UBC Theses and Dissertations

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UBC Theses and Dissertations

Electronic spectroscopy by electron impact Tam, Wing-cheung 1974

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ELECTRONIC  SPECTROSCOPY  ELECTRON  BY  IMPACT  by  WING-CHEUNG TAM  B.Sc.  ( G e n . ) , U n i v e r s i t y o f Hong Kong  (1970).  A T H E S I S SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n t h e Department o f CHEMISTRY  We a c c e p t t h i s required  t h e s i s as conforming  to the  standard  THE U N I V E R S I T Y OF B R I T I S H COLUMBIA J u l y , 1974.  In  presenting  this  an a d v a n c e d  degree  the  shall  I  Library  further  for  agree  scholarly  by  his  of  this  thesis at  partial  the U n i v e r s i t y  make  it  freely  that permission  p u r p o s e s may  representatives. thesis  in  for  financial  is  for  of  gain  Chemistry  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, C a n a d a  Date  July 22, 1974  of  Columbia,  British for  extensive by  the  Columbia  shall  not  the  requirements  reference copying of  I  agree  and  copying or  be a l l o w e d  for  that  study.  this  thesis  Head o f my D e p a r t m e n t  understood that  written permission.  Department  of  available  be g r a n t e d  It  fulfilment  or  publication  without  my  ii.  ABSTRACT  The and  d e s i g n and c o n s t r u c t i o n  variable-angle  electron  of a high-resolution,  low-energy  impact s p e c t r o m e t e r has e n a b l e d t h e s t u d y  o f t h e i n t e r a c t i o n between a low e n e r g y monochromatic e l e c t r o n and  a s e r i e s o f atoms and m o l e c u l e s .  have been s t u d i e d and  small  spectra"  f o r a r g o n and neon.  ( e l e c t r o n impact spectra)  been i n t e r p r e t e d  using  Taft  a*  obtained  T h e "vacuum  f o r some o r g a n i c  d e r i v a t i v e s o f water and carbonyl i n terms o f Rydberg t r a n s i t i o n s .  Rydberg o r b i t a l  transitions  The R y d b e r g t r a n s i t i o n s i n atoms  m o l e c u l e s have a l s o been i n v e s t i g a t e d .  such as t h e a l k y l  on  O p t i c a l l y forbidden  beam  ultraviolet molecules  compounds h a v e Substituent  effects  e n e r g i e s and i o n i z a t i o n p o t e n t i a l s a r e d i s c u s s e d  values.  i i i.  TABLE  OF  CONTENTS Page  CHAPTER I :  INTRODUCTION  1  CHAPTER I I :  THEORY OF I N E L A S T I C ELECTRON SCATTERING  4  2.1.  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  4  2.2.  Approximate Theoretical  6  2.3.  First  2.4.  L i m i t Theorem o f G e n e r a l i s e d  2.5.  Low E n e r g y E l e c t r o n  Methods  Born A p p r o x i m a t i o n  7 Oscillator  Strength.  Scattering  9 13  CHAPTER I I I : EXPERIMENTAL D E V I C E S  16  3.1.  E l e c t r o n Guns  16  3.2.  Electron  Lenses  19  3.3.  Electron  Energy A n a l y s e r s  26  CHAPTER I V :  3.3.1.  General  aspects  26  3.3.2.  T h e o r y o f t h e 127° a n a l y s e r  28  3.3.3.  E n e r g y r e s o l u t i o n o f t h e 127° a n a l y s e r . . .  33  3.3.4.  Deflection voltage  36  3.3.5.  A n a l y s e r s and t h e space charge problem...  f o r t h e 127° a n a l y s e r .  38  APPARATUS AND PERFORMANCE  41  4.1.  The S p e c t r o m e t e r  41  4.2.  A Gas-tight,  4.3.  E l e c t r o n i c s and E l e c t r o n D e t e c t i n g  4.4.  Vacuum S y s t e m a n d G a s H a n d l i n g  56  O P T I C A L L Y FORBIDDEN TRANSITIONS  59  Optical  59  CHAPTER V: 5.1.  Rotatable  S e l e c t i o n Rules  Collision  Chamber System  48 52  iv. Page 5.2.  E l e c t r o n Impact E x c i t a t i o n o f O p t i c a l l y Transitions 5.2.1. E n e r g y  Forbidden 62  dependence  63  5.2.2. A n g u l a r d e p e n d e n c e 5.3.  Optically  65  Forbidden Transitions  i n A r g o n a n d Neon  67  5.3.1. A r g o n  69  5.3.2. Neon  81  MOLECULAR RYDBERG STATES  85  6.1.  General Aspects  85  6.2.  Rydberg  88  CHAPTER V I :  S t a t e s o f Hydrogen Cyanide  6.2.1. A s s i g n m e n t  o f Rydberg  series  6.2.2. D e p e n d e n c e o f t h e s p e c t r u m energy  89  on i m p a c t ...  94  CHAPTER V I I : ELECTRON IMPACT SPECTRA OF SOME ALKYL D E R I V A T I V E S OF WATER AND RELATED COMPOUNDS  98  7.1.  Introduction  7.2.  Water, Methanol  7.3.  Ethylene Oxide  7.4.  Ethyl,  7.5.  Diethyl  7.6.  Effect of Alkyl  CHAPTER V I I I  98 and Dimethyl  Ether  99 106  I s o p r o p y l and t - B u t y l  Alcohols  E t h e r and T e t r a h y d r o f u r a n S u b s t i t u t i o n on E I S  108 114 118  ELECTRONIC SPECTRA OF SOME CARBONYL COMPOUNDS BY ELECTRON IMPACT SPECTROSCOPY  122  8.1.  Introduction  123  8.2.  Saturated Aldehydes...;  123  8.2.1. F o r m a l d e h y d e and a c e t a l d e h y d e  123  8.2.2. P r o p i o n a l d e h y d e a n d i s o b u t y r a l d e h y d e  128  8.2.3. R y d b e r g  134  and v a l e n c e a s s i g n m e n t s  8.2.4. E f f e c t o f a l k y l  substituents  135  V.  8.3.  S a t u r a t e d Ketones  137  8.3.1. A c e t o n e  137  8.3.2. H i g h e r  and 2 - b u t a n o n e ketones  8.3.3. E f f e c t o f a l k y l 8.4.  substituents  149  U n s a t u r a t e d Compounds  151  8.4.1. P r o p e n a l  151  8.4.2. M e t h y l CHAPTER I X :  146  (acrolein)  vinyl  ketone  158  CONCLUSION  161  REFERENCES: APPENDIX:  163 PHOTOELECTRON KETONES  SPECTRA OF SOME ALDEHYDES  AND 169  VI .  LIST  OF  FIGURES  Figure  Page  1.  Electron  2.  Characteristics of electron  3.  127°  4.  Diagrams t o i l l u s t r a t e c a l c u l a t i o n o f t h e energy r e s o l u t i o n o f 127° a n a l y s e r  34  5.  Schematic diagram o f the e l e c t r o n spectrometer  42  6.  Elastic  47  7.  Rotatable,  8.  The c o n t r o l  9.  E l e c t r o n impact spectra a r g o n a t 30 eV  10.  lenses  cylindrical  21  analyser  12.  13.  14.  15.  16.  17.  18.  24  field  29  s c a t t e r i n g i n h e l i u m a t 30 eV g a s - t i g h t c o l l i s i o n chamber  50  c i r c u i t diagram  Relative differential of metastable argon  11.  lenses  f o r 3p -> 4 s e x c i t a t i o n o f 70  cross  ( P2) 3  54  s e c t i o n f o r the  formation  a t 30 eV  71  Relative d i f f e r e n t i a l cross section f o r e x c i t a t i o n t o t h e 4s s t a t e s o f a r g o n a t 30 eV  72  D i f f e r e n t i a l cross section r a t i o s f o r e x c i t a t i o n to t h e 4 s s t a t e s o f a r g o n a t 30 eV  74  E l e c t r o n impact spectra a r g o n a t 30 eV  f o r 3p -> 4p e x c i t a t i o n o f 75  E l e c t r o n impact spectra a r g o n a t 3 0 eV  f o r 3p  Relative d i f f e r e n t i a l cross t o t h e 4p s t a t e s o f a r g o n  4p e x c i t a t i o n o f 77  section f o r the e x c i t a t i o n 78  D i f f e r e n t i a l cross section r a t i o s f o r e x c i t a t i o n t o 4p s t a t e s o f a r g o n a t 3 0 eV  80  E l e c t r o n i m p a c t s p e c t r u m o f n e o n a t 70 eV a n d scattering angle  82  2°  Energy dependence o f t h e r e l a t i v e d i f f e r e n t i a l s c a t t e r i n g c r o s s s e c t i o n f o r t h e 3p' (J=0) s t a t e o f neon  84  vi i .  Figure  Page  19.  E l e c t r o n impact spectra  20.  R e l a t i v e i n t e n s i t i e s o f some t r a n s i t i o n s i n t h e e l e c t r o n i m p a c t s p e c t r u m o f HCN  21.  o f HCN  90  95  E l e c t r o n i m p a c t s p e c t r a o f H 0 , CH-.0H a n d CHJ3CH.> a t 100 e V , 2° . f f 9  ?  22.  23.  24.  25.  E l e c t r o n impact spectrum o f ethylene 2° E l e c t r o n impact spectra o f e t h y l , t - b u t y l a l c o h o l s a t 100 e V , 2°  oxide  101  a t 50 eV, 107  i s o p r o p y l and t 110  E l e c t r o n impact s p e c t r a o f d i e t h y l e t h e r and t e t r a h y d r o f u r a n a t l O O e V , 2°  115  E f f e c t o f a l k y l s u b s t i t u t i o n on R y d b e r g t e r m and f i r s t i o n i z a t i o n p o t e n t i a l s i n a l k y l  values  d e r i v a t i v e s o f water  119  26.  E l e c t r o n i m p a c t s p e c t r u m o f a c e t a l d e h y d e a t 1 0 0 e V , 2°  126  27.  E l e c t r o n impact s p e c t r a o f acetaldehyde, propiona l d e h y d e a n d i s o b u t y r a l d e h y d e a t 100 e V , 2°  130  E f f e c t o f a l k y l s u b s t i t u t i o n on R y d b e r g t e r m v a l u e s a n d and f i r s t i o n i z a t i o n p o t e n t i a l s i n a l d e h y d e s  136  E l e c t r o n impact s p e c t r a a t 100 e V , 2°  139  28.  29.  30.  2  Electron 3  33.  34.  two b a n d s o f 145  impact s p e c t r a o f (CH ) CHCH C0CH , 3  2  3  and ( C H ) C C 0 C H 3  3  3  2  2  3  a t 100 e V , 2°  147  E f f e c t o f a l k y l s u b s t i t u t i o n on R y d b e r g t e r m and f i r s t i o n i z a t i o n p o t e n t i a l s i n k e t o n e s E l e c t r o n impact s p e c t r a k e t o n e a t 1 0 0 e V , 2° Energy  c  5  (CH ) CHC0CH 32.  o  E l e c t r o n impact spectra o f t h e f i r s t C H C 0 C H a t 3 0 , 70 a n d 100 eV 3  31.  o f CH^COCH^ a n d C H . X 0 C H .......  o f propenal  values 150  and methyl  vinyl ".  l e v e l s and e l e c t r o n i c t r a n s i t i o n s i n p r o p e n a l . .  152 157  o  35.  584 A p h o t o e l e c t r o n (CH ) CHCH0  spectra  584 A p h o t o e l e c t r o n  spectra  3  36.  2  (CH ) 3  CH  0  2  o f CH^CHO, CH-CH CH0 a n d T .... ?  o f CH 0CH , 3  CH CH C0CH  3  3  CHCH C0CH , (CH ) CHC0CH , ( C H ) C C 0 C H 2  = CHC0CH,.  3  3  2  3  3  3  2  3  170  3  and 172  vi i i .  LIST  OF  PLATES  Plate  Page  1.  The S p e c t r o m e t e r  43  2.  The g a s - t i g h t , r o t a t a b l e c o l l i s i o n  3.  The c o m p l e t e e x p e r i m e n t a l a r r a n g e m e n t  chamber  51 57  ix.  LIST  OF  TABLES  Table  Page  1.  Optical  selection  2.  Rydberg  transitions  3.  Electronic dimethyl  rules  spectra  60  i n HCN  transitions  91  i n w a t e r , methanol  and  ether series  102  4.  Rydberg  5.  Electronic transitions t-butyl alcohols Electronic transitions tetrahydrofuran  6.  i n atomic  i n ethylene oxide  7.  Rydberg  transitions  8.  Electronic transitions isobutyraldehyde  109  i n ethyl, isopropyl  and I l l  in diethyl  e t h e r and 117  i n acetaldehyde  129  i n propionaldehyde  and 133  9.  Rydberg  transitions  i n acetone  142  10.  Rydberg  transitions  i n 2-butanone  144  11.  E l e c t r o n i c t r a n s i t i o n s i n methyl i s o b u t y l ketone, methyl i s o p r o p y l ketone and methyl t - b u t y l k e t o n e . .  148  12.  Rydberg  transitions  i n propenal  154  13.  Rydberg  transitions  i n methyl  14.  Ionization  potentials  vinyl  ketone  f o r some c a r b o n y l c o m p o u n d s . .  159 174  X.  ACKNOWLEDGEMENTS  I would  like  t o e x p r e s s my d e e p e s t g r a t i t u d e t o my  s u p e r v i s o r , P r o f e s s o r C. E. B r i o n , f o r h i s s u p p o r t , invaluable advice  research  a s s i s t a n c e and  throughout the course o f t h i s work, as w e l l as h i s  deep and g e n u i n e c o n c e r n f o r h i s s t u d e n t s .  I would a l s o  like  t o thank  my c o l l e a g u e s , M r . G o r d o n R. W i g h t , M r . S. T o n g L e e a n d M r . D e r e k S. C. Yee  f o r h e l p f u l d i s c u s s i o n s a n d u n s e l f i s h a s s i s t a n c e on many  Special  t h a n k s a r e due t o M r . Yee f o r h i s a s s i s t a n c e  photoelectron I wish  spectra  o f many compounds  t o acknowledge g r a t e f u l l y  occasions.  i n running the  studied.  t h e s t a f f o f t h e mechanical and  e l e c t r o n i c w o r k s h o p s , e s p e c i a l l y M e s s r s . J . W y n g a a r d e n , E. Gomm, M. V a g g , J . S h i m , D. C a t t a n d K. S. Au f o r c o n s t r u c t i o n a n d m a i n t e n a n c e o f t h e a p p a r a t u s , and t h e i l l u s t r a t o r s  Mr. F. R o b e r t s a n d Mr. A.  Senzaki  f o r m a k i n g up many b e a u t i f u l d i a g r a m s . I thank t h e N a t i o n a l financial Graduate  Research Council  o f Canada f o r g e n e r o u s  s u p p o r t and t h e U n i v e r s i t y o f B r i t i s h Fellowship.  C o l u m b i a f o r a U.B.C.  1.  CHAPTER  I  INTRODUCTION  In t h e f o l l o w i n g  process:  X + e ^ E ^ -> X* + e ( E ) 2  2  w h e r e an a t o m o r m o l e c u l e X i s b o m b a r d e d by an e l e c t r o n e-j w i t h E-j g r e a t e r *  than the energy E  b e t w e e n t h e g r o u n d s t a t e a n d an e x c i t e d  state X , the incident electron doing  i t s u f f e r s an energy  i s inelastically  l o s s E-j - E  2  i n c i d e n t m o n o e n e r g e t i c beam o f e l e c t r o n s  accessible The  high,  to E .  I fthe  scattering of the  r e s u l t s i n an e n e r g y  loss  a d i s c r e t e e n e r g y l o s s i n t h e beam c o r r e s p o n d i n g t o e v e r y  f i r s t measurement o f t h e energy l o s s s u f f e r e d  by e l e c t r o n s i n  a t o m s a n d m o l e c u l e s was made by F r a n c k a n d H e r t z [ 1 ] i n  i n a multiple  collision  e x p e r i m e n t a n d was u s e d t o o b t a i n  about t h e e l e c t r o n i c energy l e v e l s o f t h e t a r g e t .  t e c h n i q u e was q u i t e  l i m i t e d due t o e x p e r i m e n t a l  information  However, t h e use o f  e n e r g y l o s s measurements i n e l e c t r o n m o l e c u l e c o l l i s i o n s  two  In so  energy s t a t e i n t h e t a r g e t .  c o l l i s i o n with 1914  scattered.  which i s equal  i n c i d e n t e l e c t r o n e n e r g y E-j i s s u f f i c i e n t l y  spectrum, with  energy  as a  difficulties.  decades have seen a r e s u r g e n c e o f i n t e r e s t i n t h i s  field  spectroscopic The  last  resulting  f r o m t h e more s o p h i s t i c a t e d e l e c t r o n i c a n d e l e c t r o n o p t i c a l  techniques  w h i c h have s i n c e  electron  spectroscopy highly impact.  been d e v e l o p e d .  (or electron  The method o f e n e r g y l o s s  i m p a c t s p e c t r o s c o p y ) h a s now p r o v e d t o be a  v e r s a t i l e t o o l f o r t h e i n v e s t i g a t i o n o f e x c i t a t i o n by e l e c t r o n P a s t a c h i e v e m e n t s and c u r r e n t  developments a r e discussed  in  2.  recent reviews The  [2,3,4].  e n e r g y l o s s s p e c t r a have been s t u d i e d w i t h t h e o b j e c t i v e o f  identifying  both  optically  f o r b i d d e n and a l l o w e d  discovering  Rydberg s t a t e s and a u t o i o n i z i n g s t a t e s i n t h e c o n t i n u a .  high impact energies and small  t r a n s i t i o n s and o f  s c a t t e r i n g a n g l e s , d i p o l e terms a r e  dominant, and as a r e s u l t , t h e s c a t t e r i n g  intensity  about t h e forward  conditions the f a i r l y  selection electron  r u l e s governing impact energies  optically  behaviour  optical  (approximately  that the differential  of optically  A t lower  and f o r helium  i t has been  c r o s s s e c t i o n s have a d i f f e r e n t  forbidden  q u a d r u p o l e ) bands.  stringent  b e l o w 50 e V ) a v a r i e t y o f  dipole allowed.  processes  observed  impact a r e s p i n exchange and t h e e x c i t a t i o n  (electric  i s s t r o n g l y peaked  transitions are followed.  from those which a r e e l e c t r i c  common t y p e s electron  Under t h e s e  f o r b i d d e n t r a n s i t i o n s may be i n d u c e d  shown [ 5 , 6 ]  can  direction.  At  angular  Two o f t h e more by l o w e n e r g y  o f symmetry  Many o f t h e n o r m a l o p t i c a l  forbidden  selection  rules  be b r o k e n by s u i t a b l e c h o i c e o f i n c i d e n t e l e c t r o n e n e r g y a n d s c a t t e r i n g  angle. By  analogy  with  the a b s o r p t i o n o f l i g h t quanta  i n optical  e l e c t r o n energy l o s s e s a r e "absorbed" i n e l e c t r o n impact (EIS).  Some o f t h e r e l a t i v e m e r i t s o f E I S a n d o p t i c a l  been d i s c u s s e d  i n recent reviews  [3,6].  spectroscopy,  spectroscopy  spectroscopy  From a n e x p e r i m e n t a l  standpoint,  t h e r e a r e a d v a n t a g e s t o t h e u s e o f E I S i n t h e vacuum u l t r a v i o l e t regard energy. do  to resolution,  i n t e n s i t y and t h e a v a i l a b i l i t y  present  only of limited  of synchrotron  availability.  with  o f a continuum o f  I n t h e e n e r g y r a n g e a b o v e 10 eV i n t e n s e c o n t i n u u m l i g h t  not e x i s t , with the exception  have  sources  r a d i a t i o n which i s a t  However, i t i s w e l l  known [ 7 ] t h a t  3.  the  interaction of a fast electron with  creation of a virtual  photon f i e l d .  a molecular target  Thus, i n using  an e l e c t r o n  spectrometer t h e energy l o s s s i m i l a t e s t h e photon energy. 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 f a s t e l e c t r o n has r e c e n t l y oscillator  been d e m o n s t r a t e d  strengths  information  about t h e e x c i t e d  states  impact  This  impact and p h o t o a b s o r p t i o n  i n the determination  by an e l e c t r o n  r e s u l t s i n the  of absolute  optical  impact technique [ 8 ] . A l s o , c a n be o b t a i n e d  through t h e i r  dependence on i n c i d e n t e l e c t r o n e n e r g i e s and s c a t t e r i n g a n g l e s . ambiguous EIS, states) states  identification  of excited  a s a means o f s t u d y i n g  states  quantum  are studied.  a c c u r a t e method  c a n be a c h i e v e d  states of neutrals  i s complimentary t o photo-electron  further  spectroscopy  Un-  i n many  cases.  ( i . e . bound  (PES) where  ionic  W i t h t h e a d v e n t o f t h e l a t t e r as a c o n v e n i e n t and  t o determine the various  ionization potentials of  m o l e c u l e s , t h e c o m b i n a t i o n o f t h e s e two methods new R y d b e r g s e r i e s a n d t o d e t e r m i n e o r b i t a l defects  and measured t e r m v a l u e s  [9,10].  applied  t o l a r g e r molecules o f chemical  c a n be u s e d t o e l u c i d a t e  symmetries from  This  quantum  p r o c e d u r e h a s now b e e n  interest.  4. CHAPTER  THEORY  OF  INELASTIC  2 . 1 . E l e c t r o n - H y d r o g e n Atom The t h e o r y  excitation  ELECTRON  cross  case o f the e l e c t r o n impact e x c i t a t i o n o f  be d e v e l o p e d f i r s t  systems.  SCATTERING  Scattering.  f o r the simplest  atomic hydrogen w i l l complicated  II  The w o r k i n v o l v e d  sections  and t h e n g e n e r a l i s e d i n obtaining  i s so f o r m i d a b l e  that  accurate  Smith  d e g r e e s o f a p p r o x i m a t i o n m u s t be i n t r o d u c e d .  [ 1 1 ] have g i v e n  Moiseiwitsch  C o n s i d e r a s y s t e m composed o f two  by 1, 2 m o v i n g i n t h e c o u l o m b f i e l d  i n f i n i t e mass.  obtained. &  the f o l l o w i n g treatment f o r the case of e l e c t r o n -  hydrogen atom s c a t t e r i n g . denoted  theoretical  no e x a c t s o l u t i o n s f o r t h e  l o w e s t s t a t e s o f a t o m i c h y d r o g e n , t h e 2s and 2 p , have been Various  t o more  The S c h r t i d i n g e r e q u a t i o n  o f a p r o t o n assumed t o have  f o r this  system i s :  [-£ "i ' '-^-fJ 4" J * 2+  2  +  electrons  E  f<?i  f2)=0  <2J)  2  where r - j ,  a  r  e  the p o s i t i o n vectors  of electrons  1, 2 r e f e r r e d t o t h e  p r o t o n as o r i g i n ; r ^ i s t h e i n t e r e l e c t r o n d i s t a n c e ; Y(r-j,r ),  energy o f the system.  2  two e l e c t r o n s , c a n be e x p r e s s e d  the total  where E  n  are the associated  wave f u n c t i o n c h a r a c t e r i s i n g t h e  i n terms o f t h e orthonormal  a t o m wave f u n c t i o n s ^ ( r ) s a t i s f y i n g n  Ey i s t h e t o t a l  the  eigen-energies.  equation  I f we  assume  s e t o f hydrogen  5.  ^VVyV  Hr r ) v  2  where t h e  symbol S  denotes a summation o v e r t h e  i n t e g r a t i o n over the multiplying the  continuum s t a t e s .  both sides  ij>  by  discrete states  Substituting  ( r - j ) and  (2.3)  integrating with  3)  and  into  an  (2.1),  respect  to r - j ,  result is  *2  O  +  n <V  F  where V ( r r ) r  is  -  (2  the  2  =  (W-W'V-tfl.  WK  =  (e /r 2  ] 2  )  -  (e /r )  (2.5)  2  2  i n t e r a c t i o n energy between the  atom composed o f e l e c t r o n  (2-4)  1 and  the  incident electron  proton.  The  2 and  a  wave number k  hydrogen is  n  given  by k  =  2 n  (2m/0 ) ( E j - E ) n  Suppose e l e c t r o n state by  2 i m p i n g e s upon a h y d r o g e n atom i n t h e  ( d e n o t e d by  subscript  t h e wave v e c t o r T<-|,  F (r)  (2.6)  2  1 ) , the  then the  f o r l a r g e r takes the  n  F (r)  *  n  exp  (1^  w h e r e f (e,<f>) i s t h e nth  referred  s t a t e of the  to the  asymptotic  incidence  behaviour of the  Is  being  given  function  form n l  + r"  1  exp  ( i l y ) f (e,<|>)  h y d r o g e n a t o m , and  d i r e c t i o n of  incidence  (2.7)  n  scattering amplitude corresponding  n  of the  . r)6  d i r e c t i o n of  ground  as  e,  <f> a r e  polar  the  axis.  to the  excitation  polar angles of 6^  is  the  Kronecker d e l t a . Introducing satisfying (v it  can  2  the  + k ) 2  n  be  shown  the  Green's f u n c t i o n G(r,r\,)  for a free  particle  equation G(r,r ) 2  that  =  6  ( r - r  2  ) ,  (2.8)  r  6.  F (r") = exp ( 1 ^ . * ) 6 n  n  l  $ f f & ( r f  +  h * $  2  }  )  V( r ^ )  x f ( r r ) dr-jdr 1  If G ( r , r )  2  (2.9)  2  i s chosen t o have t h e form  ?  (2.10)  F (r) n  has t h e c o r r e c t a s y m p t o t i c  Now f o r l a r g e r , k I r - r n  w h e r e l < i s a wave v e c t o r n  K  0  /  1  form as i n (2.9).  k r - £ n  1  . r„ n  i n t h ed i r e c t i o n o f r , then  x V ( r r ) v(r r  The  differential  cross  by  2  l  f  r ) 2  section f o r theexcitation  a h y d r o g e n atom i s g i v e n corresponding  (2.11)  /  dr dr ]  (2.12)  2  to thenthstate of  i n terms o f t h e s c a t t e r i n g amplitude  f (e,<t>h n  t o a n i n c i d e n t e l e c t r o n s c a t t e r e d t h r o u g h a n g l e s e,<|>  t h e formula  l (e,*) = ( y k ^ i y e , * ) !  2  (2.13)  n  while  thetotal  cross  s e c t i o n takes  t h e form (2.14)  2.2.Approximate T h e o r e t i c a l Methods Various [11] cross  approximations  i n some d e t a i l .  have b e e n r e v i e w e d  Many q u a n t u m t h e o r e t i c a l  s e c t i o n s h a v e b e e n made.  approximations,  which neglect  s t a t e s , and t o a l l o t h e r  For high  by M o i s e i w i t s c h a n d S m i t h calculations of excitation  i m p a c t e n e r g i e s , weak  t h e c o u p l i n g from t h e i n i t i a l  states, aresuitable.  The f i r s t  coupling  to the final  Born  approx-  7.  i m a t i o n , and and  a modified form,  a r e e f f e c t i v e a t h i g h and  the Bethe a p p r o x i m a t i o n , are w i d e l y moderate impact  c o m p l i c a t e d weak c o u p l i n g a p p r o x i m a t i o n which  energies.  A more  i s that of the d i s t o r t e d  excitation  field  o f t h e a t o m b e f o r e and  processes  i n which  t h e r e i s an  after excitation.  certain f i r s t results  than  order terms i n the i n t e r a c t i o n the Ochkur a p p r o x i m a t i o n  Strong coupling approximations  which  energy,  the u n i t a r i s e d  approximation  s t r o n g , one  classical  Born a p p r o x i m a t i o n .  can  and use  final  2 . 3 . F i r s t Born  exact  energies.  parameter resonance  t o be e x c i t e d  s t a t e s i s weak, but  i s very  When t h e c o u p l i n g that to a third  the c l o s e coupling approximation.  has  a l s o proved  The  state  semithe  valuable.  Approximation.  This approximation, the s i m p l e s t of a l l ,  assumes t h a t t h e  incident  i n t e r a c t s o n l y s l i g h t l y w i t h t h e t a r g e t a t o m , so t h a t i t s wave  f u n c t i o n may which  apply.  p e r t u r b a t i o n - i m p a c t parameter t r e a t m e n t , sometimes c a l l e d  "dipole approximation"  electron  The  to the s t a t e before e x c i t a t i o n .  between the i n i t i a l  satisfactory  are a p p l i c a b l e a t lower e l e c t r o n  a p p l i e s to c a s e s where the l e v e l  i n energy  give less  impact  from  neglects  i s a l s o simpler to  These i n c l u d e the m o d i f i e d Bethe a p p r o x i m a t i o n , the m e t h o d and  For  important c o n t r i b u t i o n  e l e c t r o n exchange, the Born-Oppenheimer a p p r o x i m a t i o n , which  is  wave,  r e p r e s e n t s t h e f r e e e l e c t r o n by f u n c t i o n s d e s c r i b i n g i t s m o t i o n  i n the s t a t i c  close  used  be c l o s e l y a p p r o x i m a t e d  w o u l d be  (ik-j . r )  the c o r r e c t f u n c t i o n i n the absence o f a l l i n t e r a c t i o n s .  T h i s s h o u l d be v a l i d i n comparison  by t h e p l a n e wave e x p  when t h e s p e e d o f t h e i n c i d e n t e l e c t r o n  w i t h that of the e l e c t r o n  i n t h e t a r g e t atom.  i s great This i s  8.  essentially range  equivalent  to the requirement  that  of the electron-atom i n t e r a c t i o n .  Y ( r r ) r  = exp(i^  2  k-ja>>!, w h e r e a i s t h e  Substituting  . r" ) * ( f , )  (2.15)  2  i n t o t h e r i g h t hand s i d e o f ( 2 . 1 2 ) y i e l d s t h e f i r s t to t h e s c a t t e r i n g  V '*) 9  where V  =  ^ | ? /  P f  x  i  t h e momentum  l  t  1  V •  M nl V V  (  2  4TT  , 2|  = -o K  }  Bethe's  f ( K ) = - (2me /tf K ) / * 2  2  n  which term  depends o n l y arising  n  6  )  Born  form  (r)dr  (2.19)  approximation with the  interaction vanishing  here  because  o f t h e a t o m i c wave f u n c t i o n .  t r a n s f e r r e d through  v a l i d when t h e e n e r g y  the c o l l i s i o n  are practically  i s at least five  of which  t h e energy  t o seven  times  t o be b e t t e r  satisfied  speeds,  f o r neutral  i s small  that i t s  that of the orbital From t h a t  approximation should hold f o r electrons At equal  i f the  m o r e p r e c i s e l y , i t s h o u l d be  i s o f t h e o r d e r o f 10 - 15 eV.  t h e o r d e r o f 150 eV o r m o r e .  valid  complex t o t h e t a r g e t  o f t h e impinging p a r t i c l e s i s such  velocity  likely  1  (2.18)  i n the simpler  compared t o t h a t o f t h e i n c i d e n t p a r t i c l e ;  is  -  (2.17)  }  * ( r ) exp ( i i t . r ) ^  from e l e c t r o n - p r o t o n  v i e w , t h e Born  2  1  I t may be shown t h a t t h e s e h y p o t h e s e s energy  (  exp ( i t . r , )  on K i n t h e f i r s t  of the orthogonality  2  integral  t h e s c a t t e r i n g a m p l i t u d e c a n be e x p r e s s e d 2  d ?  (r )dr  V ( r ^ ) ^  and u s i n g  (il£.r ) • *  -  c h a n g e o f t h e i n c i d e n t e l e c t r o n by t h e v e c t o r  tft, w h e r e t = t-j - t^,  »  (  (^)  2  exp  approximation  amplitude  ( r ~ ) =fy*  n l  Denoting  e  Born  t h e Born  electron, point of  having energies o f approximation  p a r t i c l e s than f o r e l e c t r o n s  9.  or  ions.  2 . 4 . L i m i t Theorem o f G e n e r a l i s e d The  following  derivation  Combining e q u a t i o n s an  electron  cross  12  2  k  p~  =  and in for  ^ (^n  I JJ  a r e  state;  field  the  of  the  colliding  ~ ^p)  1 to  • ^  state  [12].  to the  case  2,  differential  * *1*2  e  Ki<  and  to L a s s e t t r e  generalising  from s t a t e  eigenfunctions  t n e  K^j  V x  2  after scattering; the  ( 2 . 1 6 ) and  due  the  of  by  CC  |  9  kj  where ^ - j , final  ( 2 . 1 3 ) and  i s given  Strength.  i s e s s e n t i a l l y that  exciting a scatterer  section P  !  Oscillator  V  are  the  i s the  f o r the  the  and  dx  ^  (  2  >  2  0  )  in i t s i n i t i a l  colliding  energy of the  s c a t t e r e r ; dft d e s i g n a t e s an  electron  9 d x  scatterer  momenta o f potential  d  electron  incident  element of the  and before  electron  volume  i s a c o m p o s i t e volume element f o r  the  s c a t t e r e r , whose c e n t r e o f mass i s f i x e d . In cases o f atoms, Bethe [ 7 ] incident electron action  co-ordinate,  potential  i s the  incident electron with i n t e g r a t i o n , the given  12 u  where K K with  ft,  can  of the  be  cross  i n t e g r a t i o n over  c a r r i e d out,  actual  potential  each atomic e l e c t r o n  differential  the  section  and  with  since  the  e n e r g i e s of the  the  interthe  nucleus.  i n atomic u n i t s ,  After is  by 4k  CT  sum  showed t h a t  2  =  ee  2  F ~ K  '  ~4  i  2  2 ]  )  r  ( t = £•] = k^  ( '  +  - 1< ) 2  k  9 being the  2 2  i s t h e m a g n i t u d e o f t h e momentum  - 2k k ]  2  cos  transfer  e  scattering angle.  (2.22) The  quantity  e i s given  by  10.  /*  The  1 N  * s  *  r  t ^ , ^ as products o f t h e  Born-Oppenheimer a p p r o x i m a t i o n t o e x p r e s s  e l e c t r o n i c and n u c l e a r d e p e n d i n g on n u c l e a r 13).  state functions  co-ordinates  has been a p p l i e d .  vanish  Here r" i s t h e p o s i t i o n v e c t o r  due t o o r t h o g o n a l i t y  summation e x t e n d s o v e r a l l such e l e c t r o n s . only  (seeref.  o f t h e s t h a t o m i c e l e c t r o n and t h e  s  atomic e l e c t r o n s  The terms  and t h e e x p r e s s i o n  The i n t e g r a t i o n e x t e n d s  i s only  valid  over  f o r inelastic  scattering. When t h e momentum t r a n s f e r K i s s m a l l , t h e e x p o n e n t i a l can  be e x p a n d e d  r e  4k 12  ff  k^"  =  i n t o a power s e r i e s a n d i n t e g r a t e d  t e r m by term t o o b t a i n 2  2  [~^~  +  ( £  2 '  2  e  l 3 e  )  +  (  e  3  "  2  2 e  2 4 e  +  2  e  l  2  e  5  )  + When t h e H a m i l t o n i a n  function  real  leading  eigenfunctions, In (2.24),  i sreal,  K.  It  i s a convention to take the z-axis  matrix  to t  the coefficients  so t h a t t  s  §  element o f the e l e c t r i c  only  even  powers  by  o f the co-ordinate The q u a n t i t y  system as  e ^ i s obviously the  d i p o l e moment ( z - c o m p o n e n t ) .  s e c t i o n tends towards p r o p o r t i o n a l i t y w i t h that  (2.24)  i t i s always p o s s i b l e t o choose  are given  . r* = K z .  K  J  to a series containing  of  parallel  i n (2.23)  The c r o s s  under c o n d i t i o n s  such  K i s smal1. The  magnitude o f K  2  c a n be w e l l  a p p r o x i m a t e d by  2 K E = E  2  = 8E[sin  2  - W/2, w i t h  (|) + (^j E equal  J  when f  to the kinetic  < 0.1 energy o f t h e i n c i d e n t  (2.26) electron  11.  and  W i s t h e e x c i t a t i o n energy.  l a r g e , t h e o n l y term right of equation  a  * r| W  12  accuracy, optical  term 2  k-j, s o  2  vanishes  selection rules  hold.  However, from  v a n i s h e v e n when e-j = 0 u n l e s s  e  2  (2.24), a^  = 0 a t t h e same  (E-J = 0, e  very  does  2  time.  from t h e Born A p p r o x i m a t i o n  f  2  l a r g e i n each  n e g l i g i b l e compared t o d i p o l e a l l o w e d t r a n s i t i o n s . be c o n c l u d e d  )  0, h a v e c r o s s s e c t i o n s w h i c h a r e e s s e n t i a l l y  Hence f o r l a r g e e n o u g h E", t h e c r o s s s e c t i o n o f d i p o l e f o r b i d d e n  can  2 7  when e-j v a n i s h e s , a n d t o t h i s d e g r e e o f  have a c r o s s s e c t i o n i n d e p e d n e n t o f E (E b e i n g  is  on t h e  < -  t o E, w h e r e a s t h e f o r b i d d e n t r a n s i t i o n s  proportional  _ and E  9=0  A l s o , under these c o n d i t i o n s , k ~  t r a n s i t i o n , w i t h e-j f  Allowed  i s a minimum when  2  This approximate formula  not c o m p l e t e l y  2  o f s i g n i f i c a n t magnitude i s t h e f i r s t  (2.24).  l  e  K  0) case).  transitions  Therefore, i t  that optical  selection  rules  h o l d m o s t a c c u r a t e l y a t 9 = 0 when E i s l a r g e . The  relation  between e x c i t a t i o n  by e l e c t r o n i m p a c t  a n d by a b s o r p t i o n  o f r a d i a t i o n c a n be made m o r e p r e c i s e b y i n t r o d u c t i o n o f t h e c o n c e p t generalised o s c i l l a t o r  strength f i r s t  generalised  s t r e n g t h f-| f o r e x c i t a t i o n  1evels f  2  between  The  discrete  1, 2 i s d e f i n e d by (K ) 2  1 2  oscillator  e m p l o y e d by B e t h e [ 7 ] .  of  =  h(K)|  (2.28)  2  K where z i s d e f i n e d b e f o r e approximation be e x p r e s s e d f  By c o m p a r i s o n w i t h t h e f i r s t  c r o s s s e c t i o n CT^ i n ( 2 . 2 1 ) , i t i s o b v i o u s 2  i n terms o f a ^  12 7 \ S =  i n (2.23).  K  2  2  Born  that f ^ can 2  as  <'> 22 9  12.  (2.28) a n d (2.29)  Equations  definitions  have been used more o r l e s s a s e q u i v a l e n t  o f thegeneralised o s c i l l a t o r  strength.  b e c a u s e (2.21) i s  r e s t s on t h e v a l i d i t y o f t h e Born a p p r o x i m a t i o n , obtained  using a plane-wave r e p r e s e n t a t i o n o f t h e p e r t u r b i n g e l e c t r o n .  At e l e c t r o n e n e r g i e s where t h e Born approximation i s u s e f u l t o d e f i n e an e f f e c t i v e o s c i l l a t o r put F-j2  into c  a  F  n  (2.29) i n p l a c e o f f  be d e r i v e d  12  ( E  1'  K )  =  1  2  i t will  from measured c r o s s  £  i^"  i s no l o n g e r  strength F^  s u c h t h a t when section.  s e c t i o n as f o l l o w s :  (  approximation  give  Born  a t low energies.  the  r e g i o n where t h e Born a p p r o x i m a t i o n  and  i s independent o f energy. When t h e B o r n a p p r o x i m a t i o n  2  >  3  0  )  M e a s u r e m e n t o f F-|  2  i n f o r m a t i o n about t h e v a l i d i t y o f  the  i svalid  As t h e e n e r g y i n c r e a s e s i svalid,  F^  and a-j  from  2  (2.29), i t i s obvious t h a t as K  into  valid,i t  ^ 1 2  a f u n c t i o n o f energy should  substituted  2  give thecorrect cross  w h i c h d e p e n d s o n t h e i n c i d e n t e l e c t r o n e n e r g y E^. as  The e q u i v a l e n c e  2  into  tends towards f - j  2  (2.24) i s  -* 0  2 f  i2 = ! r  2  *  2  ?  p - •• <  .  2  <- > 2 31  2 However t h e q u a n t i t y 2We^ optical  oscillator  absorption  i n atomic' u n i t s , t o t h e  s t r e n g t h f , w h i c h c h a r a c t e r i s e s e m i s s i o n and  of radiation.  1 im f = f o  i s precisely equal,  as K  2  Hence •*•  0  Even when t h e B o r n a p p r o x i m a t i o n  does n o t h o l d a t l a r g e r v a l u e s  o f K,  i t was shown b y L a s s e t t r e e t a l . [13] t h a t t h e l i m i t o f t h e g e n e r a l i s e d oscillator  s t r e n g t h , when e x t r a p o l a t e d  z e r o momentum t r a n s f e r i s t h e o p t i c a l  into t h e non-physical oscillator  region of  strength f .  This i s  13.  q u i t e independent o f t h e Born a p p r o x i m a t i o n . optical  Therefore,  i f one has an a b s o l u t e c a l i b r a t i o n o f i m p a c t c r o s s s e c t i o n s .  