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UBC Theses and Dissertations

Electron spectroscopy using metastable helium atoms (2¹S,2³S) and 584 Å photons Yee, Derek Sui Chang 1975

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ELECTRON SPECTROSCOPY USING METASTABLE HELIUM ATOMS ^ S ^ S ) AND 584 A PHOTONS by DEREK SUI CHANG YEE .Sc. Hons., U n i v e r s i t y o f B r i t i s h Columbia, 197 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n the Department o f CHEMISTRY We accept t h i s t h e s i s as conforming to the r e q u i r e d standard UNIVERSITY OF BRITISH COLUMBIA October, 1975. 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 requirements 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 Columbia, I agree that 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 fo r re ference and study. I f u r t h e r agree t h a t permiss ion for e x t e n s i v e copying of 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 granted by the Head of my Department or by h i s r e p r e s e n t a t i v e s . I t i s understood that copy ing or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l ga in s h a l l not be a l lowed without my w r i t ten pe rm i ss i on . Department of C_Vve\,vxXs\< The U n i v e r s i t y of B r i t i s h Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 D a t e W v XS /\c\iS ABSTRACT Using a high r e s o l u t i o n 127 degree e l e c t r o n a n a l y z e r , a q u a n t i t a t i v e comparison has been made of the He (2 1S,2 3S) Penning e l e c t r o n and the 584 % p h o t o e l e c t r o n s p e c t r a o f t h i r t y molecules (H^, HD, D 2, Ng, CO, NO, 0^ NH^, PH 3, C ^ , HCN,, (CN) 2, CH^CN, BrCN, ICN, H 20, CHgOH, CT^CH^H, (CH^CHOH, (CH 3) 3COH, CH 3OCH 3, CH^OCH2, CH 3CH 2OCH 2CH 3, CH 2CH 20CH 2CH 2, OCH 2CH 2OCH 2CH 2, HCHO, CH3CHO, CH 3COCH 3, HCOOH and CH^OOH). Where v i b r a t i o n a l s t r u c t u r e has been r e s o l v e d , the v i b r a t i o n a l spacings are observed t o be the same ( w i t h i n e x p e r i -mental e r r o r ) f o r both modes o f i o n i z a t i o n . In a d d i t i o n , the r e l a t i v e v i b r a t i o n a l i n t e n s i t i e s f o r i o n i c s t a t e s o f the d i a -tomic molecules (except f o r 0 2) were found t o be the same f o r both modes o f i o n i z a t i o n . For the process He*(2 3S)/0^(X 2n ), an <^  g observed d i f f e r e n c e o f the v i b r a t i o n a l envelope f o r He*(2 3S) Penning i o n i z a t i o n and 584 X p h o t o i o n i z a t i o n has been t r a c e d t o an a u t o i o n i z i n g l e v e l o f 0 2 which i s e s s e n t i a l l y resonant with the He*(2 3S) metastable energy. D i f f e r e n c e s between the v i b r a t -i o n a l envelopes 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 have a l s o been observed f o r the ground i o n i c s t a t e o f a number o f oxygen c o n t a i n i n g molecules. These d i f f e r e n c e s have been e x p l a i n e d by p e r t u b a t i o n s o f the p o t e n t i a l s u r f a c e o f the t a r g e t molecule due to the presence o f the metastable atom. -111-When comparing the r e l a t i v e e l e c t r o n i c s t a t e p o p u l a t -ions f o r the two modes o f i o n i z a t i o n , l a r g e d i f f e r e n c e s were observed. Some o f the d i f f e r e n c e has been e x p l a i n e d by the f a c t t h a t the s t a t e p o p u l a t i o n s are a f u n c t i o n o f the e x c i t a t i o n energy, s i n c e comparisons were made between photons w i t h the energy 21.22 eV and metastable atoms with the e n e r g i e s 20.62 eV and 19.82 eV. Por the - C H N c o n t a i n i n g molecules i t was observed t h a t the r a t i o s o f the r e l a t i v e p o p u l a t i o n s o f s t a t e s correspond-i n g to the removal o f n bonding to n i t r o g e n lone p a i r were s i g n i f i c a n t l y g r e a t e r f o r p h o t o i o n i z a t i o n than f o r Penning i o n -i z a t i o n . F i n a l l y , the energy s h i f t s AE were measured f o r the obs He*(2 1S,2 3S) Penning i o n i z a t i o n p r o c e s s . When v i b r a t i o n a l s t r u c t u r e was apparent i n the e l e c t r o n s p e c t r a , i t was p o s s i b l e to evaluate the t r u e AE energy s h i f t from AE . . The magnitude obs of the t r u e AE energy s h i f t was found t o be o f thermal e n e r g i e s . - i v -TABLE OF CONTENTS Page CHAPTER ONE I n t r o d u c t i o n .. . 1 1.1. C h e m i - i o n i z a t i o n Reactions 1 1.2. Experimental Methods f o r Chemi-i o n i z a t i o n S t u d i e s 3 1.2.1. A n a l y s i s o f the Ions 3 1.2.2. A n a l y s i s o f the E l e c t r o n s ^ 1.3. E x c i t e d Atoms and Molecules 7 1.3.1. I n t r o d u c t i o n 7 1.3-2. P r o d u c t i o n and Quenching of Metastables 11 CHAPTER TWO I o n i z a t i o n Processes 1^ 2.1. P h o t o i o n i z a t i o n 1^ 2.1.1. I n t r o d u c t i o n I** 2.1.2. Pranck-Condon P r i n c i p l e 15 2.1.3. A u t o i o n i z a t i o n IB 2.2, Penning I o n i z a t i o n !9 2.2.1. Q u a l i t a t i v e D e s c r i p t i o n 19 2.2.2. P o t e n t i a l Curve Model ?2 2.2.3. Quantum Mechanical Treatment o f the P o t e n t i a l Curve Model 29 -V-Page 2.2.4. Penning E l e c t r o n S p e c t r a o f Molecules 31 a. Shape o f the V i b r a t i o n a l Envelope 31 b. R e l a t i v e E l e c t r o n i c S t a t e P o p u l a t i o n s 33 c. Energy S h i f t s 36 CHAPTER THREE Experimental . 38 3.1. I n t r o d u c t i o n 38 3.2. The Spectrometer 38 3.2.1. Metastable Source 38 3.2.2. C o l l i s i o n Region 4 l 3.2.3. E l e c t r o n Analyzer 42 3.2.4. E l e c t r o n D e t e c t i o n 45 3-2.5. L i g h t Source u 5 3.2.6. Vacuum System ^8 3.3. Treatment o f Data 50 3.3.1. Energy C a l i b r a t i o n 50 3.3.2. E l e c t r o n A n alyzer T r a n s m i s s i o n F u n c t i o n 50 3.3.3. Background S u b t r a c t i o n Technique 51 3.4. Sample P u r i t y 53 CHAPTER FOUR Diatomic Molecules 55 3.1. I n t r o d u c t i o n - v i -Page 55 3.2. Molecular Hydrogen, Deuterium Hydride and Molecular Deuterium 56 3.3. Molecular Nitrogen 62 3.4. Carbon Monoxide 6R 3.5. N i t r i c Oxide 74 3.6. Molecular Oxygen 79 CHAPTER FIVE Simple Polyatomic Molecules 87 4.1. I n t r o d u c t i o n 87 4.2. Ammonia 87 4.3- Phosphine °2 4.4. Ethylene , 9 6 CHAPTER SIX Hydrogen Cyanide and Some Related Compounds 102 5.1. I n t r o d u c t i o n 102 5.2. Hydrogen Cyanide 10 3 5.3. Dicyanogen — 105 5-4. A c e t o n i t r i l e 10° 5.5. Cyanogen Bromide and Cyanogen Iodide 110 CHAPTER SEVEN Water, Alcohols and Ethers 120 6.1. I n t r o d u c t i o n - v i i -Page 120 6.2. Water 1 2 0 6.3. M e t h a n o l and E t h a n o l 1 2 6.4. I s o p r o p y l A l c o h o l and T e r t i a r y B u t y l A l c o h o l 6.5. D i m e t h y l E t h e r and E t h y l e n e Oxide 6.6. D i e t h y l E t h e r , T e t r a h y d r o f u r a n , and 1,4 Dioxane 1 3 ' 6.7. D i s c u s s i o n o f t h e Franck-Condon En v e l o p e s f o r T r a n s i t i o n s t o t h e . Ground I o n i c S t a t e 1 -CHAPTER EIGHT C a r b o n y l C o n t a i n i n g Compounds 7.1. I n t r o d u c t i o n .... .... 7.2. Formaldehyde and A c e t a l d e h y d e 7.3. Acetone *53 7.4. Formic A c i d and A c e t i c A c i d 7.5. D i s c u s s i o n o f t h e Franck-Condon E n v e l o p e s f o r T r a n s i t i o n s t o t h e Ground I o n i c S t a t e I .3 CHAPTER NINE C o n c l u s i o n s REFERENCES 167 - v i i i -LIST OP FIGURES F i g u r e Page 1 C o r r e l a t i o n between the Franck-Condon P r i n c i p l e and the shape o f UPS bands f o r the removal of e l e c t r o n s from molecular o r b i t a l s of d i f f e r e n t bonding c h a r a c t e r .17 2 Heavy p a r t i c l e s c a t t e r i n g diagram u s i n g the impact parameter f o r m u l a t i o n , a. Re p u l s i v e system. b. A t t r a c t i v e and R e p u l s i v e system 20 3 P o t e n t i a l curve model showing the s i g n i f i c a n c e o f the t r u e AE energy s h i f t , a. R e p u l s i v e Penning i o n i z a t i o n system, b. A t t r a c t i v e Pen-ni n g i o n i z a t i o n system 23 4 P o t e n t i a l curve model showing the r a t i o of Penning i o n i z a t i o n and a s s o c i a t i v e i o n i z a t i o n . a. R e p u l s i v e Penning i o n i z a t i o n system. b. A t t r a c t i v e Penning i o n i z a t i o n system 28 5 P o t e n t i a l curve model; f o r i o n i z a t i o n by a. photons and b. metastable atoms 34 6 Schematic diagram of the e l e c t r o n s p e c t r o -meter 39 7 Schematic o f the p r e a m p l i f i e r c i r c u i t 6^ 8 Schematic o f the c o n t r o l c i r c u i t ^7 9 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 f o r the 127° e l e c t r o n ' a n a l y z e r r"?-10 Background s u b t r a c t i o n a p p l i e d t o the Penning e l e c t r o n spectrum of argon 54 11 Penning e l e c t r o n s p e c t r a of molecular hydrogen, deuterium h y d r i d e and molecular deuterium 5° 12 E s t i m a t i o n o f 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 f o r the i o n i z a t i o n o f hydrogen 61 13 E l e c t r o n s p e c t r a f o r t h e i o n i z a t i o n of molecular n i t r o g e n "3 - i x -E l e c t r o n s p e c t r a f o r the i o n i z a t i o n of carbon monoxide E l e c t r o n s p e c t r a f o r the i o n i z a t i o n of n i t r i c oxide E l e c t r o n s p e c t r a f o r the i o n i z a t i o n of molecular oxygen E l e c t r o n sjpectra f o r the i o n i z a t i o n of 0 2 t o O+CX 2 n g ) .... E l e c t r o n s p e c t r a f o r the i o n i z a t i o n of molecular oxygen E l e c t r o n s p e c t r a f o r the i o n i z a t i o n o f ammonia E l e c t r o n s p e c t r a f o r the I o n i z a t i o n of phosphine E l e c t r o n s p e c t r a f o r the i o n i z a t i o n of ethylene E l e c t r o n s p e c t r a f o r the i o n i z a t i o n of ethylene t o the ground s t a t e i o n (X 2 B 3 u ) E l e c t r o n s p e c t r a f o r the i o n i z a t i o n of hydrogen cyanide, a. f u l l s p e c t r a b. f i r s t band i n c l u d i n g 584 A c a l i b r a t i o n E l e c t r o n s p e c t r a f o r the i o n i z a t i o n of dicyanogen E l e c t r o n s p e c t r a f o r the i o n i z a t i o n o f a c e t o n i t r i l e E l e c t r o n s p e c t r a f o r the i o n i z a t i o n of cyanogen bromide E l e c t r o n s p e c t r a f o r the i o n i z a t i o n of cyanogen i o d i d e E l e c t r o n s p e c t r a f o r the i o n i z a t i o n of water E l e c t r o n s p e c t r a f o r the i o n i z a t i o n of methanol -X-•• . Page 30 E l e c t r o n spectra f o r the I o n i z a t i o n of ethanol 127 31 E lec t ron spectra f o r the I o n i z a t i o n of Isopropyl a l cohol 13? 32 E l e c t r o n spectra f o r the I o n i z a t i o n of t e r t i a r y b u t y l a l cohol 133 33 E l e c t r o n spectra f c r the I o n i z a t i o n of dimethyl ether 135 34 E l e c t r o n spectra f o r the i o n i z a t i o n of ethylene oxide 1 3 r 35 E l e c t r o n sDectra f o r the i o n i z a t i o n of d i e t h y l ether 36 E l e c t r o n spectra f o r the i o n i z a t i o n of tetrahydrofuran 1^ 2 37 E l e c t r o n spectra f o r the i o n i z a t i o n of 1,4 dioxane , l Z ,3 38 E l e c t r o n spectra f o r the ground i o n i c states of water, a l c o h o l s , and ethers . Energy s h i f t s are given i n eV 146 39 E l e c t r o n spectra f o r the i o n i z a t i o n of formaldehyde 1-° 40 E l e c t r o n spectra f o r the i o n i z a t i o n of acetaldehyde ^ 1 41 E l e c t r o n spectra f o r the i o n i z a t i o n of acetone 15 42 E l e c t r o n spectra f o r the i o n i z a t i o n of formic ac id ; 159 43 E l e c t r o n spectra f o r the i o n i z a t i o n of acet ic a c i d * - 0 44 E l e c t r o n spectra for the ground i o n i c state of some carbonyl containing compounds. Energy s h i f t s are given i n eV 1 ^  - x i -LIST OP PLATES P l a t e Page 1. The spectrometer 40 2 Complete experimental arrangement 40 - x i i -LIST OP TABLES Table P a S e 1 O p t i c a l s e l e c t i o n r u l e s i n atomic s p e c t r a .. y 2 A E o b s energy s h i f t s i n eV f o r the He*(2 1S,2 3S) Penning i o n i z a t i o n o f molecular hydrogen, deuterium h y d r i d e and mol e c u l a r deuterium at 300 °K 59 3 A E o b § energy s h i f t s i n eV f o r the He*(2 1S,2 3S) v Penning i o n i z a t i o n o f molecular n i t r o g e n at 300 °K 6ij 4 Normalized 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 2(X;v"= o) N+(X,A,B;v' ) f o r 584 8 p h o t o i o n i z a t i o n and He*(2 1S,2 3S) Penning i o n i z a t i o n 6R 5 Normalized r e l a t i v e e l e c t r o n i c s t a t e p o p u l a t i o n s f o r 584 A p h o t o i o n i z a t i o n and He*(2 1S,2^S) Penning i o n i z a t i o n o f mo l e c u l a r n i t r o g e n at 300 °K 6 7 6 A E o b energy s h i f t s i n eV f o r the He*(2 1S,2 3S) Penning i o n i z a t i o n of carbon monoxide at 300 °K ., 70 7 Normalized 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 C0(X,v"= o) •*•• C0(X,A,B,v« ) f o r 584 t p h o t o i o n i z a t i o n and He*(2 1S,2 3S) Penning i o n i z a t i o n ..... 72 8 Normalized r e l a t i v e e l e c t r o n i c s t a t e p o p u l -a t i o n s f o r 584 ft p h o t o i o n i z a t i o n and He*(2 1S,2 3S) Penning i o n i z a t i o n of carbon monoxide at 300 °K.... 73 9 A E o b energy s h i f t s i n eV f o r the He*(2 1S,2 3S) Penning i o n i z a t i o n o f n i t r i c oxide at 300 °K 77 - x i i i -Page 10 Normalized r e l a t i v e e l e c t r o n i c s t a t e p o p u l a t i o n s f o r 584 A p h o t o i o n i z a t i o n and He* (2 3S) Penning i o n i z a t i o n of n i t r i c oxide at 300 °K .............. 78 11 Normalized 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 0 2(X,v* ! = o) ot(X,v' ) f o r 584 A p h o t o i o n i z a t i o n and He* ( 2 3 S 7 Penning i o n i z a t i o n 82 12 A E o b s e n e r & y s h i f t i n eV and the normalized r e l a t i v e e l e c t r o n i c s t a t e p o p u l a t i o n s f o r 584 A photo-i o n i z a t i o n and He * ( 2 3S) Penning i o n i z a t i o n of ammonia at 300 °K 91 13 A E o b s e n e r & y s h i f t i n eV and the normalized r e l a t i v e e l e c t r o n i c s t a t e p o p u l a t i o n s f o r 584 A p h o t o i o n i z a t i o n and He * ( 2 3S) Penning i o n i z a t i o n of phosphine at 300 °K 95 14 AEob<? e n e r S Y s h i f t i n eV and the normalized r e l a t i v e e l e c t r o n i c s t a t e p o p u l a t i o n s f o r 584 A p h o t o i o n i z a t i o n and He * ( 2 3S) Penning i o n i z a t i o n o f ethylene at 300 <>K oo 15 A E o b s e n e r g y s h i f t i n eV and the normalized r e l a t i v e e l e c t r o n i c s t a t e p o p u l a t i o n s f o r 584 J? p h o t o i o n i z a t i o n and He*(2 3S) Penning i o n i z a t i o n of dicyanogen at 300 °K .. , '. 10R 16 A E n b s e n© rgy s h i f t i n eV and the normalized r e l a t i v e e l e c t r o n i c s t a t e p o p u l a t i o n s f o r 584 & p h o t o i o n i z a t i o n and He*(2 3S) Penning i o n i z a t i o n of a c e t o n i t r i l e at 300 °K 112 17 A E o b s e n e r s y s h i f t i n eV and the normalized r e l a t i v e e l e c t r o n i c s t a t e p o p u l a t i o n s f o r 584 A p h o t o i o n i z a t i o n and He*(2 3S) Penning i o n i z a t i o n of cyanogen bromide at 300 °K 116 18 A E . energy s h i f t i n eV and the normalized r e l a t i v e e l e c t r o n i c s t a t e p o p u l a t i o n s f o r 584 & p h o t o i o n i z a t i o n and He*(2 3S) Penning i o n i z a t i o n of cyanogen i o d i d e at 300 °K 118 19 & E o b s e n e i * g y s h i f t i n eV and the normalized r e l a t i v e e l e c t r o n i c s t a t e p o p u l a t i o n s f o r 584 % p h o t o i o n i z a t i o n and He*(2 3S) Penning i o n i z a t i o n of water at 300 °K 123 - x l v -Page 20 A E o b s e n e r S y s h i f t i n ev" and the normalized r e l a t i v e e l e c t r o n i c s t a t e p o p u l a t i o n s f o r 584 A p h o t o i o n i z a t i o n and He* ( 2 3S) Penning i o n i z a t i o n of methanol at 300 °K 129 2 1 A E o b s e n e r S v s n l f t i n eV and the normalized r e l a t i v e e l e c t r o n i c s t a t e p o p u l a t i o n s f o r 5 8 4 A o h o t o i o n i z a t i o n and He* ( 2 3S) Penning i o n i z a t i o n o f etha n o l at 3 0 0 °K 130 22 A E o b s e n e r & v s h i f t i n eV and the normalized r e l a t i v e e l e c t r o n i c s t a t e p o p u l a t i o n s f o r 5 8 4 A p h o t o i o n i z a t i o n and He* ( 2 3S) Penning i o n i z a t i o n o f dimethyl e t h e r at 300 °K 138 2 3 A E o b s e n e r s y s h i f t i n eV and the normalized r e l a t i v e e l e c t r o n i c s t a t e p o p u l a t i o n s f o r 5 8 4 & p h o t o i o n i z a t i o n and He* ( 2 3S) Penning i o n i z a t i o n of ethylene oxide at 300 °K 139 24. A E 0 h S energy s h i f t i n eV f o r He* ( 2 3S) Penning i o n i z a t i o n o f d i e t h y l ether,- t e t r a h y d r o f u r a n and 1 , 4 dioxane at 300 6K 144 25 A E o b s e n e r s y s h i f t i n eV and the normalized r e l a t i v e e l e c t r o n i c s t a t e p o p u l a t i o n s f o r 5 8 4 A p h o t o i o n i z a t i o n and He* ( 2 3S) Penning i o n i z a t i o n of formaldehyde at 300 °K 1 5 2 26 A E o b s e n e r s y s h i f t i n eV and the normalized r e l a t i v e e l e c t r o n i c s t a t e p o p u l a t i o n s f o r 584 A p h o t o i o n i z a t i o n and He* ( 2 3S) Penning i o n i z a t i o n o f acetaldehyde a t 3 0 0 °K 154 27 A E o b s e n e r & y s h i f t i n eV and the normalized r e l a t i v e e l e c t r o n i c s t a t e p o p u l a t i o n s f o r 584 A p h o t o i o n i z a t i o n and He* ( 2 3S) Penning I o n i z a t i o n of acetone at 3 0 0 °K 157 28 A E o b s energy s h i f t i n eV and the normalized r e l a t i v e e l e c t r o n i c s t a t e p o p u l a t i o n s f o r 584 A o h o t o i o n i z a t i o n and He* ( 2 3S) Penning i o n i z a t i o n o f formic a c i d at 300 °K 161 29 A E o b s energy s h i f t i n eV and the normalized r e l a t i v e e l e c t r o n i c s t a t e p o p u l a t i o n s f o r 5 8 4 A p h o t o i o n i z a t i o n and He* ( 2 3S) Penning i o n i z a t i o n o f a c e t i c a c i d at 300 °K 162 -XV-ACKNOWLEDGEMENTS I would l i k e t o thank s i n c e r e l y Dr. C.E. B r i o n f o r h i s continued guidance and support throughout the course o f t h i s work. I would a l s o l i k e t o thank my many c o l l e a g u e s , Mr. T. Chau, Dr. A. Hamnett, Mr. A.P. Hitch c o c k , Dr. S.T. Hood, Dr. S.-T. Lee, Dr. W. S t o l l , Dr. W.-C. Tarn and Dr. G.R. Wight 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 u n s e l f i s h a s s i s t a n c e on many o c c a s i o n s . Por the m o d i f i c a t i o n s and the many r e p a i r s t o the apparatus, I would l i k e t o acknowledge the very capable s t a f f s o f the E l e c t r o n i c and Mechanical Workshops. I thank the N a t i o n a l Research C o u n c i l o f Canada f o r f i n a n c i a l support and I g r a t e f u l l y acknowledge the r e c e i p t o f an H.R. MacMillan Family F e l l o w s h i p . F i n a l l y , I would l i k e t o thank my l a t e f a t h e r , my mother and my s i s t e r s f o r t h e i r p a t i e n c e which I must have t e s t e d o f t e n , and my wife f o r her understanding and l o v e . CHAPTER ONE INTRODUCTION 1.1. C h e m i - i o n i z a t i o n Reactions The word c h e m i - i o n i z a t i o n , f i r s t coined by C a l c o t e 1 , has been d e f i n e d by B e r r y 2 as i n c l u d i n g " a l l processes that r e s u l t i n the form a t i o n o f f r e e charges, e l e c t r o n s or i o n s , under c o n d i t i o n s of chemical r e a c t i o n s " . T h i s broad d e f i n i t i o n i n c l u d e s i o n i z a t i o n processes I n v o l v i n g heavy p a r t i c l e s c o l l i d i n g at thermal e n e r g i e s w i t h 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 , with n a t u r a l l i f e t i m e s which are comparable to the mean c o l l i s i o n times.; Por the b i n a r y system (X*,A) where X* i s an atom which i s i n a l o n g - l i v e d e x c i t e d s t a t e and A i s a t a r g e t atom which i s i n the ground s t a t e , t h e r e are thr e e c h e m i - i o n i z a t i o n processes which may occur when the e x c i t a t i o n p o t e n t i a l o f the e x c i t e d atom X, E ( X * ) , i s g r e a t e r than 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 atom A, IP ( A ) . X* + A X + A + + e + X + A + ne M u l t i p l e Penning I o n i z a t i o n Penning I o n i z a t i o n (1.1.1.1) (1.1.1.2) X* + A + XA + + e A s s o c i a t i v e I o n i z a t i o n (1.1.2) -2-X* + A -f X + + A + e C o l l i s l o n a l E l e c t r o n Release (1.1.3.1) •+ X + + A (1.1.3.2) In 1927 P e nning 3 demonstrated t h a t r e a c t i o n 1.1.1.1 was r e s p o n s i b l e f o r the l o w e r i n g o f the i g n i t i o n v o l t a g e s o f neon and argon glow d i s c h a r g e s when very s m a l l amounts of i m p u r i t y gases were added. T h i s r e a c t i o n i s g e n e r a l l y r e f e r r e d to as Pen-ni n g i o n i z a t i o n , however, i n r a d i a t i o n chemistry i t i s r e f e r r e d to as J e s s e i o n i z a t i o n * 1 . Double Penning i o n i z a t i o n has r e c e n t l y been r e p o r t e d 5 . S t u d i e s up to recent times of these three chemi-i o n i z a t i o n processes have been reviewed by a number o f a u t h o r s 2 » 6 >7. which i s i n a l o n g - l i v e d e x c i t e d s t a t e and AB Is the t a r g e t mole-c u l e which i s i n the ground s t a t e , t h e r e are two a d d i t i o n a l chemi-i o n i z a t i o n p rocesses which may occur when E(X*) > IP(AB). Por the b i n a r y system (X*,AB) where X* i s an atom X« + AB X + A + + B + e D i s s o c i a t i v e Penning I o n i z a t i o n (1.1.4.1) -»• X + A + B + + e (1.1.4.2) X* + AB •+ XA + + B + e D i s s o c i a t i v e A s s o c i a t i v e I o n i z a t i o n (1.1.5.1) -» XB + + A + e (1.1.5.2) The products o f these f i v e c h e m i - i o n i z a t i o n p rocesses may a l s o be formed by competing a u t o i o n i z a t i o n processes when there are -3-e x c i t e d s t a t e s of the t a r g e t p a r t i c l e which are resonant with X*. Lampe 6 has documented the e x i s t i n g work on d i s s o c i a t i v e Penning i o n i z a t i o n and d i s s o c i a t i v e a s s o c i a t i v e i o n i z a t i o n . 1.2. Experimental Methods f o r C h e m i - i o n i z a t i o n S t u d i e s . To study c h e m i - i o n i z a t i o n p r o c e s s e s , e i t h e r the ions or the e l e c t r o n s of the e x i t channel may be analyzed by a number of methods 8' 9. 1.2.1. A n a l y s i s o f the Ions. F i v e techniques have been used t o analyze the ions formed i n c h e m i - i o n i z a t i o n p r o c e s s e s . The f i r s t technique t o be d i s c u s s e d i s t o t a l i o n c o l l e c t i o n which measures the t o t a l i o n -i z a t i o n c r o s s - s e c t i o n , the parameter which l i n k s the c o l l i s i o n dynamics and the t o t a l i o n i z a t i o n p r o b a b i l i t y . The e v a l u a t i o n of the t o t a l i o n i z a t i o n c r o s s - s e c t i o n s i n v o l v e s the r a t h e r d i f -f i c u l t t a s k of determining the a b s o l u t e f l u x of the metastables which appears to have been s o l v e d only r e c e n t l y by Stebbings et a l . ; 1 0 " " 1 ' * . Though the i o n i z a t i o n c r o s s - s e c t i o n parameter p r o v i d e s a meeting ground f o r experiment and th e o r y , i t c o n t a i n s r e l a t -i v e l y l i t t l e i n f o r m a t i o n on the d e t a i l s of the r e a c t i o n . For example, th e r e have been a number o f t o t a l c r o s s - s e c t i o n c a l c u l -a t i o n s 1 5 - 2 0 f o r the system He*(2 3S) + H, and even though d i f -f e r e n t f u n c t i o n s f o r the p o t e n t i a l energy curves and the t r a n s i t i o n p r o b a b i l i t y were used, they obtained r a t h e r s i m i l a r answers. -4-The second technique used to analyze the product ions i n v o l v e s the use of a mass spectrometer to measure the p a r t i a l c r o s s - s e c t i o n s f o r the v a r i o u s c h e m i - i o n i z a t i o n r e a c t i o n s . In the e a r l y work on c h e m i - i o n i z a t i o n , mass a n a l y s i s was not used and as a r e s u l t , a l l of the i o n i z a t i o n was a t t r i b u t e d to Penning i o n i z a t i o n . The extent t o which the other c h e m i - i o n i z a t i o n pro-cesses c o u l d compete was not known. The mass s p e c t r o s c o p i c data up to 1969 f o r these c h e m i - i o n i z a t i o n processes have been t a b -u l a t e d by Lampe 6. I t appears t h a t Penning i o n i z a t i o n i s the predominant p r o c e s s , however, i n some cases, both a s s o c i a t i v e i o n i z a t i o n and d i s s o c i a t i v e a s s o c i a t i v e i o n i z a t i o n may occur to a s i g n i f i c a n t e x t e n t . Using the mass s p e c t r o s c o p i c time o f f l i g h t t e c h n i q u e , s e v e r a l groups 2 1 - 2 5 have measured the v e l o c i t y dependence f o r the p a r t i a l i o n i z a t i o n c r o s s - s e c t i o n s f o r Penning I o n i z a t i o n and a s s o c i a t i v e i o n i z a t i o n . Such data are o f p r a c t i c a l importance, as f o r example In d i s c h a r g e and plasma p h y s i c s where p a r t i a l c r o s s - s e c t i o n s at h i g h temperatures are d e s i r e d . Using a simple c l a s s i c a l v e r s i o n o f the t h e o r e t i c a l f o r m u l a t i o n o f Pen-ning i o n i z a t i o n 2 6 , both I l l e n b e r g e r and Niehaus 2** and P e s n e l l e et a l . 2 5 independently were able to d e s c r i b e the measured v e l o c i t y dependence of the I o n i z a t i o n c r o s s - s e c t i o n . S i m i l a r c a l c u l a t i o n s have been performed by O l s o n 2 7 . The next two techniques to be d i s c u s s e d are the a n a l y s i s o f the energy d i s t r i b u t i o n and the angular d i s t r i b u t i o n o f the ions produced i n c h e m i - i o n i z a t i o n p r o c e s s e s . These -5-experiments are d i f f i c u l t s i n c e the Ions have near thermal e n e r g i e s . A few attempts have been made to measure the energy d i s t r i b u t i o n o f ions produced l n Penning i o n i z a t i o n 2 8 - 3 0 . R e c ently, Leu and S i ' s k a 3 1 * 3 2 have r e p o r t e d the measuring of the angular d i s t r i b u t i o n o f Penning i o n s o f Ar, H 2, N 2, CO and 0^. F i n a l l y , the technique o f o p t i c a l e m ission s p e c t r o -scopy can be used t o analyze the e x c i t e d s t a t e s o f the io n s p r o-duced In the c h e m i - i o n i z a t i o n p r o c e s s e s . Using a flow system where metastables c o l l i d e w i t h t a r g e t p a r t i c l e s , the a f t e r g l o w which occurs when an e l e c t r i c d i s c harge (used t o produce the metastables) has been t u r n e d off,• i s o p t i c a l l y monitored. The p o p u l a t i o n o f the e l e c t r o n i c , v i b r a t i o n a l and r o t a t i o n a l s t a t e s of the Penning ions can be determined with t h i s method. Schmeltekopf et a l . 