Open Collections

UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

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

Your browser doesn't seem to have a PDF viewer, please download the PDF to view this item.

Item Metadata

Download

Media
831-UBC_1974_A1 S84_8.pdf [ 6.18MB ]
Metadata
JSON: 831-1.0059921.json
JSON-LD: 831-1.0059921-ld.json
RDF/XML (Pretty): 831-1.0059921-rdf.xml
RDF/JSON: 831-1.0059921-rdf.json
Turtle: 831-1.0059921-turtle.txt
N-Triples: 831-1.0059921-rdf-ntriples.txt
Original Record: 831-1.0059921-source.json
Full Text
831-1.0059921-fulltext.txt
Citation
831-1.0059921.ris

Full Text

c 1 PENNING IONIZATION ELECTRON SPECTROSCOPY OF SOME ATOMS AND MOLECULES BY WILLIAM BRIEN STEWART B.Sc. 1966, M.Sc. 1968, U n i v e r s i t y of 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 the Department of CHEMISTRY We accept t h i s t h e s i s as conforming to the requ i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA February, 1974 In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the r equ i r emen ts f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Co lumb i a , I agree t h a t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y purposes may be g r a n t e d by the Head o f my Department o r by h i s r e p r e s e n t a t i v e s . It i s u n d e r s t o o d that c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l owed w i thout my w r i t t e n p e r m i s s i o n . Department o f C — € ^ . / ^ ^ / The U n i v e r s i t y o f B r i t i s h Co lumbia Vancouver 8, Canada Date / ^ / 7 ^ ABSTRACT A comparative study has been made of Penning i o n i z a t i o n (He*(2 1S) and He*(2 3S)) and p h o t o i o n i z a t i o n (He(584 A)) of twenty-three atoms and molecules (Ar, Kr, Xe, H^, HD, D,,, N^, CO, NO, 0^, C0 2, COS, CS 2, N 20, S0 2, N0 2, NH^, CH CI, CH Br, CH I , CH 4, C 2H , C 2H 2) employing the techniques of high r e s o l u t i o n e l e c t r o n spectroscopy. E l e c t r o n s p e c t r a from mixed r a r e gas systems, at higher pressures, have also been examined and i n t e r p r e t e d on the b a s i s of f a s t 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 s . - i i -TABLE OF CONTENTS Page ABSTRACT i x ACKNOWLEDGEMENT x CHAPTER ONE - INTRODUCTION 1 1.1. 1.1.1. I n t r o d u c t i o n 1 1.1.2. Experimental Objective 3 1.2. Penning I o n i z a t i o n .. . 5 1.2.1. I n i t i a l State of A* 5 1.2.2. I n t e r p r e t a t i o n i n Terms of P o t e n t i a l Energy Curves 7 1.2.3. Ejected E l e c t r o n Energy Spectra 13 1.3. A s s o c i a t i v e I o n i z a t i o n 16 1.4. R e l a t i v e and Absolute Cross Sections 18 1.5. Photoelectron Spectroscopy 23 1.6. Franck-Condon P r i n c i p l e 27 1.7. Angular D i s t r i b u t i o n of Photoelectron and Penning E l e c t r o n s 29 1.8. A u t o i o n i z a t i o n 31 CHAPTER TWO - THEORETICAL DISCUSSION 33 2.1. Weak I n t e r a c t i o n Theories 33 2.2. Close Coupled Near A d i a b a t i c Theories 37 2.3. Exchange Model 38 2.4. Theory of the 127 Degree E l e c t r o s t a t i c Analyzer ... 39 - i i i -Page CHAPTER THREE - INSTRUMENTAL 41 3.1. Experimental Arrangement 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 Region 43 3.1.3. E l e c t r o n Analyzer and Detection System 44 3.1.4. Light Source 46 3.1.5. Vacuum System 47 3.2. Spectrometer Performance 48 3.3. C a l i b r a t i o n of Energy Scale and Pr e s e n t a t i o n of Data 56 CHAPTER FOUR - RESULTS AND DISCUSSION 60 4.1. Rare Gases 60 4.1.1. Argon 60 4.1.2. Krypton 66 4.1.3. Xenon 66 4.2. Diatomic Molecules 72 4.2.1. Molecular Hydrogen, Deuterium Hydride and Molecular Deuterium 72 4.2.2. Molecular Nitrogen 77 4.2.3. Carbon Monoxide 87 4.2.4. N i t r i c Oxide 9 2 4.2.5. Molecular Oxygen 99 4.3. T r i a t o m i c Molecules 103 4.3.1. Carbon Dioxide 105 4.3.2. Carbonyl Sulphide 107 4.3.3. Carbon Disulphide I l l - i v -Page 4.3.4. Nitr o u s Oxide , 114 4.3.5. Sulphur Dioxide and Nitrogen Dioxide 117 4.4. Polyatomic Molecules 122 4.4.1. Ammonia 122 4.4.2. Methyl C h l o r i d e , Methyl Bromide and Methyl Iodide . 125 4.5. Hydrocarbons 130 4.5.1. Methane 130 4.5.2. Ethylene 132 4.5.3. Acetylene 134 4.6. Rare Gas Mixtures 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 Page I Energy Levels f o r Rare Gas Ions, Atoms and Photons 6 II S h i f t s , AE, i n Penning E l e c t r o n Energies f o r Rare Gases (eV) 65 2 2 I I I Peak Ratios ( P,. / P, , ) f o r Rare Gas I o n i z a t i o n 67 • V 2 i / 2 IV S h i f t s , AE, i n Penning E l e c t r o n Energies 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 2£ + ) 79 ^ 8 VI V i b r a t i o n a l Spacings (meV) f o r HD +(X 2E +) 80 VII V i b r a t i o n a l Spacings (meV) f o r D 2 ( X 2 E g + ) 81 V I I I S h i f t s , AE, i n Penning E l e c t r o n Energies f o r Nitrogen feV) 84 IX R e l a t i v e Populations o f E l e c t r o n i c States (at v = 0) f o r Nitrogen 84 X 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 States o f N 2 + 86 XI S h i f t s , AE, i n Penning E l e c t r o n Energies f o r Carbon Monoxide (eV) 89 XII R e l a t i v e Populations of E l e c t r o n i c States (at v = 0) f o r Carbon Monoxide 89 XIII 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 States of C0 + 91 XIV S h i f t s , AE, i n Penning E l e c t r o n Energies f o r N i t r i c Oxide (eV) 94 XV 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 States of N0 + 97 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 98 XVII R e l a t i v e Populations of E l e c t r o n i c States (at v = 0) f o r Carbon Dioxide 106 XVIII S h i f t s , AE, i n Penning E l e c t r o n Energies f o r Carbon Dioxide (eV) 106 - v i -Table Page XIX 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 States of C 0 2 + 108 XX R e l a t i v e Population of E l e c t r o n i c States (at v = 0) f o r Carbonyl Sulphide 110 XXI S h i f t s , AE, i n Penning E l e c t r o n Energies f o r Carbonyl Sulphide (eV) 110 XXII S h i f t s , AE, i n Penning E l e c t r o n Energies f o r Carbon Dis u l p h i d e (eV) 113 XXIII R e l a t i v e Populations of E l e c t r o n i c States (at v = 0) f o r Carbon Disulphide 113 XXIV S h i f t s , AE, i n Penning E l e c t r o n Energies f o r N i t r o u s Oxide (eV) 116 XXV R e l a t i v e Populations of E l e c t r o n i c States (at v = 0) f o r N i t r o u s 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 States of N^0 + 118 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 States of NH^+ 124 XXVIII S h i f t s , AE, i n Penning E l e c t r o n Energies f o r Methyl Halides (eV) 129 XXIX S h i f t s , AE, i n Penning E l e c t r o n Energies f o r Acetylene (eV) 137 XXV 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 States of C H + 137 2 2 XXXI 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 Acetylene 138 - v i i -LIST OF FIGURES Figure Page 1. P o t e n t i a l energy curves f o r a s s o c i a t i v e and Penning i o n i z a t i o n 9 2. Ratio of s i n g l e t to t r i p l e t helium metastable atoms as a f u n c t i o n of the energy of the e x c i t i n g e l e c t r o n s .. 19 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 to photoelectron production 26 4. Schematic diagram of Penning i o n i z a t i o n e l e c t r o n spectrometer 42 o 5. High r e s o l u t i o n photoelectron spectrum (584 A) of argon 49 o 6. Photoelectron spectrum (584 A) of molecular hydrogen .. 50 o 7. Photoelectron sp e c t r a of argon at 584 A using i n t e r n a l and e x t e r n a l photon lamps 52 8. Pressure dependence of the Penning and photoelectron s i g n a l as a f u n c t i o n of t a r g e t gas pressure 53 9. Pressure dependence of the Penning s i g n a l as a f u n c t i o n of helium pressure 55 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 58 11. E l e c t r o n spectrum f o r i o n i z a t i o n of argon 61 12. Comparison of peak shapes f o r i o n i z a t i o n of argon 63 13. 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 krypton 68 14. E l e c t r o n spectra f o r i o n i z a t i o n of xenon 69 15. Comparison of peak shapes f o r i o n i z a t i o n of 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 of molecular 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 of deuterium hydride .. 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 of molecular hydrogen 78 - v i i i -Figure Page 20. 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 molecular n i t r o g e n 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 of carbon monoxide .... 88 22. 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 n i t r i c oxide 93 23. Penning e l e c t r o n and photoelectron s p e c t r a of the X^E + s t a t e of n i t r i c oxide. 96 24. 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 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 of NO to N0+(xV) 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 of molecular oxygen .. . 101 26. 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 104 27. 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 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 of carbon d i s u l p h i d e .. 112 29. 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 115 30. 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 sulphur d i o x i d e .... 119 31. 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 n i t r o g e n d i o x i d e ... 120 32. 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 123 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 of methyl c h l o r i d e .... 126 34. 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 127 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 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. 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 133 38. 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 135 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 .... 141 40. 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 a mixture of helium 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 processes 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 processes i n neon 0 ACKNOWLEDGEMENT The work described i n t h i s t h e s i s was done under the d i r e c t i o n of Dr. C. E. B r i o n , to 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 to thank Profess 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 wish to acknowledge the a s s i s t a n c e of the s t a f f of the Mechanical and E l e c t r o n i c s Workshops of the Department of 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 of the instrument. I would a l s o l i k e to thank my colleagues i n the l a b o r a t o r y , Drs. G. E. Thomas and L. A. R. Olsen, 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. I n t r o d u c t i o n . The e l e c t r o n i c s t r u c t u r e of atoms and molecules has been e x t e n s i v e l y i n v e s t i g a t e d by means of spec t r o s c o p i c methods which i n v o l v e the study of the emission or absorption of electromagnetic r a d i a t i o n a s s o c i a t e d with t r a n s i t i o n s between the various energy l e v e l s . I o n i z a t i o n of atoms (molecules) f i r s t occurs when the energy of the incoming p a r t i c l e i s equal to that needed to remove the most l o o s e l y bound e l e c t r o n . Removal of 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 formation of an ion 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 process may be a d i r e c t t r a n s i t i o n of a bound e l e c t r o n i n t o the i o n i z a t i o n continuum or i t may take place by an i n d i r e c t process ( a u t o i o n i z a t i o n ) . Several experimental methods have been developed f o r the 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 spectroscopy provides the most accurate means of determining the i o n i z a t i o n p o t e n t i a l of many atoms and simple molecules. Data from the f a r u l t r a v i o l e t absorption spectrum can often be f i t t e d to a Rydberg s e r i e s and the i o n i z a t i o n p o t e n t i a l i s then given by the convergence l i m i t of the s e r i e s . E l e c t r o n bombardment, with or without energy monochromation, has a l s o been widely used to determine i o n i z a t i o n p o t e n t i a l s from the onsets of i o n i z a t i o n e f f i c i e n c y curves which are u s u a l l y obtained using a mass spectrometer. The method of 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 of e l e c t r o n impact except that a monochromatized beam of photons i s s u b s t i t u t e d f o r the e l e c t r o n beam. By contrast photoelectron spectroscopy (PES), which employs a photon beam of f i x e d wavelength, has been widely 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 . I t has been used i n the present work f o r c a l i b r a t i o n purposes and w i l l be discussed i n a l a t e r s e c t i o n . The i o n i z a t i o n of atoms and molecules by c o l l i s i o n with 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 of thermal energy has been known f o r many years (1-9). The phenomenon i s important i n the study o f the mechanisms f o r i o n production i n flames, f o r studies 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, p h o t o l y s i s , r a d i o l y s i s and plasmas. The study of 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 important f o r the f o l l o w i n g reasons, (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 of i n t e r a c t i o n (10 se i s s e v e r a l orders of magnitude longer than i n c o l l i s i o n s with e l e c t r o n s — 16 -18 (10 sec.) or photons (10 s e c ) , (3) the r e a c t i o n s u s u a l l y take place between p a r t i c l e s w i t h near thermal k i n e t i c energies and (4) 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 of only one e l e c t r o n . Penning I o n i z a t i o n E l e c t r o n Spectroscopy (PIES) pioneered by Cermak (10), i n v o l v e s the energy a n a l y s i s of the 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 , long l i v e d n e u t r a l p a r t i c l e s (A ) and ground s t a t e atoms or molecules (M), A* + M A + M + + e (1) An a l t e r n a t i v e process, 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 to as a Hornbeck-Molnar process) (4-9) , may a l s o take place i n a c o l l i s i o n between A and M, A* + 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 to form a molecular i o n (AM+) and an e l e c t r o n i s r e l e a s e d . This r e a c t i o n can occur even i f the energy of * the metastable A i s lower than the i o n i z a t i o n p o t e n t i a l of M, f o r example, i n the formation of homonuclear diatomic ions of noble gases. F r a n k l i n (11) has reviewed the a s s o c i a t i v e i o n i z a t i o n process. In the processes of Penning and a s s o c i a t i v e i o n i z a t i o n an e l e c t r o n gains energy from the i n t e r n a l energy or from the k i n e t i c energy of r e l a t i v e motion of two c o l l i d i n g p a r t i c l e s and i n so doing makes a t r a n s i t i o n from a bound s t a t e to a free s t a t e . 1.1.2. O b j e c t i v e s . At the time t h i s study was commenced i n 1968, the PIES work of Cermak et a l . (10, 12-16) was e s s e n t i a l l y the only work of i t s type published. However, the r e s u l t s p ublished by Cermak had u t i l i z e d a c y l i n d r i c a l r e t a r d i n g analyzer of low r e s o l u t i o n (^  0.2 eV or more) with a l l the attendent disadvantages, such as, i n t e g r a l s i g n a l and the problem of "non-normal" e l e c t r o n s . A l s o at t h i s time, workers i n photo-e l e c t r o n spectroscopy had developed high r e s o l u t i o n d e f l e c t i o n analyzers and i t was intended to b u i l d a 127 degree d e f l e c t i o n analyzer to obtain higher 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 s t u d i e s . I n i t i a l workers had hoped that PIES would be another u s e f u l technique f o r measuring i o n i z a t i o n p o t e n t i a l s . However, our own and others' (17-19) experience i n d i c a t e d the existence of an energy s h i f t i n the measured i o n i z a t i o n p o t e n t i a l , i n most cases, which complicates any determination of the absolute value of the i o n i z a t i o n p o t e n t i a l . The 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 -4-t r a n s f e r between atoms and various t a r g e t atoms ("molecules) . I t i s also i n t e r e s t i n g to compare the 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 metastable c o l l i s i o n s (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 of i o n i c s t a t e s , to compare the Franck-Condon Factors f o r the two very d i f f e r e n t types of i o n i z i n g mechanism. ( i i ) To compare the 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 the photon and Penning processes, since one might i n general expect e l e c t r o n i c t r a n s i t i o n moments to d i f f e r f o r the two processes. Such a study has important s i g n i f i c a n c e w i t h regard t o the type and d i s t r i b u t i o n of energy d e p o s i t i o n i n a r c s , plasmas and atmospheres where many of the chemical processes are 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 to use the a v a i l a b l e o p t i c a l data to describe the s t a t e p o p u l a t i o n . The aim was:-(1) to employ PIES to s y s t e m a t i c a l l y study atoms and dia t o m i c s , t r i a t o m i c s and other small polyatomic molecules to provide such data f o r commonly occuring chemical s p e c i e s , s i n c e no other data 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 other than f o r a few diatomic (20) and polyatomic (21) molecules), (2) to compare helium metastable i o n i z a t i o n w i t h p h o t o i o n i z a t i o n (He S84&). (3) to 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 rare gas systems, at higher pressures, such as, 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 processes o c c u r r i n g between a metastable atom and other metastable atoms. As a r e s u l t of t h i s general survey i t was hoped to gain f u r t h e r understanding of 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 , the 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 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 re c e i v e d much a t t e n t i o n i n recent years and much experimental knowledge, concerning s e v e r a l aspects of the processes, has been obtained. This i n f o r m a t i o n i n c l u d e s , (a) the i n i t i a l s t a t e of 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 e l e c t r o n energy s p e c t r a , (c) the competition between the various processes leading to product ions and (d) c r o s s - s e c t i o n s 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 . 1.2.1. I n i t i a l State of A* * There are three types of e x c i t e d s t a t e s of 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 a short l i f e t i m e such as the 2 P s t a t e of helium (0.56 nsec. (22)), ( i i ) A metastable s t a t e , such as the 2^S 3 and 2 S s t a t e s of helium. This type of s t a t e i s the most commonly employed because of the long l i f e t i m e ( f o r example, the l i f e t i m e of * 1 He (2 S) i s 20 msec. (23)). This type of e n e r g e t i c p a r t i c l e has been used i n the work described i n t h i s t h e s i s and Table 1 l i s t s the i o n i z a t i o n , metastable and photon energies r e l e v a n t to the present work. -6-TABLE I Energy l e v e l s f o r rare gas i o n s , atoms and photons Atom He METASTABLES -I.P.(eV) Desig. Energy(eV) PHOTONS o Wavelength(A) Energy(eV) 24.586 Ne ( P , ) 21.564 C ^ / ) 21.661 Ar ( P , ) 15.759 ( 2P ) 15.937 1 2 Kr { P ) 13.999 5 / 2 ( 2 P i ; ) 14.665 2 1S 3 2 S 20.615 19.818 16.795 16.619 11.723 11.548 10.562 9.915 He I He I I Ne I Ne I I Ar I Ar I I Kr I 584.4 303.8 743.7 735.9 462.4 460.7 1066.7 1048.2 932.1 919.8 1235.8 1164.9 21.217 40.811 16.671 16.848 26.813 26.910 11.623 11.823 13.302 13.479 10.032 10.643 Xe ( P . ) 12.130 12 ( 2P . ) 13.436 12 9.447 8.315 Xe I 1469.6 1312.4 8.436 9.447 -7-Although the m a j o r i t y of Penning i o n i z a t i o n s t u d i e s have employed helium metastable atoms, some work has been reported which u t i l i z e d other metastable atoms (18, 24-26). However, the usefulness of other metastable atoms, such as those of krypton and xenon, i s s e v e r e l y l i m i t e d because of t h e i r low energies. ( i i i ) Long l i v e d h i g h l y e x c i t e d s t a t e s , the existence of which was 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 of these 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 equal to the f i r s t i o n i z a t i o n p o t e n t i a l and i t i s probable that these s t a t e s correspond to the e x c i t a t i o n of one e l e c t r o n to Rydberg s t a t e s with large p r i n c i p l e " quantum numbers. Hotop and Niehaus (17) and a l s o Kupryianov (27, 28) have pointed out that the r a d i a t i v e l i f e t i m e of Rydberg s t a t e s increases with p r i n c i p l e 4 5 quantum number n as n . Hotop and Niehaus (17) have reported absolute cross s e c t i o n data f o r c o l l i s i o n s between high l y i n g , long l i v e d atoms and ground s t a t e molecules i n which i o n i z a t i o n of the e x c i t e d atom occurs. For many molecules the i o n i z e d atom was not detected. Subsequently they proposed (18) that i n these cases the i o n i z e d atom and the molecule form a complex which decomposes by an exothermic r e a c t i o n to give the i o n i z e d molecule or fragment ions. 1.2.2. I n t e r p r e t a t i o n i n Terms of P o t e n t i a l Energy Curves. The 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 of p o t e n t i a l energy curves has been discussed by M u l l i k e n (29), 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 that i n the c o l l i s i o n of the metastable -8-* atom A , with the t a r g e t p a r t i c l e M, that the ground s t a t e p o t e n t i a l of the i o n AM + i s a t t r a c t i v e and thus e x h i b i t s a minimum. * * The p o t e n t i a l energy curve or curves of the e x c i t e d (AM) (or A - M i n t e r a c t i o n ) must be constructed and there are three cases which can be * considered. In the f i r s t case the p o t e n t i a l energy curve of (AM) o r d i n a r i l y l i e s everywhere below the curve of the ground s t a t e of AM +. In the second case, shown i n Figure 1, (AM) corresponds to an e l e c t r o n bound to an e x c i t e d AM and the p o t e n t i a l curve of (AM) l i e s above the * + curve of the i o n . T h i r d l y , (AM) corresponds to an e l e c t r o n bound to AM i n a r e p u l s i v e s t a t e . Other cases e x i s t but have not been considered i n d e t a i l . I t i s * p o s s i b l e , f o r example, to consider the case of (AM) i n which two e l e c t r o n s were e x c i t e d . However, t h i s case does not correspond to one e l e c t r o n outside an ion-molecule core. Cases of t h i s nature w i l l u l t i m a t e l y have to be i n c l u d e d i n the i n t e r p r e t a t i o n of Penning and a s s o c i a t i v e i o n i z a t i o n i n terms of the d e t a i l s of the c o r r e l a t i o n of separated atom and u n i t e d atom o r b i t a l s and s t a t e s . For the three cases p r e v i o u s l y mentioned, we should a c t u a l l y take i n t o c o n s i d e r a t i o n the b i n d i n g p r o p e r t i e s of the e x c i t e d e l e c t r o n . However, these p r o p e r t i e s o r d i n a r i l y have a r e l a t i v e l y small e f f e c t on the p o t e n t i a l when compared w i t h the s t a t e of the AM +. This occurs because the e x c i t e d e l e c t r o n , normally, i s s u f f i c i e n t l y f a r from the n u c l e i that i t s bonding forc e i s small (32). I f the o r b i t a l of an e x c i t e d e l e c t r o n has a symmetry d i f f e r e n t from any of the occupied o r b i t a l s of AM + then the bonding or antibonding force i s greater than i f there i s an occupied o r b i t a l o f AM + which has the same symmetry. M u l l i k e n has r e f e r r e d to the case where the symmetry i s d i f f e r e n t as -9-Figure 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 I o n i z a t i o n . -10-'penetrating' and the case where the symmetry i s the same as 'non-p e n e t r a t i n g ' ; the nonpenetrating o r b i t a l s are kept out of the core by the e f f e c t i v e r e p u l s i o n due to 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 of the metastable A i s i n an o r b i t a l which c o r r e l a t e s with a bonding o r b i t a l of AM then the p o t e n t i a l energy curve of AM w i l l be below the curve f o r the corresponding s t a t e of the AM core, whether the core s t a t e i s i n the ground s t a t e of AM + or some higher s t a t e . This 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 only weakly antibonding. 