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Penning ionization electron spectroscopy of some atoms and molecules Stewart, William Brien 1974

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c PENNING  1  IONIZATION OF  SOME  ELECTRON  ATOMS  AND  SPECTROSCOPY MOLECULES  BY  WILLIAM B.Sc.  BRIEN  STEWART  1966, M.Sc. 1968, U n i v e r s i t y o f B r i t i s h Columbia  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF  DOCTOR OF PHILOSOPHY i n t h e Department of  CHEMISTRY  We accept t h i s t h e s i s as conforming t o t h e required standard  THE UNIVERSITY OF BRITISH COLUMBIA F e b r u a r 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  written  thesis  in p a r t i a l  fulfilment  of  at  University  of  Columbia,  the  make  it  that permission  p u r p o s e s may  representatives. thesis  for  available  be g r a n t e d  It  is  financial  of  C  —  €  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  / ^ /  ^  shall  / ^  Columbia  7 ^  by  the  ^/  not  requirements  reference copying of  I  agree  and this  copying o r  be a l l o w e d  for  that  study. thesis  Head o f my D e p a r t m e n t  understood that  gain  .  for  for extensive  permission.  Department  Date  freely  British  the  or  publication  without  my  ABSTRACT  A comparative study has been made o f Penning i o n i z a t i o n (He*(2 S) and H e * ( 2 S ) ) and p h o t o i o n i z a t i o n 1  t h r e e atoms and m o l e c u l e s (Ar, K r , Xe, H^, C0 ,  COS,  2  (He(584 A))  3  CS , 2  N 0, 2  S0 , 2  N0 , 2  NH^,  HD,  CH C I , CH  D,,,  B r , CH  N^,  o f twentyCO,  I , CH , 4  NO, CH 2  0^, ,  C H ) employing the t e c h n i q u e s o f h i g h r e s o l u t i o n e l e c t r o n s p e c t r o s c o p y . 2  2  E l e c t r o n s p e c t r a from mixed r a r e gas a l s o been examined and interactions.  systems, at h i g h e r p r e s s u r e s , have  i n t e r p r e t e d on the b a s i s o f f a s t n e u t r a l - n e u t r a l  -ii-  TABLE  OF  CONTENTS Page  ABSTRACT  ix  ACKNOWLEDGEMENT  x  CHAPTER ONE - INTRODUCTION  1  1.1. 1.1.1. I n t r o d u c t i o n 1.1.2. E x p e r i m e n t a l 1.2.  1 Objective  Penning I o n i z a t i o n  3 .. .  5  1.2.1. I n i t i a l S t a t e o f A*  5  1.2.2. I n t e r p r e t a t i o n i n Terms o f P o t e n t i a l Energy Curves  7  1.2.3. E j e c t e d E l e c t r o n Energy S p e c t r a  13  1.3.  Associative Ionization  16  1.4.  R e l a t i v e and A b s o l u t e Cross S e c t i o n s  18  1.5.  Photoelectron Spectroscopy  23  1.6.  Franck-Condon P r i n c i p l e  27  1.7.  A n g u l a r D i s t r i b u t i o n o f P h o t o e l e c t r o n and Penning Electrons  1.8.  29  Autoionization  31  CHAPTER TWO - THEORETICAL DISCUSSION  33  2.1.  Weak I n t e r a c t i o n T h e o r i e s  33  2.2.  C l o s e Coupled Near A d i a b a t i c T h e o r i e s  37  2.3.  Exchange Model  38  2.4.  Theory o f t h e 127 Degree E l e c t r o s t a t i c A n a l y z e r  ...  39  -iii-  Page CHAPTER THREE - INSTRUMENTAL 3.1.  E x p e r i m e n t a l Arrangement  41 41  3.1.1. E x c i t a t i o n Region  41  3.1.2. C o l l i s i o n  43  Region  3.1.3. E l e c t r o n A n a l y z e r and D e t e c t i o n System  44  3.1.4. L i g h t Source  46  3.1.5. Vacuum System  47  3.2.  Spectrometer  Performance  3.3.  C a l i b r a t i o n o f Energy S c a l e and P r e s e n t a t i o n o f Data  CHAPTER FOUR - RESULTS AND DISCUSSION 4.1.  Rare Gases  48  56 60 60  4.1.1. Argon  60  4.1.2. Krypton  66  4.1.3. Xenon  66  4.2.  Diatomic Molecules  72  4.2.1. M o l e c u l a r Hydrogen, Deuterium H y d r i d e and M o l e c u l a r Deuterium  72  4.2.2. M o l e c u l a r N i t r o g e n  77  4.2.3. Carbon Monoxide  87  4.2.4. N i t r i c Oxide  9 2  4.2.5. M o l e c u l a r Oxygen  99  4.3.  Triatomic Molecules  103  4.3.1. Carbon D i o x i d e  105  4.3.2. C a r b o n y l S u l p h i d e  107  4.3.3. Carbon D i s u l p h i d e  I l l  -iv-  Page 4.3.4. N i t r o u s Oxide  ,  4.3.5. S u l p h u r D i o x i d e and N i t r o g e n D i o x i d e 4.4.  Polyatomic Molecules  114 117 122  4.4.1. Ammonia  122  4.4.2. M e t h y l C h l o r i d e , M e t h y l Bromide and M e t h y l I o d i d e .  125  4.5.  Hydrocarbons  130  4.5.1. Methane  130  4.5.2. E t h y l e n e  132  4.5.3. A c e t y l e n e  134  4.6.  Rare Gas M i x t u r e s  139  4.6.1. Helium + Helium  139  4.6.2. Helium + Argon  142  4.6.3. Argon + Argon  144  4.6.4. Neon + Neon  147  CHAPTER FIVE - CONCLUSIONS  149  -V-  LIST  OF  TABLES  Table I II  III  Page Energy L e v e l s f o r Rare Gas I o n s , Atoms and Photons S h i f t s , AE, i n Penning E l e c t r o n E n e r g i e s f o r Rare Gases (eV)  65  2 2 Peak R a t i o s ( P,. / P, , ) f o r Rare Gas I o n i z a t i o n  67  •V  IV  6  2  i /  2  S h i f t s , AE, i n Penning E l e c t r o n E n e r g i e s f o r Hydrogen, Deuterium Hydride and Deuterium (eV)  76  V  V i b r a t i o n a l Spacings  (meV) f o r H ( X £ ) ^ 8  VI  V i b r a t i o n a l Spacings  (meV) f o r H D ( X E )  80  VII  V i b r a t i o n a l Spacings  (meV) f o r D ( X E  81  VIII IX  +  2  +  2  +  2  2  79  +  + g  )  S h i f t s , AE, i n Penning E l e c t r o n E n e r g i e s f o r N i t r o g e n feV) R e l a t i v e P o p u l a t i o n s o f E l e c t r o n i c S t a t e s ( a t v = 0) f o r Nitrogen  X XI XII  V i b r a t i o n a l Spacings  84 (meV) f o r E l e c t r o n i c S t a t e s o f N  86  + 2  S h i f t s , AE, i n Penning E l e c t r o n E n e r g i e s f o r Carbon Monoxide (eV) R e l a t i v e P o p u l a t i o n s o f E l e c t r o n i c S t a t e s ( a t v = 0)  89  f o r Carbon Monoxide XIII XIV  V i b r a t i o n a l Spacings  89 (meV) f o r E l e c t r o n i c S t a t e s o f C 0  S h i f t s , AE, i n Penning E l e c t r o n  Energies  +  XVI XVII XVIII  V i b r a t i o n a l Spacings  91  for Nitric  Oxide (eV) XV  84  94 (meV) f o r E l e c t r o n i c S t a t e s o f N 0  Relative Vibrational Transition Probabilities f o r N i t r i c Oxide R e l a t i v e P o p u l a t i o n s o f E l e c t r o n i c S t a t e s ( a t v = 0) f o r Carbon D i o x i d e S h i f t s , AE, i n Penning E l e c t r o n E n e r g i e s f o r Carbon D i o x i d e (eV)  +  97  98 106 106  -vi-  Table XIX  Page V i b r a t i o n a l Spacings (meV) C0  for E l e c t r o n i c States  of 108  +  2  XX XXI XXII XXIII XXIV XXV  R e l a t i v e P o p u l a t i o n o f E l e c t r o n i c S t a t e s (at v = 0) Carbonyl Sulphide  for 110  S h i f t s , AE, i n Penning E l e c t r o n E n e r g i e s f o r C a r b o n y l S u l p h i d e (eV)  110  S h i f t s , AE, i n Penning E l e c t r o n D i s u l p h i d e (eV)  113  E n e r g i e s f o r Carbon  R e l a t i v e P o p u l a t i o n s o f E l e c t r o n i c S t a t e s (at v = 0) Carbon D i s u l p h i d e S h i f t s , AE, i n Penning E l e c t r o n E n e r g i e s f o r Oxide (eV) Relative Nitrous  for 113  Nitrous  P o p u l a t i o n s o f E l e c t r o n i c S t a t e s (at v = 0)  116 for  Oxide  116  XXVI  V i b r a t i o n a l Spacings (meV)  f o r E l e c t r o n i c S t a t e s o f N^0  XXVII  V i b r a t i o n a l Spacings (meV)  f o r E l e c t r o n i c S t a t e s o f NH^  XXVIII XXIX  XXV  S h i f t s , AE, i n Penning E l e c t r o n E n e r g i e s f o r M e t h y l H a l i d e s (eV) S h i f t s , AE, i n Penning E l e c t r o n E n e r g i e s f o r A c e t y l e n e (eV) V i b r a t i o n a l Spacings (meV) f o r E l e c t r o n i c S t a t e s o f C H 2 2 Relative Vibrational Transition Probabilities for Acetylene +  XXXI  +  118  +  124  129 137  137  138  -vii-  LIST  OF  FIGURES  Figure 1. 2. 3. 4.  Page P o t e n t i a l energy c u r v e s f o r a s s o c i a t i v e and Penning ionization  9  R a t i o o f s i n g l e t t o t r i p l e t h e l i u m m e t a s t a b l e atoms as a f u n c t i o n o f the energy o f the e x c i t i n g e l e c t r o n s ..  19  A p p l i c a t i o n o f Franck-Condon production  26  Principle to photoelectron  Schematic diagram o f Penning i o n i z a t i o n spectrometer  electron 42 o  5.  High r e s o l u t i o n p h o t o e l e c t r o n spectrum (584 A) o f argon  49  o  6.  P h o t o e l e c t r o n spectrum (584 A) o f m o l e c u l a r hydrogen  ..  50  o  7. 8. 9. 10.  P h o t o e l e c t r o n s p e c t r a o f argon at 584 A u s i n g and e x t e r n a l photon lamps  internal 52  P r e s s u r e dependence o f the Penning and p h o t o e l e c t r o n s i g n a l as a f u n c t i o n o f t a r g e t gas p r e s s u r e  53  P r e s s u r e dependence o f the Penning s i g n a l as a f u n c t i o n of helium pressure  55  R e l a t i v e t r a n s m i s s i o n c o r r e c t i o n f a c t o r f o r 127  degree  Penning s p e c t r o m e t e r  58  11.  E l e c t r o n spectrum f o r i o n i z a t i o n o f argon  61  12.  Comparison o f peak shapes f o r i o n i z a t i o n o f argon  63  13.  Electron spectra f o r i o n i z a t i o n of krypton  68  14.  E l e c t r o n s p e c t r a f o r i o n i z a t i o n o f xenon  69  15.  Comparison o f peak shapes f o r i o n i z a t i o n o f xenon  71  16.  E l e c t r o n s p e c t r a f o r i o n i z a t i o n o f m o l e c u l a r hydrogen  .  73  17.  E l e c t r o n s p e c t r a f o r i o n i z a t i o n o f d e u t e r i u m h y d r i d e ..  74  18.  E l e c t r o n spectra f o r i o n i z a t i o n of molecular deuterium  75  19.  Penning e l e c t r o n spectrum o f m o l e c u l a r hydrogen  78  -viii-  Figure  Page  20.  Electron spectra f o r i o n i z a t i o n of molecular nitrogen  82  21.  E l e c t r o n s p e c t r a f o r i o n i z a t i o n o f carbon monoxide ....  88  22.  E l e c t r o n spectra f o r i o n i z a t i o n of n i t r i c oxide  93  23.  Penning e l e c t r o n and p h o t o e l e c t r o n s p e c t r a o f t h e X^E state of n i t r i c oxide.  24.  Relative vibrational Penning i o n i z a t i o n  N0 (xV) +  +  transition probabilities for and p h o t o i o n i z a t i o n o f NO t o  96  100  25.  E l e c t r o n s p e c t r a f o r i o n i z a t i o n o f m o l e c u l a r oxygen .. .  101  26.  Electron spectra for i o n i z a t i o n of  104  27.  E l e c t r o n spectra f o r i o n i z a t i o n of carbonyl sulphide  ..  109  28.  E l e c t r o n s p e c t r a f o r i o n i z a t i o n o f carbon d i s u l p h i d e ..  112  29.  E l e c t r o n spectra f o r i o n i z a t i o n of  115  30.  Electron spectra for i o n i z a t i o n of sulphur dioxide  31.  Electron spectra for i o n i z a t i o n of nitrogen dioxide  32.  E l e c t r o n spectra f o r i o n i z a t i o n of  33.  E l e c t r o n s p e c t r a f o r i o n i z a t i o n o f methyl c h l o r i d e  34.  Electron spectra for i o n i z a t i o n of  127  35.  E l e c t r o n spectra f o r i o n i z a t i o n of  128  36.  E l e c t r o n s p e c t r a f o r i o n i z a t i o n of  131  37.  Electron spectra for i o n i z a t i o n of  133  38.  E l e c t r o n spectra f o r i o n i z a t i o n of  135  39.  E l e c t r o n spectra f o r c o l l i s i o n processes i n helium  40.  E l e c t r o n spectra f o r c o l l i s i o n processes i n a mixture o f h e l i u m and argon  41.  E l e c t r o n s p e c t r a f o r c o l l i s i o n p r o c e s s e s i n argon  42.  E l e c t r o n s p e c t r a f o r c o l l i s i o n p r o c e s s e s i n neon  ....  119  ...  120 123  ....  ....  126  141  0  ACKNOWLEDGEMENT  The  work d e s c r i b e d  i n t h i s t h e s i s was done under t h e d i r e c t i o n  o f Dr. C. E. B r i o n , t o whom I express my deepest a p p r e c i a t i o n f o r h i s support and a s s i s t a n c e .  I would a l s o l i k e t o thank P r o f e s s o r C. A.  McDowell f o r h i s i n t e r e s t i n t h i s work. In a d d i t i o n , I w i s h t o acknowledge t h e a s s i s t a n c e o f t h e s t a f f o f t h e M e c h a n i c a l and E l e c t r o n i c s Workshops o f t h e Department o f Chemistry f o r t h e i r e x p e r t i s e i n the c o n s t r u c t i o n o f the instrument. I would a l s o l i k e t o thank my c o l l e a g u e s  i n the laboratory,  Drs. G. E. Thomas and L. A. R. O l s e n , f o r t h e i r h e l p f u l d i s c u s s i o n s and  support.  -1-  CHAPTER  ONE  INTRODUCTION 1.1.1. The  Introduction. e l e c t r o n i c s t r u c t u r e o f atoms and m o l e c u l e s has been  i n v e s t i g a t e d by means o f s p e c t r o s c o p i c  extensively  methods which i n v o l v e t h e s t u d y  of the emission or absorption o f electromagnetic r a d i a t i o n w i t h t r a n s i t i o n s between t h e v a r i o u s energy l e v e l s .  associated  Ionization of  atoms ( m o l e c u l e s ) f i r s t o c c u r s when t h e energy o f t h e incoming p a r t i c l e i s e q u a l t o t h a t needed t o remove t h e most l o o s e l y bound  electron.  Removal o f more t i g h t l y bound e l e c t r o n s  r e s u l t s i n the f o r m a t i o n o f an  i o n i n an e l e c t r o n i c a l l y e x c i t e d s t a t e .  The i o n i z a t i o n p r o c e s s may be  a d i r e c t t r a n s i t i o n o f a bound e l e c t r o n i n t o t h e i o n i z a t i o n continuum or i t may take p l a c e by an i n d i r e c t p r o c e s s  (autoionization).  S e v e r a l e x p e r i m e n t a l methods have been developed f o r t h e measurement of i o n i z a t i o n p o t e n t i a l s .  O p t i c a l s p e c t r o s c o p y p r o v i d e s t h e most a c c u r a t e  means o f d e t e r m i n i n g t h e i o n i z a t i o n p o t e n t i a l o f many atoms and s i m p l e molecules. be  Data from t h e f a r u l t r a v i o l e t a b s o r p t i o n spectrum can o f t e n  f i t t e d t o a Rydberg s e r i e s and t h e i o n i z a t i o n p o t e n t i a l i s then  g i v e n by t h e convergence l i m i t o f t h e s e r i e s .  Electron  bombardment,  w i t h o r w i t h o u t energy monochromation, has a l s o been w i d e l y used t o determine i o n i z a t i o n p o t e n t i a l s from t h e o n s e t s o f i o n i z a t i o n e f f i c i e n c y c u r v e s which a r e u s u a l l y o b t a i n e d u s i n g a mass s p e c t r o m e t e r . method o f p h o t o i o n i z a t i o n  i s s i m i l a r to that o f electron  t h a t a monochromatized beam o f photons i s s u b s t i t u t e d  The  impact except  f o r the electron  beam.  By c o n t r a s t p h o t o e l e c t r o n s p e c t r o s c o p y  a photon beam o f f i x e d wavelength,  (PES), which  employs  has been w i d e l y a p p l i e d i n the  determination of i o n i z a t i o n p o t e n t i a l s . p r e s e n t work f o r c a l i b r a t i o n purposes  I t has been used i n the  and w i l l be d i s c u s s e d i n a l a t e r  section. The  i o n i z a t i o n o f atoms and m o l e c u l e s by c o l l i s i o n w i t h n e u t r a l  e l e c t r o n i c a l l y e x c i t e d p a r t i c l e s o f t h e r m a l energy has been known f o r many y e a r s ( 1 - 9 ) .  The phenomenon i s i m p o r t a n t i n the s t u d y o f t h e  mechanisms f o r i o n p r o d u c t i o n i n f l a m e s , f o r s t u d i e s i n p l a n e t a r y and s t e l l a r atmospheres, i n shock heated gas systems, r a d i o l y s i s and plasmas.  The  photolysis,  study o f i o n i z a t i o n by n e u t r a l e x c i t e d  p a r t i c l e s i s i m p o r t a n t f o r the f o l l o w i n g r e a s o n s , (1) the r e a c t i o n s -13 occur between uncharged p a r t i c l e s ,  (2) the time o f i n t e r a c t i o n (10  se  i s s e v e r a l o r d e r s o f magnitude l o n g e r than i n c o l l i s i o n s w i t h e l e c t r o n s (10  — 16  sec.) o r photons (10  -18  s e c ) , (3) the r e a c t i o n s u s u a l l y t a k e  p l a c e between p a r t i c l e s w i t h near thermal k i n e t i c e n e r g i e s and i o n i z a t i o n r e s u l t s i n the r e l e a s e o f o n l y one Penning  I o n i z a t i o n E l e c t r o n Spectroscopy  (4)  electron. (PIES) p i o n e e r e d by  Cermak ( 1 0 ) , i n v o l v e s the energy a n a l y s i s o f t h e e l e c t r o n s r e s u l t i n g from i o n i z i n g c o l l i s i o n s between e l e c t r o n i c a l l y e x c i t e d , l o n g l i v e d neutral particles  (A ) and ground s t a t e atoms o r m o l e c u l e s A* + M  A + M  +  + e  (M), (1)  An a l t e r n a t i v e p r o c e s s , a s s o c i a t i v e i o n i z a t i o n ( i n some cases r e f e r r e d t o as a Hornbeck-Molnar p r o c e s s ) i n a c o l l i s i o n between A A* + M  and +  (4-9) , may  a l s o take place  M, AM  +  + e  (2)  -3-  The  c o l l i d i n g p a r t i c l e s a s s o c i a t e t o form a m o l e c u l a r i o n (AM )  and  +  electron i s released. * the m e t a s t a b l e A  T h i s r e a c t i o n can o c c u r even i f t h e energy  (11) has r e v i e w e d the a s s o c i a t i v e  In the p r o c e s s e s o f Penning g a i n s energy  of  i s lower than the i o n i z a t i o n p o t e n t i a l o f M, f o r  example, i n the f o r m a t i o n o f homonuclear d i a t o m i c i o n s o f n o b l e Franklin  an  gases.  i o n i z a t i o n process.  and a s s o c i a t i v e i o n i z a t i o n an  electron  from the i n t e r n a l energy o r from t h e k i n e t i c energy  of  r e l a t i v e motion o f two c o l l i d i n g p a r t i c l e s and i n so d o i n g makes a t r a n s i t i o n from a bound s t a t e t o a f r e e  state.  1.1.2. O b j e c t i v e s . At the time t h i s s t u d y was Cermak et a l . (10, 12-16) was published.  commenced i n 1968, t h e PIES work o f  e s s e n t i a l l y t h e o n l y work o f i t s t y p e  However, the r e s u l t s p u b l i s h e d by Cermak had u t i l i z e d a  c y l i n d r i c a l retarding  a n a l y z e r o f low r e s o l u t i o n (^ 0.2  eV o r more)  w i t h a l l the a t t e n d e n t d i s a d v a n t a g e s , such a s , i n t e g r a l s i g n a l and problem  o f "non-normal" e l e c t r o n s .  A l s o at t h i s t i m e , workers  the  i n photo-  e l e c t r o n s p e c t r o s c o p y had developed h i g h r e s o l u t i o n d e f l e c t i o n a n a l y z e r s and i t was  i n t e n d e d t o b u i l d a 127 degree d e f l e c t i o n a n a l y z e r t o o b t a i n  h i g h e r r e s o l u t i o n and a d i f f e r e n t i a l s i g n a l f o r PIES  studies.  I n i t i a l workers had hoped t h a t PIES would be a n o t h e r u s e f u l f o r measuring  ionization potentials.  However, our own  (17-19) e x p e r i e n c e i n d i c a t e d the e x i s t e n c e o f an energy measured i o n i z a t i o n p o t e n t i a l , i n most c a s e s , which  technique  and o t h e r s ' s h i f t i n the  complicates  any  d e t e r m i n a t i o n o f the a b s o l u t e v a l u e o f the i o n i z a t i o n p o t e n t i a l . method i s u s e f u l f o r i n v e s t i g a t i n g e n e r g e t i c i o n i z a t i o n and  energy  The  -4-  t r a n s f e r between atoms and v a r i o u s t a r g e t atoms ("molecules) .  It i s  a l s o i n t e r e s t i n g t o compare t h e t r a n s i t i o n p r o b a b i l i t i e s observed f o r absorption of electromagnetic r a d i a t i o n  (PES) and m e t a s t a b l e  collisions  (PIES) f o r , (i)  V i b r a t i o n a l e x c i t a t i o n o f i o n i c s t a t e s , t o compare t h e FranckCondon F a c t o r s f o r t h e two v e r y d i f f e r e n t types o f i o n i z i n g mechanism.  ( i i ) To compare t h e p o p u l a t i o n o f e l e c t r o n i c i o n s t a t e s f o r t h e photon and Penning p r o c e s s e s , s i n c e one might i n g e n e r a l  expect  e l e c t r o n i c t r a n s i t i o n moments t o d i f f e r f o r t h e two p r o c e s s e s . Such a study has i m p o r t a n t s i g n i f i c a n c e w i t h r e g a r d t o t h e t y p e and d i s t r i b u t i o n o f energy d e p o s i t i o n i n a r c s , plasmas and atmospheres where many o f t h e c h e m i c a l p r o c e s s e s  a r e i n f a c t c o l l i s i o n s between  p a r t i c l e s and i t would be i n a p p r o p r i a t e t o use t h e a v a i l a b l e  optical  data t o describe the s t a t e p o p u l a t i o n . The (1)  aim was:t o employ PIES t o s y s t e m a t i c a l l y study atoms and d i a t o m i c s , t r i a t o m i c s and o t h e r s m a l l p o l y a t o m i c m o l e c u l e s  to provide  such d a t a f o r commonly o c c u r i n g c h e m i c a l s p e c i e s , s i n c e no o t h e r d a t a was g e n e r a l l y a v a i l a b l e (and i n f a c t i s s t i l l u n a v a i l a b l e o t h e r than f o r a few d i a t o m i c (20) and p o l y a t o m i c (21) m o l e c u l e s ) , (2)  t o compare h e l i u m m e t a s t a b l e  ionization with photoionization  (He S84&). (3)  t o i n v e s t i g a t e other c o l l i s i o n processes  i n mixed r a r e gas  systems, a t h i g h e r p r e s s u r e s , such a s , f a s t n e u t r a l - n e u t r a l  -5-  r e a c t i o n s and p r o c e s s e s o c c u r r i n g between a m e t a s t a b l e atom and o t h e r m e t a s t a b l e atoms. As a r e s u l t o f t h i s g e n e r a l  survey i t was  hoped t o g a i n f u r t h e r  u n d e r s t a n d i n g o f Penning i o n i z a t i o n , a s s o c i a t i v e i o n i z a t i o n , s i m i l a r i t i e s and d i f f e r e n c e s i n PIES and  PES  and  the  energy t r a n s f e r  processes i n general.  1.2  Penning I o n i z a t i o n Penning and  a s s o c i a t i v e i o n i z a t i o n have r e c e i v e d much a t t e n t i o n i n  r e c e n t y e a r s and much e x p e r i m e n t a l o f the p r o c e s s e s ,  knowledge, c o n c e r n i n g s e v e r a l  has been o b t a i n e d .  This information  includes,  the i n i t i a l s t a t e o f the e x c i t e d p a r t i c l e , (b) the e j e c t e d energy s p e c t r a ,  (c) the c o m p e t i t i o n  t o p r o d u c t i o n s and  aspects (a)  electron  between the v a r i o u s p r o c e s s e s  (d) c r o s s - s e c t i o n s f o r Penning and  leading  associative  ionization.  1.2.1. I n i t i a l S t a t e o f  A* *  There are t h r e e types o f e x c i t e d s t a t e s o f A which may be i n v o l v e d * when A c o l l i d e s w i t h a t a r g e t p a r t i c l e M. ( i ) A normal o p t i c a l l y allowed  s t a t e which has  h e l i u m (0.56 and  a s h o r t l i f e t i m e such as the 2 P s t a t e o f  nsec. (22)),  3 2 S states of helium.  ( i i ) A m e t a s t a b l e s t a t e , such as the  T h i s type o f s t a t e i s the most commonly  employed because o f the long l i f e t i m e * He  2^S  ( f o r example, the l i f e t i m e  of  1 (2 S) i s 20 msec. ( 2 3 ) ) .  used i n the work d e s c r i b e d  T h i s type of e n e r g e t i c p a r t i c l e has  i n t h i s t h e s i s and T a b l e 1 l i s t s  i o n i z a t i o n , m e t a s t a b l e and photon e n e r g i e s  been  the  r e l e v a n t t o the p r e s e n t work.  -6-  TABLE  I  Energy l e v e l s f o r r a r e gas i o n s , atoms and photons METASTABLES Atom  I.P.(eV)  Desig.  He  24.586  2S 3 2 S  Ne  1  o  Energy(eV) 20.615  Wavelength(A) He I  584.4  21.217  He I I  303.8  40.811  16.795  Ne I  743.7  16.671  C^/  16.619  735.9  16.848  462.4  26.813  460.7  26.910  1066.7  11.623  1048.2  11.823  932.1  13.302  919.8  13.479  1235.8  10.032  1164.9  10.643  1469.6  8.436  1312.4  9.447  Ne I I  Ar  ( P , ) 15.759  11.723  ( P  11.548  ) 15.937  2  2  1  Kr  { P  10.562  ) 14.665  9.915  ( P . ) 12.130  9.447  ( P  Kr I  2  /  2  Xe  Ar I  Ar I I  ) 13.999 5  i ;  Energy(eV)  19.818  ( P , ) 21.564 ) 21.661  PHOTONS  Xe I  2  1  ( P  . ) 13.436  2  2  1  8.315  -7-  A l t h o u g h the m a j o r i t y o f Penning i o n i z a t i o n s t u d i e s have employed h e l i u m m e t a s t a b l e atoms, some work has been r e p o r t e d which o t h e r m e t a s t a b l e atoms (18, 24-26).  utilized  However, the u s e f u l n e s s o f o t h e r  m e t a s t a b l e atoms, such as t h o s e o f k r y p t o n and xenon, i s s e v e r e l y l i m i t e d because o f t h e i r low e n e r g i e s . s t a t e s , the e x i s t e n c e o f which was  ( i i i ) Long l i v e d h i g h l y  excited  f i r s t e s t a b l i s h e d by Cermak and  Herman (10) i n e l e c t r o n impact e x c i t e d beams.  The energy o f t h e s e h i g h l y  e x c i t e d p a r t i c l e s i s almost e q u a l t o the f i r s t  ionization potential  and i t i s p r o b a b l e t h a t these s t a t e s c o r r e s p o n d t o the e x c i t a t i o n o f one e l e c t r o n t o Rydberg Hotop and Niehaus  s t a t e s w i t h l a r g e p r i n c i p l e " quantum numbers.  (17) and a l s o K u p r y i a n o v  t h a t the r a d i a t i v e l i f e t i m e o f Rydberg  (27, 28) have p o i n t e d out  states increases with p r i n c i p l e  4 5 quantum number n as n  .  Hotop and Niehaus  (17) have r e p o r t e d a b s o l u t e  c r o s s s e c t i o n d a t a f o r c o l l i s i o n s between h i g h l y i n g , l o n g l i v e d atoms and ground s t a t e m o l e c u l e s i n which i o n i z a t i o n o f the e x c i t e d atom occurs.  For many m o l e c u l e s the i o n i z e d atom was not d e t e c t e d .  Subsequently they proposed  (18) t h a t i n t h e s e cases the i o n i z e d atom  and the m o l e c u l e form a complex which decomposes by an e x o t h e r m i c to  g i v e the i o n i z e d m o l e c u l e o r fragment  ions.  1.2.2. I n t e r p r e t a t i o n i n Terms o f P o t e n t i a l Energy The  reaction  Curves.  i n t e r p r e t a t i o n of i o n i z a t i o n processes i n thermal c o l l i s i o n s i n  terms o f p o t e n t i a l energy c u r v e s has been d i s c u s s e d by M u l l i k e n ( 2 9 ) , Herman and Cermak (15, 16, 30) and a l s o by Hotop and Niehaus  (18, 19, 31,  32) . I t has g e n e r a l l y been assumed t h a t i n the c o l l i s i o n o f the m e t a s t a b l e  -8-  * atom A , w i t h the t a r g e t p a r t i c l e M, the i o n AM  +  i s a t t r a c t i v e and  t h a t the ground s t a t e p o t e n t i a l  of  t h u s e x h i b i t s a minimum.  *  *  The p o t e n t i a l energy c u r v e o r c u r v e s o f the e x c i t e d (AM) (or A i n t e r a c t i o n ) must be c o n s t r u c t e d and t h e r e are t h r e e cases which can * considered.  In the f i r s t case the p o t e n t i a l energy curve o f  (AM)  o r d i n a r i l y l i e s everywhere below the curve o f the ground s t a t e o f In the second c a s e , shown i n F i g u r e bound t o an e x c i t e d AM  1,  (AM)  - M be  c o r r e s p o n d s t o an  AM . +  electron  and  the p o t e n t i a l c u r v e o f (AM) l i e s above the * + curve o f the i o n . T h i r d l y , (AM) c o r r e s p o n d s t o an e l e c t r o n bound t o AM in a repulsive state. O t h e r cases e x i s t but have not been c o n s i d e r e d i n d e t a i l .  It i s  *  p o s s i b l e , f o r example, t o c o n s i d e r were e x c i t e d . outside  the case o f (AM)  However, t h i s case does not  an i o n - m o l e c u l e c o r e .  have t o be i n c l u d e d  i n which two  c o r r e s p o n d t o one  Cases o f t h i s n a t u r e w i l l  electrons  electron  ultimately  i n the i n t e r p r e t a t i o n o f Penning and  associative  i o n i z a t i o n i n terms o f the d e t a i l s o f the c o r r e l a t i o n o f s e p a r a t e d atom and u n i t e d atom o r b i t a l s and  states.  For the t h r e e cases p r e v i o u s l y m e n t i o n e d , we into consideration  the b i n d i n g p r o p e r t i e s  However, t h e s e p r o p e r t i e s  should a c t u a l l y take  o f the e x c i t e d  electron.  