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The photoionization and dissociation of molecules Mak, Danny Shiu Hung 1966

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THE PHOTOIONIZATION AND DISSOCIATION OF MOLECULES by DANNY, SHIU HUNG, MAK B . S c . M c G i l l U n i v e r s i t y , 1960 M . S c , U n i v e r s i t y of B r i t i s h Columbia , 1962. A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n the Department of Chemist ry We accept t h i s t h e s i s as conforming to the r equ i r ed s tandard THE UNIVERSITY OF A p r i l , BRITISH COLUMBIA 1966. In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r an advanced degree a t t he U n i v e r s i t y o f B r i t i s h C o l u m b i a , I ag ree t h a t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r ag ree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by the Head o f my Department o r by h i s r e p r e s e n t a t i v e s . It i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n Department o f C H E M I S T R Y The U n i v e r s i t y o f B r i t i s h Co lumb ia Vancouver 8, Canada Date A p r i l , 1 9 6 6 The University of B r i t i s h Columbia FACULTY OF GRADUATE STUDIES •PROGRAMME OF THE - FINAL ORAL EXAMINATION FOR THE DEGREE OF DOCTOR OF PHILOSOPHY of DANNY SHUI HUNG MAK B.Sc, M c G i l l University, 1960 M.Sc, The University of B r i t i s h Columbia, 1962 TUESDAY, JULY 19, 1966, AT 3:30 P.M. IN ROOM 261, CHEMISTRY BUILDING Research Supervisors: D. C. Frost External Examiner: G. L. Weissler Department of Physics University of Southern C a l i f o r n i a University Park Los Angeles, C a l i f o r n i a COMMITTEE IN CHARGE Chairman: C. V„ Finnegan A. V. Bree D. C. Frost L. G. Harrison C. A. McDowell R. Nodwell R. Stewart C. A. McDowell THE PHOTOIONIZATION.AND DISSOCIATION OF MOLECULES ABSTRACT The photoionization and d i s s o c i a t i o n of molecules was studied using a combination of a vacuum monochromator and a mass spectrometer. The work was performed to obtain fundamental information about some simple molecules and t h e i r ions, and i t was hoped that this method, would pro-vide a good means for the determination of accurate i o n i -zation p o t e n t i a l s . Photoionization e f f i c i e n c y curves of sixteen.atoms and molecules, namely: argon, krypton, xenon, oxygen, nitrogen, carbon monoxide, chlorine, hydrogen chloride, ammonia, water, methane, methane-d^, propylene, acetylene, methyl cyanide and methyl alcohol f or the energy range from eight to twenty-one electron v o l t s were obtained. Numerical values of i o n i z a t i o n and appearance potentials were determined from the i n i t i a l onset of the photoioni-zation e f f i c i e n c y curves, and the i o n i z a t i o n potentials are discussed and compared with those obtained by other in v e s t i g a t o r s . The threshold i o n i z a t i o n p o tentials of these molecules are i n close agreement with the spectro-scopic values and are superior to those obtained by the electron impact method. The shape of the photoionization e f f i c i e n c y curve near the threshold gives an i n d i c a t i o n as to the type of electron removed i n the photoionization process, and the correct e l e c t r o n i c configuration of the molecule can 1 some-timesbe deduced from the numerical values of the i o n i z a t i o n potentials as demonstrated i n the case of methyl cyanide. The d i s s o c i a t i o n of ammonia, methane, methane-d^, propylene, acetylene, methyl cyanide and methyl alcohol was studied, and the mechanisms for the d i s s o c i a t i o n processes were discussed. From the photoionization e f f i c i e n c y curves of the fragment ions, numberical values of bond d i s s o c i a t i o n energy, i o n i z a t i o n p o tentials of r a d i c a l s and zero-point diffe r e n c e for the i s o t o p i c ions are deduced. A u t i o n i z a t i o n processes were observed i n the study of