3 3 have observed the emission i n a helium f l o w i n g a f t e r g l o w f o r the t r a n s i t i o n N+(I 2 E + ) + N*(X 2 E * ) . *- u c. g They have concluded t h a t the v i b r a t i o n a l p o p u l a t i o n o f the N*(B 2 £ * ) i s i n accordance with the Franck-Condon f a c t o r s f o r p h o t o i o n i z a t i o n o f the ground s t a t e molecule. T h i s i s a r a t h e r s u r p r i s i n g o b s e r v a t i o n as the c l o s e p r o x i m i t y o f the helium p a r t i c l e at the time o f i o n i z a t i o n might w e l l be expected t o per-tu r b the n u c l e a r motion o f the molecule. D i f f e r e n c e s have been observed i n the Franck-Condon f a c t o r s f o r 0 o + ( A 2rt ), d u ' HC1 +(A 2 E + ) and HBr +(A 2 E + ) s t a t e s formed by He*(2 3S) Penning i o n i z a t i o n when compared t o 584 A photoionization 3**» 3 5. • -6-1»2«2. A n a l y s i s o f the E l e c t r o n s . Three techniques have been used t o analyze the e l e c t r o n s e j e c t e d i n c h e m i - i o n i z a t i o n p r o c e s s e s . The f i r s t technique i s the a n a l y s i s o f the energy d i s t r i b u t i o n o f the e l e c t r o n s . T h i s technique i s probably the most important experimental method which has been a p p l i e d t o the study o f c h e m i - i o n i z a t i o n . Since the e l e c t r o n i s e j e c t e d i n the c h e m i - i o n i z a t i o n process when the metastable and the t a r g e t p a r t i c l e are i n c l o s e p r o x i m i t y , the e l e c t r o n energy d i s t r i b u t i o n w i l l c o n t a i n v a l u a b l e i n f o r m a t i o n on the r e a c t i o n mechanism. The f o u r groups which have p u b l i s h e d such experimental data are Cermak et a l . 3 6 " 3 9 , Hotop and Niehaus 1*°~ t* 8, B r i o n et a l . * * 9 - 5 6 and Coleman et a l . 5 7 . A l l o f the e l e c t r o n s p e c t r o s c o p i c s t u d i e s t o date i n d i c a t e t h a t Penning i o n i z a t i o n i s the predominant e x i t channel. Using t h i s technique, d i r e c t i n f o r m a t i o n on the p o p u l a t i o n o f the d i f f e r e n t 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 s o f the Penning ions can be obt a i n e d . The second technique, which i s j u s t b e i n g e x p l o r e d , i s the a n a l y s i s o f the e l e c t r o n angular d i s t r i b u t i o n s measured with r e s p e c t t o the v e c t o r o f the r e l a t i v e v e l o c i t y o f the c o l l i d i n g p a r t i c l e s 5 8 ' 5 9 . In some cases, the experimental data are s t r o n g l y a n i s o t r o p i c and asymmetric. These data p r o v i d e f i n e d e t a i l s on e i t h e r the t r a n s i t i o n p r o b a b i l i t y matrix or.the c o l l i s i o n dynam-i c s of Penning i o n i z a t i o n . The f i n a l technique t o be considered i s the a n a l y s i s Of the s p i n p o l a r i z a t i o n o f the e l e c t r o n s e j e c t e d i n Penning i o n i z -a t i o n w i t h o p t i c a l l y pumped, s p i n ^ p o l a r i z e d He*(2 3S) metasta b l e s . Penning i o n i z a t i o n o f He*(2 3S), Cd, Sr and Zn w i t h He*(2 3S) m e t a s t a b l e s 6 0 ' 6 1 q u a n t i t a t i v e l y obeys the Wigner s p i n r u l e (the t o t a l s p i n must be conserved i n the r e a c t i o n ) . 1.3» E x c i t e d Atoms arid M o l e c u l e s . 1.3.1. I n t r o d u c t i o n When energy has been added t o an atom or molecule X, as f o r example by a b s o r p t i o n o f a photon w i t h a frequency y , an e l e c t r o n i n a l o w - l y i n g o r b i t a l can be promoted to a h i g h e r unoccupied o r b i t a l t o produce an e x c i t e d s p e c i e s X*. X + h v Q - X» (1.3.1.1) Some o f these e x c i t e d s p e c i e s X* w i l l pass spontaneously to lower s t a t e s by an e l e c t r i c d i p o l e t r a n s i t i o n and w i l l emit photons. Such " o p t i c a l l y allowed" s t a t e s have l i f e t i m e s o f the order of 10~9 seconds and t h e r e f o r e p l a y a very s m a l l r o l e i n c h e m i - i o n i z a t i o n p r o c e s s e s . An example o f an o p t i c a l l y a c c es-s i b l e s t a t e i s the 2  l? s t a t e o f helium which has a l i f e t i m e 6 2 o f O.56 x 10^9 seconds. 1 . E x c i t e d s t a t e s with extremely l o n g l i f e t i m e s were f i r s t observed many years ago. M u s c h l i t z 6 3 has a r b i t r a r i l y d e f i n e d such an e x c i t e d s t a t e t o be a metastable i f I t ' s l i f e t i m e f o r -8-monoraolecular decay i s g r e a t e r than one microsecond. One o f the more Important r e g i o n s where metastables p l a y an Important r o l e l s i n the upper and s t e l l a r atmospheres where the p a r t i c l e d e n s i t y i s low and b i m o l e c u l a r c o l l i s i o n s are r e l a t i v e l y i n f r e q u e n t . Metastables may a l s o p l a y an important r o l e i n flames, e l e c t r i c a l d i s c h a r g e s , and plasmas. One p r a c t i c a l a p p l i c a t i o n o f helium metastables i s i n a helium/cadmium dc d i s c h a r g e 6 1 * * 6 5 . I t appears that r e a c t i o n s o f cadmium wi t h He*(2 3S) metastables cause the 5 2D s t a t e s t o be overpopulated w i t h r e s p e c t to the 5 2 P s t a t e s r e s u l t i n g i n s t r o n g l a s e r a c t i o n at 441.6 nm and 325.0 nm. A second p r a c t i c a l a p p l i c a t i o n of metastables i s t h a t o p t i c a l l y pumped s p i n - p o l a r i z e d He*(2 3S) met'astables may p r o v i d e a high f l u x source of p o l a r i z e d e l e c t r o n s . K e l i h e r et a l . 6 6 have been able t o e x t r a c t 10"*7 A e l e c t r o n c u r r e n t w i t h up to 31% s p i n p o l a r i z a t i o n . The most important c l a s s o f metastable i s the one whose e l e c t r i c d i p o l e matrix elements f o r t r a n s i t i o n s t o a l l lower s t a t e s are zero (or very s m a l l ) . These " o p t i c a l l y f o r b i d d e n " t r a n s i t i o n s can be determined by s e l e c t i o n r u l e s governing o r b i t a l a ngular momentum and e l e c t r o n s p i n . Table 1 as taken from Gar-s t a n g 6 7 l i s t s the s e l e c t i o n ; r u l e s f o r e l e c t r i c d i p o l e , magnetic d i p o l e and e l e c t r i c quadrupole t r a n s i t i o n s f o r atoms. Two examples i n t h i s c l a s s o f metastable are the 2*S and 2 3S s t a t e s o f helium which have the r e s p e c t i v e l i f e t i m e s 6 8 » 6 9 o f 2 x 10~ 2 seconds and 6 x 1 0 5 seconds. -9-TABLE 1 O p t i c a l S e l e c t i o n Rules i n Atomic S p e c t r a E l e c t r i c d i p o l e Magnetic d i p o l e E l e c t r i c quadrupole (1) AJ = 0, * 1 (0 •*+»• 0) AJ - 0, * 1 (0 •*+*• 0) AJ - 0, * 1, * 2 (0 0, V2 l/2, 0 - H * 1) (2) AM - 0, * 1 AM = 0, * 1 AM = 0, * 1, * 2 (3) P a r i t y change No p a r i t y change No p a r i t y change (4) One e l e c t r o n Jump No e l e c t r o n Jump One or no e l e c t r o n Jump A l - * 1 A l = 0 A l • 0, * 2 An = 0 (5) AS = 0 AS = 0 AS = 0 (6) AL - 0, * 1 (0 +-H- 0) AL - 0 AL • 0, * 1, * 2 (0 ++• 0, 0 -H+ 1) -10-A second c l a s s o f metastable i s one where the o u t e r -most e l e c t r o n i s i n a Rydberg s t a t e w i t h a very l a r g e p r i n c i p a l quantum number. Although e l e c t r i c d i p o l e t r a n s i t i o n s are allowed, the c o u p l i n g o f the e l e c t r o n and the nucleus i s so weak th a t the p r o b a b i l i t y of decay i s s m a l l . Por an e l e c t r o n o f an atom with a p r i n c i p a l quantum number n = 30, the mean l i f e t i m e (averaged over a l l angular momentum su b s t a t e s ) Is about 0.02 seconds, the o i o n i z a t i o n p o t e n t i a l Is 0.015 eV, and the o r b i t a l r a d i u s i s 476 A 7 1 Hotop and Niehaus f have p o i n t e d out t h a t the r a d i a t i v e l i f e times of Rydberg s t a t e s i n c r e a s e with p r i n c i p a l quantum number n as n u * 5 . Except f o r the work of Hotop and Niehaus **l>71 l i t t l e work has been done on such s t a t e s u n t i l r e c e n t l y 7 2 . The p r o p e r t i e s o f 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 may be q u i t e d i f f e r e n t from those o f the ground s t a t e 6 3 . The p r i n c -i p a l f e a t u r e o f the e x c i t e d s p e c i e s i s t h a t they have very l a r g e r a d i i i n comparison t o t h e i r ground s t a t e s . For e x c i t e d mole-c u l e s , i n a d d i t i o n to changes i n i n t e r n u c l e a r d i s t a n c e s , geo-metries may a l s o change. F i n a l l y , atomic and m o l e c u l a r p r o p e r -t i e s o f e x c i t e d s t a t e s such as the p o l a r i z a b l l i t y ( w h i c h i l s a measure o f the extent of d i s t o r t i o n of the e l e c t r o n d i s t r i b u t i o n i n an e l e c t r i c f i e l d ) and the d i p o l e moment w i l l e x h i b i t q u i t e d i f f e r e n t values from the ground s t a t e s . For example, the s i n g l e outer 2g e l e c t r o n o f the He»(2 1S) metastable g i v e s t h i s e x c i t e d s p e c i e s p r o p e r t i e s s i m i l a r to those o f l i t h i u m ( I s 2 2s). -11-There are many atoms and molecules which have meta-s t a b l e s t a t e s and some o f these have been l i s t e d by Rundel and S t e b b i n g s 7 0 . There l s a wide v a r i a t i o n i n both the l i f e t i m e s and the e x c i t a t i o n e n e r g i e s o f the metastable s t a t e s . The l i f e t i m e s o f metastables range from as l a r g e as 6 x 10 5 seconds f o r He*(2 3S) metastables t o l e s s than 2 x 10"6 seconds f o r N0*(A 2 E + ) m e t a s t a b l e s . The e x c i t a t i o n e n e r g i e s o f the meta-s t a b l e s range from as l a r g e as 56 eV f o r L i * ( H P j _ ) metastables t o l e s s than 1 eV f o r 0 2 * ( a ~ Ag) metastables. 1.3.2. P r o d u c t i o n and Quenching of M e t a s t a b l e s . In g e n e r a l , metastables may be produced by p a r t i c l e e x c i t a t i o n i n gaseous systems. The v a r i o u s methods of p r o d u c t i o n of metastables have been reviewed by Stedman and S e t s e r 7 3 . Ther-mal metastables are most r e a d i l y produced by e l e c t r o n impact e x c i t a t i o n o f the. parent atom or molecule. High i n t e n s i t i e s o f metastables may be produced by t h i s method but many metastable s p e c i e s may be present i n the beam to g e t h e r w i t h photons. For the case o f e l e c t r o n impact e x c i t a t i o n o f helium, both He*(2 1S) and He*(2 3S) metastable s p e c i e s w i l l be p r e s e n t , i n a d d i t i o n t o o He I (584 A) photons. Fast metastables may be produced by charge e x c h a n g e 5 7 or by corona d i s c h a r g e 7 * . In a d d i t i o n t o c o l l i d i n g w i t h t a r g e t p a r t i c l e s and w a l l s , t h e r e are at l e a s t f i v e o t h e r p r o c e s s e s which may quench metasta b l e s . The d i s c u s s i o n o f the quenching p r o c e s s e s w i l l be -12-l i m l t e d t o the s p e c i e s He*(2 1S) and He*(2 3S). The s i n g l e photon decay o f the He*(2 1S) metastable i s s t r i c t l y f o r b i d d e n and the p r o c e s s which probably governs i t ' s l i f e t i m e i s the spontaneous double photon r a d i a t i v e decay. He»(2 1S) + HeC^S) + h v i + h v 2 (1.3.2.1) where two photons o f f r e q u e n c i e s v t and v 2 are emitted. I t i s only necessary t h a t the e n e r g i e s o f the two emitted photons s a t -i s f y the energy c o n s e r v a t i o n requirement hvi + h v 2 - 20.615 eV (1.3.2.2) which i s the e x c i t a t i o n p o t e n t i a l o f the He*(2 1S) metastable. The s i n g l e photon decay o f helium i n the 2 3S s t a t e though not s t r i c t l y f o r b i d d e n , i s h i g h l y improbable and decay appears t o occur p r e f e r e n t i a l l y by two-photon e m i s s i o n 7 5 . The-He*(2 1S) metastable has been r o u t i n e l y quenched 1* 3' 7 6 w i t h the u n f l l t e r e d l i g h t o f a helium lamp. The resonant (2*S - 2 1?,) 20,587 A r a d i a t i o n e x c i t e s the 2lS s t a t e t o the O 1 o 2 l P j s t a t e which can then decay t o the ground 1*S s t a t e with the o 0 emission o f 58M A photons. I t i s p o s s i b l e t o quench over 99% o f the helium s i n g l e t metastables u s i n g t h i s o p t i c a l pumping t e c h -nique. The He*(2 3S) metastables cannot be coupled t o the ground s t a t e by t h i s method. The He*(2 1S) metastables a l s o may be quenched by e l e c t r o n impact. Quenching may then occur by e x c i t a t i o n t o a -13-neighbouring r a d i a t i v e s t a t e which i s s i m i l a r t o the p r e v i o u s l y d i s c u s s e d quenching p r o c e s s . Quenching may a l s o occur as a r e -s u l t o f d i r e c t s u p e r e l a s t l c e l e c t r o n exchange. e + He«(2 1S) + e + He*(2 3S) + 0.79 eV (1.3-2.3) With thermal e l e c t r o n s , the c r o s s - s e c t i o n f o r t h i s process i s l a r g e 7 7 and thus He*(2 lS) l s e f f e c t i v e l y converted t o He*(2 3S). A dc e l e c t r i c f i e l d may be used t o quench the He*(2 1S) metastable by c o u p l i n g i t to an e x c i t e d s t a t e which can r a d i a t e to the ground s t a t e . The l a r g e s e p a r a t i o n (0.6 eV) o f the 2*S o l e v e l from the c l o s e s t r a d i a t i n g 2 1P 1 s t a t e r e q u i r e s the a p p l i c -a t i o n o f a f i e l d o f 2 x 10 5 V/cm t o achieve 90% q u e n c h i n g 7 8 . F i n a l l y , metastables may be quenched by an e x c i t a t i o n t r a n s f e r p r o c e s s . I f t h i s process i s t o occur w i t h a s i g n i f i c a n t p r o b a b i l i t y , t here must be an energy l e v e l of an e x c i t e d s t a t e o f the t a r g e t p a r t i c l e AB** which l s resonant with the e x c i t a t i o n energy o f the matastable. A w e l l known example o f an e x c i t a t i o n t r a n s f e r p rocess i s He* + Ne •+ He + Ne** (1.3.2.4) which occurs i n helium/neon l a s e r s . D i f f i c u l t i e s w i l l a r i s e i n the i n t e r p r e t a t i o n o f Penning e l e c t r o n s p e c t r a i f the e x c i t e d s t a t e o f the t a r g e t p a r t i c l e can a u t o i o n l z e . Complications w i l l a r i s e i f the e x c i t e d s t a t e d i s s o c i a t e s t o y i e l d e x c i t e d fragments which a u t o i o n l z e . These problems are analogous t o the competing a u t o i o n i z a t i o n processes which may occur i n p h o t o i o n i z a t i o n . -14-CHAPTER TWO IONIZATION PROCESSES 2.1. Photoionization 2.1.1. Introduction In 1905 E i n s t e i n 7 9 postulated the photoelectric effect which states, when an atom or molecule Interacts with a photon of frequency vl which has s u f f i c i e n t energy to eject an electron, i o n i z a t i o n w i l l occur. hv I + AB AB + + e (2.1.1.1) Any excess energy w i l l manifest i t s e l f as k i n e t i c energy which w i l l be pa r t i t i o n e d between the two p a r t i c l e s formed. Because of the large difference i n mass between the Ion and the ejected electron and through consideration of the conservation of momentum, v i r t u a l l y a l l of the excess k i n e t i c energy E* w i l l be carried e l away by the photoelectron. By u t i l i z i n g the E i n s t e i n photo-e l e c t r i c equation E f e - hv| - E e l (2.1.1.2) the binding energy E^ of electrons i n atoms or molecules can be evaluated by measuring the k i n e t i c energy of the photo-ejected electrons. Equation (2.1.1.2) can be written In a more fa m i l i a r form E e l " h v i " I p a ( A B ) - E +(v') (2.1.1.3) where IP a(AB) Is the adiabatic Ionization p o t e n t i a l which i s the -15-d i f f e r e n c e i n e l e c t r o n i c energy between the lowest v i b r a t i o n a l l e v e l s o f the i o n and the molecule, and E + ( v » ) i s the v i b r a t i o n a l energy o f the v' l e v e l o f the i o n . The experimental technique which measures the k i n e t i c e n e r g i e s o f p h o t o e l e c t r o n s i s r e f e r r e d t o as p h o t p e l e c t r o n s p e c t -roscopy and i n the l a s t t e n years i t has become a r o u t i n e t o p i f o r s t u d i e s i n m o l e c u l a r spectroscopy. In p h o t o e l e c t r o n s p e c t r o -scopy the photon source must be monochromatic, and i t must have a useable i n t e n s i t y . To analyze i n n e r or core s h e l l e l e c t r o n s , h i g h energy X-ray sources such as A l ^(1486.6 eV) are used and t h i s technique i s commonly r e f e r r e d t o as E l e c t r o n Spectroscopy f o r Chemical A n a l y s i s ( E S C A ) 8 0 or X-ray P h o t o e l e c t r o n Spectroscopy (XPS). To analyze o u t e r o r valence s h e l l e l e c t r o n s , the most o p o p u l a r photon source has been the He I resonance l i n e at 584 A (21.217 eV) and t h i s technique i s commonly r e f e r r e d t o as UV P h o t o e l e c t r o n Spectroscopy (UPS). Both methods have r e c e n t l y been reviewed by L e e 8 1 . T h i s t h e s i s i s concerned w i t h the study o f valence s h e l l e l e c t r o n s , thus d i s c u s s i o n s r e l a t e d t o photo-i o n i z a t i o n w i l l be r e s t r i c t e d t o the regime o f UPS. 2.1.2. Franck-Condon P r i n c i p l e . P h o t o i o n i z a t i o n of a diatomic molecule can be d e s c r i b e d as a v e r t i c a l or Franck-Condon t r a n s i t i o n between two m o l e c u l a r p o t e n t i a l c urves. T h i s i s a consequence of the Born-Oppenheimer a p p r o x i m a t i o n 8 2 which s t a t e s t h a t 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 t o be unchanged f o r e l e c t r o n i c -16-t r a n s l t i o n s t h a t take p l a c e In a time I n t e r v a l which Is much s h o r t e r than t h a t r e q u i r e d f o r a s i n g l e v i b r a t i o n . In F i g u r e 1 a Franck-Condon t r a n s i t i o n f o r the p h o t o i o n i z a t i o n o f a diatom i c molecule may be viewed as a v e r t i c a l t r a n s i t i o n between the po t -e n t i a l curves o f the ground s t a t e molecule and an i o n i c s t a t e . The r e g i o n i n which v e r t i c a l t r a n s i t i o n s occur with some f i n i t e p r o b a b i l i t y Is f r e q u e n t l y c a l l e d the Franck-Condon r e g i o n and i s i n d i c a t e d by the shaded r e g i o n on F i g u r e 1. V i b r a t i o n a l s t r u c t u r e I s o f t e n observed when molecules are p h o t o i o n i z e d . The v i b r a t i o n a l i n t e n s i t i e s may be q u a n t i t -a t i v e l y d e s c r i b e d by the wave mechanical d e s c r i p t i o n o f the Franck-Condon p r i n c i p l e 8 3 » 8 I*. The i n t e n s i t y o f a v i b r a t i o n a l peak i n an e l e c t r o n i c a l l y allowed t r a n s i t i o n i s p r o p o r t i o n a l t o the a b s olute square o f the ove r l a p i n t e g r a l V - v" a \j *v* V d Q | 2 (2.1.2.1) where ty^' and * v " are the v i b r a t i o n a l wavefunctions of the upper and lower quantum s t a t e s and Q i s the n u c l e a r c o - o r d i n a t e . The Franck-Condon f a c t o r w i l l be s i g n i f i c a n t when the maxima or minima of the two wave f u n c t i o n s l i e d i r e c t l y above each other. For * " the maxima o f the wavefunction occurs at the e q u i l i b r i u m v=o J i n t e r n u c l e a r d i s t a n c e o f the molecule, however f o r if ' , the max-ima or minima-occurs near t h e . c l a s s i c a l t u r n i n g p o i n t s o f motion. The p hotoelectron'energy d i s t r i b u t i o n o f an i o n i c s t a t e -17-INTERNUCLEAR DISTANCE, r — FIGURE 1. C o r r e l a t i o n between the Franck-Condon P r i n c i p l e and the shape o f UPS bands f o r the removal o f e l e c t r o n s from molecular o r b i t a l s o f d i f f e r e n t bond-i n g c h a r a c t e r . -18-o f a d i a t o m i c molecule depends on the shape and p o s i t i o n of the p o t e n t i a l curve o f the r e s u l t i n g i o n . I f f o r example, an e l e c t r o n i s removed from an a n t i b o n d i n g 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 i s t a n c e o f the i o n w i l l be s m a l l e r than t h a t f o r the molecule. Thus, the slow r i s i n g a t t r a c t i v e p o r t i o n o f the poten-t i a l curve of the i o n w i l l l i e above the Franck-Condon r e g i o n of the ground s t a t e n e u t r a l , and only a few v i b r a t i o n a l l e v e l s o f the i o n w i l l be p o p u l a t e d (see F i g u r e 1). T h i s suggests t h a t from the I n t e n s i t y and the number of v i b r a t i o n a l l e v e l s observed In the p h o t o e l e c t r o n d a t a , i t may be p o s s i b l e t o e x t r a c t i n f o r m a t i o n on the nature of the m o l e c u l a r o r b i t a l from which the e l e c t r o n was e j e c t e d . I l l u s t r a t e d I n F i g u r e 1" i s a number of e l e c t r o n energy d i s t r i b u t i o n s which r e s u l t when an e l e c t r o n l s removed from v a r i o u s types of m o l e c u l a r o r b i t a l s . I t Is p o s s i b l e to understand polyatomic molecules by extending the p r i n c i p l e s a p p l i e d t o diatomic molecules. The s i t -u a t i o n Is now very complicated as t r a n s i t i o n s occur between m u l t i -d imensional s u r f a c e s . A n a l y s i s o f the v i b r a t i o n a l s t r u c t u r e may be d i f f i c u l t as s e v e r a l v i b r a t i o n a l modes may be e x c i t e d s i m u l t -aneously. 2.1.3. A u t o i o n i z a t i o n In the p h o t o i o n i z a t i o n process i t i s p o s s i b l e to remove an e l e c t r o n from an atom or molecule by a process which does not -19-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. T h i s process i s known as a u t o i o n l z a t i o n which may be regarded as a two step p r o c e s s . F i r s t , the i n i t i a l e x c i t a t i o n o f an e l e c t r o n occurs Into a d i s c r e t e s t a t e above the i o n i z a t i o n p o t e n t i a l o f the s p e c i e s . T h i s d i s c r e t e s t a t e must have an energy l e v e l which i s resonant with the e x c i t a t i o n energy of the photon. The second step o f the pro c e s s i n v o l v e s a r a d i a t i o n l e s s t r a n s i t i o n from the d i s c r e t e s t a 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. D i s t o r t i o n of the Franck-Condon f a c t o r s f o r the photo-i o n i z a t i o n o f a molecule may be the r e s u l t o f a competing auto-i o n l z a t i o n p r o c e s s . Anomalous Franck-Condon f a c t o r s f o r the ground s t a t e i o n o f oxygen r e s u l t i n g from p h o t o i o n i z a t i o n w i t h Ne(736,7M4 %) r a d i a t i o n have been e x p l a i n e d by such a p r o c e s s 8 5 . 2.2. Penning I o n i z a t i o n . 2.2.1. Q u a l i t a t i v e D e s c r i p t i o n For Penning and a s s o c i a t i v e i o n i z a t i o n at thermal energ i e s X* + AB ->• X + AB + + e (2.2.1.1) - XAB + ; (2.2.1.2) the motion o f the heavy p a r t i c l e s can be d e s c r i b e d adequately u s i n g c l a s s i c a l t h e o r y . F i g u r e 2 shows two s c a t t e r i n g events dur-i n g which both a t t r a c t i v e and/or r e p u l s i v e f o r c e s a c t . At l a r g e d i s t a n c e s (beyond the range of f i g u r e 2) the weak van der Waals -20-X FIGURE 2. Heavy p a r t i c l e s c a t t e r i n g diagram u s i n g the impact parameter formula-t i o n , a. R e p u l s i v e system, b. A t t r a c t i v e and R e p u l s i v e system. -21-a t t r a c t i v e f o r c e s are predominant and i n both cases the metast-able p a r t i c l e approaches the t a r g e t s p e c i e s A or B. In F i g u r e 2.a, the t r a j e c t o r i e s f o r two d i f f e r e n t impact parameters i l l u s t r a t e s t h a t the metastable p a r t i c l e only sees a r e p u l s i v e p o t e n t i a l . In F i g u r e 2.b, f o r the case w i t h the r e l a t i v e l y l a r g e impact parameter p j , the metastable p a r t i c l e sees only an a t t r a c t i v e pot-e n t i a l , but f o r the case w i t h the r e l a t i v e l y s m a l l impact p a r a -meter p 2 , the metastable p a r t i c l e f i r s t sees an a t t r a c t i v e poten-t i a l and a f t e r p e n e t r a t i n g t o the r e p u l s i v e p a r t o f the p o t e n t i a l , i t i s r e p e l l e d at the c l a s s i c a l t u r n i n g p o i n t . In F i g u r e 2, R o l s the c l o s e s t d i s t a n c e o f approach f o r the v a r i o u s impact p a r a -meters o f the two s c a t t e r i n g events and i t Is at t h i s p o i n t where the r e i s the g r e a t e s t p r o b a b i l i t y t h at the energy contained i n the metastable w i l l be t r a n s f e r r e d t o the t a r g e t p a r t i c l e . Though the a c t u a l energy exchange mechanism f o r Penning i o n i z a t i o n i s unknown, th e r e i s evidence t h a t e l e c t r o n exchange, as proposed by Hotop and Niehaus 1* 1 J 1 * 3 , o c c u r s . The Hotop-Niehaus model, based on theory used t o e x p l a i n the Auger d e e x c i t a t i o n of an e x c i t e d atom at a metal s u r f a c e 8 6 , may be d e s c r i b e d by the process X«(l) + A(2) + X(2) + A + + e (1) (2.2.1.3) where X * ( l ) r e p r e s e n t s a metastable w i t h an e x c i t e d e l e c t r o n (1) and A(2) i s a ground s t a t e ; t a r g e t p a r t i c l e with a valence e l e c t r o n (2). The t r a n s i t i o n process In Penning i o n i z a t i o n can -22-be viewed as a t u n n e l l i n g of e l e c t r o n (2) to the vacant ground s t a t e o r b i t a l o f the metastable, f o l l o w e d by Auger emission of e l e c t r o n ( 1 ) . P r e l i m i n a r y s t u d i e s by K e l l h e r et a l . 