n i t r i c  oxide  7 9  '  1 0 1  ,  2p-rr*  3 s a , 6 = 0.97  i n the valence s h e l l and  spectrum  the valence s h e l l  spectrum  102 ->- 3 s a , 6 = 1.1.  o f m o l e c u l a r oxygen  ,  an e n e r g y o f 4 0 7 . 6 eV  has a quantum d e f e c t o f 1.2  ion state.  2pTr  T h i s peak c o u l d  g  g  The  fourth with  t h e n have a c o n t r i b u t i o n  s t a t e p r o d u c e d by t h e e x c i t a t i o n o f a l s a ^ e l e c t r o n orbital. could  The much l a r g e r  peak o b s e r v e d a t  respect to the  from the remaining t o t h e 3sa  Rydberg  i n t e n s i t y o f peak f o u r c o m p a r e d t o peak  three  f r o m a c o n t r i b u t i o n f r o m t h e t r a n s i t i o n l s o ^ -> 3 TT w h e r e t h e 3 l i m i t i s the n ion state. The q u a n t u m d e f e c t o f p e a k f o u r 3  result  ionization  n  P  with respect to the  n limit  i s 0.75  and  i s consistent with  t h e quantum  79 d e f e c t , 6 = 0.76  , observed f o r the corresponding valence s h e l l  i n n i t r i c o x i d e (2pTr* -> 3 p i r ) . h a s a q u a n t u m d e f e c t o f 0.80  Similarly with  p e a k f i v e o b s e r v e d a t 4 0 9 . 0 eV  respect to the  l i m i t which i s c o n s i s t -  e n t w i t h w h a t we w o u l d e x p e c t f o r 3p e x c i t a t i o n w h e r e t h e R y d b e r g converges to the  limit.  t h e 3pTr and 3pa R y d b e r g states respectively Rydberg  peaks  results  uncertain.  Rydberg 3 p o s i t i o n s o f t h e n and  t h e e x p e r i m e n t a l X - r a y PES The  b r o a d band  i n s i x and two d i p o l e a l l o w e d  ( s e e T a b l e 3) and t h e a s s i g n m e n t s o f t h e s e  probably associated with The  state  However, the p r o m o t i o n o f a I s a ^ e l e c t r o n  levels  are c l e a r l y  transition  values  to final  higher  P e a k s i x o b s e r v e d a t ^ 410.4  eV i s  s t a t e s w h i c h c o n v e r g e t o t h e "'n i o n s t a t e . 1 n K - e d g e s i n o u r s p e c t r u m a r e b a s e d on 32 o f 410.3  a n d 411.8  eV  respectively.  o f s t r u c t u r e w i t h maxima a t a p p r o x i m a t e l y 413.1  eV  and  414.5 eV i s a s s o c i a t e d w i t h t h e s h a k e - u p and s h a k e - o f f o f v a l e n c e e l e c t r o n s in conjunction with K-shell excitation. On t h e b a s i s o f t h e c o r e a n a l o g y m o d e l we e x p e c t t h e e n e r g y s p a c i n g 4 2 between t h e a v e r a g e e n e r g y o f t h e z and z s t a t e s r e s u l t i n g from l s a w  -80-  promotion states  t o the  i nn i t r i c  2pTr* o r b i t a l  and the average  oxide t o reproduce  energy  the energy  i o n s t a t e o f m o l e c u l a r o x y g e n , 12.07  Figure  16(a)].  Using  energy  level  s p a c i n g o f 12.0 ± 0 . 4 eV i n q u a l i t a t i v e  predicted  value.  ionization  Using  3  ( 5 . 2 . 3 ) , t h e n,  gives  Sit+  1  n energy  is  listed  oxygen K - s h e l l  i n Table  4.  that thei n e l a s t i c  scattering  decreases  the f i r s t  the f i r s t The  S  atomic  32 94  '  +  11  discrete  electron  .  Using t h e  oxygen exchange  spectrum  energy  loss  spectrum  This  associated with this  t o noise ratio  of nitric  scattering  oxide  intensity  by a f a c t o r , « ( e n e r g y a t 532.7  peak i n t h e n i t r o g e n K - s h e l l  FWHM o f 0.4 eV.  integrals  splitting.  The p o o r s i g n a l  peak o b s e r v e d  peak i s v e r y b r o a d  .  (5.2.1) to  equations  to 2K^ ^ *  '  Excitation.  the n i t r o g e n K - s h e l l  fact  of  i s equal  produced  i s 1 . 4 2 eV  oxide  ofnitric  shown i n F i g u r e 1 7 a n d t h e e n e r g i e s a n d p o s s i b l e a s s i g n m e n t s  are of  spacing  eV f o r t h e ESCA  b. O x y g e n K - s h e l l  o f 1.4 e V , y i e l d s  1  i nderiving  * c a l c u l a t e d with one-centre  a v a l u e o f 0.96  The  assumed  z"  agreement w i t h t h e  3  i nn i t r i c  ground  o f the  between t h e n and n i o n s t a t e s  o f a nitrogen Is electron  t h e same a p p r o x i m a t i o n  v a l u e o f K-|  +  0  ^ [see  1  4 2 z - z splitting  an e n e r g y  by  e V  peak maximum a s an i n d i c a t i o n  and our estimate o f the  The s p l i t t i n g  n and n N  s p a c i n g between the  s t a t e and f i r s t  the f i r s t  o f the  of structure  compared w i t h  that  (Figure 15) r e f l e c t s t h e  of fast loss)  electrons for 3  .  forward  The i n t e r p r e t a t i o n  eV i s a n a l o g o u s  spectrum  oxide  ofnitric  to that of  oxide  (see Table 4 ) .  a n d h a s a FWHM o f 2.1 eV c o m p a r e d w i t h a n e l a s t i c result  suggests  peak h a v e a w i d e r  t h a t the three doublet N 0 * s t a t e s K  energy  spacing than  the  corresponding  4-  A b s o l u t e b i n d i n g e n e r g i e s i n R e f e r e n c e s 9 3 a n d 9 4 a r e o n l y ± 0 . 5 eV. T h e r e f o r e i n T a b l e 4 t h e N^+O e n e r g i e s a r e f r o m R e f e r e n c e 3 2 .  Intensity  (arbitrary  units)  P  o  Oi  01 CO  G"5  CZ 73  m o X  <<  fD 7< I  o  3  (0  -i (Q  oi  o o a)  ro — Oi  CD  ro  -5 <<  o  to (/> c/>  •a ro o  fl> o i > Oi  ^  .r'r  .»» *  o  r+  -s  Oi O) -S  o o  o  QfD  -L8-  a  -82-  K*  N  0 states.  indicates  The i n t e n s i t y  that these  of the f i r s t  discrete  levels  peak r e l a t i v e  a r e expected  to the others  to give the largest  c o n t r i b u t i o n t o t h e high energy a u t o i o n i z a t i o n l i n e s observed i n t h e o x y g e n K - s h e l l A u g e r s p e c t r u m o f n i t r i c o x i d e e x c i t e d b y e l e c t r o n i m p a c t 35 . Oxygen I s e x c i t a t i o n species.  In analogy  have t h e o r e t i c a l l y 2 - 2 E ,  A and  in nitric  t o t h e case  estimated  oxide  should  p r o d u c e an " N F - l i k e "  o f nitrogen Is e l e c t r o n promotion,  we  t h e r e l a t i v e energy spacings  o f t h e ^E~,  2 + E s t a t e s as a r e s u l t o f oxygen I s promotion  t o t h e 2p-rr* K*  molecular  orbital.  calculated  A wave f u n c t i o n f o r t h e 2pn* o r b i t a l 1Q3  u s i n g a n INDO c a l c u l a t i o n  (unrestricted  i n NO  Hartree-Fock  c e n t r e e x c h a n g e ) f o r NF w i t h an i n t e r n u c l e a r s e p a r a t i o n e q u a l nitric  oxide.  Using  0.512PTTQ|<*, P T Q K *  the resulting wavefunction,  w i t h one-  to that of  <j>(2pir*) ^ 0 . 8 5 9 p T r  P ^ p j ^ Q K * ^ lSp a n d t h e o n e - c e n t r e  %  was  atomic  S  -  N  fluorine  96 exchange i n t e g r a l s were o b t a i n e d . a ^A a n d b ^ E  +  +  , the calculated  The e x p e r i m e n t a l  [see Figure 16(b)].  FWHM o f t h e f i r s t  f o rthe t r a n s i t i o n  approximation  o f t h e X E~,  1  3  the corresponding  The c a l c u l a t i o n s u g g e s t s  o x i d e , h a v e an e n e r g y s p a c i n g  cores  1  agreement w i t h t h e e s t i m a t e d  p r o d u c e d by o x y g e n I s p r o m o t i o n  required  shown i n F i g u r e 1 6 ( b )  energy s p a c i n g s ^ ' ^  s t a t e s o f NF ( l a r g e r t h a n  oxygen) a r e i n q u a l i t a t i v e  experimental  energy spacings  spacings i n  values  f o r NO  that the three doublet  t o t h e 2p-n* m o l e c u l a r  orbital  states  in nitric  o f ^ 1.8 eV i n g o o d a g r e e m e n t w i t h t h e discrete  peak  ( F i g u r e 17).  I s a ^ -> 2pir* c a n be e s t i m a t e d  and t h e thermochemical  The e n e r g y using the equivalent  method i n e x a c t  analogy  tothe  T h e e n e r g y s p a c i n g s wege a l s o c a l c u l a t e d u s i n g a w a v e f u n c t i o n c a l c u l a t e d f o r NF w i t h r = 1.3173 A, t h e e x p e r i m e n t a l i n t e r n u c l e a r s e p a r a t i o n ^ o f NF, X^E". T h e l a r g e s t d e v i a t i o n f r o m t h e e n e r g y d i f f e r e n c e s shown i n F i g u r e 1 6 ( b ) was < 0.1 eV. +  e  -83-  method u s e d f o r t h e n i t r o g e n K - s h e l l  c a s e , I s o ^ ->  The e s t i m a t e d  2p7r*.  v a l u e o f ^ 5 3 1 . 9 eV i s i n g o o d a g r e e m e n t w i t h t h e o b s e r v e d 532.7 e V , c o n s i d e r i n g t h a t t h e o b s e r v e d T h e r e a p p e a r s t o be a b r o a d  peak  energy,  FWHM o f t h e p e a k i s 2.1 eV.  b a n d o f s t r u c t u r e w i t h a maximum a t 3  approximately ionization  540.2 eV.  limits  The quantum d e f e c t s w i t h r e s p e c t t o t h e  a r e c o n s i s t e n t with those expected  1  n and  f o re x c i t a t i o n  n  o f an  oxygen I s e l e c t r o n  t o R y d b e r g s t a t e s w i t h q u a n t u m number t h r e e . 3 1 p o s i t i o n s o f t h e n a n d n K-edges i n o u r s p e c t r u m a r e b a s e d on 32  The  the experimental The  broad  X-ray  PES v a l u e s  o f 543.3 eV a n d 544.0 eV  b a n d o f s t r u c t u r e w i t h a maximum a t a p p r o x i m a t e l y  respectively. 546 eV r e p r e s -  ents t h e shake-up and s h a k e - o f f o f v a l e n c e e l e c t r o n s i n c o n j u n c t i o n w i t h ISOQ  excitation.  The c o r e a n a l o g y model s u g g e s t s t h a t t h e e n e r g y d i f f e r e n c e b e t w e e n t h e 3 1 K+ 4 2 K* a v e r a g e e n e r g i e s o f t h e n a n d n NO s t a t e s a n d t h e E a n d E NO states  [ s e e F i g u r e 1 6 ( b ) ] s h o u l d have a m a g n i t u d e s i m i l a r  ionization potential peak i n t h e NO  K*  o f NF.  spectrum  Assuming t h a t t h e onset o f t h e f i r s t  corresponds  to excitation  d e r i v e a v a l u e o f ^ 13 eV f o r t h e i o n i z a t i o n 13.1 and  ± 0.2 eV d e r i v e d ^ f r o m 1  the theoretical  with this The from  value  1 1  experimental  of the  potential  appearance  discrete  2 E s t a t e , we  o f NF.  The v a l u e o f  p o t e n t i a l s ' ' ^ 1  ^ o f 13.2 ± 0.3 eV a r e i n r e a s o n a b l e  agreement  prediction. m a g n i t u d e o f t h e m o l e c u l a r e x c h a n g e i n t e g r a l , K^  t h e INDO 2p?r* w a v e f u n c t i o n  splitting  to the f i r s t  f o r NO  K*  * derived  s  E  , i m p l i e s t h a t t h e exchange  ( t o t h e same d e g r e e o f a p p r o x i m a t i o n 3  as i n t h e c a s e o f t h e  1 K+ n NO s t a t e s i s 0.6 eV. 93 94 T h i s compares f a v o u r a b l y w i t h t h e e x p e r i m e n t a l v a l u e ' o f 0.55 eV. discrete multiplet  s p l i t t i n g s ) o f the  n and  -84-  5.2.2. O x y g e n . The  ground  electronic  s t a t e o f t h e oxygen  m o l e c u l e has  the e l e c t r o n  configuration:  (lsa )  ( l s a j  2  g  (2sa )  2  a. V a l e n c e S h e l l The  the  6.8  t o 21  electrons.  final  energy  energy  excitation  r e g i o n , has  i n oxygen  our spectrum  vertical 116).  of o p t i c a l l y  U  -  E  is X  3  -> B  G  energy  1 0 2  i s 2PTT  ' E ,  (maximum 8.3  transition  The  energy  been o b t a i n e d  eV)  '  1 1 6  .  using  6 1  25  forbidden  '  A  -* 2pTr  U  G  1 3 -  and  U  is  eV, i n  keV  incident  transitions  and  dependence  l o c a t i o n s o f peaks  resulting  g  in  ' Y.^.  The  i n six possible only  optically  -  E^  ( t h e Schuman-Runge c o n t i n u u m ) .  i s associated with  this  i s 8.6  eV;  r e g i o n where t h e f o r b i d d e n A E , C A +«-|->- s e l e c t i o n  rule  U  and  1 -  c E^  Peak  transition see  a maximum a t a p p r o x i m a t e l y 6.0 3 + 3  The  E  r e s o l u t i o n measurements.  from o t h e r workers  b r o a d band A w i t h  Reference 1 6 ) .  The  these higher  1 3 + 1 3  states;  transition  3  r e s o l u t i o n s p e c t r u m , A E ( F W H M ) = 0.01  A high  3  in  2  l o s s spectrum o f m o l e c u l a r oxygen  lower impact e n e r g i e s  electronic  allowed  ) ,  P l T g  s t a t e s h a v e b e e n made on t h e b a s i s o f a n g u l a r  using  first  (2  Spectrum.  our spectrum are c o n s i s t e n t w i t h The  (2p«/  2  g  In a d d i t i o n , assignments  some R y d b e r g studies  eV  18.  (2pa )  2  u  valence shell  shown i n F i g u r e  (2sa )  2  g  B  (the  Reference  eV  i s i n the  states occur  (see  i s not r i g o r o u s f o r nonaxial  54 scattering  i n e l e c t r o n impact  associated with  3 +  t h e A E^ s t a t e .  p r o b a b l e a t i m p a c t e n e r g y o f 2.5 the  second  (10.0  eV)  The keV  involves a spin forbidden  have n o t been o b s e r v e d C  and  and  the i n t e n s i t y 3  A  U  and  1  -  E^  o f t h e A band e x c i t a t i o n s are  since the f i r s t transition.  The  D  (10.9  eV)  less  i n v o l v e s AA = 2 ^ l * and  i n e i t h e r e l e c t r o n impact o r o p t i c a l  shoulder  i s probably  are associated with  the  ^  and  states  studies. "longest"  Peak  Valence s h e l l  energy l o s s  spectrum o f m o l e c u l a r oxygen.  -86-  (9.97 e V  D I  (10.29 e V  ) and "second"  b l  ) band r e s p e c t i v e l y .  The h i g h e r  e n e r g y p e a k s , E ( 1 3 . 0 e V ) , F ( 1 5 . 3 e V ) G ( 1 6 . 9 e V ) , H ( 1 9 . 0 e V ) , I (20.1 e V ) , J  (21.8 e V ) , K ( 2 3 . 5 e V ) a n d L ( 2 4 . 5 e V ) a r e a s s o c i a t e d w i t h  number o f o v e r l a p p i n g t r a n s i t i o n s ( s e e R e f e r e n c e 6 1 ) . first  i o n i z a t i o n p o t e n t i a l shown i n o u r s p e c t r u m  mental  a large  The l o c a t i o n o f t h e  i s based  on t h e e x p e r i -  UV PES v a l u e o f 12.07 eV.  1 0 0  b. O x y g e n K - s h e l l  Excitation.  In g e n e r a l , t h e promotion  o f a core e l e c t r o n  discrete  l e v e l s r e s u l t s i n a number o f p o s s i b l e  electron  configuration  (see Table 3 ) .  final  ( l s a = o x y g e n K) t o  states  f o r a given  The e x c h a n g e s p l i t t i n g  +  between t h e  m u l t i p l e t c o m p o n e n t s c a n be q u i t e of  l a r g e a s shown b y t h e e x p e r i m e n t a l v a l u e 4 2 1.11 eV m e a s u r e d by X - r a y PES f o r t h e s p l i t t i n g b e t w e e n E and £  ion states  produced  The K - s h e l l  by I s i o n i z a t i o n i n m o l e c u l a r  e l e c t r o n energy  T a b l e 5.  The f i r s t  promotion  o f an oxygen l s a e l e c t r o n  2piTg.  oxygen.  l o s s s p e c t r u m o f o x y g e n i s shown i n  F i g u r e 19 a n d t h e e n e r g i e s a n d p o s s i b l e a s s i g n m e n t s  the  o f peaks  are listed i n  d i s c r e t e p e a k o b s e r v e d a t 5 3 0 . 8 eV i s a t t r i b u t e d t o t h e t o t h e lowest u n f i l l e d molecular o r b i t a l ,  The r e s u l t i n g c o n f i g u r a t i o n : (lsa)  (2pu)  1  3  1  (2PTT*)  3  to a  contribute  t o o u r spectrum..  indicating  t h e e x c i t a t i o n o f a number o f v i b r a t i o n a l l e v e l s .  intensity of this  n and a  4  gives r i s e  observed  n s t a t e and o n l y t h e t r i p l e t  state  i s expected t o  T h e o b s e r v e d peak h a s a FWHM o f 0.5 e V ,  peak i s much l a r g e r t h a n t h e h i g h e r e n e r g y  Since the discrete  peaks  3 i n t h e spectrum, e x c i t a t i o n t o t h e n s t a t e i s expected t o g i v e  F o r m a l l y , g and u symmetry does n o t a p p l y t o an oxygen m o l e c u l e w i t h a l o c a l i z e d I s h o l e ( f o r example s e e R e f e r e n c e 1 1 7 ) . f  109  Intensity (arbitrary units) P 01  o CD  cr  -  • • •  m  70  CD 7^  CD  I CO  CO  o  ZT O fD -S  CQ  ro O  CO cn co  o  M  7  T3  fD o r+ -s  o -t) 3 o ro o cr _J  o> -s  CD  M  . 1  I  a  (Q CD 01  o  o X  i.Q rc>  o  -Z8-  b  TABLE 5 ABSOLUTE ENERGIES ( e V ) , R E L A T I V E ENERGIES AND POSSIBLE ASSIGNMENTS OF PEAKS OBSERVED  PEAK  IN THE K-SHELL ENERGY LOSS SPECTRUM OF 0 . 2  ENERGY  AE  ASSIGNMENT ORBITAL  1  530.8  0  2  539.2  8.4  3  541.9  11.1  2  9  STATES  %  3s  V(2),  0 g  3p, 3 d , e t c  K-EDGE  0  543.1  12.3  CO  K-EDGE  0  544.2  13.4  CO  V V  a O n l y t h e f i n a l o r b i t a l i n v o l v e d i n t h e e x c i t a t i o n and m o l e c u l a r s t a t e s d i p o l e c o n n e c t e d t o t h e g r o u n d £ g s t a t e have been i n c l u d e d . H o w e v e r , t h e E s t a t e h a s b e e n i n c l u d e d s i n c e t h e -+-/->+ r u l e d o e s not apply t o e l e c t r o n impact f o r non-axial s c a t t e r i n g . * 3  3  5 1  b A s d e t e r m i n e d by X - r a y  PES.  3 2  1  -89-  the l a r g e s t c o n t r i b u t i o n t o the high energy a u t o i o n i z a t i o n l i n e s  observed  32 35 i n t h e oxygen I s A u g e r s p e c t r u m e x c i t e d by e l e c t r o n impact second and t h i r d respectively  '  p e a k s w i t h m a x i m a a t 539.2 a n d a p p r o x i m a t e l y  are probably  . The 541.9 eV  associated with the excitation o fa Isa electron  t o t h e 3 s , 3p a n d h i g h e r e n e r g y R y d b e r g i s d e r i v e d from t h e observed  levels.  A q u a n t u m d e f e c t o f 1.25  e n e r g y p o s i t i o n o f t h e second peak and t h e  32 experimental  X - r a y PES v a l u e s  f o r t h e K-edges.  The m a g n i t u d e o f t h e  quantum d e f e c t i s s i m i l a r t o t h a t d e d u c e d f r o m t h e r e p o r t e d energy f o r t h e corresponding 2pir  + 3sag,  valence  positions o fthe  t h e X - r a y PES v a l u e s On  transition  i n oxygen  102  ,  f o r which 6 = 1.1. 4 -  The  shell  excitation  3 2  z  2 and z  o f 543.1 eV a n d 544.2 eV r e s p e c t i v e l y .  the basis o f t h e core analogy  in molecular  K-edges i n o u r s p e c t r u m a r e b a s e d o n  m o d e l we e x p e c t  oxygen t o produce an " O F - l i k e " s p e c i e s .  K-shell  excitations  The e x i s t e n c e o f  t h e o x y g e n m o n o f l u o r i d e r a d i c a l has b e e n f i r m l y e s t a b l i s h e d b y m a t r i x techniques ' , and r e c e n t l y , g a s phase d e t e c t i o n has been c l a i m e d 1 1 0  An  ionization  agrees  1 1 2  potential  w i t h t h e v a l u e o f 13.1 115  2  average energy o f t h e 3  1 n and  ionization  ± 0.3 eV e s t i m a t e d  .  114 , which  from t h e appearance  + o f OF f r o m 0 F . , From t h e e n e r g y d i f f e r e n c e b e t w e e n t h e 4 2 K+  potential  the  o f 13.1 ± 0.5 eV h a s b e e n c a l c u l a t e d 114  1 1 3  n 0  2  z~ a n d z~ 0  2  s t a t e s and t h e e s t i m a t e d  energy o f  K* 2  s t a t e s , we d e d u c e a v a l u e o f 12.7 ± 0.4 eV f o r t h e  potential  with the theoretical  o f OF.  Our e s t i m a t e d  and"experimental"  value  values.  i s i n reasonable  agreement  -90-  CHAPTER  SIX  TRIATOMIC MOLECULES  6.1.  Carbon  D i o x i d e and N i t r o u s  6.1.1. C a r b o n The and  Dioxide.  carbon d i o x i d e molecule  has t h e e l e c t r o n  <V  (  2  The  la  l  u  a  and l a  u  )  2  (  V  2  (  orbitals  { 2 a  2  la ,la  and  a r e t h e r e f o r e nonbonding.  electrons  energy  (%>  <  2  3 o  u  ) 2  ( 1  -u)  f r o m t h e c a r b o n Is  are essentially  localized  To i n d i c a t e t h e i r  'v  +  atomic atomic  on t h e i r  spectrum  orbital.  nuclei  "atomic" character, the  a r e d e s i g n a t e d oxygen K - s h e l l  A valence shell  a. V a l e n c e S h e l l  has a l s o  and c a r b o n K-shell  been r e c o r d e d .  Spectrum.  valence shell  shown i n F i g u r e 20.  state  4  We h a v e s t u d i e d b o t h t h e c a r b o n a n d o x y g e n  spectra.  The  f  i s formed  these o r b i t a l s  electrons. loss  orbitals  g  filling  electronic  a r e l i n e a r c o m b i n a t i o n s o f o x y g e n Is  The  u  a n d 2a  V  orbital  g  g  i s l i n e a r i n i t s ground  configuration  o r b i t a l s , w h i l e t h e 2a  K-shell  Oxide.  energy  loss  spectrum o f carbon d i o x i d e i s  The o b s e r v e d l o c a t i o n s o f peaks  are consistent  with  5 49 118 high r e s o l u t i o n e l e c t r o n  impact s p e c t r a  5  *  and a " h i g h "  resolution  119 optical  spectrum  is associated with  . lTT  The b r o a d g  peak A w i t h  a maximum a t a p p r o x i m a t e l y 9 eV  2-IT^ ( T T * ) t r a n s i t i o n s .  In the photoabsorption  R e c e n t q u a n t u m m e c h a n i c a l c a l c u l a t i o n s o n t h e I s - h o l e s t a t e s o f t h e O2 m o l e c u l a r i o n " ' 7 h a v e b e e n i n t e r p r e t e d as an i n d i c a t i o n t h a t t h e c o r e h o l e s are l o c a l i z e d .  +  1  ELASTIC  T  0  •  1  10  "  1  20  Energy Loss FIGURE 20.  Valence s h e l l  energy  1  1  1  30  40  (eV)  l o s s spectrum  o f carbon  p  dioxide.  -92-  spectrum  1 1 3  region;  one  transition ^->-  , two  o v e r l a p p i n g bands have been o b s e r v e d  ( p e a k maximum a t 8.41  and  the second  ^iig t r a n s i t i o n .  but  i n C2  energy peaks i n our C  (12.3  transitions  eV)  and  B E  (10.9  (16.0  eV)  ^ i * -*  energy 1  assigned  eV,  eV)  ^  u  (^Bg) the  symmetry,  component.  