A l t e r n a t i v e l y , one c a n v a l i d l y o b t a i n a b s o l u t e electron  impact c o l l i s i o n s  cross  section f o r inelastic  (at least f o r electric-dipole-allowed  processes)  normalising the value of the cross s e c t i o n , extrapolated to K  t o an a v a i l a b l e v a l u e o f f The  o  argument p r e s e n t e d  o f atoms.  In molecules,  from o p t i c a l  [14]  that equation  data.  above i s developed  s p e c i f i c a l l y f o r t h e case  i fthe eigenfunctions are separable way, t h e n  (2.21) i s v a l i d , w i t h e g i v e n  (2.23) but w i t h t h e e i g e n f u n c t i o n s being  those  excited  by a n e q u a t i o n  state lying  f o r the molecular  ^Q  „  N  they  other. limiting  theory  and f o r b i d d e n t r a n s i t i o n s .  oscillator  K  electrons.  t h e same  s e v e r a l v o l t s above t h e ground s t a t e , so t h a t  result of this  for allowed ised  similar to  a r e v a l i d when s t a t e 1 i s t h e g r o u n d s t a t e a n d s t a t e 2 i s a n  n o t p e r t u r b each The  into  i t c a n be shown  E v e n i f t h e e x c i t e d s t a t e wave f u n c t i o n s a r e n o n - s e p a r a b l e , formulae  =0,  r  e l e c t r o n i c and n u c l e a r f u n c t i o n s i n t h e u s u a l  do  obtain  t r a n s i t i o n p r o b a b i l i t i e s from e l e c t r o n impact data even a t low  energies  by  o n e may  strength F defined  F = 2W[e-j [  F = 0  i s t h a t one c a n o b t a i n t h e c r i t e r i a Considering  the effective  general-  i n (2.30)  f o r an a l l o w e d  f o r a forbidden  transition  transition.  2.5.Low E n e r g y E l e c t r o n S c a t t e r i n g . I n many c a s e s , explaining  the f i r s t  the differential  atoms and m o l e c u l e s  Born approximation  has been s u c c e s s f u l i n  cross section f o r electronic excitation of  by e l e c t r o n i m p a c t a t h i g h e n e r g i e s  (E-,>  ^ 150 e V )  14.  and  small  scattering  angles  (e< ^ 1 5 ° ) .  study of the assumptions behind to  expect  Lassettre  the f i r s t  i t t o be m o s t v a l i d , a l t h o u g h  t h e o r y d e p e n d s on  the p a r t i c u l a r  [ 1 5 ] , guided  This  +  Born a p p r o x i m a t i o n  nature  of the t r a n s i t i o n .  showed t h a t t h e d e v i a t i o n f r o m t h e f i r s t can  results  is  no  ten times  longer v a l i d  the e x c i t a t i o n  in general.  The  o b t a i n e d , i f a t a l l , by e m p l o y i n g computational procedure  procedures.  The  At  for a  lower  theoretically at a  impact Born  approximation  cross sections  r e l i a b l e r e s u l t s can  only  by one  Born a p p r o x i m a t i o n  d e p e n d on  individual  of c a l c u l a t i o n At  relative  lower  has  impact  incoming  +  importance  t r a n s i t i o n s and  a general energies  of resonance s c a t t e r i n g For r e f e r e n c e s , see  and  o f c o m p a r a b l e g e n e r a l i t y and  neglects the e f f e c t of e l e c t r o n  electron.  theory f o r slow e l e c t r o n c o l l i s i o n The  be  much m o r e e l a b o r a t e a n a l y t i c a l  been d e v i s e d t o t a k e t h e s e e f f e c t s i n t o a c c o u n t .  Burhop [ 1 7 ] .  between  I t i s not p o s s i b l e t o r e p l a c e the c l e a r - c u t  o f t h e t a r g e t by t h e  analytical  given  energies,  e x c h a n g e , d i s t o r t i o n o f s c a t t e r i n g - e l e c t r o n wave f u n c t i o n s and isation  the  transition  energy, the f i r s t  and  of the Born a p p r o x i m a t i o n  definiteness.  [16],  problem of c a l c u l a t i n g  under these c o n d i t i o n s i s d i f f i c u l t  one  example  be m o r e a p p r e c i a b l e f o r a t r a n s i t i o n  between s t a t e s w i t h d i f f e r e n t d e s i g n a t i o n s . say  For  Born approximation  s t a t e s w i t h t h e same s p e c t r o s c o p i c d e s i g n a t i o n t h a n  l e s s than  leads  the q u a n t i t a t i v e v a l i d i t y of  i n p a r t by e x p e r i m e n t a l  k i n e t i c energy o f impact  i s the r e g i o n where a  polar-  V a r i o u s methods A survey of  have  the  i s g i v e n by M a s s e y  of the three e f f e c t s  and  named a b o v e  i t seems t h a t no t h e o r e t i c a l  scheme  applicability. near the e x c i t a t i o n t h r e s h o l d , the  i s dominant.  Rice et a l . ,  "Resonances" a r e a l s o  P h y s . Rev.  A 5_ ( 1 9 7 2 )  phenomena  called  762.  15.  "compound  s t a t e s " o r "temporary  negative ions".  Their  occurrence,  a t m o r e o r l e s s w e l l - d e f i n e d e n e r g i e s when e l e c t r o n s s c a t t e r f r o m and m o l e c u l e s ,  i s due t o t h e f o r c e f i e l d  of the scatterer  l y a t t r a c t i v e t o hold the f r e e e l e c t r o n f o r a time normal can  time o f passage through  be v i e w e d  molecule.  the scattering  In sharp c o n t r a s t t o s t a t i o n a r y  l o n g compared w i t h t h e  region.  problem  These  "resonances"  "resonances"  decay  They r e s u l t  i n fine  to different exit  a s s o c i a t e d w i t h resonances  electron-molecule scattering  by N i c o l a i d e s [ 1 9 ] .  states,  photons.  i n cross sections corresponding  The t h e o r e t i c a l and  sufficient-  a s n o n - s t a t i o n a r y ( s h o r t - l i v e d ) s t a t e s o f an a t o m o r  by t h e e m i s s i o n o f e l e c t r o n s r a t h e r t h a n structures  being  atoms  h a s been r e v i e w e d  channels.  i n electron-atom by T a y l o r [ 1 8 ] a n d  S c h u l z [ 2 0 ] has r e c e n t l y g i v e n a s y s t e m a t i c  discussion of the spectroscopy  of  resonances.  16.  CHAPTER  III  EXPERIMENTAL  3.1.Electron The  DEVICES  Guns. e s s e n t i a l f u n c t i o n s o f an e l e c t r o n i m p a c t s p e c t r o m e t e r  production  o f an e n e r g y - s e l e c t e d  the energy d i s t r i b u t i o n  beam o f e l e c t r o n s a n d t h e a n a l y s i s o f  of the scattered electrons with  i m p a c t e n e r g y and t h e s c a t t e r i n g a n g l e . usually a thermionic individual  section, angular  variation of the  of electrons i s  D e p e n d i n g on t h e  e l e c t r o n beams o f a s p e c i f i c e n e r g y ,  divergence  t o meet a l l t h e s e  requirements,  t h e minimum c u r r e n t , s i n c e t h e r e a r e p h y s i c a l l a w s w h i c h  s t r i n g e n t l i m i t s o n t h e maximum o b t a i n a b l e c u r r e n t o n c e o t h e r parameters a r e f i x e d . [21].  limits  Q  separated  the presence o f a strong  I there  max  The f i r s t  = 35E  3 / 2  place beam by S i m p s o n apertures  energy E ( e V ) , (so that  t h e beam r a d i u s ) t o be  (pA) ' v  i s no m a g n e t i c f i e l d ,  optimum c o n v e r g e n c e a n g l e  i s smallerthan  especially  i s due t o s p a c e  enough c o l l i m a t i n g m a g n e t i c f i e l d  path  v  the transmitted current  (3.1) '  i s maximum a t a n  Y = t a n " - ^ - and t h e space c h a r g e f o r c e 1  2  0  Electron  a r e two f u n d a m e n t a l  t h e maximum c u r r e n t o f e l e c t r o n s w i t h  the r a d i u s o f the s p i r a l  If  by a d i s t a n c e £ . t h e r e  on t h e c u r r e n t t h a t c a n be t r a n s m i t t e d .  charge, which l i m i t s in  The f o l l o w i n g a n a l y s i s h a s b e e n g i v e n  I f an e l e c t r o n beam o f e n e r g y E i s t o p a s s t h r o u g h two  of diameter 2 r  cross  a n d minimum c u r r e n t may be r e q u i r e d .  g u n s c a n n o t a l w a y s be d e s i g n e d for  The s o u r c e  c a t h o d e i n an e l e c t r o n gun.  experiments,  are the  spreads  17.  the  beam so t h a t a t t h e c e n t r e  o f t h e s p a c e t h e beam becomes a d i s c o f  d i a m e t e r 2 r / 2 . 3 5 a n d a l l e l e c t r o n s move p a r a l l e l Q  maximum c u r r e n t  I The  m  a  second  states  limit  3 / 2  (2r /0  = 38.5E 2 t a n  2  3 /  0  2 Y  (uA)  (3.2)  i s based on t h e H e l m h o l t z - L a g r a n g e t h e o r e m  t h a t f o r a n y two c o n j u g a t e p l a n e s  of the other)  The  i s then  = 38.5E  x  to the axis.  separated  (such  t h a t one i s t h e image  by a n o n - a b s o r b i n g o p t i c a l  /E"i dx-j s i n e-j = /E~ d x 2  e  sin  2  which  path (3.3)  2  w h e r e E.., d x . a n d e . ( i = 1,2) a r e t h e e l e c t r o n e n e r g y , d i f f e r e n t i a l length  e l e m e n t and a n g l e o f c o n v e r g e n c e i n t h e p l a n e i .  i s conserved l  = J  ]  Equations R  i n a nonabsorbing channel d x ^ = J  ]  2  dx  2 2  = I  and i f J ^ i s t h e c u r r e n t  l  °1  (3.4)  1  sin  _  J 2  E sin e  When t h i s r e l a t i o n  distribution  2  R  2 E  2  2  of "Richtstrahlwert" or  i s a p p l i e d between t h e p l a n e s  of electron energies,  transmitted i s  J  2  density  2  d i a m e t e r and t h e cathode a t a t e m p e r a t u r e T p r o d u c i n g  be  current  ( 3 . 3 ) a n d ( 3 . 4 ) c a n be c o m b i n e d t o g i v e  which expresses the conservation R-.  Since  max "  J  r  cathode L  1  +  e  2  t h e maximum c u r r e n t  of the smallest a  Maxwellian d e n s i t y t h a t can  -  sin e  (3.6)  2  2  FT  where k i s t h e Boltzmann's c o n s t a n t . ness i s r e a l l y  "brightness"  This  a consequence o f t h e thermal  I n t h e c a s e o f beams u s e d  limit  on e l e c t r o n beam b r i g h t -  energy o f the emitted  electrons,  i n e l e c t r o n m i c r o s c o p y , beam w e l d i n g  and  18.  e l e c t r o n d i f f r a c t i o n , w i t h an e n e r g y o f a b o v e 10 KeV,  spot size  <  1mm  _3 and c o n v e r g e n c e  a n g l e < 10  A t v o l t a g e s b e t w e e n 10 KeV  r a d , t h e guns a r e b r i g h t n e s s and  a n g l e , t h e guns a r e u s u a l l y the  space charge  and  r e n d e r s the guns  Simpson,  300 eV,  p a r t i c u l a r l y at high  space charge dominated.  beam.  The  beam d i a m e t e r .  brightness  limited  again.  As  an a n o d e . role.  The  pointed out  Hence a c h a n g e o f p h y s i c a l i m p o r t a n c e o f t h e s e two  o f an  of  potential  distribution  (b) t h e g r i d  ictable  element  by a g r i d plays a  The  between the  f r o m them by v a r y i n g  the (c)  position  o f the e l e c t r o n from  normally l i e s  and  dominant  i t s t h i c k n e s s and  p o l a r i t y with respect to the cathode.  polarity.  the  will  (a) t h e d i s t a n c e between  a p e r t u r e and  intercept the aperture a x i s ,  of  limits.  near the cathode  t h e a n o d e , b u t c a n be moved f u r t h e r  It  only  hence f o r t h e  d i m e n s i o n o f t h e gun  i n d i r e c t l y heated cathode f o l l o w e d  v o l t a g e and  v o l t a g e and  depends  i n t h i s work i s a s i m p l e t h r e e  t h e " c r o s s - o v e r " , where t h e t r a j e c t o r i e s  cathode and  employed  T h i s d e p e n d s on t h r e e p a r a m e t e r s :  grid  and  by  b r i g h t n e s s , on t h e o t h e r h a n d , d e p e n d s i n v e r s e l y on  c a t h o d e and t h e g r i d , the  brightness  a n g l e t h e maximum c u r r e n t i s i n d e p e n d e n t o f t h e s i z e  e l e c t r o n gun  consisting  eV,  i n f r o n t o f the cathode decreases the cathode  the r e l a t i v e The  gun  300  i t i s important t o note t h a t the space charge l i m i t  same c o n v e r g e n c e  alter  convergence  Below  on t h e r a t i o o f d i m e n s i o n s a n d n o t o n t h e i r m a g n i t u d e ,  the  limited.  the  cathode  the  grid  The g r i d  voltage also controls  t h e beam  intensity.  seems t h a t t h e e l e c t r o n  source i s the weakest  and most  unpred-  p a r t o f the whole  satisfactory  solution  cathodes c o n s i s t i n g  electron  to t h i s  s p e c t r o m e t e r , and  problem.  t h e r e i s no c o m p l e t e l y  In t h i s work, d i r e c t l y  o f t u n g s t e n and r h e n i u m  heated  f i l a m e n t s were f i r s t  tried,  19.  b u t t h e y t e n d t o move when h e a t e d . use o f t u n g s t e n h a i r p i n s o f about conditioned. and  A rhenium  the performance  and/or magnetic  of  gases  to which  ribbon required  but t h e i r  they are exposed.  h e a t e d c a t h o d e has w o r k e d q u i t e w e l l of  about 6 months.  successful  have been p r e -  large heating currents  e f f e c t o f t h e gun.  stability  the  0.005" d i a m e t e r w h i c h  o f t h e s p e c t r o m e t e r was  field  possess mechanical  Kuyatt [22] reports  p e r t u r b e d by s p a c e  Indirectly  life  heated  t i m e i s d e p e n d e n t on t h e n a t u r e  i n e r t g a s e s and  back t o t h e s e c a t h o d e s i s t h e i r  expense  activation.  $200).  Elmet h e a t e r s .  of the d i r e c t i o n  t h e l o w c o s t o f t h e s e c a t h o d e s and  The  ultimate  3.2.Electron  to t h i s  passage  is tolerable  heaters permits frequent replacement.  pumped s y s t e m t o i s o l a t e  of l i g h t index.  A c r o s s a boundary V-|,  a  t h e gun  life  i s to design  from the t a r g e t  gas.  Lenses.  refractive  and  draw-  oxide cathodes  However, t h i s  problem o f s h o r t cathode  E l e c t r o n s move t h r o u g h an e l e c t r i c the  Another  o f the h e a t e r c u r r e n t causes a change i n the  and  a differentially  in that  These c a t h o d e s a r e s l i g h t l y m a g n e t i c and  f o c u s s i n g v o l t a g e of the energy a n a l y s e r s .  solution  as  With o r g a n i c molecules,  a c o m p r o m i s e h a s b e e n f o u n d by u s i n g a c o m b i n a t i o n o f RCA  reversal  time  However, p e r f o r m a n c e w i t h o r g a n i c m o l e c u l e s s u c h  e m i s s i o n decays r a p i d l y a f t e r the i n i t i a l  with P h i l i p s  indirectly  has a l i f e  a l c o h o l s , e t h e r s a n d c a r b o n y l compounds i s n o t s a t i s f a c t o r y the  charge  cathodes  A F r e n c h C.S.F. t y p e CI 10 with  6A)  i n an a n a l o g o u s  r a y s t h r o u g h a medium o f c o n t i n u o u s l y  E l e c t r o n s may separating  " S n e l l ' s Law"  field  two  be r e f l e c t e d , r e f r a c t e d equipotential  holds i n the form  fashion  to  variable  and f o c u s s e d .  regions of potentials  V  Q  20.  JT^ s i n where e normal  0  and  Q  sin e  a r e the angles  the e q u i p o t e n t i a l s u r f a c e short distance apart  The l e n s a c t i o n c a n be e x p l a i n e d b e t w e e n two c y l i n d e r s o f e q u a l  (Figure l a ) .  The e l e c t r o n p a t h  o f c o n s i s t i n g o f s t r a i g h t l i n e fragments.  a x i s from t h e l e f t  experiences  towards the a x i s .  ray experiences potential surface  a divergent  gradient  diameter a t a  i s smoothly  curved  A ray parallel  a convergent a c t i o n i n passing  t o the  through the  In passing  through the r i g h t  a c t i o n because t h e r a d i a l  i s d i r e c t e d outwards.  hand p a r t ,  component o f t h e  T h u s when t h e e q u i p o t e n t i a l  p o t e n t i a l , the a c t i o n i s convergent.  equipotential  surface  Conversely,  i s concave, the a c t i o n i s divergent.  move m o r e s l o w l y i n t h e c o n v e r g e n t r e g i o n  than  l e n s e s a r e much more v e r s a t i l e be c h a n g e d by s i m p l y  than o p t i c a l  i n the divergent  t h i c k lens terminology to Figure  lenses  electrons  region,  Electron  because t h e i r  strength  changing e l e c t r o d e p o t e n t i a l s i n s t e a d o f having  t o move l e n s c o m p o n e n t s r e l a t i v e  reference  when t h e Since  convergent e f f e c t always exceeds the d i v e r g e n t e f f e c t .  The  this  i s c o n v e x a s a p p r o a c h e d by t h e e l e c t r o n s i n t h e d i r e c t i o n o f  increasing  can  by c o n s i d e r i n g  p a r t o f t h e l e n s b e c a u s e t h e g r a d i e n t o f t h e p o t e n t i a l has a component  directed  the  t h e f o r m o f two c i r c u l a r c y l i n d e r s  o f diameter D kept a t d i f f e r e n t p o t e n t i a l s  a t a distance A apart.  instead  Hence /V may be u s e d a s a  e l e c t r o n lens u s u a l l y takes  two c i r c u l a r a p e r t u r e s  left  t h a t t h e e l e c t r o n beam makes w i t h t h e  i n d e x " , w i t h V = 0 when e l e c t r o n s a r e a t r e s t .  A simple  and  (3.7)  ]  on e i t h e r s i d e o f t h e b o u n d a r y .  "refractive  on  =  Q  t o one  another.  i s u s u a l l y used i n e l e c t r o n l e n s e s .  l b , t h e two r a y s  that leave  t o t h e a x i s r e s p e c t i v e l y , a r e known a s t h e f i r s t  and e n t e r  the lens  and second  With parallel  "principal  21.  V|  a)  <  V  2  Thick-k-iib tiTiniiioIopy.  K.R* Spangenberg, Vacuum Tubes. McGraw H i l l , N.Y.  c)  v  1  < v  2  D=1/S"  0.01"  A/D= 1  D 4.5D-  F i g u r e 1.  Electron  19Zf8  -H lenses.  6.5 D—  H  22.  rays".  For  zero, the The  lenses with  initial  principal  lens  rays  parameters.  principal  rays  final  serve  lens.  final  gradients  p o r t i o n s of the  initial  known as  the  the  and  extended u n t i l  cases both p r i n c i p a l  of the  and  to define  I f the  and  o c c u r a t what a r e all  and  initial  of p o t e n t i a l t h a t  rays w i l l  be  "principal  Furthermore, they are  planes"  the  and  foreside  crossed,  i . e . the  principal  plane.  defined  the d i s t a n c e from the  principal  plane of the  points).  There are  each s i d e of the distance  from the  two  l e n s and focal  focal  lengths,  they are designated  p o i n t to the  F-j, F^  are  sufficient  2  either  be  P and  distance  the  to describe  the  are  Q respectively. P on  the  low  One  A convenient r e l a t i o n is  The  known i s t h e  (Q - F )  values  f o r f - j , f g , F-p  [23,24].  2  Spangenberg  = f  [23]  the  the  symbol f . usually  t o use  f  1  F  object  has  f-j,  are  adopted i s t o put  image denoted the  lens.  f o r determination  o f P o r Q when e i t h e r  that (3.8)  are a v a i l a b l e i n the converted  and  can  and  2  2  on  the  thick lens  t o t h e m i d - p l a n e and  sign convention  one  (the  The  s y m b o l F.  The  point  lens  These parameters  o r by c a l c u l a t i o n .  NEWTON R E L A T I O N , w h i c h s t a t e s  (P -  lens to the  p l a n e M,  lens completely.  energy s i d e of the  is  the a x i s of the  by  reference  u s u a l l y measured r e l a t i v e  principal  length  c a r d i n a l c h a r a c t e r i s t i c s of the  found e x p e r i m e n t a l l y  distances by  are c a l l e d  side)  (usually different),  m i d - p l a n e o r e l e c t r o d e j u n c t i o n , i s i n d i c a t e d by f ,  two  almost  voltage  A focal  ray crosses  In  second  first  principal  H,,.  (low  i n f r o n t o f the  focal  thick  they i n t e r s e c t , the i n t e r s e c t i o n s  p l a n e s l i e on  a t which the corresponding  four  s t r a i g h t portions of the  plane l i e s as  straight lines.  c a r d i n a l p o i n t s and  final  are  the  literature,  f , f , F^, 1  2  F  2  e.o.  curves  to  23.  c u r v e s o f P and Q w i t h  t h e m a g n i f i c a t i o n M as a parameter.  is  These diagrams a r e v e r y  shown i n F i g u r e  2.  work when t h e o b j e c t s objects  and images a r e r e a l .  are involved, the focal  a t i o n of the lens can e a s i l y  Q- 2 f  M  magnification  When v i r t u a l  are  " f  P  m  l ' " P - F  F  l _  f  cross-section. employ long slit  Therefore,  slits  i t s length.  the o p t i c a l  i n the present  By m a k i n g t h e s l i t s These s l i t  to estimate  given  d  spectrometer, the lenses  Ic.  long  I n some w a y s ,  lenses  aperture  this lenses  enough, u n d e s i r a b l e end( c f . cylindrical  lenses i n  as d e c e l e r a t o r s  focus.  the s i z e o f the v i r t u a l From t h i s ,  p a r a m e t e r s c a n be c a l c u l a t e d . Figure  analysers  t h e performance o f t h e e l e c t r o n spectrometer, i t i s  of the energy analyser.  n  deflection  a n a l o g u e ) have f o u n d t o f u n c t i o n v e r y w e l l  analyse  essential  *„A  (3.10) 2  a n e l e c t r o n beam w h i c h i s r e c t a n g u l a r i n  accelerators giving a well-defined To  a  (3.9)  instead of c i r c u l a r apertures.  e f f e c t s can be avoided.  relations:  1  l e n s c a n be t h o u g h t o f a s a s e r i e s o f o v e r l a p p i n g  along  and  l Q - F  w o r k , 127 d e g r e e c y l i n d r i c a l  used t o energy a n a l y s e  Magnific-  f  2  In t h e p r e s e n t  images o r  be c a l c u l a t e d f r o m t h e f o l l o w i n g  2  angular  convenient f o r design  p r o p e r t i e s need t o be u s e d .  F  linear magnification  An e x a m p l e  slits  a t the entrance  t h e r e s o l u t i o n and o t h e r  plane  design  T h e d i m e n s i o n s o f t h e l e n s a r e shown i n  The c a r d i n a l p a r a m e t e r s i n u n i t s o f D ( w i d t h  o f the lens  slit)  b y Read [ 2 4 ] a r e u s e d i n t h e f o l l o w i n g s a m p l e c a l c u l a t i o n s f o r P  M _ width " width  of real s l i t of virtual s l i t  n g  i  v  e  n  Q  _ c cn. " ' 6  5 D :  100,  Cylinder lens (c/y£»=0 1) , . ,  6f  2r  10  PID The focal lengths/and/' am! foe;:! distances F and /•"' of the two-clement coaxial cylinder lens of unit diameter and 01 separation. The primed values refer to the high potential side of the lens  D.W.O. Heddle, J . Phys. E F i g u r e 2.  100  Values of object distance /'anil image ilistaiuv CJ expressed in terms of Jens diameter I) for the iwo-element coaxial cylinder lens of D/10 separation. Lines or constant voltage ratio and of constant magnification measured in the direction from low to high potential are shown  2 (1969) 10if6  Characteristics  of electron  lenses.  25.  V  1  V  =  6  l  f  =  2  ,  1  f  2  =  5  ,  1  l  F  =  3  ,  7  F  2  =  3  ,  0  (P - 3.7) ( 6 . 5 - 3.0) = 2.1 x 5.1 = 10.7 (P - 3.7) = 1 0 . 7 / 3 . 5 P  2  V  /  l  V  =  7  =  =3.0  LL  M  f-| = 1-7  f  TO  =  = 4.6  2  F  °'  =  70  = 3.1  1  F  = 2.6  2  (P - 3.1) ( 6 . 5 - 2.6) = 1.7 x 4.6 = 7.8 (P - 3.1) = 7.8/3.9 = 2.0 p  V /V 2  ]  =8  =  f  1  jy. = 1.5  FfJ  M =  f  2  = 4.3  F  °-  =  85  = 2.9  1  (P - 2.9) ( 6 . 5 - 2.2) = 1.5 x 4.3 =  F  2  = 2.2  6.5  (P - 2.9) = 6.5/4.3 = 1.5 P = 4.4 =  V /V 2  ]  =9  f  1  = 1.4  M =  f  2  = 4.0  F  |4 1.5  = 1.00 =====  = 2.7  1  F  = 2.0  2  (P - 2.7) ( 6 . 5 - 2.0) = 1.4 x 4.0 = 5.6 (P - 2.7) = 5.6/4.5 = P = JL£  1.2 M=  14  = 1,17  From t h e s e c a l c u l a t i o n s , t h e o p t i m u m r e t a r d a t i o n distances found that  o f P = 4.5 D a n d Q = 6.5 D i s V / V 2  the lenses operated best with  these conditions  the magnification  width o f the v i r t u a l  slit  1  ratio f o r the fixed  = 8.  Experimentally  a retardation  r a t i o o f 7.  i t was Under  M i s l e s s than u n i t y , meaning t h a t t h e  i s l a r g e r than the p h y s i c a l  somewhat d e g r a d e t h e r e s o l u t i o n o f t h e s p e c t r o m e t e r .  slit.  This would  26.  3.3.Electron  Energy A n a l y s e r s .  3.3.1. G e n e r a l The use  aspects.  study of e l e c t r o n s c a t t e r i n g  of techniques  electrons.  of both  Retarding  phenomena r e q u i r e s t h e s u c c e s s f u l  e n e r g y m o n o c h r o m a t i o n and  techniques  of various types  replaced  by t i m e o f f l i g h t , m a g n e t i c a n d  Of  the use  these  A detailed by H a s t e d  of e l e c t r i c  deflection  energy a n a l y s i s of  h a v e now  electric deflection  a n a l y s i s o f t h e e n e r g y s e l e c t i o n o f e l e c t r o n s has [25].  Momentum a n a l y s i s o f c h a r g e d  particles  o f f r i n g e and experiment  s t r a y magnetic f i e l d s  i s unfavourable  EIS e x p e r i m e n t s  employing  the other  affecting  double  On  shielded  from f r i n g e e f f e c t s .  special  of special  particularly  but  neighbouring  given  the  fields  problem  parts of  the  hand, e l e c t r o s t a t i c  fields  T h e r e a r e two  are e a s i l y  general  produced  types of  p r o v i s i o n s a r e made, e.g. The  integral  spherical  electrostatic  component  of  resolving  g r i d s [26]  stopping curves  useful f o r measuring r e l a t i v e t r a n s i t i o n  and  produced  probabilities  curve  differentiation  difference  (RPD)  technique  i s employed.  The  unless  retarding potential  [ 2 8 ] f o r t h e p r o d u c t i o n o f a pseudo-mono-  e n e r g e t i c e l e c t r o n beam, has  r e c e n t l y been i m p r o v e d and  can  l o w a s 0.01  now  are  but  a r e l e s s s a t i s f a c t o r y f o r a c c u r a t e measurement o f e l e c t r o n e n e r g i e s electronic  and  deflection  hence u s u a l l y e x h i b i t a l i m i t e d energy  lenses [27].  in  a n a l y s e r s and m o r e c o m p l e x e l e c t r o n  R e t a r d i n g a n a l y s e r s o n l y measure t h e normal  power u n l e s s  been  i n magnetic  e l e c t r o n e n e r g y a n a l y s e r s , n a m e l y , r e t a r d i n g a n a l y s e r s and  e l e c t r o n v e l o c i t y and  widespread.  f o r h i g h - r e s o l u t i o n , l o w - e n e r g y s t u d i e s as  optics.  analysers.  largely  techniques.  a n a l y s e r s i s the most  have been q u i t e e x t e n s i v e l y used f o r $-ray s p e c t r o m e t e r s ,  t h e use  been  g i v e a r e s o l u t i o n as  eV  automated.  This  [ 2 9 ] when u s e d c a r e f u l l y .  It  27.  is  u s u a l l y used w i t h the  trapped  e l e c t r o n technique  impact s p e c t r a , e s p e c i a l l y near Electrostatic spectrum which  analysers  merit all  to the  Sar-el  [31]  signal  after  readily gives  cylindrical  Magnetic f i e l d  mirror analyser.  from a p o i n t source,  compatible  This  but  relative  has  t h e a d v a n t a g e o f d o u b l e f o c u s s i n g and  s t r o n g l y d e c e l e r a t i n g and symmetry.  there  highest  can  collect electrons  analyser, f i r s t described  a c c e l e r a t i n g l e n s as w e l l as  S i m p s o n and  co-workers [33,34],  in  present  the  i s the  and  electrostatic  127°  w o r k and field  a path  a x i s m a k i n g an  no  the  [38].  The angle  beam e n t e r s  fringe field  An  velocity  in  EIS.  parallel  with  the analyser  Combined e l e c t r o s t a t i c  and  the  satisfactory with  also Lassettre analysers  This  be g i v e n  type  as  and  of electroby  i s used  later.  A a  uniform  electro-  beam p a r a l l e l  p l a t e s i s the main  along  disadvant-  equipotential surface,  c o n t r a r y t o most o t h e r  magnetic f i e l d s  [32],  zoom l e n s e s  plates also serves  t h r o u g h an  c o r r e c t i o n i s necessary,  Purcell  selector introduced  n e c e s s i t y of c o l l i m a t i n g the o f 45°  important.  a l t e r n a t i v e form of  Kerwin [ 3 7 ] .  a d e t a i l e d account w i l l  b e t w e e n two  analyser  but  electrostatic  i m p r o v e d by Marmet and  static  age,  of EIS.  at  in  t o most  and  for extensive  study  by  easy c o u p l i n g  advanced design  [36]  f i g u r e of  are d i f f i c u l t i e s  have produced c o n c e n t r i c h e m i s p h e r i c a l  Clarke  of  the  co-workers [35]  device  transition  n e u t r a l i z a t i o n o v e r a l a r g e volume i s p a r t i c u l a r l y  concentric hemispherical  static  differential  interpretation.  e l e c t r o n l e n s systems s u i t a b l e f o r use  The  circular  electron  i n v e s t i g a t i n g the r e l a t i v e merit  of d e f l e c t i o n analysers, assigned  polar angles  designing  produce a  i s g e n e r a l l y more a m e n a b l e t o s p e c t r o s c o p i c  probabilities. types  to study  threshold.  of the d e f l e c t i o n type  I n t e g r a t i o n of the d i f f e r e n t i a l  several  [30]  so  analysers.  are a l s o s u i t a b l e f o r  28.  e l e c t r o n energy a n a l y s i s . fields, of  i n the  better  analyser, down t h e and  pattern  of the  classical  has  with  eV  r e s o l v i n g power, i n which the  0.02  l i n e s of f o r c e  been a c h i e v e d .  in helical  e s p e c i a l l y when an  3.3.2. T h e o r y o f  axial  the  p a t h s has  ~*  point  electrons  been r e p o r t e d  f o r the  pass  by  Stamatovic  Presently,  this  analysis of electron  beams.  simple d e v i c e should f i n d wide a p p l i c a t i o n ,  consider  i n the  a two  r , where r i s the  source region  dimensional,  radial  i s injected normally  then f o r a given  e x i s t s a s p e c i f i c value of the describe  type of c r o s s - f i e l d  is desirable.  analyser. 3,  c h a r g e -e P,  new  resolution  radial  electro-  A  E = -^j  o f mass m and  not  magnetic f i e l d  127°  shown i n F i g u r e  static field  A  magnetic  [39], a  ( t r o c h o i d a l e l e c t r o n monochromator).  With f u r t h e r development, t h i s  at the  Wien f i l t e r  eV  u s e d f o r m o n o c h r o m a t i o n and  As  e l e c t r o s t a t i c and  t h a n 0.01  S c h u l z [40]  is only  Using crossed  value of  r  (along field  QP)  I f an  field  ( o r b i t 1 ).  c o n s t a n t A,  The  electron  into this  e l e c t r o n v e l o c i t y V where t h e  c i r c u l a r motion i n the  t h i s o r b i t (with radius  the  unit vector.  field there  electron  requirement  will  for  ) is  2 eE  = e£  i.e. To  a to the the  V =  (centrifugal force)  (3.12)  1  focussing  electron projected  n o r m a l and  electron  (3.11)  (eA/m) ^  i n v e s t i g a t e the  o r b i t o f an  by  = ^  can  with be  properties  i n t o the  velocity v .  resolved  field The  of  such a f i e l d ,  at the  point  acceleration  into a radial  and  a  consider  P a t an  the  angle  experienced  a tangential  component  Figure  3.  127°  cylindrical  analyser  field.  30.  >r-i$-r{&Y  (3-13)  VFH") 2  and a  2  2  + a^  r  <3J4)  = |a| .  I f v ^ i s t h e component o f t h e e l e c t r o n  velocity  p e r p e n d i c u l a r t o f , t h e n f o r an i n f i n i t e s i m a l t i m e d i s p l a c e m e n t d t ,  d<> j = v d t / r , so t h a t x  <p &  For  •  ?  *  •  ( 3 - ' 5 )  t h e case o f a c e n t r a l  from  (3.14), ^  ^cit"  =  c  o  n  s  t  a  n  under i n v e s t i g a t i o n , a, = 0 and so  (r r|-) vanishes. 2 d  t  =  n  f  r^  where h =  field  o  allt  r  (3.16)  =  2  at  r=r  ~ =Vo  c o s  _ d  d4> _ h  /- , x  d  R  ^ '  Now d e f i n i n g a new v a r i a b l e u = dr dt  1 du ~ " TJ2* d t  d r dt?  _ d_ " dt  2  _  a  r  _ "  \  1 Q  11  \J  n  2  d u 2  (3.13)  _  '  1  }  d?? —  u  2  9  n  v  (3.16)  „ h ?T  r  1  d u  . 2 2  and  .2,2 d u h  o  then  _ 1 h du . du " " u ? F2" d j " " d*  2  "  l  ( . d u \ _ h d_ / . d u \ n ^' dcp y " F 7 d * d^/  h  From e q u a t i o n s  17  (3.16)  d* d t " r ? d<j>  dt  a  3  o  Q  Changing t h e independent v a r i a b l e from t t o $ v i a r e l a t i o n d  (- )  a  2  "  _  . 2 2f d u  .,1  2  "  h  u  ^  +  uj  r  ,  9 1  ,  (3.21)  S i n c e f o r c e = mass x a c c e l e r a t i o n _  a  a  r  u2 2[d u 2  M  " "  h  u  |_dF  .2,2 fd u 2  i.e.  h  u  \W  , „] U  J  _  ~ ~ m  e  A  r  . „1 _ A e u j  "  ~nT  (3.22)  ;  r I n t r o d u c i n g t h e v a r i a b l e y = {j- =  SlZ dt|>2  +  e  d  X d<j)< i2  +  2  =  n  =  y  Ae mv^COS^a  The d i f f e r e n t i a l  at  (3.23)  ,  =  v  V Zcos^a '0  equation  ~  y V COSa  =  1  C  '0"  (3.24)  ( 3 . 2 3 ) i s t o be s o l v e d s u b j e c t e d t o t h e  conditions; <j> = 0  dy_ dd;  y = 1  _ d_ ~ d<j>  1  o r r  r_  Following  ( i .e. r =' r ) a n d  r  = _ 1 r  o  o o v  O  S  a  '  d  t  r=r  r=r.  - VoSina - -———— o v  o  dr d*  r_  Q  dr  >  C  1  ' .dr * d<|>  Hughes a n d R o j a n s k y  differential +a :  o  £ v  0  boundary  „Ae_ mj^v 2cos2ajj^y  Ae  J  i- where c  .,A_e .. mh2u 2y o  =  v  ( 3 . 2 2 ) becomes  C  0  S  _ . = tan a  a  [ 4 1 ] , approximate  e q u a t i o n a r e , f o r a n g l e s +a a n d -  y-, = c + (1 - c ) c o s /2~<f> 1  /2*  (3.25)  solutions of this a  t a n a s i n /2~c|> (3.26)  -a  y  9  = c + (1 - c ) c o s /2~<f> +  F o r two e l e c t r o n s o f equal  -  t a n a s i n /2<j>  v e l o c i t y , o n e e n t e r i n g a t a n a n g l e +a a n d  the o t h e r a t -a, t h e p o s i t i o n o f r e f o c u s o c c u r s where t h e two o r b i t s cross,  i . e . y-| = y , i n w h i c h 2  case t h e term  t a n a s i n /2<t> v a n i s h e s a n d  Jit? = 0 , T T , 2 T T , . . . . e t c . The o r b i t s c r o s s f o r t h e f i r s t <fr =  +  e  =  if/^"  Note t h a t t h i s a n g l e  time a t  = 127° 17' i s independent  (3.27) of a  32.  The two  s e c o n d f a c t o r t o be  i n v e s t i g a t e d i s the  e l e c t r o n s , entering normally  When a  =  The  slightly different  (1 - c ' )  t>Ay /2<()]  cos  separation of their orbits  (c - c ' )  This  also gives  This  treatment  intercept The  the  /2  =  =  (c  - c)  1  shows t h a t t h e  angle  r e s o l v i n g power, d ,  127°  17'  i s the  path  i n w h i c h an  as  But  from d  e  incident normally  =  2  y  f  _  c  =  o 1+23  = c +  ^  (3.24),  r  r  c = V/v  Q  (-  1)  ^  and  i f @ =  (1 - 2 3 )  = 2c  d e  =  powers o f  =  r  _  Q  the  - 4g )  3 are  at angles  ±a w i t h r e s p e c t  approximation  will an  v . Q  =  v  £  2  1 (3.30)  neglected.  path  i s the  of the e l e c t r o n s e n t e r i n g to the  n o r m a l QP,  a l l e l e c t r o n s having  to the  of  the  °=  of the  QP,  path  r  2  g  along  r  ) / V «  d e v i a t i o n from p e r f e c t r e - f o c u s s i n g , s , twin  is  - 1  (V - v  (23  Q  electron of v e l o c i t y V  0  a t <(> = <()  entering  (1 - c )  analyser.  - r , where r  Q  ( i . e . a = 0) w i t h v e l o c i t y  ^  =  * 2gr  2 6 r  where h i g h e r The  e  optimum p o s i t i o n t o  electrostatic  e  <j> = f  greatest  17'.  i s defined  g  electron  be  (3.28)  (3.29)  T T / / 2 = 127°  circular  /2(f))  s i n /2<fr = 0  t r a v e l , w h i l e r i s t h e r a d i u s v e c t o r a t $ = <j> o f  !o  velocities.  case  e l e c t r o n beam i n a c y l i n d r i c a l  radius of the  (1 - c o s  ( i . e . the d i s p e r s i o n ) w i l l  i s a maximum, i n w h i c h = -  At  of  (1 - c ) c o s /2<f>  = c' +  2  when Ay  separation  0  y-j = c + y  but w i t h  spatial  s o l u t i o n of  the  ( 3 . 2 3 ) has  from the  the path  separation electric  of the e l e c t r o n  same v e l o c i t y V. to  be  field  A  second  33.  used.  Neglecting s  The  higher  powers o f  = 4a r /3  (3.31)  2  e  Q  d e v i a t i o n i s towards the  i n s i d e of the c i r c l e  3.3.3. E n e r g y r e s o l u t i o n o f t h e It can  i s now  d e f i n e d as  i n equation (2e  =  e  L e t s-| and  s  (r  Consider slit. r  an  An  , but  be  h a l f - w i d t h s of the  the  Q  r  = 2sr  interval  + s^;  Q  infinitely  (3.32)  exit  entrance  of r a d i a l slit:  (r  Q  -  r  +  s ).  