o r d i n a r i l y have a r e l a t i v e l y s m a l l e f f e c t  the p o t e n t i a l when compared w i t h the s t a t e o f the AM . +  This occurs  because the e x c i t e d e l e c t r o n , n o r m a l l y , i s s u f f i c i e n t l y f a r from n u c l e i t h a t i t s bonding f o r c e i s s m a l l e x c i t e d e l e c t r o n has o r b i t a l s o f AM  +  I f the o r b i t a l o f  a symmetry d i f f e r e n t from any  o f the  +  which has  the an  occupied  then the b o n d i n g o r a n t i b o n d i n g f o r c e i s g r e a t e r  t h e r e i s an o c c u p i e d o r b i t a l o f AM M u l l i k e n has  (32).  on  than i f  the same symmetry.  r e f e r r e d t o the case where the symmetry i s d i f f e r e n t as  -9-  F i g u r e 1.  P o t e n t i a l energy curve f o r a s s o c i a t i v e and Penning  Ionization.  -10-  'penetrating' penetrating';  and  the case where the symmetry i s the same as  the n o n p e n e t r a t i n g o r b i t a l s are kept out o f the core by  the e f f e c t i v e r e p u l s i o n due  t o the e x c l u s i o n p r i n c i p l e .  I f the e x c i t e d e l e c t r o n o f the m e t a s t a b l e A c o r r e l a t e s w i t h a bonding o r b i t a l o f AM o f AM  'non-  i s i n an o r b i t a l which  then the p o t e n t i a l energy c u r v e  w i l l be below the c u r v e f o r the c o r r e s p o n d i n g s t a t e of the  c o r e , whether the core s t a t e i s i n the ground s t a t e o f AM higher  state.  antibonding.  This  or some  +  i s a l s o t r u e i f the e x c i t e d o r b i t a l i s o n l y weakly  I f the e x c i t e d o r b i t a l i s s t r o n g l y a n t i b o n d i n g  p o t e n t i a l curve o f AM the  AM  *  may  then  i n p r i n c i p l e r i s e above t h a t o f AM  +  .  the Parts  of  £ s t a t e s are r e p u l s i v e because the e x c i t e d o r b i t a l s c o r r e l a t e  a d i a b a t i c a l l y w i t h h i g h l y promoted o r b i t a l s i n the u n i t e d atom l i m i t t h e r e b y become a n t i b o n d i n g .  Thus i t i s u n l i k e l y t h a t the AM  above t h a t o f the c o r r e s p o n d i n g AM  and  curve r i s e s  curve because the r e p u l s i v e energy  +  would have t o be g r e a t e r than the i o n i z a t i o n energy o f the  excited  o r b i t a l f o r t h i s to occur. Mulliken He  + He  *  (33) has  and has  considered  shown how  homonuclear d i a t o m i c s ,  some o f the low  such as  l y i n g s t a t e s o f We^  have p o t e n t i a l maxima at l a r g e i n t e r n u c l e a r d i s t a n c e s .  *  must  T h i s would make  *  i t d i f f i c u l t f o r He  + He  to achieve small i n t e r n u c l e a r distances  in a  t h e r m a l c o l l i s i o n which i s r e q u i r e d t o r e a c h the p o t e n t i a l f o r We^  -  +  e  By comparison, the h i g h e x c i t e d s t a t e s are e x p e c t e d t o have p o t e n t i a l +  energy c u r v e s n e a r l y p a r a l l e l t o and Herman and Cermak (15, 16, 30) have s t u d i e d h e t e r o n u c l e a r have been s t u d i e d  cases.  s l i g h t l y below the c u r v e f o r He and Niehaus et a l . (18, 19, 31,  The  2  • 32)  r a r e gas-metal atom systems which  (16, 19, 31, 32) have p o t e n t i a l energy c u r v e s  c o r r e s p o n d i n g t o the second case ( i . e . the p o t e n t i a l energy curve o f  AM  -11-  * l i e s below the curve o f (AM)  ) , because the lowest e x c i t e d s t a t e s o f  the r a r e gases are w e l l above the i o n i z a t i o n p o t e n t i a l s o f the a l k a l i s or mercury.  E l e c t r o n s which are r e l e a s e d i n Penning i o n i z a t i o n  may  c a r r y away a l a r g e amount o f energy o r they may  leave the i o n i z e d  system w i t h c o n s i d e r a b l e t r a n s l a t i o n a l energy.  The i o n i z a t i o n o f  mercury  3  by 2 S h e l i u m atoms (31) y i e l d s e l e c t r o n s which have l e s s energy  than  3  the d i f f e r e n c e between the h e l i u m 2 S m e t a s t a b l e energy and the mercury 2 2 2 i o n i z a t i o n p o t e n t i a l s ( S . , D . o r D , ) and the e l e c t r o n •J-/ ~>/^ ^/ j * 3 d i s t r i b u t i o n s due t o i o n i z a t i o n by He (2 S) conform t o t h e known a v a i l a b l e * 1 s t a t e s o f mercury.  The s i t u a t i o n f o r i o n i z a t i o n by He  (2 S) i s more  complex, Cermak and Herman ( 1 6 ) , u s i n g a low r e s o l u t i o n a p p a r a t u s , * 1 observed t h a t the d i s t r i b u t i o n s 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 by the He (2 S) m e t a s t a b l e appear t o be s h i f t e d t o e l e c t r o n energy v a l u e s lower than * 3 expected w h i l e those due t o He (2 S) were at the expected energy; however, t h i s work l a c k e d a b s o l u t e energy c a l i b r a t i o n .  Fuchs and Niehaus  (19)  observed t h a t the c o n t r i b u t i o n t o the s p e c t r a due t o i o n i z a t i o n by the * He  1 (2 S) m e t a s t a b l e s i s spread out and e x h i b i t s t h r e e peaks, the most  i n t e n s e o f which l i e s a t the lowest e l e c t r o n energy. (31) have suggested t h a t t h i s may  Hotop and  Niehaus  be due t o one o r more p o t e n t i a l  energy curves which c o r r e s p o n d t o o t h e r e x c i t e d s t a t e s o f a q u a s i m o l e c u l e He/Hg and which c r o s s the e x c i t e d p o t e n t i a l c u r v e .  I f at a c e r t a i n  s e p a r a t i o n a t r a n s i t i o n from the i n i t i a l p o t e n t i a l c u r v e t o another curve o c c u r s and i f a f t e r a s h o r t time a u t o i o n i z a t i o n t o the  final  p o t e n t i a l c u r v e , a t a s e p a r a t i o n l e s s than b e f o r e , o c c u r s then the d i f f e r e n c e i n the s i n g l e t d i s t r i b u t i o n s may Hotop and Niehaus  be e x p l a i n e d .  (18) have a l s o attempted t o i n t e r p r e t the Penning  -12-  i o n i z a t i o n of m o l e c u l a r hydrogen.  They assume t h a t the system goes .j.  r a p i d l y i n t o a t r a n s i e n t s t a t e t h a t they d e s c r i b e  as  where the H ^  t o the  ^  +  2  v i b r a t i o n a l l y e x c i t e d according  s  CHe  - H  )  2  Franck-Condon  +t t r a n s i t i o n H^—^  H  2  .  I f the t r a n s i t i o n o c c u r s at l a r g e He  -  d i s t a n c e s , the v i b r a t i o n a l energy d i s t r i b u t i o n w i l l be the same as i n +t H^  produced by f a s t e l e c t r o n or photon impact.  occurs at s m a l l e r d i s t a n c e s  where  i s d i s t o r t e d by the e x c i t e d h e l i u m  atom, the d i s t r i b u t i o n can be d i f f e r e n t .  The  +1 f o r the  [He - H  2  most p r o b a b l e decay mode  + ) i s d i r e c t l y i n t o He + H  e x c i t e d v i b r a t i o n a l l y , then the system may has  I f the t r a n s i t i o n  + .  2  I f the H  go t o HeH  e s s e n t i a l l y no v i b r a t i o n a l energy, a H e H  + 2  +  is sufficiently  2  + H.  I f the  m o l e c u l e may  be  H  + 2  formed. **  For the c o l l i s i o n o f a h i g h l y e x c i t e d s p e c i e s , such as Ar  ,  w i t h m o l e c u l a r hydrogen the mechanism proposed by Hotop and Niehaus i s described  d i f f e r e n t l y , s i n c e the dominant s p e c i e s  produced i s A r H . +  D i r e c t c o l l i s i o n a l i o n i z a t i o n of the h i g h l y e x c i t e d atom, r a t h e r than Penning i o n i z a t i o n o f the t a r g e t p a r t i c l e , was step i n the r e a c t i o n .  The  i o n i z a t i o n was  postulated  explained  as the  first  i n terms o f a **  c r o s s i n g o f the p o t e n t i a l c u r v e s o f the systems (Ar and  electrons  +  - H) 2  and  (Ar  +  l e a v i n g the system c a r r y i n g away v e r y l i t t l e energy.  H  2  The  system i s then i n a s t a t e which l i e s w e l l above the d i s s o c i a t i o n l i m i t f o r t^e Ar - H o r H - H bond i n A r H Ar H  +  + 2  so t h a t decay o c c u r s t o  give  + H. The  r e l a t i o n s h i p between Penning and  be c o n s i d e r e d  associative ionization  i n terms o f p o t e n t i a l energy c u r v e s ; f o r example,  m e t a s t a b l e h e l i u m atoms i n c o l l i s i o n w i t h argon atoms. e l e c t r o n must c a r r y away a l a r g e amount o f energy.  We  may consider  In t h i s case should also  the  consider  -13-  the n a t u r e o f the r e l a t i o n s h i p between the i n i t i a l n u c l e a r  kinetic  energy and the f r a c t i o n o f the energy c a r r i e d away by the e l e c t r o n . Herman and Cermak (30) have i n t e r p r e t e d the r a t i o o f a s s o c i a t i v e t o Penning i o n i z a t i o n i n terms o f the p o i n t at which the Franck-Condon t r a n s i t i o n takes p l a c e .  They c o n s i d e r  t h a t the s i t u a t i o n f o r Penning  and a s s o c i a t i v e i o n i z a t i o n d i f f e r s o n l y i n the f a c t t h a t t r a n s i t i o n s from c l a s s i c a l t u r n i n g p o i n t s on the upper c u r v e s r e a c h above or below the d i s s o c i a t i o n l i m i t of AM . +  i o n i z a t i o n was  The  r a t i o o f a s s o c i a t i v e t o Penning  d e t e r m i n e d by the f r a c t i o n o f c o l l i d i n g p a i r s w i t h  r e l a t i v e k i n e t i c energy above the t r a n s i t i o n energy (which i s the  energy  above which Franck-Condon t r a n s i t i o n s g i v e Penning i o n i z a t i o n and  below  which the t r a n s i t i o n s g i v e a s s o c i a t i v e i o n i z a t i o n ) . I t i s g e n e r a l l y assumed t h a t the shape o f the two p o t e n t i a l energy curves are s u f f i c i e n t l y d i f f e r e n t t h a t most t r a n s i t i o n s o c c u r i n the r e g i o n o f the c l a s s i c a l t u r n i n g p o i n t .  The  thermal d i s t r i b u t i o n of c o l l i s i o n energies  t r a n s i t i o n energy and  the  governs the r a t i o o f a s s o c i a t i v e  t o Penning i o n i z a t i o n .  1.2.3. E j e c t e d E l e c t r o n Energy The  Spectra.  energy a n a l y s i s o f e l e c t r o n s from Penning i o n i z a t i o n p r o c e s s e s  can be u s e d , t o a f i r s t a p p r o x i m a t i o n as a s p e c t r o s c o p i c  probe t o l o c a t e  s t a t e s o f the m o l e c u l a r i o n i f m e t a s t a b l e p a r t i c l e s o f known energy are used o r c o n v e r s e l y  one may  l o c a t e m e t a s t a b l e s t a t e s o f the  projectile  i f the i o n i c s t a t e s o f d e t e c t o r m o l e c u l e s are known. The E  o  energy o f the e j e c t e d e l e c t r o n ( E ) g  = E(A*)  - [ I P ( M ) + E„ +  , (M )] +  + AE  i s given  by (3)  -14-  * * where E(A ) i s the energy o f the e x c i t e d i o n i z i n g p a r t i c l e s (A ) , I P ( M ) i s the f i r s t or h i g h e r +  or m o l e c u l e E  i o n i z a t i o n p o t e n t i a l o f an atom  (M),  . i s the v i b r a t i o n a l and  r o t a t i o n a l e x c i t a t i o n of  M, +  AE i s an energy term i n v o l v i n g the i n t e r c o n v e r s i o n o f i n t e r n a l and t r a n s l a t i o n a l energy between the v a r i o u s p a r t i c l e s i n v o l v e d i n the c o l l i s i o n I f AE= The  process.  0, I P ( M ) can be d e t e r m i n e d by measuring the e l e c t r o n energy +  f a c t t h a t AE j- 0, i n many c a s e s , s e v e r e l y l i m i t s the use  as an a c c u r a t e method f o r d e t e r m i n i n g  ionization potentials.  E  o f PIES However,  u s e f u l i n f o r m a t i o n c o n c e r n i n g the c h a r a c t e r o f the c o l l i s i o n as w e l l v i b r a t i o n a l and e l e c t r o n i c s t a t e p o p u l a t i o n s data.  The  m a j o r i t y o f the s t u d i e s r e p o r t e d  can be o b t a i n e d  the  have been made by  Cermak and Herman (12 16) and by Niehaus and h i s co-workers (19, 31, 35).  In most cases m e t a s t a b l e r a r e gas  as  from the  to date, concerning  e l e c t r o n e j e c t e d i n the Penning i o n i z a t i o n p r o c e s s ,  e <  34,  atoms o f known energy have been  used. Cermak has  a p p l i e d a L o z i e r s t o p p i n g p o t e n t i a l method t o energy  a n a l y z e e l e c t r o n s r e l e a s e d i n Penning i o n i z a t i o n p r o c e s s e s at t h e r m a l energies.  The  (approximately  energy r e s o l u t i o n o f t h i s apparatus was 0.2  rather  low  eV) which i n most cases p r e v e n t e d the s t u d y of f i n e  s t r u c t u r e such as s p i n - o r b i t s p l i t t i n g s and m o l e c u l a r v i b r a t i o n s . Cermak has used t h i s apparatus t o study the e l e c t r o n i c s t a t e s o f approximately f o r t y molecular ions. showed t h a t some peaks may  The  observed e l e c t r o n d i s t r i b u t i o n s  be s h i f t e d t o lower e l e c t r o n e n e r g i e s ,  that  i s many o f the e j e c t e d e l e c t r o n s c a r r y away l e s s than the c a l c u l a t e d  -15-  excess energy f o r t h e i o n i z a t i o n p r o c e s s .  Cermak a l s o o b s e r v e d t h a t some  p r o c e s s e s may g i v e r i s e t o i o n s w i t h excess k i n e t i c energy.  Initially  i t was b e l i e v e d t h a t Penning i o n i z a t i o n e l e c t r o n s p e c t r o s c o p y would provide  another means o f a c c u r a t e l y d e t e r m i n i n g  ionization potentials.  I t was suggested (36) t h a t t h e d i f f e r e n c e s observed i n t h e i o n i z a t i o n p o t e n t i a l s , as determined by PES and PIES, might be due t o t h e f a c t t h a t t h e Penning work was c a r r i e d out a t lower r e s o l u t i o n than most photoelectron  studies.  Niehaus and h i s co-workers have used an e l e c t r o s t a t i c l e n s i n c o n j u n c t i o n w i t h a r e t a r d i n g e l e c t r i c f i e l d t o measure t h e energy o f the e l e c t r o n s e j e c t e d i n t h e Penning p r o c e s s . a n a l y z e r was o f c o n s i d e r a b l y than t h a t used by Cermak.  The e l e c t r o n energy  higher r e s o l u t i o n (approximately  0.02 eV)  The r e s o l u t i o n was determined from photo-  e l e c t r o n energy d i s t r i b u t i o n s .  With t h i s r e s o l u t i o n Niehaus e t a l . have  been a b l e t o study r a r e gas atoms and t o measure v i b r a t i o n a l s p a c i n g s and t h e p o p u l a t i o n s ion  o f a c c e s s i b l e e l e c t r o n i c states o f the molecular  s t a t e s o f a few d i a t o m i c  molecules.  Niehaus e t a l . (19, 31) have found t h a t t h e e l e c t r o n energy d i s t r i b u t i o n s of electrons belonging  t o a s i n g l e i o n i z a t i o n process  have a f i n i t e w i d t h ( g r e a t e r than p h o t o e l e c t r o n be s h i f t e d , w i t h r e s p e c t t o t h e a b s o l u t e  peak w i d t h s ) and may  energy d i f f e r e n c e between t h e  energy o f t h e m e t a s t a b l e and the i o n i z a t i o n p o t e n t i a l o f t h e t a r g e t . *  1  *  3  I o n i z a t i o n by He (2 S) and He (2 S) m e t a s t a b l e s was found t o produce d i s t i n c t d i f f e r e n c e s i n t h e observed shapes and s h i f t s o f t h e d i s t r i b u t i o n s f o r many t a r g e t s p e c i e s .  I t was f o u n d , f o r argon, t h a t t h e d i s t r i b u t i o n s  showed a marked change w i t h t h e r e l a t i v e k i n e t i c energy o f t h e c o l l i d i n g p a r t i c l e s , t h e t r i p l e t d i s t r i b u t i o n s becoming c o n s i d e r a b l y  broader at  -16-  higher energies  ( t e m p e r a t u r e ) , whereas d i s t r i b u t i o n s produced by *  1  c o l l i s i o n s w i t h He (2 S) a r e e s s e n t i a l l y unchanged. The  s h i f t s (AE) i n t h e e l e c t r o n  energy d i s t r i b u t i o n s observed by  Niehaus e t a l . v a r y from +0.055 eV f o r n i t r o g e n (B^£* s t a t e ) ( i . e . t h e energy o f t h e e l e c t r o n  i s g r e a t e r than t h e d i f f e r e n c e  between t h e energy  o f t h e m e t a s t a b l e and t h e i o n i z a t i o n p o t e n t i a l o f the t a r g e t ) t o 2 -0.100 eV f o r mercury ( P_. s t a t e ) ( t h e energy o f t h e e l e c t r o n i s l e s s than expected}. The measured s h i f t o f the maximum o f t h e e l e c t r o n 2 energy d i s t r i b u t i o n f o r argon ( P . ) ( a t 500°K) i s +0.045 eV and has been e x p l a i n e d (31) by assuming t h a t i n most c o l l i s i o n s l e a d i n g t o Penning i o n i z a t i o n , k i n e t i c energy o f t h e n u c l e i energy o f t h e e j e c t e d The states  i s converted t o e l e c t r o n i c  electron.  r e l a t i v e p r o b a b i l i t i e s f o r ionization to various  electronic  o f t h e m o l e c u l a r i o n a r e o f t e n observed t o d i f f e r f o r Penning  i o n i z a t i o n and p h o t o i o n i z a t i o n because t h e e l e c t r o n i c  ( 1 3 , 15, 3 5 ) . T h i s may be expected  t r a n s i t i o n moments r e s p o n s i b l e f o r i o n i z a t i o n a r e  d i f f e r e n t f o r t h e two p r o c e s s e s and s h o u l d n o r m a l l y l e a d t o d i f f e r i n g r e l a t i v e populations. f o r each e l e c t r o n i c  The r e l a t i v e p o p u l a t i o n s o f v i b r a t i o n a l  states  s t a t e a r e found t o be v e r y s i m i l a r f o r t h e two p r o c e s s e s .  Niehaus (35) has attempted t o e x p l a i n  small but d e f i n i t e differences i n  the v i b r a t i o n a l p o p u l a t i o n s by a weak d i s t o r t i o n o f t h e m o l e c u l e by t h e m e t a s t a b l e atom d u r i n g i o n i z a t i o n .  1.3.  Associative Associative  Ionization. i o n i z a t i o n has f r e q u e n t l y been s t u d i e d i n c o n j u n c t i o n  w i t h Penning i o n i z a t i o n i n c o l l i s i o n s between e x c i t e d m e t a s t a b l e atoms  -17-  and atoms o r m o l e c u l e s .  A s t u d y by Hotop and Niehaus (37, 38)  employing  m e t a s t a b l e h e l i u m atoms w i t h o t h e r r a r e gases as t a r g e t p a r t i c l e s has examined the dependence o f the r e l a t i v e c r o s s s e c t i o n s f o r Penning a s s o c i a t i v e i o n i z a t i o n on the a v a i l a b l e c o l l i s i o n energy. that formation  I t was  and found  o f the a s s o c i a t i v e i o n i z a t i o n p r o d u c t became more i m p o r t a n t  at low c o l l i s i o n e n e r g i e s  and t h a t i t was  p o s s i b l e to p r e d i c t  (approximately)  the r a t i o o f a s s o c i a t i v e t o Penning i o n i z a t i o n from the amount o f o f the e l e c t r o n energy d i s t r i b u t i o n .  The  p r e d i c t i o n was  w i t h the mass s p e c t r o m e t r i c a l l y d e t e r m i n e d r a t i o .  shift  i n good agreement  However, t h i s  a p p r o x i m a t i o n , on the b a s i s o f the measured s h i f t , has been found t o v a l i d o n l y f o r the r a r e gases.  Hotop and Niehaus (38) had  r e p o r t e d the r e l a t i v e importance o f Penning and  previously  associative ionization  i n c o l l i s i o n s o f m e t a s t a b l e h e l i u m and neon atoms w i t h hydrogen deuterium hydride.  The  atoms has been r e p o r t e d  and  i o n i z a t i o n o f many o r g a n i c m o l e c u l e s by m e t a s t a b l e (12, 39, 40, 41).  P r o d u c t s r e s u l t i n g from  a s s o c i a t i v e i o n i z a t i o n were o b s e r v e d o n l y i n the case o f Cermak has  be  acetylene.  observed t h a t i n r e a c t i o n s i n v o l v i n g e x c i t e d m e t a s t a b l e  atoms, a s s o c i a t i v e i o n i z a t i o n does not  compete e f f e c t i v e l y w i t h  i o n i z a t i o n i f the t a r g e t p a r t i c l e i s a p o l y a t o m i c m o l e c u l e .  Penning  Only i f the  t a r g e t i s an atom or s i m p l e m o l e c u l e does a s s o c i a t i v e i o n i z a t i o n o c c u r with high p r o b a b i l i t y .  I f dissociation i s energetically possible i t  appears t h a t a s s o c i a t i v e i o n i z a t i o n o c c u r s w i t h low p r o b a b i l i t y . A s s o c i a t i v e and  Penning i o n i z a t i o n have, i n some c a s e s , been  found t o have comparable c r o s s s e c t i o n s i o n i z a t i o n has  ( i . e . Ar  *  + Hg)  a l s o been observed w i t h m o l e c u l a r e x c i t e d  * (42-44), f o r example, N_  * and CO  .  (30).  Associative  precursors  -18-  1.4. R e l a t i v e and A b s o l u t e Cross S e c t i o n s . * 1 * 3 The r a t i o o f t h e e x c i t a t i o n c r o s s s e c t i o n f o r He (2 S) and He (2 S) e x c i t e d by e l e c t r o n impact has been s t u d i e d by s e v e r a l workers 45-47) and t h e s e r e s u l t s a r e shown i n F i g u r e 2. the r a t i o was independent  Cermak  (37,  (47) found t h a t  o f t h e h e l i u m p r e s s u r e i n the e x c i t a t i o n  r e g i o n and t h e c u r r e n t o f e x c i t i n g e l e c t r o n s .  However, t h e r a t i o  will  i n g e n e r a l d i f f e r from t h e r a t i o o f t h e number o f s i n g l e t and t r i p l e t m e t a s t a b l e atcrns i n t h e beam because the o r i g i n a l r a t i o o f m e t a s t a b l e s * produced  1  *  3  i s m o d i f i e d by v a r i o u s e f f e c t s f o r which He (2 S) and He (2 S)  have d i f f e r e n t c r o s s s e c t i o n s .  These f a c t o r s i n c l u d e e l a s t i c  scattering,  e x c i t a t i o n t r a n s f e r , d e a c t i v a t i o n , s u p e r e l a s t i c c o l l i s i o n s w i t h slow e l e c t r o n s , p o p u l a t i o n by resonance  r a d i a t i o n and p o p u l a t i o n by c a s c a d i n g  from h i g h e r s i n g l e t and t r i p l e t s t a t e s . *  1  *  A l s o , i f the angular d i s t r i b u t i o n  3  o f the He (2 S) and He (2 S) atoms formed a t t h e c r o s s i n g p o i n t o f t h e e l e c t r o n and h e l i u m beams i s d i f f e r e n t , then t h e r a t i o o f m e t a s t a b l e s i n a beam i s a n g u l a r dependent and o n l y e q u a l s t h e r a t i o o f t h e e x c i t a t i o n c r o s s s e c t i o n s i f a l l m e t a s t a b l e s produced  are c o l l e c t e d .  T h i s may  lead  t o d i s c r e p a n c i e s between measurements u s i n g a d i r e c t e d h e l i u m beam (37, 45, 47) and measurements w i t h d i f f u s e h e l i u m gas i n t h e e x c i t a t i o n  chamber  (46). The e x p e r i m e n t a l s t u d y o f t h e c r o s s s e c t i o n f o r Penning 1  ionization  3  by 2 S and 2 S m e t a s t a b l e h e l i u m atoms has been t h e s u b j e c t o f s e v e r a l i n v e s t i g a t i o n s (37, 45, 46, 48-55) i n r e c e n t y e a r s .  The m a j o r i t y o f  these i n v e s t i g a t i o n s have been c o m p l i c a t e d by t h e f a c t t h a t t h e r e l a t i v e number o f s i n g l e t and t r i p l e t m e t a s t a b l e atoms, c o n t a i n e d i n t h e beam, has not been a c c u r a t e l y known and i t has been n e c e s s a r y t o a r b i t r a r i l y n o r m a l i z e d a t a by assuming t h a t t h e c r o s s s e c t i o n r a t i o f o r s i n g l e t  2.0  1.5  CO  1.0  0.5  DUG AM  et al  - H O L T et al 20  30 ELECTRON  F i g u r e 2.  40 ENERGY  50 (eV)  R a t i o o f s i n g l e t t o t r i p l e t h e l i u m m e t a s t a b l e atoms as a f u n c t i o n o f the energy o f the exciting electrons.  -20-  and t r i p l e t m e t a s t a b l e s f o r a p a r t i c u l a r atom o r m o l e c u l e i s u n i t y . Secondary  e l e c t r o n e j e c t i o n from metal s u r f a c e s has o f t e n been  employed t o measure t h e f l u x o f m e t a s t a b l e atoms (45, 46, 48-50) and i t was assumed t h a t t h e secondary e l e c t r o n e j e c t i o n c o e f f i c i e n t s , 6, * 1 * 3 f o r He (2 S) and He (2 S) were e q u a l .  R e c e n t l y Dunning and Smith (54)  have found t h a t t h i s i s n o t t h e case and i n f a c t t h e e j e c t i o n  efficiency  * 1 * 3 o f He (2 S) i s l e s s than t h a t o f He (2 S ) . From t h e i r r e s u l t s , t h e y were a b l e t o deduce t h a t t h e r a t i o o f e j e c t i o n e f f i c i e n c i e s i s 6(2 S)/6(2 S) 1  3  = 0.73.  S c h o l e t t e and M u s c h l i t z (49) have used a t h e r m a l beam method t o measure t h e c r o s s s e c t i o n f o r t h e Penning i o n i z a t i o n p r o c e s s . has t h e advantage t h a t t h e t a r g e t gas i s i n t h e ground  T h e i r method  s t a t e but s u f f e r s  from t h e d i s a d v a n t a g e t h a t t h e s t a t e o f the m e t a s t a b l e i s n o t r e a d i l y * determined. *  Separate c r o s s s e c t i o n s were determined  1  f o r He (2 S) and  3  He (2 S) by s t u d y i n g t h e i n e l a s t i c s c a t t e r i n g , as t h e c o m p o s i t i o n o f t h e m e t a s t a b l e beam was v a r i e d , by changing t h e energy o f t h e e x c i t i n g e l e c t r o n s and thus changing t h e r e l a t i v e amounts o f s i n g l e t and t r i p l e t m e t a s t a b l e s i n the beam.  From t h e f a c t t h a t t h e c r o s s s e c t i o n s o b t a i n e d  d i d not depend on t h e c o m p o s i t i o n o f t h e beam, except f o r hydrogen,  they  c o n c l u d e d t h a t t h e c r o s s s e c t i o n s were equal f o r t h e two m e t a s t a b l e states. Benton e t a l . (51) have used a time r e s o l v e d a f t e r g l o w t e c h n i q u e t o m o n i t e r t h e d e n s i t y o f h e l i u m m e t a s t a b l e atoms, w i t h t i m e , f o l l o w i n g a pulsed helium discharge.  The i n t r o d u c t i o n o f s m a l l amounts o f t h e  t a r g e t gas reduces t h e e f f e c t i v e l i f e t i m e o f t h e m e t a s t a b l e atoms due to t h e i o n i z i n g c o l l i s i o n s .  Schmeltekopf  and F e h s e n f e l d (52) have used  -21-  a f l o w i n g afterglow technique  t o determine t h e s i n g l e t and t r i p l e t  cross sections f o r a v a r i e t y o f t a r g e t s . afterglow technique  Using a s i m i l a r flowing  Bolden e t a l . (55, 56) have measured i o n i z a t i o n *  3  c r o s s s e c t i o n s u s i n g He (2 S ) . The a f t e r g l o w t e c h n i q u e a l . has t h e advantage t h a t t h e s t a t e o f t h e m e t a s t a b l e determined but s u f f e r s from t h e d i s a d v a n t a g e  o f Benton e t i s easily  t h a t a s i g n i f i c a n t prop-  o r t i o n o f t h e t a r g e t s p e c i e s may be v i b r a t i o n a l l y e x c i t e d o r d i s s o c i a t e d during the discharge pulse. s t a t e o f the metastable  In the f l o w i n g afterglow technique the  i s known e x p l i c i t l y and t h e r e a c t a n t gas i s  i n t r o d u c e d i n the ground s t a t e .  However, t h e method i s r e s t r i c t e d t o  s t a t e s connected t o lower s t a t e s by o p t i c a l l y a l l o w e d t r a n s i t i o n s and may be c o m p l i c a t e d by cascade p o p u l a t i o n o f t h e s t a t e s s t u d i e d . Hotop, Niehaus and Schmeltekopf (37) u s i n g an atomic beam i n c o n j u n c t i o n w i t h a mass s p e c t r o m e t e r ,  have measured t h e c r o s s s e c t i o n  r a t i o s f o r v a r i o u s s p e c i e s , by h e l i u m m e t a s t a b l e  atoms.  They  success-  f u l l y e l i m i n a t e d t h e 2*S component from t h e beam by o p t i c a l l y quenching i t w i t h l i g h t from a He d i s c h a r g e  lamp.  Because o f t h e h i g h c r o s s s e c t i o n f o r a b s o r p t i o n o f these  photons  * 1 by He (2 S) they were a b l e t o almost e l i m i n a t e t h e s i n g l e t component from t h e beam by t h e p r o c e s s e s : h v ( 2 P •> 2*S) + H e ( 2 S ) -> H e ( 2 P ) X  1 He(2 The  1  X  1 P) + H e ( I S ) + hv(584 A)  corresponding  quenching p r o c e s s  (4) f o r t r i p l e t metastables  does not  * 3 3 occur because t h e He (2 S ) , i f e x c i t e d t o t h e 2 P s t a t e , r e t u r n s t o 3 the 2 S s t a t e .  * 1 With t h e i n f o r m a t i o n from t h e mixed beam (He (2 S ) )  * 3 1 and He (2 S ) ) and t h e beam i n which t h e 2 S component had been e l i m i n a t e d  -22-  (He (2 S) beam) they were a b l e t o c a l c u l a t e i o n i z a t i o n c r o s s s e c t i o n ratios.  They c o n s i d e r e d the r a t i o s they determined were c l o s e t o one  (range 0.49 t o 1.6) thus a g r e e i n g w i t h t h e i r proposed mechanism f o r Penning i o n i z a t i o n .  Dunning and Smith (55) u s i n g a gas c e l l  technique  i n c o n j u n c t i o n w i t h a quenching lamp, s i m i l a r t o t h a t o f Hotop e t a l . (37), o b t a i n e d s i m i l a r c r o s s s e c t i o n r a t i o s and the v a r i a t i o n s (range 0.67 t o 1.31) i n the i o n i z a t i o n c r o s s s e c t i o n were c o n s i d e r e d significant.  In agreement w i t h Hotop e t a l . they found the s i n g l e t  c r o s s s e c t i o n s , i n the r a r e gases, t o be l a r g e r than t h e t r i p l e t cross s e c t i o n .  corresponding  Benton e t a l . (51) and B e l l e t a l . (57) have  p o i n t e d out t h a t t h i s i s t o be  expected.  The a b s o l u t e v a l u e s o f the c r o s s s e c t i o n s f o r Penning *  t o be  ionization  3  by He (2 S) atoms measured by v a r i o u s t e c h n i q u e s  are i n good agreement.  A f t e r g l o w measurements g i v e v a l u e s o f the c r o s s s e c t i o n r a t i o which are c o n s i s t e n t l y l a r g e r and cover a w i d e r range o f v a l u e s than o b t a i n e d by o t h e r methods.  those  I t has been suggested (55) t h a t s u p e r e l a s t i c  d e e x c i t a t i o n o f the s i n g l e t m e t a s t a b l e  has not been s u f f i c i e n t l y taken * 1 i n t o account. The c r o s s s e c t i o n f o r the r e a c t i o n o f He (2 S) w i t h * 1 e l e c t r o n s i s q u i t e l a r g e (58) and may c o n t r i b u t e t o t h e o b s e r v e d He (2 SI  d e e x c i t a t i o n r a t e and as a r e s u l t g i v e Penning i o n i z a t i o n c r o s s s e c t i o n s * 1 f o r t h e He (2 S) which are too l a r g e .  *  1  The r a t i o o f the c r o s s s e c t i o n s f o r the p r o d u c t i o n o f He (2 S) *  3  and He (2 S) s t a t e s as a f u n c t i o n o f the energy o f the e x c i t i n g e l e c t r o n s determined by the measurement o f the k i n e t i c energy o f e l e c t r o n s r e l e a s e d i n Penning i o n i z a t i o n  (37, 47) i s l a r g e r than t h a t determined i n  experiments where secondary e l e c t r o n e m i s s i o n  from metal  surfaces  -23-  (46, 47) was used t o d e t e c t m e t a s t a b l e atoms.  I f the v a l u e o b t a i n e d  by Dunning and Smith f o r the e j e c t i o n e f f i c i e n c y o f e l e c t r o n s from metal s u r f a c e s was  a p p l i e d t o the r e s u l t s o f Dugan et a l . (45) and  H o l t and Krotkov (46) the r e s u l t s would be i n b e t t e r agreement w i t h those o f Cermak (47) and Hotop e t a l . (37).  In g e n e r a l the agreement 1  3  between d i f f e r e n t i n v e s t i g a t o r s , o f the r a t i o o f 2 S and 2 S h e l i u m excitation functions i s f a i r l y 1.5.  good.  Photoelectron Spectroscopy. P h o t o e l e c t r o n s p e c t r o s c o p y (PES) has been the s u b j e c t o f s e v e r a l  r e c e n t r e v i e w s (36, 59-65), c o v e r i n g d i f f e r e n t a s p e c t s o f the s u b j e c t . E x t e n s i v e c o l l e c t i o n s o f d a t a and i n t e r p r e t a t i o n s r e s u l t i n g from m o l e c u l a r PES s t u d i e s are g i v e n i n books by T u r n e r e t a l . (66) and a l s o by  Baker  and Brundle ( 6 7 ) . H i s t o r i c a l l y , PES was  initially  groups i n the e a r l y 1960's.  developed by two  independent  V i l e s o v e t a l . (68-71) used a L o z i e r  type apparatus i n c o n j u n c t i o n w i t h a vacuum u l t r a v i o l e t monochromator, to measure the k i n e t i c energy o f p h o t o e l e c t r o n s produced A l - J o b o u r y and Turner  in ionization.  (72-74) employed the u n d i s p e r s e d h e l i u m  resonance  o  line  (Hel 584 A)as  the e x c i t a t i o n source i n the measurement o f the  k i n e t i c energy o f p h o t o - e j e c t e d e l e c t r o n s s i m i l a r s t u d i e s employing  a windowless  Schoen (75) r e p o r t e d  Seya-Namioka monochromator.  The p r i n c i p l e o f the method, the p h o t o e l e c t r i c e f f e c t , was g i v e n by E i n s t e i n , who  first  p o s t u l a t e d t h a t when a photon has more than  s u f f i c i e n t energy t o e j e c t an e l e c t r o n from an atom or m o l e c u l e , the excess energy m a n i f e s t s i t s e l f  as k i n e t i c energy o f the p a r t i c l e s  -24-  Thus i f a photon o f energy h y (h = P l a n c k s '  produced.  v = frequency  constant,  o f r a d i a t i o n ) causes t h e i o n i z a t i o n o f a gaseous atom  (molecule), hv + M ->• M  +  + e  (5)  the r e s u l t i n g p h o t o e l e c t r o n w i l l have k i n e t i c energy ( E ) g i v e n v e r y g  c l o s e l y by the e x p r e s s i o n , E  g  = hv - IP CM)  (6)  where IP(M) i s t h e i o n i z a t i o n p o t e n t i a l o f t h e atom o r m o l e c u l e . C o n s e r v a t i o n o f momentum r e s u l t s i n energy p a r t i t i o n between t h e e l e c t r o n and t h e i o n i n t h e i n v e r s e r a t i o o f t h e i r masses and thus v i r t u a l l y a l l the excess  energy i s c a r r i e d away by t h e p h o t o e l e c t r o n .  I t has been shown t h a t p h o t o e l e c t r o n s p e c t r o s c o p y  overcomes many  o f the d i f f i c u l t i e s encountered i n t h e d e t e r m i n a t i o n o f i o n i z a t i o n p o t e n t i a l s by p h o t o i o n i z a t i o n and e l e c t r o n impact methods. a p p l i c a b l e t o a wide range o f s u b s t a n c e s . mined by p h o t o e l e c t r o n s p e c t r o s c o p y  I t i s also  Ionization p o t e n t i a l s deter-  can be compared d i r e c t l y w i t h t h e  spectroscopic values reported i n the l i t e r a t u r e .  Photoelectron spectros-  copy i s g e n e r a l l y f r e e from t h e l i m i t a t i o n s imposed by t h e a u t o i o n i z a t i o n p r o c e s s which i s a f r e q u e n t c o m p l i c a t i o n i n most o t h e r methods. I n f o r m a t i o n c o n c e r n i n g t h e n a t u r e o f t h e o r b i t a l from which t h e e l e c t r o n was removed may be o b t a i n e d from p h o t o e l e c t r o n s p e c t r a . number o f i o n i c v i b r a t i o n a l l e v e l s p o p u l a t e d  i s c h a r a c t e r i s t i c o f the  type o f o r b i t a l which t h e e j e c t e d e l e c t r o n o r i g i n a l l y  occupied.  A c c o r d i n g t o t h e Franck-Condon p r i n c i p l e t h e i o n i c v i b r a t i o n a l most l i k e l y t o be p o p u l a t e d  The  level  i s t h e one which a l l o w s t h e i n t e r n u c l e a r  c o o r d i n a t e s t o approximate those o f t h e n e u t r a l .  Other i o n i c  vibrational  l e v e l s a r e p o p u l a t e d w i t h lower p r o b a b i l i t i e s governed by t h e Franck-  -25-  Condon o v e r l a p . There are two  d i f f e r e n t t y p e s o f i o n i z a t i o n p o t e n t i a l which  be o b t a i n e d f o r m o l e c u l e s , namely the ization potentials. difference  The  between the  m o l e c u l e and  the  adiabatic  vibrational  same i n t e r n u c l e a r  For  the  steeply  energy  ground  state  c o r r e s p o n d s t o the most p r o b a b l e l o w e s t v i b r a t i o n a l l e v e l o f the  l e v e l o f the m o l e c u l a r i o n  ionizing  ground  The  internuclear  Franck-Condon r e g i o n w i l l portion  o f the  removed i s from an a n t i b o n d i n g o r b i t a l the  therefore  ion potential  electron  internuclear  p a r e n t and  in  g e n e r a l a s l o w l y r i s i n g a t t r a c t i v e r e g i o n o f the m o l e c u l a r i o n  neutral. are  likely  potential  Franck-Condon r e g i o n above the ground s t a t e o f  In t h i s case r e l a t i v e l y few to be p o p u l a t e d .  I f the  vibrational  electron  or v e r y weakly bonding t h e r e i s g e n e r a l l y internuclear  intersect  energy c u r v e  When the  equilibrium  i o n i s s m a l l e r than f o r the n e u t r a l  i n the  state  corresponding  an i n c r e a s e i n  r e s u l t i n g i n e x c i t a t i o n o f many v i b r a t i o n a l quanta.  curve l i e s  The  distance.  r i s i n g repulsive  d i s t a n c e o f the  i s the  l e v e l o f the m o l e c u l a r i o n .  removal o f a bonding e l e c t r o n  separation w i l l occur.  the v e r t i c a l i o n -  ionization potential  lowest v i b r a t i o n a l  t r a n s i t i o n , t h a t i s from the  t o the  and  lowest v i b r a t i o n a l l e v e l o f the  vertical ionization potential  m o l e c u l e t o the  adiabatic  may  c o o r d i n a t e s and  l e v e l s o f the  ion  b e i n g removed i s nonbonding  l i t t l e e f f e c t on the  the most p r o b a b l y t r a n s i t i o n i s  equilibrium adiabatic  and  the Franck-Condon f a c t o r s  f o r h i g h e r v i b r a t i o n a l l e v e l s are  The  v a r i o u s p o s s i b i l i t i e s are  i l l u s t r a t e d i n Figure  PIES i s somewhat analogous t o PES  the  small.  3.  i n t h a t b o t h methods are  on the measurement o f the k i n e t i c energy o f e l e c t r o n s which are  based produced  -26-  EJECTED  R F i g u r e 3.  A p p l i c a t i o n o f Franck-Condon P r i n c i p l e t o p h o t o e l e c t r o n production.  -27-  by the i o n i z a t i o n o f atoms and m o l e c u l e s . width o f the e l e c t r o n  I n Penning i o n i z a t i o n t h e  energy d i s t r i b u t i o n depends on t h e e x c i t i n g  p a r t i c l e , t h e t a r g e t and t h e i o n i c s t a t e  investigated  w h i l e i n PES a  v e r y narrow d i s t r i b u t i o n o f e l e c t r o n s i s found f o r each d i s c r e t e of the atom o r m o l e c u l e under i n v e s t i g a t i o n .  Electron  energy  level  distributions  * 1 * 3 r e s u l t i n g from Penning i o n i z a t i o n by b o t h t h e He (2 S) and He (2 S) m e t a s t a b l e s a r e b r o a d i n comparison t o those o b t a i n e d from p h o t o e l e c t r o n s . The  r e a s o n f o r t h i s i s t h e mechanism by which t h e i o n i z i n g energy i s  transferred  t o t h e t a r g e t atom o r m o l e c u l e .  I n i o n i z a t i o n by m e t a s t a b l e  atoms t h e i n t e r a c t i o n o f t h e s p e c i e s b e f o r e i o n i z a t i o n may be d i f f e r e n t from t h a t a f t e r i o n i z a t i o n and t h e t r a n s l a t i o n a l e n e r g i e s o f p r o d u c t s may n o t n e c e s s a r i l y associative  remain t h e r m a l .  F o r some atoms and s m a l l m o l e c u l e s ,  i o n i z a t i o n can o c c u r i n p a r a l l e l t o Penning i o n i z a t i o n .  T h i s does n o t o c c u r i n p h o t o e l e c t r o n s p e c t r o s c o p y . PIES i s not a s u i t a b l e ionization potentials  In c o n t r a s t t o PES,  t e c h n i q u e f o r the d e t e r m i n a t i o n o f a c c u r a t e because t h e r e i s o f t e n a b r o a d d i s t r i b u t i o n o f  r e l a t i v e k i n e t i c energy v a l u e s .  1.6.  Franck-Condon P r i n c i p l e Transitions  i n PES between t h e two c o r r e s p o n d i n g p o t e n t i a l  curves a r e d e s c r i b e d as v e r t i c a l (Franck-Condon) t r a n s i t i o n s .  energy The  Franck-Condon p r i n c i p l e , f i r s t proposed by Franck (76) and l a t e r f o r m u l a t e d m a t h e m a t i c a l l y by Condon (77) may be s t a t e d an e l e c t r o n i c  as f o l l o w s :  i f  t r a n s i t i o n t a k e s p l a c e i n a time i n t e r v a l which i s much  s h o r t e r than t h a t r e q u i r e d f o r a s i n g l e v i b r a t i o n , then t h e r e l a t i v e position  and v e l o c i t y o f t h e n u c l e i may be assumed t o be unchanged  during the t r a n s i t i o n .  In e f f e c t we may c o n s i d e r v e r t i c a l t r a n s i t i o n s  -28-  on a p o t e n t i a l  energy diagram.  Franck-Condon f a c t o r s calculated  ( r e l a t i v e t r a n s i t i o n p r o b a b i l i t i e s ) have been  f o r e x c i t a t i o n and  d i r e c t i o n i z a t i o n by many workers  In the m a j o r i t y o f cases Morse p o t e n t i a l s potential  energy curves o f the  v a l u e s f o r the  f i r s t few  lower and  upper s t a t e s .  actual  c u r v e , s h o u l d be  a c c u r a c y d e c r e a s e s at h i g h e r v i b r a t i o n a l  overcome t h i s problem has potential  levels.  been made by Dunn (81)  energy c u r v e s f o r hydrogen and  F r o s t , McDowell and  used t o d e s c r i b e The  Vroom (82,  83),  potential  a c c u r a t e , however An  who  the  calculated  v i b r a t i o n a l l e v e l s , where the Morse  i s a good a p p r o x i m a t i o n t o the the  are  (78-80).  attempt to  used the  deuterium i n h i s  known  calculations.  using photoelectron spectroscopy,  have o b t a i n e d e x p e r i m e n t a l Franck-Condon f a c t o r s which agree q u i t e with calculated  v a l u e s ; the  e x p e r i m e n t a l v a l u e s are  s l i g h t l y larger  the h i g h e r v i b r a t i o n a l quantum numbers.  They i n t e r p r e t e d  f o r m o l e c u l a r hydrogen, i n the  manner.  following  The  i o n i z a t i o n c o e f f i c i e n t , f o r m o l e c u l a r hydrogen, was Ching (84) one  eV.  t o drop about 15 p e r c e n t per  well at  this trend,  o p t i c a l photofound by Cook  eV between e i g h t e e n and  and  twenty-  T h e r e f o r e i t i s expected t h a t the p r o b a b i l i t y o f i o n i z a t i o n  each hydrogen i o n i c v i b r a t i o n a l l e v e l w i l l  also f a l l o f f within  to  this  0  energy range.  Thus f o r a 58 4 A  quantum number o f the  final  incident  s t a t e , the  photon the h i g h e r the  l e s s w i l l the  s t a t e have f a l l e n from i t s t h r e s h o l d v a l u e .  more h e a v i l y  weighted i n the  i t a t i v e l y interpreted  electron  spectra.  and  eV  vibrational  e x p e c t e d to  T u r n e r (85)  the v a r i a t i o n as a drop i n t r a n s i t i o n  o f a p p r o x i m a t e l y 10 p e r c e n t per Robertson (86)  i o n i z a t i o n to that  Thus the h i g h  s t a t e s , which have lower k i n e t i c energy e l e c t r o n s are  vibrational  has  be  quant-  probability  above t h r e s h o l d .  Schmeltekopf (87) , u s i n g a f l o w i n g a f t e r g l o w  -29-  technique,  have found t h a t f o r the Penning r e a c t i o n s measured, the  population  o f the v i b r a t i o n a l l e v e l s f o r e l e c t r o n i c s t a t e s o f  the  m o l e c u l a r i o n are n e a r l y the same as those g i v e n by Franck-Condon f a c t o r s f o r unperturbed molecular s t a t e s . U s i n g e l e c t r o n s p e c t r o s c o p y Hotop and Niehaus (35) have compared Franck-Condon f a c t o r s o b t a i n e d by Penning i o n i z a t i o n r e a c t i o n s * He  using  3 (2 S) atoms w i t h those o b t a i n e d  with calculated values. r e s u l t s , w h i l e not  The  by p h o t o e l e c t r o n  spectroscopy  d i f f e r e n c e s they observe from  photoelectron  l a r g e , are o u t s i d e t h e i r s t a t e d e x p e r i m e n t a l  However, the Franck-Condon f a c t o r s are more d i f f i c u l t t o o b t a i n the Penning s p e c t r a than from the p h o t o e l e c t r o n  and  s p e c t r a due  error. from  t o the  w i d t h o f the Penning d i s t r i b u t i o n s and the l a r g e background, due Auger e l e c t r o n s , i n the Penning e l e c t r o n s p e c t r a . have e x p l a i n e d  Hotop and  greater  to  Niehaus  the d i f f e r e n c e s i n Penning and p h o t o i o n i z a t i o n v i b r a t i o n a l  e x c i t a t i o n i n the f o l l o w i n g manner.  The  Franck-Condon f a c t o r s are  very  s e n s i t i v e t o the r e l a t i v e p o s i t i o n s o f the i o n i c and n e u t r a l m o l e c u l a r p o t e n t i a l curves and  i f i t i s assumed t h a t the r e l a t i v e p o s i t i o n s  d i f f e r e n t f o r Penning and p h o t o i o n i z a t i o n , due  are  to i n t e r a c t i o n with  the  p r o j e c t i l e i n the case o f Penning i o n i z a t i o n , t h e n the o b s e r v e d e f f e c t can be e x p l a i n e d .  Nevertheless  Penning i o n i z a t i o n seems t o be  governed  e s s e n t i a l l y by the Franck-Condon p r i n c i p l e f o r the cases s t u d i e d t o date. 1.7.  Angular D i s t r i b u t i o n s of Photoelectrons The  and  Penning E l e c t r o n s .  measurement o f a n g u l a r d i s t r i b u t i o n s o f the  photoelectrons  e j e c t e d from v a r i o u s o r b i t a l s has been the s u b j e c t o f many i n v e s t i g a t i o n s , both experimentally  (88-96) and t h e o r e t i c a l l y (97-101).  Photoelectron  -30-  a n g u l a r d i s t r i b u t i o n s have been s t u d i e d f o r t h e r a r e gases, a few d i a t o m i c s and p o l y a t o m i c s  as w e l l as cadmium and z i n c atoms.  i o n i z a t i o n o f argon by a beam o f h e l i u m m e t a s t a b l e  The  atoms i s the o n l y  system f o r which the a n g u l a r d i s t r i b u t i o n has been r e p o r t e d f o r Penning electrons  (102).  T h e o r e t i c a l p r e d i c t i o n s o f the form o f t h e a n g u l a r f o r p h o t o e l e c t r o n s a r e as f o l l o w s .  For l i n e a r p o l a r i z e d l i g h t the  a n g u l a r d i s t r i b u t i o n has the g e n e r a l I  (6)  =  (o  t o t  /4Tr)  (1  +  3P  form,  (cos  2  distributions  6))  (7)  2 where P ( c o s 2  = / 2 ( 3 cos  9)  1  a  6-1),  t  Q  represents the t o t a l  t  cross  s e c t i o n , 6 measures the angle between the d i r e c t i o n o f t h e e j e c t e d e l e c t r o n and the p o l a r i z a t i o n o f t h e i n c i d e n t l i g h t and 3 i s an asymmetry parameter. For u n p o l a r i z e d l i g h t spectroscopy)  (which i s u s u a l l y t h e case i n p h o t o e l e c t r o n  the e x p r e s s i o n becomes.  I (6) = O  t o t  /4^)  (1 - B/  ( / 3  2  2  sin  2  0 - 1))  (8)  where 6 measures the angle between the d i r e c t i o n o f the e j e c t e d e l e c t r o n and the photon beam. Chaffee  (92) and H a l l and S i e g a l (93) have shown e x p e r i m e n t a l l y  that expression confirmed  (7) i s v a l i d and o t h e r s t u d i e s (88, 91, 94, 96) have  expression  (8) f o r u n p o l a r i z e d l i g h t .  The s i t u a t i o n f o r Penning i o n i z a t i o n i s somewhat d i f f e r e n t and no t h e o r e t i c a l p r e d i c t i o n s c o n c e r n i n g have been made.  the form o f the a n g u l a r  The beam o f m e t a s t a b l e s ,  distribution  i n c o n t r a s t t o the photon  beam, does not c o n s t i t u t e an a x i s w i t h r e s p e c t t o which an a n g u l a r d i s t r i b u t i o n o f e l e c t r o n s can be d e f i n e d .  E x p e r i m e n t a l l y Niehaus e t a l .  (96, 102) have found t h a t the p h o t o e l e c t r o n i n t e n s i t y i s symmetric w i t h  -31-  r e s p e c t t o 90 degrees ( i . e . p e r p e n d i c u l a r t o t h e photon beam) but t h a t f o r Penning e l e c t r o n s a pronounced asymmetry w i t h r e s p e c t t o 90 degrees i s observed, direction.  w i t h an i n t e n s i t y enhancement i n t h e backward  T h i s enhancement i n t h e backward d i r e c t i o n  corresponds  t o i n t e n s i t y enhancement i n t h e d i r e c t i o n o f t h e m e t a s t a b l e beam ( i . e . a t angles g r e a t e r than 90 d e g r e e s ) .  The e x t e n t o f t h e asymmetry *  seems t o i n d i c a t e t h a t t h e r e i s c o n s i d e r a b l e alignment system a t t h e moment o f i o n i z a t i o n .  o f t h e He  - Ar  The forward-backward asymmetry  * 3 * 1 was more pronounced f o r He (2 S) than f o r He (2 S ) , which was thought t o be due t o d i f f e r e n c e s r e g a r d i n g t h e mean s p a t i a l alignment  o f the  system o r p o s s i b l y w i t h a c o n s i d e r a b l e c o n t r i b u t i o n from t h e nonexchange p a r t o f t h e m a t r i x elements g o v e r n i n g *  t h e Penning  process  1  i n t h e He (2 S) case. Niehaus e t a l . (102) a l s o observed  t h a t t h e measurement o f Penning  e l e c t r o n s perpendicular t o the metastable approximately  beam d i r e c t i o n , w i l l  give  correct i n t e n s i t y r a t i o s o f the t o t a l cross s e c t i o n s f o r  f o r m a t i o n o f d i f f e r e n t e l e c t r o n i c and v i b r a t i o n a l s t a t e s o f t h e t a r g e t 1 3 s p e c i e s by t h e 2 S and 2 S s t a t e s o f h e l i u m . 1.8. A u t o i o n i z a t i o n . The  removal o f an e l e c t r o n from an atom o r m o l e c u l e  by a p r o c e s s  which does not i n v o l v e a d i r e c t t r a n s i t i o n i n t o t h e i o n i z a t i o n continuum i s known as a u t o i o n i z a t i o n . A u t o i o n i z a t i o n may be regarded step p r o c e s s .  F i r s t , the i n i t i a l  as a two  e x c i t a t i o n o f an e l e c t r o n occurs  into  a d i s c r e t e s t a t e above t h e i o n i z a t i o n p o t e n t i a l o f t h e s p e c i e s and subsequently  a r a d i a t i o n l e s s t r a n s i t i o n t a k e s p l a c e from t h e bound  s t a t e i n t o t h e a c c e s s i b l e i o n i z a t i o n continuum.  The e m i t t e d e l e c t r o n  -32-  w i l l have energy equal t o the d i f f e r e n c e between the energy o f the bound s t a t e and the i o n i z a t i o n p o t e n t i a l . (for  p h o t o a b s o r p t i o n ) and w i l l  A u t o i o n i z a t i o n i s a resonant process  only occur at energies corresponding  t o the d i s c r e t e s t a t e i n t o which t h e e l e c t r o n i s i n i t i a l l y e x c i t e d . The phenomena i s f r e q u e n t l y observed i n u l t r a v i o l e t p h o t o a b s o r p t i o n and p h o t o i o n i z a t i o n  experiments.  The p r o c e s s has a l s o been used t o  account f o r c e r t a i n s t r u c t u r e i n e l e c t r o n impact i o n i z a t i o n e f f i c i e n c y curves. The a u t o i o n i z a t i o n p r o c e s s has r e c e i v e d e x t e n s i v e t h e o r e t i c a l a t t e n t i o n (103) and the d i s c u s s i o n o f the p r o c e s s has been  included  i n t h i s t h e s i s s i n c e Penning i o n i z a t i o n has been r e g a r d e d as an autoi o n i z a t i o n p r o c e s s i n some t h e o r e t i c a l t r e a t m e n t s .  -33-  CHAPTER  THEORETICAL  TWO  DISCUSSION  There have been many e x p e r i m e n t a l  i n v e s t i g a t i o n s o f Penning  i o n i z a t i o n and t h e c r o s s s e c t i o n s f o r t h e r e a c t i o n s between v a r i o u s atoms and m o l e c u l e s have been measured.  T h e o r e t i c a l s t u d i e s , on t h e  o t h e r hand, have been c o n f i n e d , u n t i l r e c e n t l y , t o those o f p a r t i c l e i o n i z a t i o n by p r o c e s s e s  i n v o l v i n g o p t i c a l l y allowed  transitions.  Though t h e r e have been a few t h e o r e t i c a l papers d e a l i n g w i t h t h e Penning i o n i z a t i o n p r o c e s s , t h e r e i s p r e s e n t l y factory theory.  no c o m p l e t e l y  satis-  In t h i s c h a p t e r a survey o f t h e t h e o r y r e l a t i n g t o  Penning i o n i z a t i o n , and t o a l e s s e r e x t e n t a s s o c i a t i v e i o n i z a t i o n , be  will  reviewed.  2.1. Weak I n t e r a c t i o n T h e o r i e s . At t h e l e v e l o f a s e m i q u a n t i t a t i v e t h e o r y used t o t r e a t Penning i o n i z a t i o n .  two models have been  The f i r s t i s t h e s i m p l e  'critical  r a d i u s ' model, t h e second i s based on t h e d i p o l a r exchange o f a quantum from one c o l l i s i o n a l p a r t n e r t o t h e o t h e r .  In t h e c r i t i c a l  radius  model, a c h a r a c t e r i s t i c d i s t a n c e f o r t h e p a r t i c u l a r system i s c o n s t r u c t e d and t h e assumption i s made t h a t a l l c o l l i s i o n s o c c u r r i n g w i t h i n t h i s c h a r a c t e r i s t i c distance give r i s e to r e a c t i o n . Ferguson (104) has developed a model f o r c o l l i s i o n s o f atoms w i t h helium metastable  atoms on t h e c r i t i c a l r a d i u s method a s s o c i a t e d w i t h  the momentum t r a n s f e r c r o s s s e c t i o n .  Gas k i n e t i c c o l l i s i o n c r o s s s e c t i o n s  -34-  were compared t o Penning r e a c t i o n c r o s s s e c t i o n s f o r s e v e r a l  reactants.  C o l l i s i o n c r o s s s e c t i o n s were c a l c u l a t e d f o r momentum t r a n s f e r , c o n s i d e r i n g o n l y a t t r a c t i v e p o t e n t i a l s o f the van der Waals t y p e , between t h e m e t a s t a b l e h e l i u m atoms and the o t h e r r e a c t a n t s the e x p e r i m e n t a l  and i t was n o t e d t h a t  Penning i o n i z a t i o n r e a c t i o n c r o s s s e c t i o n s were o n l y  a few t e n t h s o f the c a l c u l a t e d v a l u e s . Bates e t a l . (105) a t t r i b u t e d t o Ferguson the concept t h a t  long 3  range a t t r a c t i o n s r e s u l t i n c l o s e c o l l i s i o n s o f m e t a s t a b l e h e l i u m (2 S) w i t h o t h e r r e a c t a n t s i n Penning i o n i z a t i o n .  I t i s supposed t h a t t h e  long range f o r c e s r e s u l t i n inward s p i r a l i n g o r b i t s f o r a p p r o p r i a t e impact parameters t h a t b r i n g the r e a c t a n t s the Penning r e a c t i o n s o c c u r .  i n t o c l o s e c o l l i s i o n where  They c a l c u l a t e c o e f f i c i e n t s i n a manner  s i m i l a r t o t h a t o f Ferguson, however, the v a l u e s  for collision  cross  s e c t i o n s f o r s p i r a l i n g o r b i t s a r e t h r e e o r f o u r times as l a r g e as those c a l c u l a t e d by Ferguson.  They were not a b l e t o e x p l a i n t h i s d i f f e r e n c e .  B e l l et a l . (57) computed v a l u e s  f o r the van der Waals i n t e r a c t i o n  between m e t a s t a b l e h e l i u m atoms and o t h e r atoms and m o l e c u l e s .  I t was  assumed t h a t a l l c o l l i s i o n s i n which the p a r t i c l e s pass t h e c e n t r i f u g a l b a r r i e r must l e a d t o p o s s i b l e r e a c t i o n s .  E x p e r i m e n t a l d a t a was then used  to e s t i m a t e  the p r o b a b i l i t y o f e l e c t r o n l o s s when the system i s i n c l o s e  collision.  The r a t e c o e f f i c i e n t s f o r c l o s e c o l l i s i o n were a l s o c a l c u l a t e d .  The c r o s s s e c t i o n s and p r o b a b i l i t i e s o f r e a c t i o n p e r c o l l i s i o n  obtained  are about the same o r d e r o f magnitude as those o f Bates e t a l . (105). Jones and Robertson (106) have shown t h a t the use o f van der Waals forces alone,  i n d e s c r i b i n g these i n t e r a c t i o n s , i s not an  a p p r o x i m a t i o n except a t e x t r e m e l y low t e m p e r a t u r e s .  acceptable  -35-  K a t s u u r a , Watanabe and M o r i  (107-112) have developed  a more e l a b o r a t e  treatment which i s e s s e n t i a l l y an impact parameter t r e a t m e n t .  The  process i s d e s c r i b e d as two s i m u l t a n e o u s o p t i c a l t r a n s i t i o n s , A and M - M  +  + e.  -»- A  The c o u p l i n g o p e r a t o r i s t h e product o f two e l e c t r i c  d i p o l e o p e r a t o r s and t h e t r a n s i t i o n p r o b a b i l i t y .  The c r o s s s e c t i o n s  and r a t e c o e f f i c i e n t s a r e f u n c t i o n s o f t h e t r a n s i t i o n d i p o l e m a t r i x elements  squared.  The wave f u n c t i o n i s e x p r e s s e d  states corresponding to A  + M and A + M  + e.  i n terms o f t h e  The time dependent  Schrodinger equation f o r the c o e f f i c i e n t i s i n t e g r a t e d f o r s t r a i g h t path c o l l i s i o n s w i t h a g i v e n impact parameter.  The e q u a t i o n f o r c r o s s  s e c t i o n s i s i n t e g r a t e d e x p l i c i t l y t o g i v e Penning  i o n i z a t i o n cross sections  f o r c o l l i s i o n s a t a p a r t i c u l a r v e l o c i t y , assuming t h a t t h e i n t e r p a r t i c l e i n t e r a c t i o n p o t e n t i a l can be r e p r e s e n t e d i n a manner s i m i l a r t o t h a t o f d i p o l e - d i p o l e i n t e r a c t i o n s o f s t a t i c systems. model o f K a t s u u r a  The weak i n t e r a c t i o n  (107) i s l i m i t e d i n t h a t i t i s o n l y a p p l i c a b l e when  the e l e c t r o n i c a l l y e x c i t e d A  i s Connected o p t i c a l l y t o a lower 1-  state.  3 -  I t cannot be used, f o r example, t o t r e a t t h e 2 § and 2 S s t a t e s o f helium.  The c r o s s s e c t i o n s which t h i s method p r e d i c t s a r e r a t h e r l a r g e .  Watanabe and Katsuura. (108) have developed i n t o account t h e i m p e r f e c t nuclear axis.  coupling  a method which  takes  o f a n g u l a r momentum t o t h e i n t e r -  The c r o s s s e c t i o n s Obtained U s i n g t h i s method a r e almost  one q u a r t e r s m a l l e r than those o b t a i n e d from t h e e a r l i e r t h e o r y . was  a l s o Shown  that  during the c o l l i s i o n M o r i (109) stationary  state  has  t h e assumption  §§t§ an upper  o f p e r f e c t a n g u l a r momentum C o u p l i n g , bound  en  the  gross section»  c o n s i d e r e d t h e process i n terms o f  method•, employing  It  a  f o r m a l i s m which  the  perturbed  was developed  by  -36-  Fano (113), f o r the problem o f a u t o i o n i z a t i o n i n atoms. ambiguity i n h i s treatment with respect i o n i z a t i o n and  There i s some  t o the s e p a r a t i o n  e l a s t i c s c a t t e r i n g amplitudes.  Nakamura (111)  extended Fano's f o r m a l i s m t o a m o l e c u l a r system which has nuclear  separation.  resonance s t a t e .  He  The  also defined  Miller  This  the  c r o s s s e c t i o n i s more  (114)  has  developed a  t h e o r y o f Penning i o n i z a t i o n s i m i l a r t o t h a t o f Nakamura. however, developed the a n a l y s i s f u r t h e r t o show how  He  has,  associative ionization  a l s o l o o k e d at the r e l a t i o n s h i p between the  c l a s s i c a l , s e m i c l a s s i c a l and F u j i i et a l . (115)  a fixed  c a l c u l a t e d i n terms o f  decay p r o b a b i l i t y o f t h i s resonance s t a t e .  f i t s i n t o the t h e o r y and  has  a l o c a l complex p o t e n t i a l f o r the  c r o s s s e c t i o n was  suitable for practical applications.  between  quantum t h e o r i e s .  have quantum m e c h a n i c a l l y c a l c u l a t e d the  cross  s e c t i o n f o r Penning i o n i z a t i o n o f hydrogen atoms by m e t a s t a b l e h e l i u m atoms.  The  h e l i u m atoms are t r e a t e d on the b a s i s o f the l o c a l complex  p o t e n t i a l method and bond method.  The  complex p o t e n t i a l s are e s t i m a t e d by the  valence  t o t a l c r o s s s e c t i o n c a l c u l a t e d by F u j i i et a l . i s i n  good agreement w i t h the e x p e r i m e n t a l r e s u l t s r e p o r t e d M i l l e r and S c h a e f e r (116)  by Shaw et a l . (56).  have a l s o t r e a t e d t h i s system.  curves f o r the Penning i o n i z a t i o n o f the hydrogen atom by  Their potential helium  m e t a s t a b l e s were computed by a l a r g e s c a l e c o n f i g u r a t i o n i n t e r a c t i o n They i n t r o d u c e d  a s i m p l i f i e d d e s c r i p t i o n o f the i o n i z i n g c o l l i s i o n  t h a t approximate c r o s s s e c t i o n s c o u l d be d e t e r m i n e d . has  Sheldon  so  (117)  c a l c u l a t e d a number o f c r o s s s e c t i o n s f o r i o n i z a t i o n o f a l k a l i atoms  by e x c i t e d r a r e gas method.  atoms i n the f i r s t  ^P s t a t e u s i n g a quantum  defect  -37-  2.2. C l o s e Coupled N e a r - A d i a b a t i c  Theories.  To extend a weak i n t e r a c t i o n t h e o r y i t i s n e c e s s a r y t o c o n s i d e r the way i n which t h e e x c i t e d m e t a s t a b l e atom and t h e t a r g e t p a r t i c l e i n t e r a c t during the c o l l i s i o n . the f i r s t  There a r e two extremes t o c o n s i d e r ,  i s that the c o l l i s i o n c o n s t i t u t e s a f a s t p e r t u r b a t i o n  F o u r i e r components whose f r e q u e n c i e s the c h a r a c t e r i s t i c f r e q u e n c i e s  with  may be s i m i l a r t o o r h i g h e r  than  o f the e l e c t r o n i c t r a n s i t i o n s i n the free  m e t a s t a b l e and t a r g e t p a r t i c l e s .  T h i s b e h a v i o u r i s n o t observed i n  Penning and a s s o c i a t i v e i o n i z a t i o n .  The o t h e r p r o c e s s t o c o n s i d e r i s  t h a t t h e c o l l i s i o n times a r e s u f f i c i e n t l y long so t h a t t h e Born-Oppenheimer a p p r o x i m a t i o n o r t h e a d i a b a t i c model h o l d s f o r t h e c o l l i s i o n and gives a s a t i s f a c t o r y s t a r t i n g p o i n t . Theories considering  the near-adiabatic  t r e a t m e n t have been  developed t o v a r i o u s degrees f o r a s s o c i a t i v e i o n i z a t i o n ( 1 1 8 , 1 1 9 ) , f o r d i s s o c i a t i v e r e c o m b i n a t i o n (120) and f o r m o l e c u l a r a u t o i o n i z a t i o n (121-123).  For near-adiabatic  t r e a t m e n t s , t h e i n i t i a l and f i n a l  states  o f t h e wave f u n c t i o n a r e r e p r e s e n t e d as p r o d u c t s o f e l e c t r o n i c and v i b r a t i o n a l o r v i b r a t i o n a l and r o t a t i o n a l f u n c t i o n s .  I f these  functions  are computed f o r a l l t h e bound and f r e e e l e c t r o n i c and v i b r a t i o n a l s t a t e s that are o f i n t e r e s t , i t i s p o s s i b l e t o consider  a large v a r i e t y of  p r o c e s s e s from a common b a s i s , t h e major d i s t i n c t i o n b e i n g ,  which  s t a t e s are bound and which are n o t . The p r o c e s s e s which may be c o n s i d ered i n c l u d e i n t e r a c t i o n s among e x c i t e d bound s t a t e s , a u t o i o n i z a t i o n , p r e d i s s o c i a t i o n , associative i o n i z a t i o n , d i s s o c i a t i v e recombination, Penning i o n i z a t i o n , e l e c t r o n energy t r a n s f e r and v i b r a t i o n a l o r r o t a t i o n a l r e l a x a t i o n by e l e c t r o n c o l l i s i o n s .  Berry  (118) has d i s c u s s e d t h e  -38-  a p p l i c a t i o n o f the t h e o r y t o m o l e c u l a r hydrogen. more complex than t h i s the t h e o r y i s s t i l l not Bardsley  (120)  has  considered  However f o r m o l e c u l e s  quantitative.  the problem o f a s s o c i a t i v e i o n i z a t i o n  from the p o i n t o f view o f the f o r m a t i o n  o f a resonance s t a t e .  p r o d u c t o f the c r o s s s e c t i o n f o r f o r m a t i o n  o f t h i s resonance s t a t e  a s u r v i v a l p r o b a b i l i t y toward i o n i z a t i o n g i v e s the c r o s s associative ionization. a v a i l a b l e , the method may  2.3  The and  section for  For systems i n which s p e c t r o s c o p i c  data i s  be r e a d i l y a p p l i e d .  Exchange Model. Hotop and Niehaus have proposed (18, 31, 32, 37)  an exchange  mechanism f o r Penning i o n i z a t i o n i n which the t r a n s i t i o n may s y m b o l i c a l l y as  + M  + e(l)  +  (2) c h a r a c t e r i z e the two  (9)  electrons  i n v o l v e d , namely  e x c i t e d e l e c t r o n o f the m e t a s t a b l e p r o j e c t i l e ( 1 ) , and ground s t a t e e l e c t r o n s o f the t a r g e t them as a t u n n e l l i n g o f e l e c t r o n electron  (1).  (2).  The  one  of  given by Hagstrum (124).  the  the  t r a n s i t i o n i s viewed by  (2) f o l l o w e d by an Auger e m i s s i o n of  T h e i r proposed model i s i n c l o s e analogy t o the  of Auger e m i s s i o n o f e l e c t r o n s  theory  from m e t a l s u r f a c e s by m e t a s t a b l e s , as  They proposed the exchange mechanism to account  f o r the f o l l o w i n g e x p e r i m e n t a l (1) The  written  follows:  A * ( l ) + M(2) •> A(2) where (1) and  be  observations.  c r o s s s e c t i o n does not  depend on how  s t r o n g l y the t r a n s i t i o n  * A  + A i s forbidden.  I t had been observed t h a t c r o s s s e c t i o n * 1 * 3 r a t i o s , f o r i o n i z a t i o n by He (2 S) and He (2 S ) , were a p p r o x i m a t e l y u n i t y , whereas the l i f e t i m e s are q u i t e d i f f e r e n t .  -39-  (2) The observed r e l a t i v e p o p u l a t i o n o f e l e c t r o n i c s t a t e s a r e d i f f e r e n t f o r t h e Penning and p h o t o i o n i z a t i o n (3) The measured a b s o l u t e *  1  process.  c r o s s s e c t i o n f o r Penning i o n i z a t i o n o f *  3  sodium by He (2 S) and He (2 S) m e t a s t a b l e s showed t h a t  this  v a l u e agreed, w i t h i n t h e l i m i t s o f e r r o r , w i t h a v a l u e c a l c u l a t e d from an approximate f o r m u l a d e r i v e d under t h e assumption t h a t Penning i o n i z a t i o n i s an e l e c t r o n exchange 2.4.  Theory o f 127 degree E l e c t r o s t a t i c  process.  Analyzer.  A 127 degree e l e c t r o s t a t i c v e l o c i t y s e l e c t o r has been used f o r k i n e t i c energy a n a l y s i s o f e l e c t r o n s .  The f a c t t h a t an i n v e r s e  first  power e l e c t r o s t a t i c f i e l d g e n e r a t e d between c o a x i a l c y l i n d e r s c o u l d have f o c u s i n g p r o p e r t i e s f o r charged p a r t i c l e s was f i r s t p o i n t e d out by Hughes and Rojansky (125), who d e r i v e d t h e optimum f o c u s i n g p r o p e r t i e s o f such an a n a l y z e r .  A t t h e same t i m e , t h e t h e o r e t i c a l p r e d i c t i o n s were  e x p e r i m e n t a l l y v e r i f i e d by Hughes and M c M i l l e n  (126).  For a g i v e n e l e c t r i c f i e l d s t r e n g t h between c o a x i a l c y l i n d e r s an e l e c t r o n o f given v e l o c i t y entering the f i e l d p a r a l l e l t o the surface o f the c y l i n d e r s w i l l execute c i r c u l a r motion i f t h e k i n e t i c energy E i s g i v e n by  (126).  E = V l n (b/a) ( v - Vj) 2  (10)  2  where a and b a r e t h e r a d i i o f t h e i n n e r and o u t e r c y l i n d e r s r e s p e c t i v e l y , v  1  and v  2  a r e the p o t e n t i a l s a p p l i e d t o t h e i n n e r and o u t e r c y l i n d e r s  respectively  (y\v = v  two e l e c t r o n s , h a v i n g  2  - v^.  Hughes and Rojansky (125) have shown t h a t  a common v e l o c i t y and e n t e r i n g t h e f i e l d a t t h e  same p o i n t , b u t a t a n g l e s i G w i l l have o r b i t s , t o a f i r s t of,  approximation,  -40-  = C + (1 - C) cos /2~(J) - ( t a n 0 s i n /2<t>) /T  y y  9  (11)  = C + (1 - C) cos /2~<j> + ( t a n 0 s i n /2<t>) /2  where <)> i s the angle a t which t h e e l e c t r o n s a r e r e f o c u s s e d and C i s a parameter i n t r o d u c e d t o a l l o w t h e s o l u t i o n o f the d i f f e r e n t i a l o f the o r b i t s .  equation  Thus the e l e c t r o n s a r e r e f o c u s s e d ( i . e . the p o s i t i o n  where t h e o r b i t s c r o s s f o r the f i r s t time) when y^ = y^ and t h e l a s t term i n each e x p r e s s i o n (11) v a n i s h e s .  Therefore  /2*<J>  =  TT  and § = 127° 17'  and t h i s angle i s independent o f <j>. They a l s o showed t h a t two e l e c t r o n s e n t e r i n g the f i e l d a t the same angle b u t w i t h d i f f e r e n t would a t t a i n a maximum s e p a r a t i o n a t 127° 17'.  velocities  Thus a t t h i s  the b e s t r e s o l u t i o n and r e f o c u s s i n g are o b t a i n e d .  angle  The t h e o r e t i c a l  r e s o l u t i o n o f the 127 degree s e l e c t o r has been d i s c u s s e d by L e v e n t h a l and North  (127) and a l s o by Peresse  (128).  I t has been found, by Cermak  and Ozenne (21) t h a t the approximate energy r e s o l u t i o n o f t h e s e l e c t o r may be c a l c u l a t e d from the e q u a t i o n :  (12)  where E i s the k i n e t i c energy o f t h e e l e c t r o n , s^ and s and e x i t s l i t w i d t h s , r  Q  h a l f angle o f acceptance.  ?  are the entrance  i s t h e mean e l e c t r o n t r a j e c t o r y and a i s t h e The w i d t h AE i s d e f i n e d as t h e f u l l  h a l f maximum (FWHM) o f the e l e c t r o n energy d i s t r i b u t i o n .  width  -41-  CHAPTER  THREE  INSTRUMENTATION  3.1. E x p e r i m e n t a l Arrangement. A diagram o f t h e e x p e r i m e n t a l arrangement used i n t h i s  study,  f o r t h e measurement o f t h e k i n e t i c energy o f e l e c t r o n s produced by Penning  and photon i o n i z a t i o n i s shown i n F i g u r e 4.  3.1.1. E x c i t a t i o n  Region.  The e x c i t a t i o n r e g i o n p r o v i d e d a beam o f m e t a s t a b l e atoms t o i o n i z e t h e sample under i n v e s t i g a t i o n .  M e t a s t a b l e h e l i u m atoms were  formed i n t h e e x c i t a t i o n chamber by e l e c t r o n s o f v a r i a b l e  energy  ( n o r m a l l y 170 eV, t o t a l f i l a m e n t e m i s s i o n a. 20 ma) i n c i d e n t on tank h e l i u m i n t r o d u c e d through a m u l t i c h a n n e l d i s c  (glass  f u s e d a r r a y , pore diameter 0.05 mm, t h i c k n e s s 0.5 mm).  commercial  capillary,  The h e l i u m beam  ( f i n e c o n t r o l m a i n t a i n e d by means o f a G r a n v i l l e - P h i l l i p s v a r i a b l e l e a k v a l v e ) e n t e r e d t h e e x c i t a t i o n chamber a t an a n g l e o f 45 degrees i n an attempt t o m i n i m i z e t h e e f f e c t s o f momentum t r a n s f e r from t h e e x c i t i n g electrons. E l e c t r o n s were produced  from a d i r e c t l y heated t u n g s t e n f i l a m e n t F  (0.038 mm x 0.76 mm) and a c c e l e r a t e d through a s l i t i n t o the  e x c i t a t i o n chamber.  (3.45 mm x 0.51 mm)  E l e c t r o n s and n e g a t i v e i o n s were p r e v e n t e d  from l e a v i n g t h e e x c i t a t i o n chamber by b i a s i n g t h e i o n and e l e c t r o n t r a p chamber n e g a t i v e (22.5 v o l t s ) w i t h r e s p e c t t o t h e e x c i t a t i o n chamber. The e x c i t a t i o n and a l s o t h e e l e c t r o n and i o n t r a p chambers were c a v i t i e s i n b r a s s b l o c k s o f i n t e r n a l dimensions  1.27 cm x 1.27 cm x 1.27 cm and  127° A N A B Y S E R  EXCITATION CHAMBER  EXTERNAL PHOTON  H '  GAS  ELECTRON AND ION TRAP  LAMP  COLLISION CHAMBER  i  2 0 0 0 Ips DIFFERENTIAL PUMPING  TARGET G A S end view F i g u r e 4.  Schematic diagram o f Penning i o n i z a t i o n e l e c t r o n s p e c t r o m e t e r .  -43-  1.18 cm x 1.63 cm x 1.63 cm r e s p e c t i v e l y and were e l e c t r i c a l l y i n s u l a t e d from each o t h e r by s e v e r a l mica s h e e t s . The beam o f p a r t i c l e s which emerged from t h e e x c i t a t i o n r e g i o n passed through two a p e r t u r e s (6.35 mm d i a m e t e r ) covered by t u n g s t e n mesh Gj,  G  2  (90% t r a n s m i t t a n c e ) . P o s i t i v e l y charged p a r t i c l e s were e x t r a c t e d  from t h e beam by a p p l y i n g a p o s i t i v e p o t e n t i a l  (with respect t o the  e x c i t a t i o n chamber) t o t h e d e f l e c t i n g p l a t e s P^, P e x t e r n a l l y connected.  which a r e  The s t a i n l e s s s t e e l p l a t e s were mounted i n t h e  b l o c k by means o f boron n i t r i d e  3.1.2. C o l l i s i o n  2  insulators.  Region.  The beam e n t e r e d t h e c o l l i s i o n chamber, h a v i n g passed  through  another a p e r t u r e G^, a l s o covered by t u n g s t e n mesh and s i n c e the d i s t a n c e between t h e e x c i t a t i o n chamber and t h e c e n t r e o f t h e c o l l i s i o n r e g i o n was 7.62 cm, t h e beam c o n t a i n e d e s s e n t i a l l y ground s t a t e ( l ^ S ) and m e t a s t a b l e  1 3 (2 S and 2 S) s t a t e h e l i u m atoms.  A small flux of  o  Hel,  584 A r a d i a t i o n was a l s o p r e s e n t i n the beam due t o r a d i a t i o n  produced by e l e c t r o n bombardment. the ' i n t e r n a l ' l i g h t s o u r c e ) .  ( T h i s i s s u b s e q u e n t l y r e f e r r e d t o as  Throughout t h i s t h e s i s r e f e r e n c e i s  made t o t h e h e l i u m 'beam', however, i t i s p r o b a b l y a t b e s t o n l y a quasi-beam due t o t h e p r e s s u r e s i n v o l v e d i n s i d e t h e s p e c t r o m e t e r  collision  region. The c o l l i s i o n chamber (10.16 cm x 2.54 cm d i a m e t e r ) was a t t a c h e d to  a p l a t e c o n t a i n i n g the e x i t s l i t  (5 mm x 0.5 mm).  A t e f l o n r i n g was  i n s e r t e d between t h e c o l l i s i o n chamber s l i t p l a t e and t h e e n t r a n c e  slit  p l a t e o f t h e a n a l y z e r t o p r e v e n t s t r a y e l e c t r o n s from g a i n i n g access t o the e n t r a n c e s l i t o f the a n a l y z e r .  -44-  Th e e x c i t a t i o n chamber, e l e c t r o n and i o n t r a p and a l s o t h e c o l l i s i o n chamber were g o l d p l a t e d and then t h e i n t e r n a l s u r f a c e s  covered  w i t h a l a y e r o f benzene soot i n o r d e r t o reduce t h e s c a t t e r e d e l e c t r o n background due t o Auger type p r o c e s s e s  which occured  when  metastable  atoms s t r u c k t h e w a l l s o f t h e a p p a r a t u s .  3.1.3  E l e c t r o n A n a l y z e r and D e t e c t i o n System.  A 127 degree e l e c t r o n a n a l y z e r was chosen s i n c e i t was a r e l a t i v e l y easy d e v i c e t o c o n s t r u c t and l e s s s u s c e p t i b l e t o s p u r i o u s magnetic f i e l d s than s p h e r i c a l a n a l y z e r s . i n s t a l l e d on t h e i n s t r u m e n t  H e l m h o l t z c o i l s were o r i g i n a l l y  t o p r o v i d e a magnetic f i e l d f r e e environment  but i t was found t h a t optimum performance was o b t a i n e d w i t h n e g l i g i b l e f i e l d correction.  The use o f s l i t s  (instead of c i r c u l a r apertures  i n s p h e r i c a l a n a l y z e r s ) p e r m i t t e d t h e use o f a much l a r g e r sampling and hence g r e a t e r i n t e n s i t y .  used area  The a n a l y z e r t r a n s m i t t e d a s u f f i c i e n t l y  i n t e n s e beam o f e l e c t r o n s which c o u l d be e a s i l y measured and was capable  o f s u f f i c i e n t r e s o l u t i o n such t h a t v i b r a t i o n a l l e v e l s were  easily resolved.  The use o f a d e f l e c t i o n a n a l y z e r a l s o had t h e advantage  t h a t a d i f f e r e n t i a l s i g n a l was d i r e c t l y o b t a i n e d and t h i s was a p a r t i c u l a r advantage i n s p e c t r o s c o p i c work where assignment o f energy l e v e l s was made. The a n a l y z e r was c o n s t r u c t e d o f g o l d p l a t e d b r a s s w i t h  electrodes  o f r a d i i 22.5 mm and 27.5 mm and a mean e l e c t r o n t r a j e c t o r y o f r a d i u s 25.0 mm.  The e l e c t r o d e s and end p l a t e s were i n s u l a t e d and l o c a t e d by  boron n i t r i d e s p a c e r s .  The e n t r a n c e  and e x i t s l i t p l a t e s o f t h e a n a l y z e r  were mounted d i r e c t l y t o t h e end p l a t e s ( s l i t d i m e n s i o n s , 5 mm x 0.5 mm). The h e i g h t o f t h e a n a l y z e r (129).  (8.64 cm) m i n i m i z e d  t h e problem o f end e f f e c t s  -45-  A f r a c t i o n o f the e l e c t r o n s produced i n i o n i z i n g c o l l i s i o n s between the m e t a s t a b l e  atoms and t a r g e t gas  e n t e r the a n a l y z e r .  The  l e a v e the c o l l i s i o n chamber and  a n a l y z e r was  about 2 eV energy and t h e spectrum was  normally set to transmit e l e c t r o n s of scanned by v a r y i n g the p o t e n t i a l  a p p l i e d between the c o l l i s i o n chamber (at ground p o t e n t i a l ) and e n t r a n c e s l i t p l a t e o f the a n a l y z e r . t h a t the r e s o l u t i o n  T h i s mode o f s c a n n i n g  (AE/E) remained c o n s t a n t throughout  scanning p o t e n t i a l was  the  ensured  a scan.  The  o b t a i n e d by a m p l i f i c a t i o n o f a f o u r v o l t ramp  o r i g i n a t i n g from a m u l t i c h a n n e l a n a l y z e r o r a l t e r n a t i v e l y a motor d r i v e n h e l i p o t c o u l d be used t o v a r y the r e t a r d i n g p o t e n t i a l . d i g i t a l v o l t m e t e r was  used t o measure the s c a n n i n g  A  potential.  E l e c t r o n s which passed through the a n a l y z e r were c o l l e c t e d by a M u l l a r d B419AL channel e l e c t r o n m u l t i p l i e r entrance  (having an 8 mm  cone t o i n c r e a s e c o l l e c t i o n e f f i c i e n c y ) .  The  electron multiplier  was w e l l s h i e l d e d t o e x c l u d e s p u r i o u s e l e c t r o n c u r r e n t s . output p u l s e s were m o n i t o r e d  diameter  The  multiplier  by means o f a p r e a m p l i f i e r f o l l o w e d by  a Hamner E l e c t r o n i c s p u l s e c o u n t i n g system, comprised  o f an  amplifier/  d i s c r i m i n a t o r and a l i n e a r r a t e m e t e r b o t h mounted i n a s t a n d a r d b i n (which a l s o p r o v i d e d the p r e a m p l i f i e r power). r a t e m e t e r was procedures)  The  output from the  d i s p l a y e d on a s t r i p c h a r t r e c o r d e r ( f o r s e t t i n g  o r d i r e c t e d t o the i n p u t o f a F a b r i t e k Instruments  1064 m u l t i c h a n n e l a n a l y z e r (4096 c h a n n e l s ) , o p e r a t e d i n the d i g i t i z e mode ( f o r d a t a s t o r a g e and a v e r a g i n g ) . f o r each channel was (1024 channels)  u s u a l l y 0.5  o f the memory was  s i g n a l averaged output was  sec.  NIM  up Model  signal  C o u n t i n g time p e r  In g e n e r a l o n l y one  scan  quadrant  used t o s t o r e a spectrum.  n o r m a l l y r e c o r d e d on a Moseley X-Y  While  the  plotter,  -46-  f a c i l i t i e s were a l s o a v a i l a b l e t o output s p e c t r a onto magnetic tape f o r p r o c e s s i n g at the U.B.C. main computer f a c i l i t y .  3.1.4. L i g h t Source The  e x t e r n a l l i g h t source used t o p r o v i d e the a b s o l u t e energy  c a l i b r a t i o n o f the Penning  s p e c t r a was  a low p r e s s u r e microwave d i s c h a r g e  i n h e l i u m , e a r l i e r used as a f a r u l t r a v i o l e t source f o r mass by F r o s t and McDowell (82, 130).  spectrometry  In the p r e s e n t a p p l i c a t i o n i t p r o v i d e d o  a l i n e e m i s s i o n spectrum c o n s i s t i n g e s s e n t i a l l y o f 584 A (21.217 eV) radiation.  T h i s s p e c t r a l l i n e a r i s e s from the 2^P -> l ^ S resonance  t r a n s i t i o n i n helium  (131) and has s u f f i c i e n t energy t o i o n i z e a l l  gases except neon and h e l i u m .  An Edwards n e e d l e v a l v e c o n t r o l l e d  the  f l o w o f commercial tank h e l i u m i n t o a q u a r t z tube a t a p r e s s u r e  of  approximately 1 t o r r .  stabilized  The p r e s s u r e i n the d i s c h a r g e r e g i o n was  by a c o n s t r i c t i o n at the f a r end o f the q u a r t z tube and t h i s a l s o s e r v e d t o f a c i l i t a t e d i f f e r e n t i a l pumping o f the h e l i u m , between the source and the c o l l i s i o n chamber. resonant  The  d i s c h a r g e was  c a v i t y powered by an E l e c t r o M e d i c a l  microwave g e n e r a t o r . o f b r a s s and was  The  Z e l i k o f f et a l . (134).  produced u s i n g a  ' M i c r o t r o n 200'  s i l v e r p l a t e d microwave c a v i t y was  a m o d i f i e d form (132, 133) The d i s c h a r g e was  light  2450  MHz  constructed  o f t h a t d e s c r i b e d by  i n i t i a t e d with a Tesla c o i l  and the q u a r t z d i s c h a r g e tube c o o l e d w i t h compressed a i r . The  final  p o r t i o n o f the source c o n s i s t e d o f a g l a s s c a p i l l a r y which t r a n s m i t t e d a narrow beam o f l i g h t and a l s o s e r v e d t o reduce the h e l i u m f l o w i n t o the c o l l i s i o n region.  -47-  3.1.5. Vacuum System. The vacuum system c o n s i s t e d o f t h r e e r e g i o n s , the  sample  h a n d l i n g r e g i o n , the h i g h vacuum system and the l i g h t s o u r c e r e g i o n . The sample h a n d l i n g system, o f a l l metal c o n s t r u c t i o n , c o n s i s t e d o f an i n l e t l i n e , r e s e r v o i r , pumping l i n e and a v a r i a b l e l e a k .  The  i n l e t l i n e was c o n s t r u c t e d so t h a t i n t e r c h a n g e a b l e f i t t i n g s c o u l d be used t o accomodate e i t h e r a s t o r a g e v e s s e l  ( f i t t e d w i t h a B-10  a gas c y l i n d e r o r a s t a n d a r d l e c t u r e b o t t l e .  socket) ,  The l e c t u r e b o t t l e  was  connected t o the i n l e t l i n e by a m e t a l l i n e , s i l v e r s o l d e r e d t o a Matheson s t a i n l e s s s t e e l n e e d l e v a l v e .  A four l i t r e brass r e s e r v o i r  was used t o m a i n t a i n a s t e a d y sample p r e s s u r e throughout a r u n .  Veeco  b e l l o w s v a l v e s were used t o i s o l a t e each s e c t i o n o f the sample h a n d l i n g system t o a l l o w independent e v a c u a t i o n . A G r a n v i l l e - P h i l l i p s  variable  l e a k v a l v e was used t o admit the sample t o the c o l l i s i o n chamber.  A  m e c h a n i c a l pump, pumping speed 21 l i t r e / m i n , was used t o evacuate the system. The h i g h vacuum i n the s p e c t r o m e t e r r e g i o n was m a i n t a i n e d by  two  o i l d i f f u s i o n pumps (N.R.C. VHS6, pumping speed, 2000 l i t r e / s e c ) w i t h l i q u i d n i t r o g e n c o l d t r a p s each backed by m e c h a n i c a l pumps (Welch Duo-Seal, Model 1397, pumping speed, 425 l i t r e / m i n ) .  The main vacuum  chamber, c o n t a i n i n g the Penning i o n i z a t i o n e l e c t r o n s p e c t r o m e t e r was c o n s t r u c t e d o f type 304 s t a i n l e s s s t e e l .  I s o l a t i o n o f the pumping  system, from the main chamber, was a c c o m p l i s h e d by N.R.C. High Vacuum S l i d e V a l v e s , Type 1283-6.  The p r e s s u r e i n the vacuum chamber was  measured w i t h a N.R.C. B a y a r d - A l p e r t i o n i z a t i o n guage. arrangement  With t h i s  the r e s i d u a l p r e s s u r e i n the unbaked system was  about  -48-  1.0 x 10"' t o r r . The  l i g h t source had a 4.3 cm pumping l i n e s i t u a t e d c l o s e t o t h e  end o f t h e c a p i l l a r y f o r e v a c u a t i o n o f h e l i u m , by an o i l d i f f u s i o n pump (N.R.C. HS2, pumping speed, 285 l i t r e / s e c ) coupled w i t h a d r y i c e c o o l e d t r a p and backed by a mechanical litre/min).  pump (pumping speed, 140  The Penning i o n i z a t i o n s p e c t r o m e t e r  r e s i d u a l vacuum  _7 i n c r e a s e d t o about 3 x 10  3.2.  t o r r d u r i n g l i g h t source  operation.  S p e c t r p m e t e r Performance. The  r e s o l u t i o n o f t h e 127 degree e l e c t r o n a n a l y z e r was 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 s c o p y .  