krypton, xenon, oxygen, nitrogen, carbon monoxide, hydrogen chl o r i d e and acetylene. That the peaks observed i n the photoionization e f f i c i e n c y curves of these species are indeed due to autoionization has been confirmed by comparison with corresponding peaks i n the o p t i c a l absorption spectra. The v i b r a t i o n a l . frequencies of hydro-gen chloride and acetylene i n the excited states could be deduced from the energy separation between two adjacent autoionization peaks. GRADUATE STUDIES F i e l d of Study: Chemistry Topics i n Physical Chemistry Topics i n Inorganic Chemistry Topics i n Organic Chemistry Spectroscopy and Molecular Structure Seminar i n Chemistry Quantum Chemistry. S t a t i s t i c a l Mechanics C. A. McDowell R. F. Snider J. A. R, Coope H. C. Clark N. B a r t l e t t W. R. Cullen R. Stewart J . P. Kutney R. E. I. Pincock C. Reid L. W. Reeves . K. B. Harvey J . N. Butler R. Hochstrasser R. F. Snider Related Studies: Calculus and D i f f e r e n t i a l Equations Modern Physics Elementary Quantum Mechanics S, A„ Jennings M. Bloom W. Opechowski PUBLICATION D. C. Frost, D. Mak and C. A. McDowell, "The Photoionization of Nitrogen Dioxide", Canadian Journal of Chemistry, 40y 1064 (1962) . ( i i ) ABSTRACT The present work i s concerned w i t h p h o t o i o n i z a t i o n e f f i c i e n c i e s of gases and vapors determined as a f u n c t i o n of photon energy by vacuum spect roscopy and mass a n a l y s i s . The p h o t o i o n i z a t i o n work was performed to o b t a i n fundamental i n fo rma t ion about some s imple molecu les , and i t was hoped that the r e s u l t s would p rov ide a means to e x p l a i n the apparent d i s c r e p a n c i e s of t h re sho ld i o n i z a t i o n p o t e n t i a l s p r e v i o u s l y repor ted by other workers . R e s u l t s on the p h o t o i o n i z a t i o n of s i x t e e n atoms and molecules , namely: argon, k ryp ton , • xenon, oxygen, n i t r o g e n , carbon monoxide, c h l o r i n e , hydrogen c h l o r i d e , ammonia, water , methane, methane-d4, propylene , ace ty lene , methyl cyanide and methyl a l c o h o l fo r the energy range from e igh t to twenty-one e l e c t r o n v o l t s are presented . P h o t o i o n i z a t i o n e f f i c i e n c y curves of these molecules were obta ined from which numer ica l va lues of i o n i z a t i o n p o t e n t i a l s , d i s s o c i a t i v e - i o n i z a t i o n appearance p o t e n t i a l s and d i s s o c i a t i o n energ ies are deduced, and the f i n e s t r u c t u r e and a u t o i o n i z a t i o n processes are i n t e r p r e t e d . The r e s u l t s are d i scussed and obmpared w i t h those obta ined by other i n v e s t i g a t o r s . The t h r e sho ld and inner i o n i z a t i o n p o t e n t i a l s of these molecules are i n c l o s e agreement w i t h spec t ro scop i c va lues and are supe r io r to those obta ined by the e l e c t r o n impact method. A b r i e f account of the h i s t o r i c a l developments l e a d -i n g to the present work i s d e s c r i b e d , and a few e x i s t i n g methods for the de te rmina t ion of i o n i z a t i o n p o t e n t i a l s w i t h t h e i r (iii) advantages and limitations are pointed out. The e s s e n t i a l components of the instrument and their s p e c i a l c h a r a c t e r i s t i c s are briefly discussed, and the major sources of e r r o r a re a l s o included. The limitations of the a p p a r a t u s at the p r e s e n t stage are pointed out, and improvements are suggested . The reasons for the choice of molecules for this work is m e n t i o n e d , and an outline for further work is also suggested. ( i v ) AC KNOWLEDGEMENTS I t i s a p leasure to acknowledge my g r a t i t u d e to P ro fes so r C. A. McDowell and Dr . D. C. F r o s t fo r t h e i r constant i n t e r e s t and encouragement du r ing the course of t h i s r e sea rch . I wish to express my g r a t