8 7 pf the e l e c t r o n s p i n p o l a r i z a t i o n f o r Penning i o n i z a t i o n support the e l e c t r o n exchange mechanism i f the Wigner s p i n r u l e i s o b e y e d 6 0 * 2.2.2. P o t e n t i a l Curve Model Penning I o n i z a t i o n i s a problem i n v o l v i n g heavy p a r t -i c l e I n e l a s t i c c o l l i s i o n s and i n p r i n c i p l e I t can be t r e a t e d by u s i n g a p o t e n t i a l curve model. T h i s model was f i r s t a p p l i e d t o Penning i o n i z a t i o n by Herman and C e r m a k 2 8 ' 8 8 and was l a t e r d e v e l oped by Hotop and Niehaus 4* 1 »*•2 Por low energy (thermal) c o l l i s i o n s and w i t h i n the Born Oppenheimer approximation f o r the s e p a r a t i o n o f e l e c t r o n i c and n u c l e a r motion, the p o t e n t i a l curve model f o r the system (X«,A) Is i l l u s t r a t e d I n F i g u r e 3. The incoming channel (X»,A) i s assigned a Born-Oppenheimer l i k e p o t e n t i a l V*(R) which i s a f u n c t i o n o f the i n t e r n u c l e a r d i s t a n c e R between the metastable and the t a r g e t atom (and should not be confused with the poten-t i a l curves o f the d i a t o m i c molecule which were d i s c u s s e d In s e c t i o n 2.1.2.). In F i g u r e 3.a, V»(R) Is shown as a p u r e l y r e p u l s i v e p o t e n t i a l which corresponds t o the s c a t t e r i n g events shown i n F i g u r e 2.a. In F i g u r e 3.b,V*(R) i s shown to c o n t a i n both an a t t r a c t i v e and a r e p u l s i v e p o t e n t i a l which corresponds to the s c a t t e r i n g events shown In F i g u r e 2.b. A l s o shown i s the Ca) ENERGY A •el V*(R) 0 • A V*(R)-V+CR) \ \ RaRc, COUNTS ENERGY iV*CR) (b) FIGURE 3. P o t e n t i a l curve model showing the s i g n i f i c a n c e of the t r u e AE energy s h i f t , a. Repulsive Penning i o n i -z a t i o n system, b. A t t r a c t i v e Penning i o n i z a t i o n system. COUNTS i ro I p o t e n t i a l V^CR) f o r the outgoing system (X,A +,e), Thus, fo r a l l separations R, V*(R) i s a discrete state which Is embedded i n V+tR), the continuum of states to which i t i s coupled. The two p a r t i c l e s (X*,A) enter along the curve V*(R) with some kin -e t i c energy E k . At any point along t h i s curve there i s a d e f i n i t e p r o b a b i l i t y W(R) that a v e r t i c a l t r a n s i t i o n to V+CR) w i l l occur. The energy of the ejected electron at that point w i l l be E e l ( R ) a V * ( R ) " V*(*y: (2.2.2.1) as indicated i n Figure 3 by the t h i r d curve which represents the difference between V*(R) and V +(R). For t r a n s i t i o n s at large R (R >> 10 %) the energy of the ejected electron w i l l be equal to E e l ' t n e n o m i n a l energy for the i o n i z a t i o n process. E e l ( R * "> " E (x*) _ I P (A). (2.2.2.2) « E ^ i (2.2.2.3) At smaller R, the energy of the ejected electrpn w i l l be d i f f e r -ent from E Q I i n a way which i s c h a r a c t e r i s t i c of the two poten-r t i a l curves V»(R) and V*(R). The t r a n s i t i o n probably W(R) determines the internuclear range where a t r a n s i t i o n w i l l occur with s i g n i f i c a n t p r o b a b i l i t y and t h i s w i l l strongly influence the shape of the electron energy d i s t r i b u t i o n . Within the framework of c l a s s i c a l mechanics i t i s pos-s i b l e to estimate the shape of electron energy d i s t r i b u t i o n s by assuming that the t r a n s i t i o n p r o b a b i l i t y W(R) Is largest at the c l a s s i c a l turning point and decreases exponentially with -25-increasing R. Por the repulsive Penning Ionization system (Figure 3.a), E°^ w i l l be the lower boundary of the electron energy d i s t r i b u t i o n for t r a n s i t i o n s occurring at large R o (R >> 10 A) regardless of the k i n e t i c energy between the two part-i c l e s . The upper boundary of the electron energy d i s t r i b u t i o n i s dependent on the r e l a t i v e k i n e t i c energy of the system. I f the pa r t i c l e s have a r e l a t i v e k i n e t i c energy of E and a t r a n s i t i o n occurs at the c l a s s i c a l turning point R c l> the upper boundary of the energy d i s t r i b u t i o n i s E . Since the t r a n s i t i o n p r o b a b i l i t y i s largest at the c l a s s i c a l turning point, R the electron energy d i s t r i b u t i o n w i l l have the most intense structure at the energy E •. One quantity which i s related to the four parameters V*(R); , V +(R) , W(R) and E k i s the energy difference between o the expected nominal energy E , and the peak maximum of the energy e i d i s t r i b u t i o n . This parameter i s known as the true energy s h i f t , AE, and i s indicated on Figure 3.a as having a p o s i t i v e value. When the r e l a t i v e k i n e t i c energy between the two p a r t i c l e s Is increased to E k 2 , the width of the Penning electron energy d i s -t r i b u t i o n increases and consequently, the true AE energy s h i f t value also increases. The s i t u a t i o n d i f f e r s f o r the a t t r a c t i v e Penning i o n i z -ation system. The lower boundary of the energy d i s t r i b u t i o n i s dependent on the shape of the two po t e n t i a l curves and occurs for one s p e c i f i c t r a n s i t i o n at small R (R < 10 A). When the -26-r e l a t i v e k i n e t i c energy of the two p a r t i c l e s Is small ( E k l ) > the width of the electron energy d i s t r i b u t i o n i s constant since the upper boundary of the electron energy d i s t r i b u t i o n i s the nominal energy E ^ . When the r e l a t i v e k i n e t i c energy of the two p a r t i c l e s i s large ( E k 2 ) , the width of the electron energy d i s t r i b u t i o n varies since the upper boundary of the electron energy d i s t r i b u t i o n Is dependent on E f e •, Por the portion of the electron energy d i s t r i b u t i o n which i s less than E°^, tr a n s i t i o n s occurring at two d i f f e r e n t internuclear separations R may con-tribu t e to the spectrum. F i n a l l y It i s noted that the true &E energy s h i f t value i s Indicated as having either a po s i t i v e or a negative value, as shown In Figure 3.b. The quantities V*(R) , V +(R) and W(R) are strongly coupled and i n general i t i s not possible to derive a unique set from the electron energy d i s t r i b u t i o n alone. I f "a p r i o r i " knowledge of V*(R) , from detailed e l a s t i c scattering experi-ments was ava i l a b l e , both V (R), and W (R) could be deduced from electron energy d i s t r i b u t i o n s . For the sp e c i a l case where the depth of the pot e n t i a l well,e*, for V*(fl) i s very much larger than depth of the poten-t i a l w e ll, e +, for V +(R) and the minimum of the po t e n t i a l well f o r Vf(R> occurs at a larger Internuclear separation than for V +(R) , I t i s possible that e« can be obtained from the electron energy d i s t r i b u t i o n . Hotop and N i e h a u s h a v e shown that the measured electron energy d i s t r i b u t i o n for the system (He*,Na) i s consistent with t h i s model. The po t e n t i a l V*(R) was described by a Lennard-Jones (12,6) p o t e n t i a l and the t r a n s i t i o n p r o b a b i l i t y was described by a simple exponentially decreasing function W(R) » W0exp(-c*R) (2.2.2.4) This function decreases i n the same manner as the square of the wave function for an electron bound to an atom. By varying a, Hotop and Niehaus were able to make a very good f i t between experiment and theory. The r a t i o of the p a r t i a l cross-sections f o r associative to Penning i o n i z a t i o n can be estimated from the electron energy d i s t r i b u t i o n , as shown on Figure 4. For the repulsive system where the r e l a t i v e k i n e t i c energy i s E^! (Figure 4.a), Penning i o n i z a t i o n w i l l occur at a l l Internuclear distances R > such that the energy of the ejected electron E e l * E e l + E k (2.2.2 .5) I f a t r a n s i t i o n occurs at R>Rpi to a point on the p o t e n t i a l curve V +(R) that l i e s within the p o t e n t i a l well e +, the p a r t i c l e s w i l l have s u f f i c i e n t k i n e t i c energy to escape the w e l l . Associ-ative i o n i z a t i o n w i l l occur at a l l internuclear distances R c 1 R < Rpj such that E e l > E ° x + E k (2.2.2.6) Transitions which occur at F^j « R < Rpjj- w i l l l i e on points on the p o t e n t i a l curve V+CR) that are within the po t e n t i a l well ENERGY Ca) iV*(R) ENERGY V*CR) Cb) IV) oo I E. E el FIGURE 4. P o t e n t i a l curve model showing the r a t i o o f Penning I o n i z a t i o n and a s s o c i a t i v e i o n i z a t i o n , a.Repulsive Penning i o n i z a t i o n system, b . A t t r a c -t i v e Penning i o n i z a t i o n system. V*CR)-V+(R) TE: 1 +E, COUNTS -29-e+ and the p a r t i c l e s w i l l have k i n e t i c energies which are not s u f f i c i e n t to escape the p o t e n t i a l w e l l , thus associative ions XAB+ are formed. I f the r e l a t i v e k i n e t i c energy E k Is known, the r a t i o of the two i o n i z a t i o n processes can be evaluated from the two areas under the electron energy d i s t r i b u t i o n curve. It Is noted that associative i o n i z a t i o n does not occur for the s p e c i f i c examples of the a t t r a c t i v e Penning i o n i z a t i o n system i l l u s t r a t e d i n Figure 4.b. This p o t e n t i a l curve model may be extended i n p r i n c i p l e to polyatomic molecules with t r a n s i t i o n s occurring between multidimensional p o t e n t i a l surfaces. An a d d i t i o n a l complication i s the fact that molecules may have several v i b r a t i o n a l l y excited modes. 2.2,3. Quantum Mechanical Treatment of the Potential  Curve Model. Quite recently, a number of authors 8 >9 J 1 * 8 » 7 0 > 8 9 > 9 0 ; have reviewed the t h e o r e t i c a l treatments of the Penning i o n i z a t i o n of atoms, Penning i o n i z a t i o n can be viewed as a reaction which depends upon the configuration Interaction between a discrete molecular state which i s embedded i n a continuum of molecular states. Within t h i s framework, Nakamura 9 1 has derived a rigorous quantum mechanical formalism for t h i s problem i n which the non-adiabatlc effects due to t h e ; r e l a t i v e motion of the p a r t i c l e s are properly included. However, use of t h i s formalism Involves very complicated and lengthy calculations and as of yet has not been applied to any system. -30-For Penning Ionization at thermal energies, the adlabatlc approximation which neglects the Influence of the r e l a t i v e motion, can be expected to be good. Nakamura 9 1 has also developed a quantum mechanical formalism for Penning Ionization i n the adiabatic approximation. F u j i ! et a l . * ' 6 have used t h i s formalism i n .their calculations f o r the three electron problem He* + H * He + H + + e (2.2.3.1) which i s the simplest possible Penning i o n i z a t i o n process. However, In t h e i r calculations they have appeared to neglect some avoided crossings which can s i g n i f i c a n t l y e f f e c t the poten-t i a l curve V*(R). For example, the He*(2 1S) + H p o t e n t i a l curve i s repulsive i n the zero order approximation and i t has an avoided crossing with the a t t r a c t i v e He*(2 3P) + H curve which makes the He«(2,1S)+ H p o t e n t i a l a t t r a c t i v e . F u j i i et a l . 1 6 have calculated cross-sections which are i n agreement with the experimental r e s u l t s reported by Shaw et a l . 9 2 . Errors In these calculations might be expected due to the approximations used i n c a l c u l a t i n g the p o t e n t i a l curves. M i l l e r et a l . 1 7 » 1 9 > 2 6 have also used the adlabatlc formalism In a t h e o r e t i c a l treatment of Penning i o n i z a t i o n . Using sophisticated configuration i n t e r -action calculations on the system He* + H, they obtained p o t e n t i a l curves which were quite d i f f e r e n t from F u j l i et a l . 1 6 . Cohen and L a n e 8 extended the work of M i l l e r and, using both c l a s -s i c a l and se m i - c l a s s i c a l methods, they obtained i o n i z a t i o n cross-sections which were i n good agreement with experiment -31-data. From the d e t a i l e d c a l c u l a t i o n s o f M i l l e r 1 7 ' 2 9 * 2 6 i t i s p o s s i b l e t o o b t a i n the r a t i o o f a s s o c i a t i v e t o Penning i o n i z a -t i o n . The c a l c u l a t i o n s suggest f o r the r e a c t i o n between He* + H, there should be l 8 $ s H e H + and B2% H + which i s In rough agreement w i t h the experimental data from f l o w i n g a f t e r g l o w measurements 9 2 . 2.2.4. Penning E l e c t r o n S p e c t r a o f Molecules. Because the p h y s i c a l p r o p e r t i e s o f a n e u t r a l metastable at thermal e n e r g i e s are very d i f f e r e n t from those o f a photon, the nature of the c o l l i s i o n process f o r the two modes of I o n i z a -t i o n should be very d i f f e r e n t . T h i s i s r e f l e c t e d In the f a c t t h a t both the c o l l i s i o n duration,, d e f i n e d by the time r e q u i r e d f o r the slowest p a r t i c l e t o pass through a c o l l i s i o n sphere with a diameter o f one angstrom and the c o l l i s i o n c r o s s - s e c t i o n f o r Penning i o n i z a t i o n ( 1 0 ~ 1 3 seconds, 10" 1 6 cm 2) are very much l a r g e r than f o r p h o t o i o n i z a t i o n ( 1 0 ~ 1 6 seconds, 10" 1 9 cm 2). Three g e n e r a l f e a t u r e s o f the Penning e l e c t r o n energy d i s t r i -b u t i o n t o be d i s c u s s e d with r e s p e c t t o the analogous p h o t o e l e c t r o n energy d i s t r i b u t i o n are the shapes of the v i b r a t i o n a l envelope of an i o n i c s t a t e , the r e l a t i v e e l e c t r o n i c s t a t e p o p u l a t i o n s o f the molecule and the energy s h i f t s o f the e l e c t r o n s p e c t r a . a. Shape o f the V i b r a t i o n a l Envelope. In g e n e r a l , the He*(2 1S,2 3S) Penning e l e c t r o n s p e c t r a are remarkably s i m i l a r i n appearance t o t h e i r analogous 584 K p h o t o e l e c t r o n spectra** 6 » L > 9 ~ 5 6 . Any i o n i c s t a t e observed i n p h o t o i o n i z a t i o n w i l l a l s o be observed i n Penning I o n i z a t i o n . I t i s noted t h a t s t r u c t u r e i n ; a Penning e l e c t r o n -32-spectrum i s always broader than the analogous structure i n the photoelectron spectrum. The degree to which the structure i s broadened i n Penning i o n i z a t i o n appears to be a function of the r e l a t i v e k i n e t i c energy between the metastable and the target molecule. In favourable cases where v i b r a t i o n a l structure f o r an i o n i c state i s resolved** 5' 5 1 » 5 3 , the v i b r a t i o n a l spacings for the two Ionization processes are the same within experimental error. In addition, the r e l a t i v e v i b r a t i o n a l i n t e n s i t i e s (or the Franck-Condon factors) of an i o n i c state are often very s i m i l a r f o r the two modes of i o n i z a t i o n 1 * 5 ' 5 1 » 5 3 . This suggests that the Penning i o n i z a t i o n process i s a v e r t i c a l t r a n s i t i o n , l i k e photoionization. There e x i s t a number of apparently anomalous cases where the shape of the v i b r a t i o n a l envelopes observed i n Penning i o n i z a t i o n are d i f f e r e n t from those observed i n photoionization. For example, Richardson and Setser 3*** 3 5 have observed such d i f f e r -ences i n afterglow studies which apparently are due to competing processes. A d i r e c t analysis of such an anomaly using electron spectroscopic techniques requires that v i b r a t i o n a l structure In the Penning electron spectra be resolved. For the process Ee*(23syb*(Xzftg) as studied by electron spectroscopy 5 1, an observed anomaly has been traced to an autoionizing l e v e l of 0 2 which i s e s s e n t i a l l y resonant with the He«(23S) energy. For the Penning Ionization of a number of molecules, anomalies have been observed which apparently are not due to competing autoionization p r o c e s s e s 5 5 * 5 6 . For these cases i t has been suggested that the c o l l i s i o n between the metastable and the target molecule Is s u f f i c i e n t l y " s t i c k y " t o d i s t o r t the ground s t a t e o f the t a r g e t molecule along some combination o f normal modes, but the i o n s t a t e does not i n t e r a c t s i g n i f i c a n t l y w i t h the n e u t r a l ground s t a t e metastable ( t h e r e f o r e i t ' s geometry remains e f f e c t i v e l y u n a l t e r e d ) . T h i s proposed type o f mechanism i s i l l u s t r a t e d s c h e m a t i c a l l y by the p o t e n t i a l energy curves o f F i g u r e 5. F o r the p h o t o i o n i z a t i o n p r o c e s s , a v e r t i c a l t r a n s i t i o n occurs between M, the normal ground s t a t e o f the t a r g e t and M +, the f i n a l i o n i c s t a t e o f the t a r g e t . For the Penning I o n i z a t i o n p r o c e s s , a v e r t i c a l t r a n s i t i o n occurs between M x » , the ground s t a t e o f the t a r g e t which i s p e r t u r b e d by the metastable atom and M +, the f i n a l i o n i c s t a t e o f the t a r g e t . The a b s c i s s a r e f e r s t o any normal c o o r d i n a t e o f the t a r g e t . In t h i s p a r t i -c u l a r h y p o t h e t i c a l case, the most Intense t r a n s i t i o n i s s h i f t e d from <0 n*0') f o r p h o t o i o n i z a t i o n t o (0"-»-2') f o r Penning I o n i -z a t i o n . The shape o f the v i b r a t i o n a l envelopes f o r the two processes are s i g n i f i c a n t l y d i f f e r e n t . C a l c u l a t i o n s 5 1 * suggest t h a t a p e r t u r b a t i o n o f the t a r g e t p o t e n t i a l curve o f the magni-tude of 0.2 A c o u l d account f o r the d i f f e r e n c e s observed f o r the v i b r a t i o n a l envelopes b. R e l a t i v e E l e c t r o n i c S t a t e P o p u l a t i o n s . One p o i n t where Penning i o n i z a t i o n and photo-i o n i z a t i o n g e n e r a l l y d i f f e r i s when the r e l a t i v e e l e c t r o n i c s t a t e p o p u l a t i o n s o f the i o n are compared. I t may be necessary t o c o r r e c t the e l e c t r o n energy d i s t r i b u t i o n w i t h an e l e c t r o n a n a l y z e r t r a n s m i s s i o n f u n c t i o n , b e f o r e such comparisons are made. Th i s l a b r a t o r y has s t u d i e d a number o f m o l e c u l e s 4 f 9 ~ 5 6 and l a r g e d i f f e r e n c e s i n the s t a t e p o p u l a t i o n s have been observed when (a) PHOTOIONIZATION (b) PENNING IONIZATION NORMAL CO-ORDINATE NORMAL CO-ORDINATE FIGURE 5. P a t e n t i a l curve model f o r i o n i z a t i o n by a. photons and b. m e t a s t a b l e atoms. -35-comparing the two modes o f i o n i z a t i o n . In a s e r i e s o f molecules c o n t a i n i n g -C*N, the r a t i o o f the normalized r e l a t i v e p o p u l a t i o n s of s t a t e s corresponding t o the removal o f n bonding and n i t r o g e n lone p a i r e l e c t r o n s i s s i g n i f i c a n t l y g r e a t e r f o r 58*4 A photo-i o n i z a t i o n than f o r He*(2 3S) Penning i o n i z a t i o n 5 5 . In g e n e r a l when comparing the s t a t e p o p u l a t i o n s f o r the two modes o f i o n i z a -t i o n , I t i s not p o s s i b l e t o make adjustments f o r p o s s i b l e e f f e c t s due to d i f f e r e n c e s i n e x c i t a t i o n e n e r g i e s . Instead of comparing the He*(2 1S,2 3S) Penning e l e c t r o n spectrum t o the 584 A photo-e l e c t r o n spectrum, I d e a l l y the He*(2 1S) Penning e l e c t r o n spectrum should be compared t o the 601 % (20.62 eV) p h o t o e l e c t r o n spectrum and the He*(2 3S) Penning e l e c t r o n spectrum should be compared to the 626 A (19.82 eV) p h o t o e l e c t r o n spectrum. Measurements o f the p a r t i a l c r o s s - s e c t i o n s as a f u n c t i o n o f the " p h o t o i o n i z a t i o n " energy i n d i c a t e t h a t the r e l a t i v e e l e c t r o n i c s t a t e p o p u l a t i o n s of a number o f m o l e c u l e s 9 3 - 9 5 can change d r a s t i c a l l y even over a few v o l t s . In a d d i t i o n , i t i s not p o s s i b l e t o make adjustments f o r d i f f e r e n c e s i n the angular d i s t r i b u t i o n o f e j e c t e d 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 c s t a t e s . I d e a l l y , the e l e c t r o n d i s t r i b u t i o n s f o r the p h o t o i o n i z a t i o n process should be measured at the "magic angle" of 5^.7 degrees. At t h i s angle, the e l e c t r o n energy d i s t r i b u t i o n s w i l l be independant of the angular a n i s o t r o p y parameter B. However, I t appears t h a t at 90 degrees, the 8- para-meter produces o n l y s m a l l v a r i a t i o n s i n the r e l a t i v e s t a t e p o p u l a t i o n s . Only a few Penning e l e c t r o n angular d i s t r i b u t i o n s t u d i e s 5 8 * 5 9 have been performed to date. The angular d i s t r i b u -t i o n s t u d i e s f o r the Penning i o n i z a t i o n o f a r g o n 5 8 suggest t h a t such an e f f e c t may be r e l a t i v e l y s m a l l . -36-c. Energy S h i f t s . The "expected v a l u e " o f the k i n e t i c energy o f 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 p r o c e s s , E ^ , can be d e r i v e d from an e x p r e s s i o n , E ^ = E(X«) - IP a(AB) - E + ( V ) ( c l ) which Is analogous to the e x p r e s s i o n (2.1.1.3) which evaluates the k i n e t i c energy of the e l e c t r o n s e j e c t e d i n the p h o t o i o n i -z a t i o n p r o c e s s . With r e s p e c t to a p h o t o e l e c t r o n spectrum the corresponding "expected" Penning e l e c t r o n spectrum should be s h i f t e d t o lower e l e c t r o n e n e r g i e s by an amount ( h v j - E ( X ) ) . In g e n e r a l , the "observed" Penning e l e c t r o n s p e c t r a are s h i f t e d to lower e l e c t r o n e n e r g i e s (with r e s p e c t t o t h e i r c o r r e s p o n d i n g p h o t o e l e c t r o n s p e c t r a ) , however the magnitudes o f these s h i f t s d i f f e r from the expected amount (hvx - E(X*)) by a v a l u e , A E o b s * T h i s value Is e x p e r i m e n t a l l y obtained u s i n g the e x p r e s s i o n A E o b s 3 E e l " CE(X«) - I P V ( A B ) ] (c.2) where E™^ I s the observed energy of the e j e c t e d e l e c t r o n corresponding to the peak maximum o f the s t a t e under c o n s i d e r a -t i o n f o r the Penning i o n i z a t i o n p r o c e s s , and IP V(AB) i s 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 (measured by p h o t o e l e c t r o n spectroscopy) o f the same s t a t e under c o n s i d e r a t i o n . The A E Q t ) g energy s h i f t value c o n t a i n s a number of components and has been d e f i n e d 5 1 * AE . • * AE + AE. + AET> (C.3) OPS A r where AE i s the t r u e energy s h i f t value f o r the Penning i o n i z a t i o n p r o c e s s , AE f l i s the s h i f t i n the peak maximum caused -37-by competing a u t o i o n i z a t i o n processes and A E p i s the s h i f t i n the peak maximum caused by any p e r t u r b a t i o n o f the t a r g e t p o t e n t i a l curves (see F i g u r e 5). The s i g n i f i c a n c e o f the t r u e AE energy s h i f t s f o r Penning i o n i z a t i o n has been d i s c u s s e d i n a previous s e c t i o n . E x p e r i m e n t a l l y t h i s term has g e n e r a l l y been found t o be o f the magnitude o f thermal e n e r g i e s 5 1 ' 5 3 ' 5 5 . T h e r e f o r e , Penning i o n i z a t i o n e l e c t r o n spectroscopy cannot be used as a technique t o a c c u r a t e l y determine i o n i z a t i o n poten-t i a l s o f atoms and molecules. D i f f i c u l t i e s may a r i s e i n attempting t o e v a l u a t e a t r u e AE energy s h i f t value f o r a s i n g l e s t a t e when a competing a u t o i o n i z a t i o n process occurs and/or i n the s i t u a t i o n where the p o t e n t i a l energy curves o f the t a r g e t molecule are a p p r e c i a b l y m o d i f i e d i n Penning i o n i z a t i o n . E i t h e r of these s i t u a t i o n s may a l t e r the shape o f the envelope observed f o r t r a n s i t i o n s t o an i o n i c s t a t e . I f the envelope shape i s changed, the peak maximum f o r the i o n i c s t a t e may a l s o change. I t should be noted t h a t AE A and AE p w i l l correspond to some simple m u l t i p l e o f v i b r a t i o n a l quanta. To be able t o e v a l u a t e the t r u e AE from A E o b s , v i b r a t i o n a l s t r u c t u r e must be apparent i n the Penning e l e c t r o n s p e c t r a . -38-CHAPTER THREE EXPERIMENTAL 3.1. I n t r o d u c t i o n . The study o f c h e m i - i o n i z a t i o n processes by a n a l y z i n g the energy d i s t r i b u t i o n o f the e j e c t e d e l e c t r o n s r e q u i r e s an e x p e r i -mental c o n f i g u r a t i o n c o n s i s t i n g o f a metastable source, a c o l l i s i o n chamber and an e l e c t r o n energy a n a l y z e r . A photon source i s a l s o d e s i r a b l e so th a t comparisons may be made between c h e m i - 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 p r o c e s s e s . The apparatus used i n t h i s work Is shown s c h e m a t i c a l l y In F i g u r e 6. P l a t e 1 i s a photograph o f the spectrometer. The apparatus was o r i g i n a l -l y designed by S t e w a r t 5 0 * 9 0 but I t has subsequently been modi-f i e d 5 1 . 3.2. The Spectrometer. 3.2.1. Metastable Source. The metastable source c o n s i s t s o f an e x c i t a t i o n r e g i o n where metastables are formed and a set o f g r i d s which are used t o t r a p charged p a r t i c l e s . Commercial tank helium i s i n t r o d u c e d i n t o the e x c i -t a t i o n r e g i o n by means of a v a r i a b l e l e a k v a l v e . The gas enter s the e x c i t a t i o n chamber a t an angle o f 45 degrees t o minimize the s c a t t e r i n g o f helium due t o 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 . The gas enter s through a fused a r r a y o f MU METAL SHIELDING WATER COOLING EXCITATION CHAMBER GAS (ELECTRON AND ION TRAP COLLISION CHAMBER EXTERNAL PHOTON LAMP I U) vo I FIGURE 6.. Schematic diagram o f the e l e c t r o n spectrometer. -40-PLATE 1. The spectrometer. quartz c a p i l l a r i e s (pore diameter • o.o5 mm., t h i c k n e s s • o.5mm). Studies t o determine the importance o f the quartz a r r a y showed a marked improvement i n the s i g n a l t o n o i s e r a t i o s f o r the Penning e l e c t r o n s p e c t r a . T h i s suggests t h a t the d i r e c t i o n a l p r o p e r t i e s o f the helium gas flow are Improved by the qu a r t z a r r a y . The gas i s then bombarded i n the e x c i t a t i o n r e g i o n by e l e c t r o n s o f v a r i a b l e energy. E l e c t r o n s are produced from a d i r e c t l y heated tungsten f i l a m e n t (O.038 mm x 0.76 mm) and are a c c e l e r a t e d ( t y p i c a l l y t o 600 eV) through a s l i t i n t o the e x c i t a t i o n chamber. The t o t a l e l e c t r o n e m i s s i o n v a r i e s between 2 and 20 millamps depending on the source c o n d i t i o n s . An e l e c t r i c a l l y i s o l a t e d water c o o l e d copper b l o c k surrounds the e x c i t a t i o n chamber (see P l a t e 1). The c o o l i n g b l o c k reduces both the ambient temperature at the c o l l i s i o n c e n t r e from 500°K t o 300°K and the time r e q u i r e d t o achieve thermal e q u i l i b r i u m . Any charged p a r t i c l e s produced i n the e x c i t a t i o n chamber are co n f i n e d In the metastable source by proper b i a s i n g of the i o n and e l e c t r o n t r a p p i n g grids, (90% t r a n s m i s s i o n ) . Two c o n f i g u r -a t i o n s have been used t o t r a p the charged p a r t i c l e s . In mode A, the e l e c t r o n s are the f i r s t , p a r t i c l e s t o be trapped, then the Ions are trapped. In mode B, the charged p a r t i c l e t r a p s are re v e r s e d . The best s i g n a l t o n o i s e r a t i o s are ob t a i n e d when the t r a p s are operated i n mode A w i t h a p o t e n t i a l o f 22.5 V t o t r a p the e l e c t r o n s and a p o t e n t i a l of 135 V t o t r a p the i o n s . 