placed  at r  2  2 3 ^  Q  slits  slit  r - r  Q  i n the  path  of  the and  2  entrance  radius  have a d i f f e r e n t  = +s ,  Q  entrance  2  assuming a l l e l e c t r o n s e n t e r  At the edges o f the e x i t  radius  field from  (3.32)  s  2F  0  exit  follow a circular  s  y-  - s ,  e l e c t r o n of a d i f f e r e n t v e l o c i t y w i l l  s  and  distances are, for  narrow e l e c t r o n source  v e c t o r f a t the e x i t s l i t , normally.  i s s m a l l , where V i s  o  e l e c t r o n of v e l o c i t y V w i l l  an  (V - v ) / V  r  - Sp  Q  If 6 =  v which  (3.12)  r e s p e c t i v e l y , then the slit:  analyser.  - 4e ) 2  2  127°  of radius r .  p o s s i b l e to c a l c u l a t e the range of v e l o c i t i e s  pass through the a n a l y s e r .  d  a,  =  1  - f = ^ v  = V(l  (3.33)  0  0 2 (1 ± p — s  Therefore,  e l e c t r o n s w i t h i n the v e l o c i t y range V  through w i t h equal to  be  uniform  probability  over that small  solid  line rectangle  at r  + x  Q  (where  i f the v e l o c i t y range.  i n F i g u r e 4b.  ) can  distribution  This d i s t r i b u t i o n For another s i m i l a r  |xj<fs-j|) i n the entrance  o  slit,  pass  i s assumed  i s shown a s source  electrons with  the  located velocity  34.  a)  r~ s 0  x  r +s  «*—  2  0  B  ,V  c)  2  int.  V  S2  v  r«s—H  X~+S-| F i g u r e 4.  V -±L_v 2r  X = -S-j  v  0  Diagram t o i l l u s t r a t e t h e c a l c u l a t i o n r e s o l u t i o n o f 127° a n a l y s e r .  o f t h e energy  35.  V entering  normally  (Figure 4a).  will  emerge a t t h e p o i n t r  To c a l c u l a t e t h e v e l o c i t i e s  Q  + x i n the exit  of the electrons  t h r o u g h a t t h e e d g e A, i t i s n o t e d t h a t t h e r a d i a l r  + x from A i s ( r + x) -  Q  (r  Q  v  - s ) = s 2  s + x  1 -  V  A  =  " ~Tr  V ( 1  >  Q  v  2  s  p  1 -/  = - ( s - x)  s  - "-Tr o  [(I--|F->, S  +  0  +  X  0  vo  o  Q  2  displaced  the centre  by V ( s - | / 2 r ) Q  f r o m V.  )  ( 3  -  3 5 )  J The v e l o c i t y d i s t r i b u t i o n i s  Q  line  i s displaced  s i d e i f x> 0 ( F i g u r e 4 b , d o t t e d  I f x i s then considered  side until  - x  V ( s / r ) , but the centre  the v e l o c i t y d i s t r i b u t i o n r e c t a n g l e s velocity  0  T r o—  probability.  by V ( x / 2 r ) t o t h e l o w v e l o c i t y  edge B i s  "1  )  +  a r e c t a n g l e o f base width  rectangle).  3 4 )  range  S/j — X  be t r a n s m i t t e d w i t h e q u a l  still  '  and so  2  Thus e l e c t r o n s w i t h i n t h e v e l o c i t y  can  + x from t h e other  Q  - X  9  ( 3  o  distance of the point r  + x) - ( r + s )  Q  of the point  + x.  2  o  (r  distance  come  0  =  The r a d i a l  that  s + x  0  fl  slit  t o vary  continuously  continue  f r o m 0 t o s-|,  t o move t o w a r d s t h e l o w  of the rectangular The same h o l d s  line  distribution i s  f o r the high  velocity  side  ( F i g u r e 4 c ) . T h e r e s u l t a n t d i s t r i b u t i o n c a n be f o u n d b y a d d i n g u p a l l these Case 1  rectangles. s-| ^  s  2  The r e s u l t a n t i s a t r i a n g l e w i t h s  1  + s  —2^  semi-base width  equal  to  2  V, w h i c h i s p r e c i s e l y t h e h a l f - w i d t h o f t h e v e l o c i t y o  36.  distribution.  V C a s e 2.  2r ^  s-j > The to  l ? 2r 0  s  M  A  s,  s  r  J  o  respectively.  V  r  L2.  E  0  (3 37) U.J/)  r o_  a  o f the entrance  Q  2s, = L  AEi,  = JL  2  _ s l i t width ~ mean r a d i u s i sthe full  distribution angle a,  +  d  and e x i t  slits  are equal, the expression  resolution i s  /2 E a  Ei/  n  2r  i fthe widths  where  a  0  LZ-  A E l  l ? 2r 0  s V  equal  half width o f the v e l o c i t y d i s t r i b u t i o n i s  AVi,  energy  i s a t r a p e z i u m w i t h upper and lower^base  s  I2r  for  -  (3 36)  S2  resultant  The  So  "^7 "  V  0  and E  _  w r  / '  0  but i d e n t i c a l  3 8  \ '  w i d t h a t h a l f maximum (FWHM) o f t h e e n e r g y  i s a n a l y s i n g energy.  g  3  Electrons of f i n i t e  v e l o c i t i e s V, a r e n o t a l l f o c u s e d  entrance  a t one p o i n t 2  at The  <(> = <J>j b u t a t p o i n t s s e p a r a t e d e  resolution, L  ±k-*r E.  3.3.4.  r  o  W  including  +  J  a t a distance s  angular e f f e c t s ,  g  = 4a r / 3 Q  (Eq. (3.31))  i stherefore (3.39)  D e f l e c t i o n v o l t a g e f o r t h e 127° a n a l y s e r .  The  potential  f u n c t i o n b e t w e e n two c o n c e n t r i c c y l i n d e r s o f r a d i i  a and b (a < b) i s  37.  V(r) = B At  l n r + C ( w h e r e B, C a r e c o n s t a n t s )  r = b  V(b) = B  lnb + C  r  V(a) = B  Ina+C  = a  V(b)  - V(a) = B  ln(£)  - V(b) - V(a)  P  _  Vab  .  ln(b/a)  " TnlbTiT  The e l e c t r i c f i e l d  t i s therefore  "  B  In o r d e r eW)  f o r an e l e c t r o n w i t h  to travel m F  =  V  2  " r  in a circular  ab  =  (  21n2  In o r d e r travel V(b)  V, ( k i n e t i c  path o f radius  " r  Q  along  W  =  r  e n e r g y = / m\l 1  2  Q  Q  a = 1",  the c i r c l e with  the desired  radius  Q  d e t e r m i n e d b e l o w ) so t h a t V ( r ) = W, Q  V(b) V(r ) Q  From  then  1 , 3 9 W  t i e d at  - V(a) = B l n r  Q  - Blna  =  Q  =  a fixed  t h e e n e r g y at  (3.40)  - V(r ) = B lnb - B l n r  e n e r g y to  r , the potential  - V ( r ) a n d V ( r ) - V ( a ) m u s t be  are analysed.  =  (3.42)  that electrons with  Q  u  in volts.  e x p e r i m e n t , b = 2"  ^  . 4 1 )  o II 2ew  _  TnTBTaT ' r  -  • — r  = 2W l n ( b / a )  a b  the present V  V . , tw,~\ ln(b/a)  and W a r e e x p r e s s e d  a  For  ( 3  velocity  e  3 h  =?  where both V ^ V  Vab  — v  Q  (3.40)  Bln(b/r ) Q  Bln(r /a) Q  be a n a l y s e d  differences ratio  which  (to  be  electrons  will  38.  V  out  V  V —  (  b  -  )  v  (  r 0  )  V ( r ) - V(a)  =  ln(b/r )  log(b/r )  Q  o  ln(r /a)  =  Q  =  0  out  °g( /3)  1  Y —  log(3/2)  =  0-  4  b = 2"  *  r  o  '  4  3  )  = 1.5"  ~T  3.3.5. A n a l y s e r s a n d t h e s p a c e c h a r g e The  3  5  1 2 4  0TT76  =  (  0  I n t h e p r e s e n t e x p e r i m e n t a = 1"  v  log(r /a)  problem.  127° a n a l y s e r c a n be u s e d t o p r o d u c e a beam o f l o w e n e r g y  monochromatic e l e c t r o n s from a t h e r m i o n i c experiments r e q u i r i n g  source.  To be u s e f u l  f o r most  a m o n o c h r o m a t i c beam, a s e l e c t o r m u s t be c a p a b l e o f _8  p a s s i n g c u r r e n t s o f t h e o r d e r o f ^ 10 o f l e s s t h a n 0.1 eV FWHM.  amp w i t h an e n e r g y  The problem a r i s e s  distribution  that with r e l a t i v e l y  high  c u r r e n t a n d l o w e n e r g y , t h e e l e c t r o n beam i n t h e s e l e c t o r p r o d u c e s a s p a c e charge which d i s t o r t s  the internal  field  and increases the d i s t r i b u t i o n  width. Marmet a n d K e r w i n [ 3 7 ] i n t r o d u c e d a n i m p o r t a n t b r e a k t h r o u g h t o t h e s p a c e c h a r g e p r o b l e m by r e p l a c i n g electrodes  the field-forming  by a 9 0 % t r a n s p a r e n t t u n g s t e n g r i d  (catcher electrodes) held a t a p o s i t i v e grids. solid  cylindrical  mesh.  potential  S o l i d metal  plates  were p l a c e d o u t s i d e t h e  T h o s e e l e c t r o n s w h i c h c a u s e d s p a c e c h a r g e by r e f l e c t i o n field-forming  metal  from t h e  p l a t e s a t u n d e s i r e d e n e r g i e s c o u l d t h e n pass through  t h e g r i d s a n d be r e m o v e d f r o m t h e w o r k i n g r e g i o n .  Coating the catcher  electrodes with  (such as benzene  low e l e c t r o n r e f l e c t i v i t y m a t e r i a l  soot o r " e l e c t r o n v e l v e t " A better solution analyser with "virtual  [37]) further  to this slits"  reduced t h e space charge.  space charge problem  i s to operate the  - i . e . without having r e a l ,  physical  slits  39.  at the  focal planes.  of  analyser  the  From ( 3 . 3 8 ) ,  i t i s noted  i s inversely proportional  Therefore, electrons  should enter  the  energy, which i s u s u a l l y below the To  i s adopted.  A beam o f e l e c t r o n s of the  retarding  entrance s l i t plane of the Similarly, the  ating (1)  lens. By are  The  enters  the  The  placing  exit slit  placed  a t the  physical  scattered  to match the  from s l i t  higher  slits  virtual  at  the  slit"  many h e m i s p h e r i c a l  are  virtual  device with  electron  of e l e c t r o n  placed  at  the  object  potential.  lens  lenses.  onto the  The  physical  entrance focal slit.  i s the the  image  the  are:  s e l e c t o r to cause can  be  of  reacceler-  potentials, electrons  which their  complications.  carefully collimated  electrostatic deflector.  deleterious  with  designs.  electron One  t e s t a g r i d l e s s 127° Pavlovic lenses  slits  plane  no  surface  e f f e c t s of a  Hence coating  large  electrons.  analyser  slits.  use  reacceleration  r e d u c e d t o a m i n i m u m , and  configuration  w o r k i s t o b u i l d and  possible  lose a s i g n i f i c a n t part of  c h a r a c t e r i s t i c s of the  number o f e x t r a n e o u s  and  higher  analyser  o r g r i d i s needed t o r e d u c e t h e  "virtual  -  p o t e n t i a l side of  e d g e s and  beam e n t e r i n g  unwanted e l e c t r o n s  The  energy.  lowest  e x i t focal place  prevented from entering  electron  the slit  at the  resolution  i n t e r e s t i n most e x p e r i m e n t s .  retarding  advantages of using the  energy  a t a lower p o t e n t i a l to form a v i r t u a l  virtual  slit  a t the  at a higher  the  the  analysing  - analysis  physical  lens which i s held  analyser  energy are (2)  energy of  involves  i s t h e n imaged by  the  physical  This  to the  analyser  compensate f o r t h i s , a d e c e l e r a t i o n  configuration  that  et a l . [42] but  using  optics  of the  has  been used  purposes of the  analyser  using  real s l i t s .  I t now  present  electron  have r e c e n t l y r e p o r t e d  with  lenses  a similar  seems w e l l  40.  established 127°  that  cylindrical  grids are analysers.  not  required  for efficient  operation  of  41.  CHAPTER APPARATUS  IV  AND  PERFORMANCE  4.1.The S p e c t r o m e t e r An  e n e r g y s e l e c t e d beam o f e l e c t r o n s  analysed  i s f o r m e d , s c a t t e r e d and  i n t h e a p p a r a t u s shown s c h e m a t i c a l l y  a photograph o f t h e spectrometer.  Electrons  i n Figure  a r e produced  c h a r g e l i m i t e d e l e c t r o n gun e m p l o y i n g an i n d i r e c t l y s p e c t r o m e t e r works on t h e p r i n c i p l e o f v i r t u a l and  exit  planes of the analysers.  c i r c u l a r apertures applied  t o s l i t geometry.  energy  (typically  slits  Plate 1 i s i n a space  heated cathode.  The  a t the entrance  p r i n c i p l e was f i r s t  a n d i m a g e s by K u y a t t  these ideas  object a t high  This  5.  and Simpson [ 3 4 ] .  applied to We  have  T h e e l e c t r o n beam f o r m s a r e a l  70 e V ) w h i c h i s f o c u s s e d  p l a n e o f t h e m o n o c h r o m a t o r u s i n g a 7:1 r e t a r d i n g l e n s .  onto t h e f o c a l  Similar  lenses  are  l o c a t e d a t t h e i n p u t and e x i t o f both monochromator and a n a l y s e r .  The  l e n s p a r a m e t e r s , a d a p t e d t o s l i t g e o m e t r y , have been t a k e n f r o m t h e  c a l c u l a t i o n s o f Read lens operating  [24] f o r c i r c u l a r aperture  The l e n s e l e m e n t s c o n s i s t o f a p a i r o f r e c t a n g u l a r  slits  0 . 1 2 5 " by 1", s p a c e d 0.125 " a p a r t .  width  a n d was o r i g i n a l l y  1  l i n e focus  stop  limited  The o b j e c t  t o 0.25" i n l e n g t h .  slit  i s 0.010" i n  H o w e v e r , b u r n s on  i n t h e e x i t arm o f t h e m o n o c h r o m a t o r i n d i c a t e a s h a r p  over the f u l l  1" h e i g h t  that a long  slit  apertures.  A magnification  o f the lens elements thus  may be s a t i s f a c t o r i l y occurs  image c a l c u l a t e d t o be 0.012" w i d e . incorporates  The  p o t e n t i a l s a r e found t o correspond q u i t e c l o s e l y t o t h e  calculated values.  the angular  two e l e m e n t l e n s e s .  t r e a t e d as a " s t a c k "  suggesting of circular  i n the retarding lens g i v i n g a  virtual  The l o w e n e r g y s i d e o f t h e l e n s  a Herzog c o r r e c t i o n [ 4 3 ] matching  i tto the input o f the  COLLISION CHAMBER  Plate  1.  The  Spectrometer.  44.  monochromator.  T h e 127 d e g r e e , c y l i n d r i c a l  r a d i u s o f 1.5 i n c h e s height  m o n o c h r o m a t o r h a s a mean  w i t h an i n t e r e l e c t r o d e a n n u l a r  molybdenum. divergence  Slits o f the  a minimum a n d t h e needed.  deflecting  2 , 3 , 6, 7 s e r v e beam. grid  To f u r t h e r r e d u c e b a c k g r o u n d electrodes are  The  analyser, identical  on  a rotatable circular  s c a t t e r i n g angle  t o l i m i t the  coated  with  s c a t t e r i n g the  benzene s o o t .  t o enter  5  with  the  10 eV w h i l e  collision and  p l a t f o r m a l l o w i n g the  collision capillary  array.  s e l e c t i o n o f any d e s i r e d  b e t w e e n - 3 0 a n d +100 d e g r e e s w i t h  pairs of s p l i t spectrometer.  an a c c u r a c y  system.  of  t /^ . 1  0  i n agreement  P l a t e s Qi»2»3>u  d e f l e c t i o n p l a t e s which  p l a t e s are  used t o a l i g n  the  electron  T h e mean e l e c t r o n e n e r g y i n b o t h  analyzers  the d e s i r e d e l e c t r o n impact energy i s a p p l i e d t o t h e  chamber ( w i t h r e s p e c t  between the a n a l y s e r  t o the cathode).  The p r i m a r y  b y a p p l y i n g a ramp  and the monochromator.  A t optimum  beam i n t e n s i t i e s m e a s u r e d w i t h a v i b r a t i n g  the e x i t o f the a n a l y s e r are meV  stages  i n c o n s t r u c t i o n t o the monochromator, i s mounted  the energy l o s s spectrum are obtained  primary  of the  I n the e a r l y  r e s o l u t i o n o f < 1° ( a t h a l f maximum) i s o b s e r v e d  beam t h r o u g h t h e is  surfaces  beam, f o r m e d by a 50V q u a r t z  are p a i r s o f h o r i z o n t a l and v e r t i c a l  together  are o f  angular  the c i r c u l a r  w i t h c a l c u l a t i o n s based on the geometry o f the and  and a p e r t u r e s  s t r u c t u r e used i n e a r l i e r monochromators [37] i s  chamber a s a q u a s i - m o l e c u l a r  angular  as stops  The s p e c t r o -  I n t h i s way u n w a n t e d e l e c t r o n s a r e r e d u c e d t o  of t h i s work, t a r g e t gas i s allowed  An  o f 1". T h e  o f t h e m o n o c h r o m a t o r i s 4" t o m i n i m i z e e n d e f f e c t s .  meter i s f a b r i c a t e d from c o p p e r , w h i l e a l l s l i t s  not  spacing  i n each a n a l y s e r .  approximately  10"  1 0  reed  beam  profile  voltage  performance electrometer a t  amp. a t a FWHM o f ^ 20  T h e vacuum c h a m b e r i s s u r r o u n d e d b y a h y d r o g e n  45.  a n n e a l e d mumetal s h i e l d w i t h direction  being  fine field  trimming i n the v e r t i c a l  e f f e c t e d by a p a i r o f c o i l s .  I n p r a c t i c e i t has been  found t h a t t h e s p e c t r o m e t e r e x h i b i t s optimum p e r f o r m a n c e and  intensity) with  to the c y l i n d r i c a l  a r e s i d u a l magnetic f i e l d axis.  o f ^ 200 m i l l i g a u s s  The r e a s o n f o r t h i s  i t i s probable that the p r i n c i p a l  increased  i n the energy analysers  'Wien f i l t e r ' .  analyser  mean e l e c t r o n e n e r g y electron  than t h a t c a l c u l a t e d f o r a purely  electrostatic  o f t h e same d i m e n s i o n s a n d o p e r a t e d a t t h e same (10 e V ) .  except a t very  Energy l o s s s p e c t r a  The m a g n e t i c f i e l d  low s c a t t e r e d  pulse  counting  d i r e c t l y on a c h a r t r e c o r d e r  channel a n a l y s e r .  electron  are recorded using  m u l t i p l i e r and c o n v e n t i o n a l  rather  r e s u l t i n g i n a type o f curved  perturbation  of the  t r a j e c t o r y i n t h e s c a t t e r i n g chamber i s l e s s t h a n t h e a n g u l a r  resolution  displayed  e f f e c t i s an  The o p t i m u m r e s o l v i n g power ( 0 . 0 2 0 eV FWHM) o b t a i n e d i s  3 to 4 times greater cylindrical  parallel  i s as y e t not f u l l y  understood, although dispersion  (for resolution  a channel  electron  techniques.  Spectra a r e  o r are accumulated  long c o l l e c t i o n  energy l o s s s p e c t r a .  Consequently, d i f f e r e n t i a l  excitation  states  of various  energy l o s s t o the r e q u i r e d and  recording  net  count rate i s obtained  have g e n e r a l l y value  (using  cross  the forward s c a t t e r i n g spectrum)  by b a c k g r o u n d s u b t r a c t i o n .  voltmeter.  sections f o r the  been d e t e r m i n e d by s e t t i n g t h e  t h e c o u n t r a t e as a f u n c t i o n o f a n g l e .  a digital  times f o r complete  Energy s c a l e s  A t each a n g l e t h e Energy l o s s i s  have been s e t u s i n g  p r o m i n e n t peaks i n t h e s p e c t r u m o r by s e t t i n g t h e e l a s t i c p e a k a t E = 0.  i n a multi-  A t large s c a t t e r i n g angles t h e count rates a r e u s u a l l y  low, r e s u l t i n g i n inconveniently  measured u s i n g  energies.  scattered  Measurements t a k e n a t d i f f e r e n t s c a t t e r i n g a n g l e s  46.  correspond  to d i f f e r e n t scattering  volumes.  Consequently,  the  intensity  o b t a i n e d a t a p a r t i c u l a r a n g l e 8 i s m u l t i p l i e d by s i n 6 t o g i v e a r e l a t i v e differential ^  cross section.  1% a t 5 d e g r e e s and The  The  e r r o r due  becomes n e g l i g i b l e a t l a r g e r  p e r f o r m a n c e o f t h e i n s t r u m e n t has  previous experimental L a B a h n and  Callaway  and  theoretical  [ 4 4 ] have c a l c u l a t e d  electron-helium scattering  1-95  at a s e l e c t i o n  and  shows o u r e x p e r i m e n t a l impact  energy  over  that  symmetrical  the angular range  a b o u t 0°  The  - 45°  not  L a B a h n and  error  is consistently  from  0°  scattered  The  i s a b o u t 2%. curves  is satisfactory.  are at large  and  Dolder  is  cross  10°  due  section to  electron  especially at large  the o v e r a l l agreement, a n g u l a r d i s c r i m i n a t i o n  10°  It is  scattering  angles.  values with state  angles.  beam.  45°  value.  b e s t w i t h t h e s m a l l a n g l e d a t a and  somewhat h i g h e r t h a n e x p e r i m e n t  It is  field  Beyond  [ 4 5 ] and  eV  instrument.  Over the a n g u l a r range  higher than our experimental  data of Gibson  t h e i r c a l c u l a t i o n s agree  Figure 6  electrons  than  range  c u r v e a t 30  differential less  helium.  i n the  s i g n i f i c a n t , spurious  plane.  with  section  - 180°.  t o +100°) o f o u r  [ 4 4 ] have compared t h e i r c a l c u l a t e d  absolute experimental  t h i s and  no  accurate the c a l c u l a t i o n s  Callaway  energies  from m i n o r components i n the p r i m a r y  statistical  known how  (-30°  with confidence at angles  t h e a g r e e m e n t o f t h e two  calculation  cross  t h e o r y a t an a n g l e o f 10 d e g r e e s .  i n the s c a t t e r i n g  interference  largest  and  indicating that  be d e t e r m i n e d  possible  of scattering angles  angles.  data f o r  the d i f f e r e n t i a l  our d i s t r i b u t i o n o f e l a s t i c a l l y  gradients occur cannot  scattering  at c o l l i s i o n  is  by c o m p a r i n g  r e s u l t s compared t o the c a l c u l a t e d  have n o r m a l i z e d e x p e r i m e n t apparent  scattering  been t e s t e d  electron  for elastic eV  to the s i n e c o r r e c t i o n  the  that  tend  to  be  In view  effects  i n our  of  We  (0  ; 1 -30°  i 1 - 2 0 ° -10°  1  1  1  1  1  1  1  1  CP  IO°  20°  30°  40°  50°  60°  70°  SCATTERING ANGLE 8 F i g u r e 6.  Elastic  scattering  i n h e l i u m a t 30  eV.  1  80°  j 90°  1——  100°  48.  instrument  are expected  t o be s m a l l .  Kuppermann e t a l [ 5 , 6 ] h a v e i n v e s t i g a t e d t h e i n e l a s t i c scattering cross sections f o rthe excitation forbidden  transitions  by e l e c t r o n i m p a c t  t r a n s i t i o n s from t h e ground were s t u d i e d .  of optically  on h e l i u m .  In p a r t i c u l a r t h e  3  3  t h e s e m e a s u r e m e n t s a t 44 eV  i s observed i n  Chamber.  I n a l l o w i n g t h e t a r g e t g a s t o e n t e r an o p e n c o l l i s i o n ;  beam f o r m e d b y a m u l t i c h a n n e l  the apparatus  sensitivity.  to the  replacement  s t a t e o f argon  target pressure  but a l s o permits  the spectrometer.  This  and e x i t  In e x i s t i n g  i s of particular  sufficient  a l s o o f a changing  impact  chamber  s l i t s ) not only allows a  importance  by gas m o l e c u l e s  i n the rest o f  i n the electron  gas t i g h t c o l l i s i o n  i n a complicated  higher  with regard t o the  e i t h e r by o b l i q u e l y s p l i t t i n g  chamber [ 4 6 ] r e s u l t i n g  by t h e u s e o f b e l l o w s  r e g i o n by a g a s t i g h t  e l e c t r o n spectrometers  have u s u a l l y been a c h i e v e d  reflections.  to gain  ( 1 1 . 8 3 e V ) a t an  a lower ambient pressure  effective elimination of scattering  and  torr  angle.  o f an open c o l l i s i o n  (with the exception o f entrance  collision  _ i +  pressure  U n d e r t h e s e c o n d i t i o n s a maximum c o u n t r a t e o f 6 0 0 c . p . s . was  e n e r g y o f 70 eV a n d 2° s c a t t e r i n g  analysers.  r e g i o n as a  a r r a y , the ambient  h a d t o be r a i s e d a s h i g h a s l x l O  obtained f o re x c i t a t i o n  The  impact  o f Kuppermann e t a l .  4.2.A G a s - t i g h t , R o t a t a b l e C o l l i s i o n  in  states  l  energy and s a t i s f a c t o r y agreement f o r i n t e n s i t y r a t i o s  quasi-molecular  a l l o w e d and  ( U S ) s t a t e t o 2 S , 2 ^ , 2 P and 2 P  We h a v e r e p e a t e d  comparison w i t h the date  differential  regions  the cylindrical  s c a t t e r i n g geometry o r  [ 3 4 , 3 5 , 4 7 ] w h i c h may e x h i b i t p r o b l e m s o f f l e x i b i l i t y inside c o n f i g u r a t i o n with regard  Foo e t a l . [ 4 8 ] were a b l e t o a v o i d t h e s e  t o m a i n beam p r o b l e m s by u s i n g  49.  a collision  chamber c o n s i s t i n g o f  limited  s c a t t e r i n g angle  the  i n t e r l e a v i n g vanes but  t o a r a n g e o f -1°  would have c a u s e d o v e r l a p w i t h  the entrance  t o +70°.  slit  and  this  arrangement  Larger  l o s s of  variation  gas  tightness. The gas in  following section describes  tight collision the  -30°  earlier  stages  of t h i s work.  t o +100° i s a c h i e v e d  and  is limited  This  collision  slots  (0.2" wide) cut  enter  t h r o u g h a molybdenum s l i t ,  c a r r i a g e , C, and  The  in a cylindrical  exit  slit,  t r a v e l l i n g on  below the  slot.  This  scattered x 0.75" snugly  electrons.  only  construction of  B,  brass  (0.01"  long  i n s c a t t e r i n g angle  by  the  The  strip  to the  between the  analyser  of brass  and  slit  c u t s , D and  may  be w i t h i n t h e  angular  E a r e made i n t h e r a i l s  S u i t a b l e stops  and  guides  are  i s m o u n t e d on  spectrometer two  slits  on r o t a t i o n . (0.01"  x 0.2")  Gas  on  and  now  the  a movable  turntable  (0.004"  c a r r i a g e , C,  and  (-30°  slot  fits The  wherever  t o +100°).  shim to s l i d e  only e f f e c t i v e l y  Small out.  t h a t the parts of  leak  i n a 2000-fold  the  thickness  the main c y l i n d e r .  e i t h e r s i d e so  this results  to  used f o r d e t e c t i o n o f  i t from c o n t a c t i n g other can  are  the main c y l i n d e r above  d i r e c t l y t o the  to a l l o w the  installed  backwards thus p r e v e n t i n g  range  2  Incident electrons  shim a c t s as a b l i n d w h i c h e f f e c t i v e l y c l o s e s the  the e x i t  of  i s shown i n P l a t e  shim stock  back o f the rails  spacing  x 0.2").which i s f i x e d  ( 0 . 1 0 " x 0.2")  the  s i z e and  from  hatched portions  shell.  concentric with  with  a  beam t a r g e t u s e d  chamber a l o n e  c a r r i a g e i s coupled  width) i s soldered i n t o a t h i n gap  brass  curls  A  A,  rails  t h a t i t moves s y n c h r o n o u s l y  the  A variation  F i g u r e 7 shows a s c h e m a t i c d i a g r a m o f i t .  main c y l i n d e r .  and  chamber t o r e p l a c e t h e m o l e c u l a r  the e l e c t r o n a n a l y s e r s . and  the d e s i g n  shim the  through reduction  so  SCATTERED ELECTRONS  COUPLING TO "TURNTABLE  RAIL  BRASS SHIM INCIDENT ELECTRONS F i g u r e 7.  O N E  I N C H  I t o t a t a b l e , g a s - t i g h t c o l l i s i o n chamber.  o  Plate  2.  The  gas-tight, rotatable collision  chamber.  52.  in  open a r e a o f t h e c o l l i s i o n r e g i o n .  count of  r a t e o f 5000 c . p . s .  argon  this  was o b t a i n e d f o r e x c i t a t i o n  torr.  5  w i t h a t e n - f o l d decrease  consumption i s o f importance  used.  Furthermore  chamber d e c r e a s e s scattered ally  in.ambient  the presence the acceptance  electrons.  Using  peak due t o t h e d i f f e r e n c e narrow e x i t s l i t p e a k s a r e now  o f a narrow e x i t  slit  The  reduced  samples a r e  i n the c o l l i s i o n  the molecular  beam t h e w i d t h  i n acceptance  i n t h e new c o l l i s i o n  o f an  than t h e primary t r a n s m i t t e d  angles.  However, w i t h t h e  chamber, t h e w i d t h s  o f t h e two  spectrometer.  i sa limit set  size of the other parts of  I f t h e d i s t a n c e between t h e c o l l i s i o n  chamber  s e l e c t o r s w e r e t o be i n c r e a s e d a n d / o r t h e g e o m e t r y a n d  s i z e o f t h e a n a l y s e r s changed, t h i s design o f c o l l i s i o n  w o u l d a l l o w an a n g u l a r v a r i a t i o n a t l e a s t t w i c e a s g r e a t . should prove  inelastic-  similar.  the energy  useful  not only i n electron  electron angular d i s t r i b u t i o n charged  state  angle of the electron analyser f o r  130 d e g r e e a n g u l a r r a n g e i n t h e p r e s e n t s y s t e m  the e x i s t i n g  physical  beam.  where s m a l l o r e x p e n s i v e  by t h e g e o m e t r i c a l a r r a n g e m e n t a n d p h y s i c a l  and  to the  p r e s s u r e when c o m p a r e d w i t h  s c a t t e r e d p e a k i s a b o u t 0.020 eV w i d e r  The  chamber a  T h i s r e p r e s e n t s an o r d e r o f magnitude i n c r e a s e i n  the p r e v i o u s arrangement w i t h a pseudo-molecular gas  collision  ( a t 7 0 e V , 2°) w h i l e t h e a m b i e n t p r e s s u r e i n t h e vacuum c h a m b e r  was 1 x 1 0 " signal  Using  particle  scattering  s t u d i e s as w e l l  chamber  Such a d e s i g n  but a l s o f o r photo-  as i n other types o f  spectroscopy.  4 . 3 . E l e c t r o n i c s and E l e c t r o n D e t e c t i n g System. The  v o l t a g e s u p p l i e s and c o n t r o l  circuitry  f o r the present  electron  53.  s p e c t r o m e t e r a r e shown i n F i g u r e by  low impedance r e g u l a t e d  8.  Almost a l l voltages  power s u p p l i e s .  e n c o u n t e r e d when many c o m m e r c i a l  Ripple  p o w e r s u p p l i e s on t o p o f o t h e r  resistor  network.  obtained  low  e f f e c t i v e impedances.  on  circuit with  the  operation  supplies  and have  very  when p o w e r s u p p l i e s a r e f l o a t e d  ( s a y , a r e s i s t o r network) coming through t h e interwinding  capacitance^ i n s e r i e s  A less e f f e c t i v e s o l u t i o n i s to place  a large capacitor  i m p e d a n c e t o g r o u n d , e . g . b e t w e e n one t e r m i n a l  unately The  and ground as i n t h e c i r c u i t  use o f b a t t e r i e s i s a simple  chargeable b a t t e r i e s . n o t have l o n g  data  However, t h i s  solution with the  However, one drawback o f t h i s  term s t a b i l i t y and t h e v o l t a g e s  control c i r c u i t r y  Variable voltages  of lKn per v o l t . block  using  The v a l u e  representing  unfort-  a r e r a p i d l y scanned.  tend  i s that batteries to d r i f t  when  p e r i o d o f time i s r e q u i r e d .  shown i s d e s i g n e d  so t h a t a l l power  supplies  e i t h e r g r o u n d e d one s i d e o r f l o a t e d d i r e c t l y o n t o p o f a n o t h e r  supply.  across  l i f e mercury and a l k a l i n e c e l l s as w e l l as r e -  a c q u i s i t i o n over a long The  diagram.  and i n e x p e n s i v e  to  F of the filament  p r o d u c e s s e v e r e d a m p i n g e f f e c t s when v o l t a g e s  a v a i l a b i l i t y o f long  the  Ripple occurs  f o r m e d by t h e t r a n s f o r m e r  power s u p p l y  are  floating  t h e 60 Hz p o w e r l i n e a n d t h e i m p e d a n c e f r o m t h e p o w e r s u p p l y  ground.  do  by a l w a y s  in a  i s no p r o b l e m i f p o w e r  which a r e designed f o r f l o a t i n g  a moderate impedance  frequently  power s u p p l i e s , n e v e r on t o p o f a  In t h i s way, t h e r e  are  problems,  power s u p p l i e s a r e o p e r a t e d  f l o a t e d mode a b o v e g r o u n d p o t e n t i a l c a n be s o l v e d the  are provided  are obtained  by r e m o t e r e s i s t a n c e  o f t h e programming r e s i s t o r s  t h e power s u p p l y .  v e r n i e r - d r i v e potentiometers.  Voltage  power  programming  a r e shown i n  adjustments are p o s s i b l e  T h e c a t h o d e o f t h e e l e c t r o n gun. i s  F i g u r e 8.  The c o n t r o l  circuit  diagram.  55.  held a t the potential  o f the filament centre-tap  (FCT), which i s  negative w i t h respect to ground.  The p o l a r i t y o f t h e g r i d  t h e e l e c t r o n gun i s n o r m a l l y  negative with respect  but  i t c a n be s w i t c h e d  over  kept  to positive polarity  voltages to the deflector plates Q Figure wired  5 ) a r e s u p p l i e d by d u a l  1  to  5  voltage of  to the cathode,  i f necessary.  The  and t h e s p l i t p l a t e s ( s e e  power s u p p l i e s a n d g a n g e d - p o t e n t i o m e t e r s  s o t h a t t h e v o l t a g e c a n be s c a n n e d c o n t i n u o u s l y f r o m p o s i t i v e t o  negative without  a reversing switch.  The p o l a r i t y  of the inner-electrodes  of t h e s e l e c t o r and t h e a n a l y s e r a r e p o s i t i v e w i t h r e s p e c t t o t h e i r corresponding provided  outer-electrodes.  by d u a l  centre-taps  power s u p p l i e s t r a c k e d  are tied  voltages  i n the ratio  (Focus 1 & 2) a r e o f 5 : 7 and t h e  t o t h e l o w - p o t e n t i a l s i d e o f t h e l e n s e l e m e n t s so  t h a t e l e c t r o n s t o be a n a l y s e d 3.3.4.).  The f o c u s  The c o l l i s i o n  travel  chamber  t o a v a r i a b l e power s u p p l y  along  t h e c o r r e c t path  ( C C ) c a n be s w i t c h e d  (see s e c t i o n  e i t h e r to ground,  or to a Keithley electrometer  t o measure t h e  e l e c t r o n c u r r e n t coming o u t o f t h e s e l e c t o r . Many w o r k e r s o p e r a t e the r a d i a l  plates.  127° s e l e c t o r s by s c a n n i n g  T h i s changes t h e f i e l d  strength f o rdifferent  e l e c t r o n s a n d p r o d u c e s a r e s o l v i n g power w h i c h energy.  A f u r t h e r disadvantage  calculated  from t h e geometric  the voltage  all  i n PES.  i s t h a t t h e e l e c t r o n e n e r g y m u s t be  dimensions o f t h e a n a l y s e r as i n s e c t i o n  A more s a t i s f a c t o r y  p r i o r t o t h e i r e n t r y t o the a n a l y s e r which This  i s accomplished  values  procedure i s t o r e a c c e l e r a t e  e l e c t r o n s w h i c h have l o s t e n e r g y due t o c o l l i s i o n  energy r e s o l u t i o n .  energy  i s a function of electron  3.3.4 a n d t h i s may e x p l a i n t h e s c a t t e r o f i o n i z a t i o n p o t e n t i a l [49] observed  across  i s then  t o t h e same e n e r g y  operated  at a  fixed  by a p p l y i n g a v a r i a b l e b u c k i n g  56.  and  scanning  voltage  ( F i g u r e 8) between S ( t h e h i g h - p o t e n t i a l element o f  l e n s 1) w h i c h i s h e l d a t ground p o t e n t i a l  and A ( t h e h i g h - p o t e n t i a l  e l e m e n t o f l e n s 2 ) . T h e s e two e l e m e n t s a r e shown c r o s s - h a t c h e d i n F i g u r e 5.  T h i s v o l t a g e c a n be s c a n n e d o v e r a r a n g e o f 1 0 v o l t s  motor-driven  potentiometer  programming a power s u p p l y o r swept by an  e l e c t r o n i c r a m p , a m p l i f i e d b y t h e ramp a m p l i f i e r , o p e r a t i n g with a multichannel A closed-end  necessary  channel  to monitor  electron multiplier  t h e m a i n beam a n d t u n e t h e s p e c t r o m e t e r . 10"  1 0  t o d e t e c t u s i n g t h e c o u n t i n g mode. a Faraday cage coupled  ( M u l l a r d B419AL) i s used  P r i o r t o measurement o f s c a t t e r e d e l e c t r o n s , i t  i n t e n s i t y o f t h e m a i n beam  To  synchrously  analyser.  for electron detection. is  by a  amp) i s t o o i n t e n s e f o r t h e c h a n n e l t r o n Instead, the channeltron  to a vibrating  i s used as  reed e l e c t r o m e t e r f o r t h i s  detect the scattered electrons, the channeltron  mode f o r b e t t e r s e n s i t i v i t y .  The  A positive  i s used  purpose.  i n the counting  h i g h v o l t a g e o f ^ 3KV i s a p p l i e d  to i t s output  w h i l e t h e i n p u t i s h e l d a t a b o u t 100 V a b o v e g r o u n d .  only r e l a t i v e  c r o s s s e c t i o n a r e measured, t h e a c t u a l value f o r d e t e c t i o n  efficiency  i s not important  a n d no c a l i b r a t i o n  equipment i s c o n v e n t i o n a l , comprising discriminator. as o u t p u t  A s c a l e r , ratemeter  h a s b e e n made.  Since  The c o u n t i n g  o f a p r e a m p l i f i e r , an a m p l i f i e r and  or multichannel  a n a l y s e r may be u s e d  device.  4.4.Vacuum S y s t e m a n d G a s H a n d l i n g . A p i c t u r e o f the complete experimental P l a t e 3.  The vacuum c h a m b e r h o u s i n g  a r r a n g e m e n t i s shown i n  the e l e c t r o n spectrometer  o f an a l u m i n i u m t u b e 16" i n h e i g h t and 16" i n o u t s i d e d i a m e t e r . is  1  / " t h i c k and both 2  0-ring  (without grease)  open ends a r e p o l i s h e d .  This tube s i t s  i n a g r o o v e c u t i n a 17" d i a m e t e r  consists The w a l l on a  viton  aluminium  base  P l a t e 3.  