i t u d e to Dr . C. E . B r i o n and my co l l eagues fo r t h e i r many h e l p f u l d i s c u s s i o n s . Thanks are a l so due to the t e c h n i c a l s t a f f s of the Chemistry Department at the U n i v e r s i t y of B r i t i s h Columbia fo r t h e i r s k i l f u l a s s i s t a n c e . (v) CONTENTS PAGE ABSTRACT " i i ACKNOWLEDGMENTS i v I . INTRODUCTION 1 A. Genera l 1 B. I o n i z a t i o n P o t e n t i a l s 3 1. I n t r o d u c t i o n 3 2. A d i a b a t i c and V e r t i c a l I o n i z a t i o n p o t e n t i a l s 3 3. De te rmina t ion of I o n i z a t i o n p o t e n t i a l s 5 a) O p t i c a l Spectroscopy 5 b) C y c l i c Method 6 c) E l e c t r o n Impact S tud ies 6 d) P h o t o e l e c t r o n Spectroscopy . . . . 8 e) Photon Impact Method 9 C. H i s t o r i c a l Review of P h o t o i o n i z a t i o n 11 I I . THEORETICAL 15 A. P h o t o i o n i z a t i o n 15 B . A u t o i o n i z a t i o n 17 C. Threshold Law of P h o t o i o n i z a t i o n 19 D. Theory of Mass Spectrometry 22 E . Hydrogen and Hel ium Spec t r a 24 I I I . EXPERIMENTAL A. I n t r o d u c t i o n ; 27. B . The Mass Spectrometer 30 1. Ion Source 30 2. Ana lyse r and Electromagnet 31 3. E l e c t r o n M u l t i p l i e r 33 4. V i , b r a t i bg Reed E lec t romete r . . . . . . . 33 ( v i ) CONTENTS (Continued) PAGE C. The Monochromator 35 1. L i g h t Source 35 2. G r a t i n g System 36 3. Photon Moni tor . . . ..b 37 4. Energy Convers ion Sca le 38 D. The Vacuum System 40 1. Ana lyse r Tube 40 2. Monochromator 40 3. L i g h t Source 40 4. Gas Hand l ing System 41 5. Measurement of Pressure . * 41 E . Exper imenta l Techniques 43 1. Sampling 43 2. Procedure 43 3. P h o t o i o n i z a t i o n E f f i c i e n c y Curve . . . . 44 F . CSources of E r r o r JJ46 RESULTS AND DISCUSSION IV. P h o t o i o n i z a t i o n of Atoms A. Argon >. 47 B . Kryptgn 49 C. Xenon 51 V. P h o t i o n i z a t i o n of Dia tomic Molecu les A. Oxygen 53 B . N i t rogen 58 C. Carbon Monoxide 62 D. C h l o r i n e 66 E . Hydrogen C h l o r i d e 68 ( v i i ) CONTENTS (Continued) PAGE V I . P h o t o i o n i z a t i o n of Polya tomic Molecules , A. Ammonia 71 B. Water 76 C. Methane and Deutero-methane 78 D. Propylene 84 E . Ace ty lene 87 F . Methyl Cyanide 91 G. Methanol 95 V I I . CONCLUSION 98 BIBLIOGRAPHY 102 ( v i i i ) LIST OF TABLES PAGE I . The Threshold Laws of Photon and E l e c t r o n Impact 21 I I . A u t o i o n i z a t i o n Peaks of Krypton 50 I I I . A u t o i o n i z a t i o n Peaks of Xenon 52 IV . I o n i z a t i o n P o t e n t i a l of Oxygen 54 V. A u t o i p n i z a t i o n Peaks of Oxygen 56 V I . ' . 'Threshold I . P. of N i t rogen 59 V I I . A u t o i o n i z a t i o n Peaks of N i t rogen 61 V I I I . A u t o i o n i z a t i o n Peaks of Carbon Monoxide 63 I X . A u t o i o n i z a t i o n Peaks of Hydrogen C h l o r i d e 70 X. I o n i z a t i o n P o t e n t i a l of Ammonia 73 X I . Ionization P o t e n t i a l of Methane, Deutero-^Methane. 79 X I I . I o n i z a t i o n P o t e n t i a l of Propylene . . . 85 X I I I . A u t o i o n i z a t i o n Peaks of Ace ty lene 88 XIV. R e l a t i v e I o n i z a t i o n P r o b a b i l i t i e s of Methy l Cyanide 94 XV. I o n i z a t i o n P o t e n t i a l of Methanol 96 ( i x ) LIST OF FIGURES AFTER PAGE 1. P o t e n t i a l Energy Curves 3 2. A u t o i o n i z a t i o n 17 3. S i n g l e I o n i z a t i o n Region 20 4. Hydrogen Spectrum 24 5. Hel ium Spectrum 25 6. The Monochromator and Mass Spectrometer 27 7. Mass Spectrometer Ion Source 30 8. L i g h t Source 36 9. P h o t o i o n i z a t i o n of Argon 48 10. P h o t o i o n i z a t i o n E f f i c i e n c y Curve of K r y p t o n ; . . . . 49 11. P h o t o i o n i z a t i o n E f f i c i e n c y Curve of Xenon 51 12. P h o t o i o n i z a t i o n E f f i c i e n c y Curve of Oxygen, I . . . 53 13. P h o t o i o n i z a t i o n E f f i c i e n c y Curve of Oxygen , 1 1 . 