3.2.2. C o l l i s i o n Region. The d i s t a n c e from the c e n t r e o f the c o l l i s i o n r e g i o n -42-to the centre of the e x c i t a t i o n chamber i s 7.62 cm and therefore the only processes to be observed w i l l Involve metastables with l i f e t i m e s greater than 6 x 10*"** seconds. Two annular stops with apertures of 6.35 mm and 8.89 mm have been added to the c o l l i -sion region. These stops have reduced the c o l l i s i o n volume of the o r i g i n a l apparatus by a factor of 20 (present dimensions of the c o l l i s i o n chamber : length = 1.27 cm, diameter » 2.54 cm). Studies using the stops have shown that the decrease i n the scattering of low energy secondary electrons, produced when the metastables c o l l i d e with the apparatus walls, r e s u l t s i n a s i g n i f i c a n t reduction i n the background, p a r t i c u l a r l y at lower electron energies. 3.2.3. Electron Analyzer. The k i n e t i c energies of electrons ejected i n chemi-io n i z a t i o n processes may be measured by the d e f l e c t i o n of the electrons In an e l e c t r i c and/or magnetic f i e l d . The properties and the r e l a t i v e merits of various types of electron energy analyzers have been discussed In a number of review a r t i c l e s 9 6 " ' 9 9 . The e l e c t r o s t a t i c analyzer has a number of properties which makes i t suitable for studying chemi-ionization processes. Generally speaking, e l e c t r o s t a t i c f i e l d s are easier to produce and regulate than magnetic f i e l d s . In addition, i t i s r e l a t i v e l y easy to eliminate f r i n g i n g f i e l d s . When an e l e c t r o s t a t i c analyzer of the d e f l e c t i o n : t y p e i s used, a d i f f e r e n t i a l spectrum Is obtained. This has an advantage i n that accurate assignments o f energy l e v e l s are p o s s i b l e . A 127 degree e l e c t r o s t a t i c a n a l y z e r was p r e f e r r e d over a 180 degree e l e c t r o s t a t i c a n a l y z e r because i t i s r e l a t i v e l y easy t o c o n s t r u c t and i t i s l e s s sus-c e p t i b l e to spurious magnetic f i e l d s , F i n a l l y , the use o f s l i t geometry i n the 127 degree a n a l y z e r permits a sampling o f a r e l a t i v e l y l a r g e area. The 127 degree e l e c t r o n a n a l y z e r used i n t h i s study was c o n s t r u c t e d from brass components which are spaced and l o c a t e d by p r e c i s i o n s a p p h i r e b a l l s . The a n a l y z e r p l a t e s are coated w i t h benzene soot to form a homogenous conducting s u r f a c e t h a t minimizes s c a t t e r i n g o f low energy secondary e l e c t r o n s . From f i r s t p r i n c i p l e s , T a r n 1 0 0 has d i s c u s s e d the theory o f the 127 degree a n a l y z e r . The a n a l y z i n g energy o f the e l e c t r o n , E_, w i t h i n the a n a l y z e r , can be e v a l u a t e d from the e x p r e s s i o n v a b 2 E a 1 x 1 ( b / a ) (3.2.3.D where V & b Is the p o t e n t i a l d i f f e r e n c e between the a n a l y z e r p l a t e s , b i s the o u t e r r a d i u s o f the a n a l y z e r , and a i s the i n n e r r a d i u s of the a n a l y z e r . The value of these parameters i n the present case a r e : V a b = 1.0 eV b «. 27.5 mm and a = 22.5 mm. Thus, under t y p i c a l c o n d i t i o n s , the pass energy o f the a n a l y z e r has been c a l c u l a t e d t o be 2.52 eV. The r e s o l u t i o n of the 127 degree a n a l y z e r I n c l u d i n g angular e f f e c t s has been eva l u a t e d from the e x p r e s s i o n 1 0 0 -44-AE V w |. (3.2.3.2) E a r o 3 where AE, i s the f u l l w idth at h a l f maximum (PWHM) o f the 72 e l e c t r o n energy d i s t r i b u t i o n , W i s the width o f the e x i t s l i t , r 0 i s the mean r a d i u s o f the e l e c t r o n a n a l y z e r and a i s the h a l f angle o f acceptance. The values o f these parameters a r e : W = 0.4 mm, ro » 25.0 mm, the h a l f angle o f acceptance o f the e l e c t r o n s produced by an e x t e r n a l photon source a• yf* 2° and the h a l f angle o f acceptance o f the e l e c t r o n s produced by met a s t a b l e s t aHe* " 6° • Thus, f o r p h o t o i o n i z a t i o n , the PWHM i s c a l c u l a t e d t o be 0.044 eV. T h i s can be compared t o an experimental v a l u e o f 0.048 eV PWHM on the A r + ( 2 P ^ 2 ) peak. Por Penning i o n i z a t i o n , the r e s o l u t i o n l s c a l c u l a t e d t o be 0 . 0 ^ eV. F o l d i n g the n a t u r a l width 1* 2 o f 0.035 eV f o r the process H e * ( 2 1 s y ^ r + ( 2 P j y 2 ) i n t o the observed i n s t r u m e n t a l r e s o l u t i o n r e s u l t s i n a c a l c u l a t e d PWHM o f 0.091 eV which i s In good agree-ment with the observed value o f 0.097 eV. The a n a l y z e r i s operated i n a constant r e s o l u t i o n mode, t h a t i s , V a D i s kept constant d u r i n g a scan. The scanning v o l -tage i s a p p l i e d between the c o l l i s i o n chamber which i s at ground p o t e n t i a l , and the entrance s l i t o f the a n a l y z e r . T h i s v o l t a g e i s o b t a i n e d from the a m p l i f i c a t i o n o f a f o u r v o l t ramp which o r i g i n a t e s from the m u l t i c h a n n e l a n a l y z e r i n which the spectrum i s accumulated. t the va l u e o f agg* = 8° quoted i n Reference 50 was c a l c u l a t e d from a r e l a t i o n s h i p 3 7 which evaluates the r e s o l u t i o n o f a 127 degree e l e c t r o n a n a l y z e r i n c o r r e c t l y . -45-I n s i d e the vacuum system, an un-annealed mu-metal s h i e l d surrounds both the e l e c t r o n a n a l y z e r and the c o l l i s i o n r e g i o n . W i t h i n the s h i e l d t h e r e e x i s t s a homogenous magnetic f i e l d o f 100 m i l l l g a u s s . T h i s s h i e l d i s very e f f e c t i v e i n re d u c i n g any s t r a y magnetic f i e l d s due to e x t e r n a l i n t e r -f e r e n c e . 3.2.4. E l e c t r o n D e t e c t i o n . The e l e c t r o n s which pass through the a n a l y z e r are c o l -l e c t e d by a closed-end channel e l e c t r o n m u l t i p l i e r ( M u l l a r d B419AL). The 8 mm entrance cone o f the m u l t i p l i e r and the output are b i a s e d r e s p e c t i v e l y with v o l t a g e s o f + 9 0 V and +2.4k V with r e s p e c t t o ground p o t e n t i a l . S i n c e o n l y r e l a t i v e c r o s s -s e c t i o n s are measured, the a c t u a l value o f the e f f i c i e n c y o f the m u l t i p l i e r i s unimportant and no c a l i b r a t i o n has been made. The e l e c t r o n m u l t i p l i e r Is w e l l s h i e l d e d t o exclude spurious e l e c t r o n c u r r e n t s . The co u n t i n g equipment i s c o n v e n t i o n a l and i s comprised o f a p r e a m p l i f i e r (see F i g u r e 7), an a m p l i f i e r and a d i s c r i m i n a t o r . A cha r t r e c o r d e r , ratemeter o r m u l t i c h a n n e l a n a l y z e r may be used as output d e v i c e s . F i g u r e 8 i s a schematic diagram o f the c o n t r o l c i r c u i t . 3.2.5. L i g h t Source. The e x t e r n a l l i g h t . s o u r c e i s a low p r e s s u r e 2450 MHz microwave di s c h a r g e i n helium. The microwave c a v i t y was con-s t r u c t e d from bras s and has been d e s c r i b e d by B r i o n 1 0 1 . S a m s o n 1 0 2 has r e p o r t e d the c h a r a c t e r i s t i c s o f such a source. The most - 4 6 -+ 24 V SIGNAL INPUT SIGNAL OUT + H.V. INPUT o + H.V. OUTPUT FIGURE 7. Schematic o f the p r e a m p l i f i e r c i r c u i t . COLLISION CHAMBER GROUND 9 SCAN INPUT l<M MULTIPLER CONE INNER OUTER COLLISION CHAMBER GRID FILAMENT EXCITATION CHAMBER J MANAL OR MOTOR SCAN 0-2V -®-K 25k 25k 27 V -H' / -FOCUS POTENTIAL 0-5mA LAMBDA 10V 10A TU m FLUKE 1000 V 500 mA i i FIGURE 8. Schematic of the c o n t r o l c i r c u i t . -48-Inten3e l i n e i s the 5848. He I resonance l i n e . T h i s l i n e a r i s e s from the 2l? •*• 1 1 S resonance t r a n s i t i o n i n helium and has s u f f i -c i e n t energy t o i o n i z e a l l gases except f o r helium and neon. 3.2.6. Vacuum System. A p i c t u r e o f the complete experimental arrangement i s shown i n P l a t e 2. The vacuum system c o n s i s t s o f t h r e e r e g i o n s , the sample h a n d l i n g r e g i o n , the spectrometer chamber and the l i g h t source r e g i o n , and i s e s s e n t i a l l y the same as d e s c r i b e d by S t e w a r t 9 0 . The sample h a n d l i n g r e g i o n c o n s i s t s o f two i n l e t s which are coupled to a common v a r i a b l e l e a k v a l v e ( G r a n v i l l e - P h i l l i p s 203) which leads I n t o the spectrometer. One system, f o r c o r ^ r o s i v e gases and o r g a n i c chemicals, i s c o n s t r u c t e d e n t i r e l y o f s t a i n l e s s s t e e l w h i l e the o t h e r , f o r n o n - c o r r o s i v e gases, i s c o n s t r u c t e d of b r a s s and u t i l i z e s t e f l o n s e a l s . The main spectrometer chamber was c o n s t r u c t e d from s t a i n l e s s s t e e l w i t h V i t o n 0-rings p r o v i d i n g the vacuum s e a l . E l e c t r i c a l connections were made v i a ceramic o c t a l - s e a l s or s i n g l e feed-throughs. The vacuum was maintained by two NRC 8" d i f f u s i o n pumps ( u s i n g Convalex -10, a p o l y p h e n y l e t h e r ) w i t h l i q u i d n i t r o g e n c o l d t r a p s and water b a f f l e s , each backed by mechanical pumps. The c o l d t r a p s were not g e n e r a l l y used and no apparent d e g r a d a t i o n of a n a l y z e r performance has been observed i n t h r e e years o f o p e r a t i o n . The t y p i c a l base p r e s s u r e of the system i s * 1 x 1 0 ~ 7 ; t o r r . PLATE 2. Complete experimental arrangement. -50-The l i g h t source i s pumped by a 2" NRC d i f f u s i o n pump (using Convalex -10),coupled to a dry ice/acetone trap and water b a f f l e , and i s backed by a mechanical pump. 3.3. Treatment of Data. 3»3.1. Energy C a l i b r a t i o n . The photoelectron energy scales of the various molecules studied are calibrated absolutely by using UPS 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 . An electron energy spectrum obtained f o r simultaneous Penning i o n i z a t i o n and photoionization allows c a l i b r a t i o n of the absolute energy scale of the Penning electron spectra. To minimize any possible d i s t o r t i o n of the Penning electron structure of the ground state ion due to overlap with photoelectron structure, the photon source i s regulated such that photoelectron structure Is approximately one tenth the intensity of the Penning electron structure. The accuracy of the energy scales i s lim i t e d by the shape of the structure. For i d e a l cases, the energy scale i s accurate to - 0.010 eV. 3.3.2. Electron Analyzer Transmission Function. Since the 127° electron analyzer i s operated i n a constant resolution mode, the c o l l e c t i o n e f f i c i e n c y of the electrons varies with the electron energy due to the changing electron o p t i c a l lens e f f e c t s between the analyzer entrance s i l t and the c o l l i s i o n chamber. I f the true 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 to be determined and compared to those reported elsewhere, a transmission correction factor must be determined for the apparatus. The retarding analyzer of Hotop -51-and Niehaus 1* 5 has been r e p o r t e d to have a 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° p h o t o i o n i z a t i o n e l e c t r o n i c band i n t e n s i t i e s obtained i n t h i s work have been compared t o the work o f Hotop and Niehaus f o r m o l e c u l a r n i t r o g e n 1 * 5 , carbon monoxide 1* 5, and n i t r i c o x i d e 1 0 3 , i n order to d e 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, f o r the s p e c t r a as shown i n F i g u r e 9. In independent experiments employing e l e c t r o n - e l e c t r o n c o i n -cidence t e c h n i q u e s , van der W i e l and B r i o n 9 3 have determined the r e l a t i v e i o n i c s t a t e p o p u l a t i o n s f o r the p h o t o i o n i z a t i o n o f carbon monoxide and found e x c e l l e n t agreement with the work o f Hotop and Niehaus 1* 5, as w e l l as the t o t a l c o l l e c t i o n experiments of F r o s t et a l . * 0 i * . As can be seen i n F i g u r e 9>there i s a c o n s i d e r a b l e change l n the t r a n s m i s s i o n f u n c t i o n over a 12 eV range of e l e c t r o n e n e r g i e s . F a i l u r e to use the 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 would i n t r o d u c e s e r i o u s e r r o r s i n q u a n t i -t a t i v e work, e s p e c i a l l y i n the energy range 1 t o 7 eV. The i n t e n s i t i e s of a l l e l e c t r o n ; s p e c t r a have been c o r r e c t e d , u s i n g the t r a n s m i s s i o n f a c t o r T, i n order to o b t a i n the data r e p o r t e d i n the t a b l e s . 3.3.3. Background S u b t r a c t i o n Technique. Q u a n t i t a t i v e s t u d i e s o f Penning e l e c t r o n s p e c t r a are f r e q u e n t l y complicated by 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 e n e r g i e s . An.estimation of the background e l e c t r o n spectrum can be o b t a i n e d by removing the t a r g e t gas while main-t a i n i n g a l l other parameters necessary f o r o p e r a t i n g the Penning i o n i z a t i o n e l e c t r o n spectrometer. The background s i g n a l so FIGURE 9. 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 f o r the 127° e l e c t r o n a n a l y z e r . -53-d e r i v e d may be s u b t r a c t e d from the observed "spectrum and background" t o o b t a i n a "background s u b t r a c t e d " spectrum. I n a study o f the Penning i o n i z a t i o n o f argon (see F i g u r e 10, t h i s technique was used and the s u b t r a c t i o n was performed d i r e c t l y i n the memory o f a F a b r i t e k 1074 m u l t i c h a n n e l a n a l y z e r . The removal o f the background has extended the r e g i o n where quan-t i t a t i v e a n a l y s i s may be performed by approximately 3 eV. Because o f s m a l l gas dependent changes i n the energy s c a l e , t h i s procedure i s u n r e l i a b l e below 0.5 eV where the g r a d i e n t o f the background i s very l a r g e . 3.4. Sample P u r i t y . Except f o r hydrogen cyanide and formaldehyde, a l l o f the chemical samples used i n t h i s study were purchased from commercial sources and were used without f u r t h e r p u r i f i c a t i o n . The p u r i t y o f a l l samples are l i s t e d as b e i n g b e t t e r than 98%. L i q u i d s are degassed by freeze-thaw c y c l e s . The p h o t o e l e c t r o n s p e c t r a p r o v i d e a convenient check f o r sample i m p u r i t i e s . Hydrogen cyanide was prepared a c c o r d i n g t o the procedure out-l i n e d by B r a u e r 1 0 5 . Formaldehyde was obtained by thermal decomposition o f paraformaldehyde. Spectrum -i 1 i i i i 1 6 5 4 3 2 1 0 ELECTRON ENERGY (oV) FIGURE 10. Background s u b t r a c t i o n a p p l i e d to the Penning e l e c t r o n spectrum o f argon. -55-CHAPTER FOUR DIATOMIC MOLECULES 4.1. I n t r o d u c t i o n . The o b j e c t i v e o f t h i s t h e s i s i s to continue the s y s t e -matic study i n i t i a t e d by Stewart 9; 0 o f the H e « ( 2 1 S , 2 3 S ) Penning i o n i z a t i o n o f commonly a v a i l a b l e s m a l l polyatomic molecules. To date, l e s s than s i x t y s p e c i e s have been s t u d i e d by Penning i o n i z a t i o n o f which twenty-three were s t u d i e d i n t h i s l a b o r a t o r y 9 0 . The e l e c t r o n energy d i s t r i b u t i o n of the He*(2 1S,2 3S) Penning i o n i z a t i o n process i s compared to the 584 & p h o t o i o n i z a t i o n p r o c e s s . The t h r e e g e n e r a l f e a t u r e s of the e l e c t r o n s p e c t r a which are o f i n t e r e s t i n the, comparison of the two i o n i z a t i o n p r ocesses are 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 , the normalized r e l a t i v e e l e c t r o n i c s t a t e p o p u l a t i o n s and the AE energy s h i f t s . The r a t i o s o f a s s o c i a t i v e t o Penning i o n i z a t i o n have not been e x t r a c t e d from the e l e c t r o n energy d i s t r i b u t i o n s because the r e l a t i v e k i n e t i c energy between the metastable and t a r g e t s p e c i e s i s unknown. In a d d i t i o n , the v e l o c i t y of the m etastables which leave the source have a Boltzmann's d i s t r i b u t i o n . Diatomic molecules were among the f i r s t molecules to be s t u d i e d by Penning i o n i z a t i o n with the technique o f e l e c t r o n s pectroscopy. Using a low r e s o l u t i o n e l e c t r o n a n a l y z e r , Cermak s t u d i e d the Penning i o n i z a t i o n o f H 2, N 2, CO, NO and 0 2 and observed s e v e r a l I o n i c s t a t e s u s i n g both N e * ( a P 0 , a P 2 ) and He*(2 lS,2 3S) metastable a t o m s 2 8 . More r e c e n t l y , at h i g h e r r e s o l u t i o n and u s i n g A r * ( 3 P 2 ) metastable atoms Cermak and O z enne 3 7 have s t u d i e d the Penning i o n i z a t i o n o f NO l e a d i n g t o the ground s t a t e o f N0 + (if 1 E + ) . Hotop and Niehaus 1* 5 have a l s o s t u d i e d the H e » ( 2 1 S , 2 3 S ) and N e » ( 3 P 0 , 3 P 2 ) Penning i o n i z a t i o n o f H 2 , N 2 and CO u s i n g a h i g h r e s o l u t i o n e l e c t r o n a n a l y z e r . I t i s noted t h a t i n g e n e r a l the observed v i b r a t i o n a l s t r u c t u r e o f the d i a t o m i c s p e c i e s can be unambiguously a s s i g n e d f o r the two modes o f i o n i z a t i o n . I t a l s o appears t h a t f o r these s p e c i e s both A E A and A E p are z e r o , t h e r e f o r e , A E O D S = AE. In t h i s ; chapter the He*(2 1S,2 3S) Penning e l e c t r o n s p e c t r a o f H 2 , HD, D 2 , N 2, CO, NO and 0 2 are r e p o r t e d . *4.2. M o l e c u l a r Hydrogen, Deuterium Hydride and M o l e c u l a r  Deuterium. 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 these molecules are ( I s o g ) 2 ; 1 l g ¥ s ( H 2 , D 2) (Is 0 g ) 2 ; ^ + , (HD) Using 584 % p h o t o n s 5 0 , the p h o t o i o n i z a t i o n o f the s t r o n g l y bonding ( l s o ) o r b i t a l c o n s i s t s of a s i n g l e band o f w e l l r e s o l v e d v i b r a t i o n a l components, the s p a c i n g r a p i d l y d e c r e a s i n g as the d i s s o c i a t i o n l i m i t i s approached. Using low r e s o l u t i o n e l e c -t r o n s p e c t r o s c o p y , Cermak 2 8 s t u d i e d the c o l l i s i o n process i n v o l v i n g He*(2 1S,2 3S) metastables w i t h m o l e c u l a r hydrogen and observed broad bands. He suggested t h a t competing I o n i z a t i o n processes c o u l d p o s s i b l y obscure the expected v i b r a t i o n a l s t r u c -t u r e f o r Penning i o n i z a t i o n . Using mass spectrometry, Hotop and Niehaus 1* 1 s t u d i e d the same c o l l i s i o n p r o c e s s . They meas-ured the r e l a t i v e i o n i n t e n s i t i e s f o r H2 +; HeH2 + and HeH + and concluded t h a t although Penning i o n i z a t i o n i s the dominant process both a s s o c i a t i v e i o n i z a t i o n and d i s s o c i a t i v e a s s o c i a t i v e i o n i z a t i o n occur t o a l i m i t e d e x t e n t . To e x p l a i n the r e s u l t s , they proposed a two-step model which i s confirmed by indepen-dent s t u d i e s by Neynaber et- a l . 1 0 B . Using h i g h r e s o l u t i o n e l e c t r o n s p e c t r o s c o p y , Hotop and Niehaus** 7 repeated Cermak's s t u d y 2 8 and they observed a s i n g l e band 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. The r e s u l t s f o r the-He*(2 lS,2 3S) Penning i o n i z a t i o n of deuterium h y d r i d e and m o l e c u l a r deuterium, as w e l l as f o r molecular hydrogen are now r e p o r t e d . F i g u r e 11 compares the Penning e l e c t r o n s p e c t r a f o r the three molecules. The a b s o l u t e energy s c a l e s are c a l i b r a t e d w i t h r e s p e c t to the p h o t o e l e c t r o n l i t e r a t u r e v a l u e s 1 0 7 of the I o n i z a t i o n p o t e n t i a l s . Table 2 shows the AE s h i f t s measured f o r the Penning i o n i z a t i o n p r o -cess. A l l peaks are s h i f t e d t o h i g h e r e l e c t r o n e n e r g i e s and where comparisons are p o s s i b l e , the values agree with those r e p o r t e d by Hotop and Niehaus 1* 5''* 7. The v i b r a t i o n a l spaclngs f o r i o n i z a t i o n by " i n t e r n a l " 584 % photons as w e l l as He* (2 1S,2 3S) metastable atoms,are I n d i c a t e d i n F i g u r e 11 and no d i f f e r e n c e s have been observed between the two modes of i o n i z a t i o n . -58-H, 2S "T I—I—I—I I I I I I ~l 1—I—I I I I n — i — r HD i i i i 1 1 1 1 r ^ i i i i i i hy i — i — i — i r •.• ' v. v' »; >. A • y D, H i 1 1 1 I I I ! I I I I I I I I I I I I I r- ' v VIA A/' ELECTRON ENERGY (eV) FIGURE 11. Penning e l e c t r o n s p e c t r a of m o l e c u l a r hydrogen, deuterium h y d r i d e and molecular deuterium. -59-Table 2. l 3 k AE 0bs energy s h i f t s i n eV f o r the He*(2 S,2 S) Penning i o n i z a t i o n of molecular hydrogen, deuterium h y d r i d e , and molecular deuterium at 300 °K. + 2 + + 2 + + 2 + E l e c t r o n i c State H 2 ( E g ) HD ( I ) D 2 ( E g ) He*(2 1S) l i t e r a t u r e t h i s work +0.088 * 0.020 +0.070 * 0.020 +0.085 * 0.020 He*(2 3S) a l i t e r a t u r e . +0.090 * 0.010 • l i t e r a t u r e 0 +0.070 t h i s work +0.069 * 0.010 +0.085 1 0.010 +0.051 * 0.010 a. Reference 47 b. Reference 45 at ^350 °K E x t r a c t i o n o f 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 f o r these molecules i s complicated by a number of f a c t o r s . Mixed w i t h the Penning e l e c t r o n s p e c t r a f o r He*(2 1S,2 3S) i s a low i n t e n s i t y p h o t o e l e c t r o n spectrum a r i s i n g from " i n t e r n a l " 584 A p h o t o n s 5 0 and a r a p i d l y r i s i n g background. The p o s i t i o n and shape o f t h i s background appears t o be dependent on the source c o n d i t i o n s i n the c o l l i s i o n r e g i o n . In a d d i t i o n , a l a r g e p r o p o r t i o n o f s t r u c t u r e due t o He*(2 1S) i o n i z a t i o n over-laps w i t h s t r u c t u r e due to He*(2 3S). With the a i d o f a He*(2 1S) quenching lamp, Hotop and Niehaus 1* 5» 1 , 7 have estimated 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 Penning i o n i z a t i o n p rocess He*(2 3S)/H2 from the i n t e g r a l e l e c t r o n spec-trum which i s complicated by a r i s i n g background. They conclude t h a t the Franck-Condon envelope f o r He*(2 3S) Penning i o n i z a -t i o n i s s i m i l a r t o t h a t f o r p h o t o i o n i z a t i o n by 584 X photons. I t i s not p o s s i b l e t o e v a l u a t e 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 He*(2 3S)/H2 from the Penning e l e c t r o n spectrum i n F i g u r e 11 s i n c e t h i s r e q u i r e s assumptions be made concerning the background f u n c t i o n as w e l l as the shape o f the Franck-Condon envelope f o r the Penning i o n i z a t i o n process He*(2 lS)/H 2. S i m i l a r l y i t i s not p o s s i b l e to e v a l u a t e 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 Penning i o n i z a t i o n o f deuterium h y d r i d e or molecular deuterium. Never-t h e l e s s , an attempt was made t o I n v e s t i g a t e the p o s s i b l e v a l u e s o f Franck-Condon f a c t o r s f o r He*(2 1S,2 3S) Penning i o n i z a t i o n and 584 A p h o t o i o n i z a t i o n 5 1 . In the upper curve of F i g u r e 12 the -61-J , i . 1 — - . 1 5.0 4.0 3.0 2.0 ELECTRON ENERGY (eV) FIGURE 12. E s t i m a t i o n of 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 f o r the i o n i z a -t i o n o f hydrogen. background f u n c t i o n of the composite curve f o r H 2 T i s c a l c u -l a t e d by a p p l y i n g to a l l t h r e e processes the r e l a t i v e v i b r a t i o n a l i n t e n s i t i e s ( c o r r e c t e d f o r t r a n s m i s s i o n ) as observed f o r e x t e r n a l 58l» A p h o t o i o n i z a t i o n . That i s , f o r t h i s o p e r a t i o n , i t i s assumed t h a t the same r e l a t i v e Franck-Condon f a c t o r s apply i r r e s p e c t i v e o f the mode o f i o n i z a t i o n . The background f u n c t i o n so o b t a i n e d i s shown a p p l i e d t o the N 2 + spectrum In the lower p a r t o f F i g u r e 12 and an e x c e l l e n t e m p i r i c a l f i t i s obt a i n e d . T h i s suggests t h a t the Penning i o n i z a t i o n o f H 2 i s e s s e n t i a l l y governed by the same Franck-Condon f a c t o r s as p h o t o i o n i z a t i o n . 4.3. M o l e c u l a r N i t r o g e 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 o f the n i t r o g e n molecule i s KK ( 2 s a g ) 2 ( 2 S a u ) 2 (2pftu>,» ( 2 p a g ) 2 ; lZg+ As i l l u s t r a t e d i n F i g u r e 13: the i o n i c s t a t e s X 2 E g + , A 2 I " U and B 2 I ^ are a c c e s s i b l e u s i n g 58^ % photons and H e » ( 2 1 S , 2 3 S ) metar s t a b l e atoms. The absolute energy s c a l e s are c a l i b r a t e d w i t h r e s p e c t t o data from r e f e r e n c e 107. The measured AE energy s h i f t s are l n good agreement w i t h the r e s u l t s o f Hotop and Niehaus 1* 5 and are l i s t e d i n Table 3. The v i b r a t i o n a l spacings are found t o be the same w i t h i n experimental e r r o r f o r both methods o f i o n i z a t i o n , f o r the r e s p e c t i v e i o n i c s t a t e s . A q u a n t i t a t i v e a n a l y s i s of the H e « ( 2 1 S ) / N 2 + ( A 2 l t u ) s e r i e s i s not p o s s i b l e due t o the r e l a t i v e l y low c r o s s - s e c t i o n and overlap with H e » ( 2 3 S ) / N 2 + ( X 2 l " K + ) . Table 4 l i s t s the -63-\ Penning Ionization (He*21S.23S) i ' i 1 I 1 4 • -4 • \ • 6 ELECTRON ENERGY (eV) FIGURE 13. Electron spectra for the i o n i z a t i o n of molecular nitrogen. -64-Table 3. , 1 3 . AEobs energy s h i f t s i n eV f o r the He»(2 S,2 S) Penning i o n i z a t i o n of m o lecular n i t r o g e n at 300 °K. E l e c t r o n i c S t a t e X Eg A " u +0.010 4 0.010 + 0.023 1 0.020 +0.013 4 0.010 +0.050 * 0.010 +0.053 * 0.010 +0.066 4 0.010 +0.062 * 0.010 - 2 r + B I u He*(2 S) a l i t e r a t u r e +0.015 1 0.010 t h i s work +0.020 4 0.010 He*(2 3S) . l i t e r a t u r e +0.050 4 0.010 t h i s work +0.051 4 0.010 a. Reference 45 at ^350 °K Table 4. Normalized 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 2 (X; v" = o) -»• _ o 1 3 N 2 +(X,A,B; v f ) f o r 584 A p h o t o i o n i z a t i o n and He*(2 S,2 S) Penning i o n i z a t i o n . E l e c t r o n i c S t a t e X A 2 n u - 2 „ + B z u V ' 0 1 0 1 2 3 4 5 0 1 He (584 A) l i t e r a t u r e a l i t e r a t u r e 0 t h i s work 100 100 100 7*1 10*1 .9*1 87*3 86*3 87*1 100*3 100 100 76*2 73*3 70*1 44*3 46*3 40*1 19*2 21*3 16*1 7*1 12*3 7*1 100° 100 100 10* 3 b 14*3 15*2 He « ( 2 l S ) l i t e r a t u r e * 2 t h i s work 100 100 12*2 9*2 100 57*15 _ 100 100 21*3 39*10 He»(2 3S) l i t e r a t u r e 0 t h i s work 100 100 12*2 8*2 89*4 86*9 100 100 65*5 63*6 35*5 30* 6 17*4 4*4 8*4 100 100 18*3 16*4 a. Reference 153. o b. Using 537 A r a d i a t i o n . c. Reference 45 at ^350 °K. 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 t o s p e c i f i c i o n i c s t a t e s . The data i s i n good agreement wi t h other s t u d i e s 1 * 5 and suggests t h a t 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 modes o f i o n i z a t i o n are very s i m i l a r f o r the three i o n i c s t a t e s o f n i t r o g e n . In a n a l y z i n g the r e s u l t s o f Penning i o n i z a t i o n o f m o l e c u l a r n i t r o g e n s t u d i e s by o p t i c a l e mission spectroscopy from e x c i t e d s t a t e s o f N 2 + , R o b e r t s o n 1 0 8 , Schmeltekopf et a l . 3 3 and Richardson and S e t s e r 3 5 have a l s o come to the same c o n c l u s i o n . Table 5 l i s t s the r e l a t i v e p o p u l a t i o n s o f e l e c t r o n i c s t a t e s o f N 2 + . In the case o f He*(2 1S), the r e l a t i v e ' e l e c t r o n i c p o p u l a t i o n f o r the N 2 + ( A 2 n u ) s t a t e i s estimated on the assump-t i o n that the Franck-Condon envelopes of N 2 + ( X 2 £ + ) are s i m i l a r f o r Penning i o n i z a t i o n by He*(2 3S) and p h o t o i o n i z a t i o n u s i n g 584 A photons. Thus, most of the s t r u c t u r e at 3-76 eV corresponds t o He»(2 1S)/N 2 +(A 2 n u , v* =1). I t i s a l s o assumed t h a t the r a t i o o f 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 y f o r v' = 1 and v* = 2 to the t o t a l v i b r a t i o n t r a n s i t i o n p r o b a b i l i t y f o r N 2 + ( A 2H U) i s s i m i l a r f o r Penning i o n i z a t i o n u s i n g e i t h e r H e * ^ 1 ^ ) or He*(2 3S) me t a s t a b l e s . The p o p u l a t i o n s o f e l e c t r o n i c s t a t e s o btained u s i n g He*(2 1S) metastables f o l l o w the same tr e n d as f o r He*(2 3S) metasta b l e s . Comparisons of the r e l a t i v e p o p u l a t i o n o f e l e c t r o n i c s t a t e s f o r the two modes o f i o n i z a t i o n r e v e a l s s i g n i f i c a n t p o p u l a t i o n d i f f e r e n c e s . For helium metastables, the r e l a t i v e p o p u l a t i o n of the A 2 n u s t a t e Is q u i t e s m a l l , compared to that f o r p h o t o i o n i z a t i o n . A comparison o f the p o p u l a t i o n -67-Table 5. Normalized r e l a t i v e e l e c t r o n i c s t a t e p o p u l a t i o n s f o r 584 A 1 3 p h o t o i o n i z a t i o n and He*(2 S,2 S) Penning i o n i z a t i o n o f mo l e c u l a r n i t r o g e n at 300 °K. - 2 + - 2_ - 2 + E l e c t r o n i c S t a t e X' z R A n u B £ u He (584 A) l i t e r a t u r e 3 75 * 6 100 * 6 17 1 4 t h i s work 79 * 4 100 * 6 11 * 2 He*(2 1S) l i t e r a t u r e 8 , . t h i s work 100 * 10 28 * 6 89 4 18 He*(2 3S) l i t e r a t u r e 3 85 * 10 . 61 * 12 100 * 10 t h i s work 98 * 10 46 * 5 100 * 10 a. Reference 45 at ^350 °K » b. Estimated on the b a s i s o f v =1,2 -68-r a t i o X 2 E u + / B 2 j : u + r e v e a l s d i f f e r e n c e s f o r the two modes of i o n i z a t i o n . In a recent study, Cermak 1 0 9 observed e l e c t r o n s at 1.31 eV and 1.49 eV a r i s i n g from a u t o i o n i z i n g l e v e l s of the n i t r o g e n atom. I t has been suggested that these a u t o i o n i z i n g l e v e l s are formed when e x c i t a t i o n of molecular n i t r o g e n by f a s t n e u t r a l H e d ^ ) atoms produces h i g h l y e x c i t e d molecular s t a t e s which can subsequently p r e d i s s o c i a t e . There i s some evidence f o r these two a u t o i o n i z i n g l e v e l s i n our spectrum ( l a b e l e d N + i n Figure 13). 4.4. Carbon Monoxide. Carbon monoxide i s i s o e l e c t r o n i c w i t h molecular n i t r o g e n and has a ground s t a t e c o n f i g u r a t i o n of KK ( 2 s o ) 2 ( 2 s o * ) 2 ( 2 p n ) u ( 2 p o ) 2 ; l r + Pdr C0 +, the three i o n i c s t a t e s X 2 & , A 2 n and B 2"* are a c c e s s i b l e by 584 A photons and He*(2 S ,2 S) metastables, (as shown i n Figure 14). The process He»(2 3S)/CO +(B i s hidden i n the high background at M).2 eV. The absolute energy s c a l e s are c a l i b r a t e d w i t h respect t o values given i n reference 107. There i s evidence of s t r u c t u r e f o r He*(2 lS)/CO +(A 2 n ) , but no i n f o r m a t i o n can be e x t r a c t e d s i n c e i t i s overlapped with the s t r u c t u r e of " i n t e r n a l " He(584 A V c O + ( A 2 j I ) . The measured AE energy s h i f t s are l i s t e d i n Table 6 and are i n good agreement with the work of Hotop and Niehaus** 5. -69-Penning Ionization (He*21S, 23S) CO Photoionization (584A) i i . i l l 7 i £ 4 4 k \ ELECTRON ENERGY feV) 0 FIGURE 14. E l e c t r o n s p e c t r a f o r the i o n i z a t i o n o f carbon monoxide. -70-Table 6. 1 3 A E o b s energy s h i f t s i n eV f o r the He*(2 S,2 S) Penning i o n i z a t i o n of carbon monoxide at 300 °K. - 2 + _ o - 2 + E l e c t r o n i c S t a t e X * A n B £ He*(2 S) l i t e r a t u r e 3 -0.020 * 0.010 +0.040 t h i s work -0.014 4 0.010 +0.049 * 0.010 He*(2 3S) l i t e r a t u r e 3 +0.045 1 0.010 +0.035 ^ 0.015 t h i s work +0.043 * 0.010 +0.047 * 0.010 a. Reference 45 at ^350 °K -71-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 states obtained by the removal of an e lec t ron 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 nitrogen and carbon monoxide, no c o r r e l a t i o n of the AE energy s h i f t s can be made. Por a l l three Ionic states of C0 +, the v i b r a t i o n a l spacings are the same for both modes of i o n i z a t i o n . The r e s u l t s for 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 C0 + are l i s t e d i n Table 7 and are very s i m i l a r to those reported by Hotop and Niehaus 1* 5 . As i n the case of n i t rogen , i t appears that 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 are very s i m i l a r f o r given i o n i c states of carbon monoxide, regardless of the mode of i o n i z a t i o n . This i s the same conclusion obtained from afterglow e x p e r i m e n t s 3 3 ' 3 5 ' 1 0 8 for exci ted states of C0 +. The normalized r e l a t i v e populat ion of e l e c t r o n i c states for carbon monoxide are l i s t e d i n Table 8 and agree with the values reported by Hotop and Niehaus . In the case of CO , the r a t i o X 2 E + / A 2 n i s s i g n i f i c a n t l y d i f f e r e n t f o r He ( 5 8 * 1 X) and H e * ( 2 3 S ) . Even though the two modes of i o n i z a t i o n are d i f f e r e n t , It i s i n t e r e s t i n g to note that the r a t i o s observed are what might be expected on the basis of the r e l a t i v e populations of states of CO"1" which have been measured as a funct ion of "photon" energy,by van der Wiel and B r i o n 9 3 . Comparisons of the r e l a t i v e populat ion of e l e c t r o n i c states f o r a given mode of i o n i z a t i o n are found to be very s i m i l a r for T a b l e 7. Normalized 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 CO (X, v" = o) •* C O + ( X , A , B ; v') f o r 584 A p h o t o i o n i z a t i o n and He*(2 1S,2 3S) Penning i o n i z a t i o n . E l e c t r o n i c S t a t e _ 2 X + -E - 2 A n _ 2 B + E V' 0 1 0 1 2 3 4 5 6 7 0 1 He (584 A) l i t e r a t u r e 3 l i t e r a t u r e 0 t h i s work 100 100 100 3*0.3 5*1 5*1 39*2 45*2 43*2 83*1 82*3 83*2 100*3 100 100 87*2 87*3 83*2 68*2 65*3 63*2 50*2 50*4 48*2 24*1 28*3 22*2 15*1 20*3 14*2 100 b 100 100 3 5 * l b 43*3 50*5 He « ( 2 1 S ) l i t e r a t u r e 0 t h i s work 100 100 4*2 - - - - - - - - 100 100 38*4 28*14 He«(2 3S) l i t e r a t u r e 0 t h i s work 100 100 5*2 8*1 47*4. 63*9 d 80*5„ 90*9 a 100 100 85*5 92*9 55*5 58*9 33*5 37*9 20*5 16*9 --a. Reference 153. b. 537 A. c. Reference 45 at ^350 °K. d. Some c o n t r i b u t i o n s due to He*(2 1S) and He (584 A*). -73-Table 8. Normalized r e l a t i v e e l e c t r o n i c state populations f o r 584 % 1 3 photoionizat ion and He*(2 S,2 S) Penning i o n i z a t i o n of carbon monoxide at 300 °K. - 2 + - 2 - 2 + E l e c t r o n i c State X E A n B E He (584 A) l i t e r a t u r e 3 67 t h i s work 70 He*(2 1 S) l i t e r a t u r e 3 t h i s work 96 He»(2 3S) l i t e r a t u r e 3 - 100 t h i s work 100 7 4 10 10 100 100 * 43 38 * 6 10 * 10 * 6 19 * 4 9 * 2 100 * 25 a. Reference 45 at ^350 °K - 7 4 -both carbon monoxide and n i t r o g e n . No data could be obtained f o r the process H e*(2 3 S)/C0 + ( B 2 E + ) which i s probably hidden i n the r a p i d l y r i s i n g background. However, the r a t i o X 2 E + / A 2 n f o r the populat ion of Ionic states formed using He*(2 3 S) i s found to be very s i m i l a r f o r both molecular ni trogen and carbon monoxide. There i s some evidence to suggest that carbon monoxide may be exci ted Into a highly exci ted p r e d i s s o c i a t i n g state by fast neutra l helium atoms. A f t e r d i s s o c i a t i n g , the products may be i n an a u t o i o n i z i n g s t a t e . Por example, i n the Penning e lec t ron spectra of carbon monoxide, there are two u n i d e n t i f i e d peaks i n Figure 14 l abeled 0 + at 1 . 7 7 eV and 1 . 5 5 eV. These values correspond c l o s e l y to e lec t ron e j e c t i o n energies from a u t o i o n i z i n g states of the oxygen atom as observed by Cermak and S r a m e k 1 1 0 . I t i s noted that the peak at 1 . 5 5 eV could a lso correspond to s t ructure due to B 2 E + , v ' = 0, for the 584 A photoioniza t ion of N 2 . 4 . 5 . N i t r i c Oxide. The ground s tate e lec t ron conf igura t ion f o r n i t r i c oxide i s KK ( 2 s o ) 2 ( 2 s a * ) 2 (2p°) 2 (2pH)** ( 2 P " * ) 1 ; 2 n In the He(584 -X) p h o t o i o n i z a t i o n spectrum of n i t r i c oxide, eight of the i o n i c states have been i d e n t i f i e d 1 1 1 . A number of laborator ies 3 7 »**6 >**9 have s tudied aspects of the Penning I o n i z a -t i o n e lec t ron spectroscopy of n i t r i c oxide , i n p a r t i c u l a r comparing the Penning i o n i z a t i o n and photoioniza t ion 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 NO T(X 1 ? T ) . I t has been concluded t h a t i f t h e r e i s a d i f f e r e n c e i n the Franck-Condon f a c t o r s f o r the two modes o f i o n i z a t i o n , i t i s s m a l l . F i g u r e 15 compares the Penning e l e c t r o n spectrum and the p h o t o e l e c t r o n spectrum o f n i t r i c o x i d e . In the He*(2 IS) Penning 1 4- — 3 + e l e c t r o n spectrum, s t r u c t u r e f o r X £ and b n s t a t e s o f NO i s observed. F o r He*(2 3S) i o n i z a t i o n , s t r u c t u r e i s observed f o r the X \ 1 £ + , b 3H and A l n s t a t e s . The AE energy s h i f t s f o r n i t r i c oxide are l i s t e d i n Table 9« Both the v i b r a t i o n a l spac-ings and the a s s o c i a t e d Franck-Condon f a c t o r s f o r the NO +(X + ) s t a t e f o r the two modes o f i o n i z a t i o n are found to be the same, w i t h i n experimental e r r o r , as r e p o r t e d e a r l i e r 1 * 9 . I t appears t h a t the r e l a t i v e p o p u l a t i o n s o f the thr e e e l e c t r o n i c s t a t e s X b 3 n and A 1 n as shown i n Table 10 are a l s o very s i m i l a r f o r i o n i z a t i o n by He*(2 3S) metastables and 584 % photons. There appears t o be evidence f o r a u t o i o n i z a t i o n o f d i s s o c i a t i o n products o f n i t r i c oxide e x c i t e d by f a s t n e u t r a l helium atoms i n th a t there i s s t r u c t u r e i n the r e g i o n o f 1.7 eV t o 3.0 eV which cannot be accounted f o r by Penning i o n i z a t i o n . The energy of some o f these peaks agree w i t h e l e c t r o n e j e c t i o n energies from a u t o i o n i z i n g l e v e l s of the oxygen atom as observed by C e r m a k 1 1 0 . In a d d i t i o n , t h e r e are two peaks at 0.6 eV and 1.1 eV l a b e l e d N +, which correspond to two a u t o i o n i z i n g l e v e l s i n the n i t r o g e n atom a l s o observed by C e r m a k 1 0 9 . - 7 6 -Penning Ionization (He*21S,2,S) 2'8(X) I I I I I l 213(b) i V 2lS(A) J : '[ ;/W 2>S(b) 1 O* jj/ hv(b) || A Phototonization (584 A) ^ 111 !l/ujlwM'> i! I i 1 I — T — i i i r — i 1 r—HL 12 11 10 9 8 7 6 5 4 3 2 1 0 ELECTRON ENERGY (eV) FIGURE 15 . E lec t ron spectra for the I o n i z a t i o n of n i t r i c oxide . -77-Table 9. 1 3 A E 0 b s energy s h i f t s i n eV for the He*(2 S,2 S) Penning i o n i z a t i o n of n i t r i c oxide at 300 °K. E l e c t r o n i c State X £ b H A It He*(2 S) l i t e r a t u r e 3 +0.00 * 0.010 +0.00 * 0.010 t h i s work +0.007 * 0.020 +0.018 * 0.020 H e » ( 2 3 S ) l i t e r a t u r e 3 -0.010 * 0.010 +0.035 * 0.010 t h i s work -0.009 * 0.015 +0.062 * 0.015 +0.032 * 0.015 a. Reference 103 -78-Table 10. _. o Normalized r e l a t i v e e l e c t r o n i c s t a t e p o p u l a t i o n s f o r 584 A 3 p h o t o i o n i z a t i o n and He*(2 S) Penning i o n i z a t i o n o f n i t r i c oxide at 300 °K. - 1 + _ 3 _ l E l e c t r o n i c S t a t e X I b n A n He (584 %) l i t e r a t u r e l i t e r a t u r e 1 t h i s work a 32 40 * 3 33 4 2 He*(2 S) x l i t e r a t u r e t h i s work 29 35 1 5 4 4 100 100 100 4 5 40 52 * 8 45 1 9 100 100 50 a. Reference 111 b. Reference 103 -79-4.6. M o l e c u l a r Oxygen. > Mo l e c u l a r oxygen has a ground s t a t e c o n f i g u r a t i o n o f KK ( 2 s o g ) 2 ( 2 s o u ) 2 (2po g> 2 ( 2 P n u ) ' t ( 2 P n g ) 2 ; 3 E ~ Ed q v i s t et a l . 1 1 2 have s t u d i e d both the 584 % and the 304 % p h o t o e l e c t r o n s p e c t r a o f mol e c u l a r oxygen. Using 584 A* photons, i t Is p o s s i b l e t o observe f i v e I o n i c s t a t e s (see F i g u r e 16) o f molecular oxygen. However, In the Penning e l e c t r o n spectrum only the 0 2 + (X 2 n ) s t a t e produced by He*(2 3S) metastables can be c l e a r l y i d e n t i f i e d . There i s some evidence f o r the X s t a t e of 0 2 + produced by He*(2 1S) metastables but q u a n t i t a t i v e meas-urements are not p o s s i b l e . The v i b r a t i o n a l spacings f o r the ground i o n i c s t a t e (X 2 n g ) > as i n d i c a t e d i n F i g u r e 17, are the same w i t h i n experimental e r r o r f o r both modes o f i o n i z a t i o n . The AE energy s h i f t measured f o r H e * ( 2 3 S ) / 0 2 + (X 2 n g ) i s -0.019 - 0.010 eV. F o r He(584 A ) / 0 2 + ( X 2 n ) and He*( 2 3 S ) / 0 2 + (X 2Tt ) the r e l a t i v e v i b r a t i o n a l p o p u l a t i o n s are l i s t e d i n Table 11. Comparison of the data i n Table 11 f o r the two processes i n d i -cates s i g n i f i c a n t d i f f e r e n c e s f o r Penning i o n i z a t i o n and photo-i o n i z a t i o n at h i g h e r v i b r a t i o n a l quantum numbers. D i s t o r t i o n of the Franck-Condon f a c t o r s i n the Penning i o n i z a t i o n o f mole-c u l a r oxygen has a l s o been observed by Richardson et a l . 3 % 3 5 f o r the 0 2 + ( A 2 l " u ) i o n i c s t a t e . T h i s d i s t o r t i o n o f the Franck-Condon envelopes i n Penning i o n i z a t i o n p o s s i b l y suggests t h a t the p o t e n t i a l curves f o r 0 2 and/or 0 2 + are p e r t u r b e d by the helium p a r t i c l e s . In a rec e n t study o f the angular d i s t r i b u t i o n s of Penning i o n s , Leu and S i s k a 3 2 have proposed a m o d i f i c a t i o n o f -80-9 8 7 6 5 4 3 2 1 ELECTRON ENERGY (eV) FIGURE 16. Elec t ron spectra f o r the i o n i z a t i o n molecular oxygen. P h o t o i o n i z a t i o n (584 A ) 0 1 2 3 4 5 I \ i > I II P e n n i n g Ion iza t ion (2 3 S) 0 1 2 3 4 5 6 A I w w A I w \ A -V -' • / - • '/V T" 9 I 8 ELECTRON ENERGY (eV) FIGURE 17. Electron spectra for the ionization of 0 2 to 0^(X 2n ). Table 11. Normalized 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 for 0 2 (x , v " » 0) * 0 2 + (X 2 n g , v ' ) for 584 A photoioniza t ion and 3 He*(2 S) Penning i o n i z a t i o n . v ' He (584 A) l i t e r a t u r e 3 l i t e r a t u r e " t h i s work 43 100 92 45 14 45 100 92 50 15 46 1 3 100 95 * 4 49 * 3 15 * 2 2 * 1 47 1 5 100 93 1 5 61 1 5 37 1 5 17 * 5 He*(2 3S) t h i s work  1 100 93 1 5 61. * 5 37 1 5 17 * 5 9 1 5 a. Reference 103 b. Reference 112 the p o t e n t i a l curve model f o r Penning i o n i z a t i o n . They have concluded t h a t r e l a t i v e to the He*(2 1S) Penning i o n i z a t i o n o f H , N and CO, the c o l l i s i o n w i t h 0 ? i s much harder and r e a c t i v e 2 2 ^ due to a lowering or absence o f an entrance b a r r i e r . Due t o these harder and more r e a c t i v e c o l l i s i o n s , p e r t u r b a t i o n o f the two p o t e n t i a l curves would not be unexpected. D i s t o r t i o n of the Franck-Condon f a c t o r s may a l s o be e x p l a i n e d by the p o s s i b i l i t y o f a competing a u t o i o n i z a t i o n l e a d -i n g t o 0 2 + ( X 2JI ) N a t a l i s and C o l l i n 8 5 suggest t h a t a u t o i o n i z a -t i o n w i l l e x p l a i n the anomalous Franck-Condon f a c t o r s f o r 0 2 + ( X 2 n g ) produced by p h o t o i o n i z a t i o n o f 0 2 u s i n g Ne(736,71»iJ X) r a d i a t i o n . The p o s s i b i l i t y of s i m i l a r competing modes o f i o n i z a t i o n i n metastable atom c o l l i s i o n s have been d i s c u s s e d by Herman and c'ermak 1 1 3. I t Is now suggested t h a t a u t o i o n i z a t i o n may a l s o compete w i t h the Penning i o n i z a t i o n of m o l e c u l a r oxygen when He*(2 3S) metastables are used. I t i s p o s s i b l e t h a t the oxygen molecule may be e x c i t e d t o an a u t o i o n i z i n g l e v e l by resonant e x c i t a t i o n t r a n s f e r w i t h He*(2 3S) metastable atoms. Some evidence f o r t h i s i s p r o v i d e d by the f a c t t b a t G e i g e r and S c h r o d e r 1 1 1 * have observed a member o f a Rydberg s e r i e s at 19.810 eV, converging to the i o n i z a t i o n l i m i t o f 0, + (B 2 E ~ v' = 2) (the energy o f He*(2 3S) i s 19.818 eV). Photoabsorption s t u d i e s by Huffman et a l . 1 1 5 a l s o i n d i c a t e the e x i s t e n c e of t h i s e x c i t e d s t a t e of O 2 . Since the d i f f e r e n c e i n e n e r g i e s between the Rydberg s t a t e s o f 0 2 and He*(2 3S) i s s m a l l , the c r o s s - s e c t i o n f o r e x c i t a t i o n t r a n s f e r may be l a r g e enough to populate the Rydberg -84-s t a t e t o a s i g n i f i c a n t degree. No Rydberg s t a t e s were obse r v e d 1 1 1 * i n the r e g i o n of 20.615 eV (the energy o f H e » ( 2 1 S ) ) . West et a l . 1 1 6 have i n d i c a t e d t h a t an e x c i t e d oxygen molecule, 0 2*», i s formed i n a s i g n i f i c a n t f r a c t i o n o f the He*(2 1S,2 3S) c o l l i s i o n s w i t h 0 2. With the e x c e p t i o n o f the 0 2 + (X 2 n g ) s t a t e , the Penning e l e c t r o n spectrum cannot be assigned s i n c e i t i s a p p a r e n t l y overlapped with s t r u c t u r e due t o other u n i d e n t i f i e d p r o c e s s e s . The degree t o which a s s o c i a t i v e i o n i z a t i o n competes wi t h Penning i o n i z a t i o n i s not known. In a study o f the c h e m i - i o n i z a t i o n of oxygen molecules by He*(2 1S,2 3S) metastables, West et a l . 1 1 6 have proposed the formation of a temporary ( H e 0 2 ) * molecule which then may e i t h e r a u t o i o n i z e t o the H e 0 2 + continuum or fragment. Data appear t o be c o n s i s t e n t with t h i s proposed mechanism and suggest t h a t the p r o b a b i l i t y o f forming the temp-orary ( H e 0 2 ) * molecule i s r e l a t i v e l y l a r g e . S t r u c t u r e i n the energy range 0-4 eV o f the e l e c t r o n s p e c t r a ( F i g u r e 18) resembles t h a t which Cermak and S r a m e k 1 1 0 have assigned to processes r e s u l t i n g from c o l l i s i o n s between 0 2 and f a s t n e u t r a l helium atoms. The r o l e o f f a s t n e u t r a l helium atom i n t e r a c t i o n s w i t h r a r e gas a t o m s 5 0 has been d i s c u s s e d . The l a r g e magnitude o f the s t r u c t u r e found f o r 0 2 may suggest t h a t the r a t i o o f the c r o s s -s e c t i o n s , f o r p r o c e s s e s i n v o l v i n g f a s t (600 eV) n e u t r a l helium atoms to those f o r Penning i o n i z a t i o n by thermal metastable helium atoms Is l a r g e r e l a t i v e t o the s i t u a t i o n f o r N 2, CO and NO. I t i s a l s o p o s s i b l e t h a t some o f t h i s s t r u c t u r e i s due to auto-i o n i z a t i o n o f e x c i t e d oxygen atoms formed by d i s s o c i a t i o n of -85-1 1 1 r 1 1 1 1 I 4 3 2 1 0 ELECTRON ENERGY (eV) FIGURE 18. Electron spectra for the i o n i z a t i o n of molecular oxygen. e x c i t e d 0 2** formed by e x c i t a t i o n t r a n s f e r from helium metastable atoms (below M.9 eV). The peaks observed by Cermak and Sramek 1 1 0 are shown by the v e r t i c a l marks on F i g u r e 18. Most o f these peaks have a l s o been observed by Rudd and S m i t h 1 1 7 i n the e j e c t e d 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 0 2 e x c i t a t i o n by 100 keV H + and He +. Using i o n i z a t i o n e n e r g i e s and v i b r a t i o n a l spaclngs from the 584 A p h o t o i o n i z a t i o n s t u d i e s o f 0 2 by E d q u i s t et a l . 1 1 2 i t i s p o s s i b l e t o c a l c u l a t e the expected p o s i t i o n s (shown on F i g u r e 18) o f the bands f o r v a r i o u s Penning i o n i z a t i o n p r o c e s s e s assum-i n g the energy s h i f t s , AE, to be z e r o . I t i s p o s s i b l e that the broad band between 3.7 and 2.5 eV i s due at l e a s t i n p a r t t o the process H e * ( 2 3 S ) / Q 2 + ( a **nu) (Cermak and S r a m e k 1 1 0 have a t t r i b u t e d the peak at 1.67 eV to p r o d u c t i o n o f 0(3d', 3P) f o l l o w e d by auto-i o n i z a t i o n t o 0 .) However t h i s peak may be due to He*(2 S ) / 0, + (b * * E ~ v ' = 0) which would be at 1.65 eV (assuming AE = 0). g The spectrum a l s o shows ( f i g u r e 18) a shoulder c o r r e s p o n d i n g t o v ? « 1 as w e l l as peak c l o s e t o the energy expected f o r v' « 2. I t should be noted t h a t our assignments o f the spectrum i n t h i s r e g i o n d i f f e r somewhat from those g i v e n by Cermak t o Sramek. The c a l c u l a t e d p o s i t i o n f o r the v» • 0 l e v e l o f H e * ( 2 1 S ) / 0 , + (b" 2 g. l i e s between peaks a t 2.48 and 2.36 eV r e p o r t e d by Cermak and Sramek and t h e r e i s some i n d i c a t i o n ( F i g u r e 18) o f a s m a l l peak on top o f the u n d e r l y i n g s t r u c t u r e . Apart from these d i f f e r e n c e s g e n e r a l l y good agreement i s obtained w i t h the s t r u c t u r e and assignments r e p o r t e d by Cermak and Sramek. Some d i f f e r e n c e s might be expected f o r those processes due to f a s t n e u t r a l helium atoms s i n c e i n t h i s experiment the energy of the f a s t n e u t r a l i s 600 eV compared t o 250 eV i n the work o f Cermak and Sramek. -87-CHAPTER FIVE SIMPLE POLYATOMIC MOLECULES 5.1. Introduction. Ammonia was one of the f i r s t molecules to be studied by Penning ionization using the technique of electron spectro-scopy 2 8. Using a low resolution electron spectrometer, Cermak studied the Ne*( 3P 0, 3P 2) Penning ionization of ammonia. Prom this data, an unusually large A E o b g energy shift of -0.2*4 eV can be drived for the X *A2 ionic state of ammonia. The ratio of the populations of the ionic states could not be determined. Cermak28 also has studied ethylene with both Ne*( 3P D, 3P 2) and He*(21S,23S) metastable atoms and observed several ionic states. More recently, at higher resolution and using Ar*( 3P 0, 3P 2) metastable atoms, Cermak and Ozenne37 have studied the Penning ionization of ethylene leading to the ground state of C 2 H i 4 + . In this chapter are reported the quantitative comparisons of the He*(21S,23S) Penning ionization and the 584 A photoionization of NH3, PH3 and C 2H„. ; 5.2. Ammonia. Ammonia belongs to the point group C 3 V and the ground state electron configuration can be written as ( l a j ) 2 ( 2 a i ) 2 (le)* (3a t) 2 ; ^ -88-The e a u i l i b r i u m geometry of the ammonia ion i n i t ' s ground state i s near p l a n a r 1 1 8 ' 1 1 9 and i t can therefore be considered as belonging to the point group D 3 n . The ground state e lec t ron configurat ion of the ammonia ion can be wri t ten as ( l a i ' ) 2 (2 a i') 2 ( l e ' ) - ( l a 2 " ) ; 2 A 2 " In Figure 19, the Penning e lec t ron spectrum of NH 3 i s compared to the 584 A* photoelectron spectrum. The photoelectron spectrum of NH3 i s w e l l known and has been discussed i n d e t a i l 1 1 8 - 1 2 1 . <rne f i r s t band, X 2 A " , of the photoelectron spectrum ' 2 ' consists of a w e l l resolved v i b r a t i o n a l progression i n v o l v i n g a s ingle fundamental frequency v 2 , the t o t a l l y symmetric out of plane bending m o d e 1 2 0 . 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 1 2 0 for the X 2 A 2 " i o n i c s tate at 10.87 eV (v f = 6) i n the photoelectron spectrum i s used to c a l i b r a t e the absolute energy scale of the Penning e lec t ron spectrum. The second band A 2 E ' of the photo-e lec t ron spectrum consists of a broad band with no w e l l resolved s t r u c t u r e . The Penning e l e c t r o n spectrum of NH 3 l s shown In the upper part of Figure 19. The lack of v i b r a t i o n a l s t ructure i n the f i r s t band i s probably due to the i n t r i n s i c width of Penning i o n i z a t i o n s t r u c t u r e 5 1 , therefore i t i s not p o s s i b l e to evaluate the true AE from AE . . Although the presence of the A *E OD S state i s apparent from the Penning spectrum, quant i ta t ive meas-urements are precluded because of the r a p i d l y r i s i n g background at lower e lec t ron energies . A f t e r the background subtract ion technique l s performed, q u a n t i t a t i v e analys is of both states i s \ -89-FIGURE 19. E l e c t r o n spectra for the i o n i z a t i o n of ammonia. -90-p o s s i b l e . The Penning spectrum o f NH 3 ( F i g u r e 19) e v i d e n t l y contains c o n t r i b u t i o n s from both He*(2 1S) and He*(2 3S) although the separate s t r u c t u r e s cannot be r e s o l v e d because o f the broad bands. I t i s reasonable t o assume t h a t most (^90%) o f the s t r u c t u r e i s from H e * ( 2 3 S ) , i n view o f the r e l a t i v e l y low c o n t r i b u t i o n f o r He*(2 1S) observed w i t h t h i s spectrometer f o r atoms 5 0 and m o l e c u l e s 5 1 . These c o n c l u s i o n s are a l s o supported by examining the narrower bands i n the PH 3 spectrum ( F i g u r e 20) where the He*(2 1S) c o n t r i b u t i o n s can be esti m a t e d . On t h i s b a s i s the r e l a t i v e band areas f o r Penning i o n i z a t i o n and photo-i o n i z a t i o n are compared. The normalized r e l a t i v e e l e c t r o n i c s t a t e p o p u l a t i o n s f o r each i o n i z i n g mode can be seen t o be q u i t e d i f f e r e n t (Table 12). In a c o i n c i d e n c e experiment van der Wie l and Brion 9** measured the p a r t i a l p h o t o i o n i z a t i o n c r o s s - s e c t i o n s ( i n c l u d i n g the $ dependence) f o r the three lowest i o n i c s t a t e s it o f ammonia as a f u n c t i o n o f photon energy. For the X 2 A 2 s t a t e , the c r o s s - s e c t i o n decreases w i t h energy w h i l e f o r the A 2 E ' s t a t e , the c r o s s - s e c t i o n i n c r e a s e s w i t h energy i n the range 19.8 eV t o 21.2 eV. Although these r e s u l t s are f o r p h o t o i o n i z a t i o n , t h i s type o f behaviour may i n p a r t e x p l a i n the observed d i f f e r -ences i n the r e l a t i v e s t a t e p o p u l a t i o n s . The A E O B G energy s h i f t f o r the process He*(2 3S)/NH 3 (X 2 A 2 " ) has the u n u s u a l l y l a r g e value o f - 0 . 4 l - 0.05 eV (see Fi g u r e 19). I t should be noted t h a t t h i s i s an or d e r o f magni-- 9 1 -Table 12. A Eobs energy shift in eV and the normalized relative electronic state populations for 584 X photoionization and He*(2 S) Penning ionization of ammonia at 300 °K. Relative State Populations Electronic State A E 0 b s 584 A He*(23S) (eV) (21,22 eV) (19.82 eV) X 2A2' -0.41 * 0.05 100 .* 20 100 * 10 a A V 400 * 40 64 * 6 a a. Obtained from "Background Subtracted Spectrum" and includes minor contribution from He*(21S) and He (584 % ) . tude large than A E o b s values observed for previously studied atoms 5 0 and m o l e c u l e s 5 1 , but i s comparable i n magnitude to the ^ 2 8 3 3 -0. 2.4 eV s h i f t observed by Cermak f o r the process Ne*( P 0 , P 2 ) /NH 3 (X A 2 ) . A p o s s i b l e suggestion i s that a s h i f t of the observed maximum of the Franck-Condon envelope to higher v i b r a -t i o n a l quantum numbers might produce the large A E Q B G energy s h i f t observed. This may occur as a r e s u l t of the shape of the ingoing and outgoing p o t e n t i a l surfaces and the t r a n s i t i o n p r o b a b i l i t y for Penning i o n i z a t i o n . It i s p o s s i b l e that an a l t e r n a t i v e a u t o i o n i z a t l o n process v i a e x c i t a t i o n t r a n s f e r 5 1 might s h i f t the observed maximum of the Franck-Condon envelope. No value f o r the AE . s h i f t of the A E s tate i s given because obs & of the r e l a t i v e l y large uncertainty involved i n l o c a t i n g the p o s i t i o n of the peak maximum. 5.3. Phosphine. Phosphine i s the second row analogue of ammonia and also belongs to the point group C ^ v . The ground state e lec t ron conf igurat ion of phosphine i s ( i a i ) 2 (2a x) 2 (le) 1* (3ap 2 ( H a ^ 2 (2e)V(5a 1) 2 ; 1kl Unlike ammonia, the ground state of the Ion of phosphine Is p y r a m i d a l 1 1 9 ' 1 2 0 so i t a lso belongs to the point group C 3 V . Figure 20 compares the Penning e lec t ron and photoelectron spectrum of phosphine. The photoelectron spectrum of phosphine i s very s i m i l a r to that of ammonia. The f i r s t v i b r a t i o n a l progress ion , X 2kl, of phosphine i s assigned to v 2 , the t o t a l l y symmetric out of plane bending m o d e 1 2 2 . The v e r t i c a l I o n i z a t i o n - 9 3 -FIG-URE 2 0 . Elec t ron spectra f o r the i o n i z a t i o n of phosphine. -94-p o t e n t i a l 1 2 2 i s 10.59 eV which corresponds t o v' = 9. The second band of phosphine, A 2 E , Is broad with no w e l l r e s o l v e d s t r u c t u r e . The Penning e l e c t r o n spectrum of phosphine ( F i g u r e 20) i s a l s o q u a l i t a t i v e l y very s i m i l a r t o t h a t f o r ammonia. Since v i b r a t i o n a l s t r u c t u r e i s not observed, i t i s not p o s s i b l e t o evaluate the t r u e AE from A E Q b s . A s m a l l shoulder on the X 2 A j band i s from I o n i z a t i o n by He*(2*S) metastable atoms. However, I t appears t h a t most (^9050 of the s t r u c t u r e i n the f i r s t peak i s a r e s u l t o f e l e c t r o n s e j e c t e d i n c o l l i s i o n s o f PH 3 w i t h He*(2 3S) metastable atoms. Background s u b t r a c t i o n i s used to o b t a i n the q u a n t i t a t i v e data shown i n Table 13. W i t h i n experimental e r r o r , v a l u e s from the background s u b t r a c t e d data agree w i t h v a l u e s d e r i v e d from the "spectrum p l u s background" data. T h i s lends some conf i d e n c e t o the background s u b t r a c t i o n procedure. As i n the case of ammonia, the normalized r e l a t i v e p o p u l a t i o n s o f e l e c t r o n i c s t a t e s o f PH show s i g n i f i c a n t d i f f -erences f o r p h o t o i o n i z a t i o n and Penning i o n i z a t i o n (see Ta b l e 13). For Penning i o n i z a t i o n , the r e l a t i v e p o p u l a t i o n s are estimated f o r the sum of He*(2 1S) and He*(2 3S) metastable s t a t e s , but as d i s c u s s e d e a r l i e r , the He*(2 3S) c o n t r i b u t i o n s w i l l be dominant. The A E o b g energy s h i f t o f the X 2 A X s t a t e formed by He*(2 3S) metastable i s +0.10 - 0.05 eV. The a b s o l u t e value of t h i s • A E . energy s h i f t i s f a i r l y l a r g e r e l a t i v e t o values ODS observed f o r ot h e r m o l e c u l e s 5 0 * 5 1 but i t i s somewhat s m a l l e r i n magnitude than the value of -0.41 eV found f o r ammonia. No value f o r the A E Q b s e n e r g y [ s h i f t o f the A 2 E s t a t e i s g i v e n due -95-Table 13. A E o b s energy shift in eV and the normalized relative electronic state populations for 584 X photoionization He*(23S) Penning ionization of phosphine at 300 °K. Relative State Populations Electronic State AEobs (eV) 584 A (21.22 eV) He*(2 S) (19.82 eV) X 2A. +0.10 * 0.05 100 * 22 100 * 10 a A 2E 455 * 45 67 * 7 t 7 a a. Obtained from "Background Subtracted Spectrum" and includes l o minor contribution from He*(2 S) and He (584 A). to the relatively large uncertainty involved in locating the position of the peak maximum. It is interesting to note in comparing ammonia and phosphine that the relative population of ionic states is very similar for the two molecules for a given ionization mode. 5.4. Ethylene. Ethylene i s isoelectronic with oxygen and i t is the simplest organic molecule to contain a carbon-carbonn bond. The electron configuration of ethylene is described by ••<V* ^'.n'2 ( l b 2 U > 2 <3* g) 2 <"Jg>2 <lNu>2 i '*g In the lower section of Figure 21, the 584 X photoelectron spectrum of ethylene Is shown. The spectrum Is similar to that observed by Branton et a l . 1 2 0 and by Baker et a l . 1 2 3 who have analyzed the complex fine structure of the photoelectron bands. The top section of Figure 21 shows the He*(2lS,23S) Penning electron spectra of C 2H\. Utilization of the background subtraction technique 5 2 on the ethylene data is less effective than with some other molecules but nevertheless i t allows for more accurate quantitative measurements to be made in the region o f the steeply rising background. The four ionic states observed in the photoelectron spectrum are also observed ln the He*(23S) Penning electron spectra and there i s evidence for some vibra-tional structure. This vibrational structure can be unambiguously assigned and i t appears that both AEA and AEp are zero, therefore, - 9 7 -Penning Ionization ( He* 21S, 23S ] C 2 H 4 Spectrum & Background iL 4 Q V f Al 23S(2B3g) Background Subtracted Photoionization (584 A) 2R J H 2 l .^rVrTTr Mi l ' 4 n n 1 1 1 1 1 1 1 r 11 10 9 8 7 6 5 4 3 ELECTRON ENERGY (eV) FIGURE 21. E l e c t r o n spectra for the i o n i z a t i o n of ethylene. A E o b s = A E * U s l n S He*(2 1S) metastable atoms, s t r u c t u r e i s only e v i d e n t f o r the pro c e s s H e*(2 1S)/C 2H^ +(X 2 B ). The He*(2 1S) c o n t r i b u t i o n i s approximately 20% and to t h i s extent t h e r e w i l l p r o bably be o v e r l a p p i n g He*(2 1S) c o n t r i b u t i o n s t o the spectrum. The AE energy s h i f t v a l u e s , which are parameters r e f l e c t i n g the nature o f i n t e r a c t i o n s l n the c o l l i s i o n p r o c e s s 5 1 , are l i s t e d i n Table 14. These val u e s are determined with r e s p e c t t o the measured p h o t o i o n i z a t i o n i o n i z a t i o n p o t e n t i a l s and are the aver-age values over the v i b r a t i o n a l bands. These measured val u e s are t y p i c a l o f measurements made on other m o l e c u l e s 5 1 . o A d e t a i l e d comparison of the 584 A p h o t o e l e c t r o n spectrum and the He*(2 1S,2 3S) Penning e l e c t r o n spectrum o f the f i r s t band of C 2H l t +(X 2 B 3 u ) i s shown i n F i g u r e 22. The markers i n the p h o t o i o n i z a t i o n spectrum i n d i c a t e the v i b r a t i o n a l l e v e l s f o r the v i b r a t i o n a l modes v 2 and 120,123. T n e s t r u c t u r e i n the He*(2 3S) Penning e l e c t r o n spectrum i s broadened due to the Penning process and t h i s makes the assignment of the f i r s t v i b r a -t i o n more d i f f i c u l t . Markers I n d i c a t e the assignment o f the v i b r a t i o n a l l e v e l s f o r the v i b r a t i o n a l modes v 2 and v u f o r He*(2 3S) Penning i o n i z a t i o n . No v i b r a t i o n a l s t r u c t u r e i s r e s o l v e d f o r the process H e * ( 2 1 S)/C 2 H l | + (X 2 B 3 u ) ( i n o t h e r m o l e c u l e s 5 1 * 5 2 , the bands due t o He*(2 1S) are a l s o very broad). The Franck-Condon envelope f o r the C 2H\ +(X 2 B 3 u ) s t a t e i s , w i t h i n experimental e r r o r , the same f o r both He*(2 3S) Penning i o n i z a t i o n and 584 £ p h o t o i o n i z a t i o n . F o r the H e » ( 2 3 S ) / C 2 H ^ ( A 2 B 3 g ) process ( F i g u r e 21) the Franck-Condon envelope i s a l s o found t o be very s i m i l a r - 9 9 -Table 1*4. A E o b s energy s h i f t i n eV and the normalized r e l a t i v e e l e c t r o n i c state populations for 584 X photoioniza t ion and He*(2 3 S) Penning i o n i z a t i o n of ethylene at 300 °K. E l e c t r o n i c State AEobs (eV) Relat ive State Populations 58*4 A (21.22 eV) He*(2 S) (19.82 eV) - 2 X B 3U •0.020 4 0.020 100 100 - 2 A B, 3 g +0.038 4 0.020 177 1 20 21 4 1 - 2 B A g -0.0111 * 0.030 229 4 30 23 1 2 - 2 C B. 2:U +0.037 1 0.030 150 4 20 27 1 3 a. Contains small contr ibutions from He*(2 l S) and He (584 %) -100-C 2 H 4 Photoionizat ion (584 A) Perming Ionization I ••••>x,. n — r i—r 10 ELECTRON ENERGY (eV) FIGURE 22. E lec t ron spectra f o r the i o n i z a t i o n of ethylene to the ground state ion (X 2 B -101-t o t h a t f o r the 584 % p h o t o i o n i z a t i o n p r o c e s s . S u f f i c i e n t l y accurate measurements of the Franck-Condon envelope cannot be made f o r e i t h e r o f the pro c e s s e s He«( 2 3 S ) / C 2 H l f + ( B 2A ) and He*(2 3S)/C 2H l t + (C 2 B 2 U ) . In Table 14 the normalized r e l a t i v e p o p u l a t i o n s o f the e l e c t r o n i c s t a t e s o f C 2 H i 4 + are compared f o r the two modes of i o n i z a t i o n . The data are a r b i t r a r i l y normalized with r e s p e c t t o the C 2H l t +(X 2 B 3 U ) s t a t e . I t i s noted t h a t very l a r g e r e l a t i v e d i f f e r e n c e s o c c u r . T h i s suggests t h a t t h e r e may be l a r g e d i f f e r e n c e s i n r e l a t i v e p a r t i a l c r o s s - s e c t i o n s f o r the two modes o f i o n i z a t i o n . I t i s u n l i k e l y t h a t the l a r g e d i f f e r -ences i n the r e l a t i v e p o p u l a t i o n s are due onl y t o the d i f f e r e n c e s In the' 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 5 1 . I t i s noted that i d e a l l y the He*(2 1S) 0 Penning e l e c t r o n spectrum should be compared t o the 601 A p h o t o e l e c t r o n spectrum and the He*(2 3S) Penning e l e c t r o n spectrum o should be compared t o the 626 A p h o t o e l e c t r o n spectrum. Measure-ments o f p a r t i a l c r o s s - s e c t i o n s as a f u n c t i o n of e x c i t a t i o n energy f o r the p h o t o i o n i z a t i o n o f m o l e c u l e s 9 3 " " 9 5 i n d i c a t e t h a t the r e l a t i v e s t a t e p o p u l a t i o n s can change s i g n i f i c a n t l y even over a few e l e c t r o n v o l t s . -102-CHAPTER SIX HYDROGEN CYANIDE AND SOME RELATED COMPOUNDS 6.1. I n t r o d u c t i o n . Recent s t u d i e s l n t h i s l a b o r a t o r y o f the p h o t o e l e c t r o n s pectroscopy 1 2*• and e l e c t r o n Impact e x c i t a t i o n 1 2 5 o f HCN has prompted t h i s i n v e s t i g a t i o n o f the He*(2 1S,2 3S) Penning i o n i z a t i o n of t h i s molecule and some r e l a t e d compounds. F r i d h and X s b r l n k 1 2 6 have r e c e n t l y r e p o r t e d high r e s o l u t i o n 584 A* p h o t o e l e c t r o n spectroscopy data f o r HCN. U r i s u and K u c h i t s u 1 2 7 and Coxon et a l . 1 2 8 have analyzed the emission from e x c i t e d s t a t e s o f CN produced by the d i s s o c i a t i o n o f HCN, BrCN and ICN i n A r * ( 3 P 0 , 3 P 2 ) and X e * ( 3 P 2 ) a f t e r g l o w s . T h i s suggests t h a t when us i n g He*(2 1S,2 3S) metastables that In a d d i t i o n to Penning i o n i z a t i o n , there i s a * p o s s i b i l i t y t h a t a u t o i o n i z i n g s t a t e s o f CN may be observed. D i b e l e r and L i s t o n 1 2 9 have s t u d i e d the p h o t o i o n i z a t i o n - y i e l d curves f o r dicyanogen and the cyanogen h a l i d e s from t h r e s h o l d to 600 A. In t h i s chapter the He*(2 1S,2 3S) Penning i o n i z a t i o n o f HCN, ( C N ) 2 , CH 3CN, BrCN and ICN are r e p o r t e d . The He*(2 1S,2 3S) • o Penning i o n i z a t i o n process i s compared to that o f 584 A photo-i o n i z a t i o n . A search i s a l s o made f o r evidence o f other c h e m i - i o n i z a t i o n p r o c e s s e s . -103-6.2. Hydrogen Cyanide. HCN has the ground s t a t e m o l e c u l a r o r b i t a l c o n f i g u r a t i o n 1 3 ( l o ) 2 ( 2 a ) 2 ( 3 a ) 2 (Ho) 2 (5°) 2 ( 1 * ) * ; 1 E + where the i n o r b i t a l l s the H-bonding o r b i t a l and the 5° o r b i t a l i s the n i t r o g e n lone p a i r o r b i t a l . In F i g u r e 23 the He*(2 1S,2 3S) Penning e l e c t r o n and the 5 8 4 % p h o t o e l e c t r o n s p e c t r a o f HCN are compared. The p h o t o e l e c t r o n spectrum, i n d i c a t i n g t h r e e i o n i c s t a t e s , i s very s i m i l a r t o that r e p o r t e d by Baker and T u r n e r 1 3 1 . The complex f i r s t band i n the 5 8 4 % p h o t o e l e c t r o n spectrum has been analyzed i n d e t a i l at h i g h e r r e s o l u t i o n by F r o s t et a l . 1 2 * * and F r i d h and X s b r i n k 1 2 6 . The He*(2 1S,2 3S) Penning e l e c t r o n spectrum shows s t r u c t u r e only f o r the He*(2 3S)/HCN +(X 2 n ) and the He*(2 3S)/HCN +(A 2 E ) p r o c e s s e s . The h i g h e r band observed i n the p h o t o e l e c t r o n spectrum i s too c l o s e t o the zero energy cut o f f t o be seen i n the Penning e l e c t r o n s p e c t r a . The r e l a t i v e l y l a r g e A E o b s energy s h i f t s f o r these two bands are -0.15 - 0.10 and -0.14 - 0.05 eV r e s p e c t i v e l y . A l -though there i s some i n d i c a t i o n o f v i b r a t i o n a l s t r u c t u r e f o r these two bands a unique assignment of the v i b r a t i o n a l l e v e l s i s not p o s s i b l e ( F i g u r e 23b). Consequently, AE values cannot be c a l c u l a t e d from A E O D g 5 1 * . The t e n t a t i v e v i b r a t i o n a l assignment (which assumes AE A » 0) shown above the Penning e l e c t r o n bands i n F i g u r e 23b would r e s u l t i n a AE value very c l o s e to zero f o r both s t a t e s . T h i s assignment would mean t h a t the p o s i t i o n s of the maxima of the Franck-Condon envelopes 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 correspond t o d i f f e r e n t v i b r a t i o n a l l e v e l s . -104-Penning Ionization (He"2l5,2^S) HCN 2^ (XT I )J : 1/ Photoionization (584 A) ~L 1 1 1 1 1 1 r 8 7 6 5 4 3 2 1 ELECTRON ENERGY (eV) (a) Penning Ionization (HeVS^S) 23sx2nj IFTTT Photoionization (584 A) Pi) fl I rrr A 2 ! J ——i 1 , , — , R 7 6 5 ELECTRON ENERGY (eV) (b) FIGURE 23. Electron spectra for the ionization of hydrogen cyanide, a. f u l l spectra b. f i r s t band including 584 A calibration. - 1 0 5 -Th i s type of phenomenon appears to occur for Penning i o n i z a t i o n to the ground state of the ions of water and related molecules 5** and suggests that some appreciable modification of the target p o t e n t i a l surfaces may be occurring. An al t e r n a t i v e assignment ( i . e . , s h i f t i n g by one or more v i b r a t i o n a l quanta) would mean that AE would have correspondingly large negative values, whereas for most of the other atoms and molecules 5 0 -- 5 2 studied, AE <.|0.l| eV ( i . e . , of the order of thermal energies). A quantitative analysis of the r e l a t i v e populations of electronic states of HCN+ for both Penning i o n i z a t i o n and photo-i o n i z a t i o n i s not possible because the two ionic states are not completely separated. Quantitative information for the 58*4 % photoionization process could be obtained from the higher resolution spectrum (deconvoluted) reported by Fridh and Asbrink 1 2 i f the electron transmission e f f i c i e n c y of t h e i r analyzer was known. A q u a l i t a t i v e analysis of the r e l a t i v e populations of electronic states of HCN (Figure 23) reveals large differences when comparing the two modes of i o n i z a t i o n . In p a r t i c u l a r i t appears that the p a r t i a l i o n i z a t i o n cross-section f o r production of the X 2n state of HCN+ r e l a t i v e to that for the A 2Z state i s much smaller for Penning i o n i z a t i o n than for photoionization. 6.3. Dicyanogen. The ground state electron configuration of (CN) 2 i s 1 3 0 . . . ( 3 a g ) 2 ( 3 o u ) 2 ( H a g ) 2 ( l \ ) k ( U a u ) 2 ( 5 * g ) 2 ( l ^ ) -The wavefunctlons for the i n and the i n o r b i t a l s can be g u represented by the out of phase and the i n phase combination of -106-the n bonding o r b i t a l s w h ile the 4o and the 5a„ o r b i t a l s can u g be represented by the out o f phase and the i n phase combinations of the n i t r o g e n non-bonding o r b i t a l s 1 3 1 . In F i g u r e 24 the He*(2 1S,2 3S) Penning e l e c t r o n and the o 584 A p h o t o e l e c t r o n s p e c t r a o f (CN) 2 are compared. The photo-e l e c t r o n spectrum i s very s i m i l a r t o t h a t r e p o r t e d i n other s t u d i e s 1 3 0 - 1 3 2 . The same f o u r i o n i c s t a t e s observed i n the p h o t o e l e c t r o n spectrum are a l s o observed i n the Penning e l e c t r o n spectrum. The A E Q B G energy s h i f t values are l i s t e d i n Table 15. For the ground s t a t e of the Ion, A E Q B G = AE, s i n c e the peak maximum o f the r e 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 corresponds t o the same v i b r a t i o n a l l e v e l (v* - 0). AE i s s m a l l i n magnitude ( i . e . , < | 0.1 | eV) as has been found f o r most other atoms and m o l e c u l e s 5 0 " 5 2 . V i b r a t i o n a l s t r u c t u r e i s apparent f o r the process H e » ( 2 3 S ) / ( C N ) 2 + (l 2 n ). I t i s found t h a t , w i t h i n e x p e r i -mental e r r o r , the v i b r a t i o n a l spacings are the same f o r both modes o f i o n i z a t i o n . In comparing the v i b r a t i o n a l envelope of the ground s t a t e i o n f o r the two modes of i o n i z a t i o n , d i f f e r e n c e s are noted. T h i s anomaly might be e x p l a i n e d by a competing auto-i o n i z a t i o n p r o c e s s 5 1 as was suggested f o r the process H e * ( 2 3 S ) / 0 2 + ( X 2 n g ) . The p h o t o l o n i z a t i o n - y i e l d data o f D i b e l e r and L i s t o n 1 2 9 f o r dicyanogen suggest t h a t t h e r e may be an a u t o i o n i -z i n g s t a t e at approximately 19.8 eV. Thus, i t may be p o s s i b l e to e x c i t e the dicyanogen molecule t o t h i s a u t o i o n i z i n g l e v e l by resonant e x c i t a t i o n t r a n s f e r with He*(2 3S) metastable atoms. -107-Penning Ionization ( H e ^ S ^ ) ( C N 1 i — r T x 4 2%(x2na) 2^ s(c2n.) Photoionization (584 A) * \ A 2 Z : J u WL„ 8 ' 7 ' 6 ' 5 ELECTRON ENERGY (eV) 4 FIGURE 2*4. E lec t ron spectra f o r the I o n i z a t i o n of dicyanogen. -108-Table 15-& E o b s energy s h i f t i n eV and the normalized r e l a t i v e e l e c t r o n i c o s t a t e p o p u l a t i o n s f o r 584 A p h o t o i o n i z a t i o n and He*(2 S) Penning i o n i z a t i o n o f dicyanogen at 300 °K. E l e c t r o n i c S t a t e AEobs (eV) R e l a t i v e State P o p u l a t i o n s 584 A (21.22 eV) He*(2 S) (19-82 eV) - 2 x it g A V g - 2 + u - 2 c n - o . o i * 0.03 ioo * 5 u -0.08 * 0.03 -0.07 * 0.03 -0.02 * 0.05 63 * 6 30 * 3 117 1 6 100 * 10 176 * 3 5 a 143 * 28 a 87 * 9 a Contains s m a l l c o n t r i b u t i o n s from He*(2 S) and He (584 1) -109-For the normalized r e l a t i v e populations of e l e c t r o n i c states f o r both He(2 l S,2 3 S) Penning i o n i z a t i o n and 584 A photo-i o n i z a t i o n of ( C N ) 2 , r e l a t i v e l y large di f ferences are noted as shown i n Table 15. I t i s observed that the p a r t i a l i o n i z a t i o n c ross -sec t ion for the production of the JI states of ( C N ) 2 + r e l a t i v e to that f o r the £ states i s much smaller for Penning i o n i z a t i o n than f o r p h o t o i o n i z a t i o n . There i s some evidence to suggest that other types of chemi- ioniza t ion processes may also be o c c u r r i n g , since two u n i d e n t i f i e d peaks are observed at 1.8 eV and 0.7 eV. (This i s beyond the energy range of Figure 24). These energies could correspond to a u t o i o n i z i n g l e v e l s of ( C N ) 2 . The p h o t o i o n i z a t i o n -y l e l d curves f o r (CN) 2 reported by Dibeler and L i s t o n 1 2 9 i n d i -cate the existence of a u t o i o n i z i n g leve ls of (CN) 2 which may be responsible for the observed bands but i n t h i s case some mechanism other than e x c i t a t i o n t r a n s f e r must be operat ive . For e x a m p l e 5 0 ' 1 3 3 , some processes have been a t t r i b u t e d to c o l l i s i o n s with fast neutra l helium atoms. It i s poss ible that d i s s o c i a t i v e processes may also play an important r o l e . 6.4. A c e t o n i t r i l e . A c e t o n i t r i l e has the symmetry C 3 v and ground state e l e c -tron c o n f i g u r a t i o n 1 3 4 . .. ( 2 & 1 ) 2 (3aj)2 (4a/) 2 (2e)* (5a r) 2 (3e)H ; lkx As described by Turner et a l . 1 3 2 , the 3e o r b i t a l has C = N n bonding, C-C bonding, and C-H antibonding character , the 5a j - 1 1 0 -o r b l t a l has ni trogen lone p a i r bonding character , and the 2e o r b i t a l has C-C bonding, and C-H antibonding character . In Figure 25, the H e*(2 l S,2 3 S ) Penning e l e c t r o n and the 584 A photoelectron spectra of CH3CN are compared. Four states are observed i n the photoelectron spectrum and these have been interpreted i n previous s t u d i e s 1 3 2 ' 1 3 5 . In the He*(2 1 S,2 3 S) Penning e lec t ron spectra four states are also observed. In Table 1 6 , the A E o b g energy s h i f t values are l i s t e d . There i s some evidence for v i b r a t i o n a l s t ructure i n the Penning spectrum f o r the ground state of the ion but as i n the case of HCN no unique assignment of v i b r a t i o n a l l e v e l s can be made. However, the tenta t ive assignment (which assumes AE A = 0) shown i n Figure 25, r e s u l t s i n a small value of A E (-0.09 eV) . A l t e r -native assignments would r e s u l t i n much l a r g e r negative values of A E . For the normalized r e l a t i v e populations of e l e c t r o n i c states f o r both He*(2 1 S,2 d S,) Penning i o n i z a t i o n and 584 A photo-i o n i z a t i o n of CH 3 CN, r e l a t i v e l y large dif ferences are noted and are l i s t e d i n Table 1 6 . There Is no evidence i n the e l e c t r o n spectra for a d d i t i o n a l cheml- iohlza t ion processes of the type observed i n dicyanogen. 6.5. Cyanogen Bromide and Cyanogen Iodide . The ground state outer e lec t ron conf igura t ion of the cyanogen hal ides i s 1 3 6 ( l o ) 2 ( 2 a ) 2 ( 3 o ) 2 ( i n ) 4 ( 4 o ) 2 (2JI)1* ; 1 E + where the 3o and 4c o r b i t a l s are the lone p a i r o r b i t a l s on the halogen and nitrogen atoms. The i n and 2n o r b i t a l s are - I l l -Penning Ionization (He*2S12"S) 2PS (A 2 A. ) CH3CN Spectrum & B a c k g r o u n d z W E ) V 1 Background Subtracted Photoionization ( 5 8 4 A ) B^ -1 1 1 1 1 1 1 1 r 9 8 7 6 5 4 3 2 1 ELECTRON ENERGY (eV) FIGURE 25. E l e c t r o n spectra f o r the i o n i z a t i o n of a c e t o n i t r i l e . -112-Table 1 6 . A Eobs e n e i*gy s h i f t i n eV and the normalized r e l a t i v e electronic state populations for 584 % photoionization and He*(2 3S) Penning io n i z a t i o n of a c e t o n i t r i l e at 300 °K. Electronic State AEobs (eV) Relative State Populations 584 K (21.22 eV) He*(2 S) (19.82 eV) _ 2 X E 2 A A, ~ 2 B E ~ 2 C A, -0.58 * .05 -0.30 * .03 -0.4 * .2 100 1 10 54 .