The  complete experimental  arrangement.  58.  f l a n g e on w h i c h t h e e l e c t r o n s p e c t r o m e t e r  i s located.  The t o p o f t h e  chamber i s c l o s e d w i t h an a l u m i n i u m l i d c a r r y i n g an a i r i n l e t v a l v e and an  i o n i z a t i o n guage head.  held  T h e vacuum s e a l  i n a groove c u t i n t h e l i d .  Since  i s e f f e c t e d by a v i t o n  n e i t h e r screws n o r b o l t s a r e used,  t h e vacuum c h a m b e r i s e a s i l y d e m o u n t a b l e . this  type  o f system i s very c o n v e n i e n t ,  to t h e spectrometer A transparent  i s required  (e.g.  When b a k i n g  particularly  soldered  i s not necessary  i f frequent  access  r e p l a c i n g guns, c l e a n i n g s l i t s  p l e x i g l a s s window ( h e l d by b o l t s a n d a v i t o n 0 - r i n g )  s i d e o f t h e chamber a l l o w s d i r e c t o b s e r v a t i o n Electrical  0-ring  connections  into brass  etc.).  on one  o f the s c a t t e r i n g angle.  a r e made v i a c e r a m i c o c t a l - s e a l s o r f e e d - t h r o u g h s  f l a n g e s which a r e then b o l t e d on t o t h e aluminium  base  f l a n g e and s e a l e d w i t h v i t o n 0 - r i n g s . The  vacuum i s p r o d u c e d b y a NRC 6" d i f f u s i o n  10 o i l ) a n d r o t a r y pump a r r a n g e m e n t w i t h a l i q u i d  pump ( u s i n g  n i t r o g e n c o l d t r a p and  w a t e r b a f f l e between t h e main chamber and t h e d i f f u s i o n vacuum w i t h o u t  gas sample i n t h e c o l l i s i o n  as m e a s u r e d b y a V e c c o i o n i z a t i o n A dual One  gas  flow  tubes and v a l v e s w i t h  and  o f g a s e s has been chemicals,  teflon  gases, c o n s i s t s o f brass  7  torr  constructed.  i s made up o f  seals while  the other  system  parts with viton 0-rings.  The  i s r e g u l a t e d b y a G r a n v . i l 1e;-Phi 11 i p s s e r i e s 2 0 3 v a r i a b l e l e a k ,  w h i c h c a n be b y p a s s e d  liquid  The t y p i c a l  chamber i s ^ 3 x 1 0 ~  system, f o r c o r r o s i v e gases and o r g a n i c  for non-corrosive  pump.  gauge.  i n l e t system a l l o w i n g mixing  stainless steel  Convalex  i f low;volatility i  liquids  or solids are studied.  n i t r o g e n c o l d t r a p between t h e main m a n i f o l d  t h e r o t a r y pump p r e v e n t s  chemicals  o f t h e sampling  from g e t t i n g i n t o  A  system  t h e pump o i l .  59.  CHAPTER  OPTICALLY  5.1.Optical  Selection  by  transition;  Garstang  FORBIDDEN  TRANSITIONS  Rules.  There a r e several forbidden  V  o p i n i o n s as t o t h e d e f i n i t i o n o f a l l o w e d and the following  i s a practical  terminology  suggested  [50]:  In a t o m i c s p e c t r o s c o p y , a l l t r a n s i t i o n s w h i c h v i o l a t e t h e r i g o r o u s selection  rules  f o re l e c t r i c dipole  forbidden transitions. electric  This category  include  i n free  atoms a r e termed  a l l magnetic dipole  q u a d r u p o l e t r a n s i t i o n s , two q u a n t u m p r o c e s s e s , e l e c t r i c  radiation dipole  radiation  e n f o r c e d by p e r t u r b a t i o n s e x t e r n a l  transitions  and dipole  t o t h e atom, and e l e c t r i c  caused by t h e a t o m i c n u c l e u s .  E l e c t r i c dipole  transit2 I r  ions which v i o l a t e only c e r t a i n - 4s4p  approximate s e l e c t i o n  P.j i n C a I , w h i c h v i o l a t e s  t h e r u l e AS=  rules  (e.g.  4s  ;>  0  0) a r e n o t c a l l e d  forbidden transitions. In m o l e c u l a r s p e c t r o s c o p y a l l t r a n s i t i o n s w h i c h v i o l a t e any s e l e c t i o n r u l e , whether r i g o r o u s o r n o t , a r e c a l l e d f o r b i d d e n . 3 ations  (e.g. n -  are also  z) a r e i n c l u d e d  included  among f o r b i d d e n m o l e c u l a r  among f o r b i d d e n  In atoms, t h e s e l e c t i o n and  rules  e l e c t r i c quadrupole t r a n s i t i o n s  Garstang [50].  intercombin-  1  I n p o l y a t o m i c m o l e c u l e s , t r a n s i t i o n s made p o s s i b l e action  Thus  The s e l e c t i o n  absence o f n u c l e a r p e r t u r b a t i o n  rules  by v i b r o n i c  inter-  transitions.  f o relectric dipole, are l i s t e d (1),  transitions.  magnetic  i n T a b l e 1, t a k e n  (2) and (3) a r e r i g o r o u s  a n d two q u a n t u m p r o c e s s e s .  dipole from i n the  Rule (4)  Electric (1)  dipole  AJ = 0, ± 1 (0  0)  Magnetic  dipole  AJ = 0, ± 1 (0 ^  (0  AM = 0, ± 1  AM = 0, ± 1  (3)  P a r i t y change  No p a r i t y c h a n g e  Al = ± 1  No electron jump  Al = 0 An = 0  (5)  AS = 0  (6)  AL = 0, ± 1 (0 •<+* 0 )  Table 1  AS = 0 AL = 0  S e l e c t i o n Rules i n Atomic  quadrupole  AJ = 0, ± 1, ± 2  0)  (2)  ( 4 ) One electron jump  Electric  0,  /  1  V  2  2  . 0-  AM = 0, ± 1, ± 2 No p a r i t y c h a n g e One  o r no e l e c t r o n  Al = 0, ± 2  AS = 0 AL = 0, ± 1, ± 2 (0 +t* 0, 0  Spectra,  1)  jump  1)  61.  holds and  o n l y when c o n f i g u r a t i o n i n t e r a c t i o n  ( 6 ) h o l d o n l y f o r LS c o u p l i n g .  approximate s e l e c t i o n  i s n e g l i g i b l e , and r u l e s (5)  T r a n s i t i o n s due t o v i o l a t i o n o f  r u l e s (4) - (6) a r e not u s u a l l y termed  In d i a t o m i c m o l e c u l e s ,  the general  selection  "forbidden".  rules f o r electric  dipole transitions are: (1) I f J AJ  i s the total  angular  momentum  = 0, ± 1 ( b u t n o t 0 +-> 0 )  This  i s a standard  (2) P o s i t i v e  r u l e o f general  f o r any atomic  (+) t e r m s c o m b i n e o n l y w i t h n e g a t i v e  (3) F o r i d e n t i c a l  nuclei,  symmetric  (4) F o r n u c l e i  o f equal  (g) e l e c t r o n i c  charge  (-)  ( a ) terms w i t h  system.  terms.  ( s ) terms combine o n l y  s y m m e t r i c terms and a n t i s y m m e t r i c  even  validity  with  antisymmetric.  ( b u t not n e c e s s a r i l y o f equal  mass)  s t a t e s combine o n l y w i t h odd ( u ) e l e c t r o n i c  states. Besides cases  these, there are other r u l e s which are v a l i d only f o r c e r t a i n  o f c o u p l i n g , e . g . Hund's c a s e s  [50].  There a r e three types  (a) or (b).  D e t a i l s c a n be f o u n d i n  of forbidden transitions  i n molecules:  ( i ) Those which a r e r i g o r o u s l y f o r b i d d e n f o r e l e c t r i c ions, a r i s i n g from higher m u l t i p o l e s . electric  T h e y a r e much w e a k e r t h a n  d i p o l e t r a n s i t i o n s and i n p r a c t i c e ,  selection rules are given  transitallowed  i t i s found t h a t only  m a g n e t i c d i p o l e s and 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 The  dipole  n e e d t o be  ,  considered.  i n [50].  ( i i ) Those which v i o l a t e approximate s e l e c t i o n r u l e s , u s u a l l y c a u s e d by s p i n - o r b i t  coupling or rotational-electronic  ( i i i ) Those r i g o r o u s l y f o r b i d d e n f o r f r e e m o l e c u l e s allowed  interaction. but which  become  as a r e s u l t o f e x t e r n a l p e r t u r b i n g i n f l u e n c e s , such as e x t e r n a l  62.  f i e l d s , c o l l i s i o n w i t h o t h e r m o l e c u l e s a n d a r e known a s e n f o r c e d transitions. For the  occur i n high pressure  spectrum i s proportional To a f i r s t  their equilibrium  moment.  a p p r o x i m a t i o n , t h i s may be c a l c u l a t e d position.  certain will still  the nuclei molecules,  equilibrium  be o b s e r v e d .  t r a n s i t i o n s a r e due t o m i x i n g o f t h e w a v e - f u n c t i o n s o f  vibrational  states.  .During t h e s e v i b r a t i o n s ,  have n o n - z e r o i n s t a n e o u s v a l u e s , a l t h o u g h be z e r o .  Vibronic  i n t e r a c t i o n may a l s o  o f weak t r a n s i t i o n s a r i s e s  allows f i n i t e  with  impose a z e r o v a l u e f o r t h e  the dipole  moment  the average value give appreciable  u t i o n s t o t h e band i n t e n s i t y o f w e a k l y a l l o w e d class  dipole  F o r many o f t h e m o r e s y m m e t r i c a l  I n s u c h c a s e s , weak t r a n s i t i o n s may s t i l l  These v i b r o n i c  band i n  t o the square o f t h e t r a n s i t i o n  however, symmetry c o n s i d e r a t i o n s dipole  gases.  p o l y a t o m i c m o l e c u l e s , t h e i n t e n s i t y o f an e l e c t r o n i c  moment. in  These u s u a l l y  dipole  transitions.  from s p i n - o r b i t  contrib-  Another  interaction,  t r a n s i t i o n p r o b a b i l i t i e s between s t a t e s  will  which  of different  multiplicities.  5 . 2 . E l e c t r o n Impact E x c i t a t i o n It  of O p t i c a l l y Forbidden  has been m e n t i o n e d i n s e c t i o n  hold quite  accurately  relatively  high  f o relectron  impact energies  forbidden transitions are either  2.4.that o p t i c a l  scattering  (> 100 e V ) .  angle.  At these energies, o p t i c a l l y  n o t s e e n a t a l l . Many o f t h e n o r m a l choice of  Through t h e mechanism o f e l e c t r o n  e x c i t a t i o n , low energy e l e c t r o n s  rules  a t zero a n g l e and  r u l e s , h o w e v e r , c a n be b r o k e n by j u d i c i o u s  energy and s c a t t e r i n g  selection  e x t r e m e l y weak o r a s i n t h e c a s e o f  spin-forbidden transitions, usually selection  Transitions.  (within  a few tens o f e l e c t r o n  incident exchange volts  63.  of the e x c i t a t i o n  t h r e s h o l d ) c a n be q u i t e e f f e c t i v e  i n causing  transitions  between s t a t e s o f d i f f e r e n t s p i n m u l t i p l i c i t y , w h i c h a r e h i g h l y in optical This  spectroscopy  forbidden  i n t h e absence o f a p p r e c i a b l e s p i n - o r b i t  involves the interchange  coupling.  o f t h e i n c i d e n t e l e c t r o n a n d a bound e l e c t r o n  In a d d i t i o n , t h e p r o b a b i l i t y o f p r o d u c i n g  t r a n s i t i o n s which a r e o p t i c a l l y  s y m m e t r y f o r b i d d e n c a n be d r a m a t i c a l l y i n c r e a s e d b y u s i n g  low energy  electrons.  5.2.1. E n e r g y d e p e n d e n c e . Enhancement o f o p t i c a l l y energies  forbidden  final  (2.28)  i s f = 2Wee*/K  states  ,  s  holds, thegeneralized o s c i l l a t o r  strength  a n d t h e m a t r i x , e l e m e n t e i s , f o r i n i t i a l and  s  e = o ^ l f e z  impact  c a n be e x p l a i n e d a s f o l l o w s :  When t h e B o r n a p p r o x i m a t i o n from  t r a n s i t i o n s a t lower  1  (5.1)  ^ * ! ^  i s t h e z- c o - o r d i n a t e  (which  i s taken  momentum t r a n s f e r v e c t o r K ) o f t h e s  t o be t h e d i r e c t i o n  of the  electron i n thescatterer.  On  e x p a n d i n g e a s a power s e r i e s i n K, e = f j i K ) * ^  where e  Then e-j i s t h e m a t r i x transition  = 1, ^ l l S z ^ l ^  £  (5.2)  element o f t h e e l e c t r i c  i so p t i c a l l y allowed = 2W| | E l  I f e-j = 0 b u t ££ + 0, t h e n  d i p o l e moment.  and K i s v e r y  If a  small, (5.3)  2  a t small  values  of K (neglecting higher  order  p o w e r s o f K) f Since z  2  =  1  /  3  r  2  = 2W|e | K 2  2  + (z  (5.4)  2  2  2  - /r 1  3  2  ), thequantity c  9  c a n be e x p r e s s e d  as  64.  This  i s usually referred  t o as t h e m a t r i x element  q u a d r u p o l e moment a n d a n y t r a n s i t i o n i n w h i c h been c a l l e d  term  i n (5.5) a r i s e s  e l e c t r i c quadrupole t r a n s i t i o n s . first  term.  transition  i n which t h e second  spectroscopy  the intensity of the  i s no o p t i c a l  analogue o f the  term vanishes but the f i r s t  the  first  term v a n i s h e s .  does n o t , w h i l e  p r o v i d e an example i n w h i c h  only  I t h a s b e e n shown i n e q u a t i o n ( 2 . 2 7 ) f o r v e r y  K a t zero scattering  a n g l e and h i g h impact e n e r g y , t h e c r o s s  section  transitions (a ) d  = 8  h  2 £ l  E/W  (5.6)  2  Under s i m i l a r c o n d i t i o n s , f o r e l e c t r i c q u a d r u p o l e (a ) q  which  has  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 i s a n e x a m p l e o f a  L y m a n - B i r g e - H o p f i e l d bands o f ^  for dipole  In o p t i c a l  i n considering  There  the  small  = 0 and £ 2 ^ 0  an 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 .  o n l y t h e second  of the electric  = 4e  h  2  (5.7)  2  i s independent o f t h e impact energy. (a ) /(o ) q  h  d  = W e 2  h  transitions  2 2  /(2E  2 e i  The r a t i o  )  (5.8)  i s o b v i o u s l y i n c r e a s i n g w i t h d e c r e a s e i n impact energy and hence forbidden transitions  a r e enhanced  s u b s c r i p t h emphasises  r e l a t i v e t o t h e a l l o w e d ones.  t h e c o n d i t i o n o f high impact  At lower impact e n e r g i e s , t h e s i t u a t i o n somewhat q u a l i t a t i v e l y .  I f the excitation  The  energies.  c a n o n l y be d i s c u s s e d  e n e r g y o f t h e q u a d r u p o l e and  the  dipole transition  i n question are not very d i f f e r e n t  from each  the  r a t i o o f t h e i r cross s e c t i o n s a t low impact e n e r g i e s ( q ) j / ( ( j ) a  by e q u a t i o n ( 2 . 3 0 ) q u a l i t a t i v e l y s i m i l a r t o ( F ) ^ / ( F ) , q  d  £  other,  a  a  is»  the ratio of the  65.  effective generalised of g e n e r a l i s e d  (Fj)^  o s c i l l a t o r strengths.  o s c i l l a t o r strengths  given  Due t o t h e l i m i t i n g  i n s e c t i o n 2.4, ( F )„ a n d g *•  may n o t d i f f e r d r a s t i c a l l y f r o m t h e i r v a l u e  high  impact energies  ( i . e . s u f f i c i e n t l y small  F approaches t h e g e n e r a l i s e d the o p t i c a l  Thus f r o m e q u a t i o n transitions  ) and ( F ^ ) ^ a t h  A t high  impact  energies  f , which i n turn tends t o Thus  ( 5 . 8 ) , t h e r e l a t i v e enhancement o f t h e e l e c t r i c q u a d r u p o l e i s again scattering  expected. a n g l e and d i f f e r e n t  a r e u s u a l l y s t r i k i n g l y s i m i l a r as f a r as r e l a t i v e  of e l e c t r i c dipole allowed by S k e r b e l e  a t 90 eV w i t h  K).  q  when K t e n d s t o z e r o .  impact s p e c t r a , a t zero  kinetic energies,  trated  f  at lower i n c i d e n t energies  Electron  ( F  o s c i l l a t o r strength  o s c i l l a t o r strengths  theory  transitions  are concerned.  This  initial intensities  has been  illus-  and L a s s e t t r e [ 5 1 ] , where t h e y compared t h e i r s p e c t r a  t h a t o f G e i g e r a n d S c h r t t d e r [ 5 2 ] a t 25 KeV.  of N  2  The e f f e c t o f  i m p a c t e n e r g y on r e l a t i v e i n t e n s i t i e s o f o p t i c a l l y f o r b i d d e n  transitions  has  been d i s c u s s e d  transitions  become  by B r i o n a n d O l s e n  [53].  Optically  forbidden  more a n d more p r o m i n e n t i n t h e s p e c t r a when t h e i m p a c t e n e r g y i s l o w e r e d towards  threshold.  5.2.2. A n g u l a r  dependence.  K u p p e r m a n n e t a l . [ 5 , 6 ] h a v e made e x t e n s i v e scattering variable electrons  by h e l i u m  scattering  studies of electron  a t o m s a n d some s m a l l m o l e c u l e s a t l o w e n e r g i e s angle.  They o b s e r v e d d i f f e r e n t  i n e l a s t i c a l l y s c a t t e r e d from helium  3 1 3 1 t o t h e 2 S, 2 S, 2 P a n d 2 P s t a t e s .  angular  dependence o f  associated with  These angular  and  transitions  dependence c u r v e s  have  66.  been used t o c h e c k t h e 4.1.).  performance of the  Kuppermann e t a l . [ 5 , 6 ]  justification  of a d i f f e r e n c e  The  the  nature of  ( o r any the  i n t h e a n g u l a r dependences as to the  o p t i c a l l y allowed t r a n s i t i o n ) i s the  l o n g - r a n g e d f o r c e and  the  so  l ^ S -> 2V  transition  This  is a  angular d e f l e c t i o n s , corresponding contribute  to the  differential  :  On  the  the  i n the  target with  to allow  electron the an  the  VS  2 P t r a n s i t i o n , the  relatively  plus  scattered  small  i s emitted  target  i n c i d e n t e n e r g y and  electron  impact  parameter Under  p a r t l y " f o r g e t s " where the more i s o t r o p i c a l l y .  s c a t t e r i n g can  effective central f i e l d  potential.  be  treated  Since the  as  More q u a n t i t a t i v e l y , i f i t w e r e due  range of the  leading  t o e x c i t a t i o n v i a e x c h a n g e , many more p a r t i a l  partial r max given  section  wave i s p r o p o r t i o n a l  i s the  e f f e c t i v e range.  to  because the  (r„„ ) max S i n c e the  2 l +  ^  at  phase s h i f t low  that  contribute f o r the  nth  enough e n e r g y , where  differential  cross  section  is  by °(M)  and  cross  waves  to  interaction  t o e x c i t a t i o n v i a a d i r e c t m e c h a n i s m i s much l a r g e r t h a n  differential  these  incident  leading  to the  that  i n c i d e n t e l e c t r o n must approach  range exchange process to o c c u r .  scattered  comes f r o m and  electron  cross  3  t h e much s h o r t e r  conditions,  to  o t h e r h a n d , f o r a s p i n exchange p r o c e s s s u c h as  1 involved  between  relatively  s e c t i o n , y i e l d i n g a s t r o n g l y forward-peaked d i s t r i b u t i o n of electrons.  section  follows.  coulombic repulsion  atomic e l e c t r o n .  small  impact parameters, w i l l  (see  have proposed a q u a l i t a t i v e t h e o r e t i c a l  interaction giving rise  i n c o m i n g e l e c t r o n and  large  present spectrometer  from the  = ^1  J^+l)n/ (cose)|  nature of  (5.9)  2  £  the  Legendre p o l y n o m i a l s  P ( c o s e ) , the 0  more  67.  partial  waves t h a t a r e the  i n c l u d e d , the  can  be  differential  the  a n g u l a r d i s t r i b u t i o n of  cross section.  excitation  i s more f o r w a r d  excitation  v i a an  quite  Kuppermann e t  electrons  the  a n g l e can  used t o c h a r a c t e r i s e  section  forbidden) transitions.  A is  band a r e to the  the  their ratios and  example, the  relative vibrational  for excitation  w h e r e G nv  ( E  '  9 )  be  ^ n v V  i s the  of  11.87  eV  excited. electron of  impact  scattering (spin-  t r a n s i t i o n of  generalizations  b a s e d on  Ng  has  empirical  E  of  '  electronic  p e a k s due  t o one  a n g u l a r dependence of  band i n t e n s i t i e s w i t h i n  the  e  nth  i n the  vth  intensity  a given  electronic  proportional  imply t h a t the vibrational  cross  state  )  (  state.  electronic  This fact  associated with  the  the  (00)  X (E,e) i s a function n  can  be  utilized  t r a n s i t i o n from those of  5.3.Optically Forbidden Transition  of  form  t r a n s i t i o n and  purpose of  seems t o  electronic,  F r a n c k Condon f a c t o r  a n g l e and  i s the  a function  causing  phenomenen  That these r e l a t i v e i n t e n s i t i e s are  expressed  electronic-vibrational  It  those electrons  i s a general  as  direct  identify singlet-triplet  r e s p e c t i v e Franck-Condon f a c t o r s  nv  causing  t h a t measurements o f  p o i n t to mention about the  m o l e c u l e can a  For  constant [35].  section  after  that  given.  further  t h a t the  or  i n [ 6 ] , where a s e t  is also  This  claimed  cross  results  scattered  s p e c i f i c atom o r m o l e c u l e b e i n g  a l . [5,6]  0°  a r e s u l t , i t i s expected  peaked than t h a t o f  differential  been d i s c u s s e d  As  exchange mechanism.  independent of  be  more s h a r p l y p e a k e d t o w a r d s e -  i n A r g o n and  to  of  ->  0  )  energy,  distinguish  another.  investigate  J  (nv)  Neon.  p r e s e n t work t o  5  inelastic  68.  e l e c t r o n s c a t t e r i n g by a r g o n and neon atoms a s a f u n c t i o n o f e n e r g y a n d scattering angle with states.  (L-S)  to the optically  I t i s also of i n t e r e s t to explore  discussed heavier  p a r t i c u l a r regard  by Kuppermann e t a l . [ 5 , 6 ]  rare gases.  coupling  Helium  the extent  f o r helium  i s the o n l y r a r e gas i n which  i s e x h i b i t e d . . The s p i n o r b i t  in j - j (or j - 1) coupling  splitting  [54].  f i n d a p p l i c a t i o n i n t h e G.L.C. a r g o n  ionization  s t a b l e s t a t e s have n o t been s p e c i f i c a l l y a 5 0 eV s p e c t r u m  shows some p o s s i b l e m i n i m a l  studied  (at 15°) reported  indicating study  et a l . [58]  that  while  a r e s h a r p l y peaked  triplet  ( P > PQ) » 3  2  3  +  [55,56]  ionization  detector.  The meta-  by e n e r g y l o s s  spectros-  by L a s s e t t r e e t a l . [ 3 5 ]  i n the region be o p t i m a l  s t a t e experiments 3  w o u l d be u s e f u l  and xenon, r e s u l t s  Excit-  by O l m s t e a d e t a l . [ 5 7 ] a n d  o f 3 0 eV s h o u l d  impact energies  the helium  increases  c o n t r i b u t i o n from these e x c i t a t i o n s .  a t i o n f u n c t i o n s f o rmetastable argon reported a l s o be L l o y d  Russell-Saunders  which  The m e t a s t a b l e s t a t e s  of argon a r e o f i n t e r e s t as energy sources f o r Penning  copy a l t h o u g h  t o which c o r r e l a t i o n s  c a n be a p p l i e d t o t h e  w i t h atomic w e i g h t i n t h e s e r i e s neon, a r g o n , krypton  and  inaccessible  [5,6]  o f 3 0 eV f o r such a  suggest that i t  3  t o look a t A r ( P , P ) e x c i t a t i o n at l a r g e r s c a t t e r i n g 2  Q  angles.  ? E x c i t a t i o n o f t h e t y p e - n s np are  symmetry f o r b i d d e n  impact [ 3 5 , 5 3 , 5 9 ] . [60]  although  6  7  5  -*• ns np ( n + l ) p i n a r g o n a n d n e o n  but are e a s i l y observed  Within  this  band t h e r e  by l o w e n e r g y  are a total  +  of ten J states  some o f t h e s e may be e f f e c t i v e l y d i s c o u n t e d  would r e q u i r e o c t u p o l e t r a n s i t i o n s . The c l o s e s p a c i n g u s u a l l y so d e s i g n a t e d d e s p i t e s p i n o r b i t c o u p l i n g .  electron  since  they  i n each o f t h e  69.  (n + 1 )s a n d ( n + 1 ) p bands n e c e s s i t a t e s e l e c t r o n monochromator and  5.3.1.  the use o f a high  resolution  analyser.  Argon.  We h a v e l o o k e d f o r t h e p r e s e n c e o f t h e m e t a s t a b l e s t a t e s of argon i n t h e energy l o s s spectra scattering  using  a n g l e a s s u g g e s t e d by t h e e x c i t a t i o n f u n c t i o n  i n t h e range 5 ° - 2 5 ° . the  The energy s c a l e peaks [ 6 0 ]  o p t i c a l l y allowed  3 respectively.  The  P  2  3  P  and P  ]  with  9 shows angles  reference  to  a t 1 1 . 6 2 eV a n d 1 1 . 8 3 eV  a t 1 1 . 5 5 eV f i r s t  level  Figure  scattering  i s established 1  ]  Q)  [ 5 7 , 5 8 ] and  [61,35,53,59].  o f a r g o n a t 3 0 eV f o r s e l e c t e d  energy l o s s spectra  2  l o w impact energy and l a r g e  a l s o by e a r l i e r e l e c t r o n s c a t t e r i n g s t u d i e s the  ( P  appears a s a s h o u l d e r on  3 the  low energy s i d e o f t h e  t h e m e t a s t a b l e peak r e l a t i v e as  P-j ( 1 1 . 6 2 e V ) p e a k w i t h  the intensity of  t o the o p t i c a l l y allowed  the scattering angle increases.  peak  increasing  Two d i s t i n c t p e a k s a r e o b s e r v e d a t  3 larger angles.  The  P Q p e a k a t 1 1 . 7 2 eV i s much l e s s i n t e n s e  than t h e  3 P  2  ( a p p r o x i m a t e l y an o r d e r o f m a g n i t u d e l e s s ) as has been o b s e r v e d i n  Penning [55]. 15°  ionization electron  spectroscopy using  9) a small  However, ( F i g u r e  p e a k c a n be s e e n  and 2 0 ° ) between t h e o p t i c a l l y a l l o w e d  ential  cross  section f o rproduction  states.  of the  function o f scattering angle i n Figure 10°  metastable argon  P  2  atoms  (most c l e a r l y a t The r e l a t i v e  differ-  s t a t e i s shown a s a  1 0 w h i c h shows a maximum b e t w e e n  and 1 5 ° and then d e c r e a s e s q u i t e r a p i d l y t o 6 0 ° beyond w h i c h t h e  rate o f decrease i s quite errors  involved.  small.  The d i f f e r e n t i a l  compared t o t h o s e f o r  1  P  T h e e r r o r b a r s show t h e s t a t i s t i c a l cross  section for the  ( 1 1 . 8 3 eV) and P 3  ]  1  P  2  state i s  ( 1 1 . 6 2 eV) i n F i g u r e  11  70.  Energy Loss eV F i g u r e 9.  E l e c t r o n i m p a c t s p e c t r a f o r 3p -* 4s a t 30 eV.  excitation  of  71.  Figure  10.  R e l a t i v e d i f f e r e n t i a l cross s e c t i o n f o r the o f m e t a s t a b l e a r g o n ( P ) a t 30 eV. 3  9  formation  72.  SCATTERING A N G L E 9 Figure  11.  R e l a t i v e d i f f e r e n t i a l cross s e c t i o n f o r e x c i t a t i o n to 4s s t a t e s o f a r g o n a t 30 eV.  the  73.  (note  logarithmic scale).  expected,  The  s t r o n g l y peaked i n the  monotonically transitions  at larger angles.  to those  transitions) is essentially increases  by  one  order  maximum b e f o r e  angles  the r a t i o  and  12.  r a t i o s of  As  expected,  isotropic.  decreasing  increases  3  t o 30°  t o a f l a t minimum  various  1 P^/ P^ r a t i o 3 1 V^J  P^  shown  (optical  ratio  where i t r e a c h e s a t 55°.  behaviour  o p t i c a l l y forbidden  parity  of  ^P-j e x c i t a t i o n a r e  the  This  as  decrease  intensities  Whereas t h e  slowly again.  are,  At  higher  i s quite  transition  forbidden m u l t i p l e t s  i .  ( P  The  spectra reported  3 /  in  helium  to j j coupling. 6 1 ( SQ) -»•  -3p  )4p a p p e a r i n e n e r g y l o s s s p e c t r a a t l o w e r by L a s s e t t r e e t a l . [ 3 5 ] a t 50 eV  show f o u r p e a k s a t 1 3 . 0 9 , 1 3 . 1 7 , 13.30 t h r e e bands i n v o l v e s a allowed so  the  13.48  J = 2 t r a n s i t i o n which  (dipole forbidden). has  and  The  same t e r m s y m b o l a s  impact.  A t 50  eV  L a s s e t t r e e t a l . [35] d i d not  [60]  12.91  eV  w h i c h has  at  a r g o n by  B r i o n and  01 s e n  transition  i n helium,  been r e p o r t e d  [53].  (0°  and  energies. 15°)  Each o f the  is electric  the ground s t a t e .  I^SQ  2^SQ  eV.  impact  first  quadrupole  f o u r t h peak i n v o l v e s a s t a t e w i t h J =  s i m i l a r to the  Figure  e n e r g y l o s s s p e c t r u m o f a r g o n a t 30  eV  13  i n the  Such a  i s allowed  in electron  observe the J = 1 s t a t e t h r e s h o l d spectrum  shows t h e r e l e v a n t p a r t o f  at s c a t t e r i n g angles 1  0  transition,  f r o m 2°  Two s e r i e s o f l i n e s d e s i g n a t e d n l , n l o c c u r i n t h e s p e c t r a o f t h e r a r e g a s e s l a r g e r t h a n h e l i u m due t o t h e s p i n - o r b i t s p l i t t i n g o f t h e ion core. F o r a f u l l e r d i s c u s s i o n o f t h i s n o t a t i o n see r e f e r e n c e s 54 and 60. +  a  2  -3p  and  and  p r e s u m a b l y r e f l e c t s t h e d i f f e r e n t c h a r a c t e r due +  Some members o f t h e 5  The  transitions  direction  o f m a g n i t u d e f r o m 2°  d i f f e r e n t f r o m t h a t f o r any [5,6]  forward  f o r the o p t i c a l l y allowed  (on a l o g s c a l e ) i n F i g u r e  local  o p t i c a l l y allowed  of the to  Ar, 3 0 eV  90°  SCATTERING A N G L E 6 Figure  12.  l i f f e r ' s n t i a l cross section r a t i o s for e x c i t a t i o n 4s s t a t e s o f a r g o n a t 30 eV.  to  the  F i g u r e 13.  E l e c t r o n i m p a c t s p e c t r a f o r 3p -*• 4p e x c i t a t i o n a t 30 eV.  of  argon  76.  25°.  The  energy s c a l e i s f i x e d  using  A transition  i s c l e a r l y observed  energy loss  s p e c t r a a t 50 eV  intensity  of the  discernable.  An  with scattering 13.48  eV.  b u t , as to  At  has  12.91  eV  and  that the  t o 15°.  by  intensity  a t 30  decreases  eV  t o 10°  (figure before  peak i n t h e b a n d a t The several in J  = 0 transition  rapidly  intensity  very r a p i d l y  scattering  a t 25°.  increasing again  cross  At  30 eV  and  angles  rises  we  Similar  2°  angle  have  again  at  relative is  observed  becoming  features  are  intensity  becoming the most  intense  s e c t i o n s f o r the e x c i t a t i o n  At  15 a t s c a t t e r i n g  the d i f f e r e n t i a l  beyond w h i c h  higher angles  same t r a n s i t i o n  a t 50 eV  (see  insert  section  i s s h a r p l y peaked  rapidly  t o a n e a r z e r o minimum b e t w e e n 15°  i n the forward  a t a b o u t 45°.  A  before  i t decreases  slow  i n F i g u r e 15)  direction and  of angles,  cross section f o r  20°.  before A  the  increasing  again. cross  decreasing  second,  increase i s then  the  more g e n t l y  the c r o s s s e c t i o n r i s e s  For the  angles.  i t s intensity  shows a minimum a t a b o u t 10°  maximum a t 30°  t o a minimum a t 70°.  higher  o f t h e J = 0 peak  13) w h e r e t h e J = 0 r e l a t i v e  ( 1 3 . 4 8 eV)  maximum i s o b s e r v e d  behaviour,  25°.  - 90°.  to a sharp  same  relative  when t h e s c a t t e r i n g  s t a t e s o f a r g o n a r e shown i n F i g u r e  t h e r a n g e 5°  shows t h e  s p e c t r a i s the  of the J = 0 l e v e l  relative differential 4p  [60].  such t h a t i t i s b a r e l y  L a s s e t t r e , [35]  comparable to the J = 2 s t a t e s again observed  eV  g i v e s t h e most i n t e n s e peak a t  However, a t h i g h e r  relative  F i g u r e 14  decreased  the J = 0 l e v e l  a l s o been o b s e r v e d  a t 13.48  i t i s apparent t h a t the  a n g l e s , of the r e l a t i v e  50 eV  eV.  f e a t u r e i n these  the J = 2 s t a t e s decreases  increased  a t 12.91  p e a k has  interesting  the J = 0 l e v e l  very  broader  observed  at  Ar 5 0 eV 15°  .1 •  4  V  :  •V :  .v.-  J* 1  I  t-  3 2  20°  m  10°  25°  i—I—i—i—r  130  14.  i—r  i — i — i — i — r 133 136  Energy Loss Figure  A  E l e c t r o n impact spectra a t 50 eV.  1—I—I—I—I—  130  133  T  1  136  eV f o r 3p -»• 4p e x c i t a t i o n  of  argon  8Ch 70H  SCATTERING ANGLE 9 Figure  15.  Relative d i f f e r e n t i a l cross sections t o t h e 4p s t a t e s o f a r g o n .  f o r the  excitation  79.  The The of  J  = 1 s t a t e a t 12.91 eV has a r e l a t i v e l y s m a l l c r o s s  section.  a n g u l a r b e h a v i o u r , shown i n F i g u r e 15 i s q u i t e d i f f e r e n t the J  passing  = 0 state.  A f l a t maximum e x i s t s  t h r o u g h a minimum a t 45° a s e c o n d  b e t w e e n 10° a n d 15°.  band a t 1 2 . 9 3 eV w h i c h decrease  After  s l o w l y t o 90°.  at  This  r u l e s o u t t h e p o s s i b i l i t y t h a t t h e 12.91 eV p e a k c o u l d  be due t o a n i t r o g e n i m p u r i t y .  relative  that  s h a l l o w maximum i s o b s e r v e d  60° b e f o r e t h e c r o s s s e c t i o n a g a i n d e c r e a s e s angular behaviour  from  N i t r o g e n has a s t r o n g o p t i c a l l y  allowed  i s s t r o n g l y peaked i n t h e f o r w a r d d i r e c t i o n .  i n intensity  50 eV i s a l s o f u r t h e r e v i d e n c e  o f t h e 12.91 p e a k i n g o i n g f r o m that the transition  i s an  The  30 eV t o  optically  forbidden process i n argon. The eV,  relative  cross section  ( F i g u r e 15) f o r t h e t r a n s i t i o n  a l t h o u g h m o r e i n t e n s e , shows q u a l i t a t i v e l y  the J  = 1 state.  different  angles from  those  i n the J  = 1 peak).  close to this  energy,  states.  s t a t e o f argon  has J  S i n c e t h e ground  = 3 s t a t e would  expected  t h e same b e h a v i o u r a s  Two l o c a l maxima a n d o n e m i n i m u m a r e o b s e r v e d ( a t  [ 6 0 ] t h e r e a r e two l e v e l s  J  i n v o l v e A J = 3.  t o be r e l a t i v e l y  A c c o r d i n g t o Moore due t o J  S u c h an o c t u p o l e t r a n s i t i o n  improbable  a n d on t h i s  F i g u r e 16 shows ( o n a l o g s c a l e ) t h e r a t i o s = 0 and J  = 3 and J  = 0, a t r a n s i t i o n  t h e 13.09 eV p e a k a s due t o a s i n g l e t r a n s i t i o n  the J  b a s i s we may to the J  Large  undulations are observed  differential  cross section f o r the o p t i c a l l y  = 2  to the  m i g h t be regard  = 2 level.  of the intensities  = 1 states to that of the o p t i c a l l y  (11.83 e V ) .  allowed  i n the r a t i o s .  of  state Since the  allowed t r a n s i t i o n i s  s t r i c t l y d e c r e a s i n g maxima a n d m i n i m a a r e much more a c c e n t u a t e d ratio  a t 13.09  i n the  p l o t a n d t h e y may n o t o c c u r a t t h e same a n g l e s a s i n t h e d i f f e r e n t i a l  Ar, 3 0 e V  0  0  Figure  10°  16.  20°  30°  AO°  50°  60°  70°  SCATTERING A N G L E 0  D i f f e r e n t i a l cross section ratios s t a t e s o f a r g o n a t 30 eV.  80°  for excitation  90°  t o 4p  81.  cross  sections.  5.3.2. Neon. L i t t l e work has been r e p o r t e d neon i n c o n t r a s t  impact e x c i t a t i o n o f  t o t h e o t h e r r a r e gases helium and argon.  O l s e n [ 5 3 ] h a v e shown a t h r e s h o l d [48]  on t h e e l e c t r o n  B r i o n and  e x c i t a t i o n s p e c t r u m o f neon and Foo  h a s m e a s u r e d medium r e s o l u t i o n e n e r g y l o s s s p e c t r a  34 eV a n d 1 0 0 eV a t s c a t t e r i n g a n g l e s o f 0° a n d 4 5 ° .  o f neon a t  Higher  resolution  i s n e c e s s a r y f o r t h e s t u d y o f neon due t o t h e c l o s e l y s p a c e d e n e r g y  levels.  [60]. Figure  17 shows a n e n e r g y l o s s s p e c t r u m o f n e o n we h a v e  a t 70 eV a n d 2° s c a t t e r i n g a n g l e . states  5 2 due t o t h e - 2 p ( P  obtained  The l o w e s t e n e r g y , o p t i c a l l y  , )3s configurations '2 > / 2  3 /  areclearly  allowed resolved  a t 1 6 . 6 7 a n d 1 6 . 8 5 a n d t h e n e x t R y d b e r g s e r i e s members a t 1 9 . 6 9 a n d 1 9 . 7 8 eV c a n a l s o  be s e e n .  20.0 eV w h i l e  T h e f i r s t member o f t h e - 2 p nd s e r i e s o c c u r s a t  peaks a t higher  e n e r g i e s c a n be a t t r i b u t e d p r i m a r i l y t o 5 5  f u r t h e r s t a t e s o f t h e t y p e -2p ns a n d -2p n d . e x i s t due t o t h e c o n f i g u r a t i o n s  -2p ( P ^ 3  ^  o c c u r i n t h e e n e r g y r a n g e 1 8 . 3 eV - 1 9 . 0 eV. peak a r e o b s e r v e d  i n this region  i n Figure  eV i s d u e t o t h e 3 p ' ( J = 0 ) s t a t e w h i l e states with side of this 3p(0  J values varying  lower impact energy  level states  (36 eV).  states  )3p and a r e expected t o A b r o a d hump a n d a s h a r p e r  17.  The s h a r p peak a t 18.97  t h e b r o a d hump e n c o m p a s s e s  f r o m 0 t o 3.  b r o a d band i s j u s t d i s c e r n a b l e  = 1 ) s t a t e a t 1 8 . 3 8 eV.  Ten p o s s i b l e  A small  eight  peak t o t h e l o w e n e r g y  a n d may be a t t r i b u t e d t o t h e  This  peak a p p e a r s more p r o m i n e n t l y a t a  This  s t a t e i s a n a l o g o u s t o t h e 12.91 eV 3 3  i n argon'discussed e a r l i e r  i nthis  paper.  o f neon were n o t r e s o l v e d  i nthis  work.  The  ?> 2  PQ m e t a s t a b l e  3s,3s'  •  170 Figure  17.  3p,3p'  i  180  4s,4s',3d  i  19.0  Energy Loss eV  \  20.0  i  21.0  i  22.0  E l e c t r o n i m p a c t s p e c t r a o f neon a t 70 eV and 2° s c a t t e r i n g a n g l e .  We h a v e a l s o d e t e r m i n e d for  the well  resolved  the relative differential  eV, t h e c r o s s  18).  At the higher  z e r o minimum i s o b s e r v e d a t a b o u t 3 0 ° . 50° a n d t h e c u r v e f a l l s  Finally cross  t o z e r o a g a i n beyond 80°.  t h e maximum i s s h i f t i n g  the  i s reduced.  similarly. the  This  s t a t e was s t u d i e d  the  a t 30 eV a n d 50 eV. (Figure  heavier  r a r e g a s e s may h o p e f u l l y  states The  i n atomic  section  impact  s c a t t e r i n g a n g l e s as minimum  behaves  s e c t i o n f o r t h e 3 p ' ( J = 0) The e r r o r  bars a r e q u i t e experimental  show a n upward t r e n d  i n the case o f the J = 0 s t a t e o f argon.  behavious o f the cross  that the  i n the case o f argon (see  18) b u t w i t h i n  c u r v e s a t 56 eV a n d 8 0 eV s t i l l  characteristic  to higher  The p o s i t i o n o f t h e f i r s t  where t h e c r o s s  b e y o n d t h e s e c o n d maximum  energy  f r o m 5° t o 1 0 ° .  I t seems l i k e l y  b e h a v i o u r was a l s o o b s e r v e d  previous section)  A near  peaked i n t h e f o r w a r d d i r e c t i o n a t high  e n e r g i e s and t h a t impact energy  energy,  As t h e impact  s e c t i o n d e c r e a s e s more g e n t l y  i s sharply  energies  A b r o a d maximum a p p e a r s a t  a t 3 6 eV a maximum a p p e a r s a t 1 0 ° .  section  impact  s e c t i o n d e c r e a s e s r a p i d l y f r o m 5° t o 1 0 ° .  is decreased the cross  section  3 p ' ( J = 0 ) s t a t e a t 18.97 eV a t i m p a c t  o f 3 6 , 5 6 , 8 0 a n d 100 eV ( F i g u r e 100  cross  large  error  which i s Studies  of the  b r i n g about t h e use o f t h e a n g u l a r  to characterize  the J values of excited  spectroscopy.  fact that  the cross  section o f J = 0 states  becomes m o r e a n d  more i s o t r o p i c a s t h e i m p a c t e n e r g y i s l o w e r e d c a n be e x p l a i n e d  by t h e  c o n t r i b u t i o n o f t h e exchange mechanism r e l a t i v e t o t h a t o f d i r e c t excitation. and  the cross  At higher  i m p a c t e n e r g i e s e x c h a n g e e f f e c t s s h o u l d be s m a l l  s e c t i o n more f o r w a r d  s e c o n d maximum i s r e l a t i v e l y  small.  peaked so t h a t  the height  of the  As t h e i m p a c t e n e r g y i s l o w e r e d  e x c i t a t i o n v i a e x c h a n g e becomes i n c r e a s i n g l y i m p o r t a n t a n d t h e f i r s t and  s e c o n d maxima become c o m p a r a b l e i n i n t e n s i t y .  8  0°  10° 20° 30° 40°  50° 60° 70°  80°  90°  SCATTERING A N G L E 9 Energy dependence o f t h e r e l a t i v e d i f f e r e n t i a l scatteri c r o s s s e c t i o n f o r t h e 3p' ( J = 0 ) s t a t e o f n e o n .  85.  CHAPTER  VI  MOLECULAR RYDBERG STATES  6.1.General  Aspects.  A typical orbitals  molecule with  (M.O.) ty. h a s a c o r r e s p o n d i n g  orbitals  set o f v i r t u a l  Beyond t h e v a l e n c e s h e l l , t h e r e  i n t h e LCAO a p p r o x i m a t i o n higher  a set o f occupied valence shell  molecular  valence  a r esets  shell  o f M.O.'s w h i c h  i s c o m p o s e d o f a t o m i c o r b i t a l s (A.O.) o f  p r i n c i p a l q u a n t u m number ( t h e R y d b e r g A.O.'s, R * ) .  M.O.'s a r e more a n d more l i k e A.O.'s a n d t h e r e f o r e  give  These  rise  higher  t o Rydberg  s e r i e s o f e l e c t r o n i c s t a t e s whose l i m i t c o r r e s p o n d s t o t h e c o m p l e t e removal  of theelectron considered,  molecule. the  Spectroscopists  i . e . t o an i o n i z a t i o n l i m i t  continue t o use a t o m i c - l i k e  description of  Rydberg o r b i t a l s as i f t h e c o r e p o t e n t i a l s were s p h e r i c a l The  energy o f Rydberg s t a t e s  to a Rydberg o r b i t a l  c a n be w e l l  of the  symmetric.  f o r m e d by e x c i t a t i o n o f a s i n g l e approximated  by t h e Rydberg  where A i s t h e i o n i z a t i o n l i m i t , R i s t h e Rydberg c o n s t a n t  electron  formula:  ( = 13.61 e V ) ,  n i s t h e p r i n c i p a l q u a n t u m number o f t h e e l e c t r o n a n d <5 i s t h e R y d b e r g correction  (quantum d e f e c t ) .  6 i s small  ( ^ 0.1) f o r s t a t e s d e r i v e d  (0.3  - 0.5) f o r np e l e c t r o n s  electrons facilitate  [62].  Lindholm  F o r m o l e c u l e s b u i l t up f r o m f i r s t f r o m nd e l e c t r o n s ,  and a p p r e c i a b l y  larger  (0.9  somewhat  larger  - 1.2) f o r ns  [ 9 ] h a s shown how q u a n t u m d e f e c t s  t h e i n t e r p r e t a t i o n o f Rydberg s e r i e s observed  row atoms,  c a n be u s e d t o  i nelectron  impact  86.  spectroscopy.  T h i s method u t i l i s e s  the accurate values of  p o t e n t i a l s t h a t h a v e become a v a i l a b l e f r o m p h o t o e l e c t r o n Lindholm  ionization  spectroscopy.  [ 9 ] d i d n o t q u i t e a g r e e w i t h t h e r a n g e o f quantum d e f e c t  g i v e n a b o v e [ 6 2 ] a n d he c  presented  defects f o r small molecules.  a table of typical  Using  the v i b r a t i o n a l  can  readily  e x p e r i m e n t a l l y observed  Since the e l e c t r o n s i n Rydberg o r b i t a l s bonding,  experimental  quantum  a t e n t a t i v e v a l u e o f quantum d e f e c t  [ 9 ] , the energy of the Rydberg t r a n s i t i o n c o m p a r e d w i t h t h a t f o r an  values  be e s t i m a t e d  peak i n t h e  c o n t r i b u t e very  s t r u c t u r e (or Franck  spectrum.  little  to  molecular  Condon e n v e l o p e ) a s s o c i a t e d  w i t h a Rydberg e x c i t a t i o n must c l o s e l y r e s e m b l e t h e p h o t o e l e c t r o n towards which  i t would converge.  An  acid  b e e n g i v e n by R o b i n  [63].  HNCO has  example i n the case I n an  s t a r t w i t h a s t r o n g band a t l o w e n e r g y and ionization potential  f r o m PES,  compared w i t h the e x p e c t e d determine valence the  shell  transition.  interpretation  Term v a l u e s  of e l e c t r o n impact  w i t h a knowledge o f  be to  [63,64,65] are a l s o u s e f u l i n spectra.  Harshbarger  orbital  ijj. t o R a s  the  o f t h e i/^.M.O. m i n u s t h e if;.. -> R e x c i t a t i o n e n e r g y . i s the  orbital.  et a l . [65]  i n w h i c h an e l e c t r o n  ionization Without  q u a n t u m number n ,  potential  approximation,  i o n i z a t i o n p o t e n t i a l , or the b i n d i n g energy, of the  For a g i v e n p r i n c i p a l  term v a l u e the term  the  c a n o f t e n be u s e d  v a l u e o f an e x c i t e d s t a t e [ty. ,R)  promoted from a m o l e c u l a r  this  can u s u a l l y  w h e t h e r t h e b a n d i s t h e f i r s t member o f a R y d b e r g s e r i e s o r a  have d e f i n e d t h e t e r m is  a n a l y s i s , one  This procedure  band  of isocyanic  the c a l c u l a t e d quantum d e f e c t can  values.  and  Rydberg  the d i s c u s s i o n o f  the  i s e q u i v a l e n t t o a d i s c u s s i o n o f 6 i n the Rydberg f o r m u l a , f o r  value  i s R/(n  the term value of the  - <s) . (T|;. ,R)  Harshbarger  e t a l . [65] have a l s o found  Rydberg c o n f i g u r a t i o n i n a g i v e n molecule  that is  87.  very nearly  independent o f t h e o r i g i n a l  o r b i t a l s ip..  In a series o f  a l k y l a t e d compounds s u c h a s a l c o h o l s , k e t o n e s , o l e f i n s a n d a m i n e s , i t has been f o u n d t h a t as a l k y l a t i o n p r o c e e d s , t h e 3s t e r m v a l u e d e c r e a s e s f r o m ^ 5 eV t o a l i m i t o f 2.7 e V . c o n s i s t e n t throughout the a l k y l and  1.6 eV r e s p e c t i v e l y .  alkyl  T h e 3p a n d 3d t e r m v a l u e s a r e more  s e r i e s , w i t h v a l u e s o f a p p r o x i m a t e l y 2.5  The u n e x p e c t e d  fact  that i n the l i m i t of large  g r o u p s , t h e 3s t e r m v a l u e i s i n d e p e n d e n t o f t h e c e n t r a l  has b e e n e x p l a i n e d  by R o b i n [ 6 3 , 6 6 ] .  Using the i o n i z a t i o n  group,  p o t e n t i a l s as  d e t e r m i n e d by P E S , t h e b a n d s i n t h e s p e c t r a c a n be s e p a r a t e d i n t o which  have t h e s p e c i f i c  do n o t ( v a l e n c e s h e l l  those  term v a l u e s (Rydberg t r a n s i t i o n s ) and t h o s e  that  transitions).  R o b i n [ 6 3 , 6 6 ] has p o i n t e d o u t t h a t p h y s i c a l l y , a m o l e c u l a r . R y d b e r g orbital  i s d i s t i n g u i s h e d from a valence s h e l l  orbital  by i t s c o n s i d e r a b l y  l a r g e r s i z e a n d s o i t i s more s u s c e p t i b l e t o o u t s i d e p e r t u r b a t i o n s . f o u n d t h a t s p e c t r a due t o v a l e n c e s h e l l pressure  running the optical  spectra  as a p o l y c r y s t a l l i n e f i l m strength  seems n o t t o e x c e e d Alternatively, oscillator a Rydberg  transitions  show a n  broadening t o h i g h e r f r e q u e n c y under h i g h p r e s s u r e c o n d i t i o n s .  I t has a l s o been f o u n d t h a t Rydberg  oscillator  t r a n s i t i o n s , were independent o f  (up t o P = 1 0 0 a t m . ) w h e r e a s R y d b e r g  asymmetric  He  transitions  c a n be e l i m i n a t e d b y  i n a condensed phase  [67])  #  Another useful  per degree.of s p a t i a l  ( i . e . as a . s o l u t i o n or criterion  degeneracy  0.08 i n m o l e c u l e s c o n s t r u c t e d  i n electron  i s that the  i n Rydberg  from f i r s t  transitions  row a t o m s .  i m p a c t , a minimum s h o u l d e x i s t i n t h e g e n e r a l i s e d  s t r e n g t h a s a f u n c t i o n o f t h e s q u a r e o f momentum t r a n s f e r f o r t r a n s i t i o n , whereas t h a t f o r a v a l e n c e t r a n s i t i o n  m o n o t o n i c a l l y d e c r e a s e t o z e r o w i t h t h e s q u a r e o f momentum  should transfer  88.  [68,69,70].  A study of the angular behaviour of a t r a n s i t i o n should  therefore c l a r i f y  an  assignment.  6.2.Rydberg S t a t e s o f Hydrogen The n e a r UV o p t i c a l s t u d i e d by s e v e r a l b a n d s b e t w e e n 6.2  a b s o r p t i o n spectrum o f hydrogen  authors.  bands f o r b o t h HCN - 9.2  Hilgendorff  c y a n i d e has been  [ 7 1 ] o b s e r v e d a s y s t e m o f weak  - 7.3 eV w h i l e H e r z b e r g a n d I n n e s [ 7 2 ] e x t e n d e d  s y s t e m a n d made a d e t a i l e d  r e g i o n 6.2  Cyanide.  i n v e s t i g a t i o n of the fine  and DCN.  eV.  F o u r band systems  The u p p e r  structure of the  have been f o u n d i n t h e  s t a t e s c o r r e s p o n d i n g t o two o f t h e s e  a, i  transitions are designated A  this  i  i  A" a n d B  A"  [ 7 2 ] . The t h i r d  band, w i t h  ^ "I upper is  A' was  b r i e f l y described  by H e r z b e r g [ 6 2 ] a n d t h i s  transiti  c l a i m e d t o be much s t r o n g e r t h a n t h e A - X a n d B - X t r a n s i t i o n s .  fourth is  state C  b a n d has n o t been d i s c u s s e d a n d t h e s y m m e t r y o f t h e u p p e r  n o t known.  by P r i c e  The vacuum UV a b s o r p t i o n  s p e c t r u m o f HCN  was  The  state  D,  first-studied  [ 7 3 ] a n d l a t e r by P r i c e a n d W a l s h [ 7 4 ] . A l t h o u g h s t r o n g  diffuse  • b a n d s w e r e f o u n d a t e n e r g i e s h i g h e r t h a n 11 eV no a n a l y s i s was made arid no R y d b e r g  series  spectroscopic  ionization  p o t e n t i a l s o f HCN scopy [ 7 5 ] .  have been i d e n t i f i e d potential  have r e c e n t l y  Reported i n t h i s  s p e c t r u m o f HCN  from o p t i c a l  i s available  spectra.  T h u s , no  [62], although  ionization  been d e t e r m i n e d by p h o t o e l e c t r o n s p e c t r o -  section  i s t h e e l e c t r o n impact energy  (which i s c l o s e l y r e l a t e d  t o t h e vacuum UV  loss  absorption).  T h i s s t u d y has a l s o b e e n p r o m p t e d by t h e work o f F r o s t e t a l [ 7 5 ] who, c o m p a r i s o n w i t h t h e p h o t o e l e c t r o n s p e c t r u m o f HCP,  have p r o p o s e d a  r e a s o n a b l e i n t e r p r e t a t i o n o f t h e p h o t o e l e c t r o n s p e c t r u m o f HCN. basis the f i r s t  two i o n i z a t i o n p o t e n t i a l s o f HCN  by  On  a t 1-3.61 arid 1 4 . 0 0  this eV  89.  r e s p e c t i v e l y c a n be u s e d t o f a c i l i t a t e impact  the interpretation  of the electron  spectrum.  The  electronic configuration  HCN [ 7 5 ] a r e ( l a )  2  (2a)  2  (3a)  2  o f HCN a n d o u t e r o r b i t a l  (4a)  2  (5a) ( H ) 2  4  energies of  .  - 1 4 . 0 0 , -13.61 eV It  has b e e n c l a i m e d [ 7 5 ] t h a t t h e c o m p l i c a t e d s t r u c t u r e  i n the f i r s t  photo-  2 electron  band i s d u e t o v i b r o n i c  interactions  o f t h e two i o n i c  s t a t e s X Ji  2 and A i a r i s i n g The  electron  angle  f r o m t h e i o n i z a t i o n o f t h e l i r a n d t h e 5a e l e c t r o n s  i m p a c t s p e c t r u m o f HCN a t 5 0 eV i m p a c t e n e r g y a n d 2°  (Figure  19b) s e r v e s as an example t o i l l u s t r a t e  spectrum  u s i n g quantum d e f e c t s a n d t e r m v a l u e s a s d i s c u s s e d  i n the l a s t  this  series  b a s i s m o s t b a n d s c a n be a r r a n g e d i n t o R y d b e r g  ionization  p o t e n t i a l s o b s e r v e d by PES [ 7 5 ] .  The d e r i v e d  c o n s i s t e n t with those f o r small molecules c o n t a i n i n g in  r e f . 9.  I t i s a l s o noted  levels are essentially e l e c t r o n was e x c i t e d 6.2.1. A s s i g n m e n t  scattering  analysis sections.  ( T a b l e 2) u s i n g t h e quantum d e f e c t s a r e  H, C, N, 0 a s t a b u l a t e d  a s h a s been o b s e r v e d  orbital  from which the  i n other types o f molecule [64,65].  series.  1 TT ->- n s a We p o s t u l a t e t h e f i r s t member vibrational  (n = 3) o f t h i s  series  o c c u r s a s two  b a n d s a t 1 0 . 2 0 ( B ) a n d 1 0 . 4 3 eV ( C ) . The v i b r a t i o n a l  spacing 2  of  a b o u t 0.23 eV c o m p a r e s w e l l  with  the  vibrational  i o n i c s t a t e , 0.228 eV o b s e r v e d i n PES [ 7 5 ] . eV  spacing of the X n  The term v a l u e o f t h e 10.20  ( B ) b a n d w i t h r e s p e c t t o t h e ITT I . P . i s 3.41 eV a n d f r o m t h i s t h e  calculated  quantum d e f e c t  members t o o c c u r a t a b o u t  On  ( s e e T a b l e 2) t h a t t h e term v a l u e s o f t h e Rydberg  independent o f t h e i n i t i a l  o f Rydberg  respectively,  i s 1.00.  U s i n g t h i s , we p r e d i c t t h e n = 4,5,6  1 2 . 0 9 , 1 2 . 7 6 a n d 13.06 eV.  Peaks  assignable to  A30ev, 2' 90.  \  50 eV, 2° lTr-»-np  CO  ZD  >-  <  Ql CD  CC <  >CO  |7TH»-nS  j/.  LU  I-  r—  |  i  |  i  100 eV  |  r  "1  ""|  ,  10  8  i  r  9  F i g u r e 19.  10  ENERGY  II  12  LOSS (eV)  E l e c t r o n i m p a c t s p e c t r a o f HCN.  13  1  91.  PEAK  B C  D E  F  G  OBSERVED ENERGY(eV)  TERM VALUE(eV)  10.20) 10.43 /  3.41  10.G4 )  3.36  10.73 |  CALCULATED EHERGY(eV)  ASSIGNMENT l-rr -» 3so lir -* 3sc + v  1.00 3  5a 3s 5= •* 3sc + v 5a -*• 3sc + 2v 5a -*• 3sa + v  0.99  2  10.82 ) 10.88  2  3  II I  10.93  11.06  2.68 2.55  lTT -*• 3pTT  J K  11.42 11.54  2.58 2.46  5a •*• 3po 5a -*• 3pn  L  M  11.80 11.95  1.81 1.66  lir  N  12.07  1.54  l-rr -v 4sa  0  12.30  1.31  In -> 4sc + v In 4pc In •* 4pn  P  12.48  1.52  5~ •+ 4sc  12.50  Q  12.57  1.C4  l-rr  12.64  R  12.77  1.23  In -> 3pa  0.75 0.69 n.32 n.45  0.26 0.14 12.09 3  4dc  12.30 12.33 12.37  Q.84  5= •*• 4pa 5a •*• 4pn In •*• 5sc  12.72 12.76 12.76  •  12.99  0.62 1.01  In -* 5sc + v'3 5a -> 4da  12. £9 13.03  T  13.09  0.52 0.91  In ->• 6sa 5o 4dn  13.06 13.08  U  13.18  0.82  5a -*• 5sa  13.16  Table 2  .  -> 3da In -> 3dir  S  Respective I.P.  CALCULATE:  QUANTUM DEFECT  can be obtained by adding the. term value to the t r a n s i t i o n energy.  Rydberg t r a n s i t i o n s i n HCN.  92.  [  these and  t r a n s i t i o n s are observed  13.09  eV  a s s i g n e d as quantum  (T).  Two  o t h e r p e a k s a t 12.30  t h e n = 4 and  (C = N  I.P. be  the term  (14.00 eV). (000)  initial  orbital  T h e r e f o r e , we  member o f t h i s  (G) on  the p l a t e a u f o l l o w i n g  (0.09  0.24  and  vibrational eV  levels  have been o b s e r v e d  a s s i g n the  two  The  eV  i n f l e x i o n a t 10.64  p e a k a t 10.73  eV  s m a l l p e a k s a t 10.82  t h i s may  correspond  spacing f o r v  and  2  (E)  ( F ) and  to the  (020)  determined  and  0.255 eV  observed  (U) a r e a s s i g n e d  energies expected  from the  t o be n = 4 and  lir  calculate  eV  (D)  and  ^  transition  s t a t e s i s not observed  npq  and  eV  (001)  E  is  0.99  (P)  and  series.  2.  Vibrational  i n our  spectrum,  npu  value w i t h r e s p e c t to the f i r s t t h e q u a n t u m d e f e c t t o be 0.69,  I.P.  The  a t 11.06  (13.61 eV)  eV.  i s 2.55  (I) eV.  which i s c o n s i s t e n t with a  A s m a l l s t e p o c c u r s a t 10.93  t h i s g i v e s a q u a n t u m d e f e c t o f 0.75.  to  EIS  n = 5 members o f t h i s  most i n t e n s e f e a t u r e i n the spectrum o c c u r s  upper s t a t e .  (D)  by F r o s t e t a l [ 7 5 ] ,  f r o m c a l c u l a t i o n a r e shown i n T a b l e  s t r u c t u r e a c c o m p a n y i n g t h e s e two  term  10.64  i n PES  f r o m t h i s v a l u e o f t h e q u a n t u m d e f e c t , t h e p e a k s a t 12.48  Its  2nd  10.88  from  and  The  eV  we.  corresponds  quantum d e f e c t c a l c u l a t e d  The  be  be  below the  The  eV  to  r e s p e c t i v e l y ) i s c o n s i s t e n t with values f o r the A  i o n i c s t a t e , v i z . 0.100  13.18  (S) can  of the e l e c t r o n s e x c i t e d [64,65]  band.  ( 0 1 0 ) member w h i l e t h e  The  eV  b y v i b r a t i o n a l  t r a n s i t i o n t o o c c u r a t a b o u t 3.4  to the  members.  12.99  (R)  -* nsa .  v a l u e s of Rydberg  t h e 5 c -> 3 s o  the  ( N ) , 12.77  stretch).  independent of the expect  (0) and  n = 5 s t a t e s accompanied  5a Since  i n t h e s p e c t r u m a t 12.07  eV  (H)  ( t e r m v a l u e 2.68)  q u a n t u m d e f e c t s f o r t h e 3pa  We  ;  3p and and  93.  3p-rr R y d b e r g l e v e l s  i n the i s o e l e c t r o n i c molecule N  2  a r e 0.67 a n d 0.60 [ 9 ] .  I t i s r e a s o n a b l e t o assume t h a t i n HCN t h e 3pa quantum d e f e c t i s a l s o g r e a t e r t h a n t h e 3PTT q u a n t u m d e f e c t .  T h e r e f o r e f e a t u r e s a t 10.93 eV  (H) a n d 11.06 e y ( I ) a r e a s s i g n e d ITT -> 3pg a n d In  transitions  U s i n g t h e s e v a l u e s o f q u a n t u m d e f e c t s , t h e n = 4 members  respectively. are expected  3pir R y d b e r g  a t 12.33 a n d 12.37 eV a n d t h e t r a n s i t i o n s  1 -rr ->• 4po a n d  1 TT -* 4pir may h a v e c o n t r i b u t i o n s t o t h e p e a k a t 12.30 eV (0) a s s i g n e d a s 1 TT -> 4scr + v . 3  previously  T h e n = 5 members e x p e c t e d a t a b o u t  12.87 eV  probably o v e r l a p w i t h other s t a t e s and a r e n o t r e s o l v e d i n t h e spectrum.  ->- npa & np-rr  5p  By s u b t r a c t i n g t h e 3pa t e r m v a l u e ( s e e T a b l e 2) o f ^ 2.7 eV f r o m the second  I . P . (14.0 e V ) , t h e 5a'-*- 3p  transition  i sexpected  a t a- '11.3 e V .  T h e r e f o r e t h e s t e p a t 11.42 eV ( J ) may be a s s i g n e d t o t h i s t r a n s i t i o n a n d t h e peak n e x t t o i t at-11.54, eV ( K ) i s a s s i g n e d t h e 5a •-*• 3pw t r a n s i t i o n . The two 5a -»• 4p t r a n s i t i o n s  p r o b a b l y o v e r l a p and c o n t r i b u t e t o t h e peak  a t 12.77 eV ( R ) . 1 TT -> nd & 5a  nd  Two s m a l l p e a k s a t 11.80 ( L ) a n d 11.95 eV (M) h a v e t e r m o f 1.81 a n d 1.66 eV r e s p e c t i v e l y w . r . t . t h e f i r s t  values  I . P . (13.61 e V ) . T h e  quantum d e f e c t s c a l c u l a t e d a r e 0,26 a n d 0.14 r e s p e c t i v e l y a n d t h e s e  seem  t o be c o n s i s t e n t w i t h 3da a n d 3d?r u p p e r s t a t e s r e s p e c t i v e l y .  these  Using  v a l u e s f o r quantum d e f e c t s , t h e f e a t u r e a t 12.57 eV (Q) i s a s s i g n e d t o the t r a n s i t i o n  ITT  i s not apparent Subtracting  4da.  T h e t r a n s i t i o n , 1 TT -»- 4dir, e x p e c t e d a t 12.70 e V ,  i n t h e spectrum. l  r  8 eV f r o m  i s e x p e c t e d a t ^ 12.2 eV.  14.00 eV (2nd I . P . ) , t h e 5a -> 3da t r a n s i t i o n  The spectrum  a t t h i s energy  shows no e v i d e p c e  94.  of any t r a n s i t i o n s .  The  5a •> 3 d u t r a n s i t i o n  This, i f occurring, i s buried 1 IT -* 4so 13.03 +  + v^.  and  The  13.08  eV and  ITT -> 6sa  and  spectrum whether  5a -> 4da  under and  transitions.  t h e 12.30  5a -> 4dn  these would  i s e x p e c t e d a t 12.34  eV.  eV p e a k  (0) a s s i g n e d t o  transitions  are expected at  a l s o be o v e r l a p p e d by t h e lir  I t i s therefore not c e r t a i n  t h e 5a -> nd s e r i e s  actually  5sa  from  our  makes a s i g n i f i c a n t  contrib-  ution.  6.2.2. D e p e n d e n c e o f t h e s p e c t r u m on We  have s t u d i e d  2° s c a t t e r i n g spectra  a n g l e ( F i g u r e 19a, b and c ) .  ing  t o the D - X t r a n s i t i o n mentioned  the  upper  state The  a s s i g n m e n t and  intensity  s t a t e a t 10.20  d e c r e a s e d f r o m 100 eV  5a  +  at  11.54  in  which cases the r e l a t i v e  energy.  +  eV  v  at  2  10.73  eV  (E),  impact energy  first  feature  eV.  by H e r z b e r g  of this  i n the  T h i s band d o e s  [62].  The  correspond-  symmetry o f  band, w i t h r e s p e c t t o the  (See F i g u r e 2 0 ) .  This i s different  i n t e n s i t i e s o f o t h e r Rydberg lir  +  3p-rr a t  11.06  eV  (I)  intensity  This indicates  increases with  and  states, 5o  -> 3PTT  a t 10.20  increasing  eV  t h a t the energy dependence o f the  of  of  first  I t appears t h a t the r e l a t i v e i n t e n s i t y  band i n o u r s p e c t r u m shows a b e h a v i o u r w h i c h  a forbidden t r a n s i t i o n .  (B),  impact  from  states.  from  e.g.  r e l a t i v e cross-section of this valence state i s very d i f f e r e n t the Rydberg  not  t r a n s i t i o n d e s c r i b e d i n any  ( K ) w i t h r e s p e c t t o t h e lir + 3 s a t r a n s i t i o n  (Figure 20).  and  eV, ( B ) , i n c r e a s e s a s t h e i m p a c t e n e r g y i s  t o 30 eV.  behaviour of the r e l a t i v e  30 eV  i s probably a valence t r a n s i t i o n  i s n o t known, n o r i s t h i s  relative  lir ->• 3s R y d b e r g  3sa  The  i s a b r o a d band m a x i m i s i n g a t a b o u t 8.9  any Rydberg  the  energy.  t h e s p e c t r a a t 1 0 0 , 50 a n d  fit  detail.  impact  In the case o f l i n e a r  that  this  i s characteristic  of  polyatomic molecules with  UI  20  1  1  1  1  1  1  1  30  40  50  60  70  80  90  T  1  100  ELECTRON IMPACT ENERGY (eV) i g u r e 20.  Relative  i n t e n s i t i e s o f some t r a n s i t i o n s  i n the electron  impact spectrum  o f HCN.  tn  96.  a ^E  ground  +  s t a t e , t h e group  [76] p r e d i c t s the t r a n s i t i o n u p p e r s t a t e t o be f o r b i d d e n  theoretical from t h e \  t r e a t m e n t o f Goddard ground  +  i n e l e c t r o n impact f o r a x i a l  (0° o r 180° s c a t t e r i n g a n g l e ) .  On t h i s  band c o r r e s p o n d s t o a l i n e a r  state.  basis  eV i n c r e a s e s v e r y  f r o m 2 t o 10 d e g r e e s .  From F i g u r e  The  et a l .  energy  This  are  t o t h e ITT -* 3 s a b a n d a t  transition  11.6 eV shows an i n t e r e s t i n g  W h i l e a s e r i e s o f peaks a s s i g n a b l e  s e c t i o n a t an i m p a c t e n e r g y o f 30 eV.  t o Rydberg h a v e a much  O n l y two s m a l l  o b s e r v e d a t 12.30 a n d 12.53 eV o n t o p o f a d e c r e a s i n g two  o f the type  discussed  [76].  o b s e r v e d a t 50 eV i m p a c t e n e r g y , t h e y a p p a r e n t l y  cross  1 9 c i t c a n be s e e n  behaviour f u r t h e r supports the suggestion  l o s s r e g i o n beyond  on i m p a c t e n e r g y .  scattering  r a p i d l y when t h e s c a t t e r i n g a n g l e i s i n c r e a s e d  t h a t t h e 8.9 eV band i s due t o a f o r b i d d e n by G o d d a r d  (linear)  i ti s possible that the D  t h a t t h e i n t e n s i t y o f t h e 8.9 eV b a n d r e l a t i v e 10.20  s t a t e t o a z"  eta l .  dependence states smaller  peaks a r e  continuum.  These  t r a n s i t i o n s may c o r r e s p o n d t o t h e ITT -* 4pTr a n d ITT ->• 4 d a t r a n s i t i o n s  respectively.  On t h e o t h e r  t o 100 e V , some o f t h e p e a k s hump.  F o r example,  h a n d , when t h e i m p a c t e n e r g y become b r o a d e n e d  i s increased  and merge i n t o a b r o a d  t h e 11.95 (M) a n d 12.07 (N) eV p e a k s  become a b r o a d  p e a k a n d t h e two p e a k s a t 12.57 eV (Q) a n d 12.77 ( R ) eV a r e n o t s o distinctly 100 eV.  s e p a r a t e d when t h e i m p a c t e n e r g y  In g e n e r a l , the r e l a t i v e  increases a t higher peaks  cross-section with  that the cross  of the molecules increases a t a higher  continuum  T h e b r o a d e n i n g o f some o f t h e  i n relative  i m p a c t e n e r g y o r due t o t h e f a c t  f r o m 50 eV t o  intensity of the underlying  impact energies.  may be due t o c h a n g e s  i s increased  change i n  section f o r predissociation  impact energy.  As t h e impact  97.  energy  i s increased  t o approach  the optical  so b r o a d t h a t a c o m p l e t e l y d i f f u s e reported  i n optical  work [ 7 4 ] .  limit,  t h e p e a k s may  s p e c t r u m r e s u l t s , as has been  become  98.  CHAPTER V I I  ELECTRON IMPACT SPECTRA OF SOME ALKYL D E R I V A T I V E S OF WATER AND RELATED COMPOUNDS  7.1.Introduction. In t h i s alkyl  study,  t h e e l e c t r o n i m p a c t e n e r g y l o s s s p e c t r a o f some  d e r i v a t i v e s o f w a t e r a n d r e l a t e d compounds a r e r e p o r t e d .  most o f t h e s e  compounds have b e e n s t u d i e d i n t h e g a s p h a s e by UV  spectroscopy  o r by p h o t o e l e c t r o n  and  [64]  methanol  because o f t h e i n s t r u m e n t a l  region.  containing  (PES), only water  The e l e c t r o n i c s p e c t r a o f a l i p h a t i c  i n t h e vacuum UV a n d c o n s e q u e n t l y  attention optical  spectroscopy  have e n e r g y l o s s e l e c t r o n i m p a c t s p e c t r a  in the literature. mostly  Although  Saturated  aliphatic  reported  compounds l i e  have r e c e i v e d r e l a t i v e l y  difficulties  o f working  compounds, p a r t i c u l a r l y  non-bonding e l e c t r o n p a i r s l i k e  k e t o n e s a n d h a l i d e s show c h a r a c t e r i s t i c  [77-81]  alcohols, ethers,  absorption  little  i n this those amines,  b a n d s i n t h e UV r e g i o n  o beyond t h e q u a r t z a theoretical chemical The assigned  limit  understanding  a n a l y s i s o f these spectra obtained according  ionization  interpreted UV  b a n d s may be v a l u a b l e  for  o f e l e c t r o n i c s t r u c t u r e as w e l l as f o r compounds. i n t h i s work a r e a n a l y s e d  and e x c i t e d s t a t e s  t o t h e scheme d e v e l o p e d by R o b i n a n d K u e b l e r  Harshbarger e t a l [65] various  ( -v 2 0 0 0 A ) a n d t h e s e  using  t h e measured term v a l u e s  p o t e n t i a l s obtained  with  [64] and  respect to  by P E S . T h e s p e c t r a c a n t h e n be  i n terms o f Rydberg e x c i t a t i o n s .  Comparison w i t h a v a i l a b l e  spectra i salso helpful i n the analysis of the spectra.  0.050 eV was u s e d a n d  A m a i n beam w i t h a FWHM o f a p p r o x i m a t e l y resulted  i n energy l o s s s p e c t r a o f water comparable w i t h e a r l i e r  [77-81].  E l e c t r o n impact s p e c t r a were o b t a i n e d  100 eV o r 50 eV w i t h 10°  a t an i m p a c t e n e r g y o f  o f 2° a n d 1 0 ° .  s c a t t e r i n g angles  d i d n o t show a n y g r e a t d i f f e r e n c e f r o m t h o s e  The s p e c t r a a t  a t 2° f o r a l l t h e  compounds s t u d i e d e x c e p t f o r t h e e x p e c t e d much s m a l l e r The  samples used were o b t a i n e d  further  purification.  spectra.  commercially  No s i g n i f i c a n t  compares w e l l w i t h  7.2.Water, Methanol The  optical  and Dimethyl  a n d w e r e used, w i t h o u t  respect  absorption  to the e l a s t i c a l l y  ( l a ^  (C  (2  2  a ]  )  2 v  2  Ether.  ) are well  known t o be  (lb )  )  (3  2  2  scattered  work.  g r o u n d s t a t e c o n f i g u r a t i o n and o u t e r o r b i t a l  water molecule  intensity.  i m p u r i t i e s w e r e i n d i c a t e d by t h e  The e n e r g y s c a l e , f i x e d w i t h  signal,  work  a ]  2  ( l ^ )  2  1  A  energies  of the  1  -18.54 -14.76 -12.62 eV The  orbital  is  considered  molecular  energies  a r e quoted from r e f e r e n c e  to contain  plane.  [64].  The o r b i t a l  3a^ may be d e s c r i b e d  and  energies (la ) 1  2  has a l o w e r  symmetry, C  s <  t o the  a s i n v o l v i n g H-H  2  Methanol  orbital  t h e oxygen lone p a i r (2px) p e r p e n d i c u l a r  b o n d i n g c h a r a c t e r a n d l b i n v o l v i n g H-H a n t i - b o n d i n g •  The l b ^  [82].  character  The m o l e c u l a r  orbital  ordering  a r e [64]:  (2a')  2  (3a ) 1  2  (4a ) 1  2  (5a')  2  (la  1 1  )  2  (6a')  2  (7a')  2  (2a"')  2  -32.2 - 2 2 . 6 5 -17.62 -15.64 -15.21 -12.62 -10.96 eV The  molecular  orbitals  2 a " a n d 7 a ' a r e d e r i v e d f r o m 2pir  l a r g e l y on t h e o x y g e n , w h i l e  orbitals  l a " a n d 5 a ' a r e 2pir o r b i t a l s  l a r g e l y on  100.  the  methyl  group.  Dimethyl  e t h e r a l s o has C  2 y  have g i v e n a m o l e c u l a r o r b i t a l  symmetry.  Cradock and W h i t e f o r d [ 8 3 ]  d e s c r i p t i o n o f CH -0-CH . 3  F r o m PES  3  d a t a , t h e e n e r g y o r d e r i n g o f t h e M.O.'s i s a s s i g n e d t o b e ( l  a  i  )  2  (2a ) n  4  (1b )  (CH b o n d i n g )  2  2  (2b )  1 2  (-16.5,-14.2) The  e f f e c t i v e degeneracy  F i g u r e 21 shows t h e e l e c t r o n dimethyl  levels  a ]  )  ( l ^ )  2  i s removed by  impact s p e c t r a o f w a t ^ r , methanol  e t h e r a t 100 eV i m p a c t e n e r g y a n d 2° s c a t t e r i n g  t e n t a t i v e assignment based  angle.  in  on t h e t e r m v a l u e scheme.  CH 0CH 3  3  a n d w i t h t h e a i d o f r e f . 7 7 - 8 1 , a l s o made some a d d i t i o n a l A comparison o f t h e t h r e e  2  The  i s given first  below.  prominent f e a t u r e  i n the H 0 spectrum  has b e e n a s s i g n e d a s a lb-j -+ 3s R y d b e r g t h e upper  state  transition  i s OH a n t i b o n d i n g .  broad  ( A ) a t % 7.4 eV. [64].  The s e c o n d  b r o a d w i t h a t h r e s h o l d a t ^ 8.8 eV a n d a maximum which  i s a diffuse  2  band e x t e n d i n g f r o m 6.5 eV t o 8.5 eV w i t h a maximum  because  those suggested  We h a v e e x t e n d e d t h e d i s c u s s i o n t o i n c l u d e t h e l e v e l s o f  assignments f o r t h e h i g h e r s t a t e s o f H 0. spectra  Table 3  The a s s i g n m e n t s  3  r e f . 64.  and  i n t h e s e compounds a n d a  H 0 and CH 0H a r e , w i t h a few e x c e p t i o n s , e s s e n t i a l l y 2  2  levels.  g i v e s a summary o f t h e t r a n s i t i o n s o c c u r r i n g  for  (3  - 1 3 . 4 3 -11.91 - 1 0 . 0 4 eV  o f t h e CH b o n d i n g  i n t e r a c t i o n w i t h o n e o f t h e CO b o n d i n g  2  2  This  I t i s broad  band i s a l s o  ( B ) a t 9.66 e V , a f t e r  i t i s o v e r l a p p e d by a s e r i e s o f s h a r p p e a k s .  The t e r m v a l u e f o r  t h i s s e c o n d t r a n s i t i o n w i t h r e s p e c t t o t h e 3a^ I . P . ( 1 4 . 7 6 e V ) i s 5.10 eV. Comparing  t h i s w i t h t h e 5.2 eV t e r m v a l u e o f t h e f i r s t  b r o a d band  r e s p e c t t o t h e lb-| I . P . (12.62 e V ) , i t i s e v i d e n t t h a t t h i s  with  band p r o b a b l y  100 eV, 2*  6.00  Figure 21.  8.00 10.00 12.00 14.00 ENERGY LOSS(eV)  E l e c t r o n i m p a c t s p e c t r a o f H 0 , CH 0H and 2  CH OCH 3  3  a t 100 e V , 2°  3  H0 2  (la )2(2a ) (lb )2(3a ) (1b )  energy(eV)  1  1  2  2  1  transition  2  1  term value(eV)  7.4  Ibj—>3sa  9.66  3aj—> 3saj Ibi — > 3 p a i  10.16 10.58 10.76)  lb,  11.01  I b i — > 4sax  3pb  3  Peak  5.2  1  10.00 10.38 10.76  CH OH  :  2  ,  2  1.61  E  G  F  1.09  H I  x  11.75  I b j — > 5sai  0.87  11.91  lbx—> 5p  0.71  12.07  lb lb  6sai •5d  0.55  Ibi—>7sdL\  0.38  I b i — > 6d  CH  2  3OCH3  ...(CH ) (CH ) (2fc ) (3a ) (1b I  M  I I  n  2  2  1  2a" —->3s  4.27  A  6.67  B  lbj—>3s  3.37  3.25  C  7.34 \ 7.63 ]  Ibj—*3p  2.70 2.41  D  8.46  I b i — - » 3d 3ai — > 3 s  1.58 3.45  E  9.20  3ai—}3p  2.71  F  9.95  2b —>3s  3.48  G  10.48  3ai—>3d  1.43  H  10.84  2b —}3p  2.59  I  11.58  C H T —>3p  2.6  J  12.92  CH11—>3s  3.6  2.66  9.22  2a"—}3d  1.74  9.44' 9.83,  7a'—»3p  3.18 2.79  9.63  2a"—>4p  1.33  11.90  la"—>3s  3.74  13.43  5a' —>3s  4.19  Peak  energy(eV)  transition  2  2  .  can be obtained by adding the term value to the t r a n s i t i o n energy. Table 3  2  6.69  2a"—>3p  1.24  espective I.P.  ,  term value(eV)  2.46  l b x — > 4pai  12.24  2  x  11.38  r  2  2a"-^3p  1.50  r  ) (la") (6a ) (7a ) (2a")  transition  2.62  Ibx—^ 3d Ibi—>4pb  1  energy(eV)  5.10  11.12 11.53  ...(5a  E l e c t r o n i c T r a n s i t i o n s i n water, methanol and dimethyl  ether.  o ro  103.  corresponds due  t o t h e u p p e r s t a t e b e i n g OH  methanol The  t o a 3a-j -+ 3s R y d b e r g t r a n s i t i o n .  s p e c t r u m i s a l s o d i f f u s e and  upper s t a t e i s s t i l l  OH  g i v e s a term  v a l u e o f 4.3  S i n c e i t has  been o b s e r v e d  the  transition  transition  - 4.3  = 8.3  and  eV  The  o f 3s t e r m  v a l u e s on  alkylation  c o r r e c t , by t h e 3s t e r m  and  CH-jOCH^ r e s p e c t i v e l y . the corresponding  to  than  (D)  term  v a l u e o f 3.45  to  two  and  M.O.,  Kuebler 7a'  eV  I.P.  ]  eV).  on  alkylation,  illustrated,  The  6.67  eV  i n t h e s p e c t r u m a t 8.46  4.3  and  [64].  3.4  eV  and  The  This  ]  I.P.  (11.91  five  p e a k s , ( C , F, D,  d i s t i n c t e x c i t e d s t a t e s where the v i b r a t i o n a l  sharper  transition  the f o u r t h  the b a s i s of i t s eV). i n the  2  first  3  other  peaks appear i n the spectrum o f H 0 The  CH 0H  is relatively  as  i s a s s i g n e d on  eV w i t h r e s p e c t t o t h e 3 a  assignment  ether i s  3a-j, o c c u r s  term  decrease  i n H^O,  The  band  this  i f this eV  at  resolution  gives a  Also, i t s intensity  peak i n m e t h a n o l .  eV.  The  strong  i n high  band i n d i m e t h y l  M.O.,  The  e t h e r the f i r s t  (10.04 eV)  eV.  and  to 3s, expected  p e a k ( A ) a t 6.67  v a l u e s o f 5.2,  band i n w a t e r .  l o s s r e g i o n a b o v e 10 eV.  - 7.4  (10.96  values decrease  of dimethyl  i s well  the c o r r e s p o n d i n g  A s e r i e s of sharp  I.P.  3s R y d b e r g t r a n s i t i o n .  3s f r o m t h e s e c o n d h i g h e s t o c c u p i e d  peak  ( A ) a t 6.7  b u r i e d under o t h e r  w i t h respect to the l b  is  band i n t h e  f r o m 6.0  s t r u c t u r e s have been o b s e r v e d  i s a s s i g n e d a s t h e Ib-j  much l a r g e r  occupied  In the case  transition  than  I t peaks  by R o b i n  i s probably  absorption spectra [84].  v a l u e o f 3.37  first  t o t h e 2 a " •> 3s R y d b e r g t r a n s i t i o n .  eV,  fine  The  extending  [ 6 4 ] t h a t 3s t e r m  been c o n f i r m e d  i n the spectrum.  becomes s h a r p e r optical  anti-bonding.  from the second'highest  a b o u t 12.62  broad  broadness i s a l s o  eV w i t h , r e s p e c t t o t h e 2 a "  i s assigned  R y d b e r g c h a r a c t e r has  features  anti-bonding.  The  G,  levels  E)  energy  belong  o f one  state  104.  are located  h a l f way b e t w e e n t h o s e o f t h e o t h e r .  These correspond t o  the Rydberg  t r a n s i t i o n from t h e l b ^ s t a t e  lone p a i r ) to the  two  c o m p o n e n t s o f 3p.  [85],  Johns  (oxygen  u s i n g an a r g u m e n t b a s e d  f i n e s t r u c t u r e i n UV s p e c t r o s c o p y , i d e n t i f i e d a t 10.00 10.58  on r o t a t i o n a l  t h e symmetry o f t h e s t a t e s  ( C ) a n d 10.38 ( D ) t o be ^B^, w h i l e t h e p e a k s a t 10.16 ( F ) a n d  (G) a r e o f ^  symmetry.  Based  on i n t e n s i t y [79]  s c a t t e r i n g a n g l e i n EIS S k e r b e l e and L a s s e t t r e  variations  with  considered the small  peak a t 10.76 eV ( E ) t o h a v e c o n t r i b u t i o n s f r o m b o t h c o m b i n a t i o n s , 2v-| o f t h e A s t a t e a n d v-j + v and  B states  small The  peak a t 11.91  d o u b l e t a t 11.01  o c c u r a t 11.38  eV (M) c o r r e s p o n d s  s e r i e s o v e r l a p s t r o n g l y and appear  12.07  (N) a n d 12.24 eV (0).  spectrum  o f water  T h e s e c o n d members o f t h e A  ( J ) a n d 11.53 eV ( K ) w h i l e t h e t o t h e t h i r d member  (H) a n d 11.12 eV ( I ) c o r r e s p o n d s  4 s ) a n d ( l b - j -*• 3d) r e s p e c t i v e l y .  (lb-j two  ( l b - , + 4p)  of the B state.  2  In t h e methanol  to the transitions  a s s i n g l e p e a k s a t 11.75 ( L ) ,  A f u r t h e r f e a t u r e t o be m e n t i o n e d  by S k e r b e l e e t a l [80]  optically forbidden transition spectrum,  5p).  T h e h i g h e r members o f t h e s e  i s a v e r y s m a l l peak a t 6.18 eV.  p r e v i o u s l y been o b s e r v e d  (lb-j  t o a low l y i n g t h e two b a n d s  i n the  T h i s peak h a s  a n d may c o r r e s p o n d  triplet  t o an  state.  (B a n d C ) a t 7.82 a n d  8.33 eV r e p r e s e n t t r a n s i t i o n s f r o m 2 a " t o t h e two c o m p o n e n t s o f 3p. a more d e t a i l e d  scan  ( s e e i n s e r t ) o f t h e s e two b a n d s ,  structures are revealed.  The f i r s t  band has an a v e r a g e  s p a c i n g o f ^ 0.096 eV w h i l e t h e s e c o n d The  3pu s p l i t t i n g  that i n water  i n methanol  (0.16  vibrational vibrational  o n e h a s a s p a c i n g o f 0.110 eV.  ( F i g u r e 21),  0.59 e V , i s much l a r g e r  eV) and so t h e v i b r a t i o n a l  i n CHgOH do n o t i n t e r m i n g l e .  On  states o f these  T h e t e r m v a l u e f o r t h e 7.82  than  bands  eV band ( B )  105,  (3.25  eV)  r e p r e s e n t s an e x t r e m e v a l u e f o r - 3 p - t e r m  f e a t u r e s i n the methanol peaks 9.22  (D, eV  E, F, 6 )  has  assigned  v a l u e o f 1.74  t o be a 2 a "  3d  a broad  band.  ( E ) and. 9.83  eV  transition.  A l s o on  (G) a r e a s s i g n e d  Term v a l u e s a r e g i v e n  i n Ta.ble 3.  occur above the f i r s t  I.P.  o f 3.74 as  eV w i t h r e s p e c t t o t h e  la".-> 3s w h i l e , t h e n e x t  transition a t . 17.62  on  eV.  as a s t e p 2.70  and  2.4.1  probably (D).  one  l a " I.P.  two  and  than  a t 15.64  a peak  3s t e r m  values  assigned  peaks, i n t h e i r  two  a shoulder  I and  eV  eV  3p  (C) a t 7.63  has  and  7a  is  3p.  1  (H and  a term  so c a n  I)  value  be  witha  assigned 5 a ' -»• 3s  PES  The  (see Table  eV.  ( I ) a t . 11.58.eV a n d  <The t e r m  term lb-j  I.P.  and  values 3d  :  values  of  the  on. a l k y l a t i o n  transitions  3 ) , t h e peaks. (E.,..F,,G,  be  16.5  occurs  m e n t i o n e d b e f o r e a t 8.46  3a  C r a d o c k and  spectrum-to  I I a t 14.2  Rydberg, t r a n s i t i o n  lb-| I . P . a r e c o m p a r a b l e w i t h  values.  -»• 3p r e s p e c t i v e l y .  bonding o r b i t a l s  and  v a l u e w i t h r e s p e c t t o the 5a'  10.84-eV, a r e a s s i g n e d  2b  2  bands  eV. i s - c o n s i s t e n t  o v e r l a p w i t h t h e 3a-j -»• 3s t r a n s i t i o n  9,95.,.10.48 and  at  at  c o m p o n e n t s Of broad  s h o w i n g t h a t t h e . c h a n g e o f 3p  drastic  I.P.  4p w h i l e t h o s e  a t 11.90  of dimethyl, e t h e r , the l b ^ eV  (D)  .  3a-j -> 3d and  CH  2a". +  next  peak a t 13.43  On. t h e b a s i s o f t e r m  9.20,  first  eV w i t h r e s p e c t t o t h e  values f o r water, i s much l e s s  The  ,  (B) a t 7.34  shoulder  the b a s i s of i t s term  t o t h e two  the grounds of i t s term  . In t h e x a s e  at  The  The  next  ..resolved.-,  eV w i t h r e s p e c t t o t h e 2 a "  v a l u e , t h e peak, ( F ) a„t^9.63 eV . i s a s s i g n e d 9.44  [64].>-The  spectrum are f o u r s m a l l . . p a r t i a l l y  s u p e r i m p o s e d on  a term  values  ]  •+ 3p,. 2 b  Whiteford  due e.V.  Our  EIS  H)  3s, , [83],.have  to i o n i z a t i o n  t h i s ,is a s s i g n a b l e t o  CH (I.) -> 3p., . l e a d i n g , - t o a-, term, v a l u e .of. 2t 6 e V . . The  2  eV  of  spectrum  the shows  the-transition n e x t . m a x i m u m ^ i s a t -v  106.  12.9  eV  ( J ) and  CH(II)  I.P.  CH(II) +  so  has  a t 16.5  a term  v a l u e o f 3.6  So  should  eV.  configuration  [67] i s  ( l  ( l  )  (ea^  (2  2  a  )  i  2  (2b )  2  D  l  )  (3  2  C  2 v  i  )  a  symmetry.  2  (4  9  l  )  (2b )  2  1  50 eV  2  , l a  and  2°  occurring  2  2  (lb )  2  2  (5a )  (la )  2  T  from  (3b^)  2  2  -17-4-16.6  2  -10.57  l b  to a transition  I t s ground s t a t e e l e c t r o n i c  2  -11.7 The  correspond  the  Oxide.  E t h y l e n e o x i d e ha?  i  w i t h respect to  3s  7.3.Ethylene  a  this  eV  -14.2  -13.7  eV and  2b  a r e TT o r b i t a l s .  2  scattering  i n t h e 7-8  eV  I t appears t h a t these  angle  eV.  The  spectrum two  top of a broader  vibrational  structure.  absorption which  a t a b o u t 6.5  second  band a r e a b o u t 0.095 and  0.130  8.6  have been a r r a n g e d  s p e c t r o s c o p i s t s i n t o Rydberg s e r i e s .  by UV  of the f i r s t  respectively.  first  a s s i g n m e n t by  10.81  eV  later  p h o t o i o n i z a t i o n d e t e r m i n a t i o n by W a t a n a b e [ 8 7 ] .  igating  and  ionization  r e a s s i g n i n g t h e R y d b e r g s e r i e s , Lowrey and  a p r e c i s e agreement between the  ionization  La  D u n c a n [ 8 6 ] g a v e an  value of the  I.P.  Rydberg l i m i t  Basch e t a l . [67]  photoelectron spectra of ethylene oxide. Paglia  [ 8 9 ] , they  Rydberg s e r i e s ,  being  i n c l u d e d the the f i r s t  lower  and  Features  w h i c h d o e s n o t a g r e e w i t h a v a l u e o f 1 0 - 5 6 5 eV  obtained  and  L i u and  eV  spacing  at  bands  r e g i o n show some v i b r a t i o n a l  b a n d s l i e on The  e l e c t r o n impact  i s shown i n F i g u r e 2 2 .  energy-loss  two  The  starts  eV  2  above  potential  obtained After  reinvest[88]  the  photo-  have s t u d i e d t h e  F o l l o w i n g the  b a n d s a t 7.24  members (n = 3 ) p f t h e  and  of  in a  Watanabe and  The  optical  suggestion  7.89  series  eV  of  as  ( 2 b i r -»• n s ) 2  2 CO  cn < cn  2b —^nd/nd+1/3 n 2  CO cn <  >-  50eV,2  e  7.33 eV  CO  z:  UJ  z:  2 b — • n p / n p + V j 2  6.00 Figure  22.  7 0 0 8.00  Electron  9.00 10.00 11.00 12.00 1300 1400  ENERGY LOSS (eV)  impact spectra  o f e t h y l e n e o x i d e a t 50 e V , 2°  o  108.  ( 2 b i T -* n p ) r e s p e c t i v e l y .  and  They have s u b s t a n t i a t e d  2  t h e Rydberg  c h a r a c t e r s o f t h e s e t r a n s i t i o n s by r u n n i n g t h e c o n d e n s e d phase and  by c o m p a r i n g w i t h  arranged  the features  the isomeric  molecule acetaldehyde.  i n our electron  2 b r r -* n s , n p , nd ( s e e 2  2  2  v a l u e s o f 3.37 a n d 2.69 eV r e s p e c t i v e l y  This i s consistent  i n t h e open c h a i n  e t h y l e n e o x i d e a n d 1.58 eV i n d i m e t h y l t h e Rydberg s e r i e s  a r e accompanied  v i b r a t i o n frequency v  with  splitting  o f the f i r s t  a vg v i b r a t i o n .  with  3  (0.140 e V ) a s l i s t e d  7.4.Ethyl,  i n T a b l e 4.  b a n d , 0.090 e V , c a n be e x p l a i n e d  The t-butyl  and t - B u t y l  electron alcohol  are presented  by a s s o c i a t i n g i t  weak s t r u c t u r e l e s s  for  2  isopropyl  alcohol  and  A summary o f t h e t r a n s i t i o n s  methyl a l c o h o l ,  band  a l l t h e s e compounds  ( A ) i n t h e 6.5 eV t o 7.5 eV e n e r g y  which should correspond t o t h e t r a n s i t i o n s from the highest (symmetry a") t o t h e 3s R y d b e r g o r b i t a l .  CH CH 0H, 3  transitions.  T h e i m p a c t e n e r g y i s 100 eV a n d t h e s c a t t e r i n g  show a r a t h e r  M.0.  alcohol,  i n F i g u r e 23.  A s was t h e c a s e w i t h  filled  The  t r a n s i t i o n i s g i v e n by Basch e t a l [67].  of ethyl  angle i s 2°.  loss region  associated  Alcohols.  impact spectra  i s g i v e n i n T a b l e 5.  members  T h e R y d b e r g t r a n s i t i o n s seem t o be s u p e r i m p o s e d on  of inter-valence-shell  Isopropyl  Most  transitions  b r o a d e r bands w h i c h a r e p r o b a b l y due t o i n t e r - v a l e n c e - s h e l l A discussion  (1.92 eV i n  e t h e r ) i s n o t a s good. by v i b r a t i o n a l  with the  analogue  CrtgOCHg, a l t h o u g h t h e a g r e e m e n t b e t w e e n t h e 3d t e r m v a l u e s  of  series  The t e r m v a l u e f o r t h e 2b -rr ->• 3s t r a n s i t i o n i s 3.33 eV w h i l e  f o r t h e 2b TT •> 3p t r a n s i t i o n i s 2.68 eV.  that  We h a v e  impact spectrum i n t o Rydberg  ( b a s e d on t h e a s s i g n m e n t s o f B a s c h e t a l . [67]) Table 4).  spectrum  The 3s t e r m  ( C H ^ C H O H a n d (CH )' C0H a r e 3 . 8 0 , 3.62 a n d 3.46 eV 3  3  values  2b,7T  2b,TT  1  1  ns  2b„TT  I  ns+vj  (3)  8.97  (4)  [9.10]  (4)  9.69  (5)  9.83  (5)  (6)  10.14  (6)  (7)  [10.33]  (7)  [10.29]  (8)  10.43  (8)  10.57*  H  10.70*  H  [10.19]  (3)  +  f  7.89  (3)  9.32  (4)  9.81  (5)  f  +  values o f n are given i n the b r a c k e t s ; energies are expressed i n eV.  *  p h o t o i o n i z a t i o n I.P.  [ ]  value quoted from o p t i c a l Table  4  absorption r e f .  Rydberg s e r i e s  in ethylene  21 oxide  2b 7T ?  i  nd  nd+v  np+vg  !  r e f . 20.  I  2b,7T  1  np  5  7.33  +  2b,TT  J  ns+v  7.24  TO. 00  2b„TT  8.02  (3)  9.45  (4)  +  8.64  (3)  [9.53]  (4)  [9.93]  (5)  f  8.78  3  (3)  +  no.  .  - „*,*-. •  • •• cy.-.... B*.  eV,  1 0 0  D  2 °  • CHoCHoOH  B:  F n  °  ^  A  CO  h-  P  01  ••••"V'-^.V."  <  eV,  1 0 0  B  (CH ) CHOH 3  m  rr <  A  D  B  UJ A  A-  eV,  1 0 0  F i g u r e 23.  2 °  (CH ) COH 3  6.00  2  E  CO  z:  2 °  3  8.00 10.00 12.00 ENERGY LOSS (eV) E l e c t r o n impact s p e c t r a o f e t h y l , i s o p r o p y l and t - b u t y l a l c o h o l s a t 100 eV, 2 ° .  14.00  CH CH 0H . . . ( ^ ) ( * 3 ) ( a ' ) ( a ' ' ) 2 * 3  k  2  m  n  (CH ) CH0H .  2  3  energy(eV)  transition  term value(eV)  6.82  a"—>3s  3.80  7.80 8.13  a" —>3p  Peak  (CH ) C0H  2  3  energy(eV)  transition  term value(eV)  A  6.80  a"—-»3s  3.62  2.82  B  7.92  a"—>3p  2.49  C  8.89  D  8.93  a"—>3d  1.69  9.45  a"—»4p  .1.17  a'—>3p  2.75  0.69  <|>3—^3p  2.62  1.37  ipi,—»3p  2.45  Peak  3  . ..(*J (*3) (a ' ) ( a " ) m  n  2  2  energy(eV)  transition  term value.(  A  6.79  a"—»3s  3.46  2.50  B  8.17  a"—>3p  2.08  a"—>3d  1.53  C  8.64  a" —?>3d  1.61  9.32  a'—>3p  2.39  D  9.5  a'—»3p  2.0  : E  10.07  * —»3s  3.68  E  10.18  *3—>3p  2.17  F  10.85  i>k—»3p  2.23  F  11.30  <K—>3d  1.48  G  11.51  *s—>3p  2.24  5  Jhen the symmetry o f the M.O.s are not known, tf^ designatesthe £ highest f i l l e d M.0. Respective L P . can be obtained by adding the term value to the t r a n s i t i o n energy. t  n  Table 5 . E l e c t r o n i c T r a n s i t i o n s i n e t h y l , isopropyl and t-butyl  alcohols.  112.  respectively with consistent with alkylation. H 0,  to t h e i r f i r s t  the o b s e r v a t i o n  CH 0CH  3  3  ( 5 . 2 , 4.25  3  I.P.'s  t h a t 3s  However, c o m p a r i n g w i t h  C H 0 H and  2  respect  term values  the  and  (a").  3s  3.37  eV  h y d r o g e n atom a d j a c e n t The  t o an  a t 7.71  and  While methanol 8.30  eV,  the  i v e l y more d i f f u s e and  respectively),  ethyl  I.P. to  eV.  Term v a l u e s  ( a " ) a t 10.62  the  split 3  (0.59  (CH ) CH0H the 3  peak  2  (B) a s s i g n e d  respect  to the  a"  second f e a t u r e value this  shows two  o f 2.82  3p  i n the  o f o n l y 2.08 low v a l u e ,  The  and  splitting  The  ( B ) a t 7.80  eV  and  (C)  eV  transitions o f 0.33  i s 2.50  spectrum i s a step  eV w i t h  respect  to the  possible explanation  top of a s t e e p l y r i s i n g to higher  n e x t peak  (C)  eV  energies,  i n the  eV.  3  3  3  has  (10.25 e V ) .  3p  be  and  with the a  term of  assigned  as  so t h e  apparently  one  Because  transition  (CH ) C0H spectrum occurs 3  only  This  can  half  eV).  (CH ) C0H  eV.  transition  an  M.O.  I t s term value  transition  giving  a"  (0.27  3  Coming t o  I.P.  first  i s only about  i s t h a t the a" valence  to the  from the  (B) a t 8.17 a"  a peak  e v e n s m a l l e r and  eV.  i t i s doubtful whether t h i s  maxima i s s h i f t e d  with respect  i s o b s e r v e d a t 7.92  riding  The  resolved.  i s apparently  One  value.  be  3  ( 1 0 . 4 2 eV)  progress-  can  comparable to t h a t of CH 0CH  a" -»• 3p. on  2.49 are  structure  h e a v i e r m o l e c u l e s become  splitting  splitting  a " -»• 3p I.P.  and  suggest these  eV)  methyl  than r e p l a c i n g a  bands w i t h v i b r a t i o n a l  vibrational  3p m a n i f o l d .  t h a t o f CH 0H For  eV  i t is  s p e c t r a o f t h q a l c o h o l s shows some  a l c o h o l s p e c t r u m shows a s h o u l d e r  a t 8.13  series.  t o a C a t o m by a  3s t e r m v a l u e  s p e c t r a of the  no  i n the  with  0 atom.  second f e a t u r e i n the  differences.  is  are decreasing  term values  o b v i o u s t h a t r e p l a c i n g a hydrogen atom a d j a c e n t group causes a s m a l l e r decrease i n the  This  is  peak  smaller  term  a t 8.64  eV  113.  w h i c h g i v e s a t e r m v a l u e o f 1.61 eV w i t h r e s p e c t t o t h e a " I . P . a n d s o c o r r e s p o n d s t o a a " -»• 3 d t r a n s i t i o n . in  The c o r r e s p o n d i n g t r a n s i t i o n s  ( C H ) C H 0 H and CH CH 0H a r e l e s s d i s t i n c t . 3  2  3  The ( C H ) C H 0 H  2  3  shows a s h o u l d e r ( C ) a t 8.89 eV ( t e r m v a l u e 1.53 e V ) . s p e c t r u m has a s t e e p s l o p e w i t h a change i n g r a d i e n t a b o u t 8.93 eV. this  It i sdifficult  i s due t o t h e t r a n s i t i o n The s e c o n d , t h i r d  to tell  spectrum  2  The C H C H 0 H 3  2  (D) o c c u r r i n g a t  w i t h any c e r t a i n t y as t o whether  a" ->• 3 d .  and f o u r t h  ionization  p o t e n t i a l s o f e t h a n o l have  been d e t e r m i n e d by PES [ 9 0 ] t o be 1 2 . 2 0 , 13.31 a n d 13.82 eV r e s p e c t i v e l y . Peaks  ( E , F, G) a t 9 . 4 5 , 1 0 . 6 9 a n d 1 1 . 3 7 eV i n t h e s p e c t r u m o f e t h y l  alcohol  h a v e t e r m v a l u e s o f 2 . 7 5 , 2.62 a n d 2.45 eV w i t h r e s p e c t t o t h e '.  above I . P . ' s a n d s o p r o b a b l y r e p r e s e n t t r a n s i t i o n s f r o m t h e c o r r e s p o n d i n g M.O.'s t o t h e 3 p R y d b e r g  state.  The l a s t  peak a t 12.8 eV c a n n o t be  a s s i g n e d o n t h e t e r m v a l u e scheme a n d i s p r o b a b l y a n i n t e r - v a l e n c e - s h e l l transition.  The peak  ( E ) a t 9.45 eV i s r a t h e r b r o a d a n d t h e a " ->- 4p  t r a n s i t i o n , w i t h a term value o f about this  band.  1.0 e V , may be c o n t r i b u t i n g t o  Features i n the spectrum o f (CH ) CH0H  less d i s t i n c t .  3  to a t r a n s i t i o n from ^  2  I . P . a n d s o c a n be a s s i g n e d  ( f o r convenience, ^  M.0.) t o t h e 3p R y d b e r g  state.  i s used  t o denote  eV h a v e t e r m v a l u e s o f 3.68 a n d 2.24 eV w i t h  3p R y d b e r g  states  respectively.  c a n be a s s i g n e d t o a t r a n s i t i o n  r e s p e c t t o t h e 5 t h I.P.  One f u r t h e r p e a k  from ^  A  t h e nth  Humps ( E , G) a t 10.07 a n d  (13.75 eV) and t h e r e f o r e a r e a s s i g n e d t o t r a n s i t i o n s f r o m and  12.68,  T h e s t e p (D) a t 9.32 eV has a t e r m  v a l u e o f 2.39 eV w i t h r e s p e c t t o t h e s e c o n d  11.51  a b o v e 9.0 eV a r e  T h e h i g h e r I . P . ' s o b s e r v e d by PES [ 9 0 ] a r e 1 1 . 7 1 ,  1 3 . 0 8 , 1 3 . 7 5 , 15.14 a n d 1 5 - 8 0 eV.  highest f i l l e d  2  t o t h e 3s  ( F ) a t 1 0 . 8 5 eV  t o 3p on a c c o u n t o f i t s t e r m  114.  value  o f 2.23  rises  s t e e p l y a b o v e 9.0  After this A small  eV  with  i t falls  respect eV  o f 2.08  eV  transition  from ^  t o 3p  transition  may  steeply rising i n the  100  T h e s e may  eV be  i n the  eV  eV  transition.  a term value  of  assigned  t o h a v e a 3d  upper  The  Ether  The  Rydberg t r a n s i t i o n s  has  5.Diethyl  and  observed order  eV.  Higher  I.P.  despite  the  plateau  1.47  eV  with  eV  i t s low  eV.  a t ^ 9.5  by r i d i n g  tj^ t o t h e  transition. respect  16.50  corresponding  b e t w e e n 9.6  and  eV  13-14  and  This  eV,  the  on  top of  and  10.5  of several overlapping  a eV  transitions. 3p  The  state peak  to the 4th  (F)  I.P.  and  state.  (EIS)  of diethyl  look  r e g i o n o f 6.0-8.5 eV.  i n the o p t i c a l the  region  10.5  Tetrahydrofuran.  underlying  to assign  I.P.,  plateau  from ^  a n a l o g u e , t e t r a h y d r o f u r a n ( F i g u r e 24)  of a r i s i n g  1st  inter-valence-shell  e l e c t r o n impact s p e c t r a  the energy l o s s  and  this.  as a" -> 3 p ,  energy again  result  a t 11.30 be  to the  to higher  a strong  may  i s s u p e r i m p o s e d on  i s regarded  s u p e r i m p o s e d on eV  again  to the onset of the  spectrum i s the the  becomes f l a t  (CH^COH  i s e x p e c t e d a t a b o u t 1 1 . 4 8 - 2 . 0 8 = 9.4  shifted  valence  spectrum of b e t w e e n 9.6  with respect  correspond  p e a k maximum b e i n g  The  [ 9 0 ] a t 1 1 . 4 8 , 1 2 . 3 5 , 1 2 . 7 8 , 15.42  (B) a t 8.17  term v a l u e  I.P.  comes t o a p l a t e a u  a t 11.3  have b e e n o b s e r v e d by PES transition  and  g e n t l y and  hump ( F ) p e a k i n g  I f the  to the 4th  continuum. spectra  very  Fine  EIS,  the  not  vertical  I.P.  i s determined  here.  spectra  laboratory [93]. t o be  9.63  eV  in top  r e g i o n 6-8  resolved  photoelectron  compounds h a v e b e e n m e a s u r e d i n t h i s ether  its alicyclic  particularly  s t r u c t u r e s i n the  of these  of diethyl  similar,  and  T h r e e p e a k s a r e e x h i b i t e d on  [84,91,92] are  peaks i n the  ether  while  eV, In  (PES) The  first  that  of  115.  100 eV, 2 CO  D  z:  «.»-r  >•<«'  B  >-  C2H5OC2H5  < tt:  A  DO  H  < >-  '".V  u  CO  B  CH2~CH  ;-s  CH  UJ A  ff  *  r  6.00 Figure  24.  *  ..-V'->-.v  •  2  2  CH.  *  (THF)  i  V  8.00 10.00 12.00 14.00 ENERGY LOSS (eV)  E l e c t r o n impact s p e c t r a o f d i e t h y l t e t r a h y d r o f u r a n a t 100 e V , 2°  e t h e r and  116.  tetrahydrofuran EIS a r e to  transitions  t h e 3 s , 3p,  and  i s 9.73  term  and  eV.  a term  and  t h e r e may  o r 3p is  be  M.O.  6.  assigned  i n the  (b-j a s s u m i n g C  The  third  peak  symmetry)  2 v  Transition  energies  (C)  in diethyl  some c o n t r i b u t i o n f r o m a t r a n s i t i o n  between  t o t h e 3s R y d b e r g s t a t e b e c a u s e t h i s  v a l u e o f a b o u t 3 eV w i t h r e s p e c t t o t h e 2nd  I.P.'s o f d i e t h y l  be  M.O.  ( A , B, C)  3d R y d b e r g s t a t e s r e s p e c t i v e l y .  ether are obtained  1 2 . 1 3 , 1 3 . 1 7 , 1 4 . 0 4 , 14.78 may  t h r e e peaks  from the h i g h e s t f i l l e d  the second h i g h e s t f i l l e d  Higher  first  v a l u e s a r e summarised i n T a b l e  e t h e r i s broad  has  The  and  16.55  eV.  P e a k s a b o v e 8.5  to the term  (11.09  eV).  a t 11.09, 11.35, 11.62,  to t r a n s i t i o n s from the v a r i o u s f i l l e d  Rydberg s t a t e a c c o r d i n g  I.P.  peak  value  i n the  EIS  M.O.'s t o t h e  scheme.  p o s s i b l e f o r t e t r a h y d r o f u r a n f o r which higher  eV  Similar  treatment  I.P.'s a r e observed  at  1 1 . 5 1 , 1 2 . 0 2 , 1 2 . 5 1 , 1 2 . 9 7 , 1 3 . 9 1 , 1 4 . 1 9 , 1 4 . 5 4 , 1 5 . 3 5 , 1 5 . 7 9 , and eV. In  A list  of t r a n s i t i o n  g e n e r a l , term  than  those  e t h e r and  in diethyl i t s cyclic  v a l u e s o f e a c h do should diethyl  values  a l s o be  energies  term  ether.  This  i s given  analogue,  e t h y l e n e o x i d e , w h e r e t h e 3s  by more t h a n  0.04  eV  16.84  i n Table eV  i s d i f f e r e n t from the case  and  6.  larger  of  dimethyl 3p  from the o t h e r .  t h a t w h i l e the s p e c t r a of t e t r a h y d r o f u r a n  term It  and  ether look very s i m i l a r , the spectrum of ethylene oxide i s  d r a s t i c a l l y d i f f e r e n t from t h a t of dimethyl mean t h a t t h e b o n d i n g the single-bonded n o t s i m p l y be and  values  i n t e t r a h y d r o f u r a n a r e a t l e a s t 0.15  not d i f f e r  noted  and  3s  ethers.  i n ethylene oxide  formula.  The  ether.  i s not  highest f i l l e d  as  T h i s d i f f e r e n c e must simple  M.O.  bond c h a r a c t e r s i m i l a r  has  suggested  to ethylene.  i n water,  that ethylene oxide I n d e e d , L i u and  suggested  i n ethylene oxide  t h e non-bonding e l e c t r o n o f t h e 0 atom as Walsh [94]  as  has  a  Duncan [ 8 6 ]  by may  alcohols double have  CJLOCJL Peak  enorgy(cV)  A  6.58  • • • ('1'if ) ( <i>s) (<*i ) ( b ) 2 * 7  n  m  transition  b  :  CH CH OCH in  1  term  —>3s  2  value(e~V)  Peak  2  2  energy(eV)  ...M U'3) n  2  n  transition  term  value(eV)  3.05  A  6.57  b ~>3s  3.16  1  B  7.24  b!—>3p  2.39  B  7.19  bi—>3p.  2.54  C  8.09  b  1.54  C  8.03  bi ~ > 3 d  1.70  D  8.80  ^3—^35  3.22  E  9.54  ^3  " '3D  2.48  F  11.03  i>  —>3s  3.16  G  11.40  <l>8 — > 3 s  3.14  H  13.61  *n—>3s  3.23  2  >3d/  3.00  3 i — > 3 s  D  8.98  *3—>3p  2.37  E  10.18  <!'G — ^ 3 s  2.99  F  12.39  </>o — ? 3 p  2.39  ScnJtf frSy  Respective  ry  t h e M.O.s a r e n o t k n o w n , * d e s i g n a t e s t h e * t h I.P. can b e . o b t a i n e d by adding t h e t l r m v a l u e t o t h e t r a n s i t i o n o  f  h  6  Electronic  transitions  i  q  h  e  s  t  i n diethyl  f  n  „ energy. i  d  7  Q  e t h e r and t e t r a h y d r o f u r a n .  118.  interpreted  their  spectrum of ethylene  a m o l e c u l a r bonding o r b i t a l than a non-bonding o r b i t a l  7.6.Effect  of A l k y l  Physical  i n v o l v i n g two  the  c a r b o n s and  c h e m i s t s have d e v i s e d  Taft  e x c i t a t i o n s from one  oxygen  rather  EIS  q u a n t i t a t i v e l y p o l a r e f f e c t s and resonance e f f e c t s .  b a s e d on  i n oxygen.  S u b s t i t u t i o n on  organic  oxide  means t o m e a s u r e  t o s e p a r a t e them f r o m s t e r i c  [95] gave the  polar e f f e c t of substituents  various  f o l l o w i n g equation  R on  the  rate of  normal  and  for  evaluating  hydrolysis  of  e s t e r s , RCOOR' a* a*  5  [log (k/ko)  is a substituent  the  substituent  for  the  constant  Following T a f t a*  of the  ROH,  Figure  25  I.P.  [64,83,93] of the  substituents value  attached  constant  polar e f f e c t of the  first  a l s o l i e on This  the  3s  the  oxygen atom.  I.P.  binding  of  of  With the  that refer  give  first  a*  there with  3s  for  term  is a  increasing  of water, the  energy of  the  l i n e f o r 3s  highest  filled  term M.O.  of  both  points  t h e s e compounds when p l o t t e d a g a i n s t to the  of  vertical  value  points f o r the  term value  plots  p a i r I.P.'s  the  the  exception  of  respectively.  [ 9 6 ] , who  and  to  (subscripts)  a s t r a i g h t l i n e showing that  a straight line parallel  means t h a t t h e  sum The  3s  polar effect  oxygen lone  term value  compound a g a i n s t  substituent.  vertical  A  Betteridge vs.  net  k) r e l a t i v e  a c i d i c reactions  r a t e of decrease i n the the  B and  Q  substituents  to the  upon t h e  rate constant  B a k e r and  shows a p l o t o f  l i e a p p r o x i m a t e l y on  roughly  for  alkyl  (7.1)  A  ( k , R=CH.j).  a l k a l i n e and  the example of  values  t o the  of comparison  to otherwise identical  (k/ko) ]/2.48  dependent only  (corresponding  standard  - log  B  za*,  values. decreases  119.  13  •  12-  I vertical LP. st  II-  ,<cr -a  10-  -a'  9  > CD  8^ 7  ro  o  CM  x  432-  ro  o o  nr  X o  O  X  00  ro  U  U  ro  -r OX cXvi.  ^  31 O  X  o  o  ro  X  o o^-3s term  X O  - — 3 p term V  V V  -17—3d  term  0 -0.4-0.2 0.0 0.2-04 0.6 0.8 1.0 1.2 cr F i g u r e 25.  E f f e c t ' o f a l k y l s u b s t i t u t i o n on' R y d b e r g ' t e r m v a l u e s ' a n d f i r s t i o n i z a t i o n potentials i n alkyl d e r i v a t i v e s of water.  120.  a t t h e same r a t e a s t h e b i d n i n g a l s o p l o t t h e 3p t e r m v a l u e s I.P.  i n t h e same g r a p h .  mean o f t h e two s p l i t o f t h e 3p t e r m v a l u e lower w h i l e  almost constant.  the  highest  increasing on  filled  (For s p l i t  terms.).  e n e r g y o f t h e 3p R y d b e r g o r b i t a l ) i s  ( b i n d i n g energy o f t h e 3d Rydberg o r b i t a l )  The d e c r e a s e i n t h e f i r s t  I.P. (binding  i . e . t h e non-bonding e l e c t r o n  pair binding  i s more p e n e t r a t i n g  Therefore  energy.  e n e r g y may s t i l l  polar e f f e c t s of the substituents  M.O.  An e l e c t r o n  i n negative  o x i d e ) , the p o s i t i o n o f the f i r s t  to a l k y l  substitution.  groups  i n t h e 3s R y d b e r g states.  CO b o n d s  transition  filled  i s quite insensitive  f o r i d e n t i f i c a t i o n o f CO b a n d s i n s a t u r a t e d  This  aliphatic  from the core i s l a r g e r .  fact compound.  As a r e s u l t b i n d i n g  energy i s l e s s  t o t h e i n f l u e n c e o f changes i n t h e c o r e n e g a t i v e  c h a r g e s due t o  i s only  substituents.  T h e r a t e o f d e c r e a s e o f 3p  binding  energies  binding  e n e r g y o f t h e more d i f f u s e 3d e l e c t r o n show no o b v i o u s  a b o u t 1/6 t h a t o f t h e 3 s b i n d i n g  the polar e f f e c t of alkyl  t o an o x y g e n atom i s r e p l a c e d value  extent  (except  T h e s e a l l o c c u r a t a b o u t 6.6 - 6.8 eV.  polar e f f e c t s of the alkyl  on  wi'll  i n t h e 3p a n d 3d R y d b e r g s t a t e become more and more d i f f u s e and  distance  subjected  charge  as i n t h e case o f t h e h i g h e s t  ethylene  Electrons  with  be a f f e c t e d t o t h e same  The r e s u l t i s t h a t f o r compounds c o n t a i n i n g  may be u s e f u l  energy o f  i n oxygen)  t h a n theft i n t h e 3p o r 3d R y d b e r g  t h e 3s b i n d i n g  to the f i r s t  3p l e v e l s , we c h o o s e t h e a r i t h m e t i c  t h e o x y g e n a t o m d u e t o c h a r g e d o n a t i o n by a d j a c e n t a l k y l  state  the  respect  We  I t c a n be s e e n t h a t t h e r a t e o f d e c r e a s e  ( i . e . binding  M.O.  with  p o l a r e f f e c t i s e x p e c t e d b e c a u s e an i n c r e a s e  lower the lone  by  and 3d t e r m v a l u e s  t h e 3d t e r m v a l u e s  are  e n e r g y o f t h e 3s R y d b e r g o r b i t a l .  substituents. by a m e t h y l  energy w h i l e the dependence  When a h y d r o g e n atom  adjacent  g r o u p , t h e d e c r e a s e i n 3s t e r m  i s much l a r g e r t h a n i n t h e c a s e when t h e h y d r o g e n a t o m a d j a c e n t t o  121.  a c a r b o n atom i s r e p l a c e d by a methyl g r o u p . polar  e f f e c t o f a methyl  smaller  of  of Taft  a* v a l u e s  energy p o s i t i o n o f the f i r s t common a l k y l d e r i v a t i v e s  within  ± 0.2 eV u s i n g  value i n recognizing Ea*  g r o u p on t h e c h a r g e o f t h e o x y g e n atom i s  where i t i s i n t h e $ p o s i t i o n .  With a t a b l e the  v a l u e and a l s o  [95] and t h e graph  the f i r s t  o f w a t e r c a n be p r e d i c t e d  t h e t e r m v a l u e scheme. the alkyl substituents.  i n most c a s e s t o  Alcohols  have a p o s i t i v e  f r o m e t h e r s by t h e  band i n e t h e r s .  Also,  the p r o f i l e  i s much b r o a d e r a n d d i f f u s e .  Robin has t a k e n a s l i g h t l y d i f f e r e n t approach i n c o n s i d e r i n g effect of alkyl substitution molecules. available  Some d i s c u s s i o n i n r e f . 66.  spectrum  C o n v e r s e l y , t h i s may be o f  c a n p r o b a b l y be d i s t i n g u i s h e d  band i n a l c o h o l  i n Figure 25,  few t r a n s i t i o n s i n t h e e l e c t r o n i c  higher r e l a t i v e i n t e n s i t y o f the f i r s t of  T h i s i s simply because t h e  i n the electronic  spectra  of this i s i n section  6.1.  the  of organic Details are  122.  CHAPTER  ELECTRONIC  SPECTRA  OF  ELECTRON  VIII  SOME  CARBONYL  IMPACT  COMPOUNDS  BY  SPECTROSCOPY  8.1. I n t r o d u c t i o n . O n l y a f e w c a r b o n y l compounds h a v e e n e r g y spectra reported presented  i n theliterature.  low r e s o l u t i o n  loss electron  impact  S i l v e r m a n and L a s s e t t r e [ 9 7 ]  s p e c t r a o f acetone and 2-butanone a t impact  e n e r g i e s o f 2 2 0 a n d 5 0 0 eV a n d s c a t t e r i n g a n g l e s o f 2° a n d 7 ° . was made t o l o c a t e t h e e l e c t r o n i c  states  reported  i m p a c t s p e c t r u m o f a c a r b o n y l compound  high resolution electron  is that o f formaldehyde, studied  i n t h e s e compounds.  No a t t e m p t  The o n l y  by W e i s s e t a l . [ 9 8 ] . A l t h o u g h no h i g h  resolution electron  i m p a c t work has been done on h i g h e r a l d e h y d e s and  k e t o n e s , t h e vacuum  ultra  violet absorption  some y e a r s a g o f o r f o r m a l d e h y d e [102,103] and a c r o l e i n  Some o f t h e t r a n s i t i o n s of the main d i f f i c u l t i e s  have been  [99,100], acetaldehyde [101],  (propenal) [104].  S a n d o r f y [ 1 0 5 ] have measured  spectra  o f some s i m p l e a l d e h y d e s .  have been a s s i g n e d t o v a r i o u s Rydberg s e r i e s . i n assigning  chosen i s wrong.  t o determine i o n i z a t i o n  electron  absorption  spectra.  potentials.  assignments i f t h e  Also, higher ionization  be d e t e r m i n e d e a s i l y f r o m o p t i c a l  potentials Presently,  cannot photo-  s p e c t r o s c o p y s t a n d s a s a c o n v e n i e n t a n d a c c u r a t e method t o  determine t h evarious  One  R y d b e r g s e r i e s a t t h e t i m e was t h e  T h i s c a n sometimes l e a d t o i n c o r r e c t o r c o n t r o v e r s i a l limit  acetone  More r e c e n t l y , L u c a z e a u and  t h e far-UV spectra  l a c k o f a n a c c u r a t e , i n d e p e n d e n t method  series  reported  i o n i z a t i o n p o t e n t i a l s o f compounds.  Using t h e  123.  measured  term v a l u e w i t h  respect  to various  ionization  p o t e n t i a l s and  t h e c a l c u l a t e d q u a n t u m d e f e c t , many e x c i t e d s t a t e s c a n b e a s s i g n e d the  b a s i s o f Rydberg  transitions.  . E n e r g y l o s s s p e c t r a o f t h e compounds an  s t u d i e d were d e t e r m i n e d a t  i m p a c t e n e r g y o f 100 eV a n d a s c a t t e r i n g a n g l e  used were o b t a i n e d  commercially  in  Photoelectron  8.2.Saturated  The s a m p l e s  further  purification.  to the e l a s t i c a l l y  s p e c t r a o f t h e compounds  l i t e r a t u r e ) were measured i n t h i s  s p e c t r a a r e shown  o f 2°.  and were used w i t h o u t  The e n e r g y l o s s s c a l e was f i x e d w i t h r e s p e c t peak.  on  studied  laboratory.  scattered  ( i f not reported  The  photoelectron  i n the appendix.  Aldehydes  8.2.1. F o r m a l d e h y d e and The g r o u n d  acetaldehyde.  s t a t e e l e c t r o n c o n f i g u r a t i o n and o u t e r  ( v e r t i c a l ) o f the formaldehyde molecule  (C  2 v  orbital  ) are given  energies  by T u r n e r e t a l .  [82] as (ls )  2  0  (ls )  2  c  ( l  a  ]  )  2  (2  9  ]  )  2  (3  3  l  )  2  (lb )  (lb,)  2  2  - 1 6 . 6 0 -16.01 The 2 b and  2  orbital  contains  the l b , orbital  essentially  C  2 v  the a  1  1  ]  - 1 4 . 3 9 - 1 0 . 8 8 eV electrons  t h e s t r o n g l y b o n d i n g e l e c t r o n s i n t h e CO Tr-bond.  t h e symmetry i s lowered  symmetry a r e c o r r e l a t e d w i t h b, o r b i t a l  .... A  2  2  t h e oxygen non-bonding  When o n e o f t h e h y d r o g e n a t o m s i s r e p l a c e d acetaldehyde,  (2b )  2  correlates with  and one a" o r b i t a l s  by a m e t h y l  to C . g  The a, and b  t h e a' o r b i t a l  t h e a" o r b i t a l s .  in C  g  2  orbitals in  symmetry  while  A t t h e same t i m e t w o  (CH b o n d i n g ) a r e i n t r o d u c e d .  c o n f i g u r a t i o n o f the acetaldehyde  group t o form  So t h e e l e c t r o n  m o l e c u l e c a n be r e p r e s e n t e d  as  124.  (ls )2  ( l s ^ )  (6a )  (2a")  Q  1  2  ( l s ^ )  2  (7a )  2  1  By a n a l o g y , 7 a  1  (la )  2  1  (2a«)  2  2  but  (4a«)  2  r e p r e s e n t s t h e oxygen non-bonding  i sdefinitely  i t s position  1  lower  relative  The e n e r g y  region with  i n t h e energy  7-11  series respect  studies  1  (la")  2  2  to other orbitals  l o s s range  of the l a "  i s n o t known. impact  spectrum o f  b e t w e e n 0 a n d 16 eV.  series  are observed  reported i n literature  [99,100].  has i t s f i r s t  e l e c t r o n s w h i l e 2a"  ( h i g h e r I P ) t h a n t h e 2a" and t h e 7a' o r b i t a l s ,  eV, s t r o n g Rydberg  t h e s , p and d s e r i e s  absorption  (5a )  position  Weiss e t a l . [ 9 8 ] have r e p o r t e d t h e e l e c t r o n formaldehyde  2  2  c o n t a i n s t h e CO T r - b o n d i n g e l e c t r o n s . orbital  (3a )  In t h e  a n d c a n be  from  identified  ultraviolet  A c c o r d i n g t o Weiss e t a l . [ 9 8 ] t h e f i r s t  member a t 7.10 eV and a t e r m v a l u e o f 3.78 eV w i t h  to the f i r s t  vertical  I . P . o f 1 0 . 8 8 eV d e t e r m i n e d  by PES [ 8 2 ] .  T h i s g i v e s a l a r g e q u a n t u m d e f e c t o f 1.11 a n d s o c o r r e s p o n d s  to a transfer  of  an e l e c t r o n  state 2b  to  t h e nsa-j o r b i t a l s .  correspond  from  the highest f i l l e d  a , , b, a n d b  molecules  of C  Rydberg  2 v  member o f t h e R y d b e r g 2  but the t r a n s i t i o n s  symmetry.  (0.39) i s r a t h e r  2  2b  2  t o two o f t h e  •+ n p b , a r e f o r b i d d e n i n  The quantum d e f e c t s f o r t h e s e n p b The f o u r t h  2  large f o rthis  2  a n d npa-j  Rydberg  member a t 8.88 e V , p r o b a b l y c o r r e s p o n d s  t r a n s f e r o f an e l e c t r o n f r o m 2 b  9.03  series  s e r i e s a r e 0-83 a n d 0.74 r e s p e c t i v e l y .  series, with the f i r s t  o f t h e ground  T h e n e x t t w o s t r o n g p e a k s a t 7.98 a n d 8.14 eV  to the f i r s t  orbitals:  orbital  to a  t o nd a l t h o u g h t h e quantum d e f e c t assignment.  One more s h a r p peak a t  eV may c o r r e s p o n d t o a n o t h e r c o m p o n e n t o f t h e s p l i t  3d  Rydberg  orbitals. We h a v e d e t e r m i n e d  the high resolution  electron  impact  energy  loss  125.  spectrum of acetaldehyde (Figure 26). 10.0  eV  The  vacuum u l t r a v i o l e t a b s o r p t i o n  r e g i o n and  Walsh [101]  and  t h e 6.8  L a k e and  spectrum determined of three  The  first  exhibits  to the  removal  (see F i g u r e  to the f i r s t  vibrational  these  [106]  of the  corresponding vibrational  2 6 ) T h e  vertical  I.P.  of  i s assigned The  i s too mode.  band t o c o n s i s t eV r e s p e c t i v e l y  p r e d i c t e d a t 8.70,  9.36,  a c c o u n t e d f o r by  9.67  9.43,  9.88  to these  utions to the  and  9.88  i n our  p e a k L a t 8.96  eV  eV.  The  4s +  without  B is  0.148  The  difference  3s + v^,  of  3s + v-|  the  probably  (C-H  a stretch)  c a l c u l a t e d quantum d e f e c t  eV  7 a ' -> ns  f o r n = 4,  5,  6,  spectrum w i t h energies P,  S,  U)  and  7 of  occurs  8.82,  have been  u p p e r s t a t e may  s p e c t r u m , D,  for  series  have c o n t r i b -  eV.  m o s t i n t e n s e peak i n t h e  with  In agreement  excited state, with  r e s p e c t i v e l y ( p e a k s K,  transitions.  1  also  excitation  t h e o t h e r members o f t h e  Peaks are observed eV  The  (7a ).  component  ± 0.005 eV.  while C i s assigned  respectively. and  i s 3.39  C i s 0.172  from t h i s  and  o f the f i r s t  eV  to the C = 0 s t r e t c h i n g i n the eV  7.14  s e p a r a t i o n between A and  peak A  i s 1.00  and  B i s t e n t a t i v e l y assigned  e n e r g y o f 0.148  -  photoelectron  t o a 3s R y d b e r g u p p e r s t a t e  l a r g e t o be So  10.21  e n e r g y o f 0.320 eV.  The  The  10.56  6.97  term v a l u e  with a vibrational  assigned  and  C a t 6.82,  w h i l e t h a t between B and  spacings  ^.71  i n t h e 6.8  non-bonding e l e c t r o n i n oxygen  B and  excitation.  same v i b r a t i o n a l  are  respectively.  l a b o r a t o r y shows t h e f i r s t  t h r e e c o m p o n e n t s , A,  ± 0.005 eV in  Harrison  eV  r e g i o n h a v e b e e n i n v e s t i g a t e d by  c o m p o n e n t s a t 1 0 . 2 1 , 10.37  w i t h Walsh [ 1 0 1 ] , t h i s any  eV  spectra  - 10.0  band i n t h e e l e c t r o n i m p a c t s p e c t r u m o f a c e t a l d e h y d e  respectively respect  - 8.4  in this  vibrational  corresponding  i n t h e e n e r g y l o s s r e g i o n o f 6.4  a t 7.47  eV.  8.0  Energy Loss(eV) F i g u r e 26.  Electron  9.0  i m p a c t s p e c t r u m o f a c e t a l d e h y d e a t 100 eV, 2 ° .  10.0  127.  T h i s g i v e s a t e r m v a l u e o f 2.74 (10.21  eV)  and  i s one  b^  become t h e a ,  a"  1  symmetry o f a c e t a l d e h y d e . broad  p e a k F a t 7.78  Rydberg s t a t e s  eV  (3p )  having  which are Rydberg  3p + v left  and  4  The  resolution  and  so t h e y  np'  of the  (3p).  np  4  o f two  seems t o  series  present  G can  The  4  series are  spectrometer p e a k s L, Q and regarded  then So  as  in C  be  lower  symmetry.  s  simply  t h e two  now  be  The 3p  and  i s not  p e a k s F and  and  9.74  for  9.72  eV  eV  the while  respectively.  enough t o s e p a r a t e  T a t 8.96,  9.51  G,  as  positions  9.45  due  explained  interpreted  calculated  9.50  i n the  a,,  peak D i s p r o b a b l y  a r e 8.91,  9.01,  I.P.  indicate  of the overlapping  sharper  [ 1 0 1 ] , can  (+v ).  1  allowed  upper s t a t e s .  Rydberg  be  This  vertical  Rydberg o r b i t a l s analogous  now  consists  by W a l s h  single  first  orbitals respectively  P e a k s E and  3p'•+ v  Peak M can  and  them  9.73  eV  (v^)  a v i b r a t i o n a l component  of  L. Peak H a t 8.43  first  vertical  is assigned  I.P.  eV  has  (10.21  a t e r m v a l u e o f 1.78 eV)  side. Small  T h i s may peaks  respect  and  thus  t o t h e t r a n s i t i o n t o one  peak H i s v e r y a s y m m e t r i c and  The  a'  probably  Rydberg  a p p e a r as  respectively. peak  to the  Three such o r b i t a l s i n f o r m a l d e h y d e , and  t r a n s i t i o n s 7 a ' -> 3 p  f o r the  3p  A l l three are  unassigned  h i g h e r members o f t h e those  split  w h i l e the r e l a t i v e l y  1  t o a s i n g l e component by  of the  the case of formaldehyde.  b-j and  with respect  t h e r e f o r e a q u a n t u m d e f e c t o f 0.77.  t h a t the upper s t a t e to  eV  be  I and  t o H may  caused J having  correspond  p e a k s N a t 9.24  eV  and  3d  with respect  of the  3d  shoulder  on  the  low  This energy  c o m p o n e n t s o v e r l a p p i n g one  o r two  small  the  and  vibrational  s t e p R a t 9.64  eV  0.27  can  be  another.  eV  quanta  the This  Rydberg o r b i t a l s .  e n e r g y s e p a r a t i o n s o f 0.13 t o one  to  a q u a n t u m d e f e c t o f 0.24.  shows a b r o a d  by o t h e r  eV  with  respectively.  assigned  128.  4d a n d  5d u p p e r  identified  7a  optical by W a l s h  a g r e e w i t h t h o s e g i v e n by W a l s h  ns s e r i e s  1  A summary o f t h e R y d b e r g  i n a c e t a l d e h y d e i s p r e s e n t e d i n T a b l e 7 and  o f t h e peaks The  states respectively.  i s one  spectroscopy.  The  the observed  [101] to b e t t e r  o f t h e l o n g e s t Rydberg  transitions energy  t h a n 0.02  eV.  s e r i e s observed i n  I.P. f o r t h e 7a' o r b i t a l  was  determined accurate  [ 1 0 1 ] f r o m h i s s p e c t r u m t o be 1 0 . 2 2 9 eV a n d t h i s  c o n s i s t e n t w i t h o u r d e t e r m i n a t i o n o f 10.211 ± 0.010  eV  value i s  by  PES.  8.2.2. P r o p i o n a l d e h y d e and i s o b u t y r a l d e h y d e . F i g u r e 27 shows t h e e l e c t r o n p r o p i o n a l d e h y d e and scattering all  impact s p e c t r a of a c e t a l d e h y d e ,  i s o b u t y r a l d e h y d e a t an  i m p a c t e n e r g y o f 100 eV and  angle over the energy l o s s range 3 - 1 3  eV.  The  t h r e e compounds b e t w e e n 3 t o 9 eV a r e v e r y s i m i l a r .  spectra  Also the  2°  of  electron  i m p a c t s p e c t r u m o f p r o p i o n a l d e h y d e ( F i g u r e 2 7 , m i d d l e ) o b s e r v e d by us i s i n g o o d a g r e e m e n t w i t h t h e vacuum u l t r a v i o l e t and  Simpson  [107],  In the energy  v e r y b r o a d a n d weak t r a n s i t i o n . electron configuration acetaldehyde.  f o r t h e normal  has  Barnes  there i s a  states of formaldehyde potentials  in relation  and  to these  and  the lowest configurations.  l o w e s t e n e r g y band s y s t e m c o r r e s p o n d s [ 1 0 8 ] t o t h e e x c i t a t i o n o f a  non-bonding in  by  [108] has e x p l i c i t l y g i v e n t h e  ionization  e n e r g y band s p e c t r a w e r e i n t e r p r e t e d The  studied  l o s s r e g i o n o f a b o u t 4 eV, Mulliken  Their structures,  spectra  electron  i n oxygen  t h e C = 0 b o n d , and w h i c h used  intensity  theoretically  lowest energy t r a n s i t i o n  which  is largely  has a n t i - b o n d i n g c h a r a c t e r .  calculations  predicted  t o an o r b i t a l  localised  McMurray  to help i n deciding which of the  transitions  s h o u l d be  identified  with  [109]  two  t h e weak,  c h a r a c t e r i s t i c of the unconjugated "^C  =  0  PEAK  A  OBSERVED ENERGY (eV)  TERM  3  VALUE ( e V )  CALCULATED ENERGY ( e V )  ASSIGNMENT  6.82 6.97 7.14  3.07 2.74 2.62 2.43 2.31  7a' ->• 3p  G  7.47 7.59 7.78 7.90  H I J  8.56 8.70  8.43  1.78 1.65 1.51  7a'  8.82 8.96  1.39 1.25  7a' -> 4s -»• 4s + v  L M  8.96 9.06  1.25 1.15  7a' •»• 4p 4p + v^  8.91, 9.01  N 0  9.24  • 9.38  0.97 0.83  7a' -» 4d + 4d + v  9.25  P  9.43  0.78  7a'  5s  9.36  Q  9.51  0.70  7a' -> 5p, 5p'  9.45, 9.50  R  9.64  0.57  7a' •*• 5d  9.61  S  9.71  0.50  7a'' + 6s  9.67  T  9.73  0.48  7a'  9.88  0.33 3.39  7a -t- 7s l a " + 3p  10.39  2.88  l a " + 3p  B C D  E F  K L  .U V  TABLE 7  3.39 3.24  7a'  3s -> 3 s +  CALCULATED QUANTUM DEFECT  1.00  -> 3s + v i 0.77  + 3p + vk -> 3 p ' * 3p' +  0.64  V t f  + 3d ->• 3d + vi» + 3 d + 2V!,  8.70 k  4  6p, 6p'  T  0.24  9.72, 9.74 9.83  Rydberg t r a n s i t i o n s i n acetaldehyde  a Assigned with respect to PES i o n i z a t i o n p o t e n t i a l s f o r acetaldehyde; 10.21, 13.3 14.2 is  130.  D  IOOeV,2  H  c  CH3CHO Q  x4  M I  CO  h-  100 eV, 2'  E  >CH  3  F  2  A  <• rr \m rr < >-  CH CH CHO  G  D  "  i 0_'t  H t  i-i  x4  CM f  H CO  LL)  h-  G J(CH ) CHCHO 3 2  B E  50eV,2  F  c  -'A  £ C  D *  "SI  100 eV, 2°  CM  -  6  Figure  27.  ]  1  ENERGY  1  8  1  r  LOSS (eV)  10  E l e c t r o n impact s p e c t r a o f a c e t a l d e h y d e , and i s o b u t y r a l d e h y d e a t 100 e V , 2 ° .  12  propionaldehyde  131.  group.  In terms o f t h e  localised  used,, one  of these  transitions  intensity  f o r the other allowed  transition  w i t h the  transition  s h o u l d , t h e r e f o r e , be  In t h i s  electrons  on  intensities  Tr-bond.  spectrum of  orbital.  and  50  ( F i g u r e 27,  the  intensity  eV  and  e x t e n d s up  fine  become d i f f u s e structure  i n the  and  The  any  The  of these  peaks i n the  the  TT  antiis  e l e c t r o n impact  i m p a c t e n e r g i e s , 100 50 eV  to the o p t i c a l l y eV  spectra, allowed  optical  This t r a n s i t i o n  Extensive optical  starts  s t u d i e s have  (see r e f . 62).  a l a r g e number o f b a n d s w i t h at higher  a continuous  energy the  absorption.  The  resolution  o f t h e 0-0  of our  band may  the be  i s not ,  fine structures. and  a  bands  for  spectrometer  spectra of propionaldehyde  At  vibrational  contradictory values  position  eV  impact  t o n -> TT* by  spectra of aldehydes  and  of  a-bond and  i n aldehydes  c a s e o f 100  transition  even the  transition  lone p a i r  that i n the  established.  complicated  literature.  other  i n the  transition,  merge i n t o  f r e q u e n c i e s and  s u f f i c i e n t t o see  relative  s t r u c t u r e are observed;,  i s extremely  vibrational found  region of the  by  different  t o a b o u t 5 eV..  energy s i d e of t h i s  complicated  transition  I t i s obvious  seems t o be w e l l  This  s u p e r s c r i p t * i n d i c a t e s an  a t two  transition  be  transition.  non-bonding  corroborated  i s much l a r g e r t h a n  been made o f t h i s low  be  the assignment of t h i s  spectroscopists  the  run  bottom).  o f t h e 4 eV  p e a k A a t 6.7  b e l o w . 3. eV  the  T h a t t h e 4 eV  isobutyraldehyde  So  f o r the  large to  a s c r i b e d to the forbidden  The  o p t i c a l l y f o r b i d d e n can  energy.  approximations  t h e o x y g e n a t o m ; a t h e e l e c t r o n s i n t h e C-0  bonding m o l e c u l a r  eV  t h e AO  i s much t o o  observed  notation, n represents  t h e e l e c t r o n i n t h e C-0  actually  and  i s forbidden w h i l e the c a l c u l a t e d  compatible  n -> IT*.  low  LCAO MO  isobutyr-  132.  aldehyde  c a n a l s o be  following broader  interpreted  i n terms  the example o f a c e t a l d e h y d e .  o f Rydberg  I t seems t h a t t h e p e a k s become  i n t h e s e more c o m p l e x m o l e c u l e s and  i s observed.  The  assignments  little  a r e summarised  vibrational  although not w e l l o n l y appear  eV  i s assigned  resolved,  as a b r o a d  not d i s t i n g u i s h e d .  The  8.79  eV.  the second broad may  4p a p p e a r s  s m a l l b u l g e F a t 9.13  t o e i t h e r if^ -> 5s o r ^ is likely  3s.  third  peak V i n t h e spectrum  be t h e e n v e l o p e  t h a t the term  of the o r i g i n a l  -> 3p  the d i f f e r e n t  appears  eV  on  broad  and  ramp may  eV  transitions  from The  w h i l e C and This in  similar  a t about  reasoning behind t h i s are q u i t e  to  10.4  eV  different assignment  independent  [64,65].  s m a l l p e a k s C and  s i d e o f p e a k B a t 7.40  D a r e a s s i g n e d t h e two  i s a reasonable assignment  f o r each  eV.  vibrational  i f the decrease  3p t e r m v a l u e s i n g o i n g f r o m CH^CHO t o  t h e same ( 0 . 3 eV)  9.67  t h e l a " (n o r b i t a l ) t o  transitions  s t r u c t u r e a t a l l b u t two  on t h e h i g h e n e r g y  A  and  correspond  leading  respectively.  F o r i s o b u t y r a l d e h y d e , t h e ^ - j -»• 3s t r a n s i t i o n A a t 6.69 no v i b r a t i o n a l  E a t 8.44  p e a k G w i t h a maximum a t  of acetaldehyde maximising  v a l u e s f o r Rydberg  components a r e  as a s h o u l d e r C a t  the increasing  13,2  orbital.  structure,  transitions  a s s e p a r a t e p e a k s D and  of the t r a n s i t i o n  orbital  The  o f many R y d b e r g  I . P . a t 12.4  c o m p o n e n t s o f t h e 3p R y d b e r g is  The  t o be t h e e n v e l o p e  and  and  •-»• 3d t r a n s i t i o n  eV w h i l e i ^ ' -> 4s and  eV  eV  In p r o p i o n a l d e h y d e ,  some v i b r a t i o n a l  evident.  p e a k B a t 7.42  8.21  The  3s and  is still  structure  i n T a b l e 8, w h e r e iji  represents the nth highest f i l l e d molecular o r b i t a l . t h e peak A a t 6.78  transitions  eV  D do  shows appear  So B i s a s s i g n e d  3p  c o m p o n e n t s o f ty-^ -»• 3p*. ( c o m p a r e T a b l e s 7 and  (CH^CHCHO  i s assumed t o  3p c o m p o n e n t ( s e e T a b l e s 7 and  8).  The  be  8)  CH CH CHO P r o p i o n a l d e h y d e 3  PEAK  (CH^CHCHO  2  ENERGY (eV)  TERM VALUE (eV)  isobutyraldehyde  3  TRANSITION  PEAK  TERM VALUE (eV) 3  ENERGY (eV)  TRANSITION  1  A  6.78  B  -y  *1  7.42  C  •l  8.21  , *1  D  8.44  E  8.79  F  9.13  *1  9,7  ^2  G  *1 .  '  *1  ^3  3s  3.21  3p  2.57  3d  1.78  6.69  B  7.40  C D  7.69 7.83  i  4s  1.55  4p  1.20  E  8.11  5s  0.86 3.24  \ F  8.63  -V 3s -V 3p ->  A  3s  .  2.7 3.5  G  TABLE  8  Electronic Transition  i n propionaldehyde  -> 3p  2.42  *1 *1 *1  9.4 10.0  3.13  1  •*3  H'  3s  *1  2.13 1.99  3d 4s  1.70  4p  1.19  -> 3p 3s  2.6 3.2  3p 3s  2.6 3.4  If; 3  n  3p'  -»•  r.nd i s o b u t y r a l d e h y d e  a) a s s i g n e d w i t h r e s p e c t t o PES i o n i z a t i o n p o t e n t i a l s ( f o r p r o p i o n a l d e h y d e ; 14.1 eV and f o r i s o b u t y r a l d e h y d e ; 9.82, 12.0, 12.6, 13.4 and 14.1 e V ) .  9.99, 12.4, 13.2, 13.7, and  134.  -*• 3d a n d  4 s a r e p r o b a b l y t o o c l o s e i n e n e r g y t o be r e s o l v e d a n d  t h e y a p p e a r a s t h e a s y m m e t r i c p e a k E a t 8.11 eV. p r o b a b l y a p p e a r s a s peak F a t 8.63 eV. the  e n v e l o p e o f Rydberg s t a t e s  leading  The  •> 4p  transition  B r o a d peaks G and H s h o u l d to higher  I.P.'s l i s t e d  be  i n T a b l e 8.  8.2.3. R y d b e r g a n d v a l e n c e a s s i g n m e n t s . T h e R y d b e r g a s s i g n m e n t i s by no means t h e o n l y the  spectra of aldehydes.  F o r example, t h e t h r e e h i g h e s t f i l l e d 2  orbitals  i n acetaldehyde,  and n e l e c t r o n s  interpretation of  (6a ) 1  2  2  (2a")  r e s p e c t i v e l y mentioned  molecular  (7a ) 1  c o r r e s p o n d t o t h e a , TT  i n the l a s t  paragraph.  So t h e t h r e e  t r a n s i t i o n s o f lowest energy could  be n -> TT*, n -»• a* a n d v •> TT* o n t h e  valence t r a n s i t i o n  [ 1 0 1 ] has a l s o  in  picture.  t h e 6.8 eV r e g i o n  Walsh  i n a c e t a l d e h y d e , a s b e i n g due t o t h e 7 a  t r a n s i t i o n , a n d / o r t h e n -> a * t r a n s i t i o n . at  7.5 eV was c l a s s i f i e d  the  inter-vlaence-shell  classify  transition  TT -> IT*.  e i t h e r as a p e r t u r b e d  s t a t e , e . g . , t h e 3s R y d b e r g The p h o t o e l e c t r o n  a s h a r p band a c c o m p a n i e d by  structure with the v e r t i c a l  I . P . a t 10.21 eV.  band i s b r o a d a n d p e a k s a t a b o u t 13.3 eV. at  I f t h e ir  a b o u t 7,5 e V , t h e b i n d i n g e n e r g y o f t h e TT* o r b i t a l  5.8 eV a n d we e x p e c t t h e n •> TT* t r a n s i t i o n 10.2 - 5.8 = 4.4 eV. transition  picture  This  •> 3 s  So i t a p p e a r s p o s s i b l e t o  c a n a l s o be c o n s i d e r e d a s t h e a* o r b i t a l .  vibrational  1  7 a ' -> 3p a n d / o r  t h e u p p e r s t a t e s o f t h e s e two t r a n s i t i o n s  s p e c t r u m o f a c e t a l d e h y d e shows f i r s t  the transitions  A t t h e same t i m e , t h e t r a n s i t i o n  as a Rydberg t r a n s i t i o n ,  Rydberg s t a t e o r as an i n t e r - v a l e n c e - s h e l l orbital  identified  The s e c o n d  IT* t r a n s i t i o n  occurs  i s 13.3 - 7.5 =  t o be b r o a d a n d o c c u r a t a b o u t  indicates that the inter-valence-shell  i s also consistent with the observed electron  impact  135.  s p e c t r u m and explaining by W a l s h We  photoelectron  the occurrence  there  o f p e a k s F and  G  i s some d i f f i c u l t y  ( F i g u r e 26)  left  in  unassigned  [101]. are  i n favour  mathematical  of the Rydberg assignment because of the  f i t f o r acetaldehyde  o f a l d e h y d e s can assignments are [67]  s p e c t r u m but  be  and  the  fact  i n t e r p r e t e d e a s i l y and  s u g g e s t e d by L u c a z e a u a n d  have c l a i m e d  R y d b e r g s t a t e s by  running  the  optical  that the o v e r a l l  systematically. Sandorfy [105].  t h a t Rydberg s t a t e s can  good spectra  Similar Basch e t a l .  be d i s t i n g u i s h e d f r o m  spectra at high  pressures  nonor  in a  c o n d e n s e d p h a s e ( i . e . , as a s o l u t i o n o r as a p o l y c r y s t a l l i n e f i l m ) w h e r e the  Rydberg t r a n s i t i o n s  should  be  s u p p r e s s e d and  o n l y show b r o a d f e a t u r e l e s s c o n t i n u o u s assigned in  to valence  shell  excitations.  t h e w o r k o f L u c a z e a u and  8.2.4. E f f e c t o f a l k y l In the  previous  derivatives energies  Figure  28,  a g a i n s t the  the sum  of the carbonyl  3s  net  on  the  be  s u b s t i t u e n t s on  be  clearly  seen  p o l a r e f f e c t of the  (£a*)  I.P.,  values  the  of the  Rydberg  alkyl  orbital  v a l u e s , which are a q u a n t i t substituent.  3 s , 3p  of the  are approximately  d e r i v a t i v e s of water, the  term v a l u e s  e f f e c t can  e l e c t r o n impact s p e c t r a  u s i n g T a f t a*  o f t h e T a f t a* group  bands w h i c h can  [105].  effect of alkyl  p l o t of the f i r s t  case f o r the a l k y l f o r the  chapter  been d i s c u s s e d  a t i v e measure o f the  Sandorfy  This  spectra then u s u a l l y  substituents.  of water the  has  absorption  the  and  3d  As term  s u b s t i t u e n t s on  i s greater than that f o r the  3p  values both  straight lines. slope of the  shown i n  As  sides in  the  straight line  term  values.  136.  3d term  U ~ T — — i  0.2  F i g u r e 28.  0.3  1  0.4  1  0.5  E f f e c t of a l k y l first  1  0.6  1  0.7  n—~n  0.8  s u b s t i t u t i o n on R y d b e r g t e r m  ionization potentials- in  r~  0.9 values  aldehydes.  1.0 and  137.  Finally, The  the s t r a i g h t  l i n e f o r t h e 3d t e r m v a l u e has  e f f e c t of r e p l a c i n g  one  hydrogen  the s m a l l e s t  i n formaldehyde  slope.  by an a l k y l  substit-  u e n t i s t o l o w e r t h e I . P . a n d t h e 3s t e r m v a l u e m o r e t h a n t h e 3p value.  The  c h a n g e i n t h e 3d t e r m v a l u e i s u s u a l l y  small.  term  This  effect  has b e e n e x p l a i n e d  by t h e d i f f e r e n t d e g r e e s o f p e n e t r a t i o n o f t h e  Rydberg  There  the  orbitals.  band A t o w a r d s  replaced  by m e t h y l  i s a s h i f t o f a b o u t 0.2  towards  when t h e a - h y d r o g e n i n a c e t a l d e h y d e i s r e p l a c e d reflected  eV o f t h e p o s i t i o n  l o w e r e n e r g y when t h e h y d r o g e n and a s m a l l e r s h i f t  of formaldehyde i s  lower energy by m e t h y l .  than that f o r the f i r s t  I.P.  qualitative analysis of aliphatic from the e l e c t r o n  aldehydes  i m p a c t s p e c t r u m and  s u b s t i t u e n t s from the p l o t of the f i r s t spectroscopy) against  I.P.  to determine the  aldehydic  alkyl  by p h o t o e l e c t r o n  Ea*.  and  2-butanone.  i n s e c t i o n 8.2.1.  T u r n e r e t a l . [ 8 2 ] on f o r m a l d e h y d e  Based and  on t h e p h o t o e l e c t r o n w o r k o f i t s symmetry c o r r e l a t i o n  acetaldehyde, i t i s suggested that the h i g h e s t f i l l e d t h e s e two compounds c o n t a i n s t h e n o n - b o n d i n g  atom a n d  method f o r  i s to recognise the  (obtained  impact  e l e c t r o n c o n f i g u r a t i o n s o f f o r m a l d e h y d e and a c e t a l d e h y d e have  been d i s c u s s e d  in  This i s  Ketones.  8.3.1. A c e t o n e The  i s observed  Since the e l e c t r o n  spectra of these aldehydes look very s i m i l a r , a p o t e n t i a l  8.3.Saturated  of  i n t h e s l o p e o f t h e s t r a i g h t l i n e f o r t h e 3s t e r m v a l u e b e i n g  somewhat s m a l l e r  group  different  the second  highest f i l l e d  to  molecular orbital  e l e c t r o n s on t h e  molecular orbital  contains  the  oxygen  138.  electrons of for  Tr-bond.  i n t h e C-0  We  have o b s e r v e d t h e p h o t o e l e c t r o n  a c e t a l d e h y d e a n d a c e t o n e t o be v e r y s i m i l a r , the f i r s t  two  bands,  except f o r a s h i f t  e n e r g y when one more m e t h y l  group  acetaldehyde to give acetone.  ionization.  ordering  As  bonding  M.O.  (i^)  at  and  has  over the energy  been d i s c u s s e d  loss  to Rydberg  l o s s r e g i o n 6.0  scattering  angle.  have been s t u d i e d and of  Tables  studied Duncan  orbitals  9 and  are  available that  the  acetaldeelectrons  4.3  f o r aldehydes  eV.  The  10. The  - 9.7  eV  a t an  five  I.P.'s  vacuum u l t r a v i o l e t  in section  series  h i g h e r t h a n an  I . P . o f 9.705 eV  spectra  to assign  the  spectra  found l a t e r  2-butanone  at the  spectrum o f acetone On  o f 10.26  eV  o f t h e s e compounds  are l i s t e d  Duncan [ 1 0 2 ] .  converging to a l i m i t  studies.  8.2.2.  i m p a c t e n e r g y o f 100  [ 1 0 2 ] s e l e c t e d , f r o m t h e numerous e l e c t r o n i c  i n t o a Rydberg  T h i s i s t h e n -> IT*  impact s p e c t r a o f a c e t o n e * and  photoelectron  the f i r s t  broad  transitions.  by N o y e s e t a l . [ 1 1 0 ] and  photoionization  in  o f ketones c o n t a i n a  t h e example o f a l d e h y d e s , i t i s p o s s i b l e  h i g h e r energy  2°  hydrogen  atom.  F i g u r e 29 shows t h e e l e c t r o n  and  to lower  c o n t a i n s the non-bonding  r e g i o n o f weak t r a n s i t i o n m a x i m i s i n g a t a b o u t  Following  eV  least  i s t h e same f o r a c e t o n e and  i n the case of aldehydes, the spectra  transition  0.7  the aldehydic  more C-H  highest o r b i t a l s  the highest f i l l e d  on t h e o x y g e n  o f about  Consequently, i t i s not unreasonable t o b e l i e v e  o f t h e two  hyde so t h a t  (see appendix) a t  Of c o u r s e , more bands a r e o b s e r v e d f o r  acetone than a c e t a l d e h y d e because for  replaces  spectra  bottom was  the b a s i s o f  intensity,  states, three that f i t eV.  This value i s  by W a t a n a b e [ 1 0 3 ]  from  S u b s e q u e n t l y Watanabe [ 1 0 3 ] r e a n a l y s e d t h e  V e r y r e c e n t l y a p a p e r a p p e a r e d ( H u e b n e r e t a l . , J . Chem. P h y s . 59_, 5434 ( 1 9 7 3 ) ) r e p o r t i n g t h e h i g h r e s o l u t i o n e l e c t r o n i m p a c t s p e c t r u m o f acetone. A s i m i l a r i n t e r p r e t a t i o n ( i . e . i n terms o f Rydberg t r a n s i t i o n s ) t o t h a t o f t h e p r e s e n t w o r k has b e e n g i v e n .  CH COCH -100eV,2° ' 3  39  3  cy j r—i 1 17.01 1 1 1 18.01 1—~— i • i 19.0 r  16.0 CD  §  CH COC H -100eV2° 3  j  2  5  11111 11111 11111  6.0  7.0  8.0  ENERGY F i g u r e 29.  9.0  LOSS (eV)  E l e c t r o n i m p a c t s p e c t r a o f CHgCOCHg a n d C H C 0 C H 3  • a t 100 e V , 2°  2  r  140.  optical  spectrum  [ 1 0 2 ] and f o u n d a d i f f e r e n t s e r i e s o f e l e v e n members  c o n v e r g i n g t o t h e I . P . a t 9.705 The we  f i r s t member o f t h i s  this  transition.  PES m e a s u r e m e n t o f 9.71 fit  a 4s Rydberg  eV f o r t h e f i r s t  9.32  eV  9.12  and  9.30  Tp.-j -v 3s t r a n s i t i o n  the  the f i r s t  vibrational with  t o o c c u r a t 5.96  component o f t h e f i r s t spacing of this  The  ^6.5  eV  to ^  as the v i b r a t i o n a l mode e x c i t e d  ± H.  3s p l u s v i b r a t i o n a l  So  i f we  eV.  state.  The  analysis of this  regard the f i r s t  Rydberg  [67]).  first  Lawson  unreasonable 3s  Rydberg  eV d i f f e r e n c e b e t w e e n t h e o b s e r v e d a n d  Large d e v i a t i o n s of observed energy of the  s t a t e from the c a l c u l a t e d  The  average  vibrational  band A i n t h e a c e t o n e s p e c t r u m a s t h e  molecules conforming to C  energy  band i n a c e t o n e .  to  energies.  predicts  assign the  i n the case of the spectrum o f a c e t a l d e h y d e , i t i s not  calculated  The  2 v  a f f e c t t h e energy o f Rydberg  v a l u e have a l s o  symmetry ( e . g . w a t e r  h i g h e r s y m m e t r y may  8.82,  T h i s i s comparable  As  t r a n s i t i o n d e s p i t e t h e 0.4  and  from the  d e f o r m a t i o n as s u g g e s t e d by  Duncan [ 1 1 1 ] f r o m t h e v i b r a t i o n a l  been o b s e r v e d  the 3s  i n other  [64], ethylene oxide  be t h e c a u s e o f some p e r t u r b a t i o n s  states.  Weiss  our  c o m p o n e n t s , H c a n be a s s i g n e d  c o m p o n e n t o f t h e 4s R y d b e r g  i s probably the methyl  9.14  formula then  0.005 eV.  eV  1.09.  K, L and M a t  s h a r p band by 0.4  t h e s p a c i n g between t h e peaks G and  band a t  Rydberg  a t 8.81,  eV, w h i c h d i f f e r s  b a n d i s 0.135  this  T h i s seems t o  quantum d e f e c t o f  So t h e p e a k s  eV a r e a s s i g n e d a s s u c h .  on  eV w i t h r e s p e c t t o  ns s e r i e s a r e p r e d i c t e d 7.  Based  I.P. o f a c e t o n e .  upper s t a t e w i t h a c a l c u l a t e d  r e s p e c t i v e l y f o r n = 5, 6,  eV.  i m p a c t s p e c t r u m a t 8.09  I t s t e r m v a l u e i s 1.62  H i g h e r members o f t h i s ^ -j  and  s e r i e s o c c u r s a t 8.09  c o r r e l a t e t h e s t r o n g e s t peak i n t h e e l e c t r o n  (G) w i t h  of  eV.  [ 1 2 1 ] has  suggested  the  which  141.  perturbation as  o f a t o m i c R y d b e r g o r b i t a l s by  TT -> TT*.  This  perturbation  immediate v i c i n i t y It states  3d.  0.28.  5d  a l k y l a t i o n [64].  The  The So  and  and  assigned the  6d  step  J a t 8.69  eV  9.12  and  and  7s  9.30  are  eV  t o now  series. t o be  I f we  4p,  the  B a s e d on  eV  the  3d  the  higher  eV  predicted  9.21  eV  the  p e a k s D,  assume t h e  t e r m v a l u e and  respectively.  3p  E and  The  case of the  s h o u l d e r E can Table 9 gives  be  9.10  to the  eV  d e v i a t i o n of the  I.  that  (1.25  ( c f . the 3s  a summary o f o u r  9.26  eV eV  7.42  (1.30 and  and  been  the  belong  eV  5p  3p  A  3p  4d  and The and  6p the  perturbalso  s t a t e from  a n o t h e r component o f the  to  and  d i f f e r s from  R y d b e r g s t a t e may  L  left  4s  be the the  manifold.  assignments.  spectrum of 2-butanone l o o k s  very s i m i l a r to t h a t of  the  assigned  0.77).  case f o r 3s).  p o s i t i o n of the  peaks  R y d b e r g a s s i g n m e n t i s a s s u m e d f o r peak D,  i n t e r p r e t e d as  is  and  are  T h e y may  quantum d e f e c t  peak D a t  f o r the  I f the  be  I between t h e  s t a t e a t 6.99  s i m i l a r to that  value.  spectrum to  Rydberg upper s t a t e and  features  shoulder  0.4  calculated  in  members a t 8.73,  only  a l s o by  f o r the  eV)  have c o n t r i b u t i o n s  v a l u e o f 3p  responsible  i n the  (1.78  r e s p e c t i v e l y w h i c h have p r e v i o u s l y  are  Rydberg  c a l c u l a t e d quantum d e f e c t  i s a s s i g n e d a 4d  The  the  remain e s s e n t i a l l y  term v a l u e  comparable to those of acetaldehyde  and  The  t e r m v a l u e s o f 3d  the  and  localised in  extensive.  peak F a t 7.87  Rydberg f o r m u l a then p r e d i c t s the  ation  be  be  lone pair electrons  Rydberg upper s t a t e s .  transitions  a t 8.95  can  Rydberg t r a n s i t i o n s s h o u l d  \\>i -»• np  0.76)  but  necessarily  fact that  t e r m v a l u e i s 1.84  u n a c c o u n t e d f o r up the  not  Rydberg f o r m u l a p r e d i c t s  the  M at  t o 6s  TT -* TT*  o f compounds c o n t a i n i n g  a c e t a l d e h y d e , we  eV.  the  is a well-established  c o n s t a n t on  \\>1  of  may  valence type t r a n s i t i o n s such  acetone,  OBSERVED TERM ENERGY (eV) . VALUE ( e V ) A  ASSIGNMENT  a  CALCULATED ENERGY (eV)  CALCULATED QUANTUM DEFECT  B C  6.36 6.50 6.63  3.35 3.21 3.08  i'l  D E  7.42 7.55  2.29 2.16  <h -> 3p? - 3p»  F  7.87  1.84  3d  0.28  G H  8.09 i U.22  1.62 1.49  4»i -»- 4s -> 4s + V2  1.09  I  8.41  1.30  $1  0.76  J  8.69  1.02  K  8.82  0.89  i>\  3S? ->• 3s + V2 3s + 2v2  5.96  6.99  •*• 4p •*• 4d  8.73  -»- 5s  8.81  *1 -- 5p  8.95  ;  L  9.12  0.59  *1 -> 5d * 6s  9.10 9.14  M  9.30  0.41  -•- 6p *1 -> 6d 7s  9.21 9.29 9.32  TABLE 9  Rydberg t r a n s i t i o n s i n acetone  (a) Assigned with respect to PES i o n i z a t i o n p o t e n t i a l s o f 9.71, 12.6, 13.5, 14.1,  143.  particularly  i n t h e 6.0  - 8.5  exhibits four vibrational 0. 005 eV, w h i c h first  vibrational  eV on r e p l a c i n g one  as i n 2 - b u t a n o n e .  Rydberg  spectrum. is  The  series The  1. P.'s than  The  I . P . o f 9.52  o f the a-hydrogens  nd.  t o Rydberg  continuum  transitions  ( 1 2 . 6 , 13.5  spaced  and  eV  a  into  comparison  acetone  i n 2 - b u t a n o n e a b o v e 9 eV  leading  eV).  i n terms  assignment of the  to the second  i n 2-butanone  14.1  29  i s the e x i s t e n c e of the  seems t o a f f i r m o u r R y d b e r g rising  ±  i n acetone  be f i t t e d  A summary a n d  only uncertainty  w h i c h a r e more c l o s e l y  i n acetone  band  term v a l u e of the  i n t e r p r e t a t i o n o f the 2-butanone spectrum  h i g h e r and  p r o b a b l y due  first  and o b s e r v e d e n e r g i e s i s g i v e n i n F i g u r e  ( b o t t o m ) a n d T a b l e 10. K and 0.  The  In c o n t r a s t t o the case o f a c e t o n e ,  s e r i e s ^ - j -*• n s , np a n d  between the c a l c u l a t e d  of  The  i n the spectrum o f 2-butanone can e a s i l y  t h r e e Rydberg  peaks  i n acetone.  component w i t h r e s p e c t t o t h e f i r s t  by a m e t h y l g r o u p features  loss region.  c o m p o n e n t s w i t h an a v e r a g e s p a c i n g o f 0.135  i s t h e same a s t h a t  i s d e c r e a s e d t o 3.19  the  eV e n e r g y  and h i g h e r  ( 1 2 . 3 , 12.6  and  13.0  I t seems t h a t t h e p e a k s  eV)  i n the  b i g g e r m o l e c u l e 2-butanone a r e i n t r i n s i c a l l y  broader than those i n acetone  since the v i b r a t i o n a l  band i s l e s s w e l l  for  i n the f i r s t  two a s I f we  peaks  E, F a n d  i n acetone.  The  F' a r e s e e n  s h o u l d e r F'  assume 2-butanone t o c o n f o r m  3p m a n i f o l d a r e o f a ' , a  1  electron  a n d 30 eV.  i n the  is.less to C  s  3p r e g i o n  instead  intense than the other  of two.  symmetry, t h e t h r e e components o f  symmetries  impact spectra a t d i f f e r e n t (See F i g u r e 3 0 ) .  -j  and a" symmetry.  t h r e e components t o t h e d i f f e r e n t the  resolved  t h e same v i b r a t i o n a l s p a c i n g . Three  the  structure  can  The  assignment of the  be a c h i e v e d by  studying  i m p a c t e n e r g i e s , 100 eV,  70  At these impact e n e r g i e s , the i n t e n s i t y  eV of  p ..v  OBSERVED ENERGY (eV)  TERM VALUE ( e V )  A B C D  6.33 6.46 6.59 6.72  3.19 3.06 2.93 2.80  E  7.26 7.41 7.57  2.26 2.11 1.95  H  7.71, 7.90'  1.81 1.62  I J  8.06 8.19  1.46 1.33  *1 -y 4s y 4s +  8.07  K? L  8.53  0.99  *1 -y 4p y 4p' -y 4d *1  8.38 8.43 8.55  M  8.68  0.84  N  8.85  0.67  F  F F» G  0?  CALCULATED ENERGY (eV)  A ^ i u v xm  a  *1  3s 3s + v 3s + 2v2 -y 3s +  ->  0.94  2  fl y *1 ->• *1 y y •*1 -y  3pa' 5pa" 3pa'  0.55 0.46  3d 3d»  0.26 0.10  •y 4d' 5s  8.63 8.70 . 8.83 8.S6  fi  -y 5p -y 5p* y 5d  fi  8.92  P  9.00  0.52  fi  -y 6s  8.99  Q  9.11  0.41  fi fi  y 6p •y 7s  9.07 9.15  TABLE 10 (a) Assigned  CALCULATED QUANTUM DEFECT  Rydberg t r a n s i t i o n s i n 2-butanone  with respect to PES i o n i z a t i o n p o t e n t i a l s o f 9.52, 12.3, 12.6, 13.0, 14.3 cV  F i g u r e 30.  