53 14. P h o t o i o n i z a t i o n E f f i c i e n c y Curve of N i t r o g e n . . . . 58 15. P h o t o i o n i z a t i o n E f f i c i e n c y Curve of Carbon Monoxide, I 62 16. P h o t o i o n i z a t i o n E f f i c i e n c y Curve of Carbon Monoxide, I I 62 17. P h o t o i o n i z a t i o n E f f i c i e n c y Curve of C h l o r i n e . . . . 66 18. P h o t o i o n i z a t i o n E f f i c i e n c y Curve of Hydrogen C h l o r i d e 68 19. P h o t o i o n i z a t i o n E f f i c i e n c y Curve of Ammonia 71 20. P h o t o i o n i z a t i o n E f f i c i e n c y Curve of Water 76 21. P h o t o i o n i z a t i o n E f f i c i e n c y Curve of I s o t o p i c Methane 78 22. Mass Spectrumof I s o t o p i c Methane 83 23. P h o t o i o n i z a t i o n E f f i c i e n c y Curve of P r o p y l e n e . . . 84 24. P h o t o i o n i z a t i o n E f f i c i e n c y Curve of A c e t y l e n e . . . 87 (x) LIST OF FIGURES (Continued) AFTER PAGE 25. P h o t o i o n i z a t i o n E f f i c i e n c y Curve of C2H + 90 26. P h o t o i o n i z a t i o n E f f i c i e n c y Curve of Methyl Cyanide . . . . , 91 27. I o n i z a t i o n E f f i c i e n c y Curves of Krypton and MethyiCyanide 91 28. Mass Spectrum of Methyl Cyanide 94 29. P h o t o i o n i z a t i o n E f f i c i e n c y Curve of Methanol . 95 CHAPTER ONE  INTRODUCTION A. General T h i s t h e s i s i s mainly concerned w i t h the r e s u l t s of the i o n i z a t i o n and d i s s o c i a t i o n of molecules when subjected to photon impact i n the wavelength r eg ion 15008 - 500A (about 8 to 21 e V . ) . U l t r a v i o l e t r a d i a t i o n i n t h i s energy range i s capable of removing the va lence e l e c t r o n from the atom or molecule , and sometimes e l e c t r o n s which are somewhat more s t r o n g l y bound. Th i s r a d i a t i o n i s a l so capable of b reak ing chemica l bonds. A 1-meter Seya-Namioka type scanning vacuum u l t r a v i o l e t monochromator coupled w i t h a N i e r - t y p e mass spectrometer was used to measure i o n i z a t i o n p o t e n t i a l s of molecules and appearance p o t e n t i a l s of t h e i r fragment i o n s . Mass spec t rome t r i c s t u d i e s of p h o t o i o n i z a t i o n r e s u l t s have of ten l ed to a be t t e r unders tanding of the v a r i o u s r e a c t i o n s which occur when molecular and fragment ions are formed by photon impact . C a r e f u l examinat ion of the d e t a i l e d form of the p h o t o i o n i -z a t i o n e f f i c i e n c y curves has i n genera l enabled i o n i z a t i o n and f ragmentat ion processes to be d i s t i n g u i s h e d and i d e n t i f i e d , and the shapes of the curves to be i n t e r p r e t e d i n terms of e l e c t r o n i c and v i b r a t i o n a l energy s t a t e s . Th i s work i s concerned w i t h three problems: (a) to determine a c c u r a t e l y the photon energy necessary for the removal of an e l e c t r o n from a molecule to form an i o n , t h i s amount of energy be ing equal to the i o n i z a t i o n p o t e n t i a l of the molecule , (b) to determine the photon energy r e q u i r e d to form X + i o n from a molecule XY, and from t h i s energy to o b t a i n the d i s s o c i a t i o n energy of the bond X - Y , and (c) to determine the products of molecular p h o t o i o n i z a t i o n , i f any, e . g . whether i n methane, the absorp t ion of photon of a g iven energy produces C H ^ + or CHg + i o n s , and i f both are p o s s i b l e , what the r e l a t i v e p r o b a b i l i t i e s are of forming them. B . I o n i z a t i o n P o t e n t i a l s 1. I n t r o d u c t i o n Dur ing the l a s t f o r t y years , the study of i o n i z a t i o n p o t e n t i a l s of gaseous atoms and molecules has occupied an i n c r e a s -i n g number of workers , and a cons ide r ab l e number of f a i r l y a c c u r a t e l y determined i o n i z a t i o n p o t e n t i a l s have appeared i n the l i t e r a t u r e . These have been made p o s s i b l e by methods of p p t i c a l spec t roscopy, pho toe lec t ron spec t roscopy, t h e o r e t i c a l and semi-e m p i r i c a l c a l c u l a t i o n s , charge t r ans f e r s p e c t r a , and e l e c t r o n and photon impact i o n i z a t i o n . 