* 5 123 * 24 39 1 8 100 * 20 125 * 25 a 19 16 * 4 a * 5 a a. Contains small contributions from He*(2 1S) and He (584 A ) . - I r -r e s p e c t i v e l y the i n phase and out of phase combinations of the halogen p atomic o r b i t a l and the C = N n bond. In Figure 26 and 27 the He*(2 IS,2 3S) Penning e l e c t r o n and the 584 % photoelectron spectra of BrCN and ICN r e s p e c t i v e l y are compared. The photoelectron spectra of these two molecules are very s i m i l a r to those reported by Heilbronner et a l . 1 3 7 and also by Lake and T h o m p s o n 1 3 8 , both of whom have given a s i m i l a r i n t e r p r e t a t i o n of the spect ra . The four i o n i c states of BrCN observed i n the photo-e lec t ron spectrum are a lso observed i n the He*(2 1S,2 3S) Penning e lec t ron spectra (Figure 26). The A E O D S energy s h i f t values are l i s t e d i n Table 17. For the ground state of BrCN 4*, A E O B G « AE, on the basis of the assignment (which assumes A E A « 0) shown In Figure 26. In a d d i t i o n , for t h i s state although there i s no r e s o l u t i o n of v i b r a t i o n a l s t r u c t u r e , the two components of the s p i n - o r b i t coupling are : p a r t i a l l y resolved i n the Penning e lec t ron spectra . I t appears that p a r t i a l c ross -sec t ions f o r He*(2 3S) Penning i o n i z a t i o n to these two s p i n - o r b i t coupling components of BrCN + may d i f f e r while for p h o t o i o n i z a t i o n they appear to be approximately the same. The d i f f e r e n c e may be due to a u t o i o n i z a t i o n phenomena. A s i m i l a r s i t u a t i o n occurs i n the Penning i o n i z a t i o n of x e n o n 5 0 . The normalized r e l a t i v e populations of e l e c t r o n i c states for both He«(23S) Penning i o n i z a t i o n and 584 X photo ioniza t ion of BrCN are l i s t e d i n Table 17 and large d i f f e r e n c e s are noted. -114-Penning Ionization (hV2S,ifS) BrCN Spectrum & Background , Background y JJ Subtracted Photoionization (584 A J. B ^ B T I , !• 5 1 T T 1 1 1 1 1 1 r 10 9 8 7 6 5 4 3 2 1 ELECTRON ENERGY (eV) - i 0 FIGURE 26. E l e c t r o n s p e c t r a f o r the i o n i z a t i o n o f cyanogen bromide. -115-Penning Ionization (He*21Sl23S) I C N 23S(A2I*) 2 3 s (6 2 n3B 2 n 1 ) 2' 2 23st*2n3) z^x^n, x8 Spectrum & Background 2aS(C2Z,j / , ' I I I Background Subtracted Photoionization (584 A) *2n3 x2n, i 2 I I A2I* i fi2n3 2 e2n, V C2I* I I I I I I I I I P 10 9 8 7 6 5 4 3 2 1 ELECTRON ENERGY (eV) FIGURE 27. E lec t ron spectra f o r the i o n i z a t i o n of cyanogen i o d i d e . -116-Table 17. A E o b s energy s h i f t i n eV and the normalized r e l a t i v e e l e c t r o n i c o 3 s t a t e p o p u l a t i o n s f o r 584 A p h o t o i o n i z a t i o n and He»(2 S) Penning i o n i z a t i o n of cyanogen bromide at 300 °K. E l e c t r o n i c S t a t e AEobs (eV) R e l a t i v e S t a t e P o p u l a t i o n s 584 A (21,22 eV) He*(2 S) (19.82 eV) 1 i 2 > 2 A h+ * 2 I 3 1 1*1 .a -0.08 * 0.07 100 * 5 -0.26 * 0.03 25 * 1 -0.16 * 0.05 66 * 3 •0.03 * 0.05 20 1 1 100 1 5 227 1 23 b 237 * 24 b 39 * 10 b a. An averaged v a l u e . ; " b. Contains s m a l l c o n t r i b u t i o n s from He*(21S) and He (584 R ) . -117-For p h o t o i o n i z a t i o n , the magnitude of the r e l a t i v e p o p u l a t i o n s o f e l e c t r o n i c s t a t e s are s i m i l a r i n the case of both I s t a t e s and a l s o i n the case o f both n s t a t e s but f o r Penning i o n i z a t i o n the s i t u a t i o n i s d i f f e r e n t . F i n a l l y the BrCN two u n i d e n t i f i e d peaks ( I , I I ) are observed at 1.39 eV and 0.3 eV. I t i s suggested t h a t these e l e c t r o n s c o u l d a r i s e from the a u t o i o n i z a t i o n of e x c i t e d f r a g -ments. T h i s may i n v o l v e an e x c i t e d s t a t e o f BrCN which p r e -d i s s o c i a t e s and i s then f o l l o w e d by a u t o i o n i z a t i o n o f e x c i t e d Br or CN. In a study o f B r 2 , C e r m a k 1 3 3 has observed an auto-i o n i z i n g l e v e l f o r the bromine atom (6p') which may correspond to the peak at 0.3 eV. The f o u r i o n i c s t a t e s o f ICN observed i n the photo-e l e c t r o n spectrum are a l s o observed i n the Penning e l e c t r o n s p e c t r a (Figure 27). The A E o b g energy s h i f t v alues are l i s t e d i n Table 18. For the ground s t a t e o f ICN + t h e r e i s evidence o f 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 f o r both components of the s p i n - o r b i t c o u p l i n g . The s p i n - o r b i t s p l i t t i n g i s found to be the same f o r the two modes o f i o n i z a t i o n . For the ground s t a t e of ICN +, A E o l J g * A E , (assuming AE f t •« 0 ) . S i g n i f i c a n t d i f f e r e n c e s are observed i n comparing the shapes o f the v i b r a -t i o n a l envelopes f o r the two s p i n - o r b i t c o u p l i n g components of the ground s t a t e o f ICN + 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 . A u t o i o n i z a t i o n i s an u n l i k e l y reason f o r t h i s d i f f e r e n c e s i n c e s u i t a b l e bands are not observed i n the p h o t o i o n i z a t i o n -e f f i c l e n c y curves of I CN 1 2 9> I t appears t h a t the p a r t i a l c r o s s -s e c t i o n f o r He*(2 3S) Penning i o n i z a t i o n t o the two s p i n - o r b i t -118-Table 18. A E 0 D S energy s h i f t In eV and the normalized r e l a t i v e e l e c t r o n i c s t a t e p o p u l a t i o n s f o r 584^  A* p h o t o i o n i z a t i o n and He*(2 3S) Penning i o n i z a t i o n o f cyanogen i o d i d e at 300 <>K. R e l a t i v e S t a t e P o p u l a t i o n s E l e c t r o n i c State AEobs (eV) 584 A (21.22 eV) He*(2 S) (19.82 eV) x 2 n 3 x 2 > 2 - 2 + A Z B 2 n 3 ! 2 '2 - 2 + c i -0.01 a* 0.04 100 * 5 -0.26 * 0.03 34 * 3 — - 78 * 8 -0.42 ± 0.05 26 i 1 100 * 5 355 * 50* a. An averaged value f o r the s p i n - o r b i t components. b. Contains s m a l l c o n t r i b u t i o n s from He*(2 1S) and He*(584 I). - U n -coupling components of ICN+ are approximately the same as i s observed for photoionization. The normalized relative populations of electronic o states for both He*(23S) Penning ionization and 584 A photo-ionization of ICN are listed in Table 18. It was not possible to deconvolute the bands due to the A 2 r + or B 2n, . states in the He*(23S) Penning electron spectra so they are reported as a combined value. For ICN the data for the two modes of ionization are of the same magnitude and trend as was observed for BrCN. No data was included for the process He*(23S)/ICN+ (C 2 £ + ) because the f u l l width at the half peak height of the Penning electron structure i s approximately three times larger than would be expected from a consideration of the photoelectron spectrum. This suggests that other chemi-ionization processes may be contributing to this, peak as well as the two unidentified peaks (labeled I,II). It is suggested that these electrons may arise from the autoionization of excited fragments. -120-CHAPTER SEVEN WATER, ALCOHOLS AND ETHERS 7.1. In t roduct ion . Using a r e t a r d i n g p o t e n t i a l e lec t ron spectrometer of low r e s o l u t i o n , Cermak 2 8 has s tudied the Penning e lec t ron spectrum of H20 using both He»(2 1S,2 3S) and N e « ( 3 P 0 J 3 P 2 ) metastables. Cermak 2 8 has a lso s tudied the Penning e lec t ron spectra of methanol, e thanol , dimethyl ether and ethylene oxide using N e * ( 3 P 0 , 3 P 2 ) metastables. .Only q u a l i t a t i v e i n t e r p r e t a t i o n s of these spectra are poss ible ;due to the l i m i t e d r e s o l u t i o n a v a i l -able . More r e c e n t l y , C e r m a k 1 3 9 has studied the Penning i o n i z a -t i o n to the f i r s t band of ethylene oxide using A r * ( 3 P 0 , 3 P 2 ) metastable atoms at h i g h e r , e l e c t r o n energy r e s o l u t i o n . In t h i s chapter the He*(2 1S,2 3S) Penning i o n i z a t i o n of water, a lcohols and ethers are reported and the r e s u l t s are compared to the corresponding 58*4 % photoelectron spectra . 7.2. Water. The water molecule belongs to the point group C 2 v and has the ground state e l e c t r o n c o n f i g u r a t i o n . ( i a i ) 2 ( 2 a t ) 2 ( l b 2 ) 2 O a ^ 2 ( l b j ) 2 ; lk1 The l b t o r b i t a l i s e s s e n t i a l l y the oxygen lone p a i r (2p x) which i s perpendicular to the molecular plane . The 3at o r b i t a l Includes H-H bonding character while the l b 2 involves H-H antibonding c h a r a c t e r 1 3 2 . -121-In F i g u r e 28, the He*(2 1S,2 3S) Penning e l e c t r o n and the o o 584 A p h o t o e l e c t r o n s p e c t r a o f water are compared. The 584 A p h o t o e l e c t r o n spectrum Is s i m i l a r t o t h a t r e p o r t e d by T u r n e r 1 3 2 . The v i b r a t i o n a l spaclngs f o r the X 2 B 1 e l e c t r o n i c s t a t e o f H 2 0 + formed i n the p h o t o i o n i z a t i o n process are i n d i c a t e d i n F i g u r e 28. In the upper p o r t i o n o f F i g u r e 28, the Penning spectrum r e v e a l s s t r u c t u r e f o r the H e * ( 2 3 S ) / H 2 0 + ( X 2 B l ) and the H e * ( 2 3 S ) / H 2 0 + (A 2 A l ) p r o c e s s e s . Using the background s u b t r a c t i o n t e c h n i q u e 5 2 , the r e g i o n where q u a n t i t a t i v e a n a l y s i s i s p o s s i b l e i s extended down to 0.75 eV, as shown i n the c e n t r a l p o r t i o n o f F i g u r e 28. In a d d i t i o n t o the X 2 B 1 and the A 2 A X bands i n the Penning e l e c t r o n spectrum, p a r t o f the B 2 B 2 band can a l s o be observed. I t appears t h a t Penning i o n i z a t i o n i s predominantly due to He*(2 3S) metastables and f o r the X 2 B X s t a t e i t i s estimated' t h a t approximately 5% o f the t o t a l band area i s due to He*(2 1S) metastables. Large AE Q l ; ) s energy s h i f t values ('Table 19) are observed f o r the processes H e » ( 2 3 S ) / H 2 0 + ( X 2 B X ) and H e * ( 2 3 S ) / H 2 0 + ( A .2k.x). No value i s i n d i c a t e d f o r the H e * ( 2 3 S ) / H 2 0 + ( B 2 B 2 ) process s i n c e the f u l l band i s not observed. The f i r s t band l a b e l l e d 2 3S(X 2 B L ) i n the Penning e l e c t r o n spectrum e x h i b i t s p a r t i a l l y r e 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 . A t e n t a t i v e assignment f o r the H e * ( 2 3 S ) / H 2 0 + ( X 2 B 1 ) process (assuming AE A '= 0) i s i l l u s t r a t e d i n F i g u r e 28 and p l a c e s the (1,0,0) v i b r a t i o n at the peak maximum. T h i s corresponds t o a AE energy s h i f t o f +0.040 eV which i s o f . t h e same -122-Penning Ionization (He*21S,2aS) H,0 2%(Xfy 2^ B(A2A,) FfiT i x 8 Spectrum ft Background 2aSl62BJ Background . Subtracted Photoionizatian (584 A) A A, :J\l ^VVvv^' 9 8 7 6 5 4 3 2 ^ 0 ELECTRON ENERGY (eV) FIGURE 28. E l e c t r o n spectra f o r the i o n i z a t i o n of water. -123-Table 19. A E o b s energy s h i f t i n eV and the normalized r e l a t i v e e l e c t r o n i c state populations f o r 584 K photoioniza t ion and H e *(2 3 S ) Penning i o n i z a t i o n of water at 300 <>K. E l e c t r o n i c State A E 0 b s (eV) Rela t ive State Populations 584 A (21.22 eV) He*(2 S) (19.82 eV) X 2 B, A 2 A , i 2 B . -0.37 * 0.03 -0.43 4 0.08 95 4 5 100 * 5 83 4 8 60 ± 3 100 * 5 a a. Contains small contr ibut ions from He*(2 1 S) and He (584 X) -124-magnltude as observed f o r a t oms 5 0 and most m o l e c u l e s 5 1 - 5 3 p r e -v i o u s l y s t u d i e d w i t h helium metastables. I t Is d i f f i c u l t t o r a t i o n a l i z e any a l t e r n a t i v e v i b r a t i o n a l assignments f o r t h i s band. A d i s c u s s i o n o f the apparent d i f f e r e n c e s of the Franck-Condon f a c t o r s f o r p h o t o i o n i z a t i o n and Penning i o n i z a t i o n to the ground s t a t e i o n o f H 20 and the o t h e r molecules r e p o r t e d i n t h i s s e c t i o n i s g i v e n at the end of t h i s c h a pter. F o r the normalized r e l a t i v e p o p u l a t i o n s o f e l e c t r o n i c o s t a t e s f o r He*(2 1S,2 3S) Penning i o n i z a t i o n and 584 A photo-i o n i z a t i o n of H 2 0 , r e l a t i v e l y l a r g e d i f f e r e n c e s are observed and l i s t e d In Table 19. Since the He*(2 !S) metastables c o n t r i b u t e l e s s than 10% o f the t o t a l s t r u c t u r e In the Penning spectrum, the data f o r the n ormalized r e l a t i v e p o p u l a t i o n i s e s s e n t i a l l y t h a t f o r i o n i z a t i o n by He*(2 3S) metast a b l e s . P r e v i o u s l y 5 1 I t has been suggested t h a t t h e : d i f f e r e n c e In "Impact" energie s f o r the two modes o f i o n i z a t i o n might e x p l a i n the observed d i f f e r e n c e s f o r the normalized 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 . Branton and B r l o n 9 5 have measured the energy dependance o f the p a r t i a l c r o s s - s e c t i o n s f o r the " p h o t o i o n i z a t i o n " o f H 20 and t h i s data shows t h a t the r a t i o of p a r t i a l c r o s s - s e c t i o n s , X 2B 1/A 2 A l S decreases as the p h o t o i o n i z a t i o n energy decreases i n the energy range o f 21.2 eV t o 19.8 eV. T h i s t r e n d i s i n agreement wi t h the data f o r the normalized r e l a t i v e p o p u l a t i o n s f o r the two s t a t e s X 2 B j a n d A 2A^ as shown i n Table 19. -125-7»3« Methanol and E t h a n o l . Methanol tCH-OH} belongs to the point group C" and has s the ground state e l e c t r o n c o n f i g u r a t i o n 1 4 0 . ( l a 1 ) 2 (2a') 2 (3a') 2 (4a') 2 (5a') 2 ( l a " ) 2 (6a') 2 ( 7 a ' ) 2 ; 1 A ' The molecular o r b i t a l s 2a" and 7a' are e s s e n t i a l l y the 2pn o r b i t a l s of oxygen while the l a " and the 5a' are e s s e n t i a l l y the 2pn o r b i t a l s of the methyl g r o u p 1 4 0 . Ethanol [C 2 H 5 OH] has the ground state outer e lec t ron conf igura t ion . . ( * l t ) m ( * 3 ) n ( a ' ) 2 ( a " ) 2 ; »A» Since the symmetry of the molecular o r b i t a l s i s unknown, $ t designates the l highest f i l l e d molecular o r b i t a l . In Figures 29 and 30 the He«(2 1S,2 3S) Penning e lec t ron spectra and the 584 A photoelectron spectra of methanol and 0 ethanol r e s p e c t i v e l y are compared. The 584 A photoelectron spectra are s i m i l a r to those reported by Robin and K e u b l e r 1 4 0 . Using a low r e s o l u t i o n e lec t ron analyzer , Cermak has observed some broad bands for N e * ( 3 P 0 , 3 P 2 ) Penning i o n i z a t i o n of methanol and e t h a n o l 2 8 . In the top p o r t i o n of Figure 29, four bands are observed for He*(2 1S,2 3S) Penning I o n i z a t i o n of methanol. It i s l i k e l y that two states (B + C) are c o n t r i b u t i n g to the band with the peak maximum at 4.44 eV. i n the top p o r t i o n of Figure 30, s i x bands are observed for the He*(2 1S,2 3S) Penning i o n i z a t i o n of o ethanol corresponding to the s ix observed bands In the 584 A -126-Penning Ionization (He*21S,2^S) C H 3 O H x4 23S(D2A') / ' Spectrum & Background Photoionization (584 A.) Background Subtracted X 2* n—r .... ,c t f B2A ^ 2 . . I I A H I I \ / \ / \ D2A I ~ l I I I 1 1 1 1 1 7 1 1 11 10 9 8 7 . 6 5 4 3 2 1 0 ELECTRON ENERGY (eV) FIGURE 29. Electron spectra for the i o n i z a t i o n of methanol. -127-Penning Ionization (He*2 'S , 2 3S) C H 3 C H 2 O H : x4 2%(Aft) 2,S(C) 2^ S(B) I x 1 6 2^S(E! '< i — r 2^'b; // i /V 1 Spectrum l & Background >/'\S; Background Subtracted I / "A^T •'••"•iiivj!-* Photoionization ( 5 8 4 A ) ELECTRON ENERGY (eV) FIGURE 30. E lec t ron spectra for the I o n i z a t i o n of ethanol . -128-p h o t o i o n i z a t i o n of e t h a n o l . The c e n t r a l p o r t i o n s (background s u b t r a c t e d ) o f F i g u r e s 29 and 30 r e v e a l r e g i o n s extending down to *\»1.0 eV where a n a l y s i s i s p o s s i b l e . Large A E o b s energy s h i f t s are measured f o r methanol and are l i s t e d i n Table 20. I t appears ( F i g u r e 29) t h a t r e l a t i v e t o the p a r t i a l c r o s s - s e c t i o n f o r He*(2 3S)/CH 30H +(B 2 A ' ) , t h e c r o s s -s e c t i o n f o r He«(2 3S)/CH 30H +(5" 2A") i s s m a l l s i n c e no shoulder i s observed on the peak. The normalized r e l a t i v e p o p u l a t i o n s of e l e c t r o n i c s t a t e s f o r both He*(2 1S,2 3S) Penning i o n i z a t i o n and 584 A p h o t o i o n i z a t i o n o f methanol are l i s t e d i n Table 20. R e s u l t s are normalized w i t h r e s p e c t to the f i r s t e l e c t r o n i c s t a t e (X 2 A " ) . S i n c e the B\ 2A' and C 2A" s t a t e s are not r e s o l v e d f o r e i t h e r mode o f i o n i z a t i o n they were combined t o g e t h e r when c a l c u l a t i n g the r e l a t i v e p o p u l a t i o n s of the e l e c t r o n i c s t a t e s . In comparing the normalized r e l a t i v e p o p u l a t i o n s f o r the two modes o f i o n i z a t i o n l a r g e d i f f e r e n c e s are observed. Large A E o b g energy s h i f t s a l s o are fo!ioured f o r e t h a n o l E .< re l i s t e d i n Table 21. Below 26 eV, Robin and Keubler 1** 0 have p r e d i c t e d nine i o n i z a t i o n p o t e n t i a l s but observed only seven o o u s i n g 584 A and 304 A r a d i a t i o n . In the e l e c t r o n s p e c t r a shown In F i g u r e 30, t h e r e i s some evidence of s t r u c t u r e f o r s i x bands. The normalized r e l a t i v e p o p u l a t i o n s o f e l e c t r o n i c s t a t e s f o r the 'two modes o f i o n i z a t i o n are l i s t e d i n Table 21. R e s u l t s are normalized w i t h r e s p e c t to the f i r s t e l e c t r o n i c s t a t e (X 2 A " ) . In both the Penning e l e c t r o n s p e c t r a and the p h o t o e l e c t r o n spectrum the t h i r d and f o u r t h s t a t e s , l a b e l l e d B and C, are not r e s o l v e d - 1 2 9 -Table 20. A E o b s energy s h i f t i n eV and the normalized r e l a t i v e e l e c t r o n i c o 3 s t a t e p o p u l a t i o n s f o r 584 A p h o t o i o n i z a t i o n and He*(2 S) Penning i o n i z a t i o n of methanol at 300 °K. E l e c t r o n i c S t a t e A E o b s (eV) R e l a t i v e S t a t e P o p u l a t i o n s 584 A He*(2 3S) (21.22 eV) (19.82 eV) _ 2 " X A A 2A< -0.42 * 0.05 100 * 5 -0.42 * 0.08 99 * 5 100 * 10 232 * 23 a B V c V D 2A" •0.22 * 0.08 211 * 11 -0.15 * 0.10 109 * 6 399 * 40 a 156 * 3 1 a 1 0 a. Contains s m a l l c o n t r i b u t i o n s from He*(2 S) and He (584 A). -130-Table 21. A E o b s energy s h i f t i n eV and the normalized r e l a t i v e e l e c t r o n i c o 3 state populations f o r 584 X photoioniza t ion and He*(2 S) Penning i o n i z a t i o n of ethanol at 300 °K. E l e c t r o n i c State A E 0 b s (eV) Rela t ive State Populations o 584 A (21.22 eV) He*(2 S) (19.82 eV) - 2 ' X A - 2 ' A A -0.47 * 0.08 -0.37 * 0.10 100 * 10 97 * 15 100 ± 10 136 * 20 a B C -0.55 * 0.10 262 * 50 133 1 33 a E -0.23 4 0.10 111 * 21 -0.26 * 0.10 82 * 12 128 * 25 a 100 * 20 a a. Contains small contr ibut ions from He*(2 1S) and He (584 A) -131-and so a combined value i s g i v e n f o r the r e l a t i v e p o p u l a t i o n s o f the e l e c t r o n i c s t a t e s . In comparing the normalized r e l a t i v e p o p u l a t i o n s f o r the two modes of i o n i z a t i o n , l a r g e d i f f e r e n c e s are observed. 7.4. I s o p r o p y l A l c o h o l and T e r t i a r y B u t y l A l c o h o l . Both i s o p r o p y l a l c o h o l [(CH 3) 2CHOH] and t - b u t y l a l c o h o l E(CH 3) 3COH] have ou t e r e l e c t r o n c o n f i g u r a t i o n s o f the type >A\)m (* 3> n ( a ' ) 2 ( a " ) 2 ; LA» Since the symmetry o f the molecular o r b i t a l s i s unknown, * £ designates the * t n h i g h e s t f i l l e d m o l e c u l a r o r b i t a l . In F i g u r e s 31 and 32 the He*(2 1S,2 3S) Penning s p e c t r a and the 584 % p h o t o e l e c t r o n s p e c t r a o f i s o p r o p y l a l c o h o l and t - b u t y l a l c o h o l are shown. The 584 A p h o t o e l e c t r o n s p e c t r a are s i m i l a r to those r e p o r t e d by Robin and Kuebler 1** 0. The complex, broad and o v e r l a p p i n g bands i n : t h e e l e c t r o n s p e c t r a o f both a l c o h o l s p r e c l u d e any unequivocal assignment o f the bands In the Penning s p e c t r a , except f o r the ground s t a t e s o f the i o n s . The A E o b s values f o r the ground s t a t e o f i s o p r o p y l a l c o h o l and t - b u t y l a l c o h o l are -0.41 - 0.10 eV and.-0.49 * 0.10 eV r e s p e c t i v e l y . 7.5. Dimethyl E t h e r and E thylene Oxide. Both dimethyl e t h e r [CHjOCHj] and the a l i c y c l i c analogue ethylene oxide LCH 20CH 2J, have the symmetry C"2V. The ground s t a t e e l e c t r o n c o n f i g u r a t i o n o f dimethyl e t h e r i s 1 1 * 1 . ( i a i ) 2 (2a 1)' f ( l b ) 2 (CH b o n d i n g ) 1 2 ( 2 b 2 ) 2 ( 3 a ^ 2 ( l b j ) -132-Penning Ionization (He* 2 $ , 2 3S) (CH 3 ) 2 CHOH Spectrum & Background x 4 2:\3(X2A") Background Subtracted Photoionization (584 A) ELECTRON ENERGY (eV) FIGURE 31. E l e c t r o n spectra for the i o n i z a t i o n i s o p r o p y l a l c o h o l . -133-Penning Ionization (He*21S,23S) (CK.1COH 1 / / / •V'-V Spectrum & Background Background Subtracted Photoionization (584 A) JCA" r T . . , A A ^ i i i 1 1 1 1 1 1 r 11 10 9 8 7 6 5 4 3 2 1 ELECTRON ENERGY (eV) FIGURE 32. E l e c t r o n s p e c t r a f o r the I o n i z a t i o n o f t e r t i a r y b u t y l a l c o h o l . -134-Th e e f f e c t i v e degeneracy of the C-H bonding i s removed due to an Interac t ion with another o r b i t a l , probably 1 1 * 1 the 2b2 (c-0 bonding) o r b i t a l . The ground state e lec t ron conf igura t ion of ethylene oxide i s 1 * 2 . ...(2b x) 2 ( l b 2 ) 2 (5a L) 2 ( l a 2 ) 2 (3b x) 2 ( 6 a i ) 2 (2b 2) 2 ; lAl The l b 2 , l a 2 and 2b2 are n o r b i t a l s . ^ In Figures 33 and 34 the He*(2 1S,2 3S) Penning e lec t ron o spectrum and the 584 A photoelectron spectrum of dimethyl ether o and ethylene oxide r e s p e c t i v e l y are compared. The 584 A photo-e lec t ron spectrum of dimethyl ether (Figure 33) shows f i v e i o n i c o states as reported by Cradock and Whlteford 1 *•1. The 584 A photoelectron spectrum of ethylene oxide (Figure 34) reveals four bands composed of s i x i o n i c states and i s very s i m i l a r to that reported by Basch et a l . 1 * * 2 . Some nitrogen impurity i s observed and Is l a b e l l e d as such i n Figure 34. Using N e * ( 3 P 0 3 P 2 ) metastable atoms, Cermak 2 8 has observed three broad bands f o r the Penning i o n i z a t i o n of both dimethyl ether and ethylene oxide . In the upper p o r t i o n of Figure 33 at least four bands, correspon-ding to the states X 2 B 1 , A 2 A 1 , C and D, are observed for the He*(2 3S) Penning i o n i z a t i o n , of dimethyl ether . The t h i r d band may also contain an unresolved c o n t r i b u t i o n from the B 2 B 2 s ta te . In the upper p o r t i o n of Figure 34 only four bands are observed for the He*(2 3S) Penning i o n i z a t i o n of ethylene oxide . For both of these molecules the r a t i o of He*(2 1S) to He*(2 3S) Penning I o n i z a t i o n to the ground state of the ion Is somewhat l a r g e r - 1 3 5 -Penning Ionization (He*21S,2^S) 23S(A2A1) C H 3 O C H 3 23S(C) 23S(b) I X 4 ft Background 5 8 4 A.) r a c t e d x2^ i D(OH boning) I A A, I C(C-Hbondino); I I ~l 1 1 1 1 1 1 •—1 1 r 11 1 0 9 8 7 6 5 4 3 2 ELECTRON ENERGY (eV) FIGURE 3 3 . E l e c t r o n spectra f o r the i o n i z a t i o n of dimethyl ether . -136-Penning Ionization (He*2lS^5) i ' /' 23S(B2B,) X4 x 1 6 ' 23S(X2B;) 23S(A2A1) J*'*^ l l I I j Spectrum & Background Background Subtracted ^r-srZ-Photoionization (584 A ) i B2B, ^ E 2 ^ ^ i i i I i i i i T r 11 10 9 8 7 6 5 4 3 2 1 ELECTRON ENERGY («V) FIGURE 34. E l e c t r o n spectra for the I o n i z a t i o n of ethylene oxide . -137-(^20%) than has been observed f o r most of the other molecules studied i n t h i s s e r i e s . The A E Q b s energy s h i f t s are measured for dimethyl ether and are l i s t e d i n Table 22 together with the normalized r e l a t i v e populations of e l e c t r o n i c states for both He*(2 1 S,2 3 S) Penning o i o n i z a t i o n and 584 A p h o t o i o n i z a t i o n . Results are normalized with respect to the X 2 B r e l e c t r o n i c s t a t e . Since the B and C states are not resolved i n e i t h e r mode of i o n i z a t i o n , a combined value i s given for the r e l a t i v e populat ion of these e l e c t r o n i c s ta tes . In comparing the normalized r e l a t i v e populations for the two modes of i o n i z a t i o n l a r g e , d i f f e r e n c e s are observed. The A E Q t ) g energy s h i f t s are also measured f o r ethylene oxide and are l i s t e d i n Table 23 together with the normalized r e l a t i v e populations of e l e c t r o n i c states f o r both He*(5»1S,23S) o Penning i o n i z a t i o n and 584 A p h o t o i o n i z a t i o n . Results are normalized with respect to the X 2B 2 e l e c t r o n i c s t a t e . In the e lec t ron spectrum-there are two unresolved pai rs o f i o n i c s tates (B + c) and (D + E) and therefore a combined value for the r e l a t i v e state populations In each case i s given In Table 23. Large di f ferences are observed In comparing the normalized r e l a -t i v e populations f o r the two modes of i o n i z a t i o n . 7.6. D i e t h y l Ether , Tetrahydrofuran and 1,4 Dioxane. D i e t h y l ether [ C 2 H 5 O C 2 H 5 ] and the a l i c y c l l c analogue, tetrahydrofuran [ C H 2 C H 2 O C H 2 C H 2 j , can both be considered as belonging to the point group C 2 v and to have a ground state outer -138-Table 22. A E 0 D S energy s h i f t i n eV and the normalized r e l a t i v e e l e c t r o n i c s t a t e p o p u l a t i o n s f o r 584 1 p h o t o i o n i z a t i o n and He*(2 3S) Penning i o n i z a t i o n of dimethyl e t h e r at 300 °K. E l e c t r o n i c S t a t e R e l a t i v e State P o p u l a t i o n s 584 A (21.22 eV) He*(2 3S) (19.82 eV) - 2 X . B, - 2 A A, - 2 B B. C(CH bonding) D(CH bonding) -0.34 * 0.05 100 * 10 -0.35 * 0.08 -0.1 * 0.1 77 * 8 223 * 22 -0.16 ± 0.10 218 * 22 100 * 10 333 * 50 a a 231 .* 35 1142 * 150 c a. Contains s m a l l c o n t r i b u t i o n s from He»(2 1S) and He (584 X). -139-Table 23. A E o b s energy s h i f t i n eV and the normalized r e l a t i v e e l e c t r o n i c s t a t e p o p u l a t i o n s f o r 584 & p h o t o i o n i z a t i o n and He*(2 3S) Penning i o n i z a t i o n o f et h y l e n e oxide at 300 °K. R e l a t i v e S t a t e P o p u l a t i o n s E l e c t r o n i c S t a t e AEobs (eV) 584 K (21.22 eV) He*(2 S) (19.82 eV) - 2 X B 2, - 2 A Ai -0.35 * 0.05 100 * 5 -0.45 4 0.10 87 * 4 100 * 15 138 * 2 0 a - 2 B B i C 2 A „ -0.43 * 0.10 201 * 15 152 * 23 a D 2 A j - 2 E B 2 -0.21 * 0.10  o i o | 229 * 30 517 * 100 a 1 o a. Contains s m a l l c o n t r i b u t i o n s from He*(2 S) and He (584 A) -140-elec t ron conf igurat ion of the type •••(n)m (*3)n Ux)2 ( b ^ 2 ; »A l Since the symmetry of the molecular o r b i t a l s i s unknown,$ designates the A highest f i l l e d molecular o r b i t a l . In F i g u r e s35 and 36 the H e»(2 1 S 52 3 S ) Penning e lec t ron spectra and the 584 A photoelectron spectra of d i e t h y l ether and tetrahydrofuran r e s p e c t i v e l y are compared. Since the photo-e lec t ron spectra of these molecules have not previously been reported, the absolute energy scales are c a l i b r a t e d using both argon and n i t r o g e n . For both molecules the photoelectron and the Penning e lec t ron spectra consist of a large number of over-lapping bands. 1-4 dioxane [0CH 2 C H 20CH 2 C H 2 ] has the symmetry C 2 n and has the ground state outer e lec t ron conf igurat ion • . . . < • „ ) " ( * 3 ) n ( l b g ) 2 ( l a g ) 2 ( 2 a g ) 2 ; *Ag where t|>4 designates the highest f i l l e d molecular o r b i t a l . In Figure 37 the He*(2 3 S) Penning e lec t ron spectrum of 1,4 o dioxane i s compared to the 584 A photoelectron spectrum. The photoelectron spectrum Is very s i m i l a r to that reported by Kobayashi and Nagakura 1 **3. A large number of the states observed i n the photoelectron spectrum are also observed i n the Penning e lec t ron spectra . The A E Q b s energy s h i f t s for the three molecules are l i s t e d i n Table 24. Quanti ta t ive measurements of the r e l a t i v e populations of e l e c t r o n i c states are not obtained due to the - I l l -Penning Ionization (He^^S) E t O E t x4 Spectrum & Background Z'SCCB,) Background Subtracted Photoionization (584 A.) XZB, "l I I 1 1 1 1 1 1 1 r 12 11 10 9 8 7 6 5 4 3 2 ELECTRON ENERGY («V) FIGURE 3 5 . E l e c t r o n spectra for" the i o n i z a t i o n of d i e t h y l ether . - 1 4 2 -Penning Ionization (He*21S,2 ; ,S) xte 2?SCX2B.) Spectrum & Background Z . . I V 3 -12 11 10 9 8 7 6 5 4 3 2 \ ELECTRON ENERGY (eV) FIGURE 36. E l e c t r o n spectra f o r the I o n i z a t i o n of te t rahydrofuran. -143-V Background Subtracted Photoionization (584 A) n— f — — — -1 1 1 1 1 — i 1 1 1 1 r 12 11 10 9 8 7 6 5 4 3 2 ELECTRON ENERGY (eV) FIGURE 37. E l e c t r o n spectra f o r the I o n i z a t i o n 1,4 dioxane. Table 24. 3 A E o b s e n e r S y s h i f t In eV for He*(2 S) Penning i o n i z a t i o n of d i e t h y l e ther , tetrahydrofuran and 1,4 dioxane at 300 °K. D i e t h y l Ether Tetrahydrofuran 1,4 Dioxane E l e c t r o n i c A E O D S State (eV) E l e c t r o n i c A E obs State (eV) E l e c t r o n i c A E 0 v,s State (eV) _ 2 X B i -0.17 * 0.08 Z* -0.08 * 0.10 X  2 B l -0.33 * 0.10 Z -0.29 0.10 X 2 A -0.21 * 0.10 g A 2 A -0.23 0 .08 a g B 2 B -0.19 0.10 g Z -0.32 0.10 a. Averaged value . -145-large number of overlapping bands. However, a v i s u a l estimate with reference to the ground Ionic state indica tes large d i f f e r e n c e s . Por example, the second band of states i n the spectra of these three molecules shows very s i g n i f i c a n t d i f f e r -ences between the two modes of i o n i z a t i o n . This i s p a r t i c u l a r l y obvious i n the case of 1,4-dioxane where the r a t i o , A/B , of c ross-sect ions i s much larger f o r p h o t o i o n i z a t i o n . In a d d i t i o n , f o r a l l three molecules the i n t e n s i t y r a t i o , X / Z , where Z r e f e r s to the l a s t band, i s very d i f f e r e n t for the two modes of i o n i z a t i o n . 1.1. Discussion of the Franck-Condon Envelopes f o r T r a n s i t i o n s  to the Ground Ionic State . In Figure 38 the He*(2 3S) Penning e lec t ron and 584 A photoelectron spectra are shown for the ground i o n i c states of water, a l c o h o l s , ethers and r e l a t e d compounds. The A E o b s energy s h i f t s , which are large (negative) f o r a l l molecules , are shown below the respect ive Penning spectra . The magnitude of these s h i f t s can be v i s u a l i z e d by: a comparison with the energy scale shown i n the photoelectron spectrum of water. It can be seen i n a l l cases t h a t , with respect to the photoelectron spectrum, the high energy side ( l e f t hand side) of the Penning e lec t ron peak i s s i g n i f i c a n t l y asymmetric. This asymmetry i s even more prominent when the r i s i n g background i s taken in to account. For a number of molecules there i s evidence of v i b r a t i o n a l s t r u c -ture i n the Penning e lec t ron band. The Penning spectra have been posi t ioned below the photoelectron spectra such that the -146-WATER & ALCOHOLS H , O C H 3 O H 0 1 eV I • |\ i I i / \ aE». -°-37 AE •0.04 ETHERS C H J O C H J C H 2 O C H . C H , C H 8 O H -0.47 •0.07 C H J C H J O C H J C H J ( C H 3 ) j C H O H -0.41 •0.01 C C H J J J C O H -0.49 •0.00 CHjCa-ipCHjCH, O C H C H . O C H J C H J I \ 1 J \ i AE„ AE -0.34 -0.03 -0.35 •0.02 -0.17 •0.01 -0.33 -0.04 -0.21 •0.07 FIGURE 38. E l e c t r o n spectra f o r the ground i o n i c states of water, a l c o h o l s , and ethers Energy s h i f t s are given i n eV. - 1 4 7 -tentat ive v i b r a t i o n a l assignment f o r the Penning band (AE A « 0) corresponds to that for p h o t o i o n i z a t i o n . The broken markers i n d i c a t e where v i b r a t i o n a l s tructure might be expected. This v i b r a t i o n a l assignment of the Penning spectra r e s u l t s i n small AE values (shown on Figure 38) which are of the order of thermal energies as has been observed for most other molecules'* 5 » 5 1 >5 3 . A l t e r n a t i v e assignments would r e s u l t i n large AE values of the order of one or more v i b r a t i o n a l quanta. I f these assignments are correct then the r e s u l t s suggest that there are large d i f f -erences i n the Franck-Condon factors for the two modes of i o n i z a t i o n of these molecules . For Penning i o n i z a t i o n t h i s would imply that the target p o t e n t i a l energy surfaces are modified by the inf luence of the helium ; p a r t i c l e . Even a small per turbat ion of the p o t e n t i a l surface could r e s u l t i n s i g n i f i c a n t changes i n the Franck-Condon factors as discussed i n a previous chapter. A l t e r n a t i v e l y these large di f ferences i n Franck-Condon factors might also be due to competing a u t o i o n i z a t i o n processes i n which the a u t o i o n i z i n g state i s formed by resonant charge t r a n s f e r with the helium metastable. This means, i f a competing auto-i o n i z a t i o n process i s to expla in the change i n the Franck-Condon envelope, there would have to be an a u t o i o n i z i n g l e v e l at 1 9 . 8 2 eV f o r the whole ser ies of molecules studied and t h i s seems u n l i k e l y . Furthermore, C e r m a k 1 3 9 has a lso observed a d i f f e r e n c e In shape for A r * ( 3 P 0 , 3 P 2 ) Penning i o n i z a t i o n of ethylene oxide and Invoking the a u t o i o n i z a t i o n argument would require that there be two auto ioniz ing l e v e l s i n ethylene oxide which are resonant with the helium and the argon metastable atoms r e s p e c t i v e l y . -148-CHAPTER EIGHT CARBONYL CONTAINING COMPOUNDS i 8.1. I n t r o d u c t i o n . In t h i s chapter the He*(2 1S,2 3S) Penning i o n i z a t i o n o f , HCHO,CH3CHO, CH3COCH3,HCOOH and CH3COOH, are r e p o r t e d . The He*(2 1S,2 3S) Penning i o n i z a t i o n process i s compared t o t h a t o f 584 A p h o t o i o n i z a t i o n . High r e s o l u t i o n p h o t o e l e c t r o n s p e c t r a of these f i v e c a r b o n y l - c o n t a i n i n g molecules have been r e p o r t e d i n the l i t e r a t u r e . U sing N e * ( 3 P 0 , 3 P 2 ) m e t a s t a b l e s , Sermak28 has obtained a low r e s o l u t i o n Penning e l e c t r o n spectrum o f acetone but the Penning i o n i z a t i o n o f the othe r molecules has not been s t u d i e d previously.. 8.2. Formaldehyde and Acetaldehyde. Formaldehyde [HCHO] has the symmetry C 2 V and the ground s t a t e e l e c t r o n configuration 1'*''. . . . ( l b 2 ) 2 ( S a ^ 2 ( l b x ) 2 ( 2 b 2 ) 2 ; l A t where the 2b 2 o r b i t a l i s the oxygen non-bonding o r b i t a l and the l b j o r b i t a l i s the C=0 n-bonding o r b i t a l . Acetaldehyde [CH 3CHO] has the symmetry C g and 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 1 1 * 5 . . . . ( 7 a ' ) 2 ( l a " ) 2 ( 8 a ' ) 2 C 9 a ' ) 2 ( 2 a " ) 2 ( 1 0 a ' ) 2 ; 1k1 where the 10a' o r b i t a l i s the oxygen non-bonding o r b i t a l and 2a" o r b i t a l i s the C » 0 H-bonding o r b i t a l . ; -149-In Figures 39 and 40, the H e* (2 1 S ,2 3 S ) Penning e lec t ron o and the 584 A photoelectron spectra of formaldehyde and acetalde-o hyde r e s p e c t i v e l y are compared. The 584 A photoelectron spectrum of formaldehyde which contains three bands, i s s i m i l a r to that reported by Baker et a l . 1 2 3 . The t h i r d band contains c o n t r i b u -t ions from both the B 2 B 2 and the C 2kl s t a t e s . Tn the upper port ion of Figure 39, the same three bands observed i n the photo-e lec t ron spectrum are a lso observed i n the H e* (2 1 S ,2 3 S ) Penning e l e c t r o n spectrum. It appears that Penning i o n i z a t i o n i s p r e -dominately due to He*(2 3 S) metastables however, there are traces o of s t ructure due to He*(2 l S) metastables and " i n t e r n a l " 584 A photons'* 9 . Recently, t h i s laboratory reported the photoelectron spectrum of acetaldehyde 1** 6 and the i o n i z a t i o n p o t e n t i a l s are i n good agreement with values reported by Meek et a l . 1 1 * 7 . In Figure 40, three of the f i v e bands i n the photoelectron spectrum of acetaldehyde show evidence of v i b r a t i o n s t r u c t u r e . In the upper p o r t i o n of Figure 40 , f i v e bands are a lso observed i n the H e* (2 1 S ,2 3 S ) Penning e lec t ron spectrum of acetaldehyde. The & E o b g energy s h i f t v a l u e s 5 4 of formaldehyde are l i s t e d In Table 25. The f i r s t band, l a b e l l e d 2 3 S(X 2 B 2 ) , i n the Penning e lec t ron spectrum (Figure 39) has a v i b r a t i o n a l envelope d i f f e r e n t from that i n the f i r s t band i n the photoelectron spec-trum. A t enta t ive assignment f o r the He*(2 3 S)/HCHO + (X 2 B 2 ) process , where AE f t i s assumed to be z e r o 5 4 , i s i l l u s t r a t e d i n Figure 39 and places v ' => 1: ( for e i ther the OO s t re tch or the H-C-H deformation) at the peak maximum. This corresponds to a -150-Penning Ionization (He*2,SJ2aS) [ H C H O x8 2^S(X2BJ) 2^S(AJB,) Spectrum / / 23SCB JBJ.C2A,) & B a c k g r o u n d I v . Background Subtracted Photoionization (584 A) X 2 ^ rrr B'BJ.C'A, k A B , I f A D , —i 1 1 1 1 1 1 1 r 10 9 8 7 6 5 4 3 2 ELECTRON ENERGY (eV) FIGURE 39. E l e c t r o n spectra f o r the i o n i z a t i o n of formaldehyde. -151-Penning Ionization (He*21Sl2^S) C h U C H O x 4 23scb2A") j 23SCX2A> rn— 2 3SCC 2A) / / V x16 **A*K, / Spectrum & Background l Background Subtracted Photoionization (584 A.) C 2 * 1 V_ b2A" BZA A A2A' / \ A \ ~i 1 1 1 1 1 1 1 1 r 11 10 9 8 7 6 5 4 3 2 ELECTRON ENERGY (eV) FIGURE 40. E lec t ron spectra f o r the i o n i z a t i o n of acetaldehyde. -152-Table 25. A E 0 D S energy s h i f t In eV and the normalized r e l a t i v e e l e c t r o n i c o 3 state populations for 584 A photoioniza t ion and He*(2 S) Penning i o n i z a t i o n of formaldehyde at 300 °K. Relat ive State Populations E l e c t r o n i c State AEobs (eV) 584 X (21.22 eV) He*(2 S) (19.82 eV) - 2 X B 2 A 2 B , -0.21 * 0.05 -0.09 * 0.10 100 * 10 100 * 10 100 * 10 90 * 9 a B 2 B , _ 2 C A x -0.26 * 0.08 410 * 41 550 * 100 a a. Contains small contr ibutions from He*(2 1 S) and He (584 X ) . -153-AE energy s h i f t of -0.02 eV. A d i s c u s s i o n of the apparent di f ferences of the Franck-Condon factors f o r Penning i o n i z a t i o n and photoioniza t ion to the ground state ion of HCHO and the other molecules i s given at the end of t h i s chapter . The A E Q b g energy s h i f t values measured for acetaldehyde are l i s t e d i n Table 26. The normalized r e l a t i v e populations of e l e c t r o n i c states for He*(2 1S,2 3S) Penning i o n i z a t i o n and 584 A p h o t o i o n i z a t i o n of formaldehyde and acetaldehyde r e s p e c t i v e l y are l i s t e d i n Tables 25 and 26. The B 2 B 2 and the C 2kl s tates of formaldehyde are combined since they are not resolved i n the e lec t ron spectra . In comparing the e l e c t r o n i c state populations f o r the two modes of i o n i z a t i o n , r e l a t i v e l y small d i f f e r e n c e s are observed for the two molecules. It i s a l s o noted that f o r the two molecules , the r a t i o of the r e l a t i v e e l e c t r o n i c state populations f o r the A state (which corresponds to the removal of a C=0 Jl-bonding electron) to the X state (which corresponds to the removal of the oxygen non-bonding electron) are very s i m i l a 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 . This behaviour Is d i f f e r e n t from that observed i n the case of - C H N containing m o l e c u l e s 5 5 and C O 5 1 . 8.3. Acetone. Acetone [CH 3COCH 3 ]'has the symmetry C 2 v and the ground state e lec t ron c o n f i g u r a t i o n 1 * * 5 . ...(3b2)2 ( l b L ) 2 ( T a ^ 2 ( l a 2 ) 2 (4b 2) 2 (8a,) 2 (2b x) 2 ^ b , , ) 2 ; ^ where the 5b 2 o r b i t a l i s e s s e n t i a l l y the oxygen non-bonding o r b i t a l and the 2b l o r b i t a l Is the C«=0 n-bonding o r b i t a l . Table 26. A E 6bs energy s h i f t In eV and the normalized r e l a t i v e e l e c t r o n i c state populations for 584 & photoionizat ion and He*(2 3 S) Penning i o n i z a t i o n of acetaldehyde at 300 °K. E l e c t r o n i c State A E o b s (eV) Rela t ive State Populations 584 £ (21.22 eV) He*(2 S) (19.82 eV) - 2 ' X A -0.31 * 0.05 100 * 5 100 * 10 _ 2 II A A •0.19 4 0.10 113 4 22 123 4 25 - 2 ' B A -0.48 4 0.10 175 4 35 187 4 50c - 2 • C A •0.30 4 0.10 287 4 57 338 * 67 e - 2 ' D A -0.26 * 0.10 130 * 26 349 4 70' a. Contains small contr ibutions from He*(2 1 S) and He (584 t ) . - 1 5 5 -In F i g u r e 4 l , the He*(2 1S,2 3S) Penning e l e c t r o n and the 0 o 584 A p h o t o e l e c t r o n s p e c t r a o f acetone are compared. The 584 A p h o t o e l e c t r o n spectrum o f acetone i s s i m i l a r t o t h a t r e p o r t e d by Brundle et a l . 1 1 * 8 . The upper p o r t i o n o f F i g u r e 41 shows t h a t the Penning e l e c t r o n spectrum l a c k s some o f the f e a t u r e s observed i n the p h o t o e l e c t r o n spectrum. The A E Q b g energy s h i f t values o f acetone are l i s t e d i n Table 27. No value s a re l i s t e d f o r the B and D s t a t e s o f acetone s i n c e the p o s i t i o n o f the peak maximum cannot be l o c a t e d . The normalized r e l a t i v e p o p u l a t i o n s o f e l e c t r o n i c s t a t e s f o r o He*(2 1S,2 3S) Penning i o n i z a t i o n and 584 A p h o t o i o n i z a t i o n o f acetone are a l s o l i s t e d In Table 27. F o r acetone the s t a t e s l a b e l l e d A, B, C and D, have been combined s i n c e they are not r e s o l v e d i n the e l e c t r o n s p e c t r a . T h e r e f o r e , i t was not p o s s i b l e t o compare q u a n t i t a t i v e l y the r a t i o of the e l e c t r o n i c s t a t e p o p u l a t i o n s o f the A 2Bl to the X 2 B 2 as was done f o r the two pr e v i o u s molecules. A v i s u a l comparison suggests t h a t r e l a t i v e t o p h o t o i o n i z a t i o n " the r a t i o o f the e l e c t r o n i c s t a t e s (A 2B 1/B) i s much l a r g e r f o r Penning i o n i z a t i o n . 8.4. Formic A c i d and A c e t i c A c i d . Formic a c i d [HCOOH] has the symmetry C s and 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 1 * * 9 . , . . ( 8 a ' ) 2 ( l a " ) 2 ( 9 a ' ) 2 ( 2 a " ) 2 ( 1 0 a f ) 2 ; lAx where the 10a' o r b i t a l i s e s s e n t i a l l y the non-bonding n 0 - o r b i t a l l o c a l i z e d on the oxygen atoms and the 2a" b r b i t a l i s e s s e n t i a l l y the non-bonding antisymmetric H 2 - o r b i t a l . -156-Penning Ionization (He*2 1S,2 aS) C H 3 C O C K 23S(E) 2aS(B,C,D) ' Spectrum & Background .1 •7 2:iS(F)/' / / / / / / . / / / 71 Background •> *.. Subtracted i A': Photoionization ( 5 8 4 A ) A B , I c I . D 11 1 0 9 8 I 6 5 4 3 ELECTRON ENERGY (eV) 1 2 FIGURE -111.' E lec t ron spectra f o r the i o n i z a t i o n acetone. -157-Table 27. A E o b s energy s h i f t i n eV and the normalized r e l a t i v e e l e c t r o n i c 0 3 state populations for 584 A photoioniza t ion and He*(2 S) Penning i o n i z a t i o n of acetone at 300 °K. E l e c t r o n i c State , E 0 b s (eV) Relat ive State Populations 0 584 A (21.22 eV) He*(2 S) (19.82 eV) - 2 X B 2 -0.31 * 0.08 100 * 10 100 * 10 A 2 B, +0.03 * 0.10 B C D -0.27 * 0.10 664 * 66 700 * 100 -0.36 * 0.10 331 •* 33 511 * 75 a -0.26 * 0.10 64 * 64 177 1 35 1 0 a. Contains small c o n t r i b u t i o n from He*(2 S) and He (584 A ) . - 1 5 8 -Acetlc a c i d [CH3COOH] has the symmetry C s ; . Following the photoelectron data of Sweigart and T u r n e r 1 5 0 , ace t ic a c i d has the ground state outer e lec t ron c o n f i g u r a t i o n . . .• (* l t ) m ( * 3 ) n ( l a " > 2 < l a ' ) 2 5 1 A 1 Since the symmetry of the molecular o r b i t a l s i s unknown, designates the Ith highest f i l l e d molecular o r b i t a l . The l a * o r b i t a l i s e s s e n t i a l l y the non-bonding n Q - o r b I t a l and the l a " o r b i t a l Is the non-bonding antisymmetric n 2 - o r b i t a l . In Figures 42 and 43 the H e » ( 2 1 S , 2 3 S ) Penning e l e c t r o n o and the 584 A photoelectron spectra of formic a c i d and ace t i c o ac id r e s p e c t i v e l y are compared. The 584 A photoelectron spectrum of formic a c i d (Figure 42) which contains f i v e bands, i s s i m i l a r to those reported by Brundle et a l . 1 5 1 and Watanabe et a l . 1 5 2 . In the upper p o r t i o n of Figure 42 there i s also evidence of f i v e bands In the H e * ( 2 1 S , 2 3 S ) Penning e l e c t r o n spectrum. The v i b r a -t i o n a l s t ructure observed i n the photoelectron spectrum i s not o apparent i n the Penning e lec t ron spectra . The 584 A photoelectron spectrum of ace t ic ac id (Figure 43) i s s i m i l a r to that reported by Sweigart and T u r n e r 1 5 0 . In the upper p o r t i o n of Figure 43 the same f i v e bands observed i n photoioniza t ion are also observ-ed In Penning i o n i z a t i o n . The A E o b g energy s h i f t values of formic a c i d and ace t ic ac id r e s p e c t i v e l y are l i s t e d i n Tables 28 and 2 9 . No values are l i s t e d for the C 2 A " state of formic a c i d and the B state of acet ic a c i d because the p o s i t i o n s of the respect ive peak maxima cannot be l o c a t e d . The normalized r e l a t i v e populations of -159-Penning Ionization (He*2^5,2^S) H C O O H x4 Spect rum 2%CB2A .Cfr) / 2^SCX2A) // 2^SCD2A)/ & Background ; X 4 y - -Photoionization ( 5 8 4 A ) D'A I I / Background . ' Subtracted r r r r A2A &K I'll ii!1 C2A ? -| -i 1 1 1 1 1 i r 1 0 9 8 7 6 5 4 3 2 ELECTRON ENERGY (eV) FIGURE 42. E lec t ron spectra for the i o n i z a t i o n of formic a c i d . -160-Penning Ionization (He*2X2^5) C H 3 C O O H 1 Spectrum & Background 2%<E) 2SSCX2A> 2%(A2A") \ x 4 / 2^ S(D)/ / Background Subtracted 1 I Photoionization ( 5 8 4 A.) D 1 '. E •'I I X* A2*' -1 1 1 1 1 1 1 1 1 r 11 1 0 9 8 7 6 5 4 3 2 ELECTRON ENERGY (eV) FIGURE 43. E l e c t r o n spectra f o r the i o n i z a t i o n of acet ic a c i d . -161-Table 28. A E o b s e n e r £ v s h i f t i n eV and the normalized r e l a t i v e e l e c t r o n i c state populations f o r 584 % photoioniza t ion and He*(2 3 S) Penning i o n i z a t i o n of formic a c i d at 300 °K. E l e c t r o n i c State AE - 2 t X A - 2 ?! A A - 2 t B A - 2 T! C A (eV) •0.17 * 0.10 Relat ive State Populations 584 a (21.22 eV) -0.22 * 0.08 100 * 10 -0.23 * 0.08 119 * 12 175 * 25 88 * 14 He(2 S) (19.82 eV) 100 * 10 58 * 6 a 258 * 26 a - 2 i D A -0.14 * 0.08 255 4 26 292 * 29 a a. Contains small contr ibutions from He*(2 lS) and He (584 % ) . -162-Table 29. A E Q b g energy s h i f t i n eV and the normalized r e l a t i v e e l e c t r o n i c state populations for 584 ft photo ioniza t ion and He*(2 3 S) Penning i o n i z a t i o n of ace t ic a c i d at 300 °K. E l e c t r o n i c State AEobs (eV) Relat ive State Populations 584 £ (21.22 eV) He»(2 S) (19.82 eV) - 2 X A -0.25 * 0.08 100 * 10 100 * 10 - 2 ' A A -0.11 * 0.08 118 * 12 90 * 9C B C -0.23 4 0.10 576 4 58 342 4 34 a D E -0.31 4 0.10 -0.31 4 0.88 339 4 34 392 4 39' a. Contains small contr ibut ions from He»(2XS) and He (584 %) -163-e l e c t r o n l c states for He*(2 1 S,2 3 S) Penning I o n i z a t i o n and 584 & photoionizat ion of formic ac id and acet ic a c i d are l i s t e d i n Tables 28 and 29 r e s p e c t i v e l y . The 3 2 A ' a n d the C 2 A " states of formic ac id are combined since they are not resolved i n the Penning e lec t ron spectra . A v i s u a l examination indicates that the r a t i o of the e l e c t r o n i c state populations ( B 2 A ' / C 2 A " ) for He*(2 3 S) Penning i o n i z a t i o n i s s i m i l a r to that for 584 A p h o t o i o n i z a t i o n . For ace t i c a c i d , the p a i r s of states ( B + C) and (D + E) are combined since they are not resolved i n the e lec t ron spectra . In comparing the e l e c t r o n i c state populations for the two modes of i o n i z a t i o n , d i f ferences are observed for the two molecules. 8.5. Discussion of the Franck-Condon Envelopes for Trans i t ions  to the Ground Ionic S ta te . In Figure 44 the He*(2*S) Penning e lec t ron and 584 A photoelectron spectra are shown f o r the ground i o n i c states of the aldehydes, acetone and the carboxylic a c i d s . The A E Q b s energy s h i f t s which have been measured for these molecules are shown below the respect ive Penning e lec t ron spec t ra . The magnitude of these s h i f t s can be v i s u a l i z e d by a comparison with the energy scale shown i n the photoelectron spectrum of ace t ic a c i d . It appears i n a l l cases that the v i b r a t i o n a l envelope observed for Penning i o n i z a t i o n i s d i f f e r e n t from that observed i n p h o t o i o n i z a t i o n . For acetaldehyde and acetone, there i s ' evidence for v i b r a t i o n a l s t ructure i n the Penning e lec t ron band. The Penning spectra are posi t ioned below the photoelectron spectra such that the tentat ive v i b r a t i o n a l assignment for the Penning band corresponds to that for p h o t o i o n i z a t i o n . This - 1 6 4 -ALDEHYDES & KETONES HCHO CH,< c o o N - C o •p o £ c .o *^  8 °c .o o> c 'c c £ -0.21 A E -0.02 CARBOXYLIC HCOOH ACIDS c o o 1 a. o N _o ? °c c £ A E « A E r r r r ITTT -0.22 -0.04 -0.31 •0.00 r r r r -0.25 -0.07 \. -0.31 •0.02 i — i — i ° e V 1 FIGURE 44. E lec t ron spectra f o r the ground i o n i c state of some carbonyl containing compounds. Energy s h i f t s are given i n eV. -165-t e n t a t i v e v i b r a t i o n a l assignment, which assumes AE i s z e r o 5 * 4 , Pi r e s u l t s i n s m a l l t r u e AE values (as shown i n F i g u r e 44). These values are o f the order o f thermal energies as has been observed f o r other m o l e c u l e s 5 1 " 5 5 . I f the v i b r a t i o n a l assignment i s c o r r e c t , the data suggest t h a t there are s i g n i f i c a n t d i f f e r e n c e s i n the Franck-Condon f a c t o r s f o r the two modes of I o n i z a t i o n . o f these molecules. T h i s behaviour has been p r e v i o u s l y observed i n a study o f water, a l c o h o l s and e t h e r s 5 4 . T h i s would imply t h a t the t a r g e t p o t e n t i a l energy s u r f a c e s are a p p r e c i a b l y m o d i f i e d by the presence of the helium p a r t i c l e . I t has been shown t h a t even a s m a l l p e r t u r b a t i o n of the p o t e n t i a l s u r f a c e could r e s u l t In s i g n i f i c a n t changes i n the Franck-Condon f a c t o r s 5 Large d i f f e r e n c e s In Franck-Condon f a c t o r s might a l s o be caused by competing a u t o i o n i z a t i o n processes where the a u t o i o n i z i n g s t a t e Is formed by resonant charge t r a n s f e r with the helium metastable. I f a competing a u t o i o n i z a t i o n process i s to e x p l a i n the change i n the Franck-Condon envelope, t h e r e would have to be an a u t o i o n i z i n g l e v e l at 19.82 eV f o r the whole s e r i e s o f molecules s t u d i e d and t h i s seems u n l i k e l y . -166-CHAPTER NINE CONCLUSIONS Of the many commonly occurr ing c h e m i - i o n i z a t i o n reactions which occur for the s ingle c o l l i s i o n process i n v o l v i n g a l o n g - l i v e d neutra l exci ted species and a neutra l target molecule, Penning i o n i z a t i o n appears to be the predominant r e a c t i o n . Employing the techniques of e lec t ron spectroscopy, valuable information about the physics of the c o l l i s i o n process has been obtained from the ejected Penning e lec t ron energy d i s t r i b u t i o n s of the various molecules s t u d i e d . Prom the r e s u l t s of t h i s study, there appears to be a number of experiments which must be performed before the Penning i o n i z a t i o n process i s f u l l y understood. From the previous discussions i t i s apparent that more information i s required about how the e l e c t r o n i c s tate populations of various molecules change as a funct ion of photoioniza t ion energy. A d d i t i o n a l information on the p a r t i a l c ross -sect ions f o r the various chemi-i o n i z a t i o n reactions would also be u s e f u l . One technique which would provide some of t h i s information i s the r e l a t i v e l y d i f f -i c u l t experiment which would measure the e lec t ron energy d i s t r i b u t i o n of a chemi- ioniza t ion e lec t ron i n coincidence with the r e s u l t i n g i o n . 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S . C . Yee, A . Hamnett 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 , i n p r e s s , E l e c t r o n Spectroscopy U s i n g E x c i t e d Atoms and Photons V Penning I o n i z a t i o n of Water A l c o h o l s and E t h e r s D . S . C . Yee 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 , i n p r e s s , E l e c t r o n Spectroscopy U s i n g E x c i t e d Atoms and Photons V I Penning I o n i z a t i o n of HCN and Some R e l a t e d Compounds D . S . C . Yee 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 , i n p r e s s , E l e c t r o n Spectroscopy U s i n g E x c i t e d Atoms and Photons V I I Penning I o n i z a t i o n of Some C a r b o n y l C o n t a i n i n g Compounds 

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