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 antibonding then the * + p o t e n t i a l curve of AM may i n p r i n c i p l e r i s e above that of AM . P a r t s of the £ 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 and thereby become antibonding. Thus i t i s u n l i k e l y that the AM curve r i s e s above that of the corresponding AM + curve because the r e p u l s i v e energy would have to be greater than the i o n i z a t i o n energy of the e x c i t e d o r b i t a l f o r t h i s to occur. M u l l i k e n (33) has considered homonuclear di a t o m i c s , such as * * He + He and has shown how some of the low l y i n g s t a t e s of We^ must have p o t e n t i a l maxima at large i n t e r n u c l e a r d i s t a n c e s . This 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 d i s t a n c e s i n a thermal c o l l i s i o n which i s r e q u i r e d to reach the p o t e n t i a l f o r We^  + e-By comparison, the high e x c i t e d s t a t e s are expected to have p o t e n t i a l + energy curves n e a r l y p a r a l l e l to and s l i g h t l y below the curve f o r He 2 • Herman and Cermak (15, 16, 30) and Niehaus et a l . (18, 19, 31, 32) have s t u d i e d heteronuclear cases. The rare gas-metal atom systems which have been s t u d i e d (16, 19, 31, 32) have p o t e n t i a l energy curves corresponding to the second case ( i . e . the p o t e n t i a l energy curve of AM -11-* l i e s below the curve of (AM) ), because the lowest e x c i t e d s t a t e s of 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 of the a l k a l i s or mercury. E l e c t r o n s which are released i n Penning i o n i z a t i o n may ca r r y away a large amount of energy or they may leave the i o n i z e d system with considerable 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 of mercury 3 by 2 S helium 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 helium 2 S metastable 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 . or 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 to i o n i z a t i o n by He (2 S) conform to the known a v a i l a b l e * 1 st a t e s of 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 (16), using a low r e s o l u t i o n apparatus, * 1 observed that the d i s t r i b u t i o n s corresponding to i o n i z a t i o n by the He (2 S) metastable appear to be s h i f t e d to e l e c t r o n energy values lower than * 3 expected while those due to He (2 S) were at the expected energy; however, t h i s work lacked absolute energy c a l i b r a t i o n . Fuchs and Niehaus (19) observed that the c o n t r i b u t i o n to the spectra due to i o n i z a t i o n by the * 1 He (2 S) metastables i s spread out and e x h i b i t s three peaks, the most intense of which l i e s at the lowest e l e c t r o n energy. Hotop and Niehaus (31) have suggested that t h i s may be due to one or more p o t e n t i a l energy curves which correspond to other e x c i t e d states of a quasimolecule He/Hg and which cross the e x c i t e d p o t e n t i a l curve. I f at a c e r t a i n separation 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 curve to another curve occurs and i f a f t e r a short time a u t o i o n i z a t i o n to the f i n a l p o t e n t i a l curve, at a separation l e s s than before, occurs 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 be explained. Hotop and Niehaus (18) have a l s o attempted to i n t e r p r e t the Penning -12-i o n i z a t i o n of molecular hydrogen. They assume that 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 that they describe as CHe - H 2 ) where the H 2 +^ ^ s v i b r a t i o n a l l y e x c i t e d according to the 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 occurs at large 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. I f the t r a n s i t i o n occurs at smaller distances where i s d i s t o r t e d by the e x c i t e d helium 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 most probable decay mode +1 + + f o r the [He - H 2 ) i s d i r e c t l y i n t o He + H 2 . I f the H 2 i s s u f f i c i e n t l y e x c i t e d v i b r a t i o n a l l y , then the system may go to HeH + + H. I f the H 2 + has 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 HeH 2 + molecule may be formed. ** For the c o l l i s i o n of a h i g h l y e x c i t e d s p e c i e s , such as Ar , with molecular hydrogen the mechanism proposed by Hotop and Niehaus i s described d i f f e r e n t l y , since the dominant species produced i s ArH +. 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 of the t a r g e t p a r t i c l e , was p o s t u l a t e d as the f i r s t step i n the r e a c t i o n . The i o n i z a t i o n was explained i n terms of a ** + c r o s s i n g of the p o t e n t i a l curves of the systems (Ar - H 2) and (Ar + H 2 and e l e c t r o n s l e a v i n g the system c a r r y i n g away very l i t t l e energy. 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 or H - H bond i n A r H 2 + so that decay occurs to give Ar H + + H. The r e l a t i o n s h i p between Penning and a s s o c i a t i v e i o n i z a t i o n may be considered i n terms of p o t e n t i a l energy curves; f o r example, consider metastable helium atoms i n c o l l i s i o n with argon atoms. In t h i s case the e l e c t r o n must c a r r y away a large amount of energy. We should a l s o consider -13-the nature of 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 k i n e t i c energy and the f r a c t i o n of 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 of a s s o c i a t i v e to Penning i o n i z a t i o n i n terms of 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 consider that 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 only i n the f a c t that 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 curves reach above or below the d i s s o c i a t i o n l i m i t of AM+. The r a t i o of a s s o c i a t i v e to Penning i o n i z a t i o n was determined by the f r a c t i o n of c o l l i d i n g p a i r s with 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 give 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 give 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 that the shape of 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 that most t r a n s i t i o n s occur i n the region of the c l a s s i c a l t u r n i n g p o i n t . The t r a n s i t i o n energy and 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 governs the r a t i o of a s s o c i a t i v e to Penning i o n i z a t i o n . 1.2.3. Ejected E l e c t r o n Energy Spectra. The energy a n a l y s i s of e l e c t r o n s from Penning i o n i z a t i o n processes can be used, to a f i r s t approximation as a s p e c t r o s c o p i c probe to l o c a t e states of the molecular ion i f metastable p a r t i c l e s of known energy are used or conversely one may l o c a t e metastable s t a t e s of the p r o j e c t i l e i f the i o n i c s t a t e s of detector molecules are known. The energy of the e j e c t e d e l e c t r o n (E g) i s given by E o = E(A*) - [IP(M +) + E„ , (M +)] + AE (3) -14-* * where E(A ) i s the energy of 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 ), IP(M +) i s the f i r s t or higher i o n i z a t i o n p o t e n t i a l of an atom or molecule (M), E . 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 of 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 various 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 process. I f AE= 0, IP(M +) can be determined by measuring the e l e c t r o n energy E e < The f a c t that AE j- 0, i n many cases, se v e r e l y l i m i t s the use of PIES as an accurate method f o r determining i o n i z a t i o n p o t e n t i a l s . However, u s e f u l i n f o r m a t i o n concerning the character of the c o l l i s i o n as w e l l as 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 populations can be obtained from the data. The m a j o r i t y of the s t u d i e s reported to date, concerning the 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 process, have been made by Cermak and Herman (12 16) and by Niehaus and h i s co-workers (19, 31, 34, 35). In most cases metastable rare gas atoms of known energy have been used. Cermak has a p p l i e d a L o z i e r stopping p o t e n t i a l method to energy analyze e l e c t r o n s released i n Penning i o n i z a t i o n processes at thermal energies. The energy r e s o l u t i o n of t h i s apparatus was r a t h e r low (approximately 0.2 eV) which i n most cases prevented the study 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 molecular v i b r a t i o n s . Cermak has used t h i s apparatus to study the e l e c t r o n i c s t a t e s of approx-imately f o r t y molecular ions. The observed e l e c t r o n d i s t r i b u t i o n s showed that some peaks may be s h i f t e d to lower e l e c t r o n energies, that i s many of 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 the i o n i z a t i o n process. Cermak al s o observed that some processes may give r i s e to ions with excess k i n e t i c energy. I n i t i a l l y i t was b e l i e v e d that Penning i o n i z a t i o n e l e c t r o n spectroscopy would provide another means of a c c u r a t e l y determining i o n i z a t i o n p o t e n t i a l s . I t was suggested (36) that the d i f f e r e n c e s observed i n the 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 to the f a c t that the Penning work was c a r r i e d out at lower r e s o l u t i o n than most photoelectron s t u d i e s . Niehaus and h i s co-workers have used an e l e c t r o s t a t i c lens i n conjunction with a r e t a r d i n g e l e c t r i c f i e l d to measure the energy of the e l e c t r o n s e j e c t e d i n the Penning process. The e l e c t r o n energy analyzer was of considerably higher r e s o l u t i o n (approximately 0.02 eV) than that used by Cermak. 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 et a l . have been able to study rare gas atoms and to measure v i b r a t i o n a l spacings and the populations of a c c e s s i b l e e l e c t r o n i c s t a t e s of the molecular ion s t a t e s of a few diatomic molecules. Niehaus et a l . (19, 31) have found that the e l e c t r o n energy d i s t r i b u t i o n s of e l e c t r o n s belonging to 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 width (greater than photoelectron peak widths) and may be s h i f t e d , with respect to the absolute energy d i f f e r e n c e between the energy of the metastable and the i o n i z a t i o n p o t e n t i a l of the 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) metastables was found to produce d i s t i n c t d i f f e r e n c e s i n the observed shapes and s h i f t s of the d i s t r i b u t i o n s f o r many ta r g e t species. I t was found, f o r argon, that the d i s t r i b u t i o n s showed a marked change with the r e l a t i v e k i n e t i c energy of the c o l l i d i n g p a r t i c l e s , the t r i p l e t d i s t r i b u t i o n s becoming considerably broader at -16-higher energies (temperature), 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) are e s s e n t i a l l y unchanged. The s h i f t s (AE) i n the 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 et a l . vary from +0.055 eV f o r n i t r o g e n (B^£* st a t e ) ( i . e . the energy of the e l e c t r o n i s greater than the d i f f e r e n c e between the energy of the metastable and the i o n i z a t i o n p o t e n t i a l of the t a r g e t ) to 2 -0.100 eV f o r mercury ( P_. s t a t e ) (the energy of the e l e c t r o n i s l e s s than expected}. The measured s h i f t of the maximum of the 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 . ) (at 500°K) i s +0.045 eV and has been explained (31) by assuming that i n most c o l l i s i o n s leading to Penning i o n i z a t i o n , k i n e t i c energy of the n u c l e i i s converted to e l e c t r o n i c energy of the e j e c t e d e l e c t r o n . The r e l a t i v e p r o b a b i l i t i e s f o r i o n i z a t i o n to various e l e c t r o n i c s t a t e s of the molecular i o n are oft e n observed to 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 (13, 15, 35). This may be expected because the e l e c t r o n i c 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 are d i f f e r e n t f o r the two processes and should normally lead to d i f f e r i n g r e l a t i v e p o p u l a t i o n s . The r e l a t i v e populations of v i b r a t i o n a l s t a t e s f o r each e l e c t r o n i c s t a t e are found to be very s i m i l a r f o r the two processes. Niehaus (35) has attempted to e x p l a i n small but d e f i n i t e d i f f e r e n c e s i n the v i b r a t i o n a l populations by a weak d i s t o r t i o n of the molecule by the metastable atom during i o n i z a t i o n . 1.3. A s s o c i a t i v e 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 has f r e q u e n t l y been s t u d i e d i n conjunction with 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 metastable atoms -17-and atoms or molecules. A study by Hotop and Niehaus (37, 38) employing metastable helium atoms with other rare gases as t a r g e t p a r t i c l e s has examined the dependence of the r e l a t i v e cross s e c t i o n s 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 on the a v a i l a b l e c o l l i s i o n energy. I t was found that formation of the a s s o c i a t i v e i o n i z a t i o n product became more important at low c o l l i s i o n energies and that 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 of a s s o c i a t i v e to Penning i o n i z a t i o n from the amount of s h i f t of 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 i n good agreement with the mass s p e c t r o m e t r i c a l l y determined r a t i o . However, t h i s approximation, on the b a s i s of the measured s h i f t , has been found to be v a l i d only f o r the rare gases. Hotop and Niehaus (38) had p r e v i o u s l y reported the r e l a t i v e importance of Penning and a s s o c i a t i v e i o n i z a t i o n i n c o l l i s i o n s of metastable helium and neon atoms with hydrogen and deuterium hydride. The i o n i z a t i o n of many organic molecules by metastable atoms has been reported (12, 39, 40, 41). Products 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 observed only i n the case of acetylene. Cermak has observed that 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 metastable 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 with Penning 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 polyatomic molecule. Only i f the target i s an atom or simple molecule does a s s o c i a t i v e i o n i z a t i o n occur with high p r o b a b i l i t y . I f d i s s o c i a t i o n i s e n e r g e t i c a l l y p o s s i b l e i t appears that a s s o c i a t i v e i o n i z a t i o n occurs with 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 cases, been * found to have comparable cross s e c t i o n s ( i . e . Ar + Hg) (30). A s s o c i a t i v e i o n i z a t i o n has a l s o been observed w i t h molecular e x c i t e d precursors * * (42-44), f o r example, N_ and CO . -18-1.4. R e l a t i v e and Absolute Cross S e c t i o n s . * 1 * 3 The r a t i o of the e x c i t a t i o n cross 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 (37, 45-47) and these r e s u l t s are shown i n Figure 2. Cermak (47) found that the r a t i o was independent of the helium pressure i n the e x c i t a t i o n region and the current of e x c i t i n g e l e c t r o n s . However, the r a t i o w i l l i n general d i f f e r from the r a t i o of the number of s i n g l e t and t r i p l e t metastable atcrns i n the beam because the o r i g i n a l r a t i o o f metastables * 1 * 3 produced i s modified by various 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 cross 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 s c a t t e r i n g , 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 pop u l a t i o n by cascading from higher s i n g l e t and t r i p l e t s t a t e s . A l s o , i f the angular d i s t r i b u t i o n * 1 * 3 of the He (2 S) and He (2 S) atoms formed at the c r o s s i n g p o i n t of the e l e c t r o n and helium beams i s d i f f e r e n t , then the r a t i o o f metastables i n a beam i s angular dependent and only equals the r a t i o of the e x c i t a t i o n cross s e c t i o n s i f a l l metastables produced are c o l l e c t e d . This may lead to d i s c r e p a n c i e s between measurements using a d i r e c t e d helium beam (37, 45, 47) and measurements with d i f f u s e helium gas i n the e x c i t a t i o n chamber (46). The experimental study o f the cross s e c t i o n f o r Penning i o n i z a t i o n 1 3 by 2 S and 2 S metastable helium atoms has been the subject of 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 recent years. 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 complicated by the f a c t that the r e l a t i v e number of s i n g l e t and t r i p l e t metastable atoms, contained i n the beam, has not been a c c u r a t e l y known and i t has been necessary to a r b i t r a r i l y normalize data by assuming that the cross 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 -HOLT et al 2 0 30 4 0 5 0 E L E C T R O N ENERGY (eV) Figure 2. R a t i o of s i n g l e t to t r i p l e t helium metastable atoms e x c i t i n g e l e c t r o n s . as a f u n c t i o n of the energy of the -20-and t r i p l e t metastables f o r a p a r t i c u l a r atom or molecule 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 surfaces has of t e n been employed to measure the f l u x of metastable atoms (45, 46, 48-50) and i t was assumed that the 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 equal. Recently Dunning and Smith (54) have found that t h i s i s not the case and i n f a c t the e j e c t i o n e f f i c i e n c y * 1 * 3 of He (2 S) i s l e s s than that of He (2 S). From t h e i r r e s u l t s , they were able to deduce that the r a t i o of e j e c t i o n e f f i c i e n c i e s i s 6(2 1S)/6(2 3S) = 0.73. Scholette and M u s c h l i t z (49) have used a thermal beam method to measure the cross s e c t i o n f o r the Penning i o n i z a t i o n process. T h e i r method has the advantage that the ta r g e t gas i s i n the ground s t a t e but s u f f e r s from the disadvantage that the s t a t e of the metastable i s not r e a d i l y * 1 determined. Separate cross s e c t i o n s were determined f o r He (2 S) and * 3 He (2 S) by studying the i n e l a s t i c s c a t t e r i n g , as the composition of the metastable beam was v a r i e d , by changing the energy of the e x c i t i n g e l e c t r o n s and thus changing the r e l a t i v e amounts of s i n g l e t and t r i p l e t metastables i n the beam. From the f a c t that the cross s e c t i o n s obtained d i d not depend on the composition of the beam, except f o r hydrogen, they concluded that the cross s e c t i o n s were equal f o r the two metastable s t a t e s . Benton et a l . (51) have used a time r e s o l v e d afterglow technique to moniter the d e n s i t y of helium metastable atoms, with time, 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 of small amounts of the target gas reduces the e f f e c t i v e l i f e t i m e of the metastable atoms due to the i o n i z i n g c o l l i s i o n s . Schmeltekopf and Fehsenfeld (52) have used -21-a f l o w i n g afterglow technique to determine the s i n g l e t and t r i p l e t cross s e c t i o n s f o r a v a r i e t y of t a r g e t s . Using a s i m i l a r flowing afterglow technique Bolden et a l . (55, 56) have measured i o n i z a t i o n * 3 cross s e c t i o n s using He (2 S). The afterglow technique of Benton et a l . has the advantage that the s t a t e of the metastable i s e a s i l y determined but s u f f e r s from the disadvantage that a s i g n i f i c a n t prop-o r t i o n of the t a r g e t species may be v i b r a t i o n a l l y e x c i t e d or d i s s o c i a t e d during the discharge pulse. In the fl o w i n g afterglow technique the st a t e of the metastable i s known e x p l i c i t l y and the reactant gas i s introduced i n the ground s t a t e . However, the method i s r e s t r i c t e d to states connected to lower s t a t e s by o p t i c a l l y allowed t r a n s i t i o n s and may be complicated by cascade pop u l a t i o n of the s t a t e s s t u d i e d . Hotop, Niehaus and Schmeltekopf (37) using an atomic beam i n conjunction with a mass spectrometer, have measured the cross s e c t i o n r a t i o s f o r various s p e c i e s , by helium metastable atoms. They success-f u l l y e l i m i n a t e d the 2*S component from the beam by o p t i c a l l y quenching i t with l i g h t from a He discharge lamp. Because of the high cross s e c t i o n f o r absorption of these photons * 1 by He (2 S) they were able to almost e l i m i n a t e the s i n g l e t component from the beam by the processes: hv(2 XP •> 2*S) + He(2 1S) -> He(2 XP) 1 1 He(2 P) + H e ( I S ) + hv(584 A) (4) The corresponding quenching process f o r t r i p l e t metastables does not * 3 3 occur because the He (2 S ) , i f e x c i t e d to the 2 P s t a t e , returns to 3 * 1 the 2 S s t a t e . With the informat i o n from the mixed beam (He (2 S)) * 3 1 and He (2 S)) and the beam i n which the 2 S component had been e l i m i n a t e d -22-(He (2 S) beam) they were able to c a l c u l a t e i o n i z a t i o n cross s e c t i o n r a t i o s . They considered the r a t i o s they determined were c l o s e to one (range 0.49 to 1.6) thus agreeing 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) using a gas c e l l technique i n conjunction with a quenching lamp, s i m i l a r to that of Hotop et a l . (37), obtained s i m i l a r cross 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 to 1.31) i n the i o n i z a t i o n cross s e c t i o n were considered to be s i g n i f i c a n t . In agreement with Hotop et a l . they found the s i n g l e t cross s e c t i o n s , i n the rare gases, to be l a r g e r than the corresponding t r i p l e t cross s e c t i o n . Benton et a l . (51) and B e l l et a l . (57) have pointed out that t h i s i s to be expected. The absolute values of the cross s e c t i o n s f o r Penning i o n i z a t i o n * 3 by He (2 S) atoms measured by various techniques are i n good agreement. Afterglow measurements give values of the cross 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 wider range of values than those obtained by other methods. I t has been suggested (55) that s u p e r e l a s t i c d e e x c i t a t i o n of the s i n g l e t metastable has not been s u f f i c i e n t l y taken * 1 i n t o account. The cross s e c t i o n f o r the r e a c t i o n of He (2 S) with * 1 e l e c t r o n s i s q u i t e large (58) and may c o n t r i b u t e to the observed 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 give Penning i o n i z a t i o n cross s e c t i o n s * 1 f o r the He (2 S) which are too l a r g e . * 1 The r a t i o of the cross s e c t i o n s f o r the production of He (2 S) * 3 and He (2 S) st a t e s as a f u n c t i o n of the energy of the e x c i t i n g e l e c t r o n s determined by the measurement of the k i n e t i c energy of e l e c t r o n s released i n Penning i o n i z a t i o n (37, 47) i s l a r g e r than that determined i n experiments where secondary e l e c t r o n emission from metal surfaces -23-(46, 47) was used to detect metastable atoms. I f the value obtained 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 of e l e c t r o n s from metal surfaces was a p p l i e d to the r e s u l t s of Dugan et a l . (45) and Holt and Krotkov (46) the r e s u l t s would be i n b e t t e r agreement with those of Cermak (47) and Hotop et a l . (37). In general 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 , of the r a t i o of 2 S and 2 S helium e x c i t a t i o n f u n c t i o n s i s f a i r l y good. 1.5. Photoelectron Spectroscopy. Photoelectron spectroscopy (PES) has been the subject of s e v e r a l recent reviews (36, 59-65), covering d i f f e r e n t aspects of the subject. Extensive c o l l e c t i o n s of data 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 molecular PES s t u d i e s are given i n books by Turner et a l . (66) and a l s o by Baker and Brundle (67). H i s t o r i c a l l y , PES was i n i t i a l l y developed by two independent groups i n the e a r l y 1960's. V i l e s o v et a l . (68-71) used a L o z i e r type apparatus i n conjunction 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 of photoelectrons produced i n i o n i z a t i o n . Al-Joboury and Turner (72-74) employed the undispersed helium resonance o l i n e (Hel 584 A)as the e x c i t a t i o n source i n the measurement of the k i n e t i c energy of photo-ejected e l e c t r o n s - Schoen (75) reported s i m i l a r s t u d i e s employing a windowless Seya-Namioka monochromator. The p r i n c i p l e of the method, the p h o t o e l e c t r i c e f f e c t , was f i r s t given by E i n s t e i n , who p o s t u l a t e d that when a photon has more than s u f f i c i e n t energy to e j e c t an e l e c t r o n from an atom or molecule, the excess energy manifests i t s e l f as k i n e t i c energy of the p a r t i c l e s -24-produced. Thus i f a photon o f energy h y (h = Plancks' constant, v = frequency of r a d i a t i o n ) causes the i o n i z a t i o n of a gaseous atom (molecule), hv + M ->• M + + e (5) the r e s u l t i n g photoelectron w i l l have k i n e t i c energy (E g) given very c l o s e l y by the expression, E g = hv - IP CM) (6) where IP(M) i s the i o n i z a t i o n p o t e n t i a l of the atom or molecule. Conservation of momentum r e s u l t s i n energy p a r t i t i o n between the e l e c t r o n and the ion i n the inverse r a t i o of 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 the photoelectron. I t has been shown that photoelectron spectroscopy overcomes many of the d i f f i c u l t i e s encountered 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 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. I t i s a l s o a p p l i c a b l e to a wide range of substances. I o n i z a t i o n p o t e n t i a l s deter-mined by photoelectron spectroscopy can be compared d i r e c t l y with the 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 the l i m i t a t i o n s imposed by the a u t o i o n i z a t i o n process which i s a frequent complication i n most other methods. Information concerning the nature of the o r b i t a l from which the e l e c t r o n was removed may be obtained from photoelectron s p e c t r a . The number of i o n i c v i b r a t i o n a l l e v e l s populated i s c h a r a c t e r i s t i c of the type of o r b i t a l which the 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. According to the Franck-Condon p r i n c i p l e the i o n i c v i b r a t i o n a l l e v e l most l i k e l y to be populated i s the one which allows the i n t e r n u c l e a r coordinates to approximate those of the n e u t r a l . Other i o n i c v i b r a t i o n a l l e v e l s are populated with lower p r o b a b i l i t i e s governed by the Franck--25-Condon overlap. There are two d i f f e r e n t types of i o n i z a t i o n p o t e n t i a l which may be obtained f o r molecules, namely the a d i a b a t i c and the v e r t i c a l i o n -i z a t i o n p o t e n t i a l s . 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 i s the energy d i f f e r e n c e between the lowest v i b r a t i o n a l l e v e l of the ground s t a t e molecule and the lowest v i b r a t i o n a l l e v e l of the molecular i o n . The v e r t i c a l i o n i z a t i o n p o t e n t i a l corresponds to the most probable i o n i z i n g t r a n s i t i o n , that i s from the lowest v i b r a t i o n a l l e v e l of the ground s t a t e molecule to the v i b r a t i o n a l l e v e l of the molecular ion corresponding to the same i n t e r n u c l e a r d i s t a n c e . For removal of a bonding e l e c t r o n an increase i n i n t e r n u c l e a r s e p aration w i l l occur. The Franck-Condon region w i l l t h e r e f o r e i n t e r s e c t the s t e e p l y r i s i n g r e p u l s i v e p o r t i o n of the i o n p o t e n t i a l energy curve r e s u l t i n g i n e x c i t a t i o n of many v i b r a t i o n a l quanta. When the e l e c t r o n removed i s from an antibonding o r b i t a l 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 istance of the ion i s smaller than f o r the n e u t r a l parent and i n general a slowly r i s i n g a t t r a c t i v e region of the molecular ion p o t e n t i a l curve l i e s i n the Franck-Condon region above the ground s t a t e of the n e u t r a l . In t h i s case r e l a t i v e l y few v i b r a t i o n a l l e v e l s of the ion are l i k e l y to be populated. I f the e l e c t r o n being removed i s nonbonding or very weakly bonding there i s g e n e r a l l y l i t t l e e f f e c t on the e q u i l i b r i u m i n t e r n u c l e a r coordinates and the most probably t r a n s i t i o n i s a d i a b a t i c and the Franck-Condon f a c t o r s f o r higher v i b r a t i o n a l l e v e l s are s m a l l . The v a r i ous 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 3. PIES i s somewhat analogous to PES i n that both methods are based on the measurement of the k i n e t i c energy of e l e c t r o n s which are produced -26-E J E C T E D R Figure 3. A p p l i c a t i o n of Franck-Condon P r i n c i p l e to photoelectron production. -27-by the i o n i z a t i o n o f atoms and molecules. In Penning i o n i z a t i o n the width of the e l e c t r o n energy d i s t r i b u t i o n depends on the e x c i t i n g p a r t i c l e , the t a r g e t and the i o n i c s t a t e i n v e s t i g a t e d w h i le i n PES a very narrow d i s t r i b u t i o n of e l e c t r o n s i s found f o r each d i s c r e t e l e v e l of the atom or molecule under i n v e s t i g a t i o n . E l e c t r o n energy d i s t r i b u t i o n s * 1 * 3 r e s u l t i n g from Penning i o n i z a t i o n by both the He (2 S) and He (2 S) metastables are broad i n comparison to those obtained from photoelectrons. The reason f o r t h i s i s the mechanism by which the i o n i z i n g energy i s t r a n s f e r r e d to the target atom or molecule. In i o n i z a t i o n by metastable atoms the i n t e r a c t i o n of the species before i o n i z a t i o n may be d i f f e r e n t from that a f t e r i o n i z a t i o n and the t r a n s l a t i o n a l energies of products may not n e c e s s a r i l y remain thermal. For some atoms and small molecules, a s s o c i a t i v e i o n i z a t i o n can occur i n p a r a l l e l to Penning i o n i z a t i o n . This does not occur i n photoelectron spectroscopy. In co n t r a s t to PES, PIES i s not a s u i t a b l e technique f o r the determination of accurate i o n i z a t i o n p o t e n t i a l s because there i s oft e n a broad d i s t r i b u t i o n of r e l a t i v e k i n e t i c energy values. 1.6. Franck-Condon P r i n c i p l e T r a n s i t i o n s i n PES between the two corresponding p o t e n t i a l energy curves are described as v e r t i c a l (Franck-Condon) t r a n s i t i o n s . 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 formulated mathematically by Condon (77) may be s t a t e d as f o l l o w s : i f an e l e c t r o n i c t r a n s i t i o n takes place i n a time i n t e r v a l which i s much short e r than that 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 the r e l a t i v e p o s i t i o n and v e l o c i t y of the n u c l e i may be assumed to be unchanged during the t r a n s i t i o n . In e f f e c t we may consider 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 ( 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 c a l c u l a t e d 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 (78-80). In the m a j o r i t y of cases Morse p o t e n t i a l s are used to describe the p o t e n t i a l energy curves of the lower and upper s t a t e s . The c a l c u l a t e d values f o r the f i r s t few v i b r a t i o n a l l e v e l s , where the Morse p o t e n t i a l i s a good approximation to the a c t u a l curve, should be accurate, however the accuracy decreases at higher v i b r a t i o n a l l e v e l s . An attempt to overcome t h i s problem has been made by Dunn (81) who used the known p o t e n t i a l energy curves f o r hydrogen and deuterium i n h i s c a l c u l a t i o n s . F r o s t , McDowell and Vroom (82, 83), using photoelectron spectroscopy, have obtained experimental Franck-Condon f a c t o r s which agree q u i t e w e l l with c a l c u l a t e d v a l u e s ; the experimental values are s l i g h t l y l a r g e r at the higher 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 t h i s t r e n d , f o r molecular hydrogen, i n the f o l l o w i n g manner. The o p t i c a l photo-i o n i z a t i o n c o e f f i c i e n t , f o r molecular hydrogen, was found by Cook and Ching (84) to drop about 15 percent per eV between eighteen and twenty-one eV. Therefore i t i s expected that the p r o b a b i l i t y of i o n i z a t i o n to 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 a l s o f a l l o f f w i t h i n t h i s 0 energy range. Thus f o r a 58 4 A i n c i d e n t photon the higher the v i b r a t i o n a l quantum number of the f i n a l s t a t e , the l e s s w i l l the i o n i z a t i o n to that 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 value. Thus the high v i b r a t i o n a l 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 expected to be more h e a v i l y weighted i n the e l e c t r o n s p e c t r a . Turner (85) has quant-i t a t i v e l y i n t e r p r e t e d 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 p r o b a b i l i t y of approximately 10 percent per eV above t h r e s h o l d . Robertson (86) and Schmeltekopf (87) , using a f l o w i n g afterglow -29-technique, have found that f o r the Penning r e a c t i o n s measured, the p o p u l a t i o n of 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 of the molecular ion are n e a r l y the same as those given by Franck-Condon f a c t o r s f o r unperturbed molecular s t a t e s . Using e l e c t r o n spectroscopy Hotop and Niehaus (35) have compared Franck-Condon f a c t o r s obtained by Penning i o n i z a t i o n r e a c t i o n s using * 3 He (2 S) atoms with those obtained by photoelectron spectroscopy and with c a l c u l a t e d values. The d i f f e r e n c e s they observe from photoelectron r e s u l t s , while not l a r g e , are outside t h e i r s t a t e d experimental e r r o r . However, the Franck-Condon f a c t o r s are more d i f f i c u l t to obtain from the Penning spe c t r a than from the photoelectron s p e c t r a due to the g r e a t e r width of the Penning d i s t r i b u t i o n s and the large background, due to 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 . Hotop and Niehaus have explained 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 to the r e l a t i v e p o s i t i o n s of the i o n i c and n e u t r a l molecular p o t e n t i a l curves and i f i t i s assumed that the r e l a t i v e p o s i t i o n s are 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 to i n t e r a c t i o n w i t h the p r o j e c t i l e i n the case of Penning i o n i z a t i o n , then the observed e f f e c t can be explained. Nevertheless Penning i o n i z a t i o n seems to 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 to date. 1.7. Angular D i s t r i b u t i o n s of Photoelectrons and Penning E l e c t r o n s . The measurement of angular d i s t r i b u t i o n s of the photoelectrons ejected from various o r b i t a l s has been the subject of 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-angular 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 the rare gases, a few diatomics and polyatomics as w e l l as cadmium and z i n c atoms. The i o n i z a t i o n of argon by a beam of helium metastable atoms i s the only system f o r which the angular d i s t r i b u t i o n has been reported f o r Penning el e c t r o n s (102). T h e o r e t i c a l p r e d i c t i o n s of the form of the angular d i s t r i b u t i o n s f o r photoelectrons are 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 angular d i s t r i b u t i o n has the general form, I (6) = ( o t o t / 4 T r ) (1 + 3P 2 (cos 6)) (7) 2 where P 2(cos 9) = 1/2(3 cos 6 - 1 ) , a t Q t represents the t o t a l cross s e c t i o n , 6 measures the angle between the d i r e c t i o n of the ejected e l e c t r o n and the p o l a r i z a t i o n of the 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 (which i s u s u a l l y the case i n photoelectron spectroscopy) the expression becomes. I (6) = O t o t / 4 ^ ) (1 - B/ 2 ( 3/ 2 s i n 2 0 - 1)) (8) where 6 measures the angle between the d i r e c t i o n of 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 experimentally that expression (7) i s v a l i d and other studies (88, 91, 94, 96) have confirmed 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 concerning the form of the angular d i s t r i b u t i o n have been made. The beam of metastables, i n contrast to the photon beam, does not c o n s t i t u t e an ax i s with respect to which an angular d i s t r i b u t i o n of e l e c t r o n s can be defined. Experimentally Niehaus et a l . (96, 102) have found that the photoelectron i n t e n s i t y i s symmetric with -31-respect to 90 degrees ( i . e . p e r p e n d i c u l a r to the photon beam) but that f o r Penning e l e c t r o n s a pronounced asymmetry with respect to 90 degrees i s observed, with an i n t e n s i t y enhancement i n the backward d i r e c t i o n . This enhancement i n the backward d i r e c t i o n corresponds to i n t e n s i t y enhancement i n the d i r e c t i o n of the metastable beam ( i . e . at angles greater than 90 degrees). The extent of the asymmetry * seems to i n d i c a t e that there i s considerable alignment of the He - Ar system at the moment of i o n i z a t i o n . 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 to be due to d i f f e r e n c e s regarding the mean s p a t i a l alignment of the system or p o s s i b l y with a considerable c o n t r i b u t i o n from the non-exchange part of the matrix elements governing the Penning process * 1 i n the He (2 S) case. Niehaus et a l . (102) a l s o observed that the measurement of Penning el e c t r o n s perpendicular to the metastable beam d i r e c t i o n , w i l l give approximately c o r r e c t i n t e n s i t y r a t i o s of the t o t a l cross s e c t i o n s f o r formation of 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 of the ta r g e t 1 3 species by the 2 S and 2 S sta t e s of helium. 1.8. A u t o i o n i z a t i o n . The removal of an e l e c t r o n from an atom or molecule by a process 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 the 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 as a two step process. F i r s t , the i n i t i a l e x c i t a t i o n of an e l e c t r o n occurs i n t o a d i s c r e t e s t a t e above the i o n i z a t i o n p o t e n t i a l of the species 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 takes place from the bound sta t e i n t o the a c c e s s i b l e i o n i z a t i o n continuum. The emitted e l e c t r o n -32-w i l l have energy equal to the d i f f e r e n c e between the energy of the bound sta t e and the i o n i z a t i o n p o t e n t i a l . A u t o i o n i z a t i o n i s a resonant process (fo r photoabsorption) and w i l l only occur at energies corresponding to the d i s c r e t e s t a t e i n t o which the 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 photoabsorption and p h o t o i o n i z a t i o n experiments. The process has a l s o been used to 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 process has r e c e i v e d extensive 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 of the process has been included i n t h i s t h e s i s since Penning i o n i z a t i o n has been regarded as an auto-i o n i z a t i o n process i n some t h e o r e t i c a l treatments. -33-CHAPTER TWO THEORETICAL DISCUSSION There have been many experimental i n v e s t i g a t i o n s of Penning i o n i z a t i o n and the cross s e c t i o n s f o r the r e a c t i o n s between various atoms and molecules have been measured. T h e o r e t i c a l s t u d i e s , on the other hand, have been confined, u n t i l r e c e n t l y , to those o f p a r t i c l e i o n i z a t i o n by processes i n v o l v i n g o p t i c a l l y allowed t r a n s i t i o n s . Though there have been a few t h e o r e t i c a l papers d e a l i n g with the Penning i o n i z a t i o n process, there i s p r e s e n t l y no completely s a t i s -f a c t o r y theory. In t h i s chapter a survey of the theory r e l a t i n g to Penning i o n i z a t i o n , and to a l e s s e r extent a s s o c i a t i v e i o n i z a t i o n , w i l l be reviewed. 2.1. Weak I n t e r a c t i o n Theories. At the l e v e l of a se m i q u a n t i t a t i v e theory two models have been used to t r e a t Penning i o n i z a t i o n . The f i r s t i s the simple ' c r i t i c a l r a d i u s ' model, the second i s based on the d i p o l a r exchange of a quantum from one c o l l i s i o n a l partner to the other. In the c r i t i c a l r adius model, a c h a r a c t e r i s t i c distance f o r the p a r t i c u l a r system i s constructed and the assumption i s made that 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 of atoms with helium metastable atoms on the c r i t i c a l r adius method as s o c i a t e d with the momentum t r a n s f e r cross s e c t i o n . Gas k i n e t i c c o l l i s i o n cross s e c t i o n s - 3 4 -were compared to Penning r e a c t i o n cross s e c t i o n s f o r s e v e r a l r e a c t a n t s . C o l l i s i o n cross 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 only a t t r a c t i v e p o t e n t i a l s of the van der Waals type, between the metastable helium atoms and the other reactants and i t was noted that the experimental Penning i o n i z a t i o n r e a c t i o n cross s e c t i o n s were only a few tenths of the c a l c u l a t e d values. Bates et a l . (105) a t t r i b u t e d to Ferguson the concept that long 3 range a t t r a c t i o n s r e s u l t i n clo s e c o l l i s i o n s of metastable helium (2 S) with other reactants i n Penning i o n i z a t i o n . I t i s supposed that the long range forces 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 appropriate impact parameters that b r i n g the reactants i n t o close c o l l i s i o n where the Penning r e a c t i o n s occur. 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 to that of Ferguson, however, the values f o r c o l l i s i o n 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 are three or four times as large as those c a l c u l a t e d by Ferguson. They were not able to 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 values f o r the van der Waals i n t e r a c t i o n between metastable helium atoms and other atoms and molecules. I t was assumed that 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 the c e n t r i f u g a l b a r r i e r must lead to p o s s i b l e r e a c t i o n s . Experimental data was then used to estimate the p r o b a b i l i t y of e l e c t r o n loss when the system i s i n clo s e c o l l i s i o n . The r a t e c o e f f i c i e n t s f o r close c o l l i s i o n were al s o c a l c u l a t e d . The cross s e c t i o n s and p r o b a b i l i t i e s of r e a c t i o n per c o l l i s i o n obtained are about the same order of magnitude as those of Bates et a l . (105). Jones and Robertson (106) have shown that the use of 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 acceptable approximation except at extremely low temperatures. -35-Katsuura, Watanabe and Mori (107-112) have developed a more elaborate treatment which i s e s s e n t i a l l y an impact parameter treatment. The process i s described as two simultaneous o p t i c a l t r a n s i t i o n s , A -»- A and M - M + + e. The coupling operator i s the product of two e l e c t r i c d i p o l e operators and the t r a n s i t i o n p r o b a b i l i t y . The cross 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 are fu n c t i o n s of the t r a n s i t i o n d i p o l e matrix elements squared. The wave f u n c t i o n i s expressed i n terms of the stat e s corresponding to A + M and A + M + 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 with a given impact parameter. The equation f o r cross sec 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 to give Penning i o n i z a t i o n cross s e c t i o n s f o r c o l l i s i o n s at a p a r t i c u l a r v e l o c i t y , assuming that the 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 represented i n a manner s i m i l a r to that of 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. The weak i n t e r a c t i o n model of Katsuura (107) i s l i m i t e d i n that i t i s only 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 to a lower s t a t e . 1 - 3 -I t cannot be used, f o r example, to t r e a t the 2 § and 2 S s t a t e s o f helium. The cross s e c t i o n s which t h i s method p r e d i c t s are r a t h e r l a r g e . Watanabe and Katsuura. (108) have developed a method which takes i n t o account the imperfect coupling of angular momentum to the i n t e r -nuclear a x i s . The cross s e c t i o n s Obtained Using t h i s method are almost one quarter smaller than those obtained from the e a r l i e r theory. I t was a l s o Shown that the assumption of p e r f e c t angular momentum Coupling, during the c o l l i s i o n §§t§ an upper bound en the gross section» Mori (109) has considered the process i n terms of the perturbed s t a t i o n a r y s t a t e method•, employing a formalism which was developed by -36-Fano (113), f o r the problem of a u t o i o n i z a t i o n i n atoms. There i s some ambiguity i n h i s treatment with respect to the separation between i o n i z a t i o n and e l a s t i c s c a t t e r i n g amplitudes. Nakamura (111) has extended Fano's formalism to a molecular system which has a f i x e d nuclear s e p a r a t i o n . He a l s o defined a l o c a l complex p o t e n t i a l f o r the resonance s t a t e . The cross s e c t i o n was c a l c u l a t e d i n terms of the decay p r o b a b i l i t y of t h i s resonance s t a t e . This cross s e c t i o n i s more s u i t a b l e f o r p r a c t i c a l a p p l i c a t i o n s . M i l l e r (114) has developed a theory of Penning i o n i z a t i o n s i m i l a r to that of Nakamura. He has, however, developed the a n a l y s i s f u r t h e r to show how a s s o c i a t i v e i o n i z a t i o n f i t s i n t o the theory and a l s o looked 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 quantum t h e o r i e s . F u j i i et a l . (115) have quantum mechanically 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 of hydrogen atoms by metastable helium atoms. The helium atoms are t r e a t e d on the b a s i s of the l o c a l complex p o t e n t i a l method and complex p o t e n t i a l s are estimated by the valence bond method. The t o t a l cross 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 with the experimental r e s u l t s reported by Shaw et a l . (56). M i l l e r and Schaefer (116) have a l s o t r e a t e d t h i s system. T h e i r p o t e n t i a l curves f o r the Penning i o n i z a t i o n of the hydrogen atom by helium metastables were computed by a large 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 introduced a s i m p l i f i e d d e s c r i p t i o n of the i o n i z i n g c o l l i s i o n so that approximate cross s e c t i o n s could be determined. Sheldon (117) has c a l c u l a t e d a number of cross s e c t i o n s f o r i o n i z a t i o n of a l k a l i atoms by e x c i t e d rare gas atoms i n the f i r s t ^P s t a t e using a quantum defect method. -37-2.2. Close Coupled Near-Adiabatic Theories. To extend a weak i n t e r a c t i o n theory i t i s necessary to consider the way i n which the e x c i t e d metastable atom and the ta 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 . There are two extremes to co n s i d e r , the f i r s t 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 w i t h F o u r i e r components whose frequencies may be s i m i l a r to or higher than the c h a r a c t e r i s t i c frequencies of 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 metastable and ta r g e t p a r t i c l e s . This behaviour i s not 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 other process to consider i s that the c o l l i s i o n times are s u f f i c i e n t l y long so that the Born-Oppen-heimer approximation or the a d i a b a t i c model holds f o r the 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 c o n s i d e r i n g the n e a r - a d i a b a t i c treatment have been developed to various 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 , 119), f o r d i s s o c i a t i v e recombination (120) and f o r molecular a u t o i o n i z a t i o n (121-123). For n e a r - a d i a b a t i c treatments, the i n i t i a l and f i n a l s t a t e s of the wave f u n c t i o n are represented as products of e l e c t r o n i c and v i b r a t i o n a l or 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 f u n c t i o n s are computed f o r a l l the bound and free 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 of i n t e r e s t , i t i s p o s s i b l e to consider a large v a r i e t y of processes from a common b a s i s , the major d i s t i n c t i o n b e i n g , which states are bound and which are not. The processes which may be consid-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 , a s s o c i a t i v e 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 or 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 discussed the -38-a p p l i c a t i o n of the theory to molecular hydrogen. However f o r molecules more complex than t h i s the theory i s s t i l l not q u a n t i t a t i v e . Bardsley (120) has considered 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 point of view of the formation of a resonance s t a t e . The product of the cross s e c t i o n f o r formation of t h i s resonance s t a t e and 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 gives the cross s e c t i o n f o r a s s o c i a t i v e i o n i z a t i o n . For systems i n which sp e c t r o s c o p i c data i s a v a i l a b l e , the method may be r e a d i l y a p p l i e d . 