2. A d i a b a t i c and V e r t i c a l I o n i z a t i o n P o t e n t i a l s The a d i a b a t i c i o n i z a t i o n p o t e n t i a l of a molecule i s def ined as the energy r equ i r ed to remove an e l e c t r o n comple te ly from the ground v i b r a t i o n a l l e v e l o f the lowest e l e c t r o n i c s t a t e of the molecule to the ground v i b r a t i o n a l l e v e l of the r e l e v a n t e l e c t r o n i c si iate of the m o l e c u l e - i o n . The p r o b a b i l i t y of i o n i z a t i o n of a d ia tomic molecule by e l e c t r o n impact as w e l l as p h o t o i o n i z a t i o n can be cons idered to be governed by the Born-Oppenheimer a p p r o x i m a t i o n : -where ^v' i s the v i b r a t i o n a l wavefunct ion of the l e v e l v ' of the i o n , M 'VJ" i s the v i b r a t i o n a l wavefunct ion of the ground s t a t e (v"=0) of the n e u t r a l molecule , and r i s the i n t e r n u c l e a r s e p a r a t i o n . Dia tomic molecules can be t r ea ted as anharmonic o s c i l l a -t o r s , and the v i b r a t i o n a l wavefunct ions for 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 of the molecule and i t s i o n have the form shown i n F i g u r e 1 Suppose the minima of the two e l e c t r o n i c s t a t e s are v e r t i c a l l y above one another. I t i s c l e a r that the (0-0) t r a n s i t i o n has h igh p r o b a b i l i t y , whereas (0 -1 ) , (0-2) e t c . t r a n s i t i o n s have a much lower p r o b a b i l i t y , because the nega t ive par t of the i n t e g r a l p a r t l y cance l s the p o s i t i v e p a r t . The a d i a b a t i c i o n i z a t i o n p o t e n t i a l i s the d i f f e r e n c e i n energy between the ground v i b r a t i o n a l s t a t e of AB and the ground v i b r a t i o n a l s t a t e of A B + . I f 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 -tances for the molecule and i t s i o n are the same, the i o n i z a t i o n p o t e n t i a l s obta ined by o p t i c a l spec t roscopy, e l e c t r o n impact and the p h o t o i o n i z a t i o n method should a l l correspond to the a d i a b a t i c v a l u e , provided that the ins t ruments used are s e n s i t i v e enought. But when 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 ance of the i o n i c s t a t e i s g rea te r than that of the molecular ground s t a t e , the most probable t r a n s i t i o n i s to a h igher v i b r a t i o n a l l e v e l and the i o n i z a t i o n p o t e n t i a l found by these methods may be h igher than the a d i a b a t i c v a l u e . The same i s t rue when the e q u i l i b r i u m i n t e r -nuc lear d i s t ance of the i o n i c s t a t e i s l e s s than that o f the molecu la r ground s t a t e , and the h igher va lue for the i o n i z a t i o n p o t e n t i a l i s u s u a l l y c a l l e d 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 . The (0-0) t r a n s i t i o n however, s t i l l has s m a l l f i n i t e p r o b a b i l i t y , and the va lue for the i o n i z a t i o n p o t e n t i a l obta ined by e l e c t r o n and photon impact should depend l a r g e l y on the s e n s i t i v i t y fo r d e t e c t i o n of i o n s . The genera l p i c t u r e fo r polyatomic molecules should be s i m i l a r . In p h o t o i o n i z a t i o n s t u d i e s , i o n i z a t i o n p o t e n t i a l s obta ined from the onset of the p h o t o i o n i z a t i o n e f f i c i e n c y curves are i n genera l i n good agreement w i t h the a d i a b a t i c i o n i z a t i o n p o t e n t i a l s obta ined by other methods. When i o n i z a t i o n i s caused by the removal of a bonding e l e c t r o n and the minima of the ground and i o n i c e l e c t r o n i c s t a t e s are not v e r t i c a l l y above one another, the p h o t o i o n i z a t i o n e f f i c i e n c y curve u s u a l l y shows curva tu re near the t h r e s h o l d . 