E l e c t r o n impact s p e c t r a of the f i r s t  two  bands o f C H C 0 C H 3  2  5  a t 30,  70 and  100  eV.  146.  t h e peak E and 0.51  a n d 0.51  f o r E a n d 0.35,  the r e l a t i v e i s lowered  p e a k F' r e l a t i v e  t o peak A r e m a i n q u i t e c o n s t a n t ( 0 . 5 3 ,  0.37  a n d 0.39  f o r F').  t h e o t h e r hand,  i n t e n s i t y o f peak F t o peak A i n c r e a s e s as t h e i m p a c t  ( 0 . 5 1 , 0.53  impact energy).  and  0.68  r e s p e c t i v e l y a t 100 eV,  70 ev and  T h i s seems t o i n d i c a t e t h a t p e a k E and  t h e same s y m m e t r y and  so a r e a s s i g n e d a'.  The  F' may  different  peak F on d e c r e a s i n g t h e i m p a c t e n e r g y s u g g e s t s t h a t a different  On  energy 30  ev  belong to  behaviour of  i t s h o u l d be a s s i g n e d  symmetry ( a " ) .  8.3.2. H i g h e r k e t o n e s . F i g u r e 3 1 a , b , c shows t h e e l e c t r o n ketone, methyl  isopropyl  impact s p e c t r a o f methyl  k e t o n e and m e t h y l  t-butyl  ketone  isobutyl  respectively  o v e r t h e e n e r g y l o s s r e g i o n o f 6 - 10 eV a t a n i m p a c t e n e r g y o f 100 and  2° s c a t t e r i n g a n g l e .  spectra of methyl  Duncan [ 1 1 2 ] has s t u d i e d t h e vacuum  n-propyl ketone, methyl  ketone i n the energy r e g i o n their  vibrational  2-butanone. Rydberg  6.2  eV.  The  s t r u c t u r e were d i s c u s s e d  k e t o n e and  ultraviolet diethyl  electronic transitions  and  i n comparison with acetone  and  I t i s c o n c l u d e d t h a t most t r a n s i t i o n s a r e p r o b a b l y o f a  type, leading  t o I.P.'s  and D u n c a n [ 1 1 3 ] d i s c u s s e d  i n the neighbourhood  I n T a b l e 1 1 , we  i n the s p e c t r a t o Rydberg  v a l u e s and t h e c a l c u l a t e d  Holdsworth  g r o u p s on t h e i n t e n s i t y o f t h e i r  i n t h e vacuum u l t r a - v i o l e t .  most s t r u c t u r e s  o f 10 eV.  the e f f e c t of consecutive s u b s t i t u t i o n of  a - h y d r o g e n s i n a c e t o n e by m e t h y l transitions  - 8.3  isopropyl  eV  quantum d e f e c t s .  transitions The  electronic  have a s s i g n e d  b a s e d on  broad peaks  term  at high  e n e r g y l o s s a r e p r o b a b l y t h e e n v e l o p e o f m o r e t h a n one t r a n s i t i o n . i s noted that i n these higher ketones, the spectra only  the  show b r o a d  It features  147.  |100eV,2° E  lCH ) CHCH C0CH  p  3  P  CO  0  (CH ) CHCOCH 3  E  2  B  CD  cr <  CO  o  a; H  A B  >- (a) tr < er  2  H (b)  LU  V  h-  B  D  (CH ) CCOCH 3  3  (c)  T  F i g u r e 31.  ENERGY  Electron 2  3  9  LOSS(eV)  impact spectra of  (CH ) CHC0CH 3  8  (CH^CHCf^COCH^  and ( C H ) C C 0 C H 3  3  10  3  a t 100 e V , 2 ° .  3  :  (CH ) CH COCH 3  PEAK  6.45  B  6.53  D  2  7.34 7.73  *1  -+ 3s 3s + V2  *1 + 3p *1  (CH ) CHOCH  3  ENERGY (eV) TRANSITION  A  C  2  3d  E  7.97  *1 •+ 4s  F  8.27  *1 •> 4p  G  8.48  *1 •> 4d *1 + 5s *2 -»• 3s  3  TERM VALUE (eV) 2.97  PEAK A  6.41  B  7.19  2.08  C  1.69 1.45 1.15 0.94 2.9  9.0  *3  3s  2.9  I  9.7  <"+  3s  3.2  *1  -y  (CH ) CC0CH  3  ENERGY (eV) TRANSITION 3s  3  TERM VALUE (eV)'  PEAK  3  ENERGY (eV) TRANSITION  3  TERM VALUE (eV)  2.95  A  6.40  ih •*  3s  2.81  *1 - 3p  2.17  B  7.12  *1 -> 3p  2.09  7.84  *1 •* 3d?  1.52  D  7.97  *1 •+ 4s  1.39  C? D  7.6 7.79  i-i + 3d i>i 4s  1.6 1.42  E  8.22  *1 -> 4p  1.14  E  8.05  ij>l •*• 4p  1.16  F  9.2  ^3 ->- 3s  3.1  F  8.33  G  9.9  *3 - 3p •*• 4s  2.4 2.7  2.89  H  TABLE TABLE 11 11  f  2  EE ll ee cc tt rr oo nn ii cc tt rr aa nn ss ii tt ii oo nn ss i n methyl i s o b u t y l ketone, methyl isopropyl  *1  4d 5s  G  8.55  $2 ~*  H  9.7  <l<3 •* 3s  j S  ketone and methyl t - b u t y l ketone  0.88 2.83 2.8  149.  p r o b a b l y because  t h e r e a r e s o many v i b r a t i o n a l  s h a p e o f t h e n ->• 3s band Sharp v i b r a t i o n a l  methyl  s t r u c t u r e observed  isopropyl  In p a r t i c u l a r , the  6.4 e V ) i s v e r y s e n s i t i v e  ketone  to the alkyl  (b) and methyl  The f i r s t t-butyl  band i n t h e s p e c t r a  ketone  ( c ) appears as  a c o n t i n u o u s d i f f u s e band w h i l e t h a t o f m e t h y l  isobutyl  shows some i n d i c a t i o n  i n the f i r s t  detailed  of vibrational  s t u d y o f t h e shape  of this  structure  band u s i n g o p t i c a l  been p r e s e n t e d by I t o e t a l . [ 1 1 4 ] . k e t o n e and m e t h y l  t-butyl  ketone  The second  to  (a) s t i l l  band.  i n which  isopropyl band  the a l k y l  T h i s may be a u s e f u l  b e t w e e n t h e s e two k i n d s o f k e t o n e s .  A  s p e c t r o s c o p y has  i s more i n t e n s e t h a n t h e f i r s t  a r e not branched a t the a-carbon.  distinguish  ketone  band i n m e t h y l  w h i l e t h e r e v e r s e i s t r u e f o r o t h e r ketones studied groups  group.  i n a c e t o n e a n d 2 - b u t a n o n e seems t o  disappear f o r t h e higher ketones s t u d i e d . of  modes.  feature  H o l d s w o r t h and Duncan  [ 1 1 3 ] h a v e a l s o o b s e r v e d t h a t t h e i n t e n s i t y o f t h e 6.4 eV b a n d d e c r e a s e s when t h e a - h y d r o g e n s  i n acetone are replaced  i n t e n s i t y of the t r a n s i t i o n  by m e t h y l  i n the r e g i o n a t about  7.4 eV i n c r e a s e s .  s a t i s f a c t o r y e x p l a n a t i o n has y e t been g i v e n f o r t h i s  8.3.3. E f f e c t o f a l k y l As  v a l u e s ( E a * ) o f t h e two a l k y l  (Figure 32).  ( s e c t i o n 7 . 6 ) . The s h i f t  e n e r g i e s on a l k y l a t i o n  groups a t t a c h e d  T h e b e h a v i o u r o f t h e 3 s , 3p a n d 3d  i s t h e same a s i n a l d e h y d e s ( s e c t i o n 8.2.4.)  ives o f water  plots are  I . P . , t h e 3 s , 3p a n d 3d t e r m v a l u e s a r e p l o t t e d  a g a i n s t t h e sum o f T a f t a*  terms  phenomenon.  i n the case o f aldehydes, approximately s t r a i g h t - l i n e  t h e c a r b o n y l group  No  substituents.  o b t a i n e d when t h e f i r s t  to  groups w h i l e the  and t h e a l k y l  derivat-  o f t h e 3s p e a k maxima t o l o w e r  i s s m a l l e r than i n t h e c a s e o f a l d e h y d e s and so t h e  150.  F i g u r e 32.  E f f e c t o f a l k y l s u b s t i t u t i o n on f i r s t ionization potentials  Rydberg term i n ketones.  values  and  151.  straight  lines  f o r the f i r s t  approximately p a r a l l e l . of  In o p t i c a l  t h e p e a k maxima o f t h e 7.4  a s more a - h y d r o g e n s Rydberg model Rydberg filled  eV  t h i s c a n be e a s i l y  electron  orbital  highest f i l l e d  and  molecular o r b i t a l .  100 eV a n d  As a r e s u l t ,  increasing  9.61  eV.  second  one  the t r a n s i t i o n  energy binding  alkylation.  fine  i m p a c t s p e c t r a o f p r o p e n a l and  l o s s r e g i o n o f 2 - 13 eV a t an The  s t r u c t u r e , the f i r s t I . P . ' s a r e 10.11  (see Appendix). The The  first  second  The  impact  The  first  two  bands  10.93  eV  respectively.  vinyl  We  band i s s h a r p and  vibrational  b e i n g t h e most i n t e n s e , g i v i n g  shows  have  vibrational  components, w i t h  a vertical  The  ketone i n t h i s  p e a k i s m o s t i n t e n s e and g i v e s a v e r t i c a l band has f i v e  energy  possess  peak b e i n g t h e m o s t i n t e n s e .  and  first  methyl  p h o t o e l e c t r o n spectrum of propenal  obtained the p h o t o e l e c t r o n spectrum o f methyl  structure.  3p  than the b i n d i n g energy o f the  by T u r n e r e t a l . [ 8 2 ] .  two v e r t i c a l  laboratory  i n the h i g h e s t  (acrolein).  2° s c a t t e r i n g a n g l e .  been s t u d i e d  first  than the e l e c t r o n s  3p  Compounds  ketone over the energy  vibrational  In our  i s t h e d i f f e r e n c e b e t w e e n t h e two  F i g u r e 33 shows t h e e l e c t r o n  has  groups.  e x p l a i n e d by t h e f a c t t h a t t h e  decrease less r a p i d l y  8.4.1. P r o p e n a l  of  a bigger s h i f t  therefore the binding energies of the  s t a t e , which  are  band t o l o w e r e n e r g i e s has been o b s e r v e d  energies, i s also decreasing with  vinyl  s p e c t r o s c o p y [112,113]  i s less penetrating  t h e 3p R y d b e r g  8.4.Unsaturated  t h e 3s t e r m v a l u e d e p e n d e n c i e s  i n a c e t o n e a r e r e p l a c e d by m e t h y l  molecular orbital  Rydberg  to  I . P . and  second  I.P. o f the  I.P. o f  152.  CH =CH-CHO 10OeV,2°  6  2  3.8 eV  00  M  B i  I  • • •• • .' ' i • v c '  x 8  !  G  D'  N  K Ml  J IL |  33 3 CC  cr GO  < >-  8  6  < 2  CH =CHCOCH ^ 100eV, 2° f 2  12  IO  ns  3  6,7 g  CO  UJ  N  3.6 eV  IT  •'-  V  ''•"'•v.-.V..  1  G rl J  x 4 -np  1  6  33  1  1  8  12  IO  ENERGY LOSS (eV)  F i g u r e 33.  E l e c t r o n impact s p e c t r a o f p r o p e n a l and methyl k e t o n e a t 100 eV, 2 ° .  vinyl  153.  10.62  eV.  The  second  s h a r p band i n t h e p h o t o e l e c t r o n s p e c t r u m seems t o  be a c h a r a c t e r i s t i c o f u n s a t u r a t e d c o n j u g a t e d c a r b o n y l c o m p o u n d s , f o r i t does n o t o c c u r i n t h e p h o t o e l e c t r o n s p e c t r a o f s a t u r a t e d a l d e h y d e s ketones. electron  Turner e t a l .  band  i n the  photo-  s p e c t r u m o f p r o p e n a l t o t h e l o s s o f an e l e c t r o n f r o m t h e  bonding o r b i t a l the  [ 8 2 ] have a s s i g n e d t h e f i r s t  ( i ^ ) and  the second  (ij^).  We  assume t h a t  k e t o n e , w h o s e E I S i s t o be  l a t e r , c a n be i n t e r p r e t e d  fashion.  Walsh  [ 1 0 4 ] and  series. J,  8.89,  % 9.4  assigned as the  by l e s s  i s 0.68.  s p e c t r u m o f p r o p e n a l has  t h e f e a t u r e s a b o v e 7.5  I n o u r s p e c t r u m we  L a t 7.58,  defect  in a similar  observe s e r i e s  and  9.65  np R y d b e r g The  t h a n 0.16  shoulder D  series  t h i s may  ( n = 3, 4, eV  The  with a vibrational  3p m a n i f o l d ( p r o b a b l y 3p' a s i n a c e t a l d e h y d e - s e e s e c t i o n  diffuse  transitions  formaldehyde  3.78  i s p r e s e n t e d i n T a b l e 12.  6.35  eV.  eV).  (1.10) i s a l s o c o n s i s t e n t w i t h t h i s assignment  are  then predicted  The  8.2.1.) The  The  calculated  9.22  and  strong  9.54  series  eV,  state ( c f . quantum  ( c f . formaldehyde,  h i g h e r members o f t h i s  t o o c c u r a t 8.49,  first  I t s t e r m v a l u e o f 3.76  I . P . , i s c o n s i s t e n t w i t h a 3s u p p e r  eV, a c e t a l d e h y d e 3.39  and a c e t a l d e h y d e , 1.00).  quantum.  c o r r e s p o n d t o a n o t h e r component o f  impact spectrum of propenal i s a  peak w i t h a maximum a t a b o u t  with respect to the f i r s t  G,  quantum  the  prominent f e a t u r e i n the e l e c t r o n  Rydberg  i s s e p a r a t e d f r o m t h e peak C  be a s s o c i a t e d  eV may  5, 6 ) .  small  A summary o f t h e R y d b e r g  by  I I I [ 1 0 4 ] w i t h members C,  The  1  a t ^ 7.9  three  discussed  eV r e s p e c t i v e l y w h i c h h a v e b e e n  s h o u l d e r D a t 7.74  eV and  been s t u d i e d  eV a r r a n g e d i n t o  from  the  photoelectron spectrum o f methyl v i n y l  vacuum u l t r a v i o l e t  non-  band t o t h e l o s s o f an e l e c t r o n  h i g h e r o f t h e two o c c u p i e d TT o r b i t a l s  The  and  defect  1.11  -+ ns  eV r e s p e c t i v e l y .  So  PEAK  OBSERVED ENERGY (eV)  TERM VALUE (eV)'  ASSIGNMENT  CALCULATED ENERGY (eV)  A  6.35  3.76  B  7.09  3.84  C D D'  7.58 7.74 7.9  2.53 2.37 2.2  4»i *1  3p 3p + v 3p»  E  8.49  1.62  $1  4s  F  8.63  2.30  fl  G  8.89  1.22  *1 * 4p  8.88  H  9.22  0.89  tyl •*• 5s  9.22  I  9.29  1.64  $2  4s  9.29  0.71  fl  •* 5p  9.39  J  ^ 9.4  -* 3s  1.10  ^2 -» 3s  1.12 0.68  8.49  •*• 3p  0.57  K  9.55  0.56  ^1  6s  9.54  L  9.65  0.46  fl  6p  9.63  M  10.03  0.90  i|>2 •+ 5s  10.03  N  10.53  0.40  ^2 + 7s  10.54  0  12.10  3.7  TABLE 12  CALCULATED QUANTUM DEFECT  3s  Rydberg t r a n s i t i o n s i n propenal  (a) Assigned with respect to TES i o n i z a t i o n p o t e n t i a l s o f 10.11, 10.93, 13.5, 14.8, 15.3, 16.  155.  t h e f e a t u r e s o c c u r r i n g a t 8 . 4 9 , 9.22 and 9.55 eV i n o u r s p e c t r a  (peaks  E, H a n d K) c a n be a s s i g n e d 4 s , 5s a n d 6s R y d b e r g  The  upper  states.  s h a r p p e a k B a t 7.09 eV h a s a t e r m v a l u e o f 3.84 w i t h r e s p e c t t o t h e second it  I . P . a t 1 0 . 9 3 eV.  i s a s s i g n e d as a ^  o f t h e ^2 "*"  n  The c a l c u l a t e d 3s R y d b e r g  q u a n t u m d e f e c t i s 1.12 a n d s o  transition.  s e r i e s a p p e a r a t 9.29 ( I ) a n d 1 0 . 0 3 (M) a s p r e d i c t e d ,  s  while higher states are probably contributing The  The n = 4 a n d 5 members  t o t h e b r o a d e n v e l o p e N.  t e r m v a l u e o f peak F a t 8.63 eV w i t h r e s p e c t t o t h e s e c o n d  10.93  eV i s 2.30 eV a n d s o c a n be a s s i g n e d t o t h e R y d b e r g  f r o m ^2  t  0  o  n  °f  e  contributions  t  n  e  components o f 3p.  t o peak E.  transitions  t r a n s i t i o n s from  series are probably buried weak t r a n s i t i o n  i s observed  isn  calculations [116]. ists  [ 1 1 5 ] and s t u d i e d  [117,118,119].  resolved Inuzuka  optical  That t h i s  A  low energy  by I n u z u k a , who d i d t h e o r e t i c a l  the effect of solvents on.this  transition  has a l s o been t h e i n t e r e s t o f o t h e r o p t i c a l s p e c t r o s c o p Vibrational  structure of this  band, which  i n o u r s p e c t r u m , has b e e n a n a l y s e d by E a s t w o o d  i s not  a n d Snow [ 1 1 9 ] a n d  [120].  With t h e e x c e p t i o n o f t h e ^ electron  Rydberg  i n o u r s p e c t r u m a s t h e b r o a d band w i t h a  T I* was e s t a b l i s h e d  This region  t o t h e nd  i n the spectrum.  maximum a t 3.8 eV ( c f . 4.30 eV i n a c e t a l d e h y d e ) . transition  and t h i r d  12.1 eV may be t h e  Transitions  under o t h e r f e a t u r e s  have  10.5 eV may h a v e  l e a d i n g t o t h e second  I.P.'s w h i l e t h e broad f e a t u r e m a x i m i s i n g a t about envelope o f Rydberg  transition  T h e o t h e r c o m p o n e n t s may  The b r o a d p e a k N a r o u n d  c o n t r i b u t i o n s from Rydberg  I.P. a t  np s e r i e s , o u r i n t e r p r e t a t i o n o f t h e  impact spectrum o f p r o p e n a l does n o t a g r e e w i t h s p e c t r u m g i v e n by W a l s h  that f o r the  [ 1 0 4 ] , who a s s i g n e d t h e p e a k s A ( 6 . 3 5 e V )  156.  and  B ( 7 . 0 9 eV)  From t h e a b o v e p a r a g r a p h , t h e b i n d i n g difference  between t h e b i n d i n g  and  t h e n •+ TT* t r a n s i t i o n  for  the binding  the  photoelectron  maximise  a t 10.9  T h e r e f o r e , we  spectrum. - 6.3  = 4.6  Also,  ( F i g u r e 33)  i s e x t r e m e l y weak.  the  the  the  to  transition  v a l u e o f 6.35  i n t h e 4.6  eV  region  the that  i n acetaldehyde to  TT  v a l u e ( 1 3 . 3 eV)  t h e Rydberg  of the  since  the small  transition  l i ea t the expected position and  s h o u l d e r D'  f o r t h e TT -*• a*  t h e TT ,-»- a*  transition  In  binding  expected energy f o r  e v i d e n c e i n our spectrum does not p r e c l u d e Walsh's as n -»• a*  Rydberg  has a f i r m e r f o u n d a t i o n .  leads to a s i m i l a r  TT and  to  n -> 3p.  c o i n c i d e n c e does not o c c u r i n the spectrum o f p r o p e n a l .  eV  eV.  reason  compounds ( s e e s e c t i o n s 8.2. a n d 8.3.)  r a t h e r than valence t r a n s i t i o n s  b e t w e e n t h e n -> a*  This However,  assignment  a t 7.9  eV  transition.  o f peak  happens (The  difference  energies i s expected to  T h i s a s s i g n m e n t was made i n 1945 b e f o r e t h e a d v e n t o f PES. b i n d i n g e n e r g y o f t h e TT o r b i t a l was unknown. +  from  i n which case the assignment of the prominent f e a t u r e s i n  valence transition  B a t 7.09  eV  TT -»• TT* s h o u l d  U n l e s s f o r some p e c u l i a r  case of acetaldehyde, the p a r t i c u l a r  chance  PES)  binding  ( 1 0 . 9 eV)  e x p e c t t h e TT ^ TT* t r a n s i t i o n  energy o f t h e Tr-bonding o r b i t a l the  The  i n a c e t a l d e h y d e s h o u l d be much s t r o n g e r t h a n  spectra of carbonyl  transitions  eV f r o m  o f peak A a s TT •> TT* i s  assignment  i t i s o b v i o u s t h a t any  i n p r o p e n a l , we  be weak a s w e l l ,  I.P.  i n s t e a d o f Walsh's  s u g g e s t t h a t Walsh's  TT -> TT* t r a n s i t i o n  (see F i g u r e 34).  Therefore the t r a n s i t i o n eV  (10.1  i s the  T h i s g i v e s a v a l u e o f 6.3  i s g i v e n by t h e s e c o n d  incorrect.  observed  (3.8 e V ) .  e n e r g y o f t h e TT* o r b i t a l  e n e r g y o f t h e TT e l e c t r o n  respectively.  e n e r g y o f t h e TT* o r b i t a l  energy of the n o r b i t a l  energy  n -> a*  TT -»• TT* and  to intervalence transitions*  be  Hence t h e  energy 0.0 eV  orbital ion  3p'  -  >  >\  a> CD  00  ro  n W,)  > CD  LO  ro  > CD  K  > CD  CD  >  T  >  CD  CD  CD  'VEnergy l e v e l s  A  -6.3eV  filled orbitals our assignment expected transitions on  cu  the basis of Walshs assignment.  LO  ro CD  F i g u r e 34.  valence orbitals  2.2eV •3.0 eV •3.8eV  T  3s TT  Rydberg orbitals  •10. leV •10.9 eV  p r  E  S  8  "  •l2.7eV and e l e c t r o n i c  transitions  i  „ predicted position of 7T~orbital 'TT if Walsh's assignment is correct.  propenal.  Ul  158.  of  s i m i l a r m a g n i t u d e t o t h e d i f f e r e n c e b e t w e e n t h e n and  energies.) ^1 -> 3p'  Although  the p o s s i b i l i t y  valence t r a n s i t i o n s we  cannot  tell  evidence to  B has  still  a*  n  and  from  optical  n o t TT -> TT* a s  TT o r b i t a l  suggested  i n d i c a t e s a v a l u e o f -10.9  8.4.2. M e t h y l The which  vinyl  electron  and  respectively.  At the present  I.P.  the f i r s t  by W a l s h  of methyl  to that of propenal.  ( s e c t i o n s 8.2.and 8 . 3 . ) ,  orbital. 9.12  eV  Using  eV,  The  term  shown i n T a b l e 3.4 is  eV  we  calculated  13.  to t h i s  spectrum  The  ketone  The  energy  transition vertical  transition  I.P.  and  group,  v a l u e o f t h e peak A w i t h eV  has  been o b s e r v e d  before  i s c o n s i s t e n t w i t h an  (n = 4,  5,  3s  K a t 8.02, 6).  i s expected  o f 10.6.  So  respect Since  ->- 3s R y d b e r g  I and  first  i n propenal.  transition  s Rydberg 8.70  and  A comparison  f o r a l l Rydberg t r a n s i t i o n s ^  in  d i f f e r e n c e between t h e  a s s i g n peak A t o t h e  ->- ns  ( F i g u r e 33)  i s r e p l a c e d by a m e t h y l  alkylation  quantum d e f e c t , 0.99,  below the second  assigned  vinyl  t h e R y d b e r g f o r m u l a , p e a k s F,  and  -> 3 s  eV w h e r e a s t h e PES  a s c o m p a r e d t o 3.76  are assigned t r a n s i t i o n s ^  the observed  seems  eV).  spectrum  i s 3.36  the calculated  However,  photoelectron spectroscopy  o f -12.7  t h e l o w e r i n g o f t h e 3s t e r m v a l u e on  and  time,  (Walsh's assignment would r e q u i r e the  energy  i s a b o u t 1 eV.  I.P.  are c o n t r i b u t i n g .  (A) i s t h e Rydberg t r a n s i t i o n  the a l d e h y d i c hydrogen i n propenal  second  to  TT ->• a*  as  ketone.  impact  looks very s i m i l a r  as  a l s o be a s s i g n e d  band  t o have an o r b i t a l  D'  t h a t t h e y can  s p e c t r o s c o p y and eV  •+ 3s and  exists  t o what e x t e n t e i t h e r o r both  i n d i c a t e t h a t t h e 6.35  and  a l r e a d y been a s s i g n e d a s ^  t h e IT b i n d i n g  of  observed  to occur  around  t h e p e a k D a t 7.34  t h e h i g h e r members o f t h i s ty  0  -* ns  eV  is  OBSERVED " ENERGY (eV)  TERM VALUE ( e V )  A  6.25  3.36  *1 > 3s  0.99  B C  6.66 6.85  2.95 2.76  *1 -»• 3p  0.85  D  7.34  3.28  ^2 •*• 3s  0.96  E  7.56  2.05  *1  F  8.08  1.53  • *1-> 4s  8.11  G  8.25  1.36  _*1 •* 4p  8.24  H  8.50  1.11  fl  -* 4d  8.55  I  8.70  0.91  fl  + 5s  8.76  J  8.81  0.80  *1 - 5p  8.82  K  9.12  0.49 1.50  fl fl fl  •*• 6s 6p + 4s  9.07 9.09 9.14  *  •  a  AbblolSMLM  CALCULATED ENERGY (eV)  3d  0.42  L  9.82  0.80  fl  + 5s  9.78  M  10.18  0.44  fl fl  * 6s + 7s  10.08 10.25  TABLE 13  CALCULATED QUANTUM DEFECT  Rydberg t r a n s i t i o n s i n methyl v i n y l ketone  (a) Assigned with respect to PES i o n i z a t i o n p o t e n t i a l s o f 9.61, 10.62,13.1, 13.9, 14.4, 15.1.  160.  s e r i e s a r e expected respectively. 9.83 eV. and  a t 9.14, 9.78, 10.08 a n d 1 0 . 2 5 eV f o r n = 4, 5, 6, 7  A c t u a l l y , a peak K and a s t e p L a r e o b s e r v e d  The b r o a d  p e a k a t 1 0 . 1 8 eV may be t h e c o n v o l u t i o n o f t h e  -> 7s t r a n s i t i o n s .  Rydberg t r a n s i t i o n  The o n l y f e a t u r e t h a t c a n be a s s i g n e d  i s the shoulder  with respect to the f i r s t  B a t 6.66 eV.  The t e r m  I . P . , and t h e c a l c u l a t e d  are higher than  those  of  ( 2 . 8 1 eV a n d 0.83 r e s p e c t i v e l y ) .  formaldehyde  i n propenal  quantum d e f e c t b e t w e e n p r o p e n a l  but are s t i l l  and methyl  vinyl  The R y d b e r g f o r m u l a  and  t h e s m a l l peak A a t 8.50 eV a r e a s s i g n e d  The  calculated  envelopes 13.9  of  peak N m a x i m i s i n g  Rydberg t r a n s i t i o n s from  a n d 14.4 eV r e s p e c t i v e l y ) .  methyl  vinyl  propenal  v a l u e o f 2.95 eV (0.85 eV)  comparable t o those [98] The d i f f e r e n c e i n ketone  indicates the into the  The s h o u l d e r E a t 7.56 eV -»• 3d a n d 4d  transitions.  q u a n t u m d e f e c t o f 0.42 i s a l s o c o m p a r a b l e t o t h a t o f The b r o a d  ketone  ^  and  a t a b o u t 11.8 eV (fifth  probably  and s i x t h  I.P. a t  The maximum o f t h e n -* IT* t r a n s i t i o n i n  (3.6 eV) i s s h i f t e d  (3.8 e V ) .  3p  g i v e s t h e h i g h e r members a s p e a k s  G, J , K a t 8 . 2 5 , 8.81 a n d 9.12 eV r e s p e c t i v e l y .  formaldehyde.  -*• 6 s  the  quantum d e f e c t  d i f f e r e n t d e g r e e s o f p e n e t r a t i o n o f t h e np R y d b e r g e l e c t r o n molecular core.  a t 9.12 a n d  t o lower  energy  compared t o t h a t  t  161.  CHAPTER  IX  CONCLUSION  Electron  impact  s p e c t r o s c o p y s e r v e s a s a n a l t e r n a t i v e method t o  photoabsorption f o robserving electronic wavelength  transitions.  In the short  r e g i o n ( e . g . vacuum u l t r a v i o l e t o r s o f t X - r a y  is a strong competitor with optical many d i f f i c u l t i e s continuous  encountered  spectroscopy  i n optical  region),  s i n c e i t by-passes  work such  as a v a i l a b i l i t y o f  s o u r c e s , i n monochromation and i n a b s o l u t e c a l i b r a t i o n o f  photon i n t e n s i t i e s .  Although  the resolving  spectroscopy a t longer wavelengths  power a v a i l a b l e  i n optical  i s superior to that of EIS, the  c o n d i t i o n s become more f a v o u r a b l e f o r E I S a t s h o r t e r w a v e l e n g t h s , u l a r l y w i t h recent achievements  i n a n a l y s e r and e l e c t r o n o p t i c a l  w h e r e a r e s o l u t i o n o f 0.005 - 0.010 eV w i t h u s a b l e expected  i t  i n t h e near  future.  intensity  I n t h i s w o r k , t w o o f t h e many  particdesign,  i s t o be capabilities  o f E I S have been e x p l o r e d , namely o p t i c a l l y f o r b i d d e n t r a n s i t i o n s and molecular total  Rydberg s t a t e s .  listing  transitions molecular  o f t h e energy  Thus E I S i s c a p a b l e  l e v e l s o f a molecule.  are inaccessible to optical  Rydberg t r a n s i t i o n s , t h e broad  large molecules a disadvantage  i n principle of giving  band s t r u c t u r e o f t h e s p e c t r a o f  a n d t h e r e a r e some p o s s i b i l i t i e s analysis.  o f EIS o r i t s v a r i a t i o n s which w i l l  forbidden  spectroscopy w h i l e i n the case o f  do n o t make t h e l o w e r r e s o l v i n g  t e c h n i q u e a s a means f o r c h e m i c a l  Optically  a  p o w e r o f E I S t o o much o f f o r development o f t h i s  There a r e other  provide i n f o r m a t i o n about  applications excitation  162.  cross sections, autoionizing  states, negative  energy  M.O.'s, some o f w h i c h  positions of unfilled  o t h e r methods. difficult  Although  i o n f o r m a t i o n and t h e cannot  be s t u d i e d by  e x p e r i m e n t a l l y , E I S i s i n some w a y s m o r e  than c o n v e n t i o n a l o p t i c a l  spectroscopy, the wealth of  i n f o r m a t i o n a v a i l a b l e makes t h e e f f o r t s  worthwhile.  163.  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The o n l y h i g h r e s o l u t i o n s p e c t r a r e c o r d e d  at  a c e t o n e and 2-butanone  low r e s o l u t i o n using a simple  meter and r e p o r t e d et  orbitals in high  compounds h a v e n o t been a r e those  o f form-  ( a c r o l e i n ) b y T u r n e r e t a l [ 8 2 ] . Dewar a n d  Worley [122] measured t h e p h o t o e l e c t r o n aldehyde,  ionization  indicates that  resolution  a l d e h y d e and p r o p e n a l  s p e c t r a o f many o f t h e s e  spectra  the f i r s t  spectra of acetaldehyde,  ( t o g e t h e r w i t h -63 o t h e r o r g a n i c  plate photoelectron  o p e r a t i n g a t medium r e s o l u t i o n t o m e a s u r e t h e f i r s t of a s e r i e s o f a l i p h a t i c  compounds)  retarding-potential grid-type spectro-  a n d some h i g h e r a d i a b a t i c I . P . ' s .  a l . [ 1 2 3 ] have used a p a r a l l e l  propion-  aldehydes,  ketones,  Cocksey  spectrometer  ionization potentials  a l c o h o l s , e t h e r s and i o d i d e s .  EXPERIMENTAL o  We have m e a s u r e d t h e 584 A p h o t o e l e c t r o n  s p e c t r a o f s i x k e t o n e s and  three aldehydes using a h i g h - r e s o l u t i o n photoelectron  spectrometer  ing  h a s been  a 127 d e g r e e c y l i n d r i c a l  - elsewhere  [124].  analyser.  The a p p a r a t u s  N i t r o g e n was u s e d f o r e n e r g y  employ-  described  calibration.  RESULTS AND DISCUSSION F i g u r e 35 shows t h e p h o t o e l e c t r o n  s p e c t r a , f r o m ^9 eV t o 18 e V , o f  170.  I 9  1  10  1  II  1  12  1  1  13  14-15  1  H  1  16  17  IONIZATION POTENTIAL (eV)  1  1  18  19  o  Figure  35.  584 A p h o t o e l e c t r o n s p e c t r a o f C H C H 0 , C H C H C H 0 a n d 3  (CH ) CHCH0. 3  /  •  2  3  2  171.  a c e t a l d e h y d e , p r o p i o n a l d e h y d e and i s o b u t y r a l d e h y d e . A s i n t h e c a s e o f formaldehyde  [82] v i b r a t i o n a l  structure  i s observed  i n thef i r s t  a c e t a l d e h y d e and p r o p i o n a l d e h y d e .  T h e band p o s i t i o n s  energy w i t h  The v i b r a t i o n a l  increased alkylation.  compounds i s a p p r o x i m a t e l y 0.16 eV.  The f a c t  hyde s u g g e s t t h a t t h i s due  on t h e o x y g e n a t o m .  a-carbon is  spacing i n both  t o t h e spectrum  i n the essentially  When t h e a l k y l  as i n i s o b u t y r a l d e h y d e , v i b r a t i o n a l  There  vibrational of formalde-  non-bonding  chain starts structure  t o branch a tt h e  i n the f i r s t  o b l i t e r a t e d , p r o b a b l y due t o t h e o c c u r r e n c e o f f u r t h e r  components.  these  band i n b o t h a c e t a l d e h y d e and p r o p i o n a l d e h y d e i s  t o t h e i o n i z a t i o n o f an e l e c t r o n  orbital  s h i f t t o lower  that the f i r s t  peak i s most i n t e n s e and a l s o t h e s i m i l a r i t y  band o f  a r e f o u r broad  peaks  band  vibrational  i n t h e p h o t o e l e c t r o n spectrum o f  a c e t a l d e h y d e i n t h e 12 eV t o 18 eV r e g i o n c o r r e s p o n d i n g t o i o n i z a t i o n s of  t h e i n n e r M.O.'s.  region.  Following  As a l k y l a t i o n  i n c r e a s e s , more p e a k s  t h e example o f formaldehyde  appear  at this  [ 8 2 ] , t h e second I.P.  p r o b a b l y c o r r e s p o n d s t o t h e i o n i z a t i o n o f t h e - r r - e l e c t r o n s i n t h e C=0 d o u b l e bond. No a t t e m p t are  The energy  tends t o decrease as a l k y l a t i o n  has b e e n made t o i n t e r p r e t  the individual  structure.  F i g u r e 36 shows t h e p h o t o e l e c t r o n s p e c t r a o f a c e t o n e , isobutyl  methyl  vinyl  ketone, methyl  ketone  isopropyl  ketone, methyl  vinyl  ketone) a r e very s i m i l a r  2-butanone,  t-butyl  i n t h e e n e r g y r a n g e o f ^ 9 - 19 eV.  features of the photoelectron spectra o f the saturated exception o f methyl  orbitals.  [ 1 2 5 ] have r e c e n t l y p r e s e n t e d an i n t e r p r e t a t i o n o f  PES o f a c e t a l d e h y d e i n t e r m s o f M.O.  methyl  increases.  higher I.P.'s, which  p r o b a b l y d u e t o i o n i z a t i o n s o f t h e v a r i o u s C-H b o n d i n g  Chadwick and K a t r i b the  position  k e t o n e and  The g e n e r a l  ketones  (with the  t o those o f aldehydes.  172.  CH C0CH 3  3  CH CH C0CH 3  2  3  (CHJIpHCKPOCH,  (CH ) CHC0CH  (  3 2  3  (CH ) CC0CH 3 3  3  CH =CHC0CH 2  II  F i g u r e 36.  12  1 3  1 4  1 5  1 6  17  3  1 8  19  IONIZATION POTENTIAL (eV)  2 0  584 A p h o t o e l e c t r o n s p e c t r a o f C H C O C H , 3  . CH CH COCH , (CH ) CIICH C0CH , 3  and  2  CH  3  2  3  = CHC0CH . 3  2  2  3  3  (CH^CCOCHg  173.  The  first  broad  band  and  is relatively  o v e r l a p w i t h one  sharp but the second  another.  and  h i g h e r bands  These h i g h e r bands o c c u r a t  energies than those i n the aldehydes. band t o l o w e r e n e r g i e s a s a l k y l a t i o n  There  increases.  As  i n the case  i n the f i r s t  those ketones  no b r a n c h i n g o f t h e a l k y l  chain occurs at  (i.e.  similarity  a c e t o n e , 2-butanone and m e t h y l  between t h e p h o t o e l e c t r o n s p e c t r a  seems t o s u g g e s t t h a t t h e y c a n band i s m o s t l i k e l y oxygen and bond. and  the second  their  The methyl  In  p o t e n t i a l s which and  a second  the case o f propenal  T u r n e r e t a l . [ 8 2 ] who  and  The  The  first  electron  on  i n the  C=0  bonding  M.O.'s  T a b l e 14 shows  the  i n the p h o t o e l e c t r o n spectra  of the unsaturated conjugated  from  attributed  ketone,  t h o s e o f s a t u r a t e d c a r b o n y l compounds i n  (acrolein) similar  resolved  occupied  vibrational  b e h a v i o u r has  the second  structure.  b e e n r e p o r t e d by  band t o t h e l o s s o f an  Tr-orbitals.  i n the interpretation  impact s p e c t r a effect of alkyl  k e t o n e s , has  increases.  The  electron  This i s probably a  f e a t u r e f o r an u n s a t u r a t e d c o n j u g a t e d c a r b o n y l compound  i t s significance  electron  t o C-H  the  aldehydes  of the i r - o r b i t a l  a r e p r o b a b l y due  s h a r p band w i t h  f r o m t h e h i g h e r o f t h e two distinguishing  and  of  ketones.  ketone d i f f e r s  i t exhibits  ketone).  o f the non-bonding  have been o b s e r v e d  p h o t o e l e c t r o n spectrum  vinyl  band  i n t h e same way.  to ionization  number i n c r e a s e s a s a l k y l a t i o n  aldehydes  that  I . P . due  of ketones  interpreted  to the i o n i z a t i o n  Most o f the h i g h e r I.P.'s  ionization of  due  be  isobutyl  first  of  s t r u c t u r e c a n be r e s o l v e d  a-carbon  lower  i s also a s h i f t of the  aldehydes, vibrational i n which  are  has  o f t h e vacuum u l t r a v i o l e t  been d i s c u s s e d ( s e c t i o n  substitution  of the  b e e n d i s c u s s e d i n C h a p t e r 8.  I.P.'s  The  or  8.4.1.). of aldehydes  plots of the f i r s t  and I.P.  and  COMPOUND  1st BAND  BANDS 2nd  3rd  4th  6th  7th  10.88  14 .39  10 .01  16.60  10.22  10.21  13,,2  14 .1  15.3  16.4  CH CH CHO  9.97  . 9.99  12,.4  13 .2  13.7  14.1  15.4-  16-3  (CH ) CHCHO  9.69  9.82  12..0  12 .6  13.4  14.1  15.6  16,.5  CH C0CH  9.71  9.71  12,.6  13 .5  14.1  15.6  18.0  18,,9  9.54  9.52  12.,2  12 .6  13.0  14.3  14.8  9.34  9.42  11.,4  11 .9  13.0  13.6  9.30  9.36  11.,8  12 .6  13.9  9.14  9.21  11..4  12 .5  10.11  10. 93  9.61  10. 62  A  l  3  HCHO  C  CH CH0 3  3  2  3  2  3  3  CH CH C0CH 3  2  3  (CH ) CHCH C0CH 3  2  2  (CH ) CHCOCH 3  2  (CH ) CCOCH 3  3  CH =CH-CH0 2  Table  Ik  3  3  c  CH =CH-COCH 2  3  3  Ionization  potentials  V  ]  b  (eV)  8th  9th  15,.3  15,.9  17.6  14.3  15.,5  17,.6  18.6  14.2  14.6  15.,8  17,.4  13.6  13.6  14.6  15.4  16.,1  16,.9  18.4  13,.5  14.8  15.3  16.1  16..4  17,.0  18.0  13,.1  13.9  •14.4  15.1  16. 0  17.,5  18.7  i o n i z a t i o n p o t e n t i a l , from Cocksey et al  VI  i o n i z a t i o n p o t e n t i a l , t h i s work (except  indicates vertical  10th  18.0  f o r some carbonyl compounds  a Al indicates adibatic b  5th  from photoelectron spectra reported by Turner et a l , r e f .  [6] f o r HCHO and CH =CH-CH0) 2  4.  -pa  175.  Rydberg term v a l u e s substituents discussed.  against  on b o t h s i d e s  t h e sum o f t h e T a f t a * v a l u e s  of the  o f t h e f u n c t i o n a l g r o u p s have a l s o  been  PUBLICATIONS  1.  Wing-cheung Tarn § C. E. Brion, E x c i t a t i o n o f o p t i c a l l y forbidden t r a n s i t i o n s in argon and neon by electron impact, J . Electron Spectrosc. 2_, 111 (1973).  2.  Wing-cheung Tarn § C. E. Brion, A r o t a t a b l e gas-tight c o l l i s i o n chamber f o r electron spectrometers. J . Electron Spectrosc. 3, 82 (1974).  3.  Wing-cheung Tarn $ C. E. Brion, Electron impact spectra of some a l k y l d e r i vatives of water and related compounds, J . Electron S p e c t r o s c , 3, 263 (1974).  4.  Wing-cheung Tam f C. E. Brion, Rydberg series of HCN observed by electron impact spectroscopy, J . Electron Spectrosc., 3_ 281 (1974).  5.  Wing-cheung Tam $ C. E. Brion, E l e c t r o n i c spectra of some carbonyl compounds by electron impact spectroscopy I. saturated aldehydes J . Electron S p e c t r o s c , 4_143 (1974).  6.  Wing-cheung Tam § C. E. Brion, E l e c t r o n i c spectra o f some carbonyl compounds by electron impact spectroscopy II. saturated ketones J . Electron S p e c t r o s c , i n press  7.  Wing-cheung Tam 5 C. E. Brion, E l e c t r o n i c spectra o f some carbonyl compounds by electron impact spectroscopy I I I . unsaturated compounds J . Electron S p e c t r o s c , i n press  8.  Wing-cheung Tam, D. Yee $ C. E. Brion, Photoelectron spectra of some aldehydes and ketones, J . Electron S p e c t r o s c , i n press  T  

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