2.3 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 be w r i t t e n s y m b o l i c a l l y as f o l l o w s : A * ( l ) + M(2) •> A(2) + M + + e ( l ) (9) where (1) and (2) c h a r a c t e r i z e the two e l e c t r o n s i n v o l v e d , namely the e x c i t e d e l e c t r o n o f the metastable p r o j e c t i l e ( 1 ) , and one of the ground s t a t e e l e c t r o n s of the t a r g e t (2). The t r a n s i t i o n i s viewed by them as a t u n n e l l i n g of e l e c t r o n (2) followed by an Auger emission of e l e c t r o n (1). T h e i r proposed model i s i n close analogy to the theory of Auger emission of e l e c t r o n s from metal surfaces by metastables, as given by Hagstrum (124). They proposed the exchange mechanism to account f o r the f o l l o w i n g experimental observations. (1) The cross 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 that cross 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 approximately 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 of e l e c t r o n i c s t a t e s are d i f f e r e n t f o r the Penning and p h o t o i o n i z a t i o n process. (3) The measured absolute cross s e c t i o n f o r Penning i o n i z a t i o n of * 1 * 3 sodium by He (2 S) and He (2 S) metastables showed that t h i s value agreed, w i t h i n the l i m i t s of e r r o r , w i t h a value c a l c u l a t e d from an approximate formula d e r i v e d under the 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 process. 2.4. Theory of 127 degree E l e c t r o s t a t i c 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 of e l e c t r o n s . The f a c t that an inverse f i r s t power e l e c t r o s t a t i c f i e l d generated between c o a x i a l c y l i n d e r s could have focusing 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 pointed out by Hughes and Rojansky (125), who der i v e d the optimum fo c u s i n g p r o p e r t i e s of such an analyzer. At the same time, the t h e o r e t i c a l p r e d i c t i o n s were experimentally v e r i f i e d by Hughes and McMillen (126). For a given e l e c t r i c f i e l d s trength 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 of given v e l o c i t y e n t e r i n g the f i e l d p a r a l l e l to the surface of 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 the k i n e t i c energy E i s given by (126). E = V 2 l n (b/a) ( v 2 - Vj) (10) where a and b are the r a d i i of the inner and outer 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 are the p o t e n t i a l s a p p l i e d to the inner and outer c y l i n d e r s r e s p e c t i v e l y (y\v = v 2 - v ^ . Hughes and Rojansky (125) have shown that two e l e c t r o n s , having a common v e l o c i t y and en t e r i n g the f i e l d at the same p o i n t , but at a n g l e s i G w i l l have o r b i t s , to a f i r s t approximation, o f , -40-y = C + (1 - C) cos /2~(J) - (tan 0 s i n /2<t>) /T (11) y 9 = C + (1 - C) cos /2~<j> + (tan 0 s i n /2<t>) /2 where <)> i s the angle at which the e l e c t r o n s are refocussed and C i s a parameter introduced to allow the s o l u t i o n of the d i f f e r e n t i a l equation of the o r b i t s . Thus the e l e c t r o n s are refocussed ( i . e . the p o s i t i o n where the o r b i t s cross f o r the f i r s t time) when y^ = y^ and the l a s t term i n each expression (11) vanishes. Therefore /2*<J> = T T and § = 127° 17' and t h i s angle i s independent of <j>. They a l s o showed that two e l e c t r o n s e n t e r i n g the f i e l d at the same angle but with d i f f e r e n t v e l o c i t i e s would a t t a i n a maximum separation at 127° 17'. Thus at t h i s angle the best r e s o l u t i o n and r e f o c u s s i n g are obtained. The t h e o r e t i c a l r e s o l u t i o n of the 127 degree s e l e c t o r has been discussed by Leventhal and North (127) and a l s o by Peresse (128). I t has been found, by Cermak and Ozenne (21) that the approximate energy r e s o l u t i o n of the s e l e c t o r may be c a l c u l a t e d from the equation: where E i s the k i n e t i c energy of the e l e c t r o n , s^ and s ? are the entrance and e x i t s l i t widths, r Q i s the mean e l e c t r o n t r a j e c t o r y and a i s the h a l f angle of acceptance. The width AE i s defined as the f u l l width h a l f maximum (FWHM) of the e l e c t r o n energy d i s t r i b u t i o n . (12) -41-CHAPTER THREE INSTRUMENTATION 3.1. Experimental Arrangement. A diagram of the experimental arrangement used i n t h i s study, f o r the measurement of the k i n e t i c energy of 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 Figure 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 region provided a beam of metastable atoms to i o n i z e the sample under i n v e s t i g a t i o n . Metastable helium atoms were formed i n the 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 (normally 170 eV, t o t a l f i lament emission a. 20 ma) i n c i d e n t on commercial tank helium introduced through a multichannel d i s c (glass c a p i l l a r y , fused a r r a y , pore diameter 0.05 mm, thic k n e s s 0.5 mm). The helium beam ( f i n e c o n t r o l maintained by means of 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 leak valve) entered the e x c i t a t i o n chamber at an angle of 45 degrees i n an attempt to minimize the e f f e c t s of momentum t r a n s f e r from the e x c i t i n g e l e c t r o n s . E l e c t r o n s were produced from a d i r e c t l y heated tungsten filament 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 (3.45 mm x 0.51 mm) i n t o the e x c i t a t i o n chamber. E l e c t r o n s and negative ions were prevented from l e a v i n g the e x c i t a t i o n chamber by b i a s i n g the ion and e l e c t r o n t r a p chamber negative (22.5 v o l t s ) with respect to the e x c i t a t i o n chamber. The e x c i t a t i o n and al s o the e l e c t r o n and ion trap chambers were c a v i t i e s i n brass blocks of i n t e r n a l dimensions 1.27 cm x 1.27 cm x 1.27 cm and EXCITATION C H A M B E R 127° ANABYSER G A S E L E C T R O N A N D I O N T R A P 2000 Ips C O L L I S I O N C H A M B E R H E X T E R N A L P H O T O N ' L A M P D I F F E R E N T I A L P U M P I N G T A R G E T G A S end view i F igure 4. Schematic diagram of Penning i o n i z a t i o n e l e c t r o n spectrometer. -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 other by s e v e r a l mica sheets. The beam of p a r t i c l e s which emerged from the e x c i t a t i o n region passed through two apertures (6.35 mm diameter) covered by tungsten 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 the beam by applying a p o s i t i v e p o t e n t i a l (with respect to the e x c i t a t i o n chamber) to the d e f l e c t i n g p l a t e s P^, P 2 which are e x t e r n a l l y connected. 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 the block by means of boron n i t r i d e i n s u l a t o r s . 3.1.2. C o l l i s i o n Region. The beam entered the c o l l i s i o n chamber, having passed through another aperture G^, al s o covered by tungsten mesh and since the distance between the e x c i t a t i o n chamber and the centre of the c o l l i s i o n region was 7.62 cm, the beam contained e s s e n t i a l l y ground s t a t e ( l ^ S ) 1 3 and metastable (2 S and 2 S) s t a t e helium atoms. A small f l u x of o H e l , 584 A r a d i a t i o n was a l s o present i n the beam due to r a d i a t i o n produced by e l e c t r o n bombardment. (This i s subsequently r e f e r r e d to as the ' i n t e r n a l ' l i g h t source). Throughout t h i s t h e s i s reference i s made to the helium 'beam', however, i t i s probably at best only a quasi-beam due to the pressures i n v o l v e d i n s i d e the spectrometer c o l l i s i o n r egion. The c o l l i s i o n chamber (10.16 cm x 2.54 cm diameter) was attached 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 the c o l l i s i o n chamber s l i t p l a t e and the entrance s l i t p l a t e of the analyzer to prevent s t r a y e l e c t r o n s from g a i n i n g access to the entrance s l i t of the analyzer. -44-Th e e x c i t a t i o n chamber, e l e c t r o n and ion t r a p and a l s o the c o l l i s i o n chamber were gold p l a t e d and then the i n t e r n a l surfaces covered with a l a y e r of benzene soot i n order to reduce the s c a t t e r e d e l e c t r o n background due to Auger type processes which occured when metastable atoms struck the w a l l s of the apparatus. 3.1.3 E l e c t r o n Analyzer and Detection System. A 127 degree e l e c t r o n analyzer was chosen si n c e i t was a r e l a t i v e l y easy device to construct and l e s s s u s c e p t i b l e t o spurious magnetic f i e l d s than s p h e r i c a l analyzers. Helmholtz c o i l s were o r i g i n a l l y i n s t a l l e d on the instrument to provide a magnetic f i e l d f r e e environment but i t was found that optimum performance was obtained w i t h n e g l i g i b l e f i e l d c o r r e c t i o n . The use of s l i t s ( i n s t e a d of c i r c u l a r apertures used i n s p h e r i c a l analyzers) permitted the use of a much l a r g e r sampling area and hence greater i n t e n s i t y . The analyzer t r a n s m i t t e d a s u f f i c i e n t l y intense beam of e l e c t r o n s which could be e a s i l y measured and was capable of s u f f i c i e n t r e s o l u t i o n such that v i b r a t i o n a l l e v e l s were e a s i l y r e s o l v e d . The use of a d e f l e c t i o n analyzer a l s o had the advantage that 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 obtained and t h i s was a p a r t i c u l a r advantage i n spectroscopic work where assignment of energy l e v e l s was made. The analyzer was constructed of gold p l a t e d brass w i t h e l e c t r o d e s of 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 of radius 25.0 mm. The electr o d e s and end p l a t e s were i n s u l a t e d and loc a t e d by boron n i t r i d e spacers. The entrance and e x i t s l i t p l a t e s of the analyzer were mounted d i r e c t l y to the end p l a t e s ( s l i t dimensions, 5 mm x 0.5 mm). The height of the analyzer (8.64 cm) minimized the problem of end e f f e c t s (129). -45-A f r a c t i o n of 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 metastable atoms and t a r g e t gas leave the c o l l i s i o n chamber and enter the analyzer. The analyzer was normally set to transmit e l e c t r o n s of about 2 eV energy and the spectrum was 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 the entrance s l i t p l a t e of the analyzer. This mode of scanning ensured that the r e s o l u t i o n (AE/E) remained constant throughout a scan. The scanning p o t e n t i a l was obtained by a m p l i f i c a t i o n of a four v o l t ramp o r i g i n a t i n g from a multichannel analyzer or a l t e r n a t i v e l y a motor driv e n h e l i p o t could be used to vary the r e t a r d i n g p o t e n t i a l . A d i g i t a l voltmeter was used to measure the scanning p o t e n t i a l . E l e c t r o n s which passed through the analyzer 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 (having an 8 mm diameter entrance cone to increase c o l l e c t i o n e f f i c i e n c y ) . The e l e c t r o n m u l t i p l i e r was w e l l s h i e l d e d to exclude spurious e l e c t r o n c u r r e n t s . The m u l t i p l i e r output pulses were monitored by means of 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 pulse counting system, comprised of an a m p l i f i e r / d i s c r i m i n a t o r and a l i n e a r ratemeter both mounted i n a standard NIM b i n (which a l s o provided the p r e a m p l i f i e r power). The output from the ratemeter was d i s p l a y e d on a s t r i p chart recorder ( f o r s e t t i n g up procedures) or d i r e c t e d to the input of a F a b r i t e k Instruments Model 1064 multichannel analyzer (4096 channels), operated i n the s i g n a l d i g i t i z e mode ( f o r data storage and averaging). Counting time per scan f o r each channel was u s u a l l y 0.5 sec. In general only one quadrant (1024 channels) of the memory was used to s t o r e a spectrum. While the s i g n a l averaged output was normally recorded on a Moseley X-Y p l o t t e r , -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 to output s p e c t r a onto magnetic tape f o r processing 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 to provide the absolute 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 pressure microwave discharge i n helium, 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 spectrometry by Fros t and McDowell (82, 130). In the present a p p l i c a t i o n i t provided o a l i n e emission spectrum c o n s i s t i n g e s s e n t i a l l y of 584 A (21.217 eV) r a d i a t i o n . This 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 to i o n i z e a l l gases except neon and helium. An Edwards needle valve c o n t r o l l e d the flow of commercial tank helium i n t o a quartz tube at a pressure of approximately 1 t o r r . The pressure i n the discharge region was s t a b i l i z e d by a c o n s t r i c t i o n at the f a r end of the quartz tube and t h i s a l s o served to f a c i l i t a t e d i f f e r e n t i a l pumping of the helium, between the l i g h t source and the c o l l i s i o n chamber. The discharge was produced using a resonant c a v i t y powered by an E l e c t r o Medical 'Microtron 200' 2450 MHz microwave generator. The s i l v e r p l a t e d microwave c a v i t y was constructed of brass and was a modified form (132, 133) of that described by Z e l i k o f f et a l . (134). The discharge was i n i t i a t e d w i t h a T e s l a c o i l and the quartz discharge tube cooled with compressed a i r . The f i n a l p o r t i o n of the source c o n s i s t e d of a glass c a p i l l a r y which t r a n s m i t t e d a narrow beam of l i g h t and a l s o served to reduce the helium flow 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 of three r e g i o n s , the sample handling r e g i o n , the high vacuum system and the l i g h t source r e g i o n . The sample handling system, of 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 of 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 leak. The i n l e t l i n e was constructed so that interchangeable f i t t i n g s could be used to accomodate e i t h e r a storage v e s s e l ( f i t t e d w i t h a B-10 socket) , a gas c y l i n d e r or a standard l e c t u r e b o t t l e . The l e c t u r e b o t t l e was connected to the i n l e t l i n e by a metal l i n e , s i l v e r soldered to a Matheson s t a i n l e s s s t e e l needle v a l v e . A four l i t r e brass r e s e r v o i r was used to maintain a steady sample pressure throughout a run. Veeco bellows valves were used to i s o l a t e each s e c t i o n of the sample handling system to allow independent evacuation. 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 leak valve was used to admit the sample to the c o l l i s i o n chamber. A mechanical pump, pumping speed 21 l i t r e / m i n , was used to evacuate the system. The high vacuum i n the spectrometer region was maintained 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 traps each backed by mechanical 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 spectrometer was constructed of 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 of the pumping system, from the main chamber, was accomplished by N.R.C. High Vacuum S l i d e Valves, Type 1283-6. The pressure i n the vacuum chamber was measured with a N.R.C. Bayard-Alpert i o n i z a t i o n guage. With t h i s arrangement the r e s i d u a l pressure 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 close to the end of the c a p i l l a r y f o r evacuation o f helium, 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 with a dry-i c e cooled trap and backed by a mechanical pump (pumping speed, 140 l i t r e / m i n ) . The Penning i o n i z a t i o n spectrometer r e s i d u a l vacuum _7 increased to about 3 x 10 t o r r during l i g h t source o p e r a t i o n . 3.2. Spectrpmeter Performance. The r e s o l u t i o n of the 127 degree e l e c t r o n analyzer was determined by photoelectron spectroscopy. Figure 5 shows a t y p i c a l s i n g l e scan o of the He(584 A) photoelectron spectrum of argon obtained w i t h the ext e r n a l l i g h t source. The ^P,, peak of A r + has a f u l l width at 6 1 2 h a l f maximum (FWHM) of 0.022 eV. The analyzer performance was i n good agreement with c a l c u l a t i o n s (equation 12) based on the geometry of the analyzer. Figure 6 shows a s i n g l e scan o f the photo e l e c t r o n spectrum of molecular hydrogen at lower r e s o l u t i o n . R e l a t i v e i n t e n s i t i e s are i n good agreement with other photoelectron work. For most of the work reported i n t h i s t h e s i s the energy r e s o l u t i o n was t y p i c a l l y set at 0.04 - 0.06 eV FWHM i n order to obt a i n increased s e n s i t i v i t y . There was l i t t l e advantage i n using higher r e s o l u t i o n since i n most cases the e l e c t r o n energy d i s t r i b u t i o n s obtained f o r Penning e l e c t r o n s are n a t u r a l l y much broader than those obtained f o r photoelectrons. In Penning i o n i z a t i o n the width of the d i s t r i b u t i o n s obtained depended not only on the i n c i d e n t p r o j e c t i l e but a l s o on the r e l a t i v e k i n e t i c energy of c o l l i s i o n . However, i n photoelectron -49-Ar+ 5 8 4 A -*^Ar ++e Q 1 7 8 eV o Figure 5. High r e s o l u t i o n photoelectron spectrum (584 A) of argon. o Figure 6. Photoelectron spectrum (584 A) of molecular hydrogen. -51-spectroscopy the narrower d i s t r i b u t i o n of e l e c t r o n s was determined by instrumental f a c t o r s (apart from the Doppler broadening due t o thermal motion of t h ^ t a r g e t gas). Figure 7 shows a comparison of the photoelectron spectrum of argon obtained using the r a d i a t i o n from the e x t e r n a l photon lamp with t h a t o obtained 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 ( i n t e r n a l l i g h t source). These spe c t r a were obtained w i t h a l l c o n d i t i o n s set f o r the simultaneous use of the instrument to obtain Penning s p e c t r a . The 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 large angular 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 , while using the i n t e r n a l photon source. The r e l a t i v e l y l a r ge apertures (5 mm diameter), along the metastable path, together with the source 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 analyzer over 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 to an a value of about eight degrees by the annular spacing of the e l e c t r o s t a t i c analyzer. The angular term c o n t r i b u t e d about 0.025 eV (equation 12) to the r e s o l u t i o n at an e l e c t r o n energy of ^ 2 eV. The Penning sp e c t r a were recorded under these c o n d i t i o n s of acceptance angle (a = 8°). On the other hand the narrow e x t e r n a l photon beam (1 mm diameter) subtends an a of about two degrees which under normal operating 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 (equation 12) to the r e s o l u t i o n obtained. The pressure dependence of the Penning and photoelectron s i g n a l f o r the processes He*(2 3S) + Ar -> He + A r + ( 2 P . ) + e 0 +2 and hv(584 A) + Ar -> Ar ( P , ; ) + e are shown i n Figure 8. 6 / 2 The a b s c i s s a gives the e q u i l i b r i u m pressure of argon i n the apparatus -52-- i 1 1 l _ 0.1 0 0.1 Q 2 UNCORRECTED ELECTRON ENERGY (eV) o Figure 7. Photoelectron s p e c t r a of argon at 584 A using i n t e r n a l and e x t e r n a l photon lamps. Figure 8. Pressure dependence of the Penning and photoelectron s i g n a l as a f u n c t i o n of t a r g e t gas pressure. -54-Fnr t h i s study a f i x e d helium pressure was used f o r the production of the metastable beam. I t was observed that f o r sample pressures above 8 x 10 ^ t o r r the r e s o l u t i o n of the spectrometer decreased. The * 3 + 2 pressure dependence of He (2 S) + Ar ->- He + Ar ( P_, ) + e was a l s o studied as a f u n c t i o n of the metastable beam pressure (Figure 9). The ab s c i s s a i n Figure 9 gives the t o t a l pressure of argon and helium i n the apparatus. I t was observed that f o r t o t a l pressures 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 pressures of about 4 x 10 ^ t o r r were used. Molecular gases were Matheson CP grade or P h i l l i p s '66' research grade and were stored i n commercial l e c t u r e b o t t l e s . L i q u i d s of reagent or spectroscopic grade were degassed by freeze-thaw c y c l e s . The back-ground pressure from helium used i n forming the metastable beam was about 1.5 x 10 ^ t o r r and the t o t a l pressure during a run, was approximately 2.0 x 10 t o r r . The i n t e r p r e t a t i o n of the Penning e l e c t r o n s p e c t r a was complicated i n some cases due to the f a c t that the s p e c t r a produced by the two * 1 metastable s t a t e s overlap. Attempts were made to e l i m i n a t e the He (2 S) component from the beam by o p t i c a l quenching techniques. I t has been shown (37, 54, 135) that 2.06 micron photons can be used to couple the 2^S atoms to the 2^P s t a t e from which decay to the ground s t a t e occurs o with emission of He(584 A) r a d i a t i o n . Several unsucessful attempts were made to quench the 2^S s t a t e by ( i ) surrounding the beam with a quartz glass s p i r a l , helium discharge lamp s i m i l a r i n design to that of Hotop et a l . (37), ( i i ) a q u a r t z , oval shaped, helium discharge lamp employing tungsten e l e c t r o d e s , ( i i i ) a microwave discharge lamp s i m i l a r to that -55-Figure 9. Pressure dependence of the Penning s i g n a l as a f u n c t i o n of helium pressure. -56-described by Fry and Williams (135), and ( i v ) a S y l v a n i a "Sun Gun", which u t i l i z e s a q u a r t z - i o d i n e lamp. The most intense source of 2 micron 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". However, * 1 i n no case was measureable quenching of the He (2 S) s t a t e observed. I t may be that the tank helium used was of i n s u f f i c i e n t p u r i t y and re q u i r e d f u r t h e r clean-up. 3.3. C a l i b r a t i o n of Energy Scale and P r e s e n t a t i o n of Data. Photoelectron energy s c a l e s were c a l i b r a t e d using l i t e r a t u r e values of i o n i z a t i o n p o t e n t i a l s from photoelectron spectroscopy. Simultaneous recording of Penning and photoelectron s p e c t r a , i n the present work, provided accurate c a l i b r a t i o n of the Penning e l e c t r o n energy s c a l e and the determination of energy s h i f t values. The accuracy of the energy scale was ± 0.01 eV. For a l l molecules examined i n t h i s study the f o l l o w i n g types of spectra were obtained: (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 metastable helium atoms (He (2 S) and He (2 S ) ) . (b) For the purpose of c a l i b r a t i o n of the Penning e l e c t r o n spectrum, the Penning and photoelectron s p e c t r a were recorded s i m u l t -aneously. When p o s s i b l e s e v e r a l photoelectron peaks were used to 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 r i s i n g 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 from the corresponding photoelectron peak. -57-(c) A pure photoelectron spectrum was a l s o obtained using the ex t e r n a l photon lamp. For each molecule t h i s provided a convenient check of the energy r e s o l u t i o n of the instrument and the p u r i t y of the sample. o The He 584 A photoelectron s p e c t r a of the atoms and molecules 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 various groups i n recent years. These spe c t r a and t h e i r assignments may be found i n the l i t e r a t u r e . In order that 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 could be determined and compared with those reported elsewhere, i t was necessary to apply a c o r r e c t i o n to the measured peak h e i g h t s . 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 the c o l l i s i o n r e g ion and the analyzer i n order to 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 to produce a spectrum. The c o l l e c t i o n e f f i c i e n c y of e l e c t r o n s v a r i e d with e l e c t r o n energy due to changing e l e c t r o n o p t i c a l lens e f f e c t s between the entrance and e x i t s l i t s . This i s analogous to chromatic a b e r r a t i o n i n o p t i c a l spectroscopy. I t should be noted that i n the vast m a j o r i t y of published photoelectron s p e c t r a no attempt has been made to 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 analyzer of Hotop and Niehaus has been reported (35) to have constant t r a n s m i s s i o n . The i n t e g r a t e d 90 degree photoelectron band i n t e n s i t i e s obtained i n t h i s work have been compared with the work of Hotop and Niehaus (35) f o r molecular n i t r o g e n and carbon monoxide and a l s o f o r n i t r i c oxide (136), i n order to de r i v e a 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 T. Figure 10 shows the transm 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 the analyzer used i n the present work. I t may be seen that Figure 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 of 12 eV there i s a considerable change i n the 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 to use t h i s c o r r e c t i o n f a c t o r would have introduced serious 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 the energy range 1 - 7 eV. In independent experiments employing e l e c t r o n - e l e c t r o n coincidence techniques van der Wiel and Brion (139) have determined the r e l a t i v e i o n i c s t a t e populations f o r the photoion-i z a t i o n o f carbon monoxide and found e x c e l l e n t agreement with the work of Hotop and Niehaus (35) as w e l l as the t o t a l c o l l e c t i o n experiments of Vroom f138). -60-CHAPTER FOUR  RESULTS 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 of s i n g l e t and o t r i p l e t metastable helium atoms and He(584 A) photons on argon i s shown i n Figure 11. The photoelectron spectrum produced by the ' i n t e r n a l ' 584 A r a d i a t i o n from the metastable source was of 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 of the energy s c a l e . Due to 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 of argon e x h i b i t s two peaks corresponding to the ^P , and ^P,, s t a t e s of the A r + i o n . Therefore a doublet was observed 5'2 '2 f o r each i o n i z a t i o n process. Spectroscopic values (139) f o r the i o n i z a t i o n p o t e n t i a l s were used to 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 to s i n g l e t r a t i o f o r helium metastables i n the beam, as d e r i v e d from the s p e c t r a i n Figure 11 exceeded the value expected (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 due i n part to the d i f f e r e n c e i n angular d i s t r i b u t i o n (102) of e l e c t r o n s from i o n i z a t i o n by the two metastable helium s t a t e s . Hotop and Niehaus (102) have reported angular d i s t r i b u t i o n s of Penning e l e c t r o n s f o r argon ——•• ••—= 1 : 1 l _ 5.5 5.0 4.5 4.0 ELECTRON ENERGY (eV) Figure 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 of argon. -62-and observed the d i s t r i b u t i o n to be asymmetric, favouring large angles between metastable beam and d etector d i r e c t i o n and d i f f e r e n t 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 metastable helium atoms. Unfort-unately t h e i r r e s u l t s were normalized to a r e l a t i v e i n t e n s i t y of one at a detector-metastable beam angle of 90 degrees so that a p p l i c a t i o n of t h e i r r e s u l t s to t h i s work was not p o s s i b l e . The apparently anomalous t r i p l e t to s i n g l e t r a t i o could a l s o be p a r t l y due to s u p e r e l a s t i c * 1 * 3 c o l l i s i o n s between e l e c t r o n s and He (2 S) atoms to produce He (2 S) (52, 140). Peak shapes are compared i n d e t a i l i n Figure 12 and i t i s apparent o that the width of the peaks, due to i o n i z a t i o n by i n t e r n a l He(584 A) * 1 photons and He (2 S) atoms are of s i m i l a r magnitude whereas the peaks * 3 due to i o n i z a t i o n by He (2 S) are considerably broader (by 0.025 eV) . Hotop and Niehaus (31) have a l s o reported s i m i l a r d i f f e r e n c e s between s i n g l e t and t r i p l e t e l e c t r o n d i s t r i b u t i o n s . They observed a n a t u r a l * 1 width of ^ 0.035 eV f o r the peaks due to i o n i z a t i o n by He (2 S). F o l d i n g t h i s n a t u r a l width i n t o the instrumental r e s o l u t i o n (0.080 eV) r e s u l t s i n a c a l c u l a t e d h a l f w i d t h of 0.087 eV, i n good agreement w i t h the * 1 observed width of 0.085 eV f o r i o n i z a t i o n by He (2 S) as shown i n Figure 12. The d i f f e r e n c e i n peak shapes f o r t r i p l e t and s i n g l e t i o n i z a t i o n can be a t t r i b u t e d to 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 1 3 and p o t e n t i a l energy i n t e r a c t i o n curves f o r 2 S and 2 S helium metastable * atoms with argon. In the case of He + H, M i l l e r and Schaefer (116) * 3 c a l c u l a t e d 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 than f o r He (2 S) + H. For argon a s i m i l a r e f f e c t may occur due to the f a c t that a broader e l e c t r o n d i s t r i b u t i o n i s observed i n the t r i p l e t case. This -63-J L 0.2 0.1 0 0.1 0.2 U N C O R R E C T E D E L E C T R O N ENERGY (eV) Figure 12. Comparison of peak shapes f o r i o n i z a t i o n of argon. -64-d i f f e r e n c e i n p o t e n t i a l energy curves i s al s o r e f l e c t e d i n the observed * 3 r a t i o s f o r a s s o c i a t i v e to Penning i o n i z a t i o n f o r He (2 S) + Ar and * 1 He (2 S) + Ar as a f u n c t i o n of temperature (37, 141). Hotop and co-workers (31, 37, 142 143) have observed that the t r i p l e t peaks are broadened on the high energy s i d e w i t h increase i n temperature (and t h e r e f o r e i n c r e a s i n g k i n e t i c energy of the c o l l i d i n g atoms). A s i m i l a r e f f e c t has been observed (144), a t t r i b u t e d to momentum t r a n s f e r (31, 145, 146) from the e x c i t i n g e l e c t r o n beam, i n that an increase i n the e l e c t r o n e x c i t i n g voltage was observed to broaden the t r i p l e t d i s t r i b -u t i o n s on the high energy s i d e w h i l e the 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. A d d i t i o n a l s t r u c t u r e observed by Hotop and Niehaus (31) on the high energy side of the e l e c t r o n d i s t r i b u t i o n s has not been found i n any of the s p e c t r a reported here. The presence o f d i f f e r e n t thermal groups of helium metastable atoms i n the beam may be the cause of such f i n e s t r u c t u r e on the e l e c t r o n d i s t r i b u t i o n s . The d i f f e r e n c e i n the measured Penning e l e c t r o n energy E G ^ from the nominal energy E q (where E ^ i s the d i f f e r e n c e between the e x c i t a t i o n energy of the metastable atom and the i o n i z a t i o n p o t e n t i a l of the ta r g e t species) i s defined as A E , that i s AE = E ^ - E q . S u b s c r i p t s are used to i n d i c a t e 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 metastable atoms, A E S and A E r e s p e c t i v e l y . For argon the measured e l e c t r o n energy s h i f t i s 0.044 eV f o r both 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 with other reported work (21, 31, 37). Results are shown i n Table I I . The measured r a t i o s of the i n t e n s i t i e s o f the two st a t e s of argon 2 2 ( P„, and P, , ) are i n good agreement with p h o t o i o n i z a t i o n values *'2 '2 determined i n t h i s work and with those reported by Samson and Cairns -65-TABLE 11 S h i f t s , AE, i n Penning E l e c t r o n Energies f o r Rare Gases (eV) . Species 3 1 2 S I o n i z a t i o n , AE^ 2 S I o n i z a t i o n , AE t s This work Ref. 31 This work Ref. 31 A r + +0.044 +0.04l a +0.044 +0.046 K r + +0.035 - +0.030 +0.025 Xe + +0.029 +0.032 a +0.005 +0.005 Values i n t h i s work are ± 0.005 eV. a These values have been estimated from the p r i n c i p a l maximum i n Figure 7 of Reference 31. -66-(147). Results are shown i n Table I I I and compared with l i t e r a t u r e values. No c o n t r i b u t i o n due to 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 long l i v e d Rydberg s t a t e s (10) was observed i n t h i s study. 4.1.2. Krypton The e l e c t r o n s p e c t r a obtained from krypton i o n i z a t i o n are shown i n Figure 13. The upper t r a c e was obtained by the simultaneous use of o the Penning beam and the e x t e r n a l photon lamp (He (584 A) r a d i a t i o n ) . The photoelectron peaks were used to c a l i b r a t e the energy s c a l e . The use of the e x t e r n a l photon lamp gives r i s e to peaks which are much more intense and with 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 the i n t e r n a l photon source. The lower t r a c e , "pure" Penning i o n i z a t i o n , shows only a small 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 to 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 together w i t h the ^P?/ / P-. / i n t e n s i t y 6/2 '2 r a t i o s i n Tables I I and I I I . Although the krypton spectrum i s somewhat complicated by the overlap of e l e c t r o n bands, i t was observed, as was the case f o r argon, that peaks i n the spectrum a r i s i n g from i o n i z a t i o n by t r i p l e t metastable atoms are broader, by approximately 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 metastables. 4.1.3. Xenon. The e l e c t r o n s p e c t r a obtained f o r xenon are shown i n Figure 14. As was the case f o r krypton, the upper tr a c e was obtained by the simultaneous use of the metastable beam and the e x t e r n a l photon source, TABLE I I I Peak Ratios t 2 p v 2 / 2 p i / 2 ' f o r Rare Gas I o n i z a t i o n . P h o t o i o n i z a t i o n Penning I o n i z a t i o n Species He (584 A) 21S This work Ref. 147 2 3S Argon 1.94 1.98 2.00 1.94 Krypton 1.78 1.79 1.8 a 1.8 a Xenon 1.56 1.60 1 .34 2.03 Experimental Values i n t h i s work are-0.05 a 2 These values are only approximate due to overlap of the P 1 , and '2 2 and P_. s t a t e s 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 r e s p e c t i v e l y . '2 Also because of the i n t e n s i t y of the 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 to the P_ , s t a t e formed by s i n g l e t ' 2 i o n i z a t i o n . CO z LL! h -z l±J > LLI Kr + 584A -*-Kr++e 7.5 K R Y P T O N Kr+ He*(2 'S)H^Kr + +He + e Kr+He*(2 3S) -*-Kr++ He + e '2 ON 0 0 I 7.0 6.5 6.0 5.5 E L E C T R O N E N E R G Y (eV) 5.0 Figure 13. 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 krypton. — ' 1 L _ _ 1 1 I I 9-0 8.0 7.0 6.0 ELECTRON ENERGY (eV) Figure 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 of xenon. -70-i n order to c a l i b r a t e the energy s c a l e . The lower t r a c e obtained using only the metastable source e x h i b i t s a very small c o n t r i b u t i o n o from photoelectrons due to 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 only a small c o r r e c t i o n due to overlapping peaks. I t may be seen, from Figure 15, that peaks a r i s i n g from i o n i z a t i o n * 3 * 1 by He (2 S) are broader than those due to He (2 S) i o n i z a t i o n , again by approximately 0.025 eV. This peak broadening observed f o r t r i p l e t i o n i z a t i o n of a l l the rare gases examined, i s of the order of the thermal energies of the helium atoms (148) and may r e f l e c t a d i f f e r e n c e i n the mechanism of s i n g l e t and t r i p l e t Penning i o n i z a t i o n . The e l e c t r o n energy s h i f t s (AE) are given i n Table I I and are qu i t e s m a l l , as has been p r e v i o u s l y reported (31). 2 2 I t may be seen from Table I I I that the P . / P . r a t i o s f o r the •5/2  l'2 Penning i o n i z a t i o n of xenon d i f f e r s i g n i f i c a n t l y from those observed f o r p h o t o i o n i z a t i o n . The r a t i o s observed were found to be r e p r o d u c i b l e under a wide v a r i e t y of source c o n d i t i o n s , over a long p e r i o d of time and w i t h d i f f e r e n t xenon samples. The examination of the s p e c t r a of molecules with 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 that the anomalous r a t i o s are not due to i r r e g u l a r t r a n s m i s s i o n problems i n the e l e c t r o n spectrometer. I t i s b e l i e v e d that t h i s phenomenon i s due to some i n t r i n s i c property of the cross 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 process. A small u n i d e n t i f i e d peak at approximately 6.85 eV c o n s i s t e n t l y appeared i n the xenon spectrum. -71-I I I I l _ 0.2 0.1 O O.I 0.2 UNCORRECTED ELECTRON ENERGY (eV) Figure 15. Comparison of peak shapes f o r i o n i z a t i o n of xenon. -72-4.2. Diatomic Molecules. 4.2.1. Molecular Hydrogen, Deuterium Hydride and Molecular Deuterium. The e l e c t r o n s p e c t r a obtained from the i o n i z a t i o n of H,,, HD and D 2 are shown i n Figures 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 of the three molecules may be w r i t t e n as (66) There i s only one molecular o r b i t a l , which must be s t r o n g l y bonding. The s p e c t r a of these molecules are s i n g l e bands c o n s i s t i n g of a long s e r i e s of w e l l r e s o l v e d v i b r a t i o n a l components with the v i b r a t i o n a l spacing decreasing 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 with removal of an e l e c t r o n from a s t r o n g l y bonding o r b i t a l (35, 83). The energy sc a l e s were c a l i b r a t e d using 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 given i n Table IV. A l s o i n Table 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 peaks, f o r both 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 , of H^, HD and U^. The value of AE f o r H^ reported by Hotop and Niehaus (34, 35) i s a l s o given. A l l peaks are s h i f t e d toward higher e l e c t r o n energies, as was observed by Hotop and Niehaus f o r AE f o r H^. This i n d i c a t e s that some of the k i n e t i c energy of the c o l l i d i n g p a r t i c l e s has been converted i n t o energy of the ejected e l e c t r o n . In contrast to the rare gases i t was observed that the width of the peaks due to i o n i z a t i o n by s i n g l e t metastables are c o n s i d e r a b l y broader than those due to t r i p l e t i o n i z a t i o n . This presumably i s i n d i c a t i v e of a d i f f e r i n g character f o r the p o t e n t i a l energy curves f o r the i n t e r a c t i o n . Although the Penning e l e c t r o n d i s t r i b u t i o n s are H 2, HD, D 2 -73-2 3 S V = 0 I I I 1 1 I I I I M i l l ' (c) 5 8 4 A I . I . L_ 6.0 5.0 4.0 ELECTRON ENERGY (eV) Figure 16. E l e c t r o n spectra f o r i o n i z a t i o n of molecular hydrogen. -74-5.0 4.0 3D 2.0 ELECTRON ENERGY (eV) Figure 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 of deuterium hydride. -75-2 3 S v=o ! I M I I I I M l 1 I : i i , L _ 6.0 5.0 4.0 3.0 E L E C T R O N ENERGY (eV) Figure 18. 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 molecular deuterium. TABLE IV S h i f t s , AE, i n Penning E l e c t r o n Energies f o r Hydrogen, Deuterium Hydride and Deuterium (eV) •z 1 2 S I o n i z a t i o n , AE 2 S I o n i z a t i o n , AE s Species I.P.(149) This work L i t e r a t u r e This work L i t e r a t u r e H 2 + 15.427 +0.060±0.015 +0.070 (35) +0.090±0.020 +0.09010.010(34) ^ I HD+ 15.46 +0.060±0.020 - +0.070±0.025 D 2 + 15.46 +0.05010.015 - +0.070+0.020 -77-d i s t i n c t l y broader than the corresponding photoelectron d i s t r i b u t i o n s , 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 sp e c t r a i s c l e a r l y v i s i b l e . * 1 * 3 However, due to the overlap o f the He (2 S) and He (2 S) s p e c t r a and the r i s i n g background, the determination of the v i b r a t i o n a l spacing i s l e s s p r e c i s e than that f o r the photoelectron spectrum. The v i b r a t -i o n a l spacing f o r the Penning e l e c t r o n spectrum of H^ i s shown i n d e t a i l i n Figure 19.^ In Tables V, VI and VII the v i b r a t i o n a l spacings measured f o r Penning and p h o t o i o n i z a t i o n leading to the X^E* s t a t e s of , HD+ and are compared with c a l c u l a t e d spacings (79) and values reported by other workers (35, 83). The v i b r a t i o n a l spacings are found to be independent of the means of i o n i z a t i o n , w i t h i n experimental e r r o r , i n d i c a t i n g that there was no e f f e c t i v e complex formation or nu c l e a r p e r t u r b a t i o n o f the molecule during 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 could not be measured due to the overlap of 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 of i • the Penning spectrum. 4.2.2. Molecular Nitrogen. Figure 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 obtained f o r . The photoelectron spectrum e x h i b i t s the w e l l known (35, 66, 83, 149) i o n i c 2 + 2 1 + 0 s t a t e s of 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) 6 ^ g ' u u r a d i a t i o n . 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 + This spectrum was obtained i n the c l o s i n g phase of t h i s work where a considerable improvement i n metastable i n t e n s i t y was achieved by m o d i f i c a t i o n s to the e l e c t r o n gun c o n d i t i o n s . Figure 19. Penning e l e c t r o n spectrum of molecular hydrogen. TABLE V V i b r a t i o n a l Spacings (meV) f o r (X^Z + ) . o V i b r a t i o n a l C a l c . P h o t o i o n i z a t i o n , He(584 A) I n t e r v a l (79) Ref.83 Ref.35 This work 0- 1 269 270±10 272± 3 269+ 8 1- 2 254 250±10 258± 5 2521 8 2- 3 239 240±10 240± 5 2431 8 3- 4 223 230±10 228± 8 2251 8 4- 5 208 210±10 215±10 212+ 8 5- 6 193 200±10 198110 199+ 8 6- 7 177 190110 185110 1821 8 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 * Penning I o n i z a t i o n , He 2 lS 2 3S Ref.35 This work Ref.35 This work 272115 249115 275110 255+10 240110 230115 215+15 268110 255110 239110 225H0 213115 200115 174120 -80-TABLE VI V i b r a t i o n a l Spacings (meV) f o r H D + ( X ^ I + ) . V i b r a t i o n a l Calc. I n t e r v a l (79) P h o t o i o n i z a t i o n o He(584 A) Ref.83 This work Penning I o n i z a t i o n He 21S This work 3 2°S 0- 1 1- 2 2- 3 3- 4 4- 5 5- 6 6- 7 7- 8 8- 9 9- 10 10- 11 11- 12 12- 13 13- 14 235 224 212 201 189 178 166 154 143 131 120 108 97 85 230±10 230±10 220±10 200±10 200±10 190110 170110 160+10 150110 2351 8 2191 8 214+ 8 197+ 8 1941 8 1821 8 169110 160+10 151110 141+10 129115 114115 107120 90+20 235120 229120 233115 225110 210110 201110 196+15 178115 161120 -81-TABLE VII V i b r a t i o n a l Spacings (meV) f o r D„+(X^E + ) V i b r a t i o n a l C a l c . P h o t o i o n i z a t i o n Penning I o n i z a t i o n o * I n t e r v a l (79) He(584 A) He , This work Ref.83 This work 2*S 2 3S 0- 1 194 200±10 193± 8 a a 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 a Unable to measure due to overlap. -82-, 1 i I , I I I 1 L_ 5 D 4.0 3 D 2.0 I.O ELECTRON ENERGY (eV) Figure 20. 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 molecular n i t r o g e n . -83-of the n i t r o g e n molecule can be w r i t t e n as (66): N 2 K K ( a g 2 s ) 2 ( a ^ s ) 2 ( T T ^ P ) 4 O g 2 p ) 2 , V where the terms KK represent the closed inner s h e l l s . The shape of the f i r s t band (X 2Z + ) i s c h a r a c t e r i s t i c of removal of a n e a r l y nonbonding 2 e l e c t r o n ( a g 2 p ) . The shape of the second band (A 11^ ) and the length of v i b r a t i o n a l s e r i e s confirms the removal of a s t r o n g l y bonding e l e c t r o n ( ^ p ) • T n e t h i r d band ( B 2 E u + ) i s s i m i l a r i n shape to that of the f i r s t band and i s c o n s i s t e n t with the l o s s of a nonbonding e l e c t r o n ( a ^ s ) . The energy sc a l e s were c a l i b r a t e d using the i o n i z a t i o n p o t e n t i a l values given i n Table V I I I . The Penning spectrum shows those * 1 * 3 stat e s of molecular n i t r o g e n a c c e s s i b l e using He (2 S) and He (2 S) metastable atoms. Figure 20 a l s o i l l u s t r a t e s a phenomenon observed f o r many of the molecules i n t h i s study, that i s , the observed r e l a t i v e p opulation of 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 the Penning e l e c t r o n spectrum when compared to the photoelectron spectrum. A p p l i c a t i o n of the tr 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 (Table IX) that the r a t i o o f 2 + 2 3 the X E s t a t e and A II s t a t e (X/A) i s greater i n the Penning (2 S) spectrum than i n the photoelectron spectrum. A very l a r g e d i f f e r e n c e also occurs f o r the 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 that the d i f f e r e n c e s observed i n the r e l a t i v e populations of the e l e c t r o n i c s t a t e s are due to 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 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 . As was p r e v i o u s l y s t a t e d , Hotop and Niehaus (102) have found the angular d i s t r i b u t i o n of Penning e l e c t r o n s f o r argon to 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 f o r n i t r o g e n . McGowan 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 the photoelectron angular TABLE V I I I S h i f t s , + AE, i n Penning E l e c t r o n Energies f o r Nitrogen (eV). N2 State I.P. •(149) 3 2 S I o n i z a t i o n , This work AE t Ref.35 2"*S I o n i z a t i o n , AE s This work Ref.35 x 2i + g 15. .576 +0.047±0.010 +0. .05010.010 +0.02210.010 +0.01510.010 A 2n u 16, .693 +0.04910.015 +0, .05010.010 -B 2E + u 18, .745 - +0. .05310.010 +0.00210.015 +0.01010.010 TABLE IX R e l a t i v e Populations of E l e c t r o n i c States (at v = 0) f o r Nitrogen. (Corrected f o r Transmission) N 9 + P h o t o i o n i z a t i o n Penning I o n i z a t i o n State He(584 A) 2 1 S 2 3S X 2Z + 100 100 100 g A 2n 38 - 19 u B 2£ + 18 80 u -85-d i s t r i b u t i o n s observed f o r the X^Z + and A^n s t a t e s of molecular g u n i t r o g e n , that f o r the A s t a t e being more i s o t r o p i c . Berkowitz et a l . (88), on the other hand, had p r e v i o u s l y s t u d i e d the angular d i s t r i b u t i o n of photoelectrons from molecular n i t r o g e n and carbon monoxide and had concluded that no c o r r e c t i o n f a c t o r was necessary. I f the angular d i s t r i b u t i o n of e l e c t r o n s from the 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 the Penning and photoelectron 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 to be a p p l i e d to the sp e c t r a . Further angular 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 determining the reason f o r 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 p o p u l a t i o n . The measured e l e c t r o n energy s h i f t s of the Penning e l e c t r o n peaks i n the spectrum are compared with values reported elsewhere (35) i n Table V I I I . The r e s u l t s are i n good agreement with the values reported bv Hotop and Niehaus (35) and as i n the case of hydrogen the peaks are s h i f t e d to higher e l e c t r o n energies i n d i c a t i n g that some of the k i n e t i c energy of the c o l l i d i n g p a r t i c l e s has been converted i n t o energy of the eje c t e d e l e c t r o n . The Penning e l e c t r o n peaks are d i s t i n c t l y broader than those observed f o r the corresponding photoelectron d i s t r i b u t i o n . However, 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 v i s i b l e but as before the determination of v i b r a t i o n a l spacings 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 observed, f o r the X s t a t e , that the peaks due to s i n g l e t i o n i z a t i o n are broader than those due to t r i p l e t i o n i z a t i o n . Table X compares the v i b r a t i o n a l spacings found f o r Penning and p h o t o i o n i z a t i o n f o r the X 2E + , A ^ i ^ and B^Z^ + s t a t e s of N ? with values obtained by other workers. The agreement i s good TABLE X 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 States of N E l e c t r o n i c State Spec, and I n t e r v a l (150) P h o t o i o n i z a t i o n o He (584 A) Ref.35 This work Penning I o n i z a t i o n , He 21S 3 2 S Ref.35 This work Ref.35 This work X 2E + A2!! B 2E + u 0-1 270 270± 5 269± 8 270±20 275±10 270±10 1-2 266 260±10 0-1 232 230± 5 235± 6 230±10 232±10 1-2 229 230± 5 228± 6 225±10 217±15 2-3 225 228± 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± 5 290±15 -1-2 290± 5 2-3 290±10 -87-and the v i b r a t i o n a l spacings are found to be independent of the means of i o n i z a t i o n , w i t h i n experimental e r r o r . 4.2.3. Carbon Monoxide The e l e c t r o n s p e c t r a obtained f o r carbon monoxide are shown i n Figure 21. The features o f the s p e c t r a o f carbon monoxide are very s i m i l a r to those of the i s o e l e c t r o n i c molecular n i t r o g e n . Because s i m i l a r o r b i t a l s are 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 the same i o n i c s t a t e s (X^£ +, A^n and B^E*) the same d e s c r i p t i o n o f the molecular o r b i t a l s and band shapes used f o r may be a p p l i e d to C0 +. The energy s c a l e was c a l i b r a t e d using the i o n i z a t i o n p o t e n t i a l data given i n Table XI. Due to the r a p i d l y r i s i n g background only the X and A stat e s were observed i n the Penning e l e c t r o n spectrum. A comparison of Figures 20 and 21 showed that there are both 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 the r e l a t i v e p opulations 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 molecular n i t r o g e n . As was observed f o r n i t r o g e n , a f t e r a p p l i c a t i o n of the v a r i a b l 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 , the r a t i o o f the X s t a t e and A s t a t e (X/A) i s gr e a t e r (see Table XII) i n the Penning e l e c t r o n spectrum than i n the photoelectron spectrum. Again 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 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 the two processes may c o n t r i b u t e to the observed d i f f e r e n c e . The r a t i o of t r i p l e t to 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 greater f o r carbon monoxide than f o r molecular n i t r o g e n . 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 angular d i s t r i b u t i o n s not only 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 but al s o f o r d i f f e r e n t molecules of the same s t a t e . -88-—I •——I . 1 i I , i . i 7.0 6.0 5.0 4.0 3.0 2.0 ELECTRON ENERGY (eV) Figure 21. E l e c t r o n spectra f o r i o n i z a t i o n of carbon monoxide. TABLE XI S h i f t s , AE, i n Penning E l e c t r o n Energies f o r Carbon Monoxide (eV). C(T State I.P.(149) 2 S I o n i z a t i o n , AE This work t Ref.35 2 S I o n i z a t i o n , AE This work xV A2n 14. 013 +0, ,052±0, 010 +0, .04510, .010 -0.00810.010 16, 537 +0, ,040±0, 020 +0, .03510, 015 -BV 19. ,671 _ _ Ref.35 -0.02010.010 +0.04010.020 TABLE XII R e l a t i v e Populations of E l e c t r o n i c States (at v = 0) f o r Carbon Monoxide (Corrected f o r transmission) CO State xV A 2n BV P h o t o i o n i z a t i o n o He (584 A) 100 14 17 Penning I o n i z a t i o n 2 lS 100 11 3 2 S 100 7 -90-As was observed f o r other molecules, peaks due to i o n i z a t i o n by t r i p l e t metastable atoms are narrower than those due to s i n g l e t i o n i z a t i o n 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 to 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 the p o t e n t i a l energy i n t e r a c t i o n curves. Table XI compares the measured e l e c t r o n energy s h i f t s of the Penning e l e c t r o n peaks of carbon monoxide with values reported by Hotop and Niehaus (35). The r e s u l t s are again i n good agreement. In c o n t r a s t to previous atoms and molecules, the energy s h i f t observed f o r s i n g l e t i o n i z a t i o n of the X s t a t e i s to lower e l e c t r o n energy, i n d i c a t i n g that some of the energy that the e j e c t e d e l e c t r o n normally c a r r i e s away has been r e t a i n e d by the c o l l i d i n g p a r t i c l e s or l o s t i n some other manner. Other peaks i n the Penning spectrum are s h i f t e d to higher e l e c t r o n energies i n d i c a t i n g that some of the k i n e t i c energy of the c o l l i d i n g p a r t i c l e s has been converted i n t o energy of the eje 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 the i o n i c s t a t e s obtained by removal of an e l e c t r o n are very s i m i l a r f o r the i s o e l e c t r o n i c molecules carbon monoxide and n i t r o g e n , no c o r r e l -a t i o n of the Penning e l e c t r o n energy s h i f t s can be made. Table X I I I gives the v i b r a t i o n a l spacings observed f o r Penning 2 + 2 and photoelectrons f o r the X £ and A n s t a t e s of carbon monoxide and compares the values obtained w i t h those reported elsewhere. As has been observed f o r other molecules, the r e s u l t s are 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 of the method of i o n i z a t i o n . TABLE X I I I 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 States of CO E l e c t r o n i c 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 o (151) He(584 A) Ref.35 This work Penning I o n i z a t i o n , He 2 lS 3 2°S Ref.35 This work Ref.35 This work xV A 2n 0-1 271 275± 5 271+12 260140 270110 270110 1-2 267 270110 0-1 190 190± 5 1961 9 180120 185+10 185112 1-2 187 188± 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 Oxide. The e l e c t r o n s p e c t r a obtained f o r n i t r i c oxide are shown i n Figure 22. 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 oxide may be w r i t t e n (66): - . v NO K K ( a g 2 s ) Z ( a ^ s ^ (a g2p) "O^p)' 4 (Trg2p) T^I The He(584 A) spectrum shows the X 1 X + , a 3 Z + , b 3n (g s e r i e s ) , 3A and A^n s t a t e s of N0 +. 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 molecular s t a t e and removal of t h i s e l e c t r o n , from an antibonding o r b i t a l ( I T 2p) leads to the formation of the X^Z + ion s t a t e . The remainder of the spectrum i s q u i t e complex due to the overlap of s e v e r a l bands. There are two sharp bands, with l i t t l e f i n e s t r u c t u r e 3 1 (b n(8 s e r i e s ) and A n) corresponding to the removal of n e a r l y nonbonding e l e c t r o n s (a 2p). The other bands ( a 3 Z + and 3 A ) , which e x h i b i t long . v i b r a t i o n a l s e r i e s , l i k e l y correspond to the removal of bonding e l e c t r o n s (fi'u2p) . 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 data given i n Table XIV. The Penning spectrum shows the X^Z +, b 3II (8 s e r i e s ) and A^ TI s t a t e s . A comparison of the r e l a t i v e populations of the e l e c t r o n i c s t a t e s i s d i f f i c u l t because of the r i s i n g background at low e l e c t r o n energies i n the Penning e l e c t r o n spectrum. However, i t i s apparent that the X^E + i s of much reduced i n t e n s i t y compared to the 8 s e r i e s and A^n s t a t e i n the Penning spectrum compared to that observed i n the photoelectron 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 oxide are given i n Table XIV. This i s one of the few molecules s t u d i e d where the e l e c t r o n energy s h i f t i s observed to be negative f o r both s i n g l e t and t r i p l e t ELECTRON ENERGY (eV) Figure 22. 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 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 Energies f o r N i t r i c Oxide (eV). NO State I.P. (152) xV a V 9.262 15.667 b 11(8 16.561 s e r i e s ) Ah 2 S I o n i z a t i o n , AE 18.325 This work -0.030±0.010 +0.030±0.015 0±0.025 Ref.136 -O.OlOiO.OlO +0.035±0.010 2 S I o n i z a t i o n , AE This work -0.020+0.015 -0.005±0.010 Ref.136 0±0.010 0±0.010 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 s t a t e . During the l a t t e r stages of t h i s work a s i g n i f i c a n t increase i n the i n t e n s i t y of s p e c t r a was achieved due to improvements i n the metastable source. With t h i s improvement a d e t a i l e d comparison of the Penning e l e c t r o n and photoelectron s p e c t r a f o r the X^E + s t a t e of n i t r i c oxide became p o s s i b l e (153) . The case of N0 + (X"^ E + ) i s p a r t i c u l a r l y favourable 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 places 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. Figure 23 i l l u s t r a t e s the photoelectron and Penning e l e c t r o n s p e c t r a of the X^E + s t a t e of n i t r i c oxide and Table XV shows the observed v i b r a t i o n a l spacings i n comparison with values reported by other workers. Within experimental e r r o r the v i b r a t i o n a l spacings are independent of the means of i o n i z a t i o n , as has been observed f o r other molecules and t h i s i n d i c a t e s that there i s no e f f e c t i v e complex formation or nuclear p e r t u r b a t i o n of the molecule during the i o n i z a t i o n process. In the Penning i o n i z a t i o n spectrum a small 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 metastable source. By successive s u b t r a c t i o n of t h i s c o n t r i b u t i o n and also that due to s i n g l e t i o n i z a t i o n (on the b a s i s of 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 of n i t r i c oxide by t r i p l e t metastable atoms has been obtained. 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-a b i l i t i e s (normalized at v = 1) thus obtained, together with those obtained by other methods are given i n Table XVI. 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 photoelectron and Penning e l e c t r o n (2 3S) i o n i z a t i o n to the X^E + s t a t e of n i t r i c oxide are p l o t t e d -96-N 0 + M 5 8 4 A ) — NO++e N O + ( X 1 S + ) NO+He*(23S)-v=o 1 2 3 NO++He+e 4 5 6 T NO+He*(21S) v=0 •NO + He+e 2 3 12.0 II.O lO.O 9.0 ELECTRON ENERGY (eV) i + Figure 23. Penning e l e c t r o n and photoelectron s p e c t r a of the X E st a t e of n i t r i c oxide. TABLE XV 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 States of N0 +. * E l e c t r o n i c State Calc. P h o t o i o n i z a t i o n Penning I o n i z a t i o n , He and I n t e r v a l (151) He(584 A) 2 1 S 2 3S Ref.136 This work Ref.136 This work Ref.136 This 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. E l e c t r o n i c State C a l c . and V i b r a t i o n a l (151) l e v e l of N0 + P h o t o i o n i z a t i o n o He (5.8.4 A) Penning I o n i z a t i o n * 3 * 2 He (2 JS) Ar C? ) Ne*( 3P 0) This work Ref.154 This work Ref.136 Ref.136 Ref.21 Ref.136 xV v=0 47. 8 49± 1 50± 2 50± 1 50± 6 48± 2 40 50± 3 v=l 100. 0 100 100 100 100 96± 3 93 91± 3 v=2 91. 7 94± 1 90± 3 95± 1 96± 4 100 100 100 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 v=6 0. 6 - _ _ _ _ 27±10 I OO I -99-i n Figure 24 which shows that a very 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 to v = 5 f o r both the photoelectron and the Penning process. Hotop has observed (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 (at 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 approximately 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 to 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 small step h e i g h t s , at high v i b r a t i o n a l quantum numbers i n the r e t a r d i n g p o t e n t i a l curves of Hotop (31). The d i r e c t l y obtained d i f f e r e n t i a l s i g n a l obtained i n t h i s work i s more amenable to i n t e r -p r e t a t i o n . The Penning i o n i z a t i o n of n i t r i c oxide to the X^E + s t a t e was observed to be a v e r t i c a l process, w i t h i n experimental e r r o r . Figure 23 a l s o i l l u s t r a t e s t h a t , as f o r other molecules, the peaks due to s i n g l e t i o n i z a t i o n are broader than those due to t r i p l e t i o n i z a t i o n . 4.2.5. Molecular Oxygen. The e l e c t r o n s p e c t r a obtained f o r the i o n i z a t i o n o f 0^ are shown 2 4 2TT i n Figure 25. The photoelectron spectrum shows the X II , a n A 4 ° and b E i o n i c s t a t e s of 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 the oxygen molecule has the e l e c t r o n i c c o n f i g u r a t i o n (66): 0 2 K K ( a g 2 s ) 2 ( a u 2 s ) 2 (° g2p) 2 (\2.p) 4 (Tf g2p) 2 , 3 E g " 2 The shape of the f i r s t band (X II ) i s c o n s i s t e n t with removal of an e l e c t r o n from the outer antibonding o r b i t a l (TT 2 p) . The second band (a 4n ) c o n s i s t s of 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 of a bonding e l e c t r o n ( i r ^ p ) . The t h i r d band (A 11^), which overlaps the second, a l s o a r i s e s from the removal of a bonding e l e c t r o n (iv2p) • The (100) 0 i 0 1 2 3 4 5 VIBRATIONAL QUANTUM NUMBER Figure 24. 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 + ^ + 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 of NO to NO (X E ). -101-1 . 5 7 - i I . I . I . I , 8.0 6.0 4.0 2.0 ELECTRON ENERGY (eV) Figure 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 of molecular oxygen. -102-shape of the f o u r t h band (b^T. ~) i s c o n s i s t e n t with removal of an e l e c t r o n with some bonding character (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 Edquist et a l . (155). Since most of the s t r u c t u r e observed i n the Penning e l e c t r o n spectrum l i e s on the very steep p o r t i o n of the r i s i n g Auger background i t was not p o s s i b l e to make q u a n t i t a t i v e determinations o f the r e l a t i v e populations of the e l e c t r o n i c s t a t e s . However, i t i s apparent that r e l a t i v e band i n t e n s i t i e s are 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 . In p a r t i c u l a r there 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 to the 0^ + ^ ^ n g s t a t e . In the Penning i o n i z a t i o n spectrum, the broad band at about 3 eV 4 * 3 can be assigned to the a n u s t a t e produced by He (2 S). The maximum of t h i s band corresponds to v = 6 and sin c e t h i s i s the most probable t r a n s i t i o n (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 that Penning i o n i z a t i o n of t h i s s t a t e of oxygen i s e s s e n t i a l l y a v e r t i c a l process The peak at 2.76 eV corresponds i n energy to production of the A n s t a t e (v = 0) by t r i p l e t metastable atoms. High r e s o l u t i o n photoelectron spectroscopy 2 (155) shows v = 7 of the A n s t a t e to be the most probable t r a n s i t i o n . * 3 Therefore i t appears t h a t He (2 S) i o n i z a t i o n of molecular oxygen t o 2 the A n u i o n i c s t a t e i s a n o n - v e r t i c a l process. The r e s u l t s of a f t e r -glow st u d i e s (79, 156) of oxygen support t h i s o bservation. The d i f f e r e n c e 2 i n energy K 0.7 eV) between the peak at 2.76 eV and v = 7 of A n would seem to be much too large to be a t t r i b u t e d to 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 at 2.42 eV may correspond to * 1 4 -He (2 S) i o n i z a t i o n of 0 ? to the 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, 1.76 and 1.57 eV. The peak at 1.97 eV most c l o s e l y corresponds to the 2 * 1 process i n which the A II (v = 8) i o n i c s t a t e i s formed by He (2 S) 4 * 3 or a l t e r n a t i v e l y to the b E (v = 4) s t a t e produced by He (2 S). The 4 -expected energy (1.65 eV) f o r t r i p l e t s t a t e i o n i z a t i o n to 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 correspond to the small p a r t i a l l y r e s o l v e d peak observable i n the lower tra c e of Figure 25(b). Cermak and Sramek (157) have al s o observed sharp peaks i n the Penning e l e c t r o n s p e c t r a of 0 2 i n the energy range 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 to 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 e x c i t e d 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 molecules. 4.3 Triatomic Molecules• 4.3.1. Carbon Dioxide. The e l e c t r o n s p e c t r a obtained f o r carbon d i o x i d e are shown i n Figure 26. The photoelectron 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 . The e l e c t r o n i c o r b i t a l g u' u g 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 (66): C0 2 K K K ( a g ) 2 ( a j 2 ( a g ) 2 ( a j 2 ( % ) 4 ( , g ) 4 , V 2 The shape of 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 of the removal of a nonbonding e l e c t r o n (u ) where there 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 of a bonding e l e c t r o n (ir ) . Overlap of the second and t h i r d bands prevented the determination of more than four -104-2 3 S (B ) 1 1 1 . I L _ ) 6.0 5.0 4.0 3.0 2.0 ELECTRON ENERGY (eV) Figure 26. 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 carbon d i o x i d e . -105-v i b r a t i o n a l spacings f o r the A II s t a t e . Both the t h i r d and f o u r t h bands ( B ^ E u + and C 2 E g + ) are a l s o nonbonding as i n d i c a t e d by the shape of the bands. Carbon d i o x i d e , carbonyl sulphide and carbon d i s u l p h i d e 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 outer 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 of the o r b i t a l s and bands of carbon d i o x i d e may a l s o be used to discuss the s p e c t r a of carbonyl sulphide and carbon d i s u l p h i d e . The energy s c a l e f o r CC^* 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 data p u b l i s h e d by Turner et a l . (66) and given i n Table XVIII. The Penning e l e c t r o n spectrum i n d i c a t e s those states a c c e s s i b l e using helium metastable atoms, namely the X, A and B s t a t e s . Table XVII i n d i c a t e s that the r e l a t i v e p o p u l a t i o n of the e l e c t r o n i c states i s somewhat d i f f e r e n t f o r the two i o n i z a t i o n processes. The 2 + 2 r a t i o of the i n t e n s i t i e s of the B and X s t a t e s ( B E /X JI ) i s g r e a t e r g g * 3 ° f o r i o n i z a t i o n by He (2 S) atoms than f o r i o n i z a t i o n by He(584 A) photons. The 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 popul-at i o n s may be due i n part to d i f f e r e n t angular d i s t r i b u t i o n s of e l e c t r o n s 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 angular d i s t r i b u t i o n r e s u l t s have been reported no d e t a i l e d comparison i s p o s s i b l e . I t was a l s o observed that bands due to s i n g l e t i o n i z a t i o n are of much reduced i n t e n s i t y , r e l a t i v e to those due to t r i p l e t i o n i z a t i o n , than was observed i n the case of the i n e r t gases and diatomic molecules. Table XVIII gives the e l e c t r o n energy s h i f t s measured f o r the Penning peaks obtained by t r i p l e t i o n i z a t i o n . A near zero s h i f t was measured f o r the X s t a t e with p o s i t i v e values f o r the A and B s t a t e s . The v i b r a t i o n a l spacing observed 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 Populations of E l e c t r o n i c States (at v = 0) f o r Carbon Dioxide. (Corrected f o r transmission) CO* P h o t o i o n i z a t i o n Penning I o n i z a t i o n 2 State 0 He (584 A) 3 2 S ? x"n g 68 - 30 A 2n 8 _ 12 u B 2E + 100 _ 100 u C 2E + 17 - -TABLE XVIII S h i f t s , AE, i n Penning E l e c t r o n Energies f o r Carbon Dioxide (eV). C02 I.P. State (66) AE t AE s x 2 n g 13.788 -0. ,004±0. ,012 -A 2n u 17.323 +0, ,028±0. .020 -B 2E + u 18.082 +0, .012±0. .012 -c 2z + 19.400 - -g -107-2 2 + i o n i z a t i o n to the X IT and A II s t a t e s of C0~ are given i n Table XIX g u 2 6 and compared with values reported elsewhere. As was observed with other molecules s t u d i e d , the v i b r a t i o n a l spacings are independent o f the means of i o n i z a t i o n , w i t h i n experimental e r r o r . 4.3.2. Carbonyl Sulphide. The e l e c t r o n s p e c t r a observed f o r carbonyl sulphide are shown i n Figure 27. The Penning e l e c t r o n spectrum shows those s t a t e s a c c e s s i b l e using helium metastable atoms, namely the X 2n, A 2n, B 2 Z + and C 2E +. These i o n i c s t a t e s are a l s o observed i n the photoelectron spectrum. The features of the spectrum are very s i m i l a r to those of carbon d i o x i d e , w i t h a l l bands s h i f t e d to higher e l e c t r o n energy. 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 data p u b l i s h e d by Turner et a l . (66) and given i n Table XXI. Table XX i n d i c a t e s that the r e l a t i v e e l e c t r o n i c s t a t e populations 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 . 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 to d i f f e r e n t angular 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 processes but f u r t h e r study w i l l be required to r e s o l v e the d i f f e r e n c e s . The measured values of the e l e c t r o n energy s h i f t s f o r carbonyl sulphide are given i n Table XXI. A large negative value was measured f o r the t r a n s i t i o n to the lowest i o n i c s t a t e . This i s the only molecule studi e d where a negative e l e c t r o n s h i f t was measured f o r the B s t a t e . No value was measured f o r the A s t a t e due to i t s unresolved chara c t e r . 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 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 or Penning e l e c t r o n TABLE XIX 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 States of C0 2 +. E l e c t r o n i c State Spec, and I n t e r v a l (149) x2n A 2n P h o t o i o n i z a t i o n He(584 A) (158) (66) Penning I o n i z a t i o n fl59) This work He*(2 3S) 0-1 160 180 - 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-(a) He5 2 3 S ( X ) (b) He*+ 5 8 4 A (c) 5 8 4 A x 2 n 2 3 S(B ) 2 V + B 2 2 c 2 2 + 1QO 8.0 6.0 4.0 2.0 ELECTRON ENERGY (eV) Figure 27. 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 carbonyl sulphide. - R O -TABLE XX R e l a t i v e Populations of E l e c t r o n i c States (at v = 0) f o r Carbonyl Sulphide. (Corrected f o r transmission) C0S + P h o t o i o n i z a t i o n Penning I o n i z a t i o n State He(584 A) 2 1 S 2 3S X 2n 60 - 49 A 2n - -B 2Z + 100 - 74 C 2 E + 18 - 100 TABLE XXI S h i f t s , AE, i n Penning E l e c t r o n Energies f o r Carbonyl Sulphide (eV) COS + I.P. State (66) AE. AE K t s X 2n 11.189 -0.049±0.015 A 2n B 2 E + 16.042 -0.030±0.020 C 2 E + 17.960 +0.052±0.020 - I l l -s p ectra . As i n the case of CQ^ i t was observed f o r COS + that bands due to s i n g l e t i o n i z a t i o n are of much reduced i n t e n s i t y compared to t r i p l e t i o n i z a t i o n than was observed f o r the i n e r t gases or diatomic molecules. 4.3.3. Carbon Di 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 Figure 28. The spectrum o obtained using 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 2 2 2 ^ 2 ^ X n , A l l , B E , C I and D E and the features of the s p e c t r a are g u u g u r s i m i l a r to C 0 2 + and COS +, with a l l bands s h i f t e d to higher e l e c t r o n energies. 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 2 E u + was present. In the Penning e l e c t r o n spectrum only i o n i z a t i o n produced by t r i p l e t metastable atoms gave r i s e to peaks of 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 to be measured. I t was observed that 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 of greater i n t e n s i t y than those due 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 c a l i b r a t e d from published (66) i o n i z a t i o n p o t e n t i a l data given i n Table XXII. The values of the measured e l e c t r o n energy s h i f t s are given i n Table XXII. Negative e l e c t r o n s h i f t s (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 values f o r the B and C s t a t e s . Although carbon d i o x i d e , carbonyl sulphide and carbon d i s u l p h i d e a l l possess the same number of e l e c t r o n s i n t h e i r outer 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 to observe any trend or 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 populations (Table XXIII) of the e l e c t r o n i c s t a t e s appear to be the same, with the exception of the -112-E L E C T R O N ENERGY (eV) Figure 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 of 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 Energies f o r Carbon D i s u l p h i d e (eV) . C S 2 + I . P . AE t AE s State (66) 2 n g 10. .608 -0. ,034±0.015 u 12, .694 -0, ,047±0.030 h+ u 14, .478 +0, ,022±0.015 h+ 16 .196 +0, .002+0.025 g TABLE XXIII R e l a t i v e Populations of E l e c t r o n i c States (at v = 0) f o r Carbon D i s u l p h i d e . (Corrected for. transmission) C S 2 + P h o t o i o n i z a t i o n Penning I o n i z a t i o n State He(584 A) 2 1 S 2 3S X 2n 83 - 85 g • A 2n 36 - 41 u B 2E + 100 100 u C 2E + 26 - 51 -114-C s t a t e , w i t h i n experimental e r r o r , i n both the Penning and photo-e l e c t r o n s p e c t r a . 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 separation and measurement of the v i b r a t i o n a l spacings of the bands i n e i t h e r the photoelectron or 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 oxide s p e c t r a are shown i n Figure 29. The p h o t o e l e c t r o n 2 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 outer s h e l l but i n contrast to carbon d i o x i d e and carbon d i s u l p h i d e i t i s asymmetrical. 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 may be w r i t t e n as (66): The f i r s t band (X^n) has a shape which i s c o n s i s t a n t w i t h removal of an e s s e n t i a l l y nonbonding e l e c t r o n (T T ). The second band (A^ Z+) has 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 corresponds to removal 2 of an e l e c t r o n (nu) with some bonding character. The t h i r d band (B II) contains complex v i b r a t i o n a l s t r u c t u r e which was not r e s o l v e d . 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 data p u b l i s h e d by Turner et a l . (66) and given i n Table XXIV. The Penning e l e c t r o n spectrum shows those s t a t e s obtainable using helium metastable atoms. N 90 was the only t r i a t o m i c molecule s t u d i e d f o r which the Penning spectrum was of s u f f i c i e n t i n t e n s i t y to permit measurement of the e l e c t r o n energy s h i f t f o r both 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 given i n Table XXIV. Table XXV gives the r e l a t i v e populations of the e l e c t r o n i c s t a t e s -115-I . I . I i I 1 1 > 1 ' 1 — 8D 7.0 6.0 5.0 4.0 3D 2.0 E L E C T R O N E N E R G Y (eV) Figure 29. 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 n i t r o u s oxide. -116-TABLE XXIV S h i f t s , AE, i n Penning E l e c t r o n Energies f o r N i t r o u s Oxide (eV). N„0 + I.P. AE^ AE 2 t s State (66) X 2n 12.893 -0.008±12 -0.050±0.025 A 2 E + 16.389 +0.039±10 -0.025±0.020 B 2n 17.65 TABLE XXV R e l a t i v e populations of E l e c t r o n i c States (at v = 0) f o r N i t r o u s Oxide. (Corrected f o r transmission) N_0 + P h o t o i o n i z a t i o n Penning I o n i z a t i o n ° 1 3 State He(584 A) 2 X S 2 S X 2n 100 79 75 A 2 E + 62 100 100 B n --117-and shows that there are s i g n i f i c a n t d i f f e r e n c e s . For Penning i o n i z a t i o n the r a t i o of the A to X s t a t e (A2£+/X^TI) i s gre a t e r than that observed f o r p h o t o i o n i z a t i o n . Angular 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 to determine the o r i g i n of these d i f f e r e n c e s . The v i b r a t i o n a l spacings measured f o r Penning and photoelectrons f o r the A s t a t e of N^O* are given i n Table XXVI and compared with values reported by other workers. As has been observed with a l l other molecules, the agreement i s good and i n d i c a t e d that the v i b r a t i o n a l frequencies are independent o f the means of i o n i z a t i o n . I t was al s o observed that 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 broader than d i d t r i p l e t s t a t e i o n i z a t i o n . 4.3.5. Sulphur Dioxide and Nitrogen Dioxide. The spe c t r a of the bent t r i a t o m i c molecules sulphur d i o x i d e and nit r o g e n d i o x i d e are shown i n Figures 30 and 31 r e s p e c t i v e l y . These two molecules have been in c l u d e d together because i n n e i t h e r case was i t p o s s i b l e to determine e l e c t r o n energy s h i f t s from the Penning spectrum. The cross s e c t i o n s f o r Penning i o n i z a t i o n processes are apparently much smaller f o r these two molecules than f o r the other t r i a t o m i c molecules that have been s t u d i e d . The energy scales of the S0^+ s p e c t r a were c a l i b r a t e d from i o n i z a t i o n p o t e n t i a l data p u b l i s h e d by Turner et a l . (66), f i r s t I.P. 12.29 eV ( a d i a b a t i c ) , 12.50 eV ( v e r t i c a l ) . 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 the published data of Edquist et a l . (155), 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. For both molecules the Penning s p e c t r a obtained were only -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 States of N o0 +. E l e c t r o n i c State Spec. P h o t o i o n i z a t i o n Penning o and I n t e r v a l (160) He(584 A) I o n i z a t i o n (138) This work He*(2 3S) A Z 0-1 180 150±10 161±10 166±12 1-2 170 150±10 145±10 -119-—I 1 1 1 I • i 8.0 6.0 4.0 2.0 ELECTRON ENERGY (eV) Figure 30. 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 sulphur d i o x i d e . -120-- 1 1 1 1 1 • I 10.0 8.0 6.0 4.0 E L E C T R O N E N E R G Y (eV) Figure 31. 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 o g e n d i o x i d e . -121-of comparable i n t e n s i t y to the weak photoelectron s p e c t r a produced o from i n t e r n a l He(584 A) photons. No data could be obtained from the Penning spectra. Sulphur d i o x i d e has 18 valence s h e l l e l e c t r o n s , two more than the l i n e a r t r i a t o m i c molecules s t u d i e d . 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 can be expressed as (66): S0 2 ( 3 a x ) 2 ( 3 b 2 ) 2 ( l b p 2 ( 5 a x ) 2 ( 4 b 2 ) 2 ( l a 2 ) 2 ( S a ^ 2 , 1 A 1 The two e x t r a e l e c t r o n s cause the ground s t a t e molecule to be bent. Eland and Danby (159) considered that the bond angle increased from 119 degrees to 137 degrees during i o n i z a t i o n l e a d i n g to the 6a^ band. Due to the e x t r a e l e c t r o n s the photoelectron spectrum of sulphur d i o x i d e i s more complex than that of the l i n e a r t r i a t o m i c molecules. Nitrogen d i o x i d e has one more valence e l e c t r o n than the l i n e a r t r i a t o m i c molecules 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 may be w r i t t e n (66): N0 2 ( l b ^ 2 ( 5 a x ) 2 ( l a 2 ) 2 ( 4 b 2 ) 2 ( 6 a p , \ The bond angle f o r N0 2 i s 134 degrees. 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 expected to be l i n e a r . The value of the f i r s t i o n i z a t i o n p o t e n t i a l of N0 2 has not been w e l l e s t a b l i s h e d and was not used f o r energy s c a l e c a l i b r a t i o n . Because of the unpaired e l e c t r o n a complicated spectrum was expected and as the photoelectron spectrum of Figure 31 shows t h i s was observed. -122-4.4. Polyatomic Molecules. 4.4.1. Ammonia. The spe c t r a of ammonia are shown i n Figure 32. The spectrum o obtained using He(584. A) r a d i a t i o n i s i n good agreement with that published by Branton et 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 the 2 A^ s t a t e of ammonia of 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 of the energy s c a l e ) . The ground s t a t e molecular o r b i t a l c o n f i g u r a t i o n can be expresses as (66): NH 3 ( l a x ) 2 ( 2 a i ) 2 ( l e ) 4 ( 3 & 1 ) 2 , \ The f i r s t band corresponds to removal of 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 being l o c a t e d on the n i t r o g e n atom. The NH^+ ion formed i s expected to be i n a planar 2A^ c o n f i g u r a t i o n . 2 The second band ( E) i s very broad and d i f f u s e and the v i b r a t i o n a l 2 s t r u c t u r e i s unresolved. Only the f i r s t band ( A^) r e s u l t i n g from * 3 He (2 S) i o n i z a t i o n i s observed i n the Penning e l e c t r o n spectrum. I t can be seen from Figure 32 that the Penning spectrum shows no re s o l v e d v i b r a t i o n a l s t r u c t u r e . The n a t u r a l broadness of Penning peaks would account f o r t h i s lack o f s t r u c t u r e . Table XXVII gives the v i b r a t i o n a l spacings observed from the photoelectron spectrum compared with values p u b l i s h e d by Branton et a l . (161). Ammonia has the l a r g e s t e l e c t r o n energy s h i f t of a l l atoms and 2 molecules s t u d i e d . /\Et = -0.350 ± 0.035 eV f o r the A^ State. This s h i f t was measured from the p o s i t i o n s of the band maxima. No comparison of the r e l a t i v e population of the e l e c t r o n i c s t a t e s was p o s s i b l e due to the r a p i d l y r i s i n g background. -123-I i I i I i . I I2.0 IO.O 8.0 6.0 4.0 E L E C T R O N E N E R G Y (eV) Figure 32. 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 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 State of NH^+ E l e c t r o n i c State P h o t o i o n i z a t i o n o and I n t e r v a l He (584 A) (154) This work ? 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. Methyl C h l o r i d e , Methyl Bromide and Methyl Iodide. The s p e c t r a obtained f o r the methyl h a l i d e s are shown i n Figures 33, 34 and 35. The energy s c a l e s were c a l i b r a t e d using values p u b l i s h e d by Turner et a l . (66) and given i n Table XXVIII. Traces of n i t r o g e n gas present i n the samples, as evident from the photoelectron 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 . 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 of the methyl h a l i d e s may be expressed as (66): CH3X ( l s c ) 2 ( s a p 2 ( n s ^ ) 2 Ore) 4 ( a a ^ 2 ( n p ^ e ) 4 , ^ where n = 3, 4 or 5 f o r X = CI, Br and I. The highest occupied o r b i t a l i s l a r g e l y l o c a l i z e d on the halogen atom (though there may be some halogen-hydrogen (X-H) antibonding and carbon-hydrogen (C-H) bonding c h a r a c t e r ) . Removal of an e l e c t r o n from t h i s o r b i t a l gives an ion i n 2 the E s t a t e which i s doubly degenerate and should be s u s c e p t i b l e to 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 photoelectron 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 separation ( i n the • 2 2 order CI, Br, I) between the E . and E , s t a t e s , as expected. The second and t h i r d bands i n the s p e c t r a are broad and unresolved. E l e c t r o n energy s h i f t s (Table XXVIII) were measured f o r both 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 CH,I +. No value f o r AE was obtained f o r methyl i o d i d e 3 s 2 because of the overlap of the E , s t a t e produced by the i n t e r n a l 1/^ photon source with 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 2 also the overlap of the 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 the 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 a l s o 12 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 of the -126-_ l I I L — 1 , I 1QO 8.0 6.0 4.0 ELECTRON ENERGY (eV) Figure 33. E l e c t r o n spectra f o r i o n i z a t i o n of methyl c h l o r i d e . I . I . I I 1 — lO 8 6 4 E L E C T R O N ENERGY (eV) Figure 34. 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 methyl bromide. -128-3 2 3 S ( 2 E) (a) He* (b) He*+ 5 8 4 A (c) 5 8 4 A 12.0 10.0 8.0 6.0 ELECTRON ENERGY (eV) 4.0 Figure 35. E l e c t r o n spectra f o r i o n i z a t i o n of 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 Energies f o r Methyl Halides (eV) . I.P. Species (66) AE^ AE CH3C1+^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 C H 3 B r + ( 2 E 1 / 2 ) 10. .85 -0. ,090±0, .015 -0, ,230±0, .025 C H 3 I + ( 2 e 3 / 2 ^ 9, ,55 -0, ,080±0, .020 -C H 3 I + ( 2 E 1 / 2 ) 10, .16 -0, ,090±0, .015 --130-and E st a t e s produced by Penning i o n i z a t i o n w i t h those produced by p h o t o i o n i z a t i o n . A p p l i c a t i o n of a c o r r e c t i o n f o r t h i s overlap and the tra 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 th a t the r e l a t i v e i n t e n s i t i e s are the same f o r the two processes. The r e l a t i v e e l e c t r o n i c s t a t e populations f o r Penning and P h o t o i o n i z a t i o n appear to be s i m i l a r . I t may be observed from Table XXVIII that the 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 are very n e a r l y the same and that the energy s h i f t s due to t r i p l e t i o n i z a t i o n are a l l of the same order of magnitude though considerably d i f f e r e n t from those observed f o r s i n g l e t i o n i z a t i o n . Thus f o r the methyl h a l i d e s , i t i s the type o f i o n i z i n g p a r t i c l e r a t h e r than the i o n i z e d species which determines the magnitude of the e l e c t r o n energy s h i f t . 4.5. Hydrocarbons. 4.5.1. Methane. Figure 36 shows the sp e c t r a obtained f o r methane. The photoelectron spectrum c o n s i s t s of two broad overlapping bands. The energy s c a l e s were assigned on the ba s i s of the i o n i z a t i o n p o t e n t i a l data published by Turner et a l . (66) 12.7 eV as the beginning o f the broad 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 expressed as (66): The broad band, co n t a i n i n g a double maximum, has considerable unresolved s t r u c t u r e i n d i c a t i n g that 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 o r b i t a l . The double maximum has been explained (66) by the use of the Ja h n - T e l l e r Theorem which p r e d i c t s that a t e t r a h e d r a l l y t r i p l y degenerate methane ion cannot be a g e o m e t r i c a l l y s t a b l e species. Thus the band ( l s a p 2 ( 2 s 3 l ) 2 ( 2 p t 2 ) 6 , 1 A 1 1 , 1 . 1 . I I I I 1 1 1 9.0 8.0 7.0 6.0 5.0 4.0 3.0 ELECTRON ENERGY (eV) Figure 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 methane. -132-may contain t r a n s i t i o n s from the methane ground molecular s t a t e to ions with 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 to that produced by p h o t o i o n i z a t i o n but because of the broadness of the band and the r i s i n g background o f the 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 obtained. Herce et 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 deuterated methane produced by Penning i o n i z a t i o n employing s i n g l e t and t r i p l e t helium atoms. They observed that f o r both 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 methyl ions (CH^ + and CD^+) were the most abundant, i n contrast to 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 parent ion predominates. They al s o observed that fragmentation was s i g n i f i c a n t l y greater f o r the higher energy s i n g l e t metastable atom, f o r both molecules. 4.5.2. Ethylene. The e l e c t r o n s p e c t r a obtained f o r ethylene are shown i n Figure 37. o The He(584 A) photoelectron spectrum c o n s i s t e d o f four w e l l known bands (161) with complex f i n e s t r u c t u r e . The energy s c a l e was c a l i b r a t e d using values p u b l i s h e d by Branton et a l . (161), 10.51 eV; 2B„ 12.46 eV; 2A 14.40 eV and 2 B . 15.78 eV. The only band of 3g g 2u 2 s i g n i f i c a n t i n t e n s i t y i n the Penning spectrum was the band formed by t r i p l e t i o n i z a t i o n . Ethylene, which i s i s o e l e c t r o n i c with 0^, i s the sim p l e s t organic 2 molecule to contain a double bond and the f i r s t band ( ) corresponds to removal of an e l e c t r o n ( l b 3 u ) • T h e 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 , i . I , I i I i l , I 10.0 8.0 6 0 4.0 ELECTRON ENERGY (eV) Figure 37. 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 ethylene. -134-w r i t t e n as (161) : C.H. (2a ) 2 (2b n ) 2 ( l b _ ) 2 (3a ) 2 ( l b , ) ( l b _ ) 2 , \ 2 4 g l u 2u g 3g 3u l g . The o r i g i n of the other bands i n the photoelectron spectrum have been discussed by Branton et a l . (161) and Turner et a l . (66). 2 The energy s h i f t , AE^., f o r the B_ band was found to be t 3u - 0.020 ± 0.012 eV. This was the only band o f ethylene f o r which an e l e c t r o n energy s h i f t could be measured. I t may be seen from Figure 37, that the r e l a t i v e populations of the e l e c t r o n i c s t a t e s are v a s t l y d i f f e r e n t f o r the two processes. In 2 2 p a r t i c u l a r the B 7 band which i s almost as intense as the B_ band ^ 3g 3u i n the photoelectron spectrum i s very much reduced i n i n t e n s i t y , r e l a t i v e 7 to the "B^ band i n the Penning e l e c t r o n spectrum. I t i s u n l i k e l y that the la r g e d i f f e r e n c e s i n the r e l a t i v e i n t e n s i t i e s are due only to 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 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 and the spec t r a suggest that a large 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 the two processes. 4.5.3. Acetylene. Figure 38 i l l u s t r a t e s the sp e c t r a obtained f o r acetylene. The photoelectron spectrum contains the expected bands a c c e s s i b l e using 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 using i o n i z a t i o n p o t e n t i a l data published by Turner et a l . (166) and given i n Table XXIX. The Penning e l e c t r o n spectrum shows those s t a t e s obtainable using He*(2 1S) and He*(2 3S) atoms. 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 of acetylene can be expressed as (66): -135-. • —I • 1 • ' — 8.0 6.0 4.0 2 0 E L E C T R O N ENERGY (eV) Figure 38. 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 acetylene. -136-C-H_ ( l a ) 2 ( l a ) 2 (2a ) 2 (2a ) 2 (3a ) 2 ( I T T ) 4 ,  1z * 2 2 g u g u g u g 2 The shape of the f i r s t band (X IIU) i s c h a r a c t e r i s t i c of i o n i z a t i o n from the highest occupied T T o r b i t a l i n alkynes (66) . The second and t h i r d bands contain unresolved complex s t r u c t u r e . Due to the r a p i d l y r i s i n g background at low e l e c t r o n energies i n the Penning spectrum i t was not p o s s i b l e to compare the r e l a t i v e populations of the e l e c t r o n i c s t a t e s f o r the two i o n i z a t i o n processes. I t was observed, as f o r other molecules, that 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 that bands formed by s i n g l e t i o n i z a t i o n are broader than those formed by t r i p l e t i o n i z a t i o n . 2 The measured e l e c t r o n energy s h i f t s f o r the X n u band of acetylene are given i n Table XXIX and compared with values p u b l i s h e d by Hotop (154). The 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 while that f o r 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 value given by Hotop. This may occur because the bands due to s i n g l e t i o n i z a t i o n are extremely broad and poorly r e s o l v e d i n both s t u d i e s . Table XXX l i s t s the v i b r a t i o n a l spacings measured f o r Penning and p h o t o i o n i z a t i o n to the X s t a t e of ^2^2 + a n c^ c o m P a r e s them with values reported elsewhere. The agreement i s good and as has been the case f o r other molecules s t u d i e d , the v i b r a t i o n a l spacings are independent of the means of i o n i z a t i o n , w i t h i n experimental e r r o r . 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 (normalized at 2 v = 0) f o r the X n s t a t e of acetylene are given i n Table XXXI together with those reported by other workers. 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 ° * 3 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 S) 2 i o n i z a t i o n of the X II s t a t e were found, i n t h i s study, to be the same TABLE XXIX S h i f t s , AE, i n Penning E l e c t r o n Energies f o r Acetylene (eV) . C 2 H 2 + I.P. 2 3S I o n i z a t i o n , AE^ 2 l S I o n i z a t i o n , State (66) This work (156) This work X 2 ^ 11.40 -0.040±0.012 -0.030 0.220±0.040 16.36 TABLE XXX 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 State of C 2H 2 E l e c t r o n i c State P h o t o i o n i z a t i o n Penning , -r , i rra-7 a-» I o n i z a t i o n and I n t e r v a l He(583 A) (He (2 S) (158) (164) This work This work X 2 n 0-1 240 220 226±10 226±12 u 1- 2 240 220 226±10 230±12 2- 3 - 220 215±13 -138-R e l a t i v e V i b r a t i o n a l E l e c t r o n i c State and V i b r a t i o n a l * Levels f o r C^H^ X 2n v = 0 u 1 2 TABLE XXXI T r a n s i t i o n Probabi P h o t o i o n i z a t i o n o He (584 A) (158) This work 100 100 45 43±5 15 15±7 i t i e s f o r Acetylene. Penning I o n i z a t i o n He*(2 3S) (154) This work (a) 100 100 38 43±5 9 15±7 (a) These values have been estimated from Figure 1 of Reference 154. -139-w i t h i n experimental 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 process with 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 . Hotop has reported (154) a small 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 processes and has a t t r i b u t e d i t to changes of 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 of the order of 0.01 A. 4.6. Rare Gas Mixtures. During the course of 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 f r e q u e n t l y observed i n adjacent regions of the e l e c t r o n energy spectrum. This s t r u c t u r e , which has been a t t r i b u t e d to r a r e gas i n t e r a c t i o n s , was i n v e s t i g a t e d f u r t h e r , since i t c o n s t i t u t e d a p o s s i b l e c o m p l i c a t i o n i n the i n t e r p r e t a t i o n of the Penning sp e c t r a . In t h i s part of the study one of the rare gases was subjected to e l e c t r o n bombardment i n the e x c i t a t i o n chamber and the other was admitted d i r e c t l y to the c o l l i s i o n -4 chamber v i a the sample system. T o t a l pressures of approximately 10 t o r r were used and the e l e c t r o n energy spe c t r a were searched f o r primary and secondary processes. 4.6.1. Helium + Helium. The s t r u c t u r e observed i n the spectrum at energies l e s s than 19 eV i s shown i n Figure 39. The sharp peak at 16.23 eV i s due to the o p h o t o i o n i z a t i o n of n e u t r a l helium atoms by the Hell(304 A) l i n e . The o o He (584 A) 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 of the He(304 A) r a d i a t i o n was observed to be very dependent on source c o n d i t i o n s and pressure. The peak at 16.23 eV was used f o r c a l i b r a t i o n of the energy s c a l e . No -140-change i n the spectrum was observed when the e x t e r n a l He(584 A) photon source was simultaneously 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) photons, which would give photoelectrons of 16.45 eV and 17.25 eV, was not detected. This i s not s u r p r i s i n g 4 because of the low concentration of metastable atoms (10 metastable 3 atoms/cm ) together with the low p h o t o i o n i z a t i o n cross s e c t i o n o f -19 2 ° helium metastable atoms (4 x 10 cm at 584 A (164, 165)). The o r i g i n of the broad, s t r u c t u r e l e s s peak (FWHM 1.25 eV) at 14.8 eV i s u n c e r t a i n . The encounter o f two helium t r i p l e t metastable -14 2 atoms (a ^ 1 0 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 experimentally (170, 171). This process 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 q) of 15.06 eV. However, an estimate based on the experimental 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 ge value -10 2 (10 cm ) f o r t h i s cross s e c t i o n i f the process were to be i d e n t i f i e d with the broad peak i n Figure 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 q = 15«85 eV) and s i n g l e t - s i n g l e t ( E q = 16.64 eV) d i r e c t Penning i o n i z a t i n g c o l l i s i o n s may be discounted. 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 Figure 39 may a r i s e from a c o n s i d e r a t i o n of the work of Bydin et a l . (172) who have s t u d i e d the 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 of moderately f a s t , unexcited Rb atoms (200 eV) with argon atoms. To e x p l a i n the e l e c t r o n spectrum Bydin et a l . assumed a curve c r o s s i n g mechanism leading to an e x c i t e d l e v e l of the qu a s i -* molecule RbAr ( i n v o l v i n g Ar ) 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 of Rb i n the c o l l i s i o n process. Such a mechanism r e s u l t s i n products analogous t o those produced i n Penning i o n i z a t i o n . This argument can be extended i n the f o l l o w i n g manner. Fast helium 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 of He + i n the region between the e x c i t a t i o n chamber and the i o n trap chamber (the energy d i f f e r e n c e being about 225eV) c o l l i d e w i t h n e u t r a l helium 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 , to a s t a t e w i t h a d i s s o c i a t i o n l i m i t * 1 * 3 corresponding to the two metastable helium atoms He (2 S) and He (2 S). Such a s t a t e would be unstable w i t h respect to a u t o i o n i z a t i o n , g i v i n g products, He + He + + e, i d e n t i c a l to those produced i n Penning i o n -i z a t i o n . This mechanism does not r e q u i r e the r e l a t i v e l y improbable encounter of two helium metastable atoms. A mechanism i n v o l v i n g f a s t n e u t r a l s would r e s u l t i n broad peaks (as observed) due to the Doppler e f f e c t . The p o s s i b i l i t y that the observed e l e c t r o n spectrum i s due to e l e c t r o n e j e c t i o n from metal surfaces by the helium metastables can be discounted since experiments (173, 174) show that only very broad, s t r u c t u r e l e s s , f u n c t i o n s are obtained f o r such processes. These curves are s i m i l a r i n nature to the background f u n c t i o n s observed i n the apparatus at lower e l e c t r o n energies. 4.6.2. Helium + Argon. In t h i s part of the study helium was introduced through the e x c i t -a t i o n region and argon by the sample system. The spectrum obtained i s shown i n Figure 40. The peaks at 16.23 eV and 14.8 eV are assigned to the processes discussed i n the helium + helium case i n the previous s e c t i o n . At the low energy region of the spectrum between 4 - 6 eV are the peaks c h a r a c t e r i s t i c of i o n i z a t i o n of argon by helium photons o * 1 (He(584 A) from the e x t e r n a l lamp) and by helium metastable atoms (He (2 S)) HELIUM / ARGON H e 2 3 S / A r E L E C T R O N ENERGY 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 processes i n a mixture of helium and argon. -144-and He (2°S)). The energy s c a l e was c a l i b r a t e d from the energies o f these peaks. The peak at 9.5 eV a l s o occurs with only argon i n the apparatus and w i l l be discussed i n the next s e c t i o n . I t a r i s e s i n Figure 40 due to the mixing of gases at the higher pressures used i n t h i s experiment. The prominent peak at 12.8 eV i s found only i n the helium + argon case and may be due to a u t o i o n i z a t i o n of 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 et 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 mixtures o f charge exchanged helium ions (400 eV) and argon. 4.6.3. Argon + Argon. For t h i s experiment argon was admitted through both the e x c i t a t i o n chamber and the sample system. With the simultaneous use of the e x t e r n a l o He(584A) photon source the spectrum obtained i s shown i n Figure 41. The o peaks at 5.46 eV and 5.28 eV, produced by i o n i z a t i o n o f Ar by He(584 A) photons were used to c a l i b r a t e the energy s c a l e and were the only s t r u c t u r e to disappear when use of the e x t e r n a l lamp was di s c o n t i n u e d . o No evidence was found of i o n i z a t i o n of argon metastable atoms by He(584 A) o o o o Arl(1067 A, 1048 A) or ArII(932 A, 920 A) r a d i a t i o n . The intense s t r u c t u r e i n Figure 41 i n d i c a t e s a r e l a t i v e l y high p r o b a b i l i t y f o r the processes i n v o l v e d . Berry (178) has observed s i m i l a r s t r u c t u r e i n spe c t r a obtained f o r an argon/argon study at c o l l i s i o n energies that range from 30 t o 250 eV. He found that the peaks v a r i e d with the energy o f the system. The large peak at 9.47 may be assigned to the a u t o i o n i z a t i o n (179) of 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 . This process has RELATIVE INTENSITY -146-been s t u d i e d and discussed by Gerber et a l . (179) and i t i s al s o found to occur i n A r + / A r c o l l i s i o n s at medium energies (180) . The peak at ^ 8.8 eV does not correspond to a u t o i o n i z a t i o n of any known e x c i t e d s t a t e o f argon. Siegbahn et a l . (179) have observed an u n c l a s s i f i e d peak at 8.89 eV i n the e l e c t r o n impact induced a u t o i o n i z a t i o n e l e c t r o n spectrum of argon. No d e f i n i t e assignment can be made f o r the peak at 10.4 eV, but i t i s clo s e to the value expected f o r a u t o i o n i z a t i o n of Ar(3s 3p^ 4p). A small shoulder at ^ 11.9 eV (observed at higher s e n s i t i v i t y than i n Figure 41) i s probably due to a u t o i o n i z a t i o n of Ar(3s 3p^ 3d) since angular d i s t r i b u t i o n measurements (177) i n d i c a t e that Ar(3s 3p^ 5s) i s not i n v o l v e d . Peak broadening as observed i n Figure 41 (compared with p h o t o i o n i z a t i o n peaks) would be expected due to the Doppler e f f e c t i f the p o s t u l a t e d mechanism of 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 et a l . (181) observe s i g n i f i c a n t cross s e c t i o n s 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 at l a b o r a t o r y energies o f 57—240 eV, and they a l s o report bands of e j e c t e d e l e c t r o n s at approximately 6 eV and 9.5 eV. Taking the — 16 2 n e u t r a l - n e u t r a l i o n i z i n g cross s e c t i o n (y 10 cm ) reported by Hammond et a l . (181) together with our experimental c o n d i t i o n s r e q u i r e s a charge t r a n s f e r cross s e c t i o n f o r A r + i n Ar of approximately 10 ^ 3 cm 2 to give s i g n a l s of the magnitude we observe. This value compares w e l l w i t h the cross s e c t i o n reported from charge t r a n s f e r experiments (182) and lends support to the suggested mechanism of f a s t 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 . The o r i g i n of the band at ^ 7.1 eV i s u n c e r t a i n . This band appears to have s e v e r a l components with e l e c t r o n energies i n the region expected from the encounter of two argon metastable atoms -147-producing A r + + Ar + 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 to that already discussed f o r the helium + helium system. A broad unassigned peak at about 5.7 eV which appears o as a shoulder on the argon - He(584 A) doublet i s al s o present w i t h the e x t e r n a l photon lamp exti n g u i s h e d . The peaks at 3.0, 2.69 and 2.51 eV could p o s s i b l y be due to a u t o i o n i z a t i o n o f doubly e x c i t e d states (176) of Ar to A r + ( 3 s 3 p ^ ) . 4.6.4. Neon + Neon. Figure 42 shows the e l e c t r o n spectrum observed with neon only i n the system. Broad bands are observed at ^ 11.4 eV and above 13 eV at higher pressures (upper t r a c e ) . The broad band at ^ 11.4 eV i s i n the energy range f o r processes 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 unstable s t a t e , g i v i n g products Ne + Ne + + e. The sharper peaks at a, 9.0, 6.3, 5.6 and 5.35 eV are due to photo-i o n i z a t i o n of 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 bombardment i n the e x c i t a t i o n chamber. The energy s c a l e was c a l i b r a t e d using the band at 5.35 eV ( C o n s i s t i n g o f a t r i p l e t with the expected i n t e n s i t i e s of 4:4:1) which i s due to i o n i z a t i o n by the neon ion resonance l i n e s . The higher energy peaks are due to 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 states of the neon i o n (139). 5.35 Ne + 462&-^Ne + +e NEON _ j — . ( ! ( . . 1 1 r 14 12 10 8 6 4 2 E L E C T R O N ENERGY (eV) Figure 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 processes i n neon. -149-CHAPTER FIVE CONCLUSIONS * 1 A comparative study has been made of 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)) of a large number of atoms and molecules employing the techniques of high r e s o l u t i o n e l e c t r o n spectroscopy. Good agreement i n the measured Penning e l e c t r o n energy s h i f t was found where comparison with l i t e r a t u r e values 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 molecules (acetylene, ammonia and the methyl 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 these e f f e c t s which are presumably due to the combined e f f e c t s o f the shape of the p o t e n t i a l energy i n t e r a c t i v e surface and the v a r i a t i o n of t r a n s i t i o n p r o b a b i l i t y with d i s t a n c e . A number of secondary rare gas i n t e r a c t i o n s have al s o been st u d i e d . I t was observed that the cross 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 molecules than f o r atoms. In atoms the Penning e l e c t r o n d i s t r i b u t i o n s were broader 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 opposite was observed f o r molecules. A very low cross s e c t i o n was observed f o r Penning i o n i z a t i o n of S0 ? and N0 0 and al s o to the lowest s t a t e s of 0^ and N0 +. For many molecules the r e l a t i v e populations of e l e c t r o n i c s t a t e s of the i o n are s t r o n g l y dependent on the mode of i o n i z a t i o n . Very l a r g e d i f f e r e n c e s are observed i n the case of ethylene. Although these -150-observations may be p a r t l y due to 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 of ejec 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 , i t i s probable that s i g n i f i c a n t d i f f e r e n c e s occur i n the e l e c t r o n i c t r a n s i t i o n moments f o r the two types of i o n i z a t i o n . The large d i f f e r e n c e s i n s t a t e p opulations are of s i g n i f i c a n c e i n many chemical systems e.g. discharges, l a s e r pumping and chemical r e a c t i o n s . In such systems great ca u t i o n must obviously by ex 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 are used which have been obtained from s t u d i e s of the absorption o f e l e c t r o -magnetic r a d i a t i o n . V i b r a t i o n a l spacings have been measured and found t o be independent of the method of i o n i z a t i o n . V i b r a t i o n a l i n t e n s i t i e s w i t h i n a given e l e c t r o n i c band appear i n most cases to be independent o f the mode of i o n i z a t i o n ; t h a t i s , Penning i o n i z a t i o n i s e s s e n t i a l l y a Franck-Condon process. This has been confirmed i n q u a n t i t a t i v e i n t e n s i t y s t u d i e s of the X s t a t e of NO and C 2H 9 . D i f f e r i n g v i b r a t i o n a l i n t e n s i t i e s were apparent i n the case of the A2II s t a t e of 0 2 . -151-BIBLIQGRAPHY 1. F. M. Penning, Z. Physik, 46, 225 (1928). 2. F. M. Penning, Z. Physik, 57, 723 (1929). 3. A. A. K r u i t h o f f and F. M. Penning, P h y s i c a , 4_, 430 (1937). 4. F. L. Mohler and C. Boeckner, J . Res. Nat. Bur. Stand., 5_, 51 (1930). 5. F. L. Mohler and C. Boeckner, J . Res. Nat. Bur. Stand., 5, 399 (1930). 6. F. L. Mohler, P. D. Foote and R. L. Chenault, Phys. Rev., 27, 37 (1926). 7. J . A. Hornbeck ; and J . P. Molnar, Phys. Rev., 84, 621 (1951). 8. F. L. Arnot and M. B. M'Ewen, Proc. Roy. S o c , A166, 543 (1938). 9. F. L. Arnot and M. B. M'Ewen, Proc. Roy S o c , A171 , 106 (1939). 10. V. Cermak and Z . Herman, C o l l . Czeck. Chem. Comm., 29, 953 (1964) 11 • J . L. F r a n k l i n , Advan. Chem. Ser. , 72_, 1 (1968). 12. V. Cermak and Z . Herman, Coll. Czeck. Chem. Comm., 30^ , 169 (1965). 13. V. Cermak, J . Chem. Phys., 44, 3781 (1966). 14. V. Cermak, Advances i n Mass S p e c , 4, 697 (1968). 15. V. Cermak, C o l l . Czeck. Chem. Comm., 33, 2739 (1968). 16. V. Cermak and Z . Herman, Chem. Phys. L e t t . , 2_, 359 (1968). 17. H. Hotop and A. Niehaus, J . Chem. Phys., 47, 2506 (1967). 18. H. Hotop and A. Niehaus, Z. Physik, 215, 395 (1968). 19. V. Fuchs and A. Niehaus, Phys. Rev. L e t t . , 21_, 1136 (1968). 20. H. Hotop and A. Niehaus, I n t . J . Mass Spectrom. Ion Phys., h 415 (1970). 21. V. Cermak and J . B. Ozenne, I n t . J . Mass Spectrom. Ion Phys., 7, 399 (1971). -152-22. W. L. Williams and E. S. Fry, Phys. Rev., 20_, 1335 (1968). 23. R. S. Van Dyck, J r . , C. E. Johnson and H. E. Shugart, Phys. Rev. L e t t . , 25, 1403 (1970). 24. V. Cermak and Z. Herman, Mass Spectrometry Conf., ASTM Committee E-14, New Orleans, La. (1962). 25. W. Kaul and R. Taubert, Z. Na t u r f o r s c h , 17A, 88 (1962). 26. M. S. B. Munson, J . L. F r a n k l i n and F. H. F i e l d , J . Chem. Phys., 67, 1541 (1963). 27. S. E. Kuprianov, Sov. Phys. JETP, 21_, 311 (1965). 28. S.'E. Kuprianov, Sov. Phys. JETP, 24, 674 (1967). 29. R. S. M u l l i k e n , Phys. Rev., 136A, 962 (1964). 30. Z. Herman and V. Cermak, C o l l . Czeck. Chem. Comm., 31_, 649 (1966). 31. H. Hotop and A. Niehaus, Z. Physik, 228, 68 (1969). 32. H. Hotop and A. Niehaus, Z. Physik, 238, 452 (1970). 33. R. S. M u l l i k e n , J . Am. Chem. S o c , 86>, 3183 (1964). 34. H. Hotop and A. Niehaus, Chem. Phys. L e t t . , 3_, 687 (1969). 35. H. Hotop and A. Niehaus, I n t . J . Mass Spectrom. Ion Phys., 5_, 415 (1970). 36. R. S. Berry, Ann. Rev. Phys. Chem., 20, 357 (1969). 37. H. Hotop, A. Niehaus and A. L. Schmeltekopf, Z. Physik, 229, 1 (1969). 38. H. Hotop and A. Niehaus, Z. Physik, 218, 395 (1968). 39. E. E. M u s c h l i t z , J r . and M. J . Weiss, i n "Atomic C o l l i s i o n Processes", Ed. M. R. C. McDowell, North-Holland Publ. Co., Amsterdam (1964). 40. V. Cermak and Z. Herman, C o l l . Czeck. Chem. Comm., 33_, 468 (1968). 41. E. G. Jones and A. G. H a r r i s o n , I n t . J . Mass Spectrom. Ion Phys., 5_, 137 (1970). 42. Z. Herman and V. Cermak, Nature, 199, 588 (1963). -153-43. V. Cermak, J . Chem. Phys., 43, 4527 (1965). 44. V. Cermak and Z. Herman, C o l l . Czeck. Chem. Comm., 28, 799 (1963). 45. J . L. G. Dugan, H. L. Richards and E. E. M u s c h l i t z , J r . , J . Chem. Phys. , 46_, 346 (1967) . 46. H. K. Holt and R. Ktfbtkov, Phys. Rev., 144, 82 (1966). 47. V. Cermak, J . Chem. Phys., 44, 3774 (1966). 48. D. A. Maclennan, Phys. Rev., 148, 218 (1966). 49. W. P. Shol e t t e and E. E. M u s c h l i t z , J r . , J . Chem. Phys., 36_, 3368 (1962). 50. J . A. Herce, J . R. Penton, R. J . Cross and E. E. M u s c h l i t z , J r . , Chem Phys., 49_, 958 (1968). 51. E. E. Benton, E. E. Ferguson, F. A. Matsew and W. W. Robertson, Phys. Rev., 128, 206 (1962). 52. A. L. Schmeltekopf and F. C. Fehsenfeld, J . Chem. Phys., 55, 3173 (1970). 53. F. B. Dunning and A. C. H. Smith, J . Phys. B, 3_, 260 (1970). 54. F. B. Dunning and A. C. H. Smith, Phys. L e t t . , 32A, 287 (1970). 55. R. C. Bolden, R. S. Hemsworth, M. J . Shaw and N. D. Twiddy, J . Phys. B, 3_, 61 (1970). 56. M. J . Shaw, R. C. Bolden, R. S. Hemsworth and N. D. Twiddy, Chem. Phys. L e t t . , JS, 148 (1971). 57. K. L. B e l l , A. Dalgarno and A. E. Kingston, J . Phys. B, 1_, 18 (1968) 58. A. V. Phelps, Phys. Rev., 99, 1307 (1955). 59. D. W. Turner, i n " P h y s i c a l Methods i n Advanced Inorganic Chemistry", Ed. H. A. 0. H i l l and P. Day, I n t e r s c i e n c e (1968). 60. A. D. Baker, Accts. Chem. Res., 3_, 17 (1970). 61. D. Betteridge and A. D. Baker, Anal. Chem., 42, 43 (1970). 62. W. C. P r i c e , Endeavour, 26_, 78 (1967). 63. W. C. P r i c e , i n "Molecular Spectroscopy", Ed. P. Hepple, I n s t i t u t e of Petroleum, London (1968). -154-64. C. R. Brundle and M. B. Robin, i n "Determination of Organic S t r u c t u r e by P h y s i c a l Methods", V o l . I l l , Ed. F. Nachod and G. Zuckerman, Academic Press, New York. 65. S. D. Worley, Chem. Rev., 71_, 295 (1971). 66. D. W. Turner, A. D. Baker, C. Baker and C. R. Brundle, "High R e s o l u t i o n Molecular Photoelectron Spectroscopy", John Wiley and Sons, Ltd. (1970). 67. A. D. Baker and C. R. Brundle. " I o n i z i n g Photon Impact Phenomena", Academic Press, Inc. 68. F. I. V i l e s o v , B. L. Kurbatov and A. N. Terenin, Soviet Phys. Doklady, 6_, 490 (1961) . 69. F. I. V i l e s o v , B. L. Kurbatov and A. N. Terenin, Dokl. Akad. Nauk. SSSR, 138, 1329 (1961). 70. B. L. Kurbatov, F. I. V i l e s o v and A. N. Terenin, Dokl. Akad. Nauk. SSSR, 140, 797 (1961). 71. B. L. Kurbatov, F. E. V i l e s o v and A. N. Terenin, Soviet Phys. Dokl, 6, 883 (1962). 72. D. W. Turner and M. I. Al-Joboury, J . Chem. Phys., 3J7, 3007 (1962) . 73. M. I. Al-Joboury and D. W. Turner, J . Chem. S o c , 5141 (1963). 74. M. I. Al-Joboury and D. W. Turner, J . Chem. S o c , 4434 (1964). 75. R. I. Schoen, J . Chem. Phys., 4£, 1830 (1964). 76. J . Franck, Trans. Far. S o c , 21_, 536 (1925). 77. E. U. Condon, Phys. Rev., 32, 858 (1928). 78. M. Halmann and I. L a u l i g h t , J . Chem. Phys., 43_, 1503 (1965). 79. M. E. Wacks, J . Res. Nat. Bur. Stand., A68, 631 (1964). 80. R. W. N i c h o l l s , J . Res. Nat. Bur. Stand., A65, 451 (1961). 81. G. H. Dunn, J . Chem. Phys., 44_, 2592 (1966). 82. D. C. F r o s t , C. A. McDowell and D. A. Vroom, Phys. Rev. L e t t . , 15_, 612 (1965) . 83. D. C. F r o s t , C. A. McDowell and D. A. Vroom, Proc. Roy. S o c , A296, 566 (1967). -155-84. G. R. Cook and B. K . Ching, Aerospace Corp. Rept. TDR-469 (9260-01) -4, E l Segundo, C a l i f . (1965). 85. D. W. Turner, Proc. Roy. S o c . A307, 15 (1968). 86. W. W. Robertson, J . Chem. Phys., 44, 2739 (1966). 87. A. L. Schmeltekopf, F. C. Fehsenfeld and E. E. Ferguson. J . Chem. Phys., 48_, 2966 (1968). 88. J . Berkowitz, H. Ehrhardt and T. Tekaat, Z. Physik, 200, 69 (1967). 89. J . Berkowitz and H. Ehrhardt, Phys. L e t t . , 21_, 531 (1966). 90. D- A. Vroom, A. R. Comeaux and J . W. McGowan, Chem. Phys. L e t t . , 3, 476 (1969). 91. J . W. McGowan, D. A. Vroom and A. R. Comeaux, J . Chem. Phys. 51. , 5626 (1969). 92. M. A. Chaffee, Phys. Rev., 37, 1233 (1931). 93. L. H a l l and M. W. S i e g e l , J . Chem. Phys., 48_, 943 (1968). 94. H. H a r r i s o n , J . Chem. Phys., 52, 901 (1970) 95. J . M. S i c h e l , Mol. Phys., 18, 95 (1970). 96. R. Morgenstern, A. Niehaus and M. W. Ruf, Chem. Phys. L e t t . , 4, 635 (1970). 97. P. Auger and F. P e r v i n , J . Phys. Ser. V I , 8, 93 (1927). 98. G. Sehur, Ann. Phys., 4, 433 (1930). 99. H. A. Bethe, Handbuch der Physik, 24, 483 (1935). 100. J . Cooper and R. N. Zare, J . Chem. Phys., 48_, 942 (1968). 101. J . W. Cooper and S. T. Manson, Phys. Rev., 177, 157 (1969). 102. H. Hotop and A. Niehaus, Chem. Phys. L e t t . , 8_, 497 (1971). 103. For example, " A u t o i o n i z a t i o n " , Ed. A. Temkin, Mono Book Corp., Baltimore (1966). 104. E. E. Ferguson, Phys. Rev., 128, 210 (1962). 105. D. R. Bates, K. L. B e l l and A. E. Kingston, Proc. Phys. S o c (London) , 91_, 288 (1967) . -156-106. C. R. Jones and W. W. Robertson. J . Chem. Phys., 49_, 4241 (1968). 107. K. Katsuura, J . Chem. Phys., 42, 3771 (1965). 108. T. Watanabe and K. Katsuura, J . Chem. Phys., £7_, 800 (1967). 109. M. M o r i , T. Watanabe and K. Katsuura, J . Phys. Soc. (Japan), 19, 320 (1964). 110. M. M o r i , J . Phys. Soc. (Japan), 26, 773 (1969). 111. H. Nakamura, J . Phys. Soc. (Japan), 26, 1473 (1969). 112. T. Watanabe, J . Chem. Phys., 46, 3741 (1967). 113. U. Fano, Phys. Rev., 124, 1866 (1966). 114. W. H . M i l l e r , J . Chem. Phys., 52_, 3563 (1970). 115. H. 29 F u j i i , H. Nakamura and M. M o r i , J . Phys. Soc. (Japan). , 1030 (1970). 116. W. H 1421 . M i l l e r and H. F. Schaefer, I I I , J . Chem. Phys., 53, (1970) . 117. J . W . Sheldon, J . Appl. Phys., 37_, 2928 (1966). 118. R. S Phys: . Berry i n "Proceedings of the I n t e r n a t i o n a l School of Lcs, 'Enrico Fermi'", Ed. Ch. S c h l i e r , Academic Press (1970) 1]9. S. E . N i e l s e n and J . S. Dahler, J . Chem. Phys., 4_5, 4060 (1966) . 120. J . N . Bardsley, J . Phys. B, 1_, 348 (1968). 121. S. E . N i e l s e n and R. S. Berry, Chem. Phys. L e t t . , 2, 503 (1968) . 122. R. S . Berry, J . Chem. Phys., 45_, 1228 (1966). 123. J . N . Bardsley, Chem. Phys. L e t t . , 1_, 229 (1967). 124. H. D . Hagstrum, Phys. Rev., 96, 336 (1954). 125. A. L . Hughes and V. Rojansky, Phys Rev., 34, 284 (1929). 126. A. L . Hughes and J . H. M c M i l l e n , Phys. Rev., 34_, 291 (1929). 127. J . J . Leventhal and G. R. North, Rev. Sci . I n s t . , 44_, 120 (1971) 128. J . Peresse, F. T u f f i n and A. Le Nadan, Methods Physiques d'Analyse, 3, 121 (1967). -157-129. C. E. Brion and G. E. Thomas, I n t . J . Mass Spectrom. Ion Phys., 1, 25 (1968). 130. D. C. Frost and C. A. McDowell, A i r Force Cambridge Research Laboratory, Document #TR-60-423 (1960). 131. G. Herzberg, "Atomic Spectra and Atomic S t r u c t u r e " , Second E d i t i o n , Dover P u b l i c a t i o n s , New York (1964). 132. C. E. B r i o n , Anal. Chem., 37_, 1706 (1965). 133. C. E. B r i o n , Anal. Chem., 38, 1941 (1966). 134. M. Z e l i k o f f , P. H. Wychoff, C. M. Auschenbrand and R. S. Loomis, J . Opt. Soc. Am., 49_, 338 (1952). 135. E. S. Fry and W. L. W i l l i a m s , Rev. S c i . I n s t . , 40_, 1141 (1969). 136. H. Hotop, Doctoral T h e s i s , U n i v e r s i t y of F r e i b u r g (1971). 137. M. J . van der Wiel and C. E. B r i o n , J . E l e c t r o n S p e c t r o s c , 1_, 309 (1972/73). 138. D. A. Vroom, Ph.D. Th e s i s , U n i v e r s i t y of B.C. (1966). 139. C. E. Moore, "Atomic Energy L e v e l s , V o l . I " , N a t i o n a l Bureau of Standards, C i r c u l a r No. 467 (1949). 140. D. R. Bates, Phys. Rev., 77, 718 (1950). 141. J . Herce, K. D. Foster and E. E. M u s c h l i t z , B u l l . Am. Phys. S o c , U>_, 206 (1968) . 142. H. Hotop, E. I l l e n b e r g e r , H. Morgner and A. Niehaus, i n L. Branscomb et a l . , "The Physics of E l e c t r o n i c and Atomic C o l l i s i o n s " , VII ICPEAC, North Holland Publ. Co., Amsterdam (1972). 143. H. Hotop and A. Niehaus, i n I. Amdur (ed.), "The Physics of E l e c t r o n i c and Atomic C o l l i s i o n s " , VI ICPEAC, MIT Press, Cambridge, Mass. (1969). 144. C. E. Brion and D. Yee, unpublished work. 145. R. S. Freund and W. Klemperer, J . Chem. Phys., 47_, 2897 (1967). 146. W. E. Lamb and R. C. Retherford, Phys. Rev., 79_, 549 (1950). 147. J . A. R. Samson and R. B. C a i r n s , Phys. Rev., 173, 80 (1968). 148. J . A. R. Samson, Rev. S c i I n s t . , 40_, 1174 (1969). -158-149. J . L. F r a n k l i n et a l . , " I o n i z a t i o n P o t e n t i a l s , Appearance P o t e n t i a l s and Heats of Formation of Gaseous P o s i t i v e Ions", NSRDS-NBS 26, U.S. Govt. P r i n t i n g O f f i c e , Washington, D.C. (1969). 150. L. Wallace, Astrophys. J . Suppl, 7_, 165 (1962). 151. M. E. Wacks, J . Chem. Phys., 41, 930 (1964). 152. 0. Edquist, L. Asbrink and E. Lindholm, Z. N a t u r f o r s c h . , 26a, 1407 (1971). 153. C. E. B r i o n , C. A. McDowell and W. B. Stewart, Chem. Phys. L e t t . , 13, 79 (1972). 154. H. Hotop, i n "Advances i n Mass Spectrometry" V, Ed. H. Quayle, E l s e v i e r , Essex (1971). 155. 0. Edquist, E. Lindholm, L. E. S e l i n , L. Asbrink, C. E. Kuyatt, S. R. M i e l c z a r e k , J . A. Simpson and I. Fisher-Hjalmars, Physica S c r i p t a , 1_, 172 (1970) . 156. W. C. Richardson, D. W. Setser, D. L. A l b r i t t o n and A. L. Schmeltekopf, Chem. Phys. L e t t . , 12_, 349 (1971). 157. V. Cermak and J . Sramek, J . E l e c t r o n S p e c t r o s c , 2_, 97 (1973). 158. J . S. Sandhu, Ph.D. Thesis, U n i v e r s i t y of B.C. (1969). 159. J . H. D. Eland and C. J . Danby, I n t . J . Mass Spectrom. Ion Phys., 1, 111 (1968). 160. A. B. F. Duncan, J . Chem. Phys., 4, 638 (1936). 161. G. I. R. Branton, D. C. F r o s t , T. Makita, C. A. McDowell and A. Stenhouse, P h i l . Trans. Roy. Soc. Lond. A. 268, 77 (1970). 162. J . J . A. Herce, J . R. Penton, R. J . Cross and E. E. M u s c h l i t z , J r . , Chem. Phys., A9_, 958 (1968). 163. C. Baker and D. W. Turner, Chem. Comm., 797 (1967). 164. V. Jacobs, Phys. Rev. A, 3, 289 (1971). 165. D. W. Norcross, J . Phys. B., 4, 652 (1971). 166. 0. von Ross, J . Chem. Phys., 30, 729 (1959). 167. V. A. Lo Dato, J . Chem. Phys., 50_, 2588 (1969). 168. M. Matsuzawa and K. Katsuura, J . Chem. Phys., 52, 3001 (1970). -159-169. D. J . K l e i n , J . Chem. Phys., 50, 5151 (1969). 170. A. V. Phelps and J . P. Molnar, Phys. Rev., 89, 1202 (1953). 171. W. B. Hunt, J . Chem. Phys., 45_, 2713 (1966). 172. Y. F. Bydin, V. I. Ogurtsov and V. S. S i r a z e t d i n o v , i n "The Physics of E l e c t r o n i c and Atomic C o l l i s i o n s " V I I , ICPEAC, Eds. L. Branscomb et a l . , North-Holland Publ. Co., Amsterdam (1972) . 173. D. A. MacLennan and T. A. Delchar, J . Chem. Phys., 50_, 1772 (1969) . 174. T. A. Delchar, D. A. MacLennan and A. M. Landers, J . Chem. Phys. , 50_, 1779 (1969) . 175. C. E. Brion and L. A. R. Olsen, J . Phys. B. , 3_, 1020 (1970). 176. C. E. Brion and L. A. R. Olsen, Chem. Phys. L e t t . , 22_, 400 (1973). 177. G. Gerber, R. Morgenstern, A. Niehaus and M. W. Ruf, i n "The Physics of E l e c t r o n i c and Atomic C o l l i s i o n s " , V I I , ICPEAC, Ed. L. Branscomb et a l . , North-Holland Publ. Co., Amsterdam (1972) . 178. H. W. Berry, Phys. Rev. A, 6, 1805 (1972). 179. K. Siegbahn, C. N o r d l i n g , G. Johansson, J . Hedman, P. F. Heden, K. Hamrin, V. G e l i u s , T. Bergmark, L. 0. Werme, R. Manne and Y. Baer, "ESCA Applied to Free Molecules", North-Holland Publ. Co., Amsterdam (1971). 180. G. Gerber, R. Morgenstern and A. Niehaus, Phys. Rev. L e t t . , 23_, 511 (1969). 181. R. H. Hammond, J . M. S. Henis, E. F. Green and J . Ross, J . Chem. Phys. , 55_, 3506 (1971) . 182. J . A. D i l l o n , W. F. Sheridan, H. D. Edwards and S. N. Ghosh, J . Chem. Phys., 23_, 776 (1955). 

Cite

Citation Scheme:

        

Citations by CSL (citeproc-js)

Usage Statistics

Share

Embed

Customize your widget with the following options, then copy and paste the code below into the HTML of your page to embed this item in your website.
                        
                            <div id="ubcOpenCollectionsWidgetDisplay">
                            <script id="ubcOpenCollectionsWidget"
                            src="{[{embed.src}]}"
                            data-item="{[{embed.item}]}"
                            data-collection="{[{embed.collection}]}"
                            data-metadata="{[{embed.showMetadata}]}"
                            data-width="{[{embed.width}]}"
                            async >
                            </script>
                            </div>
                        
                    
IIIF logo Our image viewer uses the IIIF 2.0 standard. To load this item in other compatible viewers, use this url:
http://iiif.library.ubc.ca/presentation/dsp.831.1-0059921/manifest

Comment

Related Items