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 can be measured from the curve at the p o i n t of s teepest s lope before the curve reaches the f i r s t maximum. 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 of a molecule w i l l always be equal to or h igher than the a d i a b a t i c v a l u e . In f a c t , exper imenta l evidence has shown that the d i f f e r e n c e i s u s u a l l y between 0.02 to 0 .5 e l e c t r o n v o l t . 3. De te rmina t ion of I o n i z a t i o n P o t e n t i a l s I o n i z a t i o n p o t e n t i a l s are among the most important p r o p e r t i e s of a molecule , and i t i s d e s i r a b l e to have methods of de te rmin ing them a c c u r a t e l y . a) The O p t i c a l Spec t roscop ic Method One of the most accurate methods fo r the de t e rmina t ion of i o n i z a t i o n p o t e n t i a l s employs o p t i c a l spec t roscopy . T h i s i n v o l v e s a study of absorp t ion s p e c t r a , and the f i t t i n g of the data i n t o Rydberg s e r i e s : -v = I — R —-_ 1.2 (n + a ) 2 where: I and a are cons tan ts s p e c i f i c to a p a r t i c u l a r molecule , v i s the wavelength of a p a r t i c u l a r molecule , R i s the Rydberg cons tan t , n i s an i n t e g r a l va lue r e p r e s e n t i n g the p a r t i c u l a r Rydberg band. Once the Rydberg (0-0) t r a n s i t i o n i s i d e n t i f i e d by a n a l y s i s of the Rydberg bands o f the absorp t ion s p e c t r a , or by comparison wi th the spec t r a of i s o t o p i c molecu les , the i o n i z a t i o n p o t e n t i a l of a molecule can r e a d i l y be c a l c u l a t e d . The wave-leng th of s p e c t r a l bands i n spec t ro scop i c work can be measured w i t h a high degree of accuracy, and the u n c e r t a i n t y i n the va lue of the i o n i z a t i o n p o t e n t i a l de r ived from i t i s u s u a l l y on ly a few pa r t s of a thousand. However, t h i s method cannot be app l i ed to q u i t e a l a r g e number of molecules which g ive cont inuous or d i f f u s e d s p e c t r a , and an unambiguous assignment of the Rydberg t r a n s i t i o n s i s not then p o s s i b l e . b) The C y c l i c Method When the abso rp t ion spectrum of a molecule i s so complex that the Rydberg s e r i e s l e a d i n g to the ground s t a t e of the i o n cannot be ob ta ined , some workers (84) have used a c y c l i c method to determine the a d i a b a t i c i o n i z a t i o n p o t e n t i a l i n d i r e c t l Th i s method i s based on the f o l l o w i n g e q u a t i o n : -I(XY) + D Q ( X Y + ) = I (X) + D Q (XY) 1.3 where I(XY) i s the i o n i z a t i o n p o t e n t i a l of XY, I(X) i s the i o n i z a t i o n p o t e n t i a l of X , (XY) and D Q ( X Y + ) are the d i s s o c i a -t i o n energ ies of the molecule and the i o n r e s p e c t i v e l y . Tf I ( X ) , D Q (XY) and D Q ( X Y + ) are known a c c u r a t e l y , the I . P . ( X Y ) of the molecule can be determined. c) The E l e c t r o n Impact Method The e s s e n t i a l s fo r making e l e c t r o n impact measurements are a beam of e l e c t r o n s of known energy which may be passed through the gas under i n v e s t i g a t i o n , and a dev ice for d e t e c t i n g the ions produced and for measuring t h e i r i n t e n s i t y . E l e c t r o n impact s t u d i e s have been h i g h l y developed, and are fa r more g e n e r a l l y a p p l i c a b l e because molecules w i t h s t r o n g l y bound e l e c t r o n s may be i n v e s t i g a t e d . , . Eoas jnany molecules , the e l e c t r o n impact method prov ides the on ly way to determine the i o n i