s h o u l d e r ^ 11.2  are probably  A  to  are forbidden i n  e a c h have an a l l o w e d  spectrum;  e V ) , D (13.3  assigned to the  ( p e a k maximum a t 9.31  Both  symmetry they  V  eV)  in this  The  higher  eV),  associated with 112  Rydberg t r a n s i t i o n s . has  The  been i n t e r p r e t e d on  of the f i r s t imental b.  the b a s i s o f a Rydberg assignment.  ionization  value  7 8  '  1 2 0  potential  o f 13.77  Carbon K - s h e l l The  higher r e s o l u t i o n e l e c t r o n energy l o s s  K-shell  shown i n F i g u r e 20  The  spectrum location  i s b a s e d on  the  eV.  Excitation. spectra of the diatomic molecules  (Chapter  Five)  w e r e i n t e r p r e t e d on  the b a s i s of a simple  "equivalent core" model, i n  which a hole i n the  K-shell  t o h a v e t h e same e f f e c t on  potential  experienced  c h a r g e on  the  by  relative  (with  i s considered  the outer valence  e l e c t r o n s as one  more  model  is valid  f o r carbon  e n e r g i e s o f the peaks observed  d i o x i d e , we  dioxide. factors i.  i n the e x c i t a t i o n  i n the carbon  o f the 6a,  e l e c t r o n t o Rydberg  However, b e f o r e c o m p a r i n g d a t a f r o m t h e should The  positive  would  K-shell  r e s p e c t to the lowest energy d i s c r e t e peak) t o reproduce  observed  the  nucleus.  I f the core analogy the  exper-  be  two  expect  spectrum  those  states in nitrogen  molecules,  several  considered.  ground e l e c t r o n i c  s t a t e of the n i t r o g e n d i o x i d e  molecule,  2 X A,,  i s b e n t ( t h e e q u i l i b r i u m bond a n g l e  vibrational  excitation  i s 134°)  and  accompanying e l e c t r o n promotion  the extent  of  i s determined  by  -93-  the  overlap  (i.e.  of the final  and i n i t i a l  the Franck-Condon f a c t o r s ) .  electron  Therefore,  i s illustrated  quanta  t h e promotion o f t h e 6a,  i n Figure  ( p a r t i c u l a r l y o f t h e b e n d i n g mode, V 2 ) . 2 1 , where a q u a l i t a t i v e  some o f t h e s t a t e s o f n i t r o g e n coordinate.  I t should  also contributes ii.  dioxide  representation of  h a s b e e n drawn i n t h e b e n d i n g  be n o t e d t h a t t h e i n d e p e n d e n t s t r e t c h i n g  to the vibrational  a r e d e t e r m i n e d by t h e F r a n c k - C o n d o n r e g i o n i s shown i n F i g u r e  state.  This  K-shell  e x c i t e d s t a t e s o f carbon d i o x i d e  those states o f nitrogen  dioxide  coordinate  structure of excited states.  In carbon K - s h e l l e x c i t a t i o n s i n carbon d i o x i d e ,  populations  valence  wavefunctions  t o t h e l i n e a r Rydberg s t a t e s and t h e i o n s t a t e i s e x p e c t e d t o  e x c i t e many v i b r a t i o n a l This  state vibrational  vibrational  of the l i n e a r  ground  2 1 , where i t has been assumed t h a t t h e have t h e same r e l a t i v e e n e r g i e s  as  r e s u l t i n g from t h e promotion o f a 6a,  electron.  In o r d e r energies  t o c o m p a r e t h e two s e t s o f d a t a ,  the nitrogen  dioxide  m u s t be c o r r e c t e d by s u b t r a c t i n g b o t h t h e b a r r i e r t o l i n e a r i t y 121  of nitrogen the  dioxide  upper s t a t e s .  imately  (1.83 eV)  and t h e e x c e s s v i b r a t i o n a l  The l a t t e r q u a n t i t y c a n be e s t i m a t e d  population  as b e i n g  of  approx-  1.6 eV on t h e b a s i s o f t h e d i f f e r e n c e b e t w e e n t h e v e r t i c a l  (11.25 e V )  1 2 2  and t h e a d i a b a t i c  ation  potential of nitrogen  apply  t o t h e Rydberg s t a t e s . The  U  9.62 e V )  dioxide.  1 2 3  values  f o r the f i r s t  We e x p e c t a s i m i l a r  ioniz-  correction to  c a r b o n K - s h e l l e n e r g y l o s s s p e c t r u m o f c a r b o n d i o x i d e i s shown i n  Figure  22 a n d t h e e n e r g i e s  listed  i n T a b l e 6.  of nitrogen  dioxide  and t e n t a t i v e a s s i g n m e n t s o f s t r u c t u r e a r e  Although complete data  on t h e v a l e n c e  i s not a v a i l a b l e , the corrected  shell excitation  relative  energies  of  -94-  e  l35°l8Cfl35°  C0  2  Carbon K-Excitation FIGURE 2 1 . Q u a l i t a t i v e r e p r e s e n t a t i o n ( n o t t o s c a l e ) o f t h e p o t e n t i a l e n e r g y s u r f a c e s , as a f u n c t i o n o f t h e b e n d i n g c o o r d i n a t e , o f some s t a t e s o f n i t r o g e n d i o x i d e a n d K - s h e l l e x c i t e d carbon dioxide. Mote: These i n d i c a t e t h e n a t u r e o f t h e energy c o r r e c t i o n s w h i c h w o u l d have t o be a p p l i e d i n o r d e r t o c o m p a r e d a t a f r o m t h e two m o l e c u l e s o n t h e b a s i s o f t h e core analogy model.  1.0 H  (a)  4-1  3  mmm  c  T  1  293  1  1—  295  4-1  5 0.5 0)  c  x8  (c^of*  k-edge  co  2  C -shell  1 2 34 : i; I  290  k  II T  300  FIGURE 22. T h e c a r b o n K - s h e l l  T  310 Energy Loss energy  320 CeV)  l o s s spectrum o f carbon  dioxide.  330  tn I  TABLE  6  ABSOLUTE  ENERGIES (eV),  CARBON AND  RELATIVE  E N E R G I E S AND  OXYGEN K-SHELL SPECTRA OF  PEAK  P O S S I B L E ASSIGNMENTS OF PEAKS OBSERVED I N THE  CARBON D I O X I D E .  CARBON K- SHELL ENERGY  1  290.7  2  292.7  3  i(294.5) I 294.9  AE  CALCULATED VALUE  0 d  OXYGEN K-•SHELL ENERGY  3  -  (535.4  2.0  294.1  (MASKED  (3.8) 4.2  294.9 295.2  538.7  CALCULATED ENERGY  AE  3  0  _  537.7  3.3  296.0 4 K-EDGE  E  296.3  5.6  297.5  6.8  5  ^301  'vlO  6  <314  ^13  296.2 296.4  539.9  -  4.5  541.1  1TT  u  ?  0  POSSIBLE . ASSIGNMENT  3sa  538.5 538.8  3 D C T  539.6  4sa  g  U  3DTT u v  539.8 539.9  9 u  v  -  5.4  (6a, + lb,)  00  lir  + SHAKE-UP  ITT  + SHAKE-UP  C a l c u l a t e d u s i n g t h e R y d b e r g f o r m u l a ; E = A - R / ( n - < 5 ) , where E i s t h e e x c i t a t i o n e n e r g y ; A , t h e K - s h e l l i o n i z a t i o n e n e r g y o f C 0 ; R, t h e R y d b e r g c o n s t a n t ; n , t h e q u a n t u m number; a n d 6 , t h e quantum d e f e c t . The quantum d e f e c t s used were t h o s e f r o m t h e v a l e n c e s h e l l Rydberg s e r i e s o f carbon d i o x i d e w i t h ; 5 ( n s a ) = 1 . 0 , fi(npa)=0.71, a n d 6(np7r)=0.56. 2  n  n  2  1 2 4  O n l y t h e f i n a l o r b i t a l / s i n v o l v e d i n t h e K - e x c i t a t i o n s h a v e been i n c l u d e d . I f the hole states are l o c a l i z e d ( s e e t e x t ) t h e g a n d u d e s i g n a t i o n s s h o u l d be o m i t t e d f o r o x y g e n K - s h e l l e x c i t a t i o n s s i n c e t h e m o l e c u l e w o u l d have C symmetry. c.  The  intense f i r s t  d. T h i s e x t r a  discrete  peak p r o b a b l y  includes  peak i s f r o m a h i g h e r r e s o l u t i o n  scan.  e. T h e s e v a l u e s a r e f r o m X - r a y PES m e a s u r e m e n t s .  1 2 5  the f i r s t  Rydberg t r a n s i t i o n  Figure 2 (insert a ) .  (see the t e x t ) .  1  cn 1  -97-  the  identified  Rydberg  states ^  which  1  converge  do n o t m a t c h t h e d a t a f r o m t h e c a r b o n K - s h e l l diagram, Figure  23).  at  dominated  by t h e l o w e s t e n e r g y  (2a )  electron  peak.  as a r i s i n g  ion state  (see the c o r r e l a t i o n  i s very s i m i l a r  and carbon monoxide, w i t h  290.7 ± 0.2 eV i s i n t e r p r e t e d  K-shell  spectrum  Q u a l i t a t i v e l y , t h e spectrum  previously observed f o r nitrogen structure  to the f i r s t  the discrete  T h i s i n t e n s e peak  observed  from the promotion o f a carbon  t o the lowest u n f i l l e d molecular o r b i t a l  g  t o those  o f carbon  d i o x i d e , t h e 2TT . U  C0  (la )  o  g  < For  linear  This and  l C T  (la )  2  g)  u  2  ( 1  °u  (2a )  2  ) 2  g  (  a  g  )  . . . .  ' ' ' '  ]  g  V  4  ]  4  ( 2 l T  u  } 1  +  G  '  V i s doubly  u  i s removed i n b e n t s t a t e s w i t h  t h e l b , R e n n e r - T e l l e r components f o r the equilibrium  carbon  (  (lrr ) , X Z  s t a t e s o f c a r b o n d i o x i d e , t h e 2TT o r b i t a l  degeneracy  ation  K-shell  electron  a p p r o x i m a t e l y 135°.  is  2  2  t h e p r o d u c t i o n o f t h e 6a-,  ( s e e F i g u r e 21).  bond a n g l e o f a s t a t e  (2a  g  becomes 2 a , i n C  This estimation  a n g l e o f 134° f o r t h e g r o u n d  degenerate.  A reasonable estim-  produced  by e x c i t i n g a  symmetry) t o t h e 6a, o r b i t a l  2y  i s based on two f a c t s ;  state o f nitrogen  dioxide,  a bond  a n d a bond a n g l e o f 120  122° f o r t h e f i r s t from t h e t r a n s i t i o n excited  state  valence excited 1 TT^(4b^)  resulting  ln^(6a,).  1  the 4b  g  -> 2TT  u  2  which  carbon  results  K-shell  2 a , -> 6 a , i s e x p e c t e d t o h a v e a  orbital  i s now f i l l e d  a n d on t h e  ?o  diagram  this  orbital  peak w h i c h we h a v e a s s o c i a t e d w i t h 2a  The a n a l o g o u s  from t h e t r a n s i t i o n  bond a n g l e l a r g e r t h a n 122° s i n c e  b a s i s o f a Walsh  s t a t e o f carbon dioxide  favours larger  the transitions  ( 6 a , + l b , ) h a s a FWHM o f 0.9 eV ( e l a s t i c  t h a t a number o f v i b r a t i o n s  are excited.  bond a n g l e s .  2 a , -> 6 a ,  The  and  peak 0.5 e V ) i n d i c a t i n g  In the bending  coordinate,  -98-  n  Excited Orbital 3po 4pa 3sa 3pn 4pn  k  +  c o  2~C>k  N0 - O 2  k  COo- Ci »  •  ••  •  ••  •  ••  •  NNO-N NNO-N  k  k  NO2  corrected 1  0 FIGURE 23.  1  5  1  10  eV  C o r r e l a t i o n o f the o b s e r v e d peaks i n the K - s h e l l energy l o s s s p e c t r a o f c a r b o n d i o x i d e and n i t r o u s o x i d e ( b o t h c a r b o n and o x y g e n K - s h e l l s ) The d a s h e d l i n e s r e p r e s e n t t h e e x p e c t e d p o s i t i o n s o f u n r e s o l v e d p e a k s (see the t e x t ) . The r e l a t i v e e n e r g i e s ( c o r r e c t e d ) o f appropriate s t a t e s from the v a l e n c e s h e l l s p e c t r u m o f n i t r o g e n d i o x i d e have a l s o been i n c l u d e d f o r c o m p a r i s o n .  -99-  maximum F r a n c k - C o n d o n o v e r l a p l i n e a r l b , component.  i s e x p e c t e d f o r t h e 0 ->- 0 t r a n s i t i o n t o t h e  Therefore,  most o f t h e observed i n t e n s i t y i s p r o b a b l y  associated with  v i b r a t i o n a l e x c i t a t i o n o f t h e l o w e r members o f t h e l i n e a r  component w h i l e  some o f t h e i n t e n s i t y o n t h e l o w e n e r g y s i d e o f t h e p e a k ,  be e x c i t a t i o n o f t h e h i g h e r  could  v i b r a t i o n a l l e v e l s o f t h e bent  component b e l o w t h e b a r r i e r t o l i n e a r i t y . Auger emission and  therefore  i n a much s h o r t e r  the  s t a t e s decay  time than t h a t r e q u i r e d  H o w e v e r , s u c h t r a n s i t i o n s may s t i l l  are f i n i t e ,  two w e l l s ) .  f o ra vibration  occur since  In Auger emission  studies  of the ejected  electrons  between  the K-shell excited states are  are higher  ion states.  The  t h a n t h e maximum e n e r g y w h i c h  be t a k e n up by a " n o r m a l " A u g e r e l e c t r o n .  can  t h e wave-  although small, a t the l i n e a r p o s i t i o n ( i . e .  o b s e r v e d when t h e y d e c a y by a u t o i o n i z i n g t o s i n g l y c h a r g e d energies  by  t h e e x c i t e d m o l e c u l e does n o t r e a c h t h e bent e q u i l i b r i u m  conformation. functions  These " h o l e "  6a-,  In the carbon K-shell  Auger  35 spectrum o f carbon d i o x i d e  268.2 ± 0.5 at  first the  eV h a v e b e e n o b s e r v e d .  290.7 ± 0.2  charged  , two h i g h  eV i s t h e i n i t i a l  that  second p r q b a b l y represents  of a valence e l e c t r o n . associated with  neutral  ionization  the f i r s t  excited state  o c c u r a t 18.1 78  necessary  implies  t o remove a 3a  state  that  singly  eV.  The  electron,  with  eV a n d  discrete  eV a n d 22.5  shake-up i n c o n j u n c t i o n  The h i g h e r  272.6 ± 0.5  while  the ionization  energy d i s c r e t e peaks i n t h e spectrum a r e  s t a t e s p r o d u c e d by p r o m o t i n g a c a r b o n K - s h e l l e l e c t r o n t o  Rydberg o r b i t a l s ,  first  Assuming t h a t  i o n states o f carbon dioxide  energy agrees w i t h  energy peaks a t  limit.  producing s t a t e s which converge t o the carbon The much l o w e r i n t e n s i t y , w i t h  d i s c r e t e peak, i s e x p e c t e d , s i n c e t h e f i r s t  respect  K-shell  to that of the  d i s c r e t e peak i s a s s o c i a t e d  -100-  with  t h e promotion  with a principal  of a K-shell  e l e c t r o n to a valence molecular  q u a n t u m number o f t w o , w h i l e t h e h i g h e r e n e r g y  p e a k s a r e a s s o c i a t e d w i t h h i g h e r q u a n t u m number ( n = 3,4) Two o r b i t a l s  orbital  which a r e sometimes i n c l u d e d i n t h e v a l e n c e  discrete  Rydberg shell,  orbitals.  t h e 5a  g  119 and  t h e 4a , u  a r e expected  Energy v a l u e s  calculated  Rydberg s e r i e s  o f carbon  t o correspond  with outer  Rydberg o r b i t a l s  u s i n g quantum d e f e c t s f r o m  the valence  ,  shell  124 dioxide  and a s e r i e s  limit of  297.5 eV a s  125 determined  by X - r a y PES  the spectrum at  6).  The l a r g e s t d e v i a t i o n i s found  f o r t h e peak  292.7 e V , w h i c h i s a s s i g n e d t o t h e f i r s t R y d b e r g t r a n s i t i o n , 2a -> 3sa . g  However, t h i s shell  i s expected  and i s p r o b a b l y  optically in  (see T a b l e  , and i n good agreement w i t h peaks o b s e r v e d i n  since the  i s very  g  n o t a " t r u e " Rydberg o r b i t a l .  f o r b i d d e n by t h e s e l e c t i o n  o u r experiment  3sa o r b i t a l  i f the f i r s t  spectroscopy  rule g  electron  impact  observed  even a t h i g h e r e n e r g i e s where t h e f i r s t  close t o the valence  This  transition i s  g and i s a l s o  Born a p p r o x i m a t i o n  g  i svalid.  forbidden However, i n  symmetry f o r b i d d e n t r a n s i t i o n s  have  been  Born approximation i s  53 normally  expected  t o be v a l i d .  proposed t h e s e l e c t i o n imation the  (seeReference  126).  In t h i s  m a n i f o l d and t h e r e f o r e d e v i a t i o n s from  expected.  and  t h a t d e v i a t i o n s from  and L a s s e t t r e the f i r s t  The t h i r d  -> 3pa , 3p-rr . u  indicates  u  peak a t  approx-  s t a t e belong t o  both  totally s t a t e s have a  t h e Born t h e o r y  a r e t o be  294.9 eV i s a s s i g n e d t o t h e R y d b e r g t r a n s i t i o n s  A h i g h e r r e s o l u t i o n s c a n i s shown i n F i g u r e 22 ( i n s e r t a )  that this  defect calculations  case,  have  Born  same s y m m e t r y s p e c i e s , i . e . t h e d e v i a t i o n d e p e n d s upon a  t e r m  20^  Skerbele  a r e l a r g e s t when t h e e x c i t e d s t a t e a n d t h e g r o u n d  symmetric o p e r a t o r 1  rule  In f a c t ,  b a n d i s composed o f a number o f p e a k s .  a r e i n good agreement w i t h t h e s e  The q u a n t u m  assignments,  giving  -101-  294.9 eV f o r t h e 3pa p e a k a n d 295.2 eV f o r t h e 3 p T r p e a k . I t  values o f is  u  possible  transition band w i t h  that 2a ^  u  t h e h i g h e n e r g y s h o u l d e r has a c o n t r i b u t i o n  -> 4sa  (calculated  g  a maximum a t  value  296.0 e V ) .  from t h e  The f o u r t h  296.4 eV i s p r o b a b l y a s s o c i a t e d  discrete  2a -> 4p  with  g  Rydberg t r a n s i t i o n s . A value of  297.5 eV h a s b e e n o b t a i n e d f o r t h e c a r b o n K - s h e l l  binding  125 e n e r g y by X - r a y PES  .  An i o n i z a t i o n p o t e n t i a l  u s i n g t h e c o r e a n a l o g y model peak i n t h e c a r b o n K - s h e l l ization the  potential  experimental  K-edge  representing  one K - s h e l l two e l e c t r o n i.  ii.  electron  a n d o n e o r more v a l e n c e s h e l l  K-shell  transitions electrons.  i n con-  where t h e  2  e x c i t a t i o n and i o n i z a t i o n ;  o t h e r remains behind  The f o l l o w i n g  contributions;  e x c i t a t i o n , d e s i g n a t e d by (C ~ 0 )  K-l denotes a hole i n t h e carbon  with  involving  i . e . shake-up o f a v a l e n c e e l e c t r o n  K-shell.  involving  an e l e c t r o n  c a r b o n K- a n d v a l e n c e s h e l l s , w h e r e o n e o f t h e e l e c t r o n s the  agreement  i s o b s e r v e d above t h e  t r a n s i t i o n s a r e e x p e c t e d t o make t h e l a r g e s t  with  superscript  Structure  discrete  value f o r the ion-  d i o x i d e ) which i s only i n f a i r  ( s e e F i g u r e 23).  double e x c i t a t i o n ;  junction  spectrum, plus a corrected  a variety of multiple  electron  298.0 eV i s o b t a i n e d  (energy o f t h e onset o f t h e f i r s t  of nitrogen value  of  i n a higher u n f i l l e d  orbital,  from both t h e  i s e j e c t e d and  d e s i g n a t e d by  (c -'o r. K  2  The b r o a d s t r u c t u r e is  associated K  i.e. as  2  discrete  discrete  i n Region  structure  I I o f t h e carbon  a r i s i n g from double  K-shell  spectrum  excitations,  1  (C " 0 ) that  with  observed  states.  The i n t e n s i t y o f t h e s e b a n d s i s r o u g h l y t h e same  o f t h e K-jump a n d a f e w p e r c e n t o f t h e i n t e n s i t y o f t h e f i r s t peak a t  290.7 eV. T h i s s u g g e s t s t h a t m o s t o f t h e i n t e n s i t y a r i s e s  -102-  from the shake-up  of valence electrons  p r o m o t i o n t o t h e 2TT  i n conjunction  molecular o r b i t a l .  with  Discrete states  promotion o f a carbon K - s h e l l e l e c t r o n t o t h e Rydberg intense  a n d t h e r e f o r e we  to c o n t r i b u t e utions  little  e x p e c t shake-up  with  t h e l o w e s t shake-up  ization,  as d e t e r m i n e d by X - r a y PES  K-edge.  The  125  , should  s t r u c t u r e observed i n Region  these t r a n s i t i o n s structure.  with  Contrib-  are  2  state associated  the  a r e much l e s s  K 1 +* (C ~ 0 ) states  s t r u c t u r e from  K-shell  r e s u l t i n g from  orbitals  i n t e n s i t y t o t h e o b s e r v e d shake-up  to the i n t e n s i t y of t h i s  improbable since  associated  carbon  K-shell ion-  s t a r t a t 10.8  I I I of the carbon  eV a b o v e K-shell  the  spectrum  K—1 is  i d e n t i f i e d with  K— 1 (C " 0 ) X - r a y PES 2  peak.  t h e o n s e t s o f i o n i z a t i o n t o (C " 0 )  states.  2  These  s t a t e s s h o u l d g i v e r i s e t o a s e r i e s o f shake-up peaks i n t h e s p e c t r u m on t h e l o w k i n e t i c e n e r g y s i d e o f t h e c a r b o n K - s h e l l  In F i g u r e  22 we  h a v e drawn t h e s h a k e - u p  peaks  associated  with  carbon  125 K - i o n i z a t i o n o f c a r b o n d i o x i d e o b s e r v e d by S i e g b a h n  et a l .  and  Carlson  81 et a l .  .  I t c a n be s e e n t h a t t h e e n e r g y  spectrum c o r r e l a t e s with observed  i n the X-ray  PES  t h e shake-up  region  peaks.  The  experiments i s roughly  peak, w h i l e  the i n t e n s i t y o f s t r u c t u r e i n Region  least twice  t h a t o f t h e K-jump.  Region  The  o f t h e band o b s e r v e d total  thresholds  (C ~ 0 ) 2  I I I of our spectrum  l a r g e shake-up  s t a t e s , which converge  [ c f . the nitrogen  K-shell  intensity  20% o f t h a t o f the K - s h e l l  to each  i s at  i n t e n s i t y observed i n  I I I o f o u r s p e c t r u m p r o b a b l y has a s i g n i f i c a n t  series of  shake-up  i n our  c o n t r i b u t i o n from  of the i n d i c a t e d  spectrum of m o l e c u l a r  nitrogen  (5.1.1). c. O x y g e n K - s h e l l E x c i t a t i o n . The  oxygen  shown i n F i g u r e  K - s h e l l energy  l o s s spectrum of carbon d i o x i d e i s  24 and t e n t a t i v e a s s i g n m e n t s o f o b s e r v e d s t r u c t u r e s  are  a  L O  HI  A  co  2  O -shell  J2  k  c  3  co^" )^ 1  >»  I  5 0.5 (0  O  co  co  * ^ * v w ^ ,  c a>  540  550 Energy  560 Loss (eV)  r  570  FIGURE 24. The oxygen K-shell energy loss spectrum of carbon d i o x i d e .  x  2  -1 CH-  listed  i n T a b l e 6.  The i n t e r p r e t a t i o n o f t h e s p e c t r u m i s a n a l o g o u s t o  that o f t h e carbon K-shell in  detail.  i n t e n s i t y on t h e h i g h  K-shell  electron)  and  shown i n F i g u r e  u  remaining  formally  some o f t h e l i n e  This  discussed  b r o a d e n i n g and with  t r a n s i t i o n i n v o l v i n g a carbon has a  orbital  symmetry  Q  with  the p o s s i b i l i t y  Rydberg t r a n s i t i o n .  that the f i r s t  The a s s i g n m e n t s o f  d i s c r e t e peaks f o l l o w t h o s e o f t h e c a r b o n K - s h e l l  good agreement w i t h  the lowest  The r e s u l t s o f t h e c o r r e l a t i o n diagram  the f i r s t  23.  orbital,  transition i s optically  one oxygen K - s h e l l  23 a r e c o n s i s t e n t  by T a b l e 6 a n d F i g u r e in  -> 3sa .  symmetry.  d i s c r e t e peak i n c l u d e s the  that  t o the corresponding  since  t h e o t h e r has a  be  In a d d i t i o n t o t h e promot-  energy s i d e o f t h e peak i s a s s o c i a t e d  energy Rydberg t r a n s i t i o n , la (in contrast  will  t o t h e t w o c o m p o n e n t s o f t h e 2TT  electron  6a, and t h e l b , , i t i s p o s s i b l e  allowed  differences  p e a k FWHM o f 0.55 eV.  the elastic  i o n o f an oxygen K - s h e l l the  only  d i s c r e t e p e a k a t 535.4 ± 0.2 eV h a s a FWHM o f 1.4 eV  The f i r s t  compared w i t h  and t h e r e f o r e  a s shown  The energy p o s i t i o n s o f t h e s e d i s c r e t e peaks a r e  t h o s e e x p e c t e d on t h e b a s i s  of calculations  involving  124 quantum d e f e c t s  from t h e valence s h e l l  l i m i t o f 541.1 e V  series  spectra  o f carbon d i o x i d e  and a  (seeTable 6 ) .  1 2 5  35 In t h e o x y g e n K - s h e l l  Auger spectrum o f carbon d i o x i d e  been o b s e r v e d a t 511.3 ± 0.3 eV w h i c h to a "normal" Auger process.  i s t o o high  The assumption t h a t  i n energy t o a t t r i b u t e the f i r s t  o b s e r v e d a t 535.4 eV i n o u r s p e c t r u m , i s t h e i n i t i a l implies  the existence  , a peak h a s  discrete  state  excited  state,  neutral  o f a s i n g l y charged s t a t e o f carbon d i o x i d e  a t an  78 e n e r g y o f 24.1 eV.  T h e c l o s e s t known i o n s t a t e o f c a r b o n d i o x i d e  19.4 eV, a r i s i n g f r o m t h e e j e c t i o n o f a 4 a state  g  electron.  i s at  Therefore, the final  p r o b a b l y a r i s e s from t h e shake-up o f a v a l e n c e e l e c t r o n  i n conjunction  -105-  with  the i o n i z a t i o n  of a second  A v a l u e o f 541.1 energy into  by X - r a y PES  two  where  125  eV  has  .  Above t h i s  +*  K-1  " )  states  extends from t h i s  K-shell  been o b t a i n e d f o r t h e oxygen edge t h e s p e c t r u m  I I e x t e n d s f r o m t h e K-edge  r e g i o n s ; Region  (CO^  valence electron.  has  divided  up t o t h e l o w e s t e n e r g y 81125 ' , and  h a v e b e e n o b s e r v e d by X - r a y PES  p o i n t t o the high energy  been  while  I I we  would  i n Region  (CO2  "  )  , and  spectrum). relative  expect to observe d i s c r e t e e x c i t a t i o n s ,  I I I the s t r u c t u r e s continuum  The  states,  intensities  I n F i g u r e 24 we  arising  from shake-up  ~  o f shake-up  to the i n t e n s i t i e s  small.  probably arise (CO2  )  have i n d i c a t e d  6„ 1.2. The  nitrous  the l i n e a r ground (la) The  la  2  (2a)  orbital  orbitals  "I  electronic (4a)  has a l s o  appear  to  be  states  '  .  The  broad r e g i o n o f t h e shake-up  structure peaks.  (5a)  2  2  (6a)  the e l e c t r o n i c (7a)  2  t h e oxygen  K-shell  K-shell  2  energy  4  loss  We  V.  4  orbital  and  t h e 2a a n d  have s t u d i e d  spectra.  and  configuration  ( I T T ) (2TT) ,  K-shell  orbitals.  carbon d i o x i d e  A valence  both  3a  the  shell  been r e c o r d e d .  a. V a l e n c e S h e l l The  i n Region I I ,  i o n i z a t i o n o f carbon  correlates with  s t a t e has  i s essentially  the oxygen  states,  pc  oxide molecule i s isoelectronic with  2  states,  (M).  represent nitrogen  n i t r o g e n and spectrum  I I I of our spectrum  (3a)  2  K-shell  a s s o c i a t e d w i t h oxygen  N i t r o u s Oxide  " )  t h e energy p o s i t i o n s where  d i o x i d e have b e e n o b s e r v e d by X - r a y PES i n Region  1 **  K-shell  ( c f . the carbon  s t r u c t u r e s observed  Q"l  observed  (CO2  Thus i n  from both d i s c r e t e  I I I and t h e K-jump,  o f Region  Region I I I  l i m i t o f the spectrum. K—  Region  binding  Spectrum.  valence shell  energy  loss  spectrum o f n i t r o u s  o x i d e i s shown  -106-  in  F i g u r e 25.  spectra  4 9  The  ' ^' 1  1 1 9  locations  .  The  o f peaks a r e c o n s i s t e n t w i t h h i g h e r  weak b a n d w i t h a maximum a t ^ 7.0  p r o b a b l y a s s o c i a t e d w i t h a TT -* TT* t r a n s i t i o n , in  the carbon  dioxide valence shell  spectrum  analogous  eV  resolution  (peak  A)  is  to the f i r s t  ( s e e F i g u r e 20).  band  In a  higher  119 resolution  photoabsorption  at  6.81  C  symmetry, both  s  eV  has  e x p l a i n s why  been a s s i g n e d  , a b r o a d , weak band w i t h a maximum -> ^A t r a n s i t i o n .  to the f o r b i d d e n ,  R e n n e r - T e l l e r c o m p o n e n t s , A'  the t r a n s i t i o n  peaks observed E (12.3  spectrum  i s observed B (8.5  i n our spectrum,  e V ) , F ( 14.1  eV), G  (14.8  and  A",  are allowed,  i n our spectrum. eV), C  e V ) , H (16.0  probably a s s o c i a t e d w i t h Rydberg t r a n s i t i o n s .  (9.6  In  The  which  higher  energy  e V ) , D(11.2 e V ) ,  eV)  and  I (18.5  A Rydberg  eV)  are  interpretation  of  118 the h i g h e r r e s o l u t i o n  although in The  electron  impact  i n the photoabsorption  our spectrum  has  spectrum  ionization 78  terminal  suggested,  , t h e peak c o r r e s p o n d i n g  potential  (a  -> ^JI  shown on o u r  and  to B  -»- T T * ) .  spectrum  o f 12.89  is  eV.  assume t h a t t h e c o r e e l e c t r o n s a r e l o c a l i z e d 3a m o l e c u l a r o r b i t a l s  nitrogen K-shell  core analogy  been  120  b a s e d on t h e e x p e r i m e n t a l v a l u e ' b. N i t r o g e n K - s h e l l E x c i t a t i o n .  n u c l e i , t h e 2a  has  been a s s i g n e d t o t h e t r a n s i t i o n  l o c a t i o n of the f i r s t  I f we  spectrum 119  orbitals  represent central  respectively.  On  their  nitrogen  the b a s i s of  model, e x c i t a t i o n of a terminal n i t r o g e n K-shell  n i t r o u s oxide should produce s t a t e s analogous  on  and  the  electron  in  to appropriate s t a t e s of  nitrogen dioxide. The and  nitrogen K-shell  tentative  analogous  energy  peak a s s i g n m e n t s  loss  spectrum  are l i s t e d  i s shown i n F i g u r e  i n Table  7.  The  to those p r e v i o u s l y g i v e n f o r the corresponding  26  assignments peaks i n the  are  Energy Loss FIGURE 2 5 .  Valence shell  energy  loss  20  (eV)  30  s p e c t r u m o f n i t r o u s o x i de.  40  k  Terminal  +  k  +  N  Central  N i^JKiiiiilif  y.  /I w I I H I I 1 2 345 6 7 1 r400 410  420  "T430  Energy Loss FIGURE 2 6 .  The n i t r o g e n  K-shell  energy l o s s  T  440  CeV)  spectrum o f n i t r o u s  oxide  -109-  TABLE 7 ABSOLUTE  ENERGIES ( e V ) , R E L A T I V E ENERGIES AND P O S S I B L E ASSIGNMENTS OF  PEAKS OBSERVED  I N THE NITROGEN  PEAK  AE  ENERGY  1  401.1  2  404.7  K-SHELL SPECTRUM  ASSIGNMENT  0  3.6  C  N  T  (N  c  (N 3  406.2  5.1  N N  T  T 1  N 407.6  6.5  T  N  3Tr(a'  ^ V A L U E ^  9  + a")  - 3Tr(a' + a")  3so ?  T  1  4  OF NITROUS OXIDE.  T  1  405.1  + 3pa  406.0  3pff  406.2  + 3da 3dTT  406.7 406.9  4sa  407.0  -> 4 p a , 4pTr 4da  407.3 407.5 407.6  4dTr  5 TERMINAL^ K-EDGE 6  408.0  6.9  N  408.5  7.4  Hj -> °°  410.0  8.9  N N N  7  411.2  10.1  c  N  c  + 3s  409.1  a  - 3pa 3PT:  410.0 410.2  3d  410.7  a  3d7T  410.9  4sa  411.0  -> 4 p a , 4 T T 4da  411.3 411.5  P  411.6  4diT  CENTRAL K-EDGE  d  4  1  2  -  5  1  1  '  4  N  c -  a Only t h e f i n a l o r b i t a l / s i n v o l v e d i n t h e K - e x c i t a t i o n s have been i n c l u d e d . b C a l c u l a t e d using t h e Rydberg f o r m u l a . The quantum d e f e c t s used were t h o s e from t h e v a l e n c e s h e l l Rydberg s e r i e s o f n i t r o u s o x i d e ( a v e r a g e d v a l u e s ) w i t h 6 ( n s o ) = 1 . 0 , 6 ( n p a ) = 0 . 6 8 , 6(npir)=0.57, 6 ( n d a ) = 0 . 2 9 a n d 6 ( n d T r ) = 0 . 0 7 c A more a c c u r a t e d e t e r m i n a t i o n i s 3.62 + 0.05 eV ( f r o m a d i f f e r e n t d a t a r u n ) . d These values a r e from X-ray PES e x p e r i m e n t s . 1 2 7  3 2  -110-  carbon d i o x i d e spectra  (see F i g u r e  23), although  nitrous oxide  spectra  will  s p e c t r u m i n t o i t s two c o m p o n e n t  and  the  separation  o f the  any f e a t u r e s which are  unique t o the  s p e c t r u m i s d o m i n a t e d by t h e imately  equal  intensities.  promotion o f a terminal molecular  orbital,  the  first  3TT.  The  o b s e r v e d energy d i f f e r e n c e between the  peak a s s o c i a t e d w i t h  a FWHM o f 1.1 eV w h i l e  the  corresponding  and  o f the  symmetry w i t h the f i r s t nitrogen  the formation  Rydberg t r a n s i t i o n  i nnitrous oxide  associated with  K - s h e l l e l e c t r o n i s masked by t h e  peaks a s s o c i a t e d w i t h  the  nitrogen the  having  central of the  l e v e l s o f both components, a  o f a' and a " c o m p o n e n t s ) .  t h e r e f o r e , may c o n t r i b u t e t o i t s w i d t h . of the  terminal  have  (FWHM o f 0.5 e V ) a n d i n d i c a t e  e x c i t a t i o n t o a number o f v i b r a t i o n a l (TT d e g e n e r a c i e s  with  produced  T h e s e a r e much l a r g e r t h a n t h e w i d t h  scattered electrons  3TT l e v e l  the  peak a s s o c i a t e d w i t h  K-shell a",  unfilled  The two d i s c r e t e peaks  the  peak f r o m e l a s t i c a l l y  approx-  K-shell electron.  two d i s c r e t e s t a t e s  with  h a s a FWHM o f 1.3 eV.  The  lowest  involving a central nitrogen  t h e s e t r a n s i t i o n s i s 3.62 ± 0.05 eV.  nitrogen  parts  e n e r g y peak i s t h e n a s s o c i a t e d  corresponding  d i f f e r e n t widths,  we  e n e r g y peak i s a t t r i b u t e d t o t h e  the  by  spectrum.  K-shell electron t o the  The h i g h e r  transition  nitrous oxide  Therefore,  t w o d i s c r e t e p e a k s w h i c h have  The l o w e s t  nitrogen  overlapped.  spectrum  i s more c o m p l e x s i n c e t w o s e p a r a t e discuss  are  the  the  are  lifted  i nC  I ti s possible  1  g  that  promotion o f a terminal  i n t e n s e s e c o n d d i s c r e t e peak and  I t i s i n t e r e s t i n g t h a t the  corresponding  ion states  widths  ( s e p a r a t e d by  32 4.0  eV),  o b s e r v e d by X - r a y PES  of a terminal 1.05  eV, w h i l e  peak h a v i n g  nitrogen  , f o l l o w the  K-shell electron gives  reverse  The a b s o l u t e  i nthat  r i s e t o a peak w i t h  ionization of a central nitrogen  a FWHM o f 0.95 eV.  order  ionization a FWHM o f  K - s h e l l e l e c t r o n produces a  m a g n i t u d e s o f t h e FWHM's  -Ill-  m e a s u r e d i n t h e two d i f f e r e n t e x p e r i m e n t s a r e n o t d i r e c t l y since  the large natural  l i n e widths o f t h e i n c i d e n t X-rays a r e the main  c o n t r i b u t o r t o t h e FWHM's o f t h e p e a k s a s s o c i a t e d separation  of the higher  associated  with  with  the ion states.  energy d i s c r e t e peaks i n t h e spectrum  each o f t h e n i t r o g e n  o b s e r v e d between c o r r e s p o n d i n g  (i.e.  the promotion o f a terminal  electron K-shell  and t h e o t h e r  with  involving  nitrogen  nitrogen  states  K-shell nitrogen  be f r o m 3.6 t o 4.0 eV.  of the i o n states.  K-edge a n d a r e a s s i g n e d  the central nitrogen  i n this  K-1  Rydberg  the promotion o f a c e n t r a l  o r b i t a l ) should  c l o s e r t o t h e 4.0 eV s e p a r a t i o n  that  peaks  f o r a " t r u e " R y d b e r g t y p e o r b i t a l , we w o u l d e x p e c t an e n e r g y  above t h e t e r m i n a l  (N  associated  e l e c t r o n t o t h e same f i n a l  In f a c t , ting  with  into  energy region  (seeTable 7 ) .  there  The  i n n e r s h e l l s h a s b e e n made on t h e b a s i s  t h a t t h e energy s p l i t t i n g one a s s o c i a t e d  comparable,  could  split-  Peaks 6 and 7 l i e  t o Rydberg t r a n s i t i o n s  However, i t i s p o s s i b l e  be a c o n t r i b u t i o n f r o m d o u b l y  excited  \** NO)  states.  tentatively assign assigned  On t h e b a s i s the other  of the f i r s t  d i s c r e t e peaks.  peaks a r e i n good agreement w i t h  assignment i t i s p o s s i b l e to The e n e r g y p o s i t i o n s o f t h e  t h o s e e x p e c t e d on t h e b a s i s o f  calculated  v a l u e s u s i n g quantum d e f e c t s f r o m t h e v a l e n c e s h e l l s p e c t r a o f 127 32 nitrous oxide a n d s e r i e s l i m i t s a s p r o v i d e d by X - r a y PES (see Table 7 ) . The  energy d i f f e r e n c e between a l l o f t h e c o r r e s p o n d i n g  r a n g e 3.6 t o 4.0 eV a n d t h e s p e c t r u m a s s o c i a t e d with  those o f carbon d i o x i d e  a t 4 0 8 . 0 eV w i t h  the f i r s t  K-shell  ( 2 a ->- 3 s a ) ,  electron  transition  (see Figure  Rydberg t r a n s i t i o n indicates that  involving a terminal  energy r e g i o n  23).  of the intense  nitrogen  with  states  i s within the  each s h e l l  correlates  The a s s o c i a t i o n o f t h e peak involving a central  nitrogen  t h e peak f r o m t h e c o r r e s p o n d i n g  K-shell e l e c t r o n should  s e c o n d d i s c r e t e peak.  be i n t h e  I n some r e g i o n s  i t is  -112-  impossible  to s p e c i f y which Rydberg t r a n s i t i o n s a r e  main i n t e n s i t y  ( f o r e x a m p l e 3p  or 3d).  For  responsible for  i n s t a n c e , i n the  the  valence  shell  127 spectra, a energies  -»-  Rydberg t r a n s i t i o n s  3dTr  of the peaks a s s i g n e d  K-shell  e l e c t r o n do  are u s u a l l y intense  (see  not match those  Figure  23).  observed  Therefore,  i n the energies  of the  not  as  i n view of the The  diatomics.  additional molecular  energy p o s i t i o n s of the  shown i n F i g u r e  26,  are  those  between the  two  e d g e s c a n n o t be  correlation  diagram, Figure  result  terminal  by  assigned  and  with  a n a l o g y model  triatomic  .  the  molecules.  Structures  observed the  d i s c r e t e assignment.  A b o v e t h e c e n t r a l n i t r o g e n K-edge t h e o b s e r v e d s t r u c t u r e s c o r r e s p o n d the  s h a k e - u p and  shake-off  of valence  electrons in conjunction with  e x c i t a t i o n or i o n i z a t i o n of e i t h e r a terminal electron.  The  i s probably 418 (N  eV K _ 1  first  could  (NN  +  i n Figure  K _ 1  (NN  ~ 0)  +  the  states.  eV  i n our  27  K-shell  spectrum  band c e n t r e d The  X-ray P E S  around  p o s i t i o n of  1 2 8  have been  26. Excitation.  oxygen K - s h e l l energy l o s s spectrum of n i t r o u s oxide  shown i n F i g u r e The  states, while  0 ) * s t a t e s as d e t e r m i n e d by  c. O x y g e n K - s h e l l The  NO)  have a c o n t r i b u t i o n f r o m  N 0 ) * and  included  (N  to  or central nitrogen K-shell  b a n d o f s t r u c t u r e o b s e r v e d a t ^ 414 K-l \**  associated with  is  K-edges,  c e r t a i n t y , although  i s consistent with  dioxide  K-shell excited  central nitrogen 32  X - r a y PES  spectrum  i s not s u r p r i s i n g  c o m p l e x i t i e s o f the  obtained  23,  This  nitrogen  nitrogen  the d e s c r i p t i o n of the  t r i a t o m i c m o l e c u l e s i n terms o f the core t h a t f o r the  relative  carbon K - s h e l l  states of these as a c c u r a t e  The  to the promotion of a terminal  of carbon d i o x i d e or the c o r r e c t e d r e l a t i v e molecule  .  and  t e n t a t i v e peak a s s i g n m e n t s a r e  i n t e r p r e t a t i o n of the  listed  is  i n Table  spectrum i s analogous to t h a t of each  8.  nitrogen  -113-  -114-  TABLE 8 ABSOLUTE ENERGIES ( e V ) , R E L A T I V E ENERGIES AND P O S S I B L E ASSIGNMENTS OF PEAKS OBSERVED I N THE OXYGEN K-SHELL SPECTRUM OF NITROUS OXIDE.  PEAK  ENERGY  AE  ASSIGNMENT  9  bi?Fh(5  CA  ED  -  1  534.6  0  2  536.5  1.9  3sa  537.8  3  538.8  4.2  3pa  538.7 538.9  3ir(a'  + a")  3piT  3da  4  540.0  5.4  3diT  539.3 539.6  4sa  539.7  4pa,  4da 4dTT  K-EDGE  541.2  0  a. O n l y t h e f i n a l o r b i t a l / s been i n c l u d e d .  4piT  540.0 540.2 540.3  6.6  i n v o l v e d i n t h e K - e x c i t a t i o n s have  b. C a l c u l a t e d u s i n g t h e R y d b e r g f o r m u l a . The quantum d e f e c t s u s e d were t h o s e from t h e v a l e n c e s h e l l Rydberg s e r i e s o f n i t r o u s o x i d e w i t h 6 ( n s o ) = 1.0, 6 ( n p a ) = 0.68, 6(npiT) = 0.57, 6 ( n d a ) = 0.29 a n d 6 ( n d a ) = 0.07. c. T h i s  value  i s from X-ray  PES  3 2  .  1 2 7  -115-  K-shell will is  spectrum and t h e r e f o r e only  be d i s c u s s e d .  similar  The f i r s t  t o those observed  provides  d i s c r e t e p e a k h a s a FWHM o f 1.2 e V , w h i c h f o r t h ecorresponding  spectrum o f n i t r o u s oxide. w e r e 0.9 eV f o r t h e  d i f f e r e n c e s and i n t e r e s t i n g f e a t u r e s  peaks i n t h e n i t r o g e n  ( I n c o n t r a s t , f o r c a r b o n d i o x i d e , t h e FWHM's  c a r b o n K - s h e l l a n d 1.4 eV f o r t h e o x y g e n K - s h e l l .  some r e i n f o r c e m e n t  o f thesuggestion  that thef i r s t  peak i n t h e  oxygen K - s h e l l spectrum o f carbon d i o x i d e  has a c o n t r i b u t i o n from a  transition  The i n t e n s i t i e s  t o t h e 3 s a Rydberg o r b i t a l . )  energy d i s c r e t e peaks i n t h e s p e c t r u m , r e l a t i v e  Part o f this orbitals  i n t e n s i t y may be a s s o c i a t e d w i t h  since thevalence  shell  o f the higher  to that o f thef i r s t  peak, a r e l a r g e r than those observed i n e i t h e r o f the  previous  discrete  spectra.  t r a n s i t i o n s t o d-type  spectrum has a s t r o n g  This  Rydberg  c o n t r i b u t i o n from  127 s i g m a t o nd R y d b e r g o r b i t a l s  •.. H o w e v e r , we w o u l d a l s o e x p e c t t h i s t o  occur i n t h e case o f t h e n i t r o g e n a  " n o r m a l " i n t e n s i t y was o b s e r v e d .  o b s e r v e d a f t e r t h e K-edge.  K - s h e l l spectrum o f n i t r o u s o x i d e , A sharp decrease i n i n t e n s i t y i s  In thenitrogen  to conclude whether a s i m i l a r  situation  K-shell spectrum i t i s d i f f i c u l t  occurs  because both  o b s c u r e d , o n e by d i s c r e t e s t r u c t u r e a n d t h e o t h e r The  p o s i t i o n o f t h e o x y g e n K-edge i n F i g u r e 32  experimental  value  provided  by X - r a y PES  K-edge i s t h e e x t r e m e l y s m a l l (N^O in  ~ )  a n d (N^O." )  conjunction  with  .  K-regions a r e  by a c o n t i n u u m .  2 7 , i s b a s e d on t h e  A s u r p r i s i n g f e a t u r e above t h e  intensity of structures associated  states.  where  The e n e r g y p o s i t i o n s w h e r e  K - s h e l l i o n i z a t i o n ( i . e . (f^O " ) 128 o b s e r v e d b y X - r a y PES a r e i n c l u d e d i n F i g u r e 27.  with  shake-up  s t a t e s ) have been  -116-  6.2.  Carbon  Disulfide  6.2.1. C a r b o n The  and Carbonyl  Disulfide.  carbon d i s u l f i d e molecule i s l i n e a r  s t a t e and has t h e e l e c t r o n c b  Sulfide.  4 ls  r  V  4 12 (5c, 1s 2s 2p  2  C  c  c  S  S  c  We have s t u d i e d spectra.  s2 ic.  x2  g  {6a  < u> 4o  y  (  N2 , u 5  a  ,2 ,  c )  0  ( 2 7 T  t h e carbon I s (K) and s u l f u r  u  o f carbon d i s u l f i d e  carbon d i o x i d e and n i t r o u s a. V a l e n c e S h e l l The  28.  4  ^4 1 + ' Eg'  g  )  2p ( L J J j  T  T  ) energy  loss  i n the region o f the  a n d a s p e c t r u m was n o t r e c o r d e d . i s isoelectronic with  those o f  oxide.  Spectrum.  valence shell  shown i n F i g u r e  N  {2lT  }  Cross-sections f o rdiscrete transitions  valence shell  electronic  configuration:  s u l f u r 2s ( L j ) e d g e a p p e a r t o b e s m a l l The  i n i t s ground  energy l o s s spectrum o f carbon d i s u l f i d e i s  The o b s e r v e d l o c a t i o n s o f p e a k s  are consistent  with  118 a higher resolution  spectrum  .  P e a k A, w i t h a maximum a t 4.1 e V , i s  a s s o c i a t e d w i t h t h e ^ R e n n e r - T e l l e r c o m p o n e n t o f t h e ''A s t a t e r e s u l t s f r o m t h e t r a n s i t i o n , 2TT -> 3ir g positively  identified  (TT*).  This transition  which  has been  u  i n t h e corresponding energy  region o f a high  129 resolution optical associated with  .  T h e weak i n t e n s i t y o f t h i s  the forbidden nature o f the t r a n s i t i o n  The  i n t e n s e peak,  2n  ->- 3TT  U  spectrum  band i s  i n D ^ symmetry. ro  B, w i t h a maximum a t 6.2 eV i s a s s o c i a t e d w i t h t h e  (TT*), ^ E * (^2)  transition  (see t h e i n t e r p r e t a t i o n  of the optical  119 129 spectra and C 2  ' V  p e a k A.  ).  This transition  i selectric  d i p o l e a l l o w e d i n both  symmetry, which a c c o u n t s f o r i t s s t r o n g The l o c a t i o n s o f h i g h e r energy peaks  intensity  D^  relative to  i n o u r spectrum a r e ;  C ( 8 . 5 e V ) , D ( 9 . 3 e V ) , E (11.1 e V ) , F ( 1 1 . 9 e V ) , G ( 1 3 . 4 e V ) a n d H (15.1 e V ) .  In t h e photoabsorption s p e c t r u m  1 1 9  and a h i g h e r r e s o l u t i o n  . ELASTIC  B 1  T 0  GURE 2 8 .  »  st  I. P  1  i  1  - i  10 20 Energy Loss (eV) Valence s h e l l  energy  loss  spectrum o f carbon  1  30 disulfide  1  r  40  -118-  electron Rydberg  impact s p e c t r u m excitations.  1 l o  ,  c o r r e s p o n d i n g peaks  The l o c a t i o n o f t h e f i r s t  have b e e n a s s i g n e d t o ionization  potential  78 shown i n o u r s p e c t r u m i s b a s e d o n t h e UV-PES v a l u e b. C a r b o n The  K-shell  o f 10.06 e V .  Excitation.  carbon K-shell  energy  l o s s spectrum o f carbon d i s u l f i d e i s  shown i n F i g u r e 29 a n d t h e e n e r g i e s a n d p o s s i b l e a s s i g n m e n t s o f p e a k s a r e listed  i n T a b l e 9.  as a r i s i n g  T h e i n t e n s e p e a k o b s e r v e d a t 286.1  from the promotion o f a carbon K - s h e l l  unfilled molecular o r b i t a l ,  t h e 3TT  U  (TT*).  eV i s i n t e r p r e t e d  electron  t o the lowest  T h e p e a k h a s a FWHM o f 0.56 eV  c o m p a r e d w i t h a FWHM o f 0.38 eV f o r t h e peak a s s o c i a t e d w i t h scattered electrons. ional in  levels.  This indicates  T h i s peak i s a n a l o g o u s t o t h e f i r s t  the carbon K - s h e l l  peaks  the e x c i t a t i o n  spectrum o f carbon d i o x i d e .  elastically  o f a number o f v i b r a t d i s c r e t e peak o b s e r v e d The second  and t h i r d  l o c a t e d a t ^ 289.6 eV a n d 290.6 eV r e s p e c t i v e l y , a r e p r o b a b l y a s s o c -  iated with the promotion o f a carbon K-shell S i n c e carbon and s u l f u r b e l o n g t o d i f f e r e n t choice o f principal  quantum numbers e x i s t s .  electron  t o Rydberg  rows o f t h e p e r i o d i c The l o w e s t Rydberg  orbitals. table, a orbital  may  be d e s i g n a t e d 3s a p p r o p r i a t e f o r c a r b o n o r 4 s a p p r o p r i a t e f o r s u l f u r .  The  q u a n t u m d e f e c t s may be a p p r e c i a b l y d i f f e r e n t f r o m t h o s e d e r i v e d f o r  m o l e c u l e s c o n t a i n i n g o n l y s e c o n d row a t o m s ( s e e R e f e r e n c e 1 3 0 ) . quantum d e f e c t s  The  (assuming n = 3) d e r i v e d from t h e e x p e r i m e n t a l e n e r g i e s o f  p e a k s t w o a n d t h r e e a n d t h e X - r a y P E S v a l u e f o r t h e c a r b o n K-edge i n c a r b o n 125 disulfide  a r e 1.03 a n d 0.67 r e s p e c t i v e l y .  I f a Rydberg  c o r r e c t , p e a k t w o i s p r o b a b l y a s s o c i a t e d w i t h 3s e x c i t a t i o n with may  3p e x c i t a t i o n . also  assignment i s a n d peak t h r e e  The b r o a d s h o u l d e r on t h e l o w e n e r g y s i d e o f peak  be a s s o c i a t e d w i t h  Rydberg  transitions.  four  1XH  CO 4ml  c  K-edge 1} 1.  CD to I  CO c  CD 4-1  c  280  290  300  Energy Loss FIGURE 2 9 .  Carbon  K-shell  energy l o s s  310  (eV) spectrum o f carbon  320  disulfide.  TABLE 9 ABSOLUTE  ENERGIES  ( e V ) , R E L A T I V E ENERGIES AND P O S S I B L E ASSIGNMENTS  THE CARBON K-SHELL ENERGY LOSS SPECTRUM  OF PEAKS OBSERVED IN  OF CARBON D I S U L F I D E AND THE CARBON AND OXYGEN  K-SHELL ENERGY LOSS SPECTRA OF CARBONYL S U L F I D E .  CARBON  DISULFIDE  POSSIBLE  CARBON  K-SHELL  ASSIGNMENT  PEAK  ENERGY  1  286.1  2  289.6  3  290.6  EDGE  AE  0  CARBONYL  9  SULFIDE  CARBON K - S H E L L PEAK  OXYGEN  ENERGY  AE  TT*  1  288.2  3.5  ns?  2  291.0  2.8  4.5  np?  3  291.5  3.3  0  4  ^ 293.7  5.5  5  294.4  6.2  295.2  7.0  293.1  7.0  4  293.4  7.3  6  297.5  9.3  5  295.8  9.7  7  298.3  10.1  6  a, 299.4  0  00  K-EDGE  0  ENERGY  involved  b These v a l u e s a r e from X-ray  PES  i n the K-excitations 1 2 5  .  have been  AE  533.7  0  540.3  6.6  ^ 13.3  a Only the outer o r b i t a l s  K-SHELL  included,  -121-  The  p o s i t i o n of the  carbon  K-edge i n d i c a t e d on  our  spectrum i s based  1?5 on  the experimental  X - r a y PES  a maximum a t 2 9 3 . 4 ± 0 . 2 may  be  associated with  in conjunction with 3TT  molecular  effective  b a r r i e r w h i c h may  two  excitation  and,  ii.  potential barrier  References 131-134).  ation  has  of a carbon  the  i n the  the  Peak f o u r i .  regions  of valence  can  peak  electrons  K - s h e l l e l e c t r o n to  of the occur  s u l f u r atoms  up  to the  limit.  the have  an  (see  top of  The  with  the  c a r b o n d i s u l f i d e m o l e c u l e may  Discrete levels  by  eV.  possible explanations:  be w e l l a b o v e t h e i o n i z a t i o n  i s supported  ± 0.1  the shake-up and/or s h a k e - o f f  the  orbital  eV  o f 293.1  value  the  first  interpret-  shake-up l i n e s o b s e r v e d i n c o n j u n c t i o n  with  125 c a r b o n K - s h e l l i o n i z a t i o n i n c a r b o n d i s u l f i d e as PES. The two l o w e s t s h a k e - u p s t a t e s , (C ~ S )  determined s t a t e s , occur  2  by at  X-ray 6.5  1 25 and  9.1  eV  above the  intensities  K-shell ion state  (with respect  respectively.  The  (see  shake-up s t a t e s a s s o c i a t e d w i t h  20% o f t h a t o f t h e m a i n p e a k .  are c o n s i s t e n t with ( p e a k 6)  ure  u  orbital  observed  and  the  eV  2 9 9 . 4 eV  t r i a t o m i c molecules,  5 to  f i v e , which  i n t e n s e 3ir  Similarly c o u l d be  energies  the  u  are  peak  broad  struct-  associated with  K-shell excitation  to  the the  S i m i l a r shake-up s t r u c t u r e s were  (carbon,  (5.1.1),  roughly  p e a k s f o u r and  electrons in conjunction with  K-shell spectra  16.4%  discrete excitation  intensities  a shake-up i n t e r p r e t a t i o n .  nitrogen, Section  the  r e s p e c t i v e l y above t h e  and/or K - s h e l l i o n i z a t i o n .  i n the  molecules,  9.7  Therefore,  located at approximately  shake-up o f v a l e n c e 3TT  and  relative  t o have s i m i l a r r e l a t i v e  ( w i t h r e s p e c t t o t h e m a i n d i s c r e t e p e a k ) and  eV  with  t o t h e main K - s h e l l i o n p e a k ) o f 7 and  of a carbon K - s h e l l e l e c t r o n are expected  o b s e r v e d a t 7.3  29)  Figure  and  n i t r o g e n and  oxygen) of the  carbon monoxide, S e c t i o n  carbon d i o x i d e , S e c t i o n  (6.1.1),  and  diatomic (5.1.2),  nitrous  -122-  oxide, Section rather  (6.1.2).  However, i n t h e s e c a s e s t h e shake-up bands a r e  broad i n contrast  second i n t e r p r e t a t i o n ,  t o t h e r e l a t i v e l y s h a r p n a t u r e o f peak f o u r .  that  t h e r e i s an e f f e c t i v e p o t e n t i a l  b a s e d on t h e o b s e r v a t i o n t h a t disulfide  has p r o p e r t i e s  of a central  barrier, i s  spectrum o f carbon  s i m i l a r t o those observed i n the inner s h e l l  absorption spectra of S F consisting  the carbon K-shell  1 6 g  "  1 9  ,  BF  2 0 3  "  and o t h e r  2 2  molecules  a t o m " s u r r o u n d e d " by e l e c t r o n e g a t i v e  '  1 5 , 2 1  i n some c a s e s t h e s u r r o u n d i n g a t o m s ) a r e g e n e r a l l y  strong discrete  uted to the existence  and s m a l l  131-134  outer rim o f these molecules. well the  states  These e f f e c t s  o f an e f f e c t i v e p o t e n t i a l  of large  radius  o f BFg s u p p o r t a p o t e n t i a l  132  .  barrier  s u l f u r Lj j j j j absorption spectrum which are u s u a l l y  115  atoms.  barrier  well attrib-  potential  Calculations  133  for  i n t h i s molecule. of a  central  In f a c t t h e  o f SO^ h a s some o f t h e  associated with a potential  as  by  b a r r i e r on t h e  T h i s phenomenon i s n o t l i m i t e d t o m o l e c u l e s w h i c h c o n s i s t atom c o m p l e t e l y s u r r o u n d e d by e l e c t r o n e g a t i v e  atom  have been  T h i s b a r r i e r s e p a r a t e s an i n n e r  f r o m an o u t e r , s h a l l o w w e l l excited  K-jumps.  For  characterized  peaks b o t h above and below t h e i o n i z a t i o n l i m i t ,  as weak R y d b e r g s e r i e s  2 3 - 2 5  atoms.  these molecules the inner s h e l l absorption spectra o f the central (and  The  131  .  characteristics  Dehmer  131  has  15 21 pointed out that  the s u l f u r L J J J J J photoabsorption spectrum  carbon d i s u l f i d e i s not consistent barrier.  with the existence of a  the  of  potential  However, t h i s r e s u l t does n o t n e g a t e t h e p o s s i b i l i t y o f a  b a r r i e r t o the promotion o f a carbon K-shell central  '  atom) o f c a r b o n d i s u l f i d e .  carbon K-shell  narrow s t r u c t u r e s  The f a c t t h a t  spectrum a r e located indicates  electron  the possible  (i.e.,  potential  from the  peaks f o u r and f i v e i n  a b o v e t h e K-edge a n d a r e r e l a t i v e l y existence of a potential  barrier.  -123-  The  presence of a b a r r i e r  o v e r l a p between Rydberg K-shell  orbital  molecule.  i s not expected t o s i g n i f i c a n t l y  orbitals  (see Reference 131). characteristics  ( o u t e r - w e l l ) and t h e i n n e r - w e l l  s i n c e the b a r r i e r would  T h e r e f o r e , Rydberg  excitations  a r e a l s o e x p e c t e d t o be o b s e r v e d  W i t h t h e e x c e p t i o n o f t h e NF^, B F ^ a n d B C l g m o l e c u l e s ,  attributed  to a potential  b a r r i e r have o n l y b e e n  molecules, the participation of drorbitals  Section  The c a r b o n  K-shell  energy  sulfur or silicon  observed  In these  i n t h e b o n d i n g may b e a n i m p o r t -  loss  spectrum  o f carbon  ( 6 . 1 . 1 ) , d o e s n o t s u p p o r t s u c h an i n t e r p r e t a t i o n  These r e s u l t s  carbon  not completely surround the  (see R e f e r e n c e 131) i n m o l e c u l e s c o n t a i n i n g  ant f a c t o r .  reduce t h e  dioxide,  f o r this  molecule.  suggest that the e l e c t r o n e g a t i v i t y of the p e r i p h e r a l  n o t t h e o n l y c o n s i d e r a t i o n , s i n c e o x y g e n i s more e l e c t r o n e g a t i v e  atoms i s  than  sulfur. c. S u l f u r L J J J J J ( 2 p ) S h e l l  Excitation.  sulfur L J J jjj-shell  The  energy  loss spectrum  o f carbon  i s shown i n F i g u r e 30 a n d t h e e n e r g i e s a n d p o s s i b l e a s s i g n m e n t s are l i s t e d  i n T a b l e 10.  The o p t i c a l  a b s o r p t i o n spectrum  disulfide  o f peaks  o f carbon  disulfide  15 in  t h i s energy  continuum.  has p r e v i o u s l y b e e n o b t a i n e d  The i n s t r u m e n t a l  our spectrum) optical  region  r e s o l u t i o n was % 0.4 eV ( i . e .  and t h e a b s o l u t e c a l i b r a t i o n  results  are l i s t e d  using a Bremstrahlung  i n T a b l e 10.  t h e same a s i n  i s r e p o r t e d t o be ± 0.1 eV.  The  Below t h e L J J j j j - e d g e , t h e o p t i c a l  15 spectrum  and o u r energy  the absolute c a l i b r a t i o n s  loss  spectrum  differ  by 0.4 eV ( t h i s  sum o f t h e e x p e r i m e n t a l u n c e r t a i n t i e s ) . i s t i c s which a r e u s u a l l y  show i d e n t i c a l  structure,  although  i s 0.1 eV l a r g e r  than the  The s p e c t r a do n o t show  associated with a potential  barrier.  character-  I t has b e e n  131 suggested  that the d i s c r e t e s t r u c t u r e observed  i n the optical  spectrum  c/>  c1.0 H  IF  4-1 a mmm  5S  Ml  Z3 >>  ******  2  n • ••••  (CS£- ) * 1  +  X-ray PES  £ 0.54  c  ro  i  v !  0  II  IMI I I  1 2 3456 7 8  OJ  160  T  T  170  180  Energy Loss (eV) FIGURE 3 0 .  Sulfur L  n  I H  190  ( 2 p ) energy l o s s spectrum o f carbon d i s u l f i d e .  TABLE  10  ABSOLUTE FUR  ENERGIES  (L  2p  T T  T  (eV),  J -SHELL)  ENERGY  T  CARBON THIS PEAK  RELATIVE  163.1  2  LOSS  AND P O S S I B L E  SPECTRA  OF  CARBON  ASSIGNMENTS  DISULFIDE  DISULFIDE WORK  ENERGY  1  ENERGIES  ENERGY  a  PEAKS  CARBONYL  POSSIBLE"  THIS  ORBITAL AE  ASSIGNMENT  OBSERVED  IN  THE  SUL-  CARBONYL.  . OPTICAL  AE  AND  OF  SULFIDE WORK  PEAK  ENERGY  AE  0  163.5  0  IT*  1  164.2  164.2  1.1  164.6  1.1  TT*  2  165.6  1.4  3  165.9  2.8  166.4  2.9  IT*  3  166.9  2.7  4  166.5  3.4  167.0  3.5  4  168.1  3.9  5  167.4  4.3  167.7  4.2  5  168.6  4.4  6  168.2  5.1  168.6  5.1  6  170.0  5.8  7  169.5  6.4  169.9  6.4  ) 169.8  6.7  170.6  6.4  o/171.0  6.8  171.8  7.6  L-EDGE  C  ( n 2  3 / 2  8 L-EDGE  170.8 0  7.4  ( n,,.) 171.0 2  00  171.1  7.9  00  a,14.0  a. R e f e r e n c e 1 5 . b. O n l y t h e o u t e r o r b i t a l c. R e f e r e n c e 1 2 5 .  involved  C  < "3 2  / 2  )  7.6  / 2  ^177.1  L-EDGE  SHAKE-UP  i n the transition  i s given.  L-EDGE  0  )  M91  0  a.26.8  -126-  probably d e r i v e s from superimposed ment o f t h e p e a k s sulfur  2p s h e l l  Rydberg  has n o t b e e n a t t e m p t e d .  o f carbon d i s u l f i d e  l i n e s , b u t an i n d i v i d u a l Spin-orbit coupling  p  observed  by X - r a y PES b e t w e e n t h e  n  p  n  and  3 /  12 ion s t a t e s o f carbon d i s u l f i d e . n o t a p p l y and Reference  i n the  i s l a r g e , a s shown by t h e 1.2 eV  1 pr  splitting  assign-  sulfur  1 ;  2p  12  Therefore Russell-Saunders coupling  does  c o u p l i n g g i v e s a more a p p r o p r i a t e d e s c r i p t i o n ( s e e  75).  The l o w e s t e n e r g y d i s c r e t e p e a k s  observed  i n the sulfur  spectrum  a r e e x p e c t e d t o be a s s o c i a t e d w i t h t h e p r o m o t i o n o f a s u l f u r  electron  to the  valence molecular o r b i t a l  spectrum o f carbon d i s u l f i d e ) .  S i x groups  ( c f . the carbon  2p  2p  K-shell  of molecular states are  75 expected  as a r e s u l t o f t h i s e x c i t a t i o n ,  may be a a\.  , TT  12 with  3TT  u  or TTI, .  3  12  Peaks  since the lone s u l f u r  o n e a n d two a r e p r o b a b l y  2p e l e c t r o n  associated  12  excitation.  P e a k one i s a p p r o x i m a t e l y t h e same e n e r g y  below t h e  2 n1 3  edge, as t h e f i r s t  carbon d i s u l f i d e  d i s c r e t e peak i n t h e c a r b o n  i s below  probably associated with  t h e c a r b o n K-edge. Rydberg  Peaks  K-shell  spectrum o f  t h r e e t o seven a r e  e x c i t a t i o n s , a l t h o u g h some o f t h e i n t e n s i t y  ( p a r t i c u l a r l y i n t h e l o w e n e r g y r e g i o n o f t h i s g r o u p o f p e a k s ) may be associated with excitation. 2 2 The p o s i t i o n s o f t h e n ^ and L-edges i n d i c a t e d i n o u r s p e c t r u m 3  125  a r e based  on t h e X - r a y  respectively.  PES v a l u e s  o f 169.8 ± 0.1 a n d 1 7 1 . 0 ± 0.1  The b a n d o f s t r u c t u r e w i t h  an o n s e t a t ^ 177 eV i s p r o b a b l y  a s s o c i a t e d w i t h t h e shake-up o f v a l e n c e e l e c t r o n s sulfur  2p i o n i z a t i o n .  agreement w i t h  PES f o r t h e s u l f u r shake-up s t a t e s ,  L ?  of.carbon d i s u l f i d e .  ~ ) *, observed 1  n  3  ^  edge i n e x a c t  o f t h e l o w e s t shake-up s t a t e observed  2p s h e l l  (CS  i n conjunction with  The o n s e t i s 7.3 eV a b o v e t h e  t h e energy  +  1 2 5  eV  by X - r a y  The p o s i t i o n s o f t h e  by X - r a y PES a r e i n d i c a t e d i n  -127-  Figure  30.  6.2.2. C a r b o n y l The  carbonyl  s t a t e and  S  We  ls  has  °ls  Sulfide.  s u l f i d e molecule  the e l e c t r o n  l s 2s  C  S  S  2p  energy  loss  ( 6 a ) 2  not  recorded.  The  those o f carbon shell  spectrum  The F i g u r e 31. electron  2s  Is  ( 3  ( K ) and  ^  ) 4  '  shell  of carbonyl o x i d e and  s m a l l and  sulfide  carbon  1 e +  sulfur  Cross-sections for discrete  shell  2p  2TT •> 3TT (TT*) [ ^  and  1 1 8  +  (LJJ JJJ)  transitions  i n the  a spectrum  was  is isoelectronic A  disulfide.  spectrum  of carbonyl  sulfide  with  valence  i s shown i n  l o c a t i o n s o f peaks are c o n s i s t e n t w i t h h i g h e r optical  a maximum a t a p p r o x i m a t e l y ->-  1  e V ) , F (13.2  5.7  A( A')], 1  i n our spectrum  (12.1  ( 2 i t ) 4  Spectrum.  valence  impact  observed  electronic  recorded.  Shell  The  ( 9 a ) 2  ( L j ) edge a p p e a r t o be  valence  was  { 8 a ) 2  ( K ) , carbon  dioxide, nitrous  a. V a l e n c e  E  ( 7 a ) 2  spectra.  region of the s u l f u r  i n i t s ground  configuration:  have s t u d i e d t h e oxygen I s  shell  is linear  eV)  are; and  G  spectra eV  1 1 9  .  The  resolution  band, A ,  weak b r o a d  i s probably a s s o c i a t e d w i t h the  see  Reference  B  (7.4 e V ) , C  (13.8  eV).  119. (8.1  The  transition  h i g h e r energy  eV), D  (9.5  with  peaks  eV),  In the h i g h e r r e s o l u t i o n  electron  118 impact  spectrum  the corresponding  peaks have been a s s i g n e d t o  Rydberg  119 transitions. B and  C i n our  -> £ 1  +  potential  However, i n t h e o p t i c a l spectrum  transitions  spectrum  , peaks c o r r e s p o n d i n g  have b e e n a s s i g n e d t o n o n - R y d b e r g ,  respectively.  shown i n F i g u r e 31  The  1  E  +  •+ \  to  and  l o c a t i o n of the f i r s t i o n i z a t i o n 120 i s b a s e d on t h e o p t i c a l and e x p e r i m e n t a l  I  0 FIGURE 3 1 .  I  1  •  I  •  10 20 Energy Loss (eV) Valence shell  energy l o s s  spectrum o f carbonyl  s  30 sulfide.  I  I 40  -129-  UV-PES v a l u e ' " o f 1 1 . 2 eV. b.  Oxygen K - s h e l l  Excitation.  The o x y g e n K - s h e l l  energy  loss  spectrum o f carbonyl  shown i n F i g u r e 32 a n d t h e e n e r g i e s a n d p o s s i b l e are  listed  ±0.3  i n T a b l e 9.  eV.  thei n e l a s t i c  oc ( e n e r g y l o s s ) interpreted the of  t o noise and signal  due t o t h e f a c t t h a t  scattering,  -  .  scattering  as a r i s i n g from  vibrational  t h e p r o m o t i o n o f an oxygen K - s h e l l t h e 4u ( I T * ) .  PES v a l u e  Carbon K - s h e l l  i n T a b l e 9.  carbon  1 2 5  K-shell  Excitation. energy  loss  spectrum o f carbonyl  the  peak o b s e r v e d Section  i s similar to  spectrum o f carbon d i s u l f i d e , although t h e r e l a t i v e are different.  molecular orbital  This interpretation i n the carbon  The i n t e n s e d i s c r e t e  peak o b s e r v e d  as a r i s i n g from t h e promotion o f a carbon  t o the lowest u n f i l l e d  4TT ( I T * ) .  sulfidei s  assignments o f peaks a r e  The g e n e r a l appearance o f t h e s p e c t r u m  288.2 eV i s i n t e r p r e t e d  electron  i s based  o f 5 4 0 . 3 ± 0.1 eV.  K-shell  energies o f structures at  The p e a k h a s a FWHM  on o u rspectrum  shown i n F i g u r e 33 a n d t h e e n e r g i e s a n d p o s s i b l e  the  electron t o  FWHM 0.5 e V ) i n d i c a t i n g t h e e x c i t a t i o n o f a number o f  The c a r b o n  listed  by a f a c t o r ,  levels.  the X-ray c.  1 1  a maximum a t 533.7 ± 0.3 eV i s  T h e p o s i t i o n o f t h e o x y g e n K-edge i n d i c a t e d on  ratios are  impact and f o r w a r d  intensity decreases  molecular o r b i t a l ,  1.2 eV ( e l a s t i c  t o background  f o r fast electron  The b r o a d peak w i t h  lowest u n f i l l e d  assignments o f s t r u c t u r e s  The c a l i b r a t i o n a c c u r a c y o f t h e spectrum i s  The poor s i g n a l  partially  sulfidei s  i s analogous  K-shell  ( 6 . 2 . 1 ) , and carbon d i o x i d e ,  o f carbonyl  K-shell  sulfide,  to that o f the f i r s t  discrete  spectrum o f carbon d i s u l f i d e , Section  (6.1.1).  I n each  case t h e  Intensity  -0£L-  (arbitrary  units)  1.0 I  K-edge (C - OS) * X-ray PES K  1  +  5 0.5 03  i  CO I  •l  li  II  :1 . 2 3 4 5  II 6 7  T  290  300 Energy Loss  FIGURE 33.  Carbon  K-shell  energy l o s s  310 (eV) spectrum o f carbonyl  320 sulfide.  -132-  peak i s a p p r o x i m a t e l y t h e peak i n t h e  carbonyl  same e n e r g y b e l o w t h e  derived 1.2  K-edge a r e  probably  associated  promotion o f a carbon K - s h e l l e l e c t r o n t o Rydberg o r b i t a l s . quantum d e f e c t s  o f p e a k s two a n d t h r e e  a n d 1.08 r e s p e c t i v e l y .  promotion t o the defect  The  e x c i t a t i o n o f a number o f v i b r a t i o n a l l e v e l s .  Higher energy d i s c r e t e s t r u c t u r e s below the the  K-edge.  s u l f i d e s p e c t r u m h a s a FWHM o f 0.85 eV ( e l a s t i c  FWHM 0.56 e V ) i n d i c a t i n g t h e  with  respective  (assuming  n = 3) a r e  T h e r e f o r e p e a k s two a n d t h r e e  3s a n d 3p R y d b e r g o r b i t a l s  The  could  respectively.  represent  The quantum  o f 1.08 i s somewhat l a r g e f o r a 3p R y d b e r g e x c i t a t i o n a n d s u g g e s t s  that this  Rydberg o r b i t a l  does t h e  3s.  h a s more p e n e t r a t i o n  from the  shake-up o f v a l e n c e e l e c t r o n s  promotion o f a carbon K - s h e l l e l e c t r o n t o the is also possible that  the  carbonyl  b a r r i e r ( c f . the  than  i nconjunction  with the  4TT m o l e c u l a r o r b i t a l .  It  s u l f i d e m o l e c u l e has an e f f e c t i v e  possible  spectrum o f carbon d i s u l f i d e ) . are expected  s u l f u r core  I n a d d i t i o n t o R y d b e r g e x c i t a t i o n s , peak f i v e may h a v e a  contribution  potential  i n t o the  i n t e r p r e t a t i o n o f the  I fthis  t o be a m i x t u r e o f R y d b e r g  i sthe case, the (outer-well  carbon excited  K-shell states  s t a t e s ) and i n n e r - w e l l  states. The  p o s i t i o n o f the  K-edge i n d i c a t e d i n o u r s p e c t r u m i s b a s e d o n t h e  125 X - r a y PES v a l u e the  o f 295.2 ± 0.1 eV.  The broad s t r u c t u r e o b s e r v e d  K-edge ( p e a k s s i x a n d s e v e n ) may be a s s o c i a t e d  where the  promotion o f a carbon K - s h e l l  orbital  i sinvolved.  respect  t o the  first  The r e l a t i v e  e l e c t r o n t o the  energies  shake-up  states  4TT m o l e c u l a r  o f these structures  with  d i s c r e t e p e a k i s ^ 9 eV w h i c h i s c o n s i s t e n t w i t h t h e  lov/est shake-up s t a t e o b s e r v e d ionization  with  above  125  ( i . e . 8.3 eV a b o v e t h e  i n conjunction K-edge).  with  carbon  125 The X-ray PES  K-shell shake-up  -133-  lines corresponding  to C  N _ l  0S)  states  +  A l t e r n a t i v e l y , p e a k s s i x a n d s e v e n may  h a v e been i n c l u d e d represent discrete  a b o v e t h e K-edge by an e f f e c t i v e p o t e n t i a l d.  Sulfur  LJJ jj j (2p)-shell  are  i n T a b l e 10.  energy l o s s  spectrum o f carbonyl  The i n t e r p r e t a t i o n  to t h a t o f t h e s u l f u r  structure  below t h e  o f the spectrum i s s i m i l a r  spectrum o f carbon d i s u l f i d e .  t h r e e peaks a r e r e l a t i v e l y  sulfide  assignments o f peaks  F i g u r e 35 shows t h e d i s c r e t e  edge on an e x p a n d e d s c a l e .  raised  barrier.  shown i n F i g u r e 34 a n d t h e e n e r g i e s a n d p o s s i b l e listed  states  Excitation.  The s u l f u r L J J j j j - s h e l l is  i n F i g u r e 33.  The  first  i n t e n s e and a r e p r o b a b l y a s s o c i a t e d w i t h t h e  p r o m o t i o n o f a s u l f u r 2p e l e c t r o n  t o t h e 4TT ( T T * ) m o l e c u l a r o r b i t a l .  exact analogy t o carbon d i s u l f i d e , t o t h e 4TT m o l e c u l a r o r b i t a l  t h e p r o m o t i o n o f a s u l f u r 2p  results  In  electron  i n s i x groups o f m o l e c u l a r s t a t e i n  120 coupling  .  Higher energy d i s c r e t e  a s s o c i a t e d w i t h Rydberg t r a n s i t i o n s . exist  i n the carbonyl  peaks ( f o u r - s i x )  S h o u l d an e f f e c t i v e  sulfide molecule, i t i s unlikely  are probably  potential  barrier  (as i n t h e case o f  c a r b o n d i s u l f i d e ) t h a t i t w o u l d h a v e a s i g n i f i c a n t e f f e c t on t h e e x c i t a t i o n o f a s u l f u r 2p  electron. 2  The p o s i t i o n s  of the  n ^ 3  2 and  and  35 a r e b a s e d on t h e e x p e r i m e n t a l  and  171.8 ± 0.1 eV r e s p e c t i v e l y .  125  L-edges i n d i c a t e d  X - r a y PES v a l u e s o f 170.6 ± 0.1 125  The s h a k e - u p l i n e s o b s e r v e d  c o n j u n c t i o n w i t h s u l f u r 2p i o n i z a t i o n c o r r e s p o n d i n g h a v e been i n c l u d e d  i n F i g u r e 34.  The b r o a d b a n d o f s t r u c t u r e  structures,  The X - r a y PES s p e c t r u m  i s n o t r e p o r t e d a b o v e 190 eV.  eV  states,  w i t h an  the e x c i t a t i o n  , showing  34  in  t o (COS " )  o n s e t a t a p p r o x i m a t e l y 191 eV i s p r o b a b l y a s s o c i a t e d w i t h 125 shake-up/shake-off s t a t e s .  i n Figures  shake-up  of  'n  (COS " ) * X-ray PES L  1  +  MINI 1 2 345 6  T  160  FIGURE 34.  T  170 180 Energy Loss (eV) Sulfur  L  I  I  j  n  ( 2 p )  energy  loss  H  190  spectrum o f carbonyl  r  200  sulfide.  c/>  c 3  1.0-  lit  S5  l i p  CD  •*  CO  c  V v CO  cn  0.52  3  45  7  ~i—r  160  164  172  Energy Loss FIGURE 35.  6  Sulfur L  n  j  n  ( 2 p )  (eV)  energy l o s s spectrum o f carbonyl  energy s c a l e i n the region of the  ^  edges.  176 s u l f i d e with  an  expanded  -136-  CHAPTER  SEVEN  POLYATOMIC MOLECULES.  7.1.  Introduction. The  shell  prominent features  regions)  with  observed  of saturated  i n the absorption  spectra  polyatomic molecules are usually  these molecules. K-shell  those observed  e n e r g y and t h e c o r r e s p o n d i n g  ionization potential) since  quantum number, n, a d i s c u s s i o n  o r the term value  R/(n  - 6)  i s equivalent  since  t o 3p a n d 3d R y d b e r g l e v e l s , approximately constant  they  spectra.  o f t h e quantum  t h e term value  where R i s t h e Rydberg c o n s t a n t .  shell  ( d i f f e r e n c e between t h e  have been e x t e n s i v e l y u s e d i n i n t e r p r e t i n g t h e v a l e n c e s h e l l same p r i n c i p a l  spectra of  i n the promotion o f a valence  i t i s c o n v e n i e n t t o use t h e term v a l u e s  excitation  basis,  In comparing t h e Rydberg s t a t e s o b s e r v e d as a r e s u l t o f  promotion with  electron,  6,  associated  R y d b e r g t r a n s i t i o n s ( f o r e x a m p l e s s e e R e f e r e n c e 1 2 0 ) . On t h i s  Rydberg t r a n s i t i o n s a r e a l s o expected t o dominate t h e K - s h e l l  the  (valence  For  defect,  i s equal t o  For valence shell e x c i t a t i o n  i t h a s been f o u n d t h a t t h e t e r m v a l u e s a r e  i n a w i d e range o f compounds, w h i l e  t h e 3s t e r m  135 values  vary  considerably  .  The o b s e r v e d 3s d e v i a t i o n s  correlate with the  n a t u r e o f t h e s u b s t i t u e n t groups o f t h e m o l e c u l e and o c c u r because t h e penetration  o f t h e 3s o r b i t a l  and  therefore  the  a d d i t i o n o f an e l e c t r o n e g a t i v e  ively.  t h e term value  e i t h e r increases increases)  (thebinding  o r decreases  (lower  energy  term value) f o r  or electropositive substituent  The 3p a n d 3d R y d b e r g o r b i t a l s  increases  respect-  a r e much l e s s s e n s i t i v e t o t h e n a t u r e  -137-  of  t h e s u b s t i t u e n t s s i n c e t h e y h a v e much l e s s p e n e t r a t i o n t h a n t h e  In  principle,  observe ation of  f o r K-shell  excitation  t o Rydberg  l a r g e r term values than those observed  i n t h e same m o l e c u l e .  a K-shell  molecule  has  This result  7.2.  has  i s e x p e c t e d s i n c e , as a  result  effectively  more p o s i t i v e  (nucleus + K-shell) of charge.  i s t h e r e f o r e expected  term v a l u e ) f o r a m o l e c u l e w i t h a K - s h e l l a valence  to  excit-  of the cores  one  expect  f o r valence shell  e x c i t a t i o n , one  s u c h as a 3s R y d b e r g  molecule  o r b i t a l s , we  3s.  A penetrating orbital  t o be more t i g h t l y vacancy  the  bound ( h i g h e r  t h a n i t i s when  the  vacancy.  M e t h a n e , Ammonia, W a t e r , M e t h a n o l ,  Dimethyl  Ether  and  Monomethylamine. 7.2.1. M e t h a n e . The  ground  s y m m e t r y and  is  s t a t e o f t h e methane m o l e c u l e  the e l e c t r o n  (la,) The  electronic  2  (2a,)  6  2  l a , molecular orbital essentially  localized  ]  A,.  i s formed  from  on t h e c a r b o n  Is atomic o r b i t a l  In r e c o g n i t i o n  this orbital  of  and  this  are designated  carbon  electrons.  a. V a l e n c e The  Shell  resolution  spectrum  peak p o s i t i o n s  Spectrum.  valence shell  shown i n F i g u r e 36.  The  the carbon  nucleus.  "atomic" c h a r a c t e r the e l e c t r o n s f i l l i n g K-shell  tetrahedral  configuration:  (lt ) ,  2  has  The  electron  energy  spectrum  o f methane i s  l o c a t i o n s o f peaks a r e c o n s i s t e n t w i t h a h i g h e r  , where a Rydberg  i n our spectrum  l o c a t i o n of the f i r s t  loss  are:  ionization  A  assignment (10.0 e V ) , B  potential  has  been p r o p o s e d .  (11.6  eV)  and  shown i n F i g u r e 36  The  C (13.4 i s based  eV).  .• ELASTIC  Energy Loss FIGURE 3 6 . V a l e n c e s h e l l  energy loss  (eV) spectrum o f methane.  -139-  on t h e a d i a b a t i c v a l u e ^ 1  b. C a r b o n  K-shell  Absorption been i n v e s t i g a t e d  o f 13.0 eV.  Excitation.  i n t h e r e g i o n o f t h e c a r b o n K-edge i n m e t h a n e h a s  using Bremsstrahlung continuua  137-140  and more  recently pQ  with  the continuous radiation  spectra obtained with  by an e l e c t r o n  Bremsstrahlung radiation  absorptions superimposed l e n g t h r e g i o n , making  produced  synchrotron  are characterized  .  The  by weak  by t h e s e c o n d o r d e r s p e c t r u m o f t h e l o w e r wave-  i tdifficult  to identify  carbon-K  absorption  bands.  pQ  H o w e v e r , t h e much " c l e a n e r " s y n c h r o t r o n s p e c t r u m shows two d i s c r e t e absorptions. E n e r g y l e v e l s f o r some o f t h e c o r e e x c i t e d s t a t e s o f m e t h a n e have a l s o been c a l c u l a t e d The c a r b o n K - s h e l l  141  142 .  energy  l o s s s p e c t r u m o f m e t h a n e i s shown i n  F i g u r e 37 a n d t h e e n e r g i e s a n d t e n t a t i v e a s s i g n m e n t s o f p e a k s in  T a b l e 11.  T a b l e 11 a l s o  are l i s t e d  includes e x c i t a t i o n energies observed  using  28 electron  synchrotron radiation  and c a l c u l a t e d  v a l u e s u s i n g SCF w a v e -  142 functions  .  O u r s p e c t r u m shows more d i s c r e t e s t r u c t u r e t h a n t h e o p t i c a l  28 spectrum  and e x t e n d s f u r t h e r  i n t o t h e continuum  region.  peak o b s e r v e d a t 287.0 eV i s i n t e r p r e t e d a s a r i s i n g a carbon K - s h e l l  electron  from  K-shell  Rydberg  ionization  state.  potential  The m a g n i t u d e  observed f o r e x c i t a t i o n s 1 a-j -»• 3sa-| i s o p t i c a l l y first  energy  v a l u e o f 290.7 eV f o r  i m p l i e s a q u a n t u m d e f e c t o f 1.08 f o r t h e 3s  t o an ns R y d b e r g  level  f o r b i d d e n and i s f o r b i d d e n  and e ^  This experimental  o f t h e quantum d e f e c t i s c o n s i s t e n t w i t h 130  Born a p p r o x i m a t i o n i s v a l i d  excitation  discrete  the promotion o f  (la-,) t o t h e 3 s a , Rydberg l e v e l . 32  v a l u e f o r t h e e x c i t a t i o n e n e r g y a n d t h e X - r a y PES the  The f i r s t  0°).  .  The  transition  i n our experiment  ( t h e impact energy  those  i fthe  i s 8 times the  However, both t h e i n i t i a l  and f i n a l  states  TABLE 11 ABSOLUTE ENERGIES CARBON K-SHELL  PEAK  ( e V ) , R E L A T I V E ENERGIES AND  TENTATIVE  ASSIGNMENTS OF PEAKS OBSERVED IN THE  SPECTRUM OF METHANE.  ENERGY  AE  TERM  VALUE  ASSIGNMENT  9  1  287.0  0  3.7  3sa-|  2  288.0  1.0  2.7  3pt 4sa  CALCULATED ENERGYC  5  2  OPTICAL DATA d  SCF  287.2  287.3  288.3  288.4  289.1  1  3d  289.2  3  289.4  2.4  1.3  4pt  2  289.4  -  -  4  289.8  2.8  0.9  5pt  2  289.9  -  -  290.7  3.7  0  K-EDGE  f  5  o,303  o,16  6  0,311  o,24  e  00  (SHAKE-UP < AND (SHAKE-OFF  a. D e f i n e d a s t h e d i f f e r e n c e b e t w e e n t h e i o n i z a t i o n p o t e n t i a l and t h e e x c i t a t i o n e n e r g y . b. O n l y t h e f i n a l o r b i t a l i s l i s t e d ( i n i t i a l o r b i t a l i s la-, = c a r b o n K ) . c. C a l c u l a t e d u s i n g t h e R y d b e r g f o r m u l a E = A - R / ( n - < 5 ) w h e r e E i s t h e e x c i t a t i o n e n e r g y f o r t h e R y d b e r g l e v e l h a v i n g q u a n t u m number n a n d quantum d e f e c t 6 , A i s t h e c a r b o n K - s h e l l i o n i z a t i o n p o t e n t i a l and R i s t h e Rydberg c o n s t a n t . T h e q u a n t u m d e f e c t s u s e d f o r n s and np w e r e d e r i v e d f r o m t h e e n e r g y p o s i t i o n s o f t h e f i r s t two p e a k s . <s(nd) was assumed t o be 0. d. From R e f e r e n c e 28. e. From R e f e r e n c e 1 4 2 . f . X - r a y PES v a l u e ( R e f e r e n c e 3 2 ) . n  2  n  -142-  belong t o the theory  are  spectrum value  28  same s y m m e t r y s p e c i e s  o f 287.3 eV f o r t h e  absorption  l a , -> 3 s a , .  From  intensity  spectrum should  be a s s i g n e d  being  first  to the  observed because o f  3sa,  Rydberg s t a t e due  m o d e s , vg a n d v ^ .  F i n a l l y , the  term value  i s 3.7 eV i n c o m p a r i s o n w i t h  a term value  o f 3.95 eV  2  transition  synchrotron  they suggest that the  between the ground s t a t e and t h e  two,T ,vibrational  electron  Born  142 Bagus e t a l . have c a l c u l a t e d a  Rydberg t r a n s i t i o n ,  synchrotron  d e v i a t i o n s from the  i n the  Rydberg t r a n s i t i o n , the t r a n s i t i o n  vibronic coupling to the  absorption  and the c a l c u l a t e d r e s u l t s ,  peak i n t h e  ->- 3 s a ,  this  The f i r s t  was o b s e r v e d a t 287.2 eV.  considerations  la,  53  expected.  and t h e r e f o r e  for  136 observed  f o r the  o f methane, l t  corresponding  t o the  2  and the  ^  large intensity  therefore expected.  2  .  This  relative  The e x c i t a t i o n  which i s reasonable  valence  shell  spectrum  spectrum observed a t  promotion o f a l a , e l e c t r o n t o the  p - R y d b e r g o r b i t a l , l a , -v 3 p t ,  0.75  i n the  3 s a , . T h e s e c o n d peak i n o u r  2  288.0 eV i s a s s i g n e d  allowed  transition  transition  first  is electric  t o t h a t o f the  first  dipole  peak i s  e n e r g y i m p l i e s a quantum d e f e c t o f  f o r a 3p R y d b e r g l e v e l  (see  Reference 130).  p e a k h a s a FWHM o f 1.0 eV ( i n c o n t r a s t t o a FWHM o f 0.5 eV f o r t h e associated with high  two  (see  the  some o f t h e  a Jahn-Teller  peak  e l a s t i c a l l y s c a t t e r e d e l e c t r o n s ) and i s asymmetric on t h e  energy s i d e  excitation,  The  splitting  peaks observed  insert  i nFigure  37).  In addition to vibrational  b r o a d e n i n g and asymmetry c o u l d o f the degenerate V  2  be a s s o c i a t e d  electronic state.  i n the e l e c t r o n impact spectrum o f the 1 o r -I *5  with  The f i r s t  valence  shell  A  energy r e g i o n an  o f methane  '  (corresponding  t o peak A i n F i g u r e  36) have  e n e r g y d i f f e r e n c e o f 0.68 eV a n d h a v e b e e n i n t e r p r e t e d a s J a h n - T e l l e r  components o f the  ^T  ?  state arising  from the  transition  l t  0  3sa-,.  A  -143-  Jahn-Teller splitting  o f 0.8  eV  has  been o b s e r v e d  f o r the f i r s t  ion  state  144 o f m e t h a n e by PES  .  The  observed  e n e r g i e s f o r peak 2 i n o u r  spectrum  28 and  the second  peak i n t h e s y n c h r o t r o n s p e c t r u m  a r e i n good  agreement.  142 Bagus e t a l . h a v e c a l c u l a t e d a v a l u e o f 288.4 eV f o r t h e l a - j 3pt t r a n s i t i o n and have s u g g e s t e d t h a t t h i s i s t h e c o r r e c t i n t e r p r e t a t i o n o f 2  the photoabsorption  Hartree-Fock  peak o b s e r v e d 141  calculation  v a l u e o f 284.7 ± 0.3 result. this  transition.  The  28  a t 2 8 8 . 3 eV.  o f t h e l a ^ -> 3 p t  eV, w h i c h  F i n a l l y , a term  by Chun  A  transition  2  one-centre  energy  i s a p p r e c i a b l y lower than our  v a l u e o f 2.7  eV  magnitude of t h i s  i s o b t a i n e d from term  gave a  experimental  our data f o r  value i s s i m i l a r  to  those I  observed  i n the valence s h e l l  s p e c t r a of the fluoromethane  f o r the promotion  o f an o u t e r m o s t  ( e . g . CF^:  3p,  excitation  It-j  term  v a l u e 2.61  a v a l u e o f 290.7 eV  are l i s t e d  i n Table  higher energy o f a peak (peak  11  We  npt  2  our experimental  f o r the carbon and  t o a 3p  eV).  e n e r g i e s f o r h i g h e r nsa-j and  quantum d e f e c t s d e r i v e d f r o m  and  electron  levels  K-edge o f m e t h a n e  .  The  observed  values f o r the 4 p t  structure  relative  intensity  indicates that transitions  w o u l d be v e r y weak. in this  the  The  levels  results  This structure  and  a lower energy  energies are 2  and  5pt  2  Finally,  r e g i o n , as  o f t h e 3p  the  consists shoulder  in excellent  Rydberg  s u g g e s t i n g t h a t t h e s e peaks c o u l d have c o n t r i b u t i o n s from  3s t r a n s i t i o n  using  expected  h a v e b e e n u s e d as an a i d i n i n t e r p r e t i n g  number 3) a t 289.4 eV.  The  the  values f o r the n = 3 32  (number 4 ) w i t h a maximum a t 289.8 eV  these o r b i t a l s .  orbital  have c a l c u l a t e d  d i s c r e t e s t r u c t u r e i n the spectrum.  ment w i t h t h e c a l c u l a t e d  molecules  Rydberg  Rydberg  oc  agree-  levels,  transitions  transition  to  to that of  the  t o h i g h e r q u a n t u m number ns s t a t e s  l a - j t o 3d t r a n s i t i o n s c o u l d c o n t r i b u t e t o i n d i c a t e d by t h e c a l c u l a t e d  transition  energy  -144-  ( t h e quantum d e f e c t The the  was a s s u m e d t o be z e r o ) .  p o s i t i o n o f t h e K-edge i n d i c a t e d i n o u r s p e c t r u m i s b a s e d on  value  o f 290.7 eV f o r t h e K - s h e l l  i o n i z a t i o n p o t e n t i a l d e t e r m i n e d by  32 X - r a y PES 311  .  The v e r y  eV a r e a s s o c i a t e d  valence shell  with  electrons  in conjunction  with  have been o b s e r v e d (see S e c t i o n  broad s t r u c t u r e s l o c a t e d  a t a p p r o x i m a t e l y 303 and  t h e s i m u l t a n e o u s t r a n s i t i o n s o f a K - s h e l l and  ( i . e . t h e shake-up and s h a k e - o f f  K-shell excitation or i o n i z a t i o n . i n the case o f the diatomic  o f valence  Similar  electrons  structures  and t r i a t o m i c m o l e c u l e s  (5.1.1) f o r d e t a i l s ) .  7.2.2. Ammonia. The  g r o u n d e l e c t r o n i c s t a t e o f t h e ammonia m o l e c u l e has p y r a m i d a l  g e o m e t r y , b u t i s more a p p r o p r i a t e l y d e s c r i b e d inversion.  However, t h e s m a l l  vibrational  level  by D^  inversion splitting  symmetry because o f  h  of the  i n t o a symmetric and a n t i s y m m e t r i c l e v e l  = 0 results in 145  selection  r u l e s which are e f f e c t i v e l y  t h e same a s t h o s e f o r C ^  v  symmetry  The  e l e c t r o n c o n f i g u r a t i o n o f t h e g r o u n d e l e c t r o n i c s t a t e o f ammonia i n  Cg  symmetry i s  v  (la,) The  (2a,)  2  (le)  2  l a , molecular orbital  4  (3a,) , 2  1  A,.  i s formed from t h e n i t r o g e n  Is atomic  orbital.  P r o m o t i o n o f a l a , e l e c t r o n t o n s a , , npe a n d n p a , R y d b e r g o r b i t a l s i s electric  dipole allowed.  One i n t e r e s t i n g f e a t u r e  s p e c t r u m o f ammonia i s t h a t a l l o f t h e s t a t e s resulting  of the valence  (both  shell  Rydberg and i o n ) ,  from t h e promotion o f a 3a, e l e c t r o n , a r e e i t h e r p l a n a r  o r very  146 nearly  planar  .  This  produces long  progressions  in v  9  and i s a r e s u l t o f  -145-  the  l o s s o f a n e l e c t r o n f r o m an o r b i t a l  geometry. of  a 1 a-j  Such  l a r g e changes  electron  i n geometry  (essentially  pyramidal  are not expected f o r the promotion  atomic and nonbonding)  and, t h e r e f o r e , these t r a n s i t i o n s ional  which s t r o n g l y s t a b i l i z e s  t o Rydberg  are expected t o r e s u l t . i n  less  orbitals vibrat-  excitation. a.  Valence Shell The  Spectrum.  valence shell  e l e c t r o n energy  loss  shown i n F i g u r e 3 8 . T h e l o c a t i o n s o f p e a k s resolution  are consistent with higher  120 1 4 5 photoabsorption ' and e l e c t r o n  where t h e peaks  have been a s s o c i a t e d w i t h  spectrum, c o r r e s p o n d i n g peaks C (9.2 e V ) , D (11.3 eV)  s p e c t r u m o f ammonia i s  impact r e s u l t s  Rydberg  are observed a t :  and E (15.2 e V ) .  4 9 50 5 6 ' '  transitions.  In our  A ( 6 . 3 e V ) , B (^ 8.0 e V ) ,  The l o c a t i o n o f t h e  first 120 147  ionization potential of  i n F i g u r e 38 i s b a s e d o n t h e e x p e r i m e n t a l v a l u e  10.2 eV. b. Nitrogen K-shell The  nitrogen  Excitation.  K-shell  energy  loss  s p e c t r u m o f ammonia i s shown i n  F i g u r e 39 a n d t h e e n e r g i e s a n d t e n t a t i v e a s s i g n m e n t s o f p e a k s T a b l e 12.  The g e n e r a l appearance  carbon K - s h e l l in  o f the spectrum  orbitals.  o f a nitrogen  Transition  "K-shell"  a r el i s t e d i n  resembles t h a t o f t h e  s p e c t r u m o f methane and t h e s p e c t r u m has been  terms o f the e x c i t a t i o n  Rydberg  '  electron  interpreted (la,) to  e n e r g i e s e s t i m a t e d u s i n g quantum  defects  120 1 4 5 d e r i v e d from the valence s h e l l in  T a b l e 12. The f i r s t  assigned (compared  w i t h an e l a s t i c levels  '  o f ammonia h a v e b e e n  included  d i s c r e t e p e a k o b s e r v e d a t 4 0 0 . 6 eV h a s b e e n  t o the t r a n s i t i o n ,  vibrational  spectrum  l a , -> 3sa-,.  T h e p e a k h a s a FWHM o f 0.8 eV  peak FWHM o f 0.5 e V ) i n d i c a t i n g  are e x c i t e d .  t h a t a number o f  The 5.0 eV t e r m v a l u e i m p l i e s  a quantum  ELASTIC  1  st  IP  NH  : B  T  0  10  20  Energy Loss FIGURE 3 8 . V a l e n c e s h e l l  (eV)  e n e r g y l o s s s p e c t r u m o f ammonia.  3  Intensity  (arbitrary units)  -LH-  TABLE  12  ABSOLUTE ENERGIES OBSERVED IN THE  PEAK  ( e V ) , R E L A T I V E ENERGIES AND  NITROGEN K-SHELL  ENERGY  AE  TENTATIVE ASSIGNMENTS OF THE  SPECTRUM OF AMMONIA  TERM VALUE  ASSIGNMENT  3  ESTIMATED ENERGY 0  1  400.6  0  5.0  3sa-|  401.2  2  402.2  1.6  3.4  3pe  402.8  3  403.5  2.9  2.1  3pa,  403.4  4  404.1  3.5  1.5  4sa /3d  404.1  4pe  404.3  5pe  404.8  ^ K-EDGE  C  404.6  4.0  1.0  405.6  5.0  0  5  <\, 4 1 4  d  * 13.5  6  ^ 428  d  ^ 27.5  a. O n l y t h e f i n a l o r b i t a l l a , = Nitrogen K).  PEAKS  1  CO  (SHAKE-UP < AND (SHAKE-OFF  involved  i n the e x c i t a t i o n  i s given (the i n i t i a l  orbital  is  b. E s t i m a t e d u s i n g q u a n t u m d e f e c t s d e r i v e d f r o m t h e v a l e n c e s h e l l s p e c t r u m ' ' ; n = 3; 1.02 n > 3, 6 ( n p e ) = 0.8, 6 ( n p a , ) = 0.54, and 6 ( n d ) was a s s u m e d t o be 0. 5 0  c. From X - r a y d.  Onset.  PES  3 2  .  1 4 3  1 4 5  5(nsa,) =  1.25  -149-  d e f e c t o f 1.35 w h i c h i s c o m p a r a b l e w i t h t h e i n the valence f o r the  shell  spectrum f o r the  1.25 q u a n t u m d e f e c t  transition  3a-j -> 3sa-j.  3s q u a n t u m d e f e c t t o be a p p r e c i a b l y h i g h e r t h a n  f o r t h e h i g h e r members o f t h e s e r i e s f o r valence  shell  excitation  i npolyatomic  I t i s normal  that  determined  molecules.  i n a m m o n i a , t h e 3sa-j o r b i t a l  observed  In f a c t ,  i s n o t a pure cc  Rydberg o r b i t a l  a n d shows a p p r e c i a b l e a n t i b o n d i n g  number 2, o b s e r v e d electron  t o the  a t 404.2 e V , i s a s s i g n e d  i n Reference  Rydberg o r b i t a l .  (3pe).  The energy d i f f e r e n c e o b s e r v e d  f o r the  FWHM o f t h e  s e c o n d peak i n o u r s p e c t r u m d o e s n o t s u p p o r t 3pa-| u n l e s s f o r K - s h e l l  energy d i f f e r e n c e i s small o r the suggest  t h a t the t h i r d  p e a k a t 403.5 eV c o u l d r e p r e s e n t  This  The  e n e r g y o f t h e f o u r t h p e a k , 401.1  energy c a l c u l a t e d Rydberg o r b i t a l s . the e x c i t a t i o n The  excitation  .  i m p l i e s a 3p s p l i t t i n g  f o r the e x c i t a t i o n  energy  shell  The o b s e r v e d  a c o n t r i b u t i o n from t h e 3 p e a n d 3pa-j  i n t e n s i t y o f one t r a n s i t i o n  l a - | •> 3pa-|. observed  peak h a s a  two v a l e n c e  ( 6 = 0.54.) i s < 0.6 eV  3 a + 3 p e a n d 3a-| + 3pa-|  1 a-j  This  o f a 3a-j e l e c t r o n t o t h e 3 p e  transitions  the t r a n s i t i o n  o f a la-j  from the a d i a b a t i c t r a n s i t i o n  50) f o r the promotion  1  Peak  ( p e a k maximum) o f 1.0 i n c o n t r a s t t o  t h e q u a n t u m d e f e c t o f 0.8 ( c a l c u l a t e d reported  .  t o the promotion  l o w e s t e n e r g y 3p R y d b e r g o r b i t a l  FWHM o f 0.7 eV a n d a quantum d e f e c t  character  the  i s weak.  We  transition  o f 1.3 eV f o r K - s h e l l  excitation.  eV, i s c o n s i s t e n t w i t h t h e  o f a la-j e l e c t r o n t o 3 d , 4 s a n d 4p  The h i g h e n e r g y s h o u l d e r p r o b a b l y  o f n - 5 and h i g h e r Rydberg  has c o n t r i b u t i o n s from  orbitals.  p o s i t i o n o f t h e K-edge i n o u r s p e c t r u m i s b a s e d o n t h e  experimental  32 X - r a y PES v a l u e broad  o f 405.6 eV f o r t h e la-j b i n d i n g e n e r g y i n ammonia.  The  a t ^ 4 1 4 eV a n d o, 4 2 8 eV a r e  with  s t r u c t u r e s with onsets  the simultaneous  transitions  of a K-shell  and valence  identified  shell  electrons.  -150-  7.2.3. H a t e r . The g r o u n d e l e c t r o n i c s t a t e o f t h e w a t e r m o l e c u l e has C and t h e e l e c t r o n (la,)  configuration: (2a,)  2  The  l a , orbital  the  oxygen  and  have a , , b, a n d  (lb )  2  i s formed  nucleus.  The b  2  2  (3a,)  three  Valence Shell The  Figure D  40.  optical  120  Is o r b i t a l  Transitions  and  i s l o c a l i z e d on i n C^  symmetry  v  i n v o l v i n g the promotion of  o r b i t a l s are e l e c t r i c  A  dipole  allowed.  (7.5 e V ) , B ( 9 . 7 e V ) , C (10.1  F ( 1 7 . 2 eV) a r e c o n s i s t e n t  and e l e c t r o n  impact r e s u l t s  have b e e n i n t e r p r e t e d  i n terms o f Rydberg  location  ionization potential  of the f i r s t  A,.  e n e r g y l o s s s p e c t r u m o f w a t e r i s shown i n  l o c a t i o n s of peaks;  (11.1 e V ) , E ( 1 3 . 6 eV) a n d  resolution  1  Spectrum.  valence shell  The  2  p - o r b i t a l s are nondegenerate  symmetries.  2  (lb,) ,  2  from t h e oxygen  a l a , e l e c t r o n t o ns a n d np R y d b e r g a.  symmetry  2 v  49  '  148  transitions i n Figure  with ' 1  149 2  0  '  higher  . 1  eV),  These 5  0  ,  1  5  1  .  40 i s b a s e d on  spectra The the  120 experimental  value  o f 12.61  b. O x y g e n K - s h e l l The the  K-shell  Excitation. e n e r g y l o s s s p e c t r u m o f w a t e r i s shown i n F i g u r e  e n e r g i e s and t e n t a t i v e a s s i g n m e n t s o f p e a k s  general  appearance  K-shell  spectra  quantum d e f e c t a t 5 3 4 . 0 eV  are l i s t e d  of the spectrum i s s i m i l a r to that  o f m e t h a n e a n d ammonia.  terms o f Rydberg  (la,)  eV.  The  i n T a b l e 13.  observed f o r the  spectrum i s i n t e r p r e t e d i n  e x c i t a t i o n s and e x c i t a t i o n e n e r g i e s e s t i m a t e d  method a r e i n c l u d e d  i s assigned  i n T a b l e 13.  The  t o t h e p r o m o t i o n o f an o x y g e n  t o t h e 3sa, Rydberg  orbital.  The  41  first  using  the  peak o b s e r v e d  K-shell  p e a k has a FWHM o f 1.0  electron eV,  and The  1  I.P  st  0  1  10  1  20  Energy Loss FIGURE 4 0 . V a l e n c e s h e l l  energy  '  30  (eV)  l o s s spectrum o f water.  40  1.0-1  K- edge  H  I  I I I  1 234  oi  530  T T T 540 550 560 Energy Loss (eV )  FIGURE 4 1 . O x y g e n K - s h e l l  energy loss  spectrum o f water  TABLE  13  ABSOLUTE ENERGIES  ( e V ) , R E L A T I V E ENERGIES AND  IN THE OXYGEN K-SHELL  PEAK  P O S S I B L E ASSIGNMENTS OF PEAKS OBSERVED  SPECTRUM OF WATER.  ENERGY  AE  1  534.0  •0  2  535.9  3  537.1  TERM  POSSIBLE , ASSIGNMENT  VALUE  538.5  K-EDGE  3sa-|  534.5  1.9  3.8  3pb^  -  3.1  2.6  |3pa-,  537.1  4.5  539.7  C  ^  1.2  5.7  1  3d  537.5  j4s  538.1  J4p  538.5  CO  (SHAKE-UP < AND (SHAKE-OFF  555  a. O n l y t h e f i n a l  0  5.7  /3pb  4  ESTIMATED ENERGY  orbitals  i n v o l v e d i n t h e K - e x c i t a t i o n s have been i n c l u d e d .  b. E s t i m a t e d u s i n g t h e q u a n t u m d e f e c t m e t h o d w i t h quantum d e f e c t s f r o m t h e v a l e n c e s h e l l spectrum o f w a t e r . ( 3 s ) = 1.38 ( f r o m t h e t e r m v a l u e o f 5.2 eV r e p o r t e d R e f e r e n c e 1 5 0 , 6 ( n s ) = 1.05 n > 3, 6 ( n p a / b ) = 0.7, fi(nd) = 0.05. 1 4 6  ]  From X - r a y  PES  3 2  .  1  by  -154-  indicating value  t h a t a number o f v i b r a t i o n a l  i s 5.7  state.  eV w h i c h  levels  are excited.  i m p l i e s a q u a n t u m d e f e c t o f 1.45  T h i s term v a l u e i s comparable  with  The  term  f o r t h e 3s  t h e t e r m v a l u e o f 5.2  Rydberg eV  observ-  150 ed  i n the valence s h e l l  spectrum of water f o r the promotion of  e l e c t r o n from the outermost o r b i t a l lb-j •> 3sa-j.  The  second peak,  ( d i f f e r e n t data run) which ional  levels.  o b s e r v e d a t 535.9 e V ,  indicates  eV.  s p e c t r u m l b ^ -»• 3 p b  is electric  2  H o w e v e r , t h e l b - j -»- 3 p b 152  has  the e x c i t a t i o n  a FWHM o f 0.9  o f a number o f  2  The  corresponding transition d i p o l e f o r b i d d e n and  e x c i t a t i o n energy  1 a^ ->• 3 p b  vibratand  2  i n the valence has  eV  has shell  n o t been o b s e r v e d .  has b e e n c a l c u l a t e d  by  the  153 and  energy  level, i.e.  T h i s peak i s a s s i g n e d t o t h e t r a n s i t i o n  a t e r m v a l u e o f 3.8  INDO  t o t h e 3s Rydberg  an  t h e IVO  3p R y d b e r g  component.  The  methods.  excitation  third  Both c a l c u l a t i o n s  should result  indicate  that the lowest  from promotion to the  peak i n o u r s p e c t r u m a t 537.1  eV  i s then  b  2  associated  w i t h p r o m o t i o n o f a l a - j e l e c t r o n t o t h e 3pa-| and 3pb-j R y d b e r g o r b i t a l s a n d has a t e r m v a l u e o f 2.6 eV. The e n e r g y d i f f e r e n c e b e t w e e n t h e s e o r b i t a l s 1 50 in  the valence s h e l l  2.62  eV  and  2.46).  agreement w i t h The  fourth  spectrum The  term v a l u e f o r the K - s h e l l  eV  peak o b s e r v e d a t 538.5 eV 4p  Rydberg  ( t e r m v a l u e 1.2  transitions  (term v a l u e s  transition  the term v a l u e s f o r the c o r r e s p o n d i n g v a l e n c e  a s s o c i a t e d w i t h 4s a n d Table  ( l b - j p r o m o t i o n ) i s 0.16  eV)  i s i n good transitions.  i s probably  ( c f . the estimated values i n  13). The  position  o f t h e K-edge shown on o u r s p e c t r u m  i s b a s e d on  the  32 X - r a y PES  v a l u e o f 539.7 eV  f o r the K-shell  broad s t r u c t u r e observed i n the continuum with  the simultaneous t r a n s i t i o n s  binding energy of water.  r e g i o n ^ 555 eV  of a K-shell  and  The  i s associated  valence shell  electrons.  -155-  7.2.4. M e t h a n o l . The  ground e l e c t r o n i c s t a t e o f the methanol  and t h e e l e c t r o n i c c o n f i g u r a t i o n (la ) 1  The  (2a )  2  1  l a ' and  (3a')  2  (4a )  2  1  the C  s  p o i n t group  an e l e c t r o n  shell  1  The  symmetry  g  2  (la")  (6a )  2  1  (7a')  2  3s a n d 3p R y d b e r g ( t w i c e ) and  (2a ) ,  2  1 1  I s and  orbital  3pa".  orbitals is electric  b o t h t h e c a r b o n and o x y g e n s p e c t r u m was  The F i g u r e 42.  carbon Is  symmetries  The  promotion  allowed.  We  regions of methanol.  of  to  have  invest-  A valence  recorded. Spectrum.  valence shell  The  K-shell  dipole  V.  2  from any o f t h e o c c u p i e d m o l e c u l a r o r b i t a l s o f methanol  a. V a l e n c e S h e l l  D  (5a )  a r e 3 s a ' , 3pa'  each o f t h e s e Rydberg igated  2  C  ;  2a' m o l e c u l a r o r b i t a l s r e p r e s e n t t h e oxygen  atomic o r b i t a l s r e s p e c t i v e l y . in  150  m o l e c u l e has  locations  energy  l o s s spectrum o f methanol  of peaks, A  i s shown i n  (6.8 e V ) , B(7.9 e V ) , C (8.3 e V ) ,  ( 9 . 8 e V ) , E ( 1 2 . 0 e V ) , F ( 1 3 . 8 eV) and G ( 1 5 . 8 e V ) , a r e c o n s i s t e n t  higher resolution electron been a s s o c i a t e d w i t h  i m p a c t r e s u l t s ^ ' ^ , where t h e peaks 1  Rydberg  transitions.  have  1  The  l o c a t i o n of the  with  first 120  ionization 10.85  potential  eV. b. C a r b o n The  i n F i g u r e 42 i s b a s e d on t h e a d i a b a t i c  K-shell carbon  value  Excitation.  K-shell  energy  l o s s spectrum of methanol  F i g u r e 43 a n d t h e e n e r g i e s a n d t e n t a t i v e a s s i g n m e n t s o f p e a k s i n T a b l e 14. ions.  We  Transition  of  have i n t e r p r e t e d  the spectrum  i s shown i n are  i n terms o f Rydberg  listed transit-  e n e r g i e s e s t i m a t e d u s i n g term v a l u e s observed i n the 150  valence shell orbital  spectrum  ) are also  f o r 2a" p r o m o t i o n  listed  i n T a b l e 14.  ( m a i n l y an o x y g e n  The  first  discrete  lone  pair  peak o b s e r v e d  : ELASTIC  1 I. P st  Energy Loss IGURE 4 2 .  Valence s h e l l  (eV)  energy l o s s spectrum o f methanol.  K-edge  CH30H  C -shell K  I  III  1234  290 FIGURE 43.  Carbon  300 Energy Loss K-shell  energy l o s s  —1  310  (eV)  spectrum o f methanol  320  TABLE 14 ABSOLUTE  ENERGIES ( e V ) , R E L A T I V E ENERGIES AND TENTATIVE ASSIGNMENTS OF PEAKS OBSERVED  THE CARBON AND OXYGEN K-SHELL SPECTRUM  OF METHANOL •  CARBON K- SHELL PEAK  ENERGY  1  288.1  2  TERM VALUE  AE  OX YGEN K- SHELL ESTIMATED VALUE 9  0  4.2  288.1  289.4  1.3  2.9  289.1  3  290.3  2.2  2.0  289.7 290.6  4  291.3  3.2  1.0  291.O  292.3  4.2  K-EDGE  C  a. E s t i m a t e d u s i n g b. O n l y t h e f i n a l c. X - r a y PES  PEAK  value.  3 2  534.1  AE 0  TERM VALUE 4.8  ASSIGNMENT ESTIMATED VALUE 3  534.6  3sa' 3p  2  537.1  3,0  1.8  3p 3d  d  the term v a l u e s observed involved  ENERGY  1  4s/4p K-EDGE  orbital  IN  0  538.9  4.8  i n the valence s h e l l  i n the e x c i t a t i o n  i s given.  CO  spectrum.  1 5 0  5  -159-  a t 288.1  eV  has a t e r m v a l u e o f 4.2  of a carbon  Is e l e c t r o n  (6  The  = 1.2).  v a l u e o f 4.22 2 a " -> 3 s a ' .  eV  i s assigned to the  ( 2 a ) t o the 3sa' Rydberg 1  term v a l u e f o r t h i s  eV o b s e r v e d The  and  150  s e c o n d and  transition  orbital,  promotion  2 a ' -> 3 s a '  i s very close to the  f o r the corresponding valence s h e l l third  d i s c r e t e peaks  o b s e r v e d e n e r g y d i f f e r e n c e b e t w e e n t h e two  3p  transition,  i n our spectrum  a s s i g n e d t o t h e p r o m o t i o n o f a l a ' e l e c t r o n t o 3p R y d b e r g levels  term  are  orbitals.  i s 0.8  eV.  The  In the  150 valence shell  electron  impact spectrum o f methanol  ( o b t a i n e d w i t h much  h i g h e r r e s o l u t i o n ) two p e a k s w i t h an e n e r g y d i f f e r e n c e o f 0.4 i n g t o peaks  B and C i n F i g u r e 42)  a t i o n s , 2 a " -> 3p.  The  h a v e b e e n a s s i g n e d t o 3p R y d b e r g  o b s e r v e d t e r m v a l u e s w e r e 3.24  eV a n d  a r e somewhat h i g h e r t h a n t h o s e o b s e r v e d i n o u r K - s h e l l 2.0  eV).  The  2.64  spectrum  f o u r t h b a n d o f s t r u c t u r e w i t h a maximum a t 2 9 1 . 3  has c o n t r i b u t i o n s The  eV ( c o r r e s p o n d -  position  f r o m 3 d , 4s and 4p  Rydberg  excit-  eV  which  (2.9  and  eV p r o b a b l y  transitions.  o f t h e c a r b o n K-edge i n o u r s p e c t r u m  i s based  on  the  32 X - r a y PES arising  value  o f 292.3 eV f o r t h e K - s h e l l  ionization  from the simultaneous promotion o f a K - s h e l l  energy.  and  Structures  valence shell  e l e c t r o n s a p p e a r t o be weak. c. Oxygen K - s h e l l E x c i t a t i o n . The  oxygen K - s h e l l  energy  loss  spectrum o f methanol  F i g u r e 44 a n d t h e e n e r g i e s a n d t e n t a t i v e a s s i g n m e n t s o f p e a k s i n T a b l e 14. arising  The  s p e c t r u m has a s l o p i n g  b a s e l i n e which  from the l a r g e c o n t i n u o u s background  apparatus scattered e l e c t r o n s . as a f u n c t i o n o f e n e r g y background  checked  s c a t t e r e d e l e c t r o n s w i t h o u t any  are  listed  i s instrumental,  o f s e c o n d a r y e m i t t e d and  T h i s background  l o s s and was  i s shown i n  i s monotonicallydecreasing  by r e c o r d i n g  t a r g e t gas.  the signal  The  from  background  is  Intensity  ( arbitrary  units )  p  o  CD  CZ TO  -7 v  4>  O X •<  CQ  m  CD  zn  ro —  I t/>  fD  •.v  7  a  0  CD  ro -s  CD CD  CQ  O  :  IO  </) fD O r+ -S  T3  < ©  cz  3  fD =3  O  K..  eft-  cn  ©  o n  .:y.v-  I  0)  u  CD  3  -09L-  CO  O  -161-  more p r o m i n e n t  i n t h e oxygen  K-shell  energy  decrease i n s c a t t e r i n g i n t e n s i t y with as  (energy l o s s )  from t h a t  .  The a p p e a r a n c e  f o rt h e carbon K-shell  region  because  energy l o s s  1 1  of the rapid  , i . e . a t l e a s t as f a s t  o f the spectrum i s appreciably  (Figure  43).  The f i r s t  different  p e a k a t 534.1 eV  has a t e r m v a l u e o f 4.6 eV a n d i s i n t e r p r e t e d a s a r i s i n g f r o m t h e p r o m o t i o n o f an o x y g e n The  electron  t o t h e 3sa' Rydberg  orbital,  l a  1  -> 3 s a ' .  p e a k h a s a FWHM o f 1.2 eV i n d i c a t i n g t h e e x c i t a t i o n o f many v i b r a t i o n a l  levels. at  K-shell  Higher energy s t r u c t u r e c o n s i s t s  537.1 eV.  On t h e b a s i s  o f a b r o a d peak w i t h  a maximum  o f the assignments o f the previous s p e c t r a ,  e x p e c t 3p R y d b e r g  excitations to contribute  broad s t r u c t u r e .  I f this  we  t h e most i n t e n s i t y t o t h i s  i s t h e c a s e , t h e r e l a t i v e i n t e n s i t i e s o f t h e 3p  c o m p o n e n t s m u s t be s i g n i f i c a n t l y d i f f e r e n t f r o m t h o s e o b s e r v e d i n t h e carbon K - s h e l l orbitals  spectrum o f methanol.  are also  Transitions  expected t o c o n t r i b u t e  t o 3 d , 4s a n d 4p  i n t e n s i t y i n the region  Rydberg  o f t h e band  maximum. The  p o s i t i o n o f t h e K-edge i n d i c a t e d on o u r s p e c t r u m  i s b a s e d on t h e  32 X - r a y PES v a l u e  o f 538.9 eV f o r t h e o x y g e n  K-shell  binding  energy o f  methanol.  7.2.5. D i m e t h y l E t h e r . The g r o u n d  e l e c t r o n i c s t a t e o f t h e d i m e t h y l e t h e r m o l e c u l e has C^  symmetry and t h e e l e c t r o n c o n f i g u r a t i o n ( l a ^ The  2  ( 2 a /(lb )  la-j a n d 2 a - j / l b  orbitals  0  2  2  (CH  b  o  n  d  i  n  g  )  1  2  154  :  (2b ) 2  2  (3  a  ]  )  2  (lb ) ,  m o l e c u l a r o r b i t a l s r e p r e s e n t oxygen  respectively.  v  I n t h e e x c i t a t i o n o f an o x y g e n  2  2  1  A . 1  I s and carbon I s Is electron, the  -162-  final  Rydberg  states  have t h e same s y m m e t r i e s a s t h e c o r r e s p o n d i n g s t a t e s  in water [see Section should r e s u l t  (7.2.3.)].  The  promotion of a carbon Is e l e c t r o n  i n a l o w e r i n g o f t h e m o l e c u l a r symmetry t o C . $  point group, e x c i t a t i o n o f a K-shell  electron  3s and 3p R y d b e r g  dipole  a.  levels  Valence Shell The  (carbon o r oxygen) t o a l l  allowed.  Spectrum.  valence shell  shown i n F i g u r e 4 5 . C  is electric  In e i t h e r  The  energy l o s s  spectrum of dimethyl ether i s  l o c a t i o n s o f peaks, A  (6.7 e V ) , B (7.6 e V ) ,  (8.5 e V ) , D (9.2 e V ) , E (11.0 eV) and F (12.9 e V ) , a r e c o n s i s t e n t  with  148 a higher resolution  energy l o s s spectrum  assigned  transitions.  t o Rydberg  associated with  nitrogen  impurity  P e a k s G ( 1 4 . 0 eV) and (verified  F a l s o have a c o n t r i b u t i o n f r o m t h i s ionization potential  , where s t r u c t u r e s  shown i n F i g u r e 45  H ( 1 5 . 5 eV)  by UV-PES) a n d  source.  The  have been  peaks D  l o c a t i o n of the  i s b a s e d on t h e  are and first  experimental,  adiabatic v a l u e o f 9.96 eV. b. Carbon K - s h e l l E x c i t a t i o n . 1 2 0  The  carbon K-shell  energy l o s s  spectrum o f dimethyl ether i s  shown i n F i g u r e 46 a n d t h e e n e r g i e s a n d t e n t a t i v e a s s i g n m e n t s o f p e a k s listed at  i n T a b l e 15.  a p p r o x i m a t e l y 288.5 eV  peak a n d (2a  1  in C  first  $  ( t e r m v a l u e 3.75  p o i n t group) t o t h e 3sa' Rydberg  a carbon K-shell  ( t e r m v a l u e 2.85  electron  structure  a p p e a r s as a s h o u l d e r  eV) on t h e more i n t e n s e  orbital.  1  eV  f r o m 4s a n d 4p R y d b e r g  The  second  electron second  peak  eV) i s a s s i g n e d t o t h e p r o m o t i o n o f  ( 2 a ) t o a 3p R y d b e r g  s p e c t r u m o b s e r v e d a t 291.1  ributions  energy l o s s  i s assigned to the promotion of a carbon K-shell  o b s e r v e d a t 289.4 eV  the  The  are  orbital.  ( t e r m v a l u e 1.15 transitions.  The  third  peak i n  e V ) p r o b a b l y has c o n t -  : ELASTIC a ••••  c  E  CH30CH3  4-1  • ••••  (U  c  (0  CO  I  0  0  10  20  Energy Loss FIGURE 4 5 .  Valence s h e l l  energy loss  1  30  (eV)  spectrum o f dimethyl  ether.  40  1.(H  K-edge  co  mmmm  c 3  v.  CD  b  CH3OCH3  0.5  \  C -shell K  J-  CU 1  CO  c  CD  II I  /12 3 290 FIGURE 4 6  Carbon  300 Energy Loss K-shell  energy l o s s  310 (eV) spectrum o f dimethyl  320 ether.  TABLE  15  ABSOLUTE ENERGIES THE  ( e V ) , R E L A T I V E ENERGIES AND  CARBON AND OXYGEN  CARBON  PEAK  ENERGY  SPECTRA  TERM  ESTIMATED  AE  VALUE  VALUE  9  0  3.75  288.9  2  289.4  0.9  2.85  (289.6 (289.8 290.7  C  291.1  2.6  292.25  3.75  a. E s t i m a t e d  using  b. O n l y t h e f i n a l X - r a y PES  DIMETHYL  ETHER  (CH 0CH 3  OXYGEN  288.5  K-EDGE  OF  K- SHELL  1  3  c.  K-SHELL  TENTATIVE ASSIGNMENTS OF PEAKS OBSERVED IN  1.15  PEAK  ENERGY  1  535.5  j  2 K-EDGE  1538.6  0  538.59  K-- S H E L L  AE  0  values  1 5 5  .  involved  i n the e x c i t a t i o n  ASSIGNMENT  TERM  ESTIMATED  VALUE  VALUE  3.1  3  535.2 /535.9 (536.2 537,0  3sa-j 3p 3d  3.1  4s/4p  3.1  CO  the term values observed i n the valence s h e l l orbital  3>-  is listed.  spectrum of dimethyl  e t h e r ^ 1  5  -166-  The  p o s i t i o n o f t h e c a r b o n K-edge i n d i c a t e d  b a s e d on t h e X - r a y PES c.  value  Oxygen K - s h e l l The  oxygen  o f 292.25  1 5 5  i n our spectrum i s  0.05  eV.  Excitation.  K-shell  energy l o s s spectrum of dimethyl e t h e r i s  shown i n F i g u r e 47 a n d t h e e n e r g i e s a n d t e n t a t i v e a s s i g n m e n t s o f p e a k s listed  i n T a b l e 15.  The  spectrum  i s very d i f f e r e n t from the carbon  s p e c t r u m o f d i m e t h y l e t h e r ( F i g u r e 46) and r e s e m b l e s t h e oxygen spectrum o f methanol The  first  p r o m o t i o n o f an o x y g e n  K-shell  ( t e r m v a l u e 3.1  eV)  h i g h e r q u a n t u m number R y d b e r g  position  on t h e X - r a y PES  K-shell  o f the oxygen value  orbital.  presumably  has  The  contribut-  transition.  K-edge i n d i c a t e d  o f 538.6 ± 0.05  1 5 5  i s assigned to the  e l e c t r o n t o t h e 3sa-| R y d b e r g  b r o a d band o f s t r u c t u r e w i t h a maximum a t 5 3 8 . 6 eV  The  K-shell  (Figure 44).  p e a k a t 535.5 eV  i o n s f r o m 3p and  are  on o u r s p e c t r u m  i s based  eV.  7.2.6. M o n o m e t h y l a m i n e . The symmetry (la ) 1  The  2  ground  electronic  s t a t e o f t h e m o n o m e t h y l a m i n e m o l e c u l e has  ( s t a g g e r e d c o n f o r m a t i o n ) and (2a ) 1  l a ' and  2  (3a')  2  (4a ) 1  2  (la")  2  the e l e c t r o n (5a')  2  (6a ) 1  2  configuration (2a")  2  (7a ) 1  2 a ' o r b i t a l s r e p r e s e n t n i t r o g e n I s and c a r b o n I s  2  156  C  $  ;  (3a") , 2  V.  orbitals  respectively. a.  Valence Shell The  valence shell  i s shown i n F i g u r e 48. been r e p o r t e d  Spectrum. e l e c t r o n energy  loss  spectrum o f monomethylamine  E l e c t r o n i m p a c t d a t a f o r monomethylamine has  i n the l i t e r a t u r e and, o p t i c a l l y , o n l y the X  A  not  transition,  120 w i t h an o n s e t a t 5.2  eV,  has b e e n o b s e r v e d  .  This transition  has  been  120 assigned  to the e x c i t a t i o n of a nitrogen lone p a i r e l e c t r o n  (3a ) to the M  1.0 H  J K-edge  co 4-1  c  CH30CH3 0 -shell  CO  K  k.  4-i  !5 03  I  I  CO  c  0  I  CM I  2  1  0.5-^  530  540  T  Energy Loss FIGURE 47.  Oxygen K - s h e l l  550  T  (eV )  560  energy l o s s spectrum o f dimethyl  570 ether.  Intensity  -89 L-  (arbitrary units)  -169-  3s R y d b e r g shell  orbital,  spectrum  analogous t o t h e f i r s t  (peak A i n F i g u r e 3 8 ) .  b a n d i n t h e ammonia v a l e n c e  Peak A , w i t h a maximum a t a p p r o x -  i m a t e l y 5.7 eV i n o u r s p e c t r u m , i s t h e r e f o r e a s s o c i a t e d w i t h transition.  Peak B  h i g h e r energy  7.0 e V ) p r o b a b l y r e p r e s e n t s t h e e x c i t a t i o n o f a  Rydberg  state.  The l o c a t i o n o f t h e f i r s t  shown i n F i g u r e 4 8 i s b a s e d o n t h e a d i a b a t i c v a l u e b. C a r b o n The  t h e X •> A  K-shell  120  ionization  potential  o f 8.97 eV.  Excitation.  carbon K-shell  energy  loss  spectrum o f monomethylamine i s  shown i n F i g u r e 49 a n d t h e e n e r g i e s a n d t e n t a t i v e a s s i g n m e n t s o f p e a k s a r e listed is  i n T a b l e 16.  The f i r s t  peak o b s e r v e d a t 2 8 7 . 5 eV ( t e r m v a l u e 4.1 e V )  i n t e r p r e t e d as r e p r e s e n t i n g t h e promotion o f a carbon I s e l e c t r o n  3sa'  Rydberg  o r b i t a l , w h i l e t h e s e c o n d p e a k o b s e r v e d a t 2 8 8 . 5 eV ( t e r m v a l u e  3.1 e V ) i s a s s o c i a t e d w i t h c a r b o n I s e x c i t a t i o n Finally,  np R y d b e r g  the  t o a 3p R y d b e r g  3d a n d h i g h e r q u a n t u m number ns a n d  transitions.  p o s i t i o n o f t h e c a r b o n K-edge i n d i c a t e d  i n our spectrum  i s b a s e d on  1 ^5 X - r a y PES v a l u e o f 291.6 ± 0.05 eV. 1 0  c. N i t r o g e n K - s h e l l The  nitrogen  shown i n F i g u r e listed  Excitation.  K-shell  50 a n d t h e  i n T a b l e 16.  energy  loss  spectrum o f monomethylamine i s  e n e r g i e s and t e n t a t i v e  The f i r s t  assignments o f peaks a r e  p e a k a t 4 0 0 . 6 eV h a s a t e r m v a l u e o f 4.5 eV  (6 = 1.3) a n d i s a s s i g n e d t o t h e p r o m o t i o n o f a n i t r o g e n 3sa'  level.  t h e b r o a d b a n d w i t h a peak maximum a t 2 9 1 . 5 eV a n d s h o u l d e r  ^ 290.4 eV i s p r o b a b l y a s s o c i a t e d w i t h  The  tothe  Rydberg  orbital.  orbital.  t o the  The s e c o n d p e a k a t 4 0 1 . 9 eV h a s a t e r m v a l u e o f 3.2 eV  (5 = 0.9) a n d i s a s s o c i a t e d w i t h a 3p R y d b e r g  Is electron  the promotion o f a nitrogen  Is electron to  The b r o a d s t r u c t u r e w i t h a p e a k maximum a t 4 0 4 . 6 eV  to H  K-edge IIP  0)  c 3  CH NH  03 s-  3  5 0.5 H  2  C -shell K  03  o  I  c  co  :l I l l  0  /12 34 280  FIGURE 4 9 .  290 300 310 Energy Loss (eV ) Carbon K-shell energy loss spectrum of monomethylamine.  320  TABLE 16 ABSOLUTE ENERGIES  ( e V ) , R E L A T I V E ENERGIES AND TENTATIVE ASSIGNMENTS OF PEAKS OBSERVED IN  THE CARBON AND NITROGEN K-SHELL SPECTRA OF MONOMETHYLAMINE  CARBON PEAK  TERM VALUE  ENERGY  1 2  K-SHELL  287.5  0  288.5 1.0 s h o u l d e r M .8?  K-EDGE  (CH NH ). 3  2  NITROGEN K-SHELL ESTIMATED VALUE 3  PEAK  TERM VALUE  ENERGY  4.1  1  400.6  3.1 2.3  2  401.9  1.3  3.2  3  o403.4  o,2.8  1.7  4  404.6  4.0  0.5  405.1  4.5  290.4  2.9  1.2  291.5  4.0  0.1  291.6  4.1  289.2 290.2  K-EDGE  0  ASSIGNMENT ESTIMATED VALUE 3  4.5  3s 3p  403.3 403.6  a. E s t i m a t e d f r o m t h e q u a n t u m d e f e c t s d e r i v e d f r o m t h e e n e r g y p o s i t i o n s o f t h e f i r s t 6 ( n d ) was a s s u m e d t o be 0. b. O n l y  the final  orbital  c. X - r a y PES  value  d. X - r a y PES  value  1 5 5  involved i n the e x c i t a t i o n  .  1 5 5  ,  1 5 7  .  i s given.  4s 3d/4p  two p e a k s .  -172-  ( siiun  AjBjjjqje )  Ajjsueiui  -173-  (term  value  probably  0.5  a shoulder  T a b l e 16  3d,  and  3p  includes estimates  peaks i n our  The  position  b a s e d on  t h e X - r a y PES  7.2.7. Term The  3s  charge. values  '  as  i n the  is  Rydberg  energies  from the observed e n e r g i e s  of  on the  eV  f o r the  spectrum i s  nitrogen Is binding  energy  the  K - s h e l l s p e c t r a o f m e t h a n e , ammonia and  3s  term values 1 2 0  '  CH^  s i n c e term values  1 3 6  '  < NH  1 4 5  3  '  1 4 8  derived '  < HgO  increase with  s e r i e s , w a t e r , methanol  d e r i v e d from both the valence  f o l l o w the  np  excitation  K-edge i n d i c a t e d i n o u r  o f 405.1  same m o l e c u l e s  In the  and  eV)  157  bound e l e c t r o n ) w i t h  expected,  ^ 1.7  Values.  same o r d e r  s p e c t r a of the  of these  derived  nitrogen  value  term values  f o l l o w the  tightly  155  value  spectrum.  o f the  i n monomethylamine  a t ^ 403.4 ( t e r m  h i g h e r q u a n t u m number ns  b a s i s o f t h e quantum d e f e c t s  3s a n d  is  and  associated with  transitions. the  eV)  same o r d e r w i t h C H 0 C H 3  3  and  shell  1 5 0  valence  shell  ( f o r promotion of the  (see T a b l e 17).  This  dimethyl  ether, the  s p e c t r a and The  the  nuclear  3s  term  K-shell  trend  least  result  increasing effective  < C H 0 H < H,,0. 3  from the  water  spectra  i n the  valence  135 shell  s p e c t r a has  increases less  penetration  Carbon  i n t o the  i s observed  m e t h y l amine w i t h  The  on  i t s carbon character w i t h  similar trend  7.3.  been e x p l a i n e d  the  core  and,  i n the  term values  the  b a s i s t h a t the  3s  Rydberg  orbital  i n c r e a s i n g a l k y l a t i o n , which r e s u l t s t h e r e f o r e , a lower  K - s h e l l s p e c t r a o f ammonia and i n the  expected order,  A  term value.  CH NH Q  0  mono<  NH,,.  Tetraf1uoride.  carbon t e t r a f 1 u o r i d e molecule i s t e t r a h e d r a l i n i t s ground  in  TABLE 17. 3s AND  3p RYDBERG TERM VALUES  ELECTRON) IN C H ,  K-Shell  2  3  EXCITATION AND  NH.  (a)  3.95  2  it:  3a 29 61  5.0  2.7  CH NH 3  (c)  5.7  2.81 2.24  (3.8  5.2  3  C  K  4.5  4.1  3.2  (3.1 12.3  C  °K 4.8  K  3.1  c. R e f e r e n c e s  120,145  ..(d)  2a  4.2  4.22  3s  (2.9 12.1  (3.24 12.64  3p  3  'K  136; 148.  3s  CH 0H  3  b. R e f e r e n c e e. R e f e r e n c e  Final Orbital  (2.62 [2.46  12.6  CH 0CH  158; 150;  (d)  lb  4.43  2  h  a. R e f e r e n c e d. R e f e r e n c e  (OUTERMOST  H 0  It, 3.7  VALENCE SHELL EXCITATION  C H ^ .  3  CH,  3.04  K-SHELL  N H 3 , H 0 , C H 0 H , C H 0 C H , AND  4  Ne  OBSERVED FOR  3.75 2.85  lb-  (e)  3.37  3s  70 41  3p  {i:  -175-  electronic (lt )  (la,)  6  2  The  l t  s t a t e and  and  2  fluorine  2  has  (2a,)  the e l e c t r o n  (3a,)  2  (2t )  2  l a , molecular orbitals  I s (K) a t o m i c o r b i t a l s .  b e t w e e n t h e l a , and orbitals will  l t  2  It  (3t )  is negligible  Similarly  I s (K) a t o m i c o r b i t a l  be d e s i g n a t e d c a r b o n  energy  an e f f e c t i v e  Reference  two  the e l e c t r o n s  filling  i s formed  this  orb-  6.2.  t h a t the i n n e r atom  shell  "surrounded"  have been  attributed  b a r r i e r on t h e o u t e r r i m o f t h e s e m o l e c u l e s  The  fluorine  1 4 4  electrons.  131). T h e r e f o r e , a p o t e n t i a l  t e t r a f l u o r i d e molecule.  The  A,.  of  2  eV).  ]  of t h e i r atomic character  f o r m o l e c u l e s composed o f a c e n t r a l  potential  6  difference"^ '  by e l e c t r o n e g a t i v e a t o m s , show a n a l o m o u s f e a t u r e s w h i c h to  (It,) ,  l i n e a r combinations  (^ 0.001  has a l r e a d y b e e n p o i n t e d o u t i n S e c t i o n  absorption spectra  6  2  t h e 2a, m o l e c u l a r o r b i t a l  and  K-shell  4  from  calculated  :  ( l e ) (4t )  6  2  be d e s i g n a t e d f l u o r i n e - K b e c a u s e  from the carbon will  2  are formed The  orbitals  a n d a s s u m e d t o be d e g e n e r a t e .  ital  (4a,)  6  2  144  configuration  b a r r i e r may K-shell  exist  i n the  (see  carbon  absorption spectra of the 26  fluoromethanes, including  carbon t e t r a f l u o r i d e ,  using Bremsstrahlung r a d i a t i o n . of  the carbon  of  a possible potential a.  Valence Shell The  is  K-edges, which  However, a b s o r p t i o n s p e c t r a  a r e the most i n t e r e s t i n g  i n the  region  from the p o i n t o f  view  b a r r i e r , were not r e p o r t e d , Spectrum.  valence shell  shown i n F i g u r e 51  have b e e n o b t a i n e d  energy  loss  spectrum  and t h e e n e r g i e s o f peaks  of carbon  tetrafluoride  are given i n Table  18.  The  v a l e n c e s h e l l s p e c t r u m o f c a r b o n t e t r a f l u o r i d e has p r e v i o u s l y been 1 ozr obtained w i t h 400 eV i n c i d e n t e l e c t r o n s , z e r o d e g r e e s c a t t e r i n g a n g l e 1 36  and  a resolution  o f ^ 0.045 eV.  The  o b s e r v e d peaks  have b e e n a s s i g n e d  1.0 ELASTIC  T0  1st  1  10  »  |p  1  20  Energy Loss FIGURE 5 1 .  Valence s h e l l  electron  energy  loss  1  1  30  1  r  40  (eV) spectrum o f carbon  tetrafluoride.  -177-  TABLE 18 ABSOLUTE ENERGIES ( e V ) OF PEAKS OBSERVED  I N THE VALENCE SHELL  ENERGY LOSS SPECTRUM OF CARBON TETRAFLUORIDE.  THIS WORK 'EAK  REFERENCE 136°  5  ENERGY  ENERGY  1  12.5  12.51  2  13.7  113.59 (13.89  3  4  15.9  15.  16.8  1  6  .  8  8 1  6  5  17.2  6  17.8)  7  18.4)  8  19.3  19.42  9  20.6  20.53  18.01  a. 2.5 keV i n c i d e n t e n e r g y , 0.5 eV FWHM e l a s t i c average s c a t t e r i n g angle 2 x 1 0 " r a d .  peak and  2  b. 4 0 0 eV i n c i d e n t e n e r g y , 0.045 FWHM e l a s t i c peak a n d z e r o d e g r e e s c a t t e r i n g a n g l e . The s p e c t r u m has been a s s i g n e d i n R e f e r e n c e 136.  -178-  to  Rydberg  transitions  u s i n g t h e t e r m v a l u e scheme and t h i s  interpretation  1 is  consistent with  oc  that of the other fluoromethane molecules  s p e c t r u m compares f a v o u r a b l y w i t h  the higher r e s o l u t i o n  .  Our  spectrum  (see  Table 18). b. C a r b o n K - s h e l l The is are  Excitation.  carbon K-shell  energy  loss  spectrum of carbon  tetrafluoride  shown i n F i g u r e 52 a n d t h e e n e r g i e s and p o s s i b l e a s s i g n m e n t s o f p e a k s listed  structure  i n T a b l e 19. located just  The  spectrum i s dominated  b e l o w t h e c a r b o n K-edge.  has a number o f c o m p o n e n t s w h i c h a r e c l e a r l y spectrum  shown i n t h e i n s e r t  i n F i g u r e 52.  i n t e r p r e t a t i o n s of the carbon K-shell Section  7.2.1) and  the valence s h e l l  by a b r o a d band o f  T h i s band o f  visible On  structure  i n t h e expanded  the basis of the  spectrum o f methane  Rydberg  (see  s p e c t r a o f methane and  the  fluoro-  12g methanes  , we  expect the lowest energy  spectrum o f carbon t e t r a f l u o r i d e transition  is optically  transition  t o be, 2a,  f o r b i d d e n and  i n the carbon  ( c a r b o n - K ) -> 3sa-,.  i s forbidden  K-shell  This  i n our experiment  i f the  f i r s t Born a p p r o x i m a t i o n i s v a l i d ( t h e i n c i d e n t energy i s e i g h t t i m e s the e x c i t a t i o n e n e r g y ) . H o w e v e r , b o t h i n i t i a l and f i n a l s t a t e s b e l o n g t o t h e 53 same s y m m e t r y s p e c i e s and d e v i a t i o n s The (see  spectrum  from the Born t h e o r y are e x p e c t e d  i s expected to resemble the carbon K - s h e l l  F i g u r e 37) w h e r e t h e 3s peak has much l e s s  shown by F i g u r e 5 2 , t h i s  i s not observed.  u r e s on t h e band have quantum d e f e c t s o f 1.2, v a l u e s , 4.0,  3.7,  excitation.  The  in  3.4  and  3.1  eV  1.1,  t h a n t h e 3p.  the f i r s t  1.0  and  0.9  four  ( F i g u r e 37) was  3.7  eV.  carbon  K-shell  In the v a l e n c e  As  struct-  (term  respectively), a l l consistent with  term v a l u e observed f o r the f i r s t  t h e methane s p e c t r u m  s p e c t r u m o f methane  intensity  Moreover,  .  3s  absorption shell  -6ZL-  -180-  TABLE 19 ABSOLUTE  ENERGIES  ( e V ) , R E L A T I V E ENERGIES AND  PEAKS OBSERVED IN THE CARBON AND  P O S S I B L E ASSIGNMENTS  OF  FLUORINE K-SHELL ENERGY LOSS SPECTRA  OF CARBON TETRAFLUORIDE.  CARBON K-SHELL  FLUORINE K-SHELL —  PEAK  ENERGY  1  297.8  2  AE  TERM VALUE  PEAK  0  4.0  298.1  0.3  3.7  3  298.4  0.6  3.4  4.  298.7  0.9  3.1  5  299.3  1.5  2.5  1  6  300.2  2.7  1.6  2  301.8  4.0  K-EDGE  d  n2o?™  ,—  ENERGY  AE  K-EDGE  o,694 d  TERM VALUE  ^4  shoulder  692.9  695.2  ASSIGNMENT  3s  0  2.3  3p  1.1  1.2  3d  l  2.3  a. D e f i n e d a s t h e d i f f e r e n c e b e t w e e n t h e e x c i t a t i o n e n e r g y a n d t h e i o n i z a t i o n p o t e n t i a l ( i . e . t h e binding energy o f the e l e c t r o n i n t h e e x c i t e d o r b i t a l ) . b. O n l y t h e f i n a l  orbital  i s given.  c. T h i s a s s i g n m e n t d o e s n o t a p p l y d. X - r a y PES v a l u e s  3 2  .  ^  _ (JKDIIAL  to the carbon K-shell  spectrum  (see t e x t ) .  -181-  spectrum 4.1  of carbon t e t r a f l u o r i d e  - 3.5  eV.  ,  However, t h e energy  o f t h e b a n d , ^ 0.3 structure.  1 0 0  3s t e r m v a l u e s w e r e i n t h e  s p a c i n g s between t h e f i r s t  eV, a r e t o o l a r g e t o be a s s o c i a t e d w i t h  T h e r e f o r e , the appearance  and  s i x have q u a n t u m d e f e c t s o f 0.67  2.5  and  carbon  1.6  eV.  K-shell  T h e s e f e a t u r e s may electron  t o 3p and  term v a l u e s a r e c o n s i s t e n t w i t h  i s highly  and  0.08  four  components  vibrational  of these m u l t i p l e t  r e g i o n where a s i n g l e peak i s e x p e c t e d  range  features  unusual.  i n the  Features  respectively  five  (term v a l u e s  be a s s o c i a t e d w i t h t h e p r o m o t i o n  3d R y d b e r g  orbitals  of a  respectively.  those observed f o r corresponding  The  Rydberg 1 3fi  excitations ( e . g . CF^, ively). The  i n the valence s h e l l I t , •+ 3p and  spectra o f the fluoromethane  l e -> 3d h a v e t e r m v a l u e s o f 2.6  and  molecules  1.6  eV  l o c a t i o n o f t h e c a r b o n K-edge i n d i c a t e d on o u r s p e c t r u m  respect-  i s based  32 on t h e e x p e r i m e n t a l X - r a y PES  value  o f 301.8  eV.  The  extremely  broad  s t r u c t u r e o b s e r v e d a b o v e t h e K-edge i s p o s s i b l y a s s o c i a t e d w i t h s h a k e - u p shake-off processes i n conjunction with  K-shell  excitation  and/or  and  ioniz-  ation. With carbon  regard to 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  K-shell  energy  loss  spectrum  not observed i n the K-shell and w a t e r , w h e r e a p o t e n t i a l and  an u n u s u a l  barrier  i s not expected  number o f c o m p o n e n t s a r e o b s e r v e d  The l a r g e s t v i b r a t i o n a l = 0.16 e V . 1 5 9  0  has  two  features  s p e c t r a o f m o l e c u l e s s u c h as m e t h a n e , ammonia,  where a s i n g l e peak a s s o c i a t e d w i t h  v  of carbon t e t r a f l u o r i d e  the  (see F i g u r e s 37,  39  41): i.  +  barrier,  3s R y d b e r g  s p a c i n g f o r CF4  i n the energy  excitation  i n i t s ground  region  i s expected.  electronic  This  state i s  -182-  is apparently  not  of the higher  e n e r g y components  Jahn-Teller  vibrational  splitting  of the  2a-, •> 3pt£» w h i c h has  s t r u c t u r e and  i t i s also unlikely that  ( i . e . p e a k s 3 and  ^  state arising  been a s s o c i a t e d w i t h  4)  are  Jahn-Teller  i s l a r g e r i n methane than i n c a r b o n t e t r a f l u o r i d e .  rated  by  observed  valence  shell  i n methane ( l t  2  spectra  -> 3 s , AE  - where a ' d i s t i n c t = 0.68  eV)  while  apparent i n the carbon t e t r a f l u o r i d e spectrum. splitting Figure  i s not  37)  we  obvious  do  not  i n the  expectvto  iated with ii.  splitting, the  observe a s p l i t t i n g  the  there.can  Tr, s t a t e . ratio  .•  observed Figures  i n the 37,  39  41).  i n m o l e c u l e s where the I t a r i s e s because the suppressed barrier on  the  onlybe  1  "direct"  is  been  not  (see  carbon K - s h e l l is  a maximum o f t h r e e  i n the  i s small  Thjs feature existence  appreciable features  continuum region  i n comparison w i t h  similar  water  o f a p o t e n t i a l b a r r i e r has  t h e e j e c t e d e l e c t r o n has Using  assoc-  ratios  eV  been  spectra  proposed. is  enough e n e r g y t o overcome  may  be  increase  K-edge  (see  direct ionization  t h i s model, the 308  of the  i s common t o a l l i n n e r s h e l l  intensity associated with  R e f e r e n c e 1.31).  has  •  K-continuum a t approximately  onset of c.  until  (see  i n the  K - s h e l l s p e c t r a o f m e t h a n e , ammonia and and  illust-  Jahn-Teller  However, even i f t h e r e  of the . i n t e n s i t y  to t h a t of d i s c r e t e s t r u c t u r e s  is  splitting  a  instab-  carbon K - s h e l l spectrum o f methane,  spectrum of carbon t e t r a f l u o r i d e ; Jahn-Teller  This  a splitting  Since  in  the  intensity  associated with  the  ionization.  Fluorine K-shell Excitation. The  tetrafluoride  a  transition,  ility  the  associated with  from the  peak f i v e .  any  f l u o r i n e .K-shel1 e l e c t r o n energy l o s s s p e c t r u m o f i s shown i n F i g u r e  53  and  the energies  and  carbon  possible  K-edge  1.0 H  CF  4  l= -shell  4  CO CO  —. • • t* ... .: .  - 0.6-I 1  2  T  690  •i  700 Energy  FIGURE 53. F l u o r i n e  I  K-shell  energy  710  .»^/.V */-A ,  ..;iv  720  Loss (eV) loss  spectrum o f carbon  tetrafluoride.  -184-  a s s i g n m e n t s o f s t r u c t u r e s a r e g i v e n i n T a b l e 19.  The o p t i c a l  absorption  s p e c t r u m has p r e v i o u s l y been o b t a i n e d u s i n g B r e m s s t r a h l u n g r a d i a t i o n I t c o n s i s t s o f one b r o a d a b s o r p t i o n band l o c a t e d j u s t below several  b r o a d bands i n t h e continuum r e g i o n .  .  t h e K-edge a n d  O u r s p e c t r u m shows a b r o a d  band b e l o w t h e K-edge w i t h a maximum l o c a t e d a t 692.9 eV a n d a h i g h e n e r g y s h o u l d e r l o c a t e d a t a p p r o x i m a t e l y 694 eV. i s asymmetric  and appears  The  discrete  was  attributed  and  t o have a c o n t r i b u t i o n  s t r u c t u r e observed i n the K-shell to the promotion o f a f l u o r i n e  bonding v a l e n c e o r b i t a l s . likely.  The l o w e n e r g y  high energy  from u n r e s o l v e d s t r u c t u r e .  photoabsorption spectrum K-shell  We s u g g e s t t h a t a R y d b e r g  The quantum d e f e c t s d e r i v e d  s i d e o f t h e peak  electron  interpretation  i s more  f r o m t h e l o c a t i o n s o f t h e peak maximum  s h o u l d e r a r e 0.57 a n d 0 r e s p e c t i v e l y  ( t e r m v a l u e s 2.3 a n d  1.2 e V ) , c o n s i s t e n t w i t h t h o s e e x p e c t e d f o r 3p a n d 3d R y d b e r g The  to anti-  excitation.  s t r u c t u r e on t h e l o w e n e r g y s i d e o f t h e peak may be a s s o c i a t e d w i t h 3s  Rydberg  excitation.  carbon K - s h e l l  This interpretation  that of the  s p e c t r u m o f methane ( s e e T a b l e 11) and t h e v a l e n c e s h e l l  spectra o f the fluoromethanes The  i s consistent with  (including CF^).  l o c a t i o n o f t h e f l u o r i n e K-edge i n d i c a t e d  on o u r s p e c t r u m i s  32 b a s e d on t h e X - r a y j u s t beyond  PES v a l u e  o f 695.2 eV.  The i n t e n s i t y  of structure  t h e K-edge i s a p p r o x i m a t e l y o n e - h a l f t h a t o f t h e m a i n  discrete  26 peak ( s e e a l s o t h e o p t i c a l  absorption spectrum  to t h e low r a t i o o f continuum K-shell  to discrete  This i s i n sharp  This suggests that i fa p o t e n t i a l  barrier exists  t e t r a f l u o r i d e molecule (see the carbon K-shell e f f e c t on t h e e x c i t a t i o n  contrast  s t r u c t u r e observed i n t h e carbon  spectrum o f c a r b o n t e t r a f 1 u o r i d e and t h e f l u o r i n e  of SFg.  little  ).  of a fluorine  K-shell  spectrum  i n the carbon  d i s c u s s i o n ) i t p r o b a b l y has K-shell  electron.  1 6  -185-  7.4.  Carbon The  K-shell  ground  and t h e e l e c t r o n ( l a / The  orbitals 3a-j and  o f Acetone.  s t a t e o f t h e a c e t o n e m o l e c u l e has C  2  symmetry  2 v  configuration: ( 3 a /  ( l b / (valence s h e l l )  2a-j m o l e c u l a r o r b i t a l s  molecular orbitals  atomic o r b i t a l s  their  Loss Spectrum  a r e formed  2 4  ,  1  A  ]  f r o m t h e I s (K) a t o m i c  o f oxygen and t h e c a r b o n y l c a r b o n r e s p e c t i v e l y . l b  orbitals  electronic  ( 2 a /  la-j a n d  Energy  represent linear  o f t h e two m e t h y l  are designated K-shell  respective nuclei  carbons.  The  the  combinations of the Is electrons  e l e c t r o n s because  (nonbonding)  Similarly,  filling  these  they are l o c a l i z e d  and a r e m a i n l y a t o m i c  (K)  on  i n character.  32 The  X - r a y PES  the carbon  spectrum o f acetone  K-edge s e p a r a t e d by 2.6  represent the i o n i z a t i o n the methyl larger and  l b  of 3a-j/lb  On  recently Rydberg  molecular orbitals  valence shell  and  2  ratio  1 6 0  On  '  i n the region  2:1).  2a-j e l e c t r o n s  0.5  1 6 1  this  and  binding  the K-shell spectrum. carbon K - s h e l l energy l o s s  of  peaks with  e n e r g y and  the  s p e c t r u m , t h e 3a-j degenerate  +  eV). loss  prominent  b a s i s we  These  respectively,  a r e c o n s i d e r e d t o be e f f e c t i v e l y  e l e c t r o n energy  been r e p o r t e d transitions.  dominate The  (intensity  peaks  t h e b a s i s o f t h e X - r a y PES  at our e x p e r i m e n t a l r e s o l u t i o n The  eV  carbon a s s o c i a t e d w i t h the lower K - s h e l l  i n t e n s i t y peak. 2  c o n s i s t s o f two  spectrum of acetone features  e x p e c t Rydberg  has  have b e e n a s s i g n e d t o transitions  spectrum of acetone  to  i s shown i n  F i g u r e 54 and t h e e n e r g i e s a n d p o s s i b l e a s s i g n m e n t s o f p e a k s  are given i n  + T h e o r e t i c a l l y a small energy d i f f e r e n c e i s expected. A s i m i l a r s i t u a t i o n occurs i n t h e CF^ m o l e c u l e f o r t h e f l u o r i n e I s ( K ) e l e c t r o n s . In t h i s case, the c a l c u l a t e d ' ^ energy s p l i t t i n g i s very small 0.001 e V ) . 3 2  3 2  1  K-edge  (METHYL)  K-edge (CARBONYL)  CH3COCH3 C - shell K  280 FIGURE 54.  T  290 Carbon  300 Energy Loss K-shell  energy  loss  310 (eV)  spectrum o f acetone.  320  -187-  T a b l e 20. The f i r s t preted  as a r i s i n g  d i s c r e t e peak w i t h  from t h e promotion  3a-j/lb2> t o t h e 3sa-j R y d b e r g the  X - r a y PES v a l u e  32  a maximum a t 2 8 6 . 8 eV i s i n t e r -  o f a carbon  orbital.  K-shell electron  The o b s e r v e d e x c i t a t i o n  f o rthe series l i m i t  (methyl),  energy and  i m p l i e s a q u a n t u m d e f e c t o f 1.2. 130  The m a g n i t u d e for  a 3s R y d b e r g  Rydberg and  o f t h i s quantum d e f e c t s t a t e and s i m i l a r  161).  that  t o t h e quantum d e f e c t  s e r i e s i nthe valence s h e l l  1.09, R e f e r e n c e  i s consistent with  spectrum o f acetone  The f i r s t  peak o b s e r v e d  expected  derived  f o r t h e ns  ( 1 . 0 3 , R e f e r e n c e 160  i n o u r spectrum  has a  FWHM o f 1.0 eV c o m p a r e d w i t h a FWHM o f 0.6 eV f o r t h e p e a k a s s o c i a t e d elastically  scattered electrons.  number o f v i b r a t i o n a l is  associated with  levels.  an e n e r g y  This  indicates the excitation o fa  I ti s unlikely  t h a t any o f t h i s  Rydberg  the e x c i t a t i o n  K - s h e l l e l e c t r o n ( m e t h y l ) t o one o r more c o m p o n e n t s o f t h e 3p  orbital  consistent with the  broadening  d i f f e r e n c e b e t w e e n t h e 3a-j a n d l b , , o r b i t a l s .  Peak t w o , w i t h a maximum a t 288.4 e V , may be a s s o c i a t e d w i t h of a carbon  with  ( a - j , b-j a n d b^).  The d e r i v e d quantum d e f e c t  t h e quantum d e f e c t o b s e r v e d  valence shell  spectrum  o f acetone  i s 0.8,  f o r t h e np R y d b e r g  (0.81, Reference  seriesi n  160 a n d 0 . 7 6 ,  Reference  161).  I n a d d i t i o n , we e x p e c t t h e p r o m o t i o n o f a 2a-j e l e c t r o n  (carbonyl  carbon  K - s h e l l ) t o t h e 3sa-j R y d b e r g  i n t e n s i t y observed defect  of this  Assignments are  i n t h i s region o f t h e spectrum.  o f peak two w i t h  magnitude  clearly  a methyl  respect  quantum d e f e c t  t o the carbonyl  to contribute to the  The d e r i v e d  carbon  However, peaks  quantum  K-edge i s 1.4. T h e  i s p o s s i b l e f o r a 3s R y d b e r g  o f s t r u c t u r e l o c a t e d above t h e f i r s t  speculative.  carbon  orbital  state.  peak i n o u r s p e c t r u m  associated with  t h epromotion o f  K - s h e l l e l e c t r o n a r e e x p e c t e d t o be a p p r o x i m a t e l y  as i n t e n s e a s t h o s e a s s o c i a t e d w i t h  the excitation  o f a carbonyl  twice K-shell  TABLE  20  ABSOLUTE ENERGIES  ( e V ) , R E L A T I V E ENERGIES AND  POSSIBLE ASSIGNMENTS OF PEAKS OBSERVED IN THE  CARBON K-SHELL ENERGY LOSS SPECTRUM OF ACETONE.  POSSIBLE ASSIGNMENT 9  5  PEAK  ENERGY  1  286.8  0  C  1  •+ 3 s a  2  288.4  1.6  C  1  -* 3p  0.8  -y 3sa-j  1.4  3  A  290.0  K-EDGE ( C /  E  DERIVED QUANTUM DEFECT  C  2  C  ]  291.2  4.4  291.9  5.5  293.8  C  2  5  * 296.4  6  -v- 301  a. C| = C a r b o n  K (methyl),  1.2  3.2  2  7.0  C  a, 9.6) > ^ 14.2)  C  2  = Carbon  + »  (C + ,C  K-EDGE ( C )  ]  K  2  ?  4s  -> 3d  1.3 0.32  - »  SHAKE-UP AND SHAKE-OFF  (carbonyl)  b. D e r i v e d f r o m t h e R y d b e r g f o r m u l a , E = A - R / ( n - 6 ) ^ w h e r e E i s t h e o b s e r v e d e x c i t a t i o n e n e r g y ; A, t h e i o n i z a t i o n p o t e n t i a l ; R, t h e R y d b e r g c o n s t a n t ; n, t h e p r i n c i p a l quantum number a n d t h e quantum d e f e c t . c. X - r a y PES v a l u e s  3 2  .  -189-  electron  ( c f . the X-ray  The  PES  spectrum  l o c a t i o n s o f t h e two  of  carbon  acetone^).  K-edges shown on  our spectrum  are  32 b a s e d on PES.  the experimental  P e a k 4,  transition  carbon  Is b i n d i n g energies determined  l o c a t e d b e t w e e n t h e two  carbon  K ( c a r b o n y l ) •> 4s  c a r b o n - K ( c a r b o n y l ) -> 3d  e d g e s , may  (6 = 1.3)  (6 = 0 . 3 2 ) .  t h e 3d  Reference  161).  and  Rydberg s e r i e s The  broad  6) a r e a t t r i b u t e d  valence shell 7.5. and  s t r u c t u r e s observed  R a d i c a l s Using  o f 0.32  of  (0.28,  a b o v e t h e K-edges  transitions  e l e c t r o n s ( i . e . s h a k e - u p and  spectrum  CO  a quantum d e f e c t  to the simultaneous  the  and/or  In the valence s h e l l  E s t i m a t i o n o f t h e E x c i t a t i o n and H^F  Loss  has  X-ray  be a s s o c i a t e d w i t h  "I  acetone,  by  (peaks  o f a carbon  Is  5  and  shake-off processes).  Ionization  E n e r g i e s o f NH^,  Core A n a l o g i e s A p p l i e d t o K - s h e l l  H^O  E l e c t r o n Energy  Spectra. 27 Nakamura e t a l .  of  molecular  have i n t e r p r e t e d  The  resemble  oxide  nitric  urations of K-shell  K-shell  excited  i n two  nitrogen molecule  respects;  and,  both molecules  i s expected  i i . the core p o t e n t i a l  results  presented  s p e c t r a can  excited  s t a t e s of n i t r o g e n a r e found in this  nitric  spectrum  by one  config-  of the K - s h e l l s unit.  oxide  very s i m i l a r .  t h e s i s demonstrate that inner s h e l l  be more e a s i l y o b t a i n e d u s i n g t e c h n i q u e s  to  oxide states  positive  states of n i t r i c t o be  core  plus K-shells) in  t o be s i m i l a r , s i n c e a h o l e i n one  spacings of valence shell  excited  valence  (nuclei  n i t r o g e n i n c r e a s e s the e f f e c t i v e core charge  K-shell  i s expected  i . the outer e l e c t r o n i c  e x c i t e d n i t r o g e n s t a t e s and  are i d e n t i c a l ,  the energy  photoabsorption  nitrogen (obtained using synchrotron radiation) using a  analogy model.  of  the K-shell  o f energy  Thus  and  The  absorption loss,  electron  -190-  impact spectroscopy a t high loss spectra ical  (see  model  of nitrogen  Figures  (both  Furthermore, the is consistent  and  9 and  K-shell  (2.5  13)  keV)  carbon monoxide as  e x p e c t e d on  oxygen K - s h e l l  with  a "CF  the  of the  case of  dioxide  (see  spacings of the  the  i n ^0  and  able  6.1.)  there  excited  excited  with  oxygen K - s h e l l  states  and  electron  i n C0^  absorption  basis  of  K-shell  produce  i n t h e s e c a s e s may  be  l a r g e changes i n m o l e c u l a r geometry which  possible  to p r e d i c t the  using  core analogies  excited  In  s t a t e and  applied  favourionization  to inner  shell  spectra. c o r e a n a l o g y model  i s expected to apply to the  K-shell  "central"  heavy n u c l e u s w h i c h i s e x p e c t e d t o dominate the  "atomic-like" i n the  structure  K-shell  is evident  lowest energy K - s h e l l  K-shell excited  "hole"  w a t e r m o l e c u l e s b e c a u s e t h e y have potential  one field.  from the w e l l - b e h a v e d Rydberg l e v e l s  s p e c t r u m o f methane (see  r e l a t i v e e n e r g i e s of the the  energy  the  should  t h e m e t h a n e , ammonia and  observed  carbon  nitrogen  states of  This  molecule.  However, i n  t h o s e e x p e c t e d on terminal  14)  ionization  a r e s u l t of e l e c t r o n i c e x c i t a t i o n i n these molecules.  energies of r a d i c a l species  analogy  excited  s t a t e e n e r g i e s and  b r e a k d o w n o f t h e model the  ident-  oxide).  i s poor agreement between the  carbon K-shell  The  c a s e s i t s h o u l d be  The  core  "resemble" n i t r i c  ( i . e . e x c i t a t i o n of the  the  associated  o c c u r as  of the  energy  spectrum of carbon monoxide (Figure  d e s c r i p t i o n " of the  K-shell  "NO2-1ike" s p e c i e s ) . partially  basis  almost  l i n e a r t r i a t o m i c m o l e c u l e s , n i t r o u s o x i d e and  c o r e a n a l o g y model  electron  K-shell  carbon m o n o f l u o r i d e r a d i c a l were o b t a i n e d .  Section the  The  (carbon-K) are  the  excited molecules should  Thus, s a t i s f a c t o r y e s t i m a t e s o f the potential  impact e n e r g i e s .  excited state);  states  T a b l e 11). o f methane  Therefore, (with  respect  the to  -191-  ( l a / are of  (2  a i  )  (2t )  2  (3sa/,  6  2  \  e x p e c t e d t o be s i m i l a r t o t h e r e l a t i v e e n e r g i e s o f t h e e x c i t e d t h e ammonium r a d i c a l , NH^, ( w i t h ( l a /  ( 2 a /(2t )  ( 3 s a / ,  6  2  assuming t e t r a h e d r a l Similarly,  r e s p e c t t o t h e ground \  s y m m e t r y ) p r o d u c e d b y t h e e x c i t a t i o n o f a 3s e l e c t r o n , excited  and  w a t e r a r e e x p e c t e d t o be s i m i l a r t o t h o s e o b s e r v e d  ing  f r o m 3s e l e c t r o n  The  promotion  phase.  claimed  Mass s p e c t r o m e t r y  a n d H^O  matrix-stabilized  1 C O 1 C *3 ' could  1 6 6  respectively, Bader ^  1  been  1 6  ^.  a s an i n t e r m e d i a t e i n w a t e r  A recent r e p o r t  grounds '' . 1  0  f o rH 0 3  respect to dissociation  indicate  path, suggesting  unstable with  not  found.  the s t a b i l i t y  i n t o NH  the p o s s i b i l i t y  of  experimo f gas  + H- a n d H^O + H«  3  The c a l c u l a t i o n s  a b a r r i e r o f 6.6 K c a l s / m o l e a l o n g  o f Gangi  the d i s -  o f low temperature i s o l a t i o n . 172  by L a t h a n e t a l .  indicate  respect t o d i s s o c i a t i o n , since  Experimental  that  potential  NH^ a n d H 0 3  minima were  evidence to support the existence o f the H F 172  has n o t b e e n r e p o r t e d  the radical  o f t h e ESR s p e c t r a  Theoretically,  has n o t been c l e a r l y e s t a b l i s h e d .  are  1 6 8  3  However, t h e c a l c u l a t i o n s  that  '  3  3  sociation  s u r f a c e s has a l s o  experi-  e x i s t i n the  H 0 a n d D 0 r a d i c a l s h a s been c h a l l e n g e d on b o t h  and H 0 w i t h  1  a n d H^O  4  has been p o s t u l a t e d  ^ and t h e o r e t i c a l 4  NH  has p r o v i d e d  both  165 '  phase NH  that  The f o r m a t i o n o f NH^ on s o l i d  radiolysis experiments  radical  result-  ammonium a n d h y d r o g e n o x i d e r a d i c a l s h a v e b e e n i n v e s t i g a t e d  164  and  f o rstates  respectively.  2  "ICO  1 6  o f ammonia  3  fluoronium r a d i c a l , H F,  mental e v i d e n c e s u g g e s t i n g  ental  states  i n t h e hydrogen o x i d e r a d i c a l , H 0 , and t h e  e x p e r i m e n t a l l y and t h e o r e t i c a l l y .  gas  state:  ,  the r e l a t i v e energies o f the K-shell  hypothetical  states  2  and t h e o r e t i c a l  does n o t have a t i g h t l y  calculations  bound s t r u c t u r e .  indicate The f l u o r o n i u m  -192-  +  i o n , H^F by  , has  recently  (low temperature) Table  t h e NH^,  21  H^O  infrared  lists and  173  been o b s e r v e d  in a solid  and  in  solution  spectroscopy.  the p r e d i c t e d e x c i t a t i o n  h^F  mixture  and  ionization  r a d i c a l s using the core analogy  energies  model.  of  Theoretical  174 values and  f o r t h e ammonium r a d i c a l  core analogy  ental  have a l s o been i n c l u d e d .  values f o r the e x c i t e d  accuracy.  This  is illustrated  s t a t e s o f NH^  corresponds  loss  spectrum  K-edge i n d i c a t e d o n  NH^  o f methane.  to i t s ground e l e c t r o n i c the spectrum  agree  s t a t e s on The  state.  i s b a s e d on  have  position  experim-  indicated  the carbon  z e r o o f t h e NH^  The  theoretical  within  i n F i g u r e 5 5 , w h e r e we  the r e l a t i v e e n e r g i e s o f the c a l c u l a t e d e l e c t r o n energy  The  K-shell  energy  scale  o f t h e methane  the experimental  value deter-  32 m i n e d by  X-ray  ionization and  3.8  3.7  ± 0.3  ization  photoelectron spectroscopy  potential  eV^ '^ 2  eV.  7 7  ,  The  o f NH  give values of 3 . 9 2  4  experimental 162  v a l u e o f 5.9  i s much l a r g e r t h a n  values.  and  not agree  predicted values.  w i t h our  5.2  eV  f o r t h e two The  ( u s i n g a l a r g e r b a s i s s e t ) i s 4.6  eV  predicted  Other  v a l u e o f 5.0  ± 0.3  of H 0 3  eV.  and  and  3.9  eV  .  appearance p o t e n t i a l Calculations  experimental o f H^0  for excited  +  value  1 7 5  '  1 7 6  eV  ,  the  3.97  e s t i m a t e d by s u r f a c e i o n -  f o r t h e H^O  lowest excited calculated  calculated 1 7 2  1 7 5  of  s t a t e s , which  ionization  1 7 1  , 4.4  1 7 5  ,  do  potential^  values f o r the  , 4.6  and  r a d i c a l ^  i s i n b e t t e r agreement w i t h  1 The  , 3.94  of  the p r e d i c t e d core analogy  give values of 5 . 8  1 cp  1 7 2  calculations  predicted value  Excited state calculations  g i v e v a l u e s o f 4.4  ization potential  Other  i n good agreement w i t h o u r  techniques  theoretical  .  our  ion-  4.2  175  '  178  cp  o f ^ 10.9  eV  i s a p p r e c i a b l y l a r g e r than  estimated  from  the  our p r e d i c t e d v a l u e .  s t a t e s o f t h e f l u o r o n i u m r a d i c a l , H^F,  have  not  TABLE 21 ESTIMATED  ENERGY L E V E L S ( e V ) OF THE  EXCITED  NH  ORBITAL  THIS AE  WORK  H 0  AND  3  HYPOTHETICAL  H 0  4  AE  THIS  9  2  RADICALS.  2  WORK  THIS  AE-(eV)  (eV)  H F  H F  3  THEORY  (eV)  NH^,  AE  WORK  (eV)  GROUND STATE  3p  0  0  0  1.0  1.3  H.6 12.9  3d/4s 4p  2.4  5p  2.8  - (IP)  3.7  2.6  3.8  b.  Other c a l c u l a t i o n s 3.94 , , 3.97 1 7 5  1 7 6  Calculations 4.4 , 4.2 1 7 5  3.5  5.0  b  A v a l u e o f 8.57  5.7  C  ionization potential  of the i o n i z a t i o n potential , a n d 3.8 e V , \  1 7 5  1  6  2  1  1 7 8  eV h a s b e e n  1 6 2  o f NH. g i v e  calculated  1 7 2  o f H,0 a i v e 3  d  increased  7  of the i o n i z a t i o n potential , a n d 3.9 e V .  1 7 5  4.5  4.0  R e f e r e n c e 174, t h e c a l c u l a t e d with a larger basis s e t .  d.  H.9 13.1  2.2  a.  c.  0  t o 4.0 eV  values of 3 . 9 2  1 7 2  ,  4  values of 5 . 8  1 7 2  ,  4.6  1 7 1  ,  -194-  •  286 FIGURE 5 5 .  3s  3p  I  I  4p 5p 1  T  oo  CH  1  ( )  4  T"  288 290 292 Energy L o s s ( e V )  The c a r b o n K - s h e l l e l e c t r o n e n e r g y l o s s s p e c t r u m o f m e t h a n e and c a l c u l a t e d e n e r g y l e v e l s o f t h e ammonium r a d i c a l ( N H J .  -195-  been r e p o r t e d .  The  calculated  ionization potential  l a r g e r than our  p r e d i c t e d v a l u e , 5.7  ± 0.3  eV.  , 8.57  eV,  is  -196-  CHAPTER  EIGHT  CONCLUSION  The K - s h e l l been s t u d i e d  energy loss  using fast electron  impact energy, e l e c t r o n to  spectra  in the  soft  of the incident  magnitude as t h e n a t u r a l  nitrogen the  basis  i s a viable  high  alternative processes  I n f a c t , t h e r e a r e some p r a c t i c a l  impact s p e c t r o s c o p y , and w i t h  only  modest  beam, t h e r e s o l u t i o n w o u l d be t h e same  l i n e widths o f these highly  h a s been shown t h a t  have  The r e s u l t s d e m o n s t r a t e t h a t  spectroscopy  X-ray and X-ray r e g i o n s .  energy s e l e c t i o n  molecules  techniques f o r studying e x c i t a t i o n  advantages t o t h e use o f e l e c t r o n  It  impact.  energy l o s s  t h e use o f p h o t o a b s o r p t i o n  f o r a v a r i e t y o f small  the K-shell  spectra  and c a r b o n monoxide, a r e c o n s i s t e n t o f a simple core analogy model.  excited  of the diatomic  with  T h i s model  i n the K-shell  molecules,  t h e r e s u l t s e x p e c t e d on has been used t o  e s t i m a t e e x c i t a t i o n a n d i o n i z a t i o n e n e r g i e s f o r some e x o t i c from t h e r e l a t i v e e n e r g i e s observed  states.  chemical  energy l o s s  species  spectra  of  a number o f m o l e c u l e s . 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