F i g u r e 5 shows a t y p i c a l s i n g l e  scan  o  o f t h e He(584 A) p h o t o e l e c t r o n spectrum o f argon o b t a i n e d w i t h t h e e x t e r n a l l i g h t source.  The ^P,, peak o f A r has a f u l l w i d t h a t 2 +  6 1  h a l f maximum (FWHM) o f 0.022 eV. The a n a l y z e r performance was i n good agreement w i t h c a l c u l a t i o n s ( e q u a t i o n 12) based on t h e geometry of the analyzer.  F i g u r e 6 shows a s i n g l e scan o f t h e p h o t o e l e c t r o n  spectrum o f m o l e c u l a r hydrogen at lower r e s o l u t i o n .  Relative intensities  are i n good agreement w i t h o t h e r p h o t o e l e c t r o n work. For most o f t h e work r e p o r t e d i n t h i s t h e s i s t h e energy r e s o l u t i o n was t y p i c a l l y s e t a t 0.04 - 0.06 eV FWHM i n o r d e r t o o b t a i n i n c r e a s e d sensitivity.  There was l i t t l e advantage i n u s i n g h i g h e r r e s o l u t i o n  s i n c e i n most cases t h e e l e c t r o n energy d i s t r i b u t i o n s o b t a i n e d f o r Penning e l e c t r o n s a r e n a t u r a l l y much b r o a d e r photoelectrons.  than those o b t a i n e d f o r  I n Penning i o n i z a t i o n t h e w i d t h o f t h e d i s t r i b u t i o n s  o b t a i n e d depended n o t o n l y on t h e i n c i d e n t p r o j e c t i l e b u t a l s o on t h e r e l a t i v e k i n e t i c energy o f c o l l i s i o n .  However, i n p h o t o e l e c t r o n  -49-  A r + 5 8 4 A -*^Ar +e +  Q 1 7 8 eV  o  F i g u r e 5.  High r e s o l u t i o n p h o t o e l e c t r o n spectrum (584 A) o f argon.  o  F i g u r e 6.  P h o t o e l e c t r o n spectrum (584 A) o f m o l e c u l a r  hydrogen.  -51-  spectroscopy  the n a r r o w e r d i s t r i b u t i o n o f e l e c t r o n s was  d e t e r m i n e d by  i n s t r u m e n t a l f a c t o r s ( a p a r t from the D o p p l e r b r o a d e n i n g due t o  thermal  motion o f t h ^ t a r g e t g a s ) . F i g u r e 7 shows a comparison o f the p h o t o e l e c t r o n spectrum o f argon o b t a i n e d u s i n g the r a d i a t i o n from the e x t e r n a l photon lamp w i t h t h a t o  o b t a i n e d w i t h the 584 A r a d i a t i o n produced i n the e x c i t a t i o n chamber (internal l i g h t source). s e t f o r the simultaneous The  These s p e c t r a were o b t a i n e d w i t h a l l c o n d i t i o n s use o f the i n s t r u m e n t  t o o b t a i n Penning s p e c t r a .  d i f f e r e n c e i n h a l f w i d t h s between the two s p e c t r a was  caused by  a  l a r g e a n g u l a r c o n t r i b u t i o n which reduced the r e s o l u t i o n , w h i l e u s i n g i n t e r n a l photon s o u r c e . along the m e t a s t a b l e  The  r e l a t i v e l y large apertures  p a t h , t o g e t h e r w i t h the source  (5 mm  the  diameter),  geometry, r e s u l t e d i n  e l e c t r o n s e n t e r i n g the 127 degree a n a l y z e r o v e r a s o l i d angle which i s e f f e c t i v e l y l i m i t e d t o an a v a l u e o f about e i g h t degrees by the s p a c i n g o f the e l e c t r o s t a t i c a n a l y z e r . 0.025 eV The  The  annular  a n g u l a r term c o n t r i b u t e d about  ( e q u a t i o n 12) t o the r e s o l u t i o n at an e l e c t r o n energy o f ^ 2  eV.  Penning s p e c t r a were r e c o r d e d under these c o n d i t i o n s o f a c c e p t a n c e  angle  (a = 8°).  beam (1 mm  On the o t h e r hand the narrow e x t e r n a l photon  d i a m e t e r ) subtends an a o f about two degrees which under  normal o p e r a t i n g c o n d i t i o n s made an almost n e g l i g i b l e c o n t r i b u t i o n ( e q u a t i o n 12) t o the r e s o l u t i o n o b t a i n e d . The the  pressure  dependence o f the Penning and p h o t o e l e c t r o n s i g n a l f o r  processes H e * ( 2 S ) + A r -> He + A r ( P 3  +  0  and  +2  hv(584 A) + A r -> A r ( P , ; ) 6 /  The  . ) + e  2  +  e  are shown i n F i g u r e  8.  2  a b s c i s s a g i v e s the e q u i l i b r i u m p r e s s u r e  o f argon i n the apparatus  -52-  1  -i  0.1  0  UNCORRECTED  1  l _  0.1  Q2  E L E C T R O N E N E R G Y (eV) o  F i g u r e 7.  P h o t o e l e c t r o n s p e c t r a o f argon a t 584 A u s i n g and e x t e r n a l photon lamps.  internal  Figure  8.  P r e s s u r e dependence o f the Penning and p h o t o e l e c t r o n s i g n a l as a f u n c t i o n o f t a r g e t gas p r e s s u r e .  -54-  Fnr t h i s study a f i x e d h e l i u m p r e s s u r e was used f o r t h e p r o d u c t i o n o f t h e m e t a s t a b l e beam.  I t was o b s e r v e d t h a t f o r sample p r e s s u r e s  above 8 x 10 ^ t o r r 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 d e c r e a s e d .  The  * 3 +2 p r e s s u r e dependence o f He (2 S) + A r ->- He + A r ( P_, ) + e was a l s o s t u d i e d as a f u n c t i o n o f t h e m e t a s t a b l e beam p r e s s u r e ( F i g u r e 9 ) .  The  a b s c i s s a i n F i g u r e 9 g i v e s t h e t o t a l p r e s s u r e o f argon and h e l i u m i n the a p p a r a t u s .  I t was observed t h a t f o r t o t a l p r e s s u r e s above 2.5 x 10  t o r r the r e s o l u t i o n  decreased.  In g e n e r a l , sample p r e s s u r e s o f about 4 x 10 ^ t o r r were used. M o l e c u l a r gases were Matheson CP grade o r P h i l l i p s and were s t o r e d i n commercial  lecture bottles.  '66' r e s e a r c h grade  L i q u i d s o f reagent o r  s p e c t r o s c o p i c grade were degassed by f r e e z e - t h a w c y c l e s .  The back-  ground p r e s s u r e from h e l i u m used i n f o r m i n g t h e m e t a s t a b l e beam was about 1.5 x 10 ^ t o r r and t h e t o t a l p r e s s u r e d u r i n g a r u n , was a p p r o x i m a t e l y 2.0 x 10 The  torr.  i n t e r p r e t a t i o n o f t h e Penning e l e c t r o n s p e c t r a was c o m p l i c a t e d  i n some cases due t o the f a c t t h a t t h e s p e c t r a produced  by t h e two *  metastable s t a t e s overlap.  Attempts  1  were made t o e l i m i n a t e t h e He (2 S)  component from t h e beam by o p t i c a l quenching  techniques.  I t has been  shown (37, 54, 135) t h a t 2.06 m i c r o n photons can be used t o c o u p l e t h e 2^S atoms t o t h e 2^P s t a t e from which decay t o t h e ground s t a t e o c c u r s o  w i t h e m i s s i o n o f He(584 A) r a d i a t i o n .  S e v e r a l u n s u c e s s f u l attempts were  made t o quench t h e 2^S s t a t e by ( i ) s u r r o u n d i n g t h e beam w i t h a q u a r t z g l a s s s p i r a l , h e l i u m d i s c h a r g e lamp s i m i l a r i n d e s i g n t o t h a t o f Hotop et a l . ( 3 7 ) , ( i i ) a q u a r t z , o v a l shaped, h e l i u m d i s c h a r g e lamp  employing  t u n g s t e n e l e c t r o d e s , ( i i i ) a microwave d i s c h a r g e lamp s i m i l a r t o t h a t  -55-  F i g u r e 9.  P r e s s u r e dependence o f the Penning of helium pressure.  s i g n a l as a f u n c t i o n  -56-  d e s c r i b e d by F r y and W i l l i a m s ( 1 3 5 ) , and ( i v ) a S y l v a n i a "Sun which u t i l i z e s a q u a r t z - i o d i n e  Gun",  lamp.  The most i n t e n s e source o f 2 m i c r o n r a d i a t i o n , as measured by a H i l g e r and Watts monochrometdr, was the S y l v a n i a "Sun Gun". * i n no case was measureable quenching o f the He  However,  1 (2 S) s t a t e o b s e r v e d .  I t may be t h a t the tank h e l i u m used was o f i n s u f f i c i e n t p u r i t y  and  required f u r t h e r clean-up. 3.3. C a l i b r a t i o n o f Energy S c a l e and P r e s e n t a t i o n o f Data. P h o t o e l e c t r o n energy s c a l e s were c a l i b r a t e d u s i n g l i t e r a t u r e v a l u e s o f i o n i z a t i o n p o t e n t i a l s from p h o t o e l e c t r o n s p e c t r o s c o p y .  Simultaneous  r e c o r d i n g o f Penning and p h o t o e l e c t r o n s p e c t r a , i n the p r e s e n t work, p r o v i d e d a c c u r a t e c a l i b r a t i o n o f the Penning e l e c t r o n energy s c a l e and the d e t e r m i n a t i o n o f energy s h i f t v a l u e s . s c a l e was  ± 0.01  The a c c u r a c y o f the energy  eV.  For a l l m o l e c u l e s examined  i n t h i s s t u d y the f o l l o w i n g t y p e s o f  s p e c t r a were o b t a i n e d : (a)  A "pure" Penning e l e c t r o n spectrum r e s u l t i n g from i o n i z a t i o n * 1 * 3 by m e t a s t a b l e h e l i u m atoms (He (2 S) and He (2 S ) ) .  (b)  For t h e purpose o f c a l i b r a t i o n o f t h e Penning e l e c t r o n spectrum, the Penning and p h o t o e l e c t r o n s p e c t r a were r e c o r d e d s i m u l t aneously.  When p o s s i b l e s e v e r a l p h o t o e l e c t r o n peaks were used  t o c a l i b r a t e the energy s c a l e .  Peaks on the r a p i d l y  background ramp were not used f o r c a l i b r a t i o n .  Penning e l e c t r o n  energy s h i f t s were measured, where p o s s i b l e , d i r e c t l y the c o r r e s p o n d i n g p h o t o e l e c t r o n peak.  rising  from  -57-  (c)  A pure p h o t o e l e c t r o n spectrum was a l s o o b t a i n e d u s i n g t h e e x t e r n a l photon lamp. convenient  F o r each m o l e c u l e t h i s p r o v i d e d a  check o f t h e energy r e s o l u t i o n o f t h e i n s t r u m e n t  and t h e p u r i t y o f t h e sample. o  The He 584 A p h o t o e l e c t r o n s p e c t r a o f t h e atoms and m o l e c u l e s used i n t h i s work have been e x t e n s i v e l y s t u d i e d by v a r i o u s groups i n recent years.  These s p e c t r a and t h e i r assignments may be found i n t h e  literature. In o r d e r t h a t r e l a t i v e t r a n s i t i o n p r o b a b i l i t i e s c o u l d be d e t e r m i n e d and compared w i t h those r e p o r t e d e l s e w h e r e , i t was n e c e s s a r y c o r r e c t i o n t o t h e measured peak h e i g h t s .  t o apply a  The c o r r e c t i o n arose  since  a v a r i a b l e p o t e n t i a l was a p p l i e d between t h e c o l l i s i o n r e g i o n and t h e a n a l y z e r i n o r d e r t o operate  the e l e c t r o n analyzer i n the constant  energy r e s o l u t i o n mode t o produce a spectrum.  The c o l l e c t i o n  efficiency  o f e l e c t r o n s v a r i e d w i t h e l e c t r o n energy due t o changing e l e c t r o n o p t i c a l l e n s e f f e c t s between t h e e n t r a n c e analogous t o c h r o m a t i c  and e x i t s l i t s .  a b e r r a t i o n i n o p t i c a l spectroscopy.  This i s I t s h o u l d be  noted t h a t i n t h e v a s t m a j o r i t y o f p u b l i s h e d p h o t o e l e c t r o n s p e c t r a no attempt has been made t o c o r r e c t f o r t h i s e f f e c t , w h i c h can be q u i t e l a r g e . The r e t a r d i n g a n a l y z e r o f Hotop and Niehaus has been r e p o r t e d (35) t o have c o n s t a n t  transmission.  The i n t e g r a t e d 90 degree p h o t o e l e c t r o n  band i n t e n s i t i e s o b t a i n e d i n t h i s work have been compared w i t h t h e work o f Hotop and Niehaus (35) f o r m o l e c u l a r and a l s o f o r n i t r i c o x i d e c o r r e c t i o n f a c t o r T.  n i t r o g e n and carbon monoxide  (136), i n o r d e r t o d e r i v e a r e l a t i v e  transmission  F i g u r e 10 shows t h e t r a n s m i s s i o n c o r r e c t i o n  f a c t o r f o r t h e a n a l y z e r used i n t h e p r e s e n t work.  I t may be seen t h a t  F i g u r e 10.  R e l a t i v e t r a n s m i s s i o n c o r r e c t i o n f a c t o r f o r 127 degree Penning  spectrometer.  -59-  over a range o f 12 eV t h e r e i s a c o n s i d e r a b l e change i n t h e c o l l e c t i o n e f f i c i e n c e s and f a i l u r e t o use t h i s c o r r e c t i o n f a c t o r would have i n t r o d u c e d s e r i o u s e r r o r s i n q u a n t i t a t i v e work, e s p e c i a l l y i n t h e energy range 1 - 7 eV.  I n independent e x p e r i m e n t s employing  e l e c t r o n - e l e c t r o n c o i n c i d e n c e t e c h n i q u e s van d e r W i e l and B r i o n (139) have determined t h e r e l a t i v e i o n i c s t a t e p o p u l a t i o n s f o r t h e p h o t o i o n i z a t i o n o f carbon monoxide and found e x c e l l e n t agreement w i t h the work o f Hotop and Niehaus o f Vroom f138).  (35) as w e l l as t h e t o t a l c o l l e c t i o n experiments  -60-  CHAPTER  RESULTS  FOUR  AND  DISCUSSION  4.1. Rare Gases.  4.1.1. Argon. The e l e c t r o n spectrum r e s u l t i n g from the impact o f s i n g l e t  and  o  t r i p l e t m e t a s t a b l e h e l i u m atoms and He(584 A) photons on argon i s shown i n F i g u r e 11.  The p h o t o e l e c t r o n spectrum produced by the  'internal'  584 A r a d i a t i o n from the m e t a s t a b l e source was o f s u f f i c i e n t i n t e n s i t y f o r c a l i b r a t i o n o f the energy s c a l e .  Due t o s p i n - o r b i t s p l i t t i n g  the  i o n i z a t i o n spectrum o f argon e x h i b i t s two peaks c o r r e s p o n d i n g t o the ^P , and ^P,, s t a t e s o f the A r i o n . T h e r e f o r e a d o u b l e t was o b s e r v e d +  '2  5  '2  f o r each i o n i z a t i o n p r o c e s s .  S p e c t r o s c o p i c v a l u e s (139) f o r t h e  i o n i z a t i o n p o t e n t i a l s were used t o c a l i b r a t e the energy s c a l e and are given i n Table I. The t r i p l e t  t o s i n g l e t r a t i o f o r h e l i u m m e t a s t a b l e s i n the beam,  as d e r i v e d from t h e s p e c t r a i n F i g u r e 11 exceeded t h e v a l u e e x p e c t e d (119) f o r 170 eV e l e c t r o n s ( ^ 0.42).  The d i f f e r e n c e i n r a t i o may be  i n p a r t t o the d i f f e r e n c e i n a n g u l a r d i s t r i b u t i o n  due  (102) o f e l e c t r o n s  from i o n i z a t i o n by the two m e t a s t a b l e h e l i u m s t a t e s .  Hotop and Niehaus  (102) have r e p o r t e d a n g u l a r d i s t r i b u t i o n s o f Penning e l e c t r o n s f o r argon  ——••  5.5  ••—=  1  :  5.0  1  4.5  l _  4.0  E L E C T R O N ENERGY (eV) F i g u r e 11.  E l e c t r o n s p e c t r a f o r i o n i z a t i o n o f argon.  -62-  and  observed the  d i s t r i b u t i o n t o be  angles between m e t a s t a b l e beam and f o r i o n i z a t i o n by  s i n g l e t and  asymmetric, f a v o u r i n g  large  d e t e c t o r d i r e c t i o n and  different  t r i p l e t m e t a s t a b l e h e l i u m atoms.  Unfort-  u n a t e l y t h e i r r e s u l t s were n o r m a l i z e d t o a r e l a t i v e i n t e n s i t y o f at a d e t e c t o r - m e t a s t a b l e beam a n g l e o f 90 degrees so t h a t o f t h e i r r e s u l t s t o t h i s work was triplet  possible.  The  t o s i n g l e t r a t i o c o u l d a l s o be p a r t l y due  c o l l i s i o n s between e l e c t r o n s and (52,  not  He  *  one  application  a p p a r e n t l y anomalous  to  superelastic  1 * 3 (2 S) atoms t o produce He (2 S)  140). Peak shapes are  compared i n d e t a i l i n F i g u r e 12 and  i t i s apparent o  t h a t the w i d t h o f the p e a k s , due * photons and due  He  1 (2 S) atoms are * 3  t o i o n i z a t i o n by He  Hotop and  Niehaus (31)  s i n g l e t and  this natural  distributions.  f o r the peaks due  halfwidth  t o i o n i z a t i o n by He  instrumental resolution  o f 0.087 eV,  i n good agreement w i t h * 1  F i g u r e 12.  i n peak shapes f o r t r i p l e t  i o n i z a t i o n can be and  potential  attributed  (2 S ) .  (0.080 eV)  f o r i o n i z a t i o n by He  difference  between  They observed a n a t u r a l * 1  observed w i d t h o f 0.085 eV The  peaks  c o n s i d e r a b l y b r o a d e r (by 0.025 eV) .  have a l s o r e p o r t e d s i m i l a r d i f f e r e n c e s  w i d t h i n t o the  in a calculated  i n t e r n a l He(584 A)  o f s i m i l a r magnitude whereas the  (2 S) are  t r i p l e t electron  w i d t h o f ^ 0.035 eV  t o i o n i z a t i o n by  Folding  results the  (2 S) as shown i n and  singlet  to d i f f e r e n c e s  in transition probabilities 1 3 energy i n t e r a c t i o n c u r v e s f o r 2 S and 2 S h e l i u m m e t a s t a b l e *  atoms w i t h argon. calculated He  In the  case o f He  + H, M i l l e r and * 3  a s i g n i f i c a n t l y deeper w e l l f o r He  (2 S) + H.  For  a broader e l e c t r o n  argon a s i m i l a r e f f e c t may  Schaefer  (116)  (2 S) + H t h a n f o r o c c u r due  d i s t r i b u t i o n i s observed i n the  t o the  triplet  fact  case.  that  This  -63-  J  0.2  0.1  0  UNCORRECTED F i g u r e 12.  0.1  L  0.2  E L E C T R O N E N E R G Y (eV)  Comparison o f peak shapes f o r i o n i z a t i o n o f argon.  -64-  difference  in potential  energy c u r v e s i s a l s o r e f l e c t e d i n the o b s e r v e d *  ratios for associative * He  t o Penning i o n i z a t i o n f o r He  3 (2 S) + Ar and  1 (2 S) + A r as a f u n c t i o n  workers  o f temperature  Hotop and co-  (31, 37, 142 143) have o b s e r v e d t h a t the t r i p l e t peaks a r e  broadened  on the h i g h energy s i d e w i t h i n c r e a s e i n temperature  therefore increasing  k i n e t i c energy o f t h e c o l l i d i n g atoms).  e f f e c t has been o b s e r v e d (144), a t t r i b u t e d 145, 146) from the e x c i t i n g e l e c t r o n electron  (37, 141).  (and A similar  t o momentum t r a n s f e r  (31,  beam, i n t h a t an i n c r e a s e i n the  e x c i t i n g v o l t a g e was o b s e r v e d t o broaden the t r i p l e t  distrib-  u t i o n s on the h i g h energy s i d e w h i l e t h e s i n g l e t d i s t r i b u t i o n s were e s s e n t i a l l y unchanged. Niehaus  Additional  structure  observed by Hotop and  (31) on the h i g h energy s i d e o f the e l e c t r o n  not been found i n any o f the s p e c t r a r e p o r t e d h e r e .  distributions  has  The presence o f  d i f f e r e n t t h e r m a l groups o f h e l i u m m e t a s t a b l e atoms i n the beam may the  cause o f such f i n e s t r u c t u r e The d i f f e r e n c e  the nominal energy E  on the e l e c t r o n  distributions.  i n the measured Penning e l e c t r o n q  (where E ^ i s the d i f f e r e n c e  energy E ^ from G  between t h e  energy o f the m e t a s t a b l e atom and the i o n i z a t i o n p o t e n t i a l t a r g e t s p e c i e s ) i s d e f i n e d as A E , t h a t i s A E = E ^ - E . q  are used t o i n d i c a t e AE  S  and A E  be  excitation  o f the Subscripts  i o n i z a t i o n by s i n g l e t and t r i p l e t m e t a s t a b l e atoms,  respectively.  F o r argon the measured e l e c t r o n  energy  shift  i s 0.044 eV f o r b o t h s i n g l e t and t r i p l e t i o n i z a t i o n , i n good agreement w i t h o t h e r r e p o r t e d work (21, 31, 37).  R e s u l t s are shown i n T a b l e I I .  The measured r a t i o s o f the i n t e n s i t i e s o f the two s t a t e s 2 ( P„, *'2  o f argon  2 and  P, , ) are i n good agreement w i t h p h o t o i o n i z a t i o n '2  values  determined i n t h i s work and w i t h t h o s e r e p o r t e d by Samson and C a i r n s  -65-  TABLE  S h i f t s , AE, i n Penning E l e c t r o n  11  E n e r g i e s f o r Rare Gases (eV) .  3 2 S I o n i z a t i o n , AE^ t  1 2 S I o n i z a t i o n , AE s  Species T h i s work  Ref. 31  Ar  +  +0.044  +0.04l  Kr  +  +0.035  -  Xe  +  +0.029  +0.032  a  a  T h i s work  Ref. 31  +0.044  +0.046  +0.030  +0.025  +0.005  +0.005  Values i n t h i s work a r e ± 0.005 eV. a  These v a l u e s have been e s t i m a t e d from t h e p r i n c i p a l maximum i n F i g u r e 7 o f Reference 31.  -66-  (147). values.  R e s u l t s are shown i n T a b l e I I I and compared w i t h l i t e r a t u r e No c o n t r i b u t i o n due t o i o n i z a t i o n r e a c t i o n s i n v o l v i n g l o n g  l i v e d Rydberg  s t a t e s (10) was observed i n t h i s s t u d y .  4.1.2. Krypton The e l e c t r o n s p e c t r a o b t a i n e d from k r y p t o n i o n i z a t i o n a r e shown i n F i g u r e 13.  The upper t r a c e was  o b t a i n e d by the s i m u l t a n e o u s use o f o  the Penning beam and the e x t e r n a l photon  lamp (He (584 A)  The p h o t o e l e c t r o n peaks were used t o c a l i b r a t e use o f the e x t e r n a l photon  radiation).  the energy s c a l e .  The  lamp g i v e s r i s e t o peaks which are much  more i n t e n s e and w i t h a narrower energy d i s t r i b u t i o n than those a r i s i n g from t h e i n t e r n a l photon s o u r c e .  The lower t r a c e , "pure" Penning  i o n i z a t i o n , shows o n l y a s m a l l c o n t r i b u t i o n from p h o t o i o n i z a t i o n due t o the i n t e r n a l  photons.  The measured s h i f t i n e l e c t r o n energy i s i n good agreement w i t h 2 2 e a r l i e r work (31) and i s shown t o g e t h e r w i t h the ^P?/ / P-. / i n t e n s i t y 2  6/  r a t i o s i n T a b l e s I I and I I I .  '2  A l t h o u g h the k r y p t o n spectrum i s somewhat  c o m p l i c a t e d by the o v e r l a p o f e l e c t r o n bands, i t was o b s e r v e d , as the case f o r argon, t h a t peaks i n the spectrum a r i s i n g from  was  ionization  by t r i p l e t m e t a s t a b l e atoms are b r o a d e r , by a p p r o x i m a t e l y 0.025 eV, than those a r i s i n g from i o n i z a t i o n by s i n g l e t m e t a s t a b l e s . 4.1.3. Xenon. The e l e c t r o n s p e c t r a o b t a i n e d f o r xenon a r e shown i n F i g u r e 14. As was  the case f o r k r y p t o n , the upper t r a c e was  o b t a i n e d by the  s i m u l t a n e o u s use o f the m e t a s t a b l e beam and the e x t e r n a l photon  source,  TABLE I I I  Peak R a t i o s  t  2  p  v  /  2  p  2  i / '  f o r Rare Gas I o n i z a t i o n .  2  Photoionization  Penning I o n i z a t i o n  He (584 A)  Species  T h i s work  Ref.  147  2S  2S  3  1  Argon  1.94  1.98  2.00  1.94  Krypton  1.78  1.79  1.8  1.8  Xenon  1.56  1.60  1 .34  a  a  2.03  E x p e r i m e n t a l V a l u e s i n t h i s work are-0.05  a These v a l u e s a r e o n l y approximate due t o o v e r l a p o f t h e  2 P , and '2 1  2 and  P_.  states  formed by s i n g l e t and t r i p l e t i o n i z a t i o n  respectively.  '2  A l s o because o f the i n t e n s i t y o f t h e i n t e r n a l He(584 A) r a d i a t i o n 2 a c o r r e c t i o n was a p p l i e d t o the P_ , s t a t e formed by s i n g l e t '2 ionization.  Kr + 5 8 4 A -*-Kr +e +  KRYPTON  Kr+ H e * ( 2 ' S ) H ^ K r + H e + e +  z  CO  Kr+He*(2 S) -*-Kr + He + e 3  LL!  +  '  h-  z  2  l±J  ON 00  I  > LLI  7.5 F i g u r e 13.  7.0  6.5  6.0  E L E C T R O N E N E R G Y (eV)  E l e c t r o n spectra f o r i o n i z a t i o n of krypton.  5.5  5.0  —'  9-0 F i g u r e 14.  1  L__  8.0  1  1  7.0  ELECTRON ENERGY (eV)  E l e c t r o n s p e c t r a f o r i o n i z a t i o n o f xenon.  I  I  6.0  -70-  i n o r d e r t o c a l i b r a t e the energy s c a l e .  The  lower t r a c e o b t a i n e d  u s i n g o n l y the m e t a s t a b l e s o u r c e e x h i b i t s a v e r y s m a l l c o n t r i b u t i o n o  from p h o t o e l e c t r o n s due t o the i n t e r n a l He(584 A) r a d i a t i o n , n e c e s s i t a t i n g o n l y a s m a l l c o r r e c t i o n due t o o v e r l a p p i n g peaks. I t may be seen, from F i g u r e 15, t h a t peaks a r i s i n g from by He  *  ionization  3 * 1 (2 S) are b r o a d e r than t h o s e due t o He (2 S) i o n i z a t i o n , a g a i n by  a p p r o x i m a t e l y 0.025 eV.  T h i s peak b r o a d e n i n g observed f o r t r i p l e t  i o n i z a t i o n o f a l l the r a r e gases examined, i s o f the o r d e r o f the t h e r m a l e n e r g i e s o f the h e l i u m atoms (148) and may i n the mechanism o f s i n g l e t and t r i p l e t Penning The e l e c t r o n energy s h i f t s  reflect a difference  ionization.  (AE) are g i v e n i n T a b l e I I and are  q u i t e s m a l l , as has been p r e v i o u s l y r e p o r t e d ( 3 1 ) . 2 I t may  be seen from T a b l e I I I t h a t the  2 P  . / •5/  P  .  r a t i o s f o r the  '2  l  2  Penning i o n i z a t i o n o f xenon d i f f e r s i g n i f i c a n t l y from those observed for photoionization.  The r a t i o s o b s e r v e d were found t o be r e p r o d u c i b l e  under a wide v a r i e t y o f s o u r c e c o n d i t i o n s , o v e r a l o n g p e r i o d o f t i m e and w i t h d i f f e r e n t xenon samples.  The e x a m i n a t i o n o f the s p e c t r a o f  m o l e c u l e s w i t h e l e c t r o n bands i n t h i s energy range i n d i c a t e s t h a t the anomalous r a t i o s are not due t o i r r e g u l a r t r a n s m i s s i o n problems e l e c t r o n spectrometer.  i n the  I t i s b e l i e v e d t h a t t h i s phenomenon i s due t o  some i n t r i n s i c p r o p e r t y o f the c r o s s s e c t i o n dependences f o r the  two  p o s s i b l e i o n i c s t a t e s r e s u l t i n g from the c o l l i s i o n p r o c e s s . A s m a l l u n i d e n t i f i e d peak at a p p r o x i m a t e l y 6.85 appeared i n the xenon spectrum.  eV  consistently  -71-  I  0.2  I  0.1  O  UNCORRECTED  F i g u r e 15.  I  I  O.I  l _  0.2  E L E C T R O N E N E R G Y (eV)  Comparison o f peak shapes f o r i o n i z a t i o n o f xenon.  -72-  4.2. D i a t o m i c M o l e c u l e s .  4.2.1. M o l e c u l a r Hydrogen, Deuterium H y d r i d e and M o l e c u l a r Deuterium. The e l e c t r o n s p e c t r a o b t a i n e d from the i o n i z a t i o n o f H,,, HD D  2  are shown i n F i g u r e s 16, 17 and 18.  The ground 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 o f the t h r e e m o l e c u l e s may be w r i t t e n as H, 2  HD,  D  and  (66)  2  There i s o n l y one m o l e c u l a r o r b i t a l , which must be s t r o n g l y b o n d i n g . The s p e c t r a o f t h e s e m o l e c u l e s are s i n g l e bands c o n s i s t i n g o f a l o n g s e r i e s o f w e l l r e s o l v e d v i b r a t i o n a l components w i t h t h e v i b r a t i o n a l s p a c i n g d e c r e a s i n g r a p i d l y toward the d i s s o c i a t i o n l i m i t .  The band  shape i s c o n s i s t e n t w i t h removal o f an e l e c t r o n from a s t r o n g l y b o n d i n g orbital  (35, 83).  The energy s c a l e s were c a l i b r a t e d u s i n g the a d i a b a t i c  i o n i z a t i o n p o t e n t i a l s g i v e n i n T a b l e IV.  A l s o i n T a b l e IV are the  energy s h i f t s , AE, observed f o r the Penning e l e c t r o n p e a k s , f o r b o t h s i n g l e t and t r i p l e t i o n i z a t i o n , o f H^, HD and U^. f o r H^ r e p o r t e d by Hotop and Niehaus  The v a l u e o f AE  (34, 35) i s a l s o g i v e n .  A l l peaks  are s h i f t e d toward h i g h e r e l e c t r o n e n e r g i e s , as was observed by Hotop and Niehaus f o r AE  f o r H^.  T h i s i n d i c a t e s t h a t some o f the k i n e t i c  energy o f the c o l l i d i n g p a r t i c l e s has been c o n v e r t e d i n t o energy o f the ejected electron. In c o n t r a s t t o the r a r e gases i t was o b s e r v e d t h a t the w i d t h o f the peaks due t o i o n i z a t i o n by s i n g l e t m e t a s t a b l e s are c o n s i d e r a b l y b r o a d e r than t h o s e due t o t r i p l e t i o n i z a t i o n .  T h i s presumably i s  i n d i c a t i v e o f a d i f f e r i n g c h a r a c t e r f o r the p o t e n t i a l energy c u r v e s f o r the i n t e r a c t i o n .  A l t h o u g h the Penning e l e c t r o n d i s t r i b u t i o n s a r e  -73-  2 S 3  V=0 I  I  I  .  I  1 1 I II  I  Mill'  (c) 5 8 4 A  6.0  F i g u r e 16.  I  5.0  .  L_  4.0  E L E C T R O N ENERGY (eV)  E l e c t r o n s p e c t r a f o r i o n i z a t i o n o f m o l e c u l a r hydrogen.  -74-  5.0  4.0  3D  2.0  E L E C T R O N E N E R G Y (eV) F i g u r e 17.  E l e c t r o n s p e c t r a f o r i o n i z a t i o n o f deuterium hydride.  -75-  2 S 3  v=o ! I M  l  6.0  1  I  5.0  :  i  i  I I I I M  ,  4.0  L _  3.0  E L E C T R O N E N E R G Y (eV) F i g u r e 18.  Electron spectra f o r i o n i z a t i o n of molecular  deuterium.  TABLE  IV  S h i f t s , AE, i n Penning E l e c t r o n E n e r g i e s f o r Hydrogen, Deuterium H y d r i d e and Deuterium (eV) •z 2 S I o n i z a t i o n , AE Species  + 2  15.427  +0.060±0.015  Literature  +0.070  (35)  T h i s work  D  +  + 2  Literature  +0.090±0.020  +0.09010.010(34)  HD  s  I.P.(149) T h i s work  H  1 2 S Ionization, AE  ^ I  15.46  +0.060±0.020  -  +0.070±0.025  15.46  +0.05010.015  -  +0.070+0.020  -77-  d i s t i n c t l y b r o a d e r than the c o r r e s p o n d i n g  photoelectron  distributions,  the v i b r a t i o n a l s t r u c t u r e i n the Penning s p e c t r a i s c l e a r l y  visible.  * 1 * 3 However, due t o t h e o v e r l a p o f t h e He (2 S) and He (2 S) s p e c t r a and t h e r i s i n g background, t h e d e t e r m i n a t i o n  o f the v i b r a t i o n a l  i s l e s s p r e c i s e than t h a t f o r the p h o t o e l e c t r o n spectrum.  spacing  The v i b r a t -  i o n a l s p a c i n g f o r the Penning e l e c t r o n spectrum o f H^ i s shown i n d e t a i l i n F i g u r e 19.^ In T a b l e s V, VI and V I I the v i b r a t i o n a l s p a c i n g s measured f o r Penning and p h o t o i o n i z a t i o n l e a d i n g t o the X^E* s t a t e s o f are compared w i t h c a l c u l a t e d s p a c i n g s workers (35, 8 3 ) .  , HD  +  and  (79) and v a l u e s r e p o r t e d by o t h e r  The v i b r a t i o n a l s p a c i n g s  a r e found t o be independent  o f the means o f i o n i z a t i o n , w i t h i n e x p e r i m e n t a l  error, indicating that  t h e r e was no e f f e c t i v e complex f o r m a t i o n o r n u c l e a r p e r t u r b a t i o n o f t h e m o l e c u l e d u r i n g the Penning i o n i z a t i o n  process.  R e l a t i v e t r a n s i t i o n p r o b a b i l i t i e s c o u l d not be measured due t o the o v e r l a p o f the s i n g l e t and t r i p l e t s p e c t r a and the r i s i n g background o f i• the Penning spectrum.  4.2.2. M o l e c u l a r  Nitrogen.  F i g u r e 20 i l l u s t r a t e s the e l e c t r o n s p e c t r a o b t a i n e d f o r  .  The  p h o t o e l e c t r o n spectrum e x h i b i t s t h e w e l l known (35, 66, 83, 149) i o n i c 2 + 2 1 + s t a t e s o f n i t r o g e n (X E . A n and B E ) a c c e s s i b l e u s i n g He(584 A) ^ g ' u u 0  6  radiation.  The e l e c t r o n i c o r b i t a l c o n f i g u r a t i o n f o r the ground s t a t e  + T h i s spectrum was o b t a i n e d i n the c l o s i n g phase o f t h i s work where a c o n s i d e r a b l e improvement i n m e t a s t a b l e i n t e n s i t y was a c h i e v e d by m o d i f i c a t i o n s t o the e l e c t r o n gun c o n d i t i o n s .  F i g u r e 19.  Penning e l e c t r o n spectrum o f m o l e c u l a r hydrogen.  TABLE V i b r a t i o n a l Spacings  (meV) f o r  V  (X^Z ) . +  o  Vibrational  Calc.  Interval  (79)  *  P h o t o i o n i z a t i o n , He(584 A)  Penning I o n i z a t i o n , He 2 S  2S  3  l  Ref.83  Ref.35  T h i s work  Ref.35  T h i s work  Ref.35  T h i s work  0- 1  269  270±10  272± 3  269+ 8  272115  275110  268110  1- 2  254  250±10  258± 5  2521 8  249115  255+10  255110  2- 3  239  240±10  240± 5  2431 8  240110  239110  3- 4  223  230±10  228± 8  2251 8  230115  225H0  4- 5  208  210±10  215±10  212+ 8  215+15  213115  5- 6  193  200±10  198110  199+ 8  200115  6- 7  177  190110  185110  1821 8  174120  7- 8  162  170±10  170110  1711 8  8- 9  146  160±10  155110  156110  9- 10  135  143110  10- 11  116  118110  11- 12  100  113110  12- 13  85  100+15  13- 14  70  92115  -80-  TABLE  VI  V i b r a t i o n a l S p a c i n g s (meV) f o r H D ( X ^ I ) . +  Vibrational Interval  Calc. (79)  +  Photoionization o  He(584 A) Ref.83  T h i s work  Penning He 2S 1  Ionization T h i s work 3 2°S  0- 1  235  230±10  2351 8  235120  233115  1- 2  224  230±10  2191 8  229120  225110  2- 3  212  220±10  214+ 8  210110  3- 4  201  200±10  197+ 8  201110  4- 5  189  200±10  1941 8  196+15  5- 6  178  190110  1821 8  178115  6- 7  166  170110  169110  161120  7- 8  154  160+10  160+10  8- 9  143  150110  151110  9- 10  131  141+10  10- 11  120  129115  11- 12  108  114115  12- 13  97  107120  13- 14  85  90+20  -81-  TABLE V I I  V i b r a t i o n a l S p a c i n g s (meV) f o r D„ (X^E ) +  Vibrational Interval  Calc. (79)  Photoionization o  Penning I o n i z a t i o n *  He(584 A) Ref.83  a  +  He , T h i s work  T h i s work  2*S  2 S  a  a  3  0- 1  194  200±10  193± 8  1- 2  186  190±10  188± 8  192120  190110  2- 3  178  180±10  180± 8  176120  176+10  3- 4  171  170±10  171± 8  173110  4- 5  163  170±10  168± 8  162110  5- 6  155  160±10  153± 8  155115  6- 7  148  150±10  151± 8  153115  7- 8  140  150±10  145± 8  8- 9  132  140±10  130± 8  9-10  125  130110  123± 8  10-11  117  119±10  11-12  109  116±10  12-13  102  107±10  13 14  94  100±10  14-15  86  88110  15-16  84115  16-17  81115  17-18  82+15  Unable t o measure due t o o v e r l a p .  -82-  ,  1  5D  i  I  4.0  ,  I  3D  I  I  1  2.0  E L E C T R O N ENERGY (eV) F i g u r e 20.  Electron spectra f o r i o n i z a t i o n of molecular nitrogen.  L_  I.O  -83-  o f t h e n i t r o g e n m o l e c u l e can be w r i t t e n as ( 6 6 ) : N  2  KK(a 2s)  (a^s)  2  g  2  where t h e terms KK r e p r e s e n t  (TT^P)  O 2p)  4  ,  2  g  V  the closed inner s h e l l s .  The shape o f t h e  f i r s t band ( X Z ) i s c h a r a c t e r i s t i c o f removal o f a n e a r l y nonbonding 2  +  2 electron (a 2p).  The shape o f t h e second band (A 11^) and t h e l e n g t h  g  of v i b r a t i o n a l s e r i e s c o n f i r m s t h e removal o f a s t r o n g l y b o n d i n g electron ( ^ p ) •  T  n  e  t h i r d band ( B E ) i s s i m i l a r i n shape t o t h a t 2  +  u  o f t h e f i r s t band and i s c o n s i s t e n t w i t h t h e l o s s o f a nonbonding e l e c t r o n ( a ^ s ) . The energy s c a l e s were c a l i b r a t e d u s i n g t h e i o n i z a t i o n p o t e n t i a l v a l u e s g i v e n i n T a b l e V I I I . The Penning spectrum shows t h o s e * 1 * 3 s t a t e s o f m o l e c u l a r n i t r o g e n a c c e s s i b l e u s i n g He (2 S) and He (2 S) m e t a s t a b l e atoms.  F i g u r e 20 a l s o i l l u s t r a t e s a phenomenon o b s e r v e d f o r  many o f t h e m o l e c u l e s i n t h i s s t u d y , t h a t i s , t h e o b s e r v e d r e l a t i v e population  o f e l e c t r o n i c s t a t e s i s d i f f e r e n t i n t h e Penning e l e c t r o n  spectrum when compared t o t h e p h o t o e l e c t r o n the t r a n s m i s s i o n 2 the X E  spectrum.  c o r r e c t i o n f a c t o r shows (Table  + 2 s t a t e and A II  state  Application of  IX) t h a t t h e r a t i o o f  3 (X/A) i s g r e a t e r i n t h e Penning (2 S)  spectrum than i n t h e p h o t o e l e c t r o n  spectrum.  A very large d i f f e r e n c e  a l s o o c c u r s f o r t h e X/B r a t i o f o r Penning (2^S) and p h o t o i o n i z a t i o n . I t may be t h a t t h e d i f f e r e n c e s o b s e r v e d i n t h e r e l a t i v e p o p u l a t i o n s  of  the e l e c t r o n i c s t a t e s a r e due t o d i f f e r e n c e s i n a n g u l a r d i s t r i b u t i o n s for  s i n g l e t and t r i p l e t i o n i z a t i o n .  As was p r e v i o u s l y s t a t e d , Hotop  and Niehaus (102) have found t h e a n g u l a r d i s t r i b u t i o n o f Penning e l e c t r o n s f o r argon t o be asymmetric and d i f f e r e n t f o r s i n g l e t and t r i p l e t i o n i z a t i o n and t h i s may a l s o be o c c u r r i n g  for nitrogen.  et a l . (91) have found s l i g h t d i f f e r e n c e s i n t h e p h o t o e l e c t r o n  McGowan angular  TABLE S h i f t s , AE, i n Penning E l e c t r o n E n e r g i e s  VIII  f o r Nitrogen (eV).  +  2 State N  I.P. •(149)  3 2 S Ionization, T h i s work  AE  2"*S I o n i z a t i o n , AE  t  Ref.35  T h i s work  x ig  15..576  +0.047±0.010  +0..05010.010  An  16,.693  +0.04910.015  +0,.05010.010  2  +  2  u BE 2  +  u  18,.745  -  +0.02210.010  s Ref.35 +0.01510.010  -  +0..05310.010  +0.00210.015  +0.01010.010  TABLE IX Relative Populations of E l e c t r o n i c States N  ( a t v = 0) f o r N i t r o g e n .  9  Photoionization  State  He(584 A)  +  X Z 2  +  g  A nu 2  B£ 2  +  u  Penning I o n i z a t i o n  2 S 1  2 S 3  100  100  100  38  -  19  18  80  (Corrected f o r Transmission)  -85-  d i s t r i b u t i o n s o b s e r v e d f o r t h e X^Z  +  and A^n  g  states of molecular u  n i t r o g e n , t h a t f o r t h e A s t a t e b e i n g more i s o t r o p i c .  Berkowitz et a l .  (88), on t h e o t h e r hand, had p r e v i o u s l y s t u d i e d t h e a n g u l a r d i s t r i b u t i o n of photoelectrons  from m o l e c u l a r n i t r o g e n and carbon monoxide and had  c o n c l u d e d t h a t no c o r r e c t i o n f a c t o r was n e c e s s a r y .  I f the angular  d i s t r i b u t i o n o f e l e c t r o n s from t h e d i f f e r e n t i o n i c s t a t e s i s d i f f e r e n t f o r t h e Penning and p h o t o e l e c t r o n  processes then a f u r t h e r c o r r e c t i o n  f a c t o r would have t o be a p p l i e d t o t h e s p e c t r a .  Further  d i s t r i b u t i o n s t u d i e s w i l l perhaps a s s i s t i n d e t e r m i n i n g the observed d i f f e r e n c e s i n e l e c t r o n i c s t a t e The i n the  angular the reason f o r  population.  measured e l e c t r o n energy s h i f t s o f t h e Penning e l e c t r o n peaks spectrum a r e compared w i t h v a l u e s  i n Table V I I I .  r e p o r t e d e l s e w h e r e (35)  The r e s u l t s a r e i n good agreement w i t h t h e v a l u e s  r e p o r t e d bv Hotop and Niehaus (35) and as i n t h e case o f hydrogen t h e peaks a r e s h i f t e d t o h i g h e r  e l e c t r o n energies  i n d i c a t i n g t h a t some o f  the k i n e t i c energy o f t h e c o l l i d i n g p a r t i c l e s has been c o n v e r t e d i n t o energy o f the e j e c t e d e l e c t r o n . The  Penning e l e c t r o n peaks a r e d i s t i n c t l y b r o a d e r than those  observed f o r t h e c o r r e s p o n d i n g p h o t o e l e c t r o n  distribution.  However, t h e  v i b r a t i o n a l s t r u c t u r e i n t h e Penning s p e c t r a i s c l e a r l y v i s i b l e b u t as before  the determination  o f v i b r a t i o n a l s p a c i n g s i s l e s s p r e c i s e than  that f o r photoelectrons.  I t was a l s o o b s e r v e d , f o r t h e X s t a t e , t h a t  the peaks due t o s i n g l e t i o n i z a t i o n are b r o a d e r than those due t o t r i p l e t i o n i z a t i o n . T a b l e X compares t h e v i b r a t i o n a l s p a c i n g s found f o r Penning and p h o t o i o n i z a t i o n f o r t h e X E , A ^ i ^ and B^Z^ 2  of N  ?  with values  +  o b t a i n e d by o t h e r w o r k e r s .  +  states  The agreement i s good  TABLE V i b r a t i o n a l Spacings Electronic State and I n t e r v a l  (meV) f o r E l e c t r o n i c S t a t e s o f N Spec,  (150)  Photoionization o  He (584 A) Ref.35  X E 2  +  A !! 2  BE 2  +  u  X  0-1  270  270± 5  1-2  266  260±10  0-1  232  230± 5  1-2  229  2-3  T h i s work 269± 8  Penning I o n i z a t i o n , He 3  2 S  2S 1  Ref.35  T h i s work  T h i s work  275±10  270±10  235± 6  230±10  232±10  230± 5  228± 6  225±10  217±15  225  228± 5  227± 6  225±15  235120  3-4  221  228± 8  220± 8  215±15  4-5  217  220±10  220±10  200±20  5-6  214  215±10  205±15  0-1  294  300±10  290± 5  1-2  290± 5  2-3  290±10  270±20  Ref.35  290±15  -  -87-  and t h e v i b r a t i o n a l s p a c i n g s a r e found t o be independent o f t h e means of i o n i z a t i o n , w i t h i n experimental  error.  4.2.3. Carbon Monoxide The  e l e c t r o n spectra obtained  F i g u r e 21.  f o r carbon monoxide a r e shown i n  The f e a t u r e s o f t h e s p e c t r a o f carbon monoxide a r e v e r y  s i m i l a r t o those o f t h e i s o e l e c t r o n i c m o l e c u l a r n i t r o g e n .  Because  s i m i l a r o r b i t a l s a r e a v a i l a b l e f o r i o n i z a t i o n , r e s u l t i n g i n t h e same i o n i c s t a t e s (X^£ , A^n and B^E*) t h e same d e s c r i p t i o n o f t h e m o l e c u l a r +  o r b i t a l s and band shapes used f o r  may be a p p l i e d t o C 0 . +  The  energy s c a l e was c a l i b r a t e d u s i n g t h e i o n i z a t i o n p o t e n t i a l d a t a i n Table XI.  given  Due t o t h e r a p i d l y r i s i n g background o n l y t h e X and A  s t a t e s were o b s e r v e d i n t h e Penning e l e c t r o n spectrum. A comparison o f F i g u r e s  20 and 21 showed t h a t t h e r e a r e b o t h  s i m i l a r i t i e s and d i f f e r e n c e s i n t h e r e l a t i v e p o p u l a t i o n s  o f the e l e c t -  r o n i c s t a t e s f o r carbon d i o x i d e and m o l e c u l a r n i t r o g e n .  As was o b s e r v e d  f o r nitrogen, a f t e r a p p l i c a t i o n o f the v a r i a b l e transmission  correction  f a c t o r , t h e r a t i o o f t h e X s t a t e and A s t a t e (X/A) i s g r e a t e r X I I ) i n the Penning e l e c t r o n spectrum than i n t h e p h o t o e l e c t r o n  (see T a b l e spectrum.  A g a i n d i f f e r e n c e s i n a n g u l a r d i s t r i b u t i o n s f o r e l e c t r o n s e j e c t e d from the d i f f e r e n t i o n i c s t a t e s by t h e two p r o c e s s e s may c o n t r i b u t e t o t h e observed d i f f e r e n c e .  The r a t i o o f t r i p l e t t o s i n g l e t peak i n t e n s i t y  f o r the X s t a t e i s considerably molecular nitrogen.  g r e a t e r f o r carbon monoxide than f o r  The d i f f e r e n c e i n r a t i o may be due t o d i f f e r e n c e s  i n a n g u l a r d i s t r i b u t i o n s n o t o n l y f o r i o n i z a t i o n by s i n g l e t and t r i p l e t atoms b u t a l s o f o r d i f f e r e n t m o l e c u l e s o f t h e same s t a t e .  -88-  —I  7.0  •——I  6.0  .  1  5.0  i  I  4.0  ,  i  3.0  .  i  2.0  E L E C T R O N ENERGY (eV) F i g u r e 21.  E l e c t r o n s p e c t r a f o r i o n i z a t i o n o f carbon monoxide.  TABLE  XI  S h i f t s , AE, i n Penning E l e c t r o n E n e r g i e s f o r Carbon Monoxide ( e V ) .  C(T State  2 S I o n i z a t i o n , AEt  I.P.(149)  Ref.35  T h i s work  xV An 2  BV  14..013 16,,537 19.,671  2 S I o n i z a t i o n , AE T h i s work  +0,,052±0,,010 +0,,040±0,,020  +0,.04510,.010 +0,.03510,,015  _  _  -0.00810.010 -  Ref.35  -0.02010.010 +0.04010.020  TABLE X I I R e l a t i v e P o p u l a t i o n s o f E l e c t r o n i c S t a t e s ( a t v = 0) f o r Carbon Monoxide (Corrected f o r transmission) CO State  xV An 2  BV  Photoionization o He  (584 A) 100 14 17  Penning  Ionization  2S  23 S  100 11  100 7  l  -90-  As was observed t r i p l e t metastable  f o r o t h e r m o l e c u l e s , peaks due t o i o n i z a t i o n by  atoms a r e n a r r o w e r than those due t o s i n g l e t  ionization  The d i f f e r e n c e s i n peak shapes may be a t t r i b u t e d t o d i f f e r e n c e s i n t r a n s i t i o n p r o b a b i l i t i e s and i n t h e p o t e n t i a l energy i n t e r a c t i o n  curves.  T a b l e XI compares the measured e l e c t r o n energy s h i f t s o f t h e Penning e l e c t r o n peaks o f carbon monoxide w i t h v a l u e s r e p o r t e d by Hotop and Niehaus ( 3 5 ) . The r e s u l t s a r e a g a i n i n good agreement.  In contrast  t o p r e v i o u s atoms and m o l e c u l e s , t h e energy s h i f t observed  f o r singlet  i o n i z a t i o n o f t h e X s t a t e i s t o lower e l e c t r o n energy, i n d i c a t i n g t h a t some o f t h e energy t h a t t h e e j e c t e d e l e c t r o n n o r m a l l y c a r r i e s away has been r e t a i n e d by t h e c o l l i d i n g p a r t i c l e s o r l o s t i n some o t h e r manner. Other peaks i n t h e Penning  spectrum a r e s h i f t e d t o h i g h e r e l e c t r o n  e n e r g i e s i n d i c a t i n g t h a t some o f t h e k i n e t i c energy o f t h e c o l l i d i n g p a r t i c l e s has been c o n v e r t e d i n t o energy o f t h e e j e c t e d e l e c t r o n . Although the o r b i t a l s a v a i l a b l e f o r i o n i z a t i o n , t h e i r r e l a t i v e  energies  and t h e i o n i c s t a t e s o b t a i n e d by removal o f an e l e c t r o n a r e v e r y f o r the i s o e l e c t r o n i c molecules a t i o n o f t h e Penning  carbon monoxide and n i t r o g e n , no c o r r e l -  e l e c t r o n energy s h i f t s can be made.  T a b l e X I I I g i v e s t h e v i b r a t i o n a l s p a c i n g s observed 2 and p h o t o e l e c t r o n s f o r the X £  +  f o r Penning  2  and A n s t a t e s o f carbon monoxide and  compares t h e v a l u e s o b t a i n e d w i t h those r e p o r t e d e l s e w h e r e . observed  similar  As has been  f o r o t h e r m o l e c u l e s , t h e r e s u l t s a r e i n good agreement and  w i t h i n experimental e r r o r i n d i c a t e that the v i b r a t i o n a l spacings are independent o f t h e method o f i o n i z a t i o n .  TABLE V i b r a t i o n a l Spacings Electronic and  State  Interval  (meV) f o r E l e c t r o n i c S t a t e s o f CO  Spec. (151)  Photoionization o  He(584 A) Ref.35  xV An 2  XIII  0-1  271  275± 5  1-2  267  270110  0-1  190  1-2  T h i s work  Penning I o n i z a t i o n , He 3  2S  2°S  l  Ref.35  T h i s work  Ref.35  T h i s work  271+12  260140  270110  270110  190± 5  1961 9  180120  185+10  185112  187  188± 5  1851 9  180120  195110  185112  2-3  184  182± 5  1791 9  180120  195110  177112  3-4  180  180± 5  1711 9  185110  171115  4-5  177  1801 8  1741 9  185115  176115  5-6  173  170110  168+ 9  185+15  6-7  170  170110  168112  ID  l—»  I  -92-  4.2.4. N i t r i c O x i d e . The e l e c t r o n s p e c t r a o b t a i n e d f o r n i t r i c o x i d e a r e shown i n F i g u r e 22. be w r i t t e n NO  The ground 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 f o r n i t r i c o x i d e may (66): KK(a 2s)  -  .  v  ( a ^ s ^ (a 2p)  Z  g  "O^p)'  g  The He(584 A) spectrum shows the X X , 1  and A^n s t a t e s o f N0 . +  4  (Tr 2p)  a Z , b n  +  3  ^TI  g  +  3  (g s e r i e s ) ,  3  A  N i t r i c oxide possesses a s i n g l e unpaired e l e c t r o n  i n i t s ground m o l e c u l a r s t a t e and removal o f t h i s e l e c t r o n , from an antibonding o r b i t a l  (IT 2p) l e a d s t o the f o r m a t i o n o f the X^Z  ion state.  +  The remainder o f the spectrum i s q u i t e complex due t o t h e o v e r l a p o f s e v e r a l bands.  There are two sharp bands, w i t h l i t t l e f i n e  structure  3 1 (b n(8 s e r i e s ) and A n) c o r r e s p o n d i n g t o the removal o f n e a r l y electrons  (a 2 p ) .  The o t h e r bands ( a Z 3  +  nonbonding  and A ) , which e x h i b i t l o n g 3  .  v i b r a t i o n a l s e r i e s , l i k e l y c o r r e s p o n d t o the removal o f b o n d i n g e l e c t r o n s (fi' 2p) . u  The energy s c a l e was c a l i b r a t e d u s i n g i o n i z a t i o n p o t e n t i a l d a t a  g i v e n i n T a b l e XIV.  The Penning spectrum shows the X^Z , +  b II (8 s e r i e s ) 3  and A^TI s t a t e s . A comparison o f the r e l a t i v e p o p u l a t i o n s o f the e l e c t r o n i c  states  i s d i f f i c u l t because o f the r i s i n g background at low e l e c t r o n e n e r g i e s i n the Penning e l e c t r o n spectrum.  However, i t i s apparent t h a t the  X^E  i s o f much reduced i n t e n s i t y compared t o the 8 s e r i e s and A^n s t a t e i n the Penning spectrum compared t o t h a t o b s e r v e d i n the p h o t o e l e c t r o n spectrum. The measured e l e c t r o n energy s h i f t s f o r n i t r i c o x i d e are g i v e n i n Table XIV.  T h i s i s one o f the few m o l e c u l e s s t u d i e d where t h e e l e c t r o n  energy s h i f t i s o b s e r v e d t o be n e g a t i v e f o r b o t h s i n g l e t and  triplet  +  E L E C T R O N ENERGY (eV) F i g u r e 22.  Electron spectra f o r ionization of n i t r i c  oxide.  TABLE  XIV  S h i f t s , AE, i n Penning E l e c t r o n E n e r g i e s f o r N i t r i c Oxide ( e V ) .  2 S I o n i z a t i o n , AE  NO State  2 S I o n i z a t i o n , AE  I.P. (152) T h i s work  Ref.136  T h i s work  -0.030±0.010  -O.OlOiO.OlO  -0.020+0.015  0±0.010  +0.035±0.010  -0.005±0.010  0±0.010  xV  9.262  a V  15.667  b 11(8 series)  16.561  +0.030±0.015  Ah  18.325  0±0.025  Ref.136  to i  -95-  i o n i z a t i o n to a p a r t i c u l a r  state.  D u r i n g the l a t t e r stages o f t h i s work a s i g n i f i c a n t i n c r e a s e i n the i n t e n s i t y o f s p e c t r a was source.  a c h i e v e d due t o improvements i n the  metastable  With t h i s improvement a d e t a i l e d comparison o f the Penning  e l e c t r o n and p h o t o e l e c t r o n s p e c t r a f o r the X^E became p o s s i b l e (153) .  The  case o f N0  +  state of n i t r i c  oxide  (X"^E ) i s p a r t i c u l a r l y f a v o u r a b l e  +  +  f o r such a comparison because the low i o n i z a t i o n p o t e n t i a l p l a c e s  the  Penning e l e c t r o n spectrum i n an energy range f a r removed from the s t e e p l y r i s i n g background.  F i g u r e 23 i l l u s t r a t e s the p h o t o e l e c t r o n  and Penning e l e c t r o n s p e c t r a o f the X^E XV shows the observed  +  s t a t e o f n i t r i c o x i d e and  v i b r a t i o n a l s p a c i n g s i n comparison w i t h v a l u e s  r e p o r t e d by o t h e r workers.  Within experimental  e r r o r the  vibrational  s p a c i n g s are independent o f the means o f i o n i z a t i o n , as has been f o r other molecules  Table  observed  and t h i s i n d i c a t e s t h a t t h e r e i s no e f f e c t i v e complex  f o r m a t i o n o r n u c l e a r p e r t u r b a t i o n o f the m o l e c u l e  d u r i n g the  ionization  process. In the Penning i o n i z a t i o n spectrum a s m a l l c o n t r i b u t i o n i s  observed  o  from the 584 A r a d i a t i o n produced i n the m e t a s t a b l e s u b t r a c t i o n o f t h i s c o n t r i b u t i o n and a l s o t h a t due  source. to s i n g l e t  By  successive  ionization  (on the b a s i s o f the known p h o t o i o n i z a t i o n r e l a t i v e i n t e n s i t i e s ) a c o r r e c t e d spectrum f o r i o n i z a t i o n o f n i t r i c o x i d e by t r i p l e t atoms has been o b t a i n e d . abilities  The  r e l a t i v e v i b r a t i o n a l t r a n s i t i o n prob-  ( n o r m a l i z e d at v = 1) thus o b t a i n e d , t o g e t h e r w i t h  o b t a i n e d by o t h e r methods are g i v e n i n T a b l e XVI.  The  3  i o n i z a t i o n t o the X^E  +  those  relative vibrat-  i o n a l t r a n s i t i o n p r o b a b i l i t i e s , f o r the p h o t o e l e c t r o n and e l e c t r o n (2 S)  metastable  Penning  s t a t e o f n i t r i c o x i d e are p l o t t e d  -96-  N 0 + M 5 8 4 A ) — NO +e +  NO (X S ) +  1  NO+He*(2 S)3  1  v=o  NO+He*(2 S) 1  v=0  12.0  2  3  T  +  NO +He+e +  4  5  6  •NO + He+e 2  II.O  3  lO.O  9.0  E L E C T R O N E N E R G Y (eV) F i g u r e 23.  i + Penning e l e c t r o n and p h o t o e l e c t r o n s p e c t r a o f t h e X E state of n i t r i c oxide.  TABLE V i b r a t i o n a l Spacings  XV  (meV) f o r E l e c t r o n i c S t a t e s o f N 0 . +  *  E l e c t r o n i c State and I n t e r v a l  Calc. (151)  Photoionization He(584 A) Ref.136 T h i s work  Penning I o n i z a t i o n , He  2 S  2 S 3  1  Ref.136 T h i s work  Ref.136 T h i s work  0-1  290  290± 3  294± 5  290± 5  290± 8  288±10  293± 5  1-2  287  290± 5  294± 5  290± 5  288± 8  287± 5  285± 5  2-3  283  285 + 5  279± 5  285±10  283±10  280± 5  3-4  278  280± 5  277± 5  278±10  268± 5  TABLE  XVI  R e l a t i v e V i b r a t i o n a l T r a n s i t i o n P r o b a b i l i t i e s f o r N i t r i c Oxide. Electronic State and V i b r a t i o n a l level of N0  xV  Calc. (151)  +  v=0  Photoionization o  He (5.8.4 A) T h i s work Ref.154  47. 8  49± 1  50± 2  Penning 3 He ( 2 S ) *  J  T h i s work Ref.136 50± 1  50± 6  Ionization * 2 A r C? )  Ne*( P ) 3  0  Ref.136  Ref.21  Ref.136  48± 2  40  50± 3  96± 3  93  91± 3  v=l  100. 0  v=2  91. 7  94± 1  90± 3  95± 1  96± 4  v=3  48. 4  51± 1  48± 4  55± 1  62± 6  65± 6  62  84± 5  v=4  16. 3  20± 1  17± 3  20± 1  33± 6  44± 8  29  67± 6  v=5  3. 6  5± 2  4± 2  15± 6  25±10  11  39± 8  _  27±10  v=6  0. 6  100  100  -  100  -  100  _  _  100  100  _  100 I OO I  -99-  i n F i g u r e 24 which shows t h a t a v e r y c l o s e correspondence e x i s t s f o r the v i b r a t i o n a l t r a n s i t i o n p r o b a b i l i t i e s up t o v = 5 f o r b o t h the p h o t o e l e c t r o n and the Penning p r o c e s s .  Hotop has o b s e r v e d (31) a  s i g n i f i c a n t d e v i a t i o n above v = 2 ( a t v = 4 the p r o b a b i l i t y f o r Penning i o n i z a t i o n i s shown as a p p r o x i m a t e l y double t h a t f o r p h o t o i o n i z a t i o n ) . The d i f f e r e n c e s observed may be due t o the d i f f i c u l t y i n a c c u r a t e l y measuring the s m a l l s t e p h e i g h t s , a t h i g h v i b r a t i o n a l quantum numbers i n t h e r e t a r d i n g p o t e n t i a l c u r v e s o f Hotop ( 3 1 ) .  The d i r e c t l y o b t a i n e d  d i f f e r e n t i a l s i g n a l o b t a i n e d i n t h i s work i s more amenable t o i n t e r pretation.  The Penning i o n i z a t i o n o f n i t r i c o x i d e t o the X^E  +  state  was observed t o be a v e r t i c a l p r o c e s s , w i t h i n e x p e r i m e n t a l e r r o r . F i g u r e 23 a l s o i l l u s t r a t e s t h a t , as f o r o t h e r m o l e c u l e s , the peaks due t o s i n g l e t i o n i z a t i o n a r e b r o a d e r than t h o s e due t o t r i p l e t  ionization.  4.2.5. M o l e c u l a r Oxygen. The e l e c t r o n s p e c t r a o b t a i n e d f o r the i o n i z a t i o n o f 0^ a r e shown 2 4 2TT i n F i g u r e 25. The p h o t o e l e c t r o n spectrum shows the X II , a n A 4 ° and b E i o n i c s t a t e s o f oxygen a c c e s s i b l e u s i n g He(584 A) r a d i a t i o n The ground s t a t e o f t h e oxygen m o l e c u l e has t h e e l e c t r o n i c  configuration  (66): 0  2  KK(a 2s) g  2  (a 2s) u  2  (° 2p) g  2  (\2.p)  4  (Tf 2p) g  2  ,  3  E " g  2 The shape o f the f i r s t band (X II ) i s c o n s i s t e n t w i t h removal o f an e l e c t r o n from the o u t e r a n t i b o n d i n g o r b i t a l  (TT 2 p) . The second band  ( a n ) c o n s i s t s o f a long v i b r a t i o n a l s e r i e s r e s u l t i n g from the removal 2 4  o f a bonding e l e c t r o n ( i r ^ p ) .  The t h i r d band (A 11^), which o v e r l a p s t h e  second, a l s o a r i s e s from the removal o f a b o n d i n g e l e c t r o n ( i v 2 p ) •  The  (100)  0  i  0  1  2  3  4  5  VIBRATIONAL QUANTUM NUMBER F i g u r e 24.  Relative vibrational transition probabilities for Penning i o n i z a t i o n and p h o t o i o n i z a t i o n o f NO t o NO  +  ^ (X E ) . +  -1011.57  -i  I  8.0  .  I  6.0  .  I  .  4.0  ELECTRON ENERGY (eV) F i g u r e 25.  E l e c t r o n s p e c t r a f o r i o n i z a t i o n o f m o l e c u l a r oxygen.  I  ,  2.0  -102-  shape o f t h e f o u r t h band (b^T. ~) i s c o n s i s t e n t w i t h removal o f an e l e c t r o n w i t h some bonding c h a r a c t e r  (a ;jp)•  The energy s c a l e was  c a l i b r a t e d using i o n i z a t i o n p o t e n t i a l values published  by E d q u i s t  et a l .  (155). S i n c e most o f t h e s t r u c t u r e o b s e r v e d i n t h e Penning e l e c t r o n spectrum l i e s on t h e v e r y s t e e p p o r t i o n o f t h e r i s i n g Auger background i t was n o t p o s s i b l e t o make q u a n t i t a t i v e d e t e r m i n a t i o n s o f t h e r e l a t i v e populations  o f the e l e c t r o n i c s t a t e s .  However, i t i s apparent t h a t  r e l a t i v e band i n t e n s i t i e s a r e r a t h e r d i f f e r e n t f o r Penning i o n i z a t i o n and p h o t o i o n i z a t i o n .  I n p a r t i c u l a r t h e r e appears t o be a low r e l a t i v e  p r o b a b i l i t y f o r Penning i o n i z a t i o n t o t h e 0^  +  ^^ g state. n  In t h e Penning i o n i z a t i o n spectrum, t h e b r o a d band a t about 3 eV 4 can be a s s i g n e d t o t h e a n  u  * 3 s t a t e produced by He (2 S ) . The maximum  o f t h i s band c o r r e s p o n d s t o v = 6 and s i n c e t h i s i s t h e most p r o b a b l e transition  (155) i n p h o t o i o n i z a t i o n  i t i n d i c a t e s t h a t Penning i o n i z a t i o n  o f t h i s s t a t e o f oxygen i s e s s e n t i a l l y a v e r t i c a l p r o c e s s at 2.76 eV c o r r e s p o n d s i n energy t o p r o d u c t i o n by t r i p l e t m e t a s t a b l e atoms. (155)  o f the A n  High r e s o l u t i o n photoelectron  2 shows v = 7 o f t h e A n *  The peak s t a t e (v = 0) spectroscopy  s t a t e t o be t h e most p r o b a b l e t r a n s i t i o n . 3  T h e r e f o r e i t appears t h a t He (2 S) i o n i z a t i o n o f m o l e c u l a r oxygen t o 2 the A n  u  i o n i c state i s a n o n - v e r t i c a l process.  The r e s u l t s o f a f t e r -  glow s t u d i e s  (79, 156) o f oxygen s u p p o r t t h i s o b s e r v a t i o n .  The d i f f e r e n c e 2  i n energy K  0.7 eV) between t h e peak a t 2.76 eV and v = 7 o f A n  would seem t o be much t o o l a r g e t o be a t t r i b u t e d t o a d i f f e r e n c e i n t r a n s l a t i o n a l energy (AE). The peak a t 2.42 eV may c o r r e s p o n d t o * 1 4 He (2 S) i o n i z a t i o n o f 0 t o t h e b z„ (v = 0) s t a t e . ?  -103-  No c e r t a i n assignment can be made f o r the peaks at 1.97, and 1.57  eV.  The peak at 1.97  2 p r o c e s s i n which the A II  eV most c l o s e l y c o r r e s p o n d s t o the  (v = 8) i o n i c s t a t e i s formed by He  4 or a l t e r n a t i v e l y t o the b E  1.76  (v = 4) s t a t e produced by He  *  *  1 (2 S)  3 (2 S ) . 4  expected energy  (1.65 eV) f o r t r i p l e t  The -  s t a t e i o n i z a t i o n t o the b E  (v = 0) i o n i c s t a t e l i e s between peaks at 1.76  and 1.57  eV and  may  c o r r e s p o n d t o the s m a l l p a r t i a l l y r e s o l v e d peak o b s e r v a b l e i n the lower t r a c e o f F i g u r e  25(b).  Cermak and Sramek (157) have a l s o o b s e r v e d  sharp peaks i n the Penning e l e c t r o n s p e c t r a o f 0  i n the energy range  2  0 - 1.8 eV and have a t t r i b u t e d t h e i r o r i g i n t o d i s s o c i a t i v e e x c i t a t i o n i n t o h i g h l y e x c i t e d r e p u l s i v e s t a t e s and e x c i t a t i o n i n t o h i g h l y  excited  p r e d i s s o c i a t i n g bound s t a t e s o f oxygen m o l e c u l e s .  4.3 T r i a t o m i c M o l e c u l e s •  4.3.1. Carbon  Dioxide.  The e l e c t r o n s p e c t r a o b t a i n e d F i g u r e 26.  The p h o t o e l e c t r o n  f o r carbon d i o x i d e are shown i n  spectrum shows the w e l l known (66)  2 2 2 + 2 + i o n i c s t a t e s , X II , A II , B E and C E . g u' u g  The e l e c t r o n i c o r b i t a l  c o n f i g u r a t i o n f o r carbon d i o x i d e may be w r i t t e n as ( 6 6 ) : C0  2  KKK(a ) g  2  ( a j  2  (a ) g  2  ( a j  2  (  %  )  4  (, )  4  g  ,  V  2 The shape o f the f i r s t band (X II ) i s c h a r a c t e r i s t i c o f t h e removal o f a nonbonding  e l e c t r o n (u ) where t h e r e  i s l i t t l e change i n the i n t e r 2  nuclear  separation  on i o n i z a t i o n .  The second band (A I I ) has a shape  which i n d i c a t e s the removal o f a bonding e l e c t r o n (ir ) . second and t h i r d bands p r e v e n t e d the d e t e r m i n a t i o n  O v e r l a p o f the  o f more than f o u r  -104-  2 S(B) 3  1  )  6.0  1  5.0  1  4.0  .  I  3.0  ELECTRON ENERGY (eV)  F i g u r e 26.  L_  2.0  E l e c t r o n s p e c t r a f o r i o n i z a t i o n o f carbon d i o x i d e .  -105-  v i b r a t i o n a l s p a c i n g s f o r the A II bands ( B ^ E  + u  of the bands.  and C E 2  + g  state.  Both the t h i r d and  fourth  ) are a l s o nonbonding as i n d i c a t e d by the  Carbon d i o x i d e , c a r b o n y l  sulphide  and  carbon  shape  disulphide  each possess s i x t e e n e l e c t r o n s i n t h e i r o u t e r s h e l l and have a l i n e a r s t r u c t u r e , hence the same d e s c r i p t i o n o f the o r b i t a l s and bands o f carbon d i o x i d e may sulphide  and  a l s o be used t o d i s c u s s the s p e c t r a o f  carbon d i s u l p h i d e .  The  energy s c a l e f o r CC^*  using i o n i z a t i o n p o t e n t i a l data published given  i n Table XVIII.  The  carbonyl was  calibrated  by T u r n e r et a l . (66)  and  Penning e l e c t r o n spectrum i n d i c a t e s those  s t a t e s a c c e s s i b l e u s i n g h e l i u m m e t a s t a b l e atoms, namely the X, A  and  B states. T a b l e XVII i n d i c a t e s t h a t the r e l a t i v e p o p u l a t i o n s t a t e s i s somewhat d i f f e r e n t f o r the two r a t i o o f the i n t e n s i t i e s o f the B and f o r i o n i z a t i o n by He photons.  The  a t i o n s may  *  o f the e l e c t r o n i c  i o n i z a t i o n processes.  X states  2 (BE  The  +  2 /X JI ) i s g g  greater  3 ° (2 S) atoms than f o r i o n i z a t i o n by He(584 A)  observed d i f f e r e n c e s i n r e l a t i v e e l e c t r o n i c s t a t e p o p u l -  be due  i n p a r t t o d i f f e r e n t a n g u l a r d i s t r i b u t i o n s of  electrons  from the d i f f e r e n t i o n i c s t a t e s but u n t i l a n g u l a r d i s t r i b u t i o n r e s u l t s have been r e p o r t e d  no d e t a i l e d comparison i s p o s s i b l e .  observed t h a t bands due  t o t r i p l e t i o n i z a t i o n , than was  i n the case o f the i n e r t gases and  diatomic  observed  molecules.  g i v e s the e l e c t r o n energy s h i f t s measured f o r the  Penning peaks o b t a i n e d  by t r i p l e t i o n i z a t i o n .  A near z e r o s h i f t  measured f o r the X s t a t e w i t h p o s i t i v e v a l u e s f o r the A and The  also  t o s i n g l e t i o n i z a t i o n are o f much reduced  i n t e n s i t y , r e l a t i v e t o those due  Table XVIII  I t was  v i b r a t i o n a l spacing  was  B states.  o b s e r v e d f o r Penning i o n i z a t i o n and  photo-  -106-  TABLE  XVII  R e l a t i v e P o p u l a t i o n s o f E l e c t r o n i c S t a t e s ( a t v = 0) f o r Carbon (Corrected f o r transmission) CO* 2 State  0  ?  -  30  8  _  12  +  100  _  100  +  17  -  -  2  u  2  u  CE 2  A)  68  x"n g An BE  Penning I o n i z a t i o n 3 2 S  Photoionization He (584  TABLE  Dioxide.  XVIII  S h i f t s , AE, i n Penning E l e c t r o n E n e r g i e s f o r Carbon D i o x i d e ( e V ) . I.P.  2  C0  State  xn 2  (66)  2  2  c z 2  +  u +  g  s  -0.,004±0.,012  -  17.323  +0,,028±0..020  -  18.082  +0,.012±0..012  -  19.400  -  -  u BE  AE  t  13.788 g  A n  AE  -107-  2 2 + i o n i z a t i o n t o the X IT and A II s t a t e s o f C0~ a r e g i v e n g u 2 6  and  compared w i t h v a l u e s r e p o r t e d  elsewhere.  i n T a b l e XIX  As was o b s e r v e d w i t h  o t h e r m o l e c u l e s s t u d i e d , t h e v i b r a t i o n a l s p a c i n g s a r e independent o f t h e means o f i o n i z a t i o n , w i t h i n e x p e r i m e n t a l e r r o r .  4.3.2. C a r b o n y l  Sulphide.  The e l e c t r o n s p e c t r a o b s e r v e d f o r c a r b o n y l Figure  27.  sulphide  a r e shown i n  The Penning e l e c t r o n spectrum shows those s t a t e s  u s i n g h e l i u m m e t a s t a b l e atoms, namely t h e X n , A n , B Z 2  2  i o n i c s t a t e s are a l s o observed i n the photoelectron  2  +  accessible  and C E . 2  spectrum.  +  The  These features  of t h e spectrum a r e v e r y s i m i l a r t o t h o s e o f carbon d i o x i d e , w i t h a l l bands s h i f t e d t o h i g h e r using  e l e c t r o n energy.  i o n i z a t i o n p o t e n t i a l data published  The energy s c a l e was c a l i b r a t e d by T u r n e r e t a l . (66) and  g i v e n i n T a b l e XXI. T a b l e XX i n d i c a t e s t h a t t h e r e l a t i v e e l e c t r o n i c s t a t e d i f f e r s i g n i f i c a n t l y f o r Penning and p h o t o i o n i z a t i o n .  populations  These d i f f e r e n c e s  may p o s s i b l y be a s c r i b e d t o d i f f e r e n t a n g u l a r d i s t r i b u t i o n s f o r e l e c t r o n s a r i s i n g from d i f f e r e n t i o n i z a t i o n p r o c e s s e s b u t f u r t h e r s t u d y w i l l be r e q u i r e d t o r e s o l v e the d i f f e r e n c e s . The measured v a l u e s o f the e l e c t r o n energy s h i f t s f o r c a r b o n y l sulphide  are given  i n T a b l e XXI.  A l a r g e n e g a t i v e v a l u e was measured  f o r t h e t r a n s i t i o n t o the lowest i o n i c s t a t e .  This i s the only molecule  s t u d i e d where a n e g a t i v e e l e c t r o n s h i f t was measured f o r t h e B s t a t e . v a l u e was measured f o r the A s t a t e due t o i t s u n r e s o l v e d Instrumental  character.  r e s o l u t i o n was i n s u f f i c i e n t t o p e r m i t t h e r e s o l u t i o n  of v i b r a t i o n a l l e v e l s i n e i t h e r the photoelectron  o r Penning e l e c t r o n  No  TABLE  XIX  V i b r a t i o n a l S p a c i n g s (meV) f o r E l e c t r o n i c S t a t e s o f C 0 . +  2  Electronic State and I n t e r v a l  Spec,  P h o t o i o n i z a t i o n He(584 A)  (149)  Ionization (158)  xn 2  0-1  An 2  160  Penning  (66)  fl59)  T h i s work  He*(2 S) 3  180  -  154  163±10  -  0-1  140  130  141  138± 8  -  1 2  140  140  139  140± 8  143±12  2-3  130  120  140  138± 8  133±12  3-4  130  130  131  138± 8  135±12  -109-  2 S(B) 3  (a) He  5  2 S(X) 3  (b) He*+ 5 8 4 A  V B2 2  (c) 5 8 4 A  2  +  c 2  x n  2  2  1QO  8.0  6.0  4.0  +  2.0  ELECTRON ENERGY (eV) F i g u r e 27.  Electron spectra f o r i o n i z a t i o n of carbonyl  sulphide.  -RO-  TABLE  XX  Relative Populations of E l e c t r o n i c States (Corrected f o r transmission) C0S  +  Photoionization  State  He(584 A)  ( a t v = 0) f o r C a r b o n y l  Penning I o n i z a t i o n  2 S  2 S 49  1  Xn  60  -  An  -  -  2  2  3  B Z 2  +  100  -  74  C E  +  18  -  100  2  TABLE  XXI  S h i f t s , AE, i n Penning E l e c t r o n E n e r g i e s COS  +  State Xn 2  f o r Carbonyl  I.P. (66)  K  AE. t  11.189  -0.049±0.015  An 2  B E 2  +  16.042  -0.030±0.020  C E  +  17.960  +0.052±0.020  2  Sulphide.  AE s  S u l p h i d e (eV)  -Ill-  spectra . As i n the case o f CQ^  i t was  observed f o r COS  +  t h a t bands due  to  s i n g l e t i o n i z a t i o n are o f much reduced i n t e n s i t y compared t o  triplet  i o n i z a t i o n than was  molecules.  o b s e r v e d f o r the i n e r t gases o r d i a t o m i c  4.3.3. Carbon D i s u l p h i d e . Carbon d i s u l p h i d e s p e c t r a are shown i n F i g u r e 28.  The  spectrum  o  o b t a i n e d u s i n g He(584 A) r a d i a t i o n shows the w e l l known (66) i o n i c s t a t e s , 2 Xn, g  2 A l l , u  2 BE, u  s i m i l a r to C 0 energies.  + 2  2 CI  ^  2 ^ and D E and the f e a t u r e s o f the s p e c t r a are u  g  r  and COS , w i t h a l l bands s h i f t e d t o h i g h e r e l e c t r o n +  For carbon d i s u l p h i d e an a d d i t i o n a l band, D E 2  + u  was  In the Penning e l e c t r o n spectrum o n l y i o n i z a t i o n produced by metastable  present. triplet  atoms gave r i s e t o peaks o f s u f f i c i e n t i n t e n s i t y f o r the  e l e c t r o n s h i f t t o be measured.  I t was  observed t h a t the peaks due  to  o  i n t e r n a l He(584 A) i o n i z a t i o n are o f g r e a t e r i n t e n s i t y than those to s i n g l e t i o n i z a t i o n .  The  energy s c a l e was  due  c a l i b r a t e d from p u b l i s h e d  (66) i o n i z a t i o n p o t e n t i a l d a t a g i v e n i n T a b l e X X I I . The  v a l u e s o f the measured e l e c t r o n energy s h i f t s are g i v e n i n  Table XXII.  Negative  electron shifts  (AE^.) were measured f o r the X  and A s t a t e s and p o s i t i v e v a l u e s f o r the B and C s t a t e s .  Although  carbon d i o x i d e , c a r b o n y l s u l p h i d e and carbon d i s u l p h i d e a l l possess the same number o f e l e c t r o n s i n t h e i r o u t e r s h e l l and have a l i n e a r s t r u c t u r e i t was  not p o s s i b l e t o observe any t r e n d o r p a t t e r n f o r t h e i r e l e c t r o n  energy s h i f t s . For carbon d i s u l p h i d e the r e l a t i v e p o p u l a t i o n s  (Table X X I I I )  of  the e l e c t r o n i c s t a t e s appear t o be the same, w i t h the e x c e p t i o n o f the  -112-  ELECTRON F i g u r e 28.  E N E R G Y (eV)  E l e c t r o n s p e c t r a f o r i o n i z a t i o n o f carbon d i s u l p h i d e .  -113-  TABLE  XXII  S h i f t s , AE, i n Penning E l e c t r o n E n e r g i e s f o r Carbon D i s u l p h i d e CS  + 2  I.P.  AE  AE  t  State  (66)  n  10..608  -0.,034±0.015  12,.694  -0,,047±0.030  +  14,.478  +0,,022±0.015  +  16 .196  +0,.002+0.025  2  g u  h h ug  TABLE  + 2  State  Xn 2  An BE 2  CE 2  Photoionization  ( a t v = 0) f o r Carbon  Penning I o n i z a t i o n  He(584 A)  2 S  83  -  85  36  -  41  1  2 S 3  g•  2  u +  u +  100 26  s  XXIII  Relative Populations of E l e c t r o n i c States (Corrected for. transmission) CS  (eV) .  100 -  51  Disulphide.  -114-  C s t a t e , w i t h i n experimental electron spectra.  e r r o r , i n b o t h the Penning and  Instrumental  r e s o l u t i o n was  i n s u f f i c i e n t to permit  the s e p a r a t i o n and measurement o f the v i b r a t i o n a l s p a c i n g s bands i n e i t h e r the p h o t o e l e c t r o n  photo-  o f the  o r Penning e l e c t r o n s p e c t r a .  4.3.4. N i t r o u s Oxide. N i t r o u s o x i d e s p e c t r a are shown i n F i g u r e 29.  The 2  photoelectron 2+  2  spectrum shows the w e l l known (66) i o n i c s t a t e s , X II, A Z  and  B n.  Nitrous oxide i s a l i n e a r t r i a t o m i c molecule with s i x t e e n e l e c t r o n s i n i t s o u t e r s h e l l but i t i s asymmetrical. be w r i t t e n as The  i n c o n t r a s t t o carbon d i o x i d e and  carbon d i s u l p h i d e  The  ground s t a t e e l e c t r o n o r b i t a l c o n f i g u r a t i o n  has  a shape which i s c o n s i s t a n t w i t h removal o f  may  (66):  f i r s t band (X^n)  an e s s e n t i a l l y nonbonding e l e c t r o n  (TT  ).  The  second band  some v i b r a t i o n a l s t r u c t u r e a s s o c i a t e d w i t h i t and  (A^Z) has +  c o r r e s p o n d s t o removal 2  o f an e l e c t r o n (nu) w i t h some bonding c h a r a c t e r . c o n t a i n s complex v i b r a t i o n a l s t r u c t u r e which was energy s c a l e was  The  t h i r d band (B  not r e s o l v e d .  c a l i b r a t e d using i o n i z a t i o n p o t e n t i a l data  by T u r n e r et a l . (66) and g i v e n i n T a b l e XXIV.  The  II)  The  published  Penning e l e c t r o n  spectrum shows those s t a t e s o b t a i n a b l e u s i n g h e l i u m m e t a s t a b l e atoms. N0 9  was  spectrum was  the o n l y t r i a t o m i c m o l e c u l e s t u d i e d f o r which the  Penning  o f s u f f i c i e n t i n t e n s i t y t o p e r m i t measurement o f the  e l e c t r o n energy s h i f t f o r b o t h s i n g l e t and t r i p l e t i o n i z a t i o n .  The  measured energy s h i f t s are g i v e n i n T a b l e XXIV. T a b l e XXV  g i v e s the r e l a t i v e p o p u l a t i o n s  o f the e l e c t r o n i c s t a t e s  -115-  I  8D  .  I  .  I  7.0  6.0  ELECTRON F i g u r e 29.  i  I  5.0  1  1  4.0  ENERGY  >  1  3D  '  (eV)  Electron spectra f o r i o n i z a t i o n of nitrous oxide.  1  —  2.0  -116-  TABLE  XXIV  S h i f t s , AE, i n Penning E l e c t r o n E n e r g i e s f o r N i t r o u s Oxide ( e V ) . N„0 2 State +  X n 2  A E 2  +  Bn 2  I.P.  AE^ t  12.893  -0.008±12  -0.050±0.025  16.389  +0.039±10  -0.025±0.020  17.65  XXV  Relative populations o f E l e c t r o n i c States (Corrected f o r transmission) N_0  +  State  Xn 2  2  s  (66)  TABLE  A E  AE  +  Bn  Photoionization ° He(584 A)  ( a t v = 0) f o r N i t r o u s Oxide.  Penning I o n i z a t i o n 1 3 2 S 2 S X  100  79  75  62  100  100  -  -117-  and shows t h a t t h e r e a r e s i g n i f i c a n t d i f f e r e n c e s .  F o r Penning  ionization  the r a t i o o f t h e A t o X s t a t e (A £ /X^TI) i s g r e a t e r than t h a t o b s e r v e d 2  for photoionization.  +  A n g u l a r d i s t r i b u t i o n s t u d i e s w i l l be r e q u i r e d t o  determine t h e o r i g i n o f t h e s e d i f f e r e n c e s . The v i b r a t i o n a l s p a c i n g s measured f o r Penning and p h o t o e l e c t r o n s f o r t h e A s t a t e o f N^O* a r e g i v e n i n T a b l e XXVI and compared w i t h v a l u e s r e p o r t e d by o t h e r w o r k e r s .  As has been o b s e r v e d w i t h a l l o t h e r m o l e c u l e s ,  the agreement i s good and i n d i c a t e d t h a t t h e v i b r a t i o n a l f r e q u e n c i e s a r e independent o f t h e means o f i o n i z a t i o n . I t was a l s o observed t h a t s i n g l e t s t a t e i o n i z a t i o n gave bands much b r o a d e r than d i d t r i p l e t s t a t e  ionization.  4.3.5. S u l p h u r D i o x i d e and N i t r o g e n D i o x i d e . The s p e c t r a o f t h e bent t r i a t o m i c m o l e c u l e s s u l p h u r d i o x i d e and n i t r o g e n d i o x i d e a r e shown i n F i g u r e s 30 and 31 r e s p e c t i v e l y .  These  two m o l e c u l e s have been i n c l u d e d t o g e t h e r because i n n e i t h e r case was i t p o s s i b l e t o determine e l e c t r o n energy s h i f t s from t h e Penning  spectrum.  The c r o s s s e c t i o n s f o r Penning i o n i z a t i o n p r o c e s s e s a r e a p p a r e n t l y much s m a l l e r f o r t h e s e two m o l e c u l e s than f o r t h e o t h e r t r i a t o m i c m o l e c u l e s t h a t have been s t u d i e d . The energy s c a l e s o f t h e S0^  +  s p e c t r a were c a l i b r a t e d from  p o t e n t i a l d a t a p u b l i s h e d by T u r n e r e t a l . ( 6 6 ) , f i r s t ( a d i a b a t i c ) , 12.50 eV ( v e r t i c a l ) .  ionization  I.P. 12.29 eV  The n i t r o g e n d i o x i d e s p e c t r a were  c a l i b r a t e d from t h e p u b l i s h e d d a t a o f E d q u i s t e t a l . ( 1 5 5 ) , second I.P. 13.02 eV, t h i r d I.P. 13.60 eV, f o u r t h I.P. 14.52 eV and n i n t h I.P. 18.864 eV.  F o r b o t h m o l e c u l e s t h e Penning s p e c t r a o b t a i n e d were o n l y  -118-  TABLE  XXVI  V i b r a t i o n a l Spacings (meV) f o r E l e c t r o n i c S t a t e s o f N 0 . +  o  Electronic State and I n t e r v a l  Spec. (160)  Photoionization o  He(584 A) (138)  A Z  T h i s work  0-1  180  150±10  161±10  1-2  170  150±10  145±10  Penning Ionization He*(2 S) 3  166±12  -119-  —I  8.0  1  1  6.0  I  1  •  4.0  E L E C T R O N E N E R G Y (eV) F i g u r e 30.  E l e c t r o n spectra f o r i o n i z a t i o n o f sulphur  dioxide.  i  2.0  -120-  -  1  1  10.0  1  1  8.0  ELECTRON F i g u r e 31.  1  •  6.0  ENERGY  I  4.0  (eV)  Electron spectra f o r i o n i z a t i o n of nitrogen dioxide.  -121-  o f comparable i n t e n s i t y t o the weak p h o t o e l e c t r o n  s p e c t r a produced  o  from i n t e r n a l He(584 A) photons.  No d a t a c o u l d be o b t a i n e d  from  the  Penning s p e c t r a . S u l p h u r d i o x i d e has  18 v a l e n c e s h e l l e l e c t r o n s , two more than  l i n e a r t r i a t o m i c molecules studied. u r a t i o n can be e x p r e s s e d as S0 The  (3a )  2  two  ground s t a t e e l e c t r o n c o n f i g -  (66):  (3b )  2  x  The  the  2  2  (lbp  2  (5a )  2  x  (4b )  2  2  (la ) 2  2  (Sa^  ,  2  1  A  1  e x t r a e l e c t r o n s cause the ground s t a t e m o l e c u l e t o be b e n t .  E l a n d and Danby (159)  considered  t h a t the bond a n g l e i n c r e a s e d  119  degrees t o 137  degrees d u r i n g  Due  t o the e x t r a e l e c t r o n s the p h o t o e l e c t r o n  from  i o n i z a t i o n l e a d i n g t o the 6a^ band. spectrum o f s u l p h u r  dioxide  i s more complex than t h a t o f the l i n e a r t r i a t o m i c m o l e c u l e s . Nitrogen  d i o x i d e has  one  more v a l e n c e e l e c t r o n than the  linear  t r i a t o m i c m o l e c u l e s and the ground s t a t e e l e c t r o n i c c o n f i g u r a t i o n be w r i t t e n N0 The  may  (66): (lb^  2  bond angle f o r N0  2  2  (5a )  2  x  i s 134  (la )  2  2  (4b )  degrees.  2  2  (6 p a  ,  \  Removal o f the most l o o s e l y  bound e l e c t r o n leaves n i t r o g e n d i o x i d e i s o e l e c t r o n i c w i t h carbon d i o x i d e and  i t i s e x p e c t e d t o be  p o t e n t i a l o f N0  2  t h i s was  The  v a l u e o f the f i r s t i o n i z a t i o n  has not been w e l l e s t a b l i s h e d and was  energy s c a l e c a l i b r a t i o n . spectrum was  linear.  e x p e c t e d and  observed.  Because o f the u n p a i r e d as the p h o t o e l e c t r o n  not used f o r  electron a  complicated  spectrum o f F i g u r e 31 shows  -122-  4.4.  Polyatomic Molecules.  4.4.1. Ammonia. The s p e c t r a o f ammonia a r e shown i n F i g u r e  32.  The  spectrum  o  obtained published  u s i n g He(584. A) r a d i a t i o n i s i n good agreement w i t h  that  by Branton e t a l . (161) ( i o n i z a t i o n p o t e n t i a l s (161) f o r t h e  2 A^ s t a t e o f ammonia o f 10.14  eV ( a d i a b a t i c ) and 10.87 eV ( v e r t i c a l )  were used f o r c a l i b r a t i o n o f the energy s c a l e ) .  The ground  state  m o l e c u l a r o r b i t a l c o n f i g u r a t i o n can be e x p r e s s e s as ( 6 6 ) : NH  3  (la )  (2  2  x  a  i  )  2  (le) (3 4  & 1  )  ,  2  \  The f i r s t band c o r r e s p o n d s t o removal o f an e l e c t r o n from a l a r g e l y nonbonding o r b i t a l , the e l e c t r o n b e i n g l o c a t e d on the n i t r o g e n The NH^  +  i o n formed i s e x p e c t e d t o be i n a p l a n a r A^ 2  atom.  configuration.  2 The second band ( E) i s v e r y b r o a d and d i f f u s e and the v i b r a t i o n a l 2 structure i s unresolved. * He  O n l y the f i r s t band ( A^) r e s u l t i n g from  3 (2 S) i o n i z a t i o n i s o b s e r v e d i n the Penning e l e c t r o n I t can be seen from F i g u r e  resolved v i b r a t i o n a l structure.  32 t h a t the Penning spectrum shows no The n a t u r a l broadness o f Penning peaks  would account f o r t h i s l a c k o f s t r u c t u r e .  T a b l e XXVII g i v e s  v i b r a t i o n a l s p a c i n g s o b s e r v e d from the p h o t o e l e c t r o n with values published  spectrum.  the  spectrum compared  by Branton e t a l . (161).  Ammonia has t h e l a r g e s t e l e c t r o n energy s h i f t o f a l l atoms and 2 molecules studied.  /\E = -0.350 ± 0.035 eV f o r the t  A^ S t a t e .  This  s h i f t was measured from t h e p o s i t i o n s o f the band maxima. No comparison o f the r e l a t i v e p o p u l a t i o n was p o s s i b l e due t o the r a p i d l y r i s i n g  o f the e l e c t r o n i c s t a t e s  background.  -123-  I  i  I2.0  I  IO.O  i  I  i  8.0  6.0  ELECTRON F i g u r e 32.  ENERGY  .  I  4.0  (eV)  E l e c t r o n s p e c t r a f o r i o n i z a t i o n o f ammonia.  -124-  TABLE  XXVII  V i b r a t i o n a l Spacings (meV) f o r E l e c t r o n i c S t a t e o f NH^  Electronic State  Photoionization  and I n t e r v a l  He (584 A) (154) T h i s work  ?  o  0-1  120  140+15  1-2  110  110±15  2-3  120  120±12  3-4  130  120±12  4-5  130  140±12  5-6  120  130±12  6-7  130  120±12  7-8  140  130±12  8-9  130  140±12  9-10  130  140112  10-11  130  140±12  11-12  140  140±15  12-13  140  130115  +  -125-  4.4.2. M e t h y l C h l o r i d e , M e t h y l The  Bromide and M e t h y l  Iodide.  s p e c t r a o b t a i n e d f o r the m e t h y l h a l i d e s are shown i n F i g u r e s  33, 34 and 35.  The  energy s c a l e s were c a l i b r a t e d u s i n g v a l u e s p u b l i s h e d  by T u r n e r et a l . (66) and g i v e n i n T a b l e X X V I I I .  Traces  of n i t r o g e n  gas p r e s e n t i n the samples, as e v i d e n t from the p h o t o e l e c t r o n s p e c t r a , d i d not i n t e r f e r e w i t h i n t e r p r e t a t i o n and were used as a check on the e l e c t r o n energy c a l i b r a t i o n . methyl h a l i d e s may CH X 3  (ls ) c  2  The  ground 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 o f the  be e x p r e s s e d (sap  2  as  ( n s ^ )  2  (66): Ore)  where n = 3, 4 o r 5 f o r X = C I , Br and  (aa^  4  I.  The  2  (np^e)  4  character).  ^  highest occupied  i s l a r g e l y l o c a l i z e d on the h a l o g e n atom (though t h e r e may halogen-hydrogen (X-H)  ,  orbital  be some  a n t i b o n d i n g and carbon-hydrogen (C-H)  bonding  Removal o f an e l e c t r o n from t h i s o r b i t a l g i v e s an i o n i n  2 the  E s t a t e which i s doubly  degenerate and s h o u l d be s u s c e p t i b l e t o  both s p i n - o r b i t and J a h n - T e l l e r e f f e c t s .  Both the p h o t o e l e c t r o n  and  Penning e l e c t r o n s p e c t r a e x h i b i t i n c r e a s i n g energy s e p a r a t i o n ( i n the • 2 2 o r d e r C I , B r , I) between the  E  .  and  E  ,  s t a t e s , as expected.  second and t h i r d bands i n the s p e c t r a are b r o a d and E l e c t r o n energy s h i f t s  The  unresolved.  (Table X X V I I I ) were measured f o r b o t h  s i n g l e t and t r i p l e t i o n i z a t i o n f o r CH^Cl* and CH^Br and f o r t r i p l e t i o n i z a t i o n f o r C H , I . No v a l u e f o r AE was o b t a i n e d f o r methyl i o d i d e 3 s 2 because o f the o v e r l a p o f the E , s t a t e produced by the i n t e r n a l 1/^ photon source w i t h the ^E,, s t a t e produced by s i n g l e t i o n i z a t i o n and +  +  2 a l s o the o v e r l a p o f the  2  E.. , s t a t e formed by s i n g l e t i o n i z a t i o n and 2  2 E  .  s t a t e formed by t r i p l e t i o n i z a t i o n .  This l a t t e r overlap also  2  1  e x p l a i n s the apparent d i f f e r e n c e i n the r e l a t i v e i n t e n s i t i e s o f the  the  -126-  _ l  1QO  F i g u r e 33.  I  I  L—  1  8.0 6.0 ELECTRON ENERGY (eV)  ,  E l e c t r o n s p e c t r a f o r i o n i z a t i o n o f methyl  I  4.0  chloride.  I  .  lO  I  8  ELECTRON F i g u r e 34.  .  I  1—  I  6  ENERGY  4  (eV)  E l e c t r o n s p e c t r a f o r i o n i z a t i o n o f methyl bromide.  -128-  2 S ( E)  3  (a)  3  2  He*  (b) He*+ 5 8 4 A  (c)  584  A  12.0  10.0  8.0  6.0  4.0  ELECTRON ENERGY (eV)  F i g u r e 35.  E l e c t r o n s p e c t r a f o r i o n i z a t i o n o f methyl i o d i d e .  -129-  TABLE  XXVIII  S h i f t s , AE, i n Penning E l e c t r o n E n e r g i e s f o r M e t h y l H a l i d e s (eV) .  I.P. Species  (66)  AE^  AE  CH C1 ^E )  11..28  -0.,120±0,.015  -0.,230±0..025  C H X ^ E ^ )  10..54  -0,,080±0,.015  -0.,220+0,.025  CH Br ( E  10..85  -0.,090±0,.015  -0,,230±0,.025  9,,55  -0,,080±0,.020  -  10,.16  -0,,090±0,.015  -  +  3  +  2  3  C H  3  I +  (  2 e  3/ ^  +  )  2  CH I ( E 3  1 / 2  2  1 / 2  )  -130-  and  s t a t e s produced by Penning i o n i z a t i o n w i t h t h o s e  E  produced by p h o t o i o n i z a t i o n .  Application of a correction for this  o v e r l a p and t h e t r a n s m i s s i o n c o r r e c t i o n f a c t o r shows t h a t t h e r e l a t i v e i n t e n s i t i e s a r e t h e same f o r t h e two p r o c e s s e s . state populations  The r e l a t i v e e l e c t r o n i c  f o r Penning and P h o t o i o n i z a t i o n appear t o be s i m i l a r .  I t may be observed from T a b l e X X V I I I t h a t t h e energy s h i f t s f o r s i n g l e t i o n i z a t i o n a r e v e r y n e a r l y t h e same and t h a t t h e energy  shifts  due t o t r i p l e t i o n i z a t i o n a r e a l l o f t h e same o r d e r o f magnitude though c o n s i d e r a b l y d i f f e r e n t from t h o s e o b s e r v e d f o r s i n g l e t i o n i z a t i o n . Thus f o r t h e m e t h y l h a l i d e s , i t i s t h e t y p e o f i o n i z i n g p a r t i c l e  rather  than t h e i o n i z e d s p e c i e s which determines t h e magnitude o f t h e e l e c t r o n energy  4.5.  shift.  Hydrocarbons.  4.5.1. Methane. F i g u r e 36 shows t h e s p e c t r a o b t a i n e d  f o r methane.  spectrum c o n s i s t s o f two b r o a d o v e r l a p p i n g bands. were a s s i g n e d  The p h o t o e l e c t r o n  The energy s c a l e s  on t h e b a s i s o f t h e i o n i z a t i o n p o t e n t i a l d a t a p u b l i s h e d by  T u r n e r e t a l . (66) 12.7 eV as t h e b e g i n n i n g  o f t h e b r o a d band.  The  ground 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 f o r CH^ may be e x p r e s s e d as ( 6 6 ) : (lsap  2  (2s  3 l  )  2  (2pt ) 2  6  , A 1  1  The b r o a d band, c o n t a i n i n g a double maximum, has c o n s i d e r a b l e  unresolved  s t r u c t u r e i n d i c a t i n g t h a t i o n i z a t i o n i s o c c u r r i n g from a h i g h l y bonding orbital.  The double maximum has been e x p l a i n e d  (66) by t h e use o f t h e  J a h n - T e l l e r Theorem which p r e d i c t s t h a t a t e t r a h e d r a l l y t r i p l y methane i o n cannot be a g e o m e t r i c a l l y s t a b l e s p e c i e s .  degenerate  Thus t h e band  1  9.0  F i g u r e 36.  ,  1  8.0  .  1  .  7.0  I  6.0  I  I  5.0  E L E C T R O N ENERGY (eV)  E l e c t r o n s p e c t r a f o r i o n i z a t i o n o f methane.  I  1  4.0  1  1  3.0  -132-  may  c o n t a i n t r a n s i t i o n s from the methane ground m o l e c u l a r s t a t e t o i o n s  w i t h l e s s than t e t r a h e d r a l symmetry. The Penning i o n i z a t i o n spectrum i s s i m i l a r t o t h a t produced by p h o t o i o n i z a t i o n but because o f the broadness o f the band and the r i s i n g background o f t h e Penning e l e c t r o n spectrum no e l e c t r o n energy s h i f t was o b t a i n e d . Herce e t a l . (162) have measured the r e l a t i v e i o n abundances, from methane and f u l l y d e u t e r a t e d methane produced by Penning employing s i n g l e t and t r i p l e t h e l i u m atoms. s i n g l e t and t r i p l e t  ionization  They o b s e r v e d t h a t f o r b o t h  i o n i z a t i o n the m e t h y l i o n s (CH^  +  and CD^ ) +  were t h e  most abundant, i n c o n t r a s t t o e l e c t r o n impact and p h o t o i o n i z a t i o n where the p a r e n t i o n predominates.  They a l s o o b s e r v e d t h a t f r a g m e n t a t i o n  was s i g n i f i c a n t l y g r e a t e r f o r t h e h i g h e r energy s i n g l e t m e t a s t a b l e atom, for both molecules.  4.5.2. E t h y l e n e . The e l e c t r o n s p e c t r a o b t a i n e d f o r e t h y l e n e a r e shown i n F i g u r e 37. o  The He(584 A) p h o t o e l e c t r o n spectrum c o n s i s t e d o f f o u r w e l l known bands (161) w i t h complex f i n e s t r u c t u r e .  The energy s c a l e was  u s i n g v a l u e s p u b l i s h e d by B r a n t o n e t a l . ( 1 6 1 ) , 2  B„ 12.46 eV; A 14.40 3g g 2  eV and B . 2  2u  15.78 eV.  10.51  2  by t r i p l e t i o n i z a t i o n . E t h y l e n e , which i s i s o e l e c t r o n i c w i t h 0^,  3 u  )•  T h  e  band formed  i s the simplest o r g a n i c 2  m o l e c u l e t o c o n t a i n a double bond and the f i r s t band ( b  eV;  The o n l y band o f  s i g n i f i c a n t i n t e n s i t y i n t h e Penning spectrum was t h e  to removal o f an e l e c t r o n ( l  calibrated  ) corresponds  e l e c t r o n c o n f i g u r a t i o n may  be  -133-  I  10.0  ,  i  .  I  8.0  ,  I  i  I  60  i  l  ,  ELECTRON ENERGY (eV)  F i g u r e 37.  Electron spectra f o r i o n i z a t i o n of ethylene.  I  4.0  -134-  w r i t t e n as (161) : C.H. 2 4  (2a ) g  (2b ) lu  2  n  2  (lb_ ) 2u  2  (3a ) g  2  (lb, ) (lb_ ) 3g 3u  2  ,  \ lg.  The o r i g i n o f t h e o t h e r bands i n t h e p h o t o e l e c t r o n spectrum have been d i s c u s s e d by Branton  et a l .  (161) and T u r n e r e t a l .  (66).  2 The energy s h i f t , AE^., f o r t h e t - 0.020 ± 0.012 eV.  B_ band was found t o be 3u  T h i s was t h e o n l y band o f e t h y l e n e f o r which an  e l e c t r o n energy s h i f t c o u l d be measured. I t may be seen from F i g u r e 37, t h a t t h e r e l a t i v e p o p u l a t i o n s o f the e l e c t r o n i c s t a t e s a r e v a s t l y d i f f e r e n t f o r t h e two p r o c e s s e s . p a r t i c u l a r the ^  2  B  3g 7  band which i s almost as i n t e n s e as t h e  2  In  B_ band 3u  i n t h e p h o t o e l e c t r o n spectrum i s v e r y much reduced i n i n t e n s i t y ,  relative  7  t o t h e "B^  band i n t h e Penning e l e c t r o n spectrum.  I t i s unlikely  t h a t t h e l a r g e d i f f e r e n c e s i n t h e r e l a t i v e i n t e n s i t i e s a r e due o n l y to d i f f e r e n c e s i n a n g u l a r d i s t r i b u t i o n s f o r e l e c t r o n s e j e c t e d from t h e d i f f e r e n t i o n i c s t a t e s and t h e s p e c t r a suggest t h a t a l a r g e d i f f e r e n c e i n e l e c t r o n i c t r a n s i t i o n moment e x i s t s f o r t h e two p r o c e s s e s . 4.5.3. A c e t y l e n e . F i g u r e 38 i l l u s t r a t e s t h e s p e c t r a o b t a i n e d f o r a c e t y l e n e . p h o t o e l e c t r o n spectrum c o n t a i n s t h e expected  The  bands a c c e s s i b l e u s i n g  o  He(584 A) r a d i a t i o n .  The energy s c a l e was c a l i b r a t e d u s i n g i o n i z a t i o n  p o t e n t i a l d a t a p u b l i s h e d by T u r n e r e t a l . (166) and g i v e n i n Table The Penning e l e c t r o n spectrum shows those s t a t e s o b t a i n a b l e u s i n g H e * ( 2 S ) and H e * ( 2 S ) atoms. 1  3  The ground s t a t e e l e c t r o n i c c o n f i g u r a t i o n o f a c e t y l e n e can be expressed  as ( 6 6 ) :  XXIX.  -135-  .  •  8.0  —I  •  6.0  E L E C T R O N E N E R G Y (eV)  F i g u r e 38.  Electron spectra f o r i o n i z a t i o n of acetylene.  1  4.0  •  '—  20  -136-  C-H_ 2  The  2  (la ) g  shape o f the  2  (la ) u  (2a ) g 2  2  (2a ) u  2  f i r s t band (X II)  2  (3a ) g  (ITT )  2  i s c h a r a c t e r i s t i c of  U  from the h i g h e s t o c c u p i e d T T o r b i t a l i n a l k y n e s (66) . t h i r d bands c o n t a i n u n r e s o l v e d complex Due i n the  t o the  p o p u l a t i o n s o f the  electronic  not  possible  states  1  g  *  ionization  The  second  and  structure.  r a p i d l y r i s i n g background at low  Penning spectrum i t was  z  ,  4  u  electron  energies  t o compare the  f o r the  two  relative  i o n i z a t i o n processes.  I t was o b s e r v e d , as f o r o t h e r m o l e c u l e s , t h a t the v i b r a t i o n a l s t r u c t u r e was l e s s w e l l r e s o l v e d i n the Penning spectrum and t h a t bands formed by s i n g l e t i o n i z a t i o n are b r o a d e r than those formed by t r i p l e t  ionization.  2 The  measured e l e c t r o n  are g i v e n i n T a b l e The  energy s h i f t s f o r the X n  XXIX and  u  band o f  acetylene  compared w i t h v a l u e s p u b l i s h e d by Hotop  r e s u l t f o r t r i p l e t i o n i z a t i o n i s i n good agreement w h i l e t h a t  s i n g l e t i o n i z a t i o n d i f f e r s somewhat from the v a l u e g i v e n by T h i s may broad and  o c c u r because the bands due p o o r l y r e s o l v e d i n both  T a b l e XXX photoionization  Hotop.  t o s i n g l e t i o n i z a t i o n are  extremely  studies.  t o the X s t a t e o f ^ 2 ^ 2 The  +  anc  ^  c  o  m  agreement i s good and  P  a  r  e  s  the means o f i o n i z a t i o n , w i t h i n  experimental  and  them w i t h v a l u e s  as has  o t h e r m o l e c u l e s s t u d i e d , the v i b r a t i o n a l s p a c i n g s are  The  for  l i s t s the v i b r a t i o n a l s p a c i n g s measured f o r Penning  reported elsewhere.  (154).  been the  case f o r  independent  of  error.  relative vibrational transition probabilities  (normalized  at  2 v = 0) f o r the X n s t a t e o f a c e t y l e n e are g i v e n i n T a b l e XXXI t o g e t h e r w i t h those r e p o r t e d by o t h e r w o r k e r s . The r e l a t i v e v i b r a t i o n a l t r a n s i t i o n p r o b a b i l i t i e s , f o r the p h o t o i o n i z a t i o n  ° (He(584 A))  and  Penning He  *  (2  3  S)  2 i o n i z a t i o n o f the X II  s t a t e were found, i n t h i s s t u d y , t o be  the  same  TABLE  XXIX  S h i f t s , AE, i n Penning E l e c t r o n E n e r g i e s f o r A c e t y l e n e  (eV) .  C H  2 S Ionization,  2  + 2  State  I.P.  2 S I o n i z a t i o n , AE^ 3  (66) T h i s work  X ^ 2  l  11.40  (156)  -0.040±0.012  T h i s work  -0.030  0.220±0.040  16.36  TABLE V i b r a t i o n a l Spacings  (meV) f o r E l e c t r o n i c S t a t e o f C H 2  E l e c t r o n i c State and  ,  ,  Interval  2  u  2  Photoionization  -r  i  rra-7  a-»  He(583 A) (158)  X n  XXX  Penning Ionization (He (2 S)  (164) T h i s work T h i s work  0-1  240  220  226±10  226±12  1- 2  240  220  226±10  230±12  2- 3  -  220  215±13  -138-  TABLE  XXXI Probabi i t i e s f o r Acetylene.  Relative Vibrational Transition E l e c t r o n i c State and  Penning  o  Vibrational  L e v e l s f o r C^H^  Photoionization  *  He*(2 S) 3  He (584 A) (158)  Ionization  (154)  T h i s work  T h i s work (a)  Xn 2  (a)  u  100  100  100  v = 0  100  1  45  43±5  38  43±5  2  15  15±7  9  15±7  These v a l u e s have been e s t i m a t e d from F i g u r e 1 o f R e f e r e n c e 154.  -139-  w i t h i n e x p e r i m e n t a l e r r o r , i n d i c a t i n g an e s s e n t i a l l y v e r t i c a l w i t h no change i n the e q u i l i b r i u m i n t e r n u c l e a r d i s t a n c e . reported  (154)  process  Hotop  has  a s m a l l d e v i a t i o n i n the r e l a t i v e v i b r a t i o n a l t r a n s i t i o n  p r o b a b i l i t i e s f o r the two  p r o c e s s e s and has  a t t r i b u t e d i t t o changes o f o  the e q u i l i b r i u m i n t e r n u c l e a r d i s t a n c e o f the o r d e r o f 0.01  4.6.  Rare Gas  A.  Mixtures.  D u r i n g the c o u r s e o f t h i s work s i g n i f i c a n t s t r u c t u r e was observed i n a d j a c e n t r e g i o n s  frequently  o f the e l e c t r o n energy spectrum.  s t r u c t u r e , which has been a t t r i b u t e d t o r a r e gas  This  i n t e r a c t i o n s , was  investigated further, since i t constituted a possible complication the i n t e r p r e t a t i o n o f the Penning s p e c t r a . one  o f the r a r e gases was  e x c i t a t i o n chamber and  subjected  the o t h e r was  In t h i s p a r t o f the  in  study  t o e l e c t r o n bombardment i n the a d m i t t e d d i r e c t l y t o the  collision -4  chamber v i a the sample system. t o r r were used and and  T o t a l pressures of approximately  10  the e l e c t r o n energy s p e c t r a were s e a r c h e d f o r p r i m a r y  secondary p r o c e s s e s .  4.6.1. Helium + Helium. The  s t r u c t u r e observed i n the spectrum at e n e r g i e s l e s s than  19 eV i s shown i n F i g u r e  39.  The  sharp peak at 16.23  eV i s due  to  the  o  photoionization  o f n e u t r a l h e l i u m atoms by the H e l l ( 3 0 4  o  He (584 A)  A)  line.  The  o  and He(304 A) r a d i a t i o n are produced by e l e c t r o n bombardment o  i n the e x c i t a t i o n chamber.  The  i n t e n s i t y o f the He(304 A) r a d i a t i o n  observed t o be v e r y dependent on s o u r c e c o n d i t i o n s peak at 16.23  eV was  and p r e s s u r e .  used f o r c a l i b r a t i o n o f the energy s c a l e .  The No  was  -140-  change i n t h e spectrum was observed when t h e e x t e r n a l He(584 A) photon s o u r c e was s i m u l t a n e o u s l y employed.  In p a r t i c u l a r p h o t o i o n i z a t i o n o f  * 1 * 3 ° He (2 S) and He (2 S) by He(584 A) p h o t o n s , which would g i v e p h o t o e l e c t r o n s o f 16.45 eV and 17.25 eV, was n o t d e t e c t e d .  This i s not s u r p r i s i n g 4  because o f t h e low c o n c e n t r a t i o n o f m e t a s t a b l e  atoms (10  metastable  3 atoms/cm ) t o g e t h e r w i t h t h e low p h o t o i o n i z a t i o n c r o s s s e c t i o n o f -19 2 ° h e l i u m m e t a s t a b l e atoms (4 x 10 cm a t 584 A (164, 1 6 5 ) ) . The o r i g i n o f t h e b r o a d , s t r u c t u r e l e s s peak (FWHM 1.25 eV) a t 14.8 eV i s u n c e r t a i n . -14 atoms (a ^ 1 0  The e n c o u n t e r  o f two h e l i u m t r i p l e t  metastable  2 cm ) has been s t u d i e d t h e o r e t i c a l l y  (166-169) and  e x p e r i m e n t a l l y (170, 171). T h i s p r o c e s s would r e s u l t i n an u n s h i f t e d e l e c t r o n energy ( E ) o f 15.06 eV. q  However, an e s t i m a t e based on t h e  e x p e r i m e n t a l c o n d i t i o n s used would r e q u i r e an u n r e a l i s t i c a l l y l a r g e v a l u e -10 (10  2 cm ) f o r t h i s c r o s s s e c t i o n i f t h e p r o c e s s were t o be i d e n t i f i e d  w i t h t h e b r o a d peak i n F i g u r e 39.  On s i m i l a r grounds s i n g l e t - t r i p l e t  (E  ( E = 16.64 eV) d i r e c t  q  = 15«85 eV) and s i n g l e t - s i n g l e t  q  i o n i z a t i n g c o l l i s i o n s may be d i s c o u n t e d .  Penning  A p o s s i b l e e x p l a n a t i o n f o r the  broad s t r u c t u r e i n F i g u r e 39 may a r i s e from a c o n s i d e r a t i o n o f t h e work o f Bydin e t a l . (172) who have s t u d i e d t h e e l e c t r o n energy s p e c t r a r e s u l t i n g from c o l l i s i o n s o f m o d e r a t e l y w i t h argon atoms.  f a s t , u n e x c i t e d Rb atoms (200 eV)  To e x p l a i n t h e e l e c t r o n spectrum Bydin e t a l . assumed  a curve c r o s s i n g mechanism l e a d i n g t o an e x c i t e d l e v e l o f t h e q u a s i * molecule  RbAr ( i n v o l v i n g A r ) which r a p i d l y undergoes d i s s o c i a t i o n  and i o n i z a t i o n o f Rb i n t h e c o l l i s i o n p r o c e s s .  Such a mechanism r e s u l t s  i n p r o d u c t s analogous t o those produced i n Penning  ionization.  This  argument can be extended i n t h e f o l l o w i n g manner.  F a s t h e l i u m atoms  F i gure 39.  E l e c t r o n s p e c t r a f o r c o l l i s i o n processes i n helium.  -142-  produced by charge exchange o f He  +  i n the r e g i o n between the e x c i t a t i o n  chamber and the i o n t r a p chamber (the energy d i f f e r e n c e b e i n g about 225eV) c o l l i d e w i t h n e u t r a l h e l i u m atoms i n the c o l l i s i o n chamber l e a d i n g , by curve c r o s s i n g , t o a s t a t e w i t h a d i s s o c i a t i o n * c o r r e s p o n d i n g t o the two m e t a s t a b l e h e l i u m atoms He  limit  1  *  (2 S) and He  3 (2 S ) .  Such a s t a t e would be u n s t a b l e w i t h r e s p e c t t o a u t o i o n i z a t i o n ,  giving  p r o d u c t s , He + He  ion-  ization.  +  + e, i d e n t i c a l t o t h o s e produced i n Penning  T h i s mechanism does not r e q u i r e t h e r e l a t i v e l y  encounter o f two h e l i u m m e t a s t a b l e atoms. n e u t r a l s would r e s u l t i n b r o a d peaks  improbable  A mechanism i n v o l v i n g  fast  (as observed) due t o the D o p p l e r  effect. The p o s s i b i l i t y t h a t the observed e l e c t r o n spectrum i s due t o e l e c t r o n e j e c t i o n from m e t a l s u r f a c e s by the h e l i u m m e t a s t a b l e s can be d i s c o u n t e d s i n c e experiments  (173, 174) show t h a t o n l y v e r y b r o a d , s t r u c t u r e l e s s ,  f u n c t i o n s are o b t a i n e d f o r such p r o c e s s e s . i n n a t u r e t o the background  These c u r v e s a r e s i m i l a r  f u n c t i o n s o b s e r v e d i n the apparatus at  lower e l e c t r o n e n e r g i e s . 4.6.2. Helium + Argon. In t h i s p a r t o f the s t u d y h e l i u m was  i n t r o d u c e d t h r o u g h the e x c i t -  a t i o n r e g i o n and argon by the sample system. shown i n F i g u r e 40.  The peaks at 16.23  The spectrum o b t a i n e d i s  eV and 14.8 eV a r e a s s i g n e d t o  the p r o c e s s e s d i s c u s s e d i n t h e h e l i u m + h e l i u m case i n t h e p r e v i o u s section.  At the low energy r e g i o n o f the spectrum between 4 - 6 eV are  the peaks c h a r a c t e r i s t i c o f i o n i z a t i o n o f argon by h e l i u m photons o * 1 (He(584 A) from the e x t e r n a l lamp) and by h e l i u m m e t a s t a b l e atoms (He (2 S))  HELIUM / A R G O N  ELECTRON  He23S/Ar  E N E R G Y eV  e c t r o n s p e c t r a f o r c o l l i s i o n p r o c e s s e s i n a m i x t u r e o f h e l i u m and argon.  -144-  and He (2°S)). t h e s e peaks.  The energy s c a l e was c a l i b r a t e d from t h e e n e r g i e s o f The peak at 9.5 eV a l s o o c c u r s w i t h o n l y argon i n t h e  apparatus and w i l l be d i s c u s s e d i n t h e next s e c t i o n .  I t arises i n  F i g u r e 40 due t o t h e m i x i n g o f gases a t t h e h i g h e r p r e s s u r e s used i n t h i s experiment.  The prominent peak a t 12.8 eV i s found o n l y i n the h e l i u m +  argon case and may be due t o a u t o i o n i z a t i o n o f doubly e x c i t e d argon atoms (175, 176) formed by n e u t r a l - n e u t r a l i n t e r a c t i o n .  A s i m i l a r peak  has been observed by Gerber e t a l . (177) i n e l e c t r o n s p e c t r a r e s u l t i n g from atom-atom c o l l i s i o n s i n m i x t u r e s o f charge exchanged h e l i u m i o n s (400 eV) and argon.  4.6.3. Argon + Argon. For t h i s experiment argon was a d m i t t e d through b o t h the e x c i t a t i o n chamber and the sample system.  With the s i m u l t a n e o u s use o f t h e e x t e r n a l  o  He(584A) photon source the spectrum o b t a i n e d i s shown i n F i g u r e 41.  The o  peaks a t 5.46 eV and 5.28 eV, produced by i o n i z a t i o n o f A r by He(584 A) photons were used t o c a l i b r a t e the energy s c a l e and were t h e o n l y s t r u c t u r e t o d i s a p p e a r when use o f t h e e x t e r n a l lamp was d i s c o n t i n u e d . o  No e v i d e n c e was found o f i o n i z a t i o n o f argon m e t a s t a b l e atoms by He(584 A) o  o  o  o  A r l ( 1 0 6 7 A, 1048 A) o r A r I I ( 9 3 2 A, 920 A) r a d i a t i o n .  The i n t e n s e s t r u c t u r e  i n F i g u r e 41 i n d i c a t e s a r e l a t i v e l y h i g h p r o b a b i l i t y f o r t h e p r o c e s s e s involved.  B e r r y (178) has observed s i m i l a r s t r u c t u r e i n s p e c t r a o b t a i n e d  f o r an argon/argon s t u d y a t c o l l i s i o n e n e r g i e s t h a t range from 30 t o 250 eV.  He found t h a t the peaks v a r i e d w i t h t h e energy o f t h e system.  The l a r g e peak a t 9.47 may be a s s i g n e d t o t h e a u t o i o n i z a t i o n Ar(3s 3p  6  4s) produced i n n e u t r a l - n e u t r a l c o l l i s i o n s .  (179) o f  T h i s p r o c e s s has  R E L A T I V E INTENSITY  -146-  been s t u d i e d and d i s c u s s e d  by Gerber e t a l .  (179) and i t i s a l s o found  t o o c c u r i n A r / A r c o l l i s i o n s a t medium e n e r g i e s (180) . +  The peak a t  ^ 8.8 eV does not c o r r e s p o n d t o a u t o i o n i z a t i o n o f any known e x c i t e d s t a t e o f argon.  Siegbahn e t a l . (179) have o b s e r v e d an u n c l a s s i f i e d  peak a t 8.89 eV i n t h e e l e c t r o n impact i n d u c e d a u t o i o n i z a t i o n spectrum o f argon.  electron  No d e f i n i t e assignment can be made f o r t h e peak at  10.4 eV, but i t i s c l o s e t o t h e v a l u e expected f o r a u t o i o n i z a t i o n o f A r ( 3 s 3p^ 4 p ) .  A s m a l l s h o u l d e r a t ^ 11.9 eV (observed a t h i g h e r  s e n s i t i v i t y than i n F i g u r e 41) i s p r o b a b l y due t o a u t o i o n i z a t i o n o f A r ( 3 s 3p^ 3d) s i n c e a n g u l a r d i s t r i b u t i o n measurements t h a t A r ( 3 s 3p^ 5s) i s not i n v o l v e d .  (177) i n d i c a t e  Peak b r o a d e n i n g as observed i n  F i g u r e 41 (compared w i t h p h o t o i o n i z a t i o n peaks) would be expected due t o the D o p p l e r e f f e c t i f the p o s t u l a t e d mechanism o f n e u t r a l - n e u t r a l c o l l i s i o n occurs.  In a beam experiment Hammond e t a l . (181) observe  s i g n i f i c a n t cross sections f o r n e u t r a l - n e u t r a l i o n i z i n g c o l l i s i o n s i n argon a t l a b o r a t o r y e n e r g i e s o f 57—240 eV, and t h e y a l s o r e p o r t o f e j e c t e d e l e c t r o n s a t a p p r o x i m a t e l y 6 eV and 9.5 eV. — 16 n e u t r a l - n e u t r a l i o n i z i n g cross section  (y 10  +  cm ) r e p o r t e d  3  cm  to give  2  T h i s v a l u e compares w e l l w i t h t h e  from charge t r a n s f e r experiments (182) and lends  support t o the suggested mechanism o f f a s t n e u t r a l - n e u t r a l collisions.  by Hammond  r e q u i r e s a charge  i n A r o f a p p r o x i m a t e l y 10 ^  s i g n a l s o f the magnitude we o b s e r v e . cross s e c t i o n reported  Taking the  2  et a l . (181) t o g e t h e r w i t h our e x p e r i m e n t a l c o n d i t i o n s t r a n s f e r cross s e c t i o n f o r A r  bands  ionizing  The o r i g i n o f the band a t ^ 7.1 eV i s u n c e r t a i n .  T h i s band  appears t o have s e v e r a l components w i t h e l e c t r o n e n e r g i e s i n the r e g i o n expected from t h e e n c o u n t e r o f two argon m e t a s t a b l e atoms  -147-  producing A r  + A r + e.  +  I t i s n o s s i b l e that t h i s process occurs v i a a  c r o s s i n g mechanism s i m i l a r t o t h a t a l r e a d y h e l i u m system.  discussed  f o r the helium +  A b r o a d u n a s s i g n e d peak a t about 5.7 eV which appears o  as a s h o u l d e r on t h e argon - He(584 A) d o u b l e t i s a l s o p r e s e n t the e x t e r n a l photon lamp e x t i n g u i s h e d .  with  The peaks a t 3.0, 2.69 and  2.51 eV c o u l d p o s s i b l y be due t o a u t o i o n i z a t i o n o f d o u b l y e x c i t e d states  (176) o f A r t o  Ar (3s3p^). +  4.6.4. Neon + Neon. F i g u r e 42 shows t h e e l e c t r o n spectrum o b s e r v e d w i t h neon o n l y i n the system.  Broad bands a r e observed a t ^ 11.4 eV and above 13 eV a t  h i g h e r p r e s s u r e s (upper t r a c e ) .  The b r o a d band a t ^ 11.4 eV i s i n the  energy range f o r p r o c e s s e s i n v o l v i n g f a s t neon atoms and n e u t r a l neon atoms c o l l i d i n g and producing, an u n s t a b l e Ne + N e  +  s t a t e , g i v i n g products  + e.  The s h a r p e r peaks a t a, 9.0, 6.3, 5.6 and 5.35 eV a r e due t o photoi o n i z a t i o n o f neon by N e l l r a d i a t i o n produced by e l e c t r o n i n t h e e x c i t a t i o n chamber.  bombardment  The energy s c a l e was c a l i b r a t e d u s i n g t h e  band a t 5.35 eV ( C o n s i s t i n g o f a t r i p l e t w i t h t h e e x p e c t e d i n t e n s i t i e s o f 4:4:1) which i s due t o i o n i z a t i o n by the neon i o n resonance The h i g h e r  lines.  energy peaks a r e due t o i o n i z a t i o n by r a d i a t i o n from e x c i t e d  s t a t e s o f t h e neon i o n (139).  5.35 Ne + 4 6 2 & - ^ N e + e +  NEON  _j—.  14  (  12  !  10 ELECTRON  F i g u r e 42.  (  .  .  8 ENERGY  1  6 (eV)  E l e c t r o n s p e c t r a f o r c o l l i s i o n p r o c e s s e s i n neon.  1  4  r  2  -149-  CHAPTER  FIVE  CONCLUSIONS * 1 A comparative s t u d y has been made o f Penning i o n i z a t i o n (He (2 S) * 3 ° and He (2 S)) and p h o t o i o n i z a t i o n (He(584 A)) o f a l a r g e number o f atoms and m o l e c u l e s employing t h e t e c h n i q u e s o f h i g h r e s o l u t i o n e l e c t r o n spectroscopy.  Good agreement i n t h e measured Penning e l e c t r o n  energy  s h i f t was found where comparison w i t h l i t e r a t u r e v a l u e s was p o s s i b l e . Very l a r g e e l e c t r o n energy s h i f t s were observed f o r some m o l e c u l e s (acetylene,  ammonia and the m e t h y l h a l i d e s ) but no c o r r e l a t i o n s have  been found f o r t h e s e e f f e c t s which are presumably due t o t h e combined e f f e c t s o f the shape o f the p o t e n t i a l energy i n t e r a c t i v e s u r f a c e and the v a r i a t i o n o f t r a n s i t i o n p r o b a b i l i t y w i t h d i s t a n c e . secondary r a r e gas i n t e r a c t i o n s have a l s o been  A number o f  studied.  I t was observed t h a t t h e c r o s s s e c t i o n r a t i o f o r Penning i o n i z a t i o n *  1  *  3  (He (2 S)/He (2 S ) ) was r e l a t i v e l y lower f o r m o l e c u l e s than f o r atoms. In atoms t h e Penning e l e c t r o n d i s t r i b u t i o n s were b r o a d e r f o r t r i p l e t i o n i z a t i o n than f o r s i n g l e t i o n i z a t i o n .  The o p p o s i t e was observed f o r  molecules. A v e r y low c r o s s s e c t i o n was observed f o r Penning i o n i z a t i o n o f S0  ?  and N 0  0  and a l s o t o t h e lowest s t a t e s o f 0^  and N0 . +  For many m o l e c u l e s t h e r e l a t i v e p o p u l a t i o n s o f e l e c t r o n i c s t a t e s o f the i o n a r e s t r o n g l y dependent on the mode o f i o n i z a t i o n . d i f f e r e n c e s are observed i n t h e case o f e t h y l e n e .  Very  large  A l t h o u g h these  -150-  o b s e r v a t i o n s may be p a r t l y due t o d i f f e r e n c e s  i n angular d i s t r i b u t i o n s o f  e j e c t e d e l e c t r o n s i n Penning and p h o t o i o n i z a t i o n , significant differences the  occur i n the e l e c t r o n i c  two t y p e s o f i o n i z a t i o n .  are o f s i g n i f i c a n c e  i t i s probable that t r a n s i t i o n moments f o r  The l a r g e d i f f e r e n c e s  i n state  populations  i n many c h e m i c a l systems e.g. d i s c h a r g e s ,  pumping and c h e m i c a l r e a c t i o n s .  laser  I n such systems g r e a t c a u t i o n must  o b v i o u s l y by e x e r c i s e d i f r e l a t i v e t r a n s i t i o n p r o b a b i l i t i e s a r e used which have been o b t a i n e d from s t u d i e s o f t h e a b s o r p t i o n o f e l e c t r o magnetic  radiation.  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