z a t i o n p o t e n t i a l . The e l e c t r o n impact method s u f f e r s from s e v e r a l de f ec t s . The f i r s t a r i s e s through us ing an e l e c t r o n beam emit ted from a hot f i l a m e n t . Th i s e l e c t r o n beam w i l l not be monoenerger t i c i n cha rac te r but w i l l possess an energy spread of about one e l e c t r o n v o l t which w i l l be mainly Maxwel l -Bol tzman i n nature , and i s , of course , governed by the temperature of the f i l a m e n t . In a d d i t i p n , a fu r the r energy spread w i l l be imparted to the e l e c t r o n beam by the v a r i a t i o n of temperature a long the f i l ament due to conduct ion of heat through the suppo r t i ng l eads , and by the v o l t a g e drop across the f i l a m e n t . S ince e l e c t r o n s are charged p a r t i c l e s , the e l e c t r i c f i e l d which i s necessary to produce an i on beam i n the i o n source u s u a l l y a l so per turbs the e l e c t r o n energy. The d i f f i c u l t y of o b t a i n i n g an e l e c t r o n beam w i t h s u f f i c i e n t l y low energy spread causes much of the informat t i o n obta ined by the e l e c t r o n impact method to be of low p r e c i -s i o n , and f i n e d e t a i l s i n the i o n i z a t i o n e f f i c i e n c y curves to be un reso lved . The d i m i n u t i o n of the energy spread i n the e l e c t r o n beams used i n the e l e c t r o n impact method i s at present under i n v e s t i g a t i o n i n t h i s and olther l a b o r a t o r i e s u s ing e l e c t r o s t a t i c s e l e c t o r s . Success i n t h i s f i e l d of study w i l l g r e a t l y improve the accuracy of va lues of the i o n i z a t i o n p o t e n t i a l s obta ined by the e l e c t r o n impact method. Wannier (131) proposed a theory for the i o n i z a t i o n of molec | i le by e l e c t r o n impact near the t h r e s h o l d . He s t a ted that jfche two slow e l e c t r o n s , upon emerging from the i o n , remain 8. w i t h i n the r e a c t i o n zone at the t h r e sho ld energy of the impac t ing e l e c t r o n . Only when each slow e l e c t r o n has apprec iab le k i n e t i c energy can i t escape from t h i s r e g i o n and on ly then does the i o n cur ren t s t a r t to grow. For t h i s reason, i t i s q u i t e p o s s i b l e that i o n i z a t i o n c ross s e c t i o n s are zero or v a n i s h i n g l y s m a l l for e l e c t r o n ( impact ing) of an energy equal to or j u s t exceeding the t h r e sho ld i o n i z a t i o n energy. I f t h i s i s so, i t p laces a r e s t r i c t i o n on the u l t i m a t e accuracy ob t a inab l e for e l e c t r o n impact measurements of i o n i z a t i o n p o t e n t i a l s , even w i t h improved methods of o b t a i n i n g e f f e c t i v e l y monoenergetic e l e c t r o n beams. d) Pho toe l ec t ron Spectroscopy T h i s i s a r a the r new technique repor ted by Kurbatov, V i l e s o r and Teren in (66) , Schoen (107) and Turner and A l - J o b o u r y (127-129), for the d i r e c t measurement of i o n i z a t i o n p o t e n t i a l s of a molecule l e s s than 21.21 eV. The gas under study i s i l l u m i n a t e d by a beam of photons of energy 21.21 eV. These photons can cause the emiss ion of pho toe l ec t rons , and a c y l i n d r i c a l energy ana lyzer i s used to study the pho toe l ec t ron energy d i s t r i b u t i o n . A photo-e l e c t r o n energy spectrum c o n s i s t s o f peaks which can be shown to lead d i r e c t l y to the v i b r a t i o n a l and e l e c t r o n i c energy l e v e l s of the molecu le . The i o n i z a t i o n p o t e n t i a l s of qu i t e a number of mole-c u l e s have been repor ted u s ing t h i s method and i n some favorab le cases v i b r a t i o n a l s t r u c t u r e can a l so be seen. F r o s t , McDowell and Vroom (41) have repor ted r e c e n t l y the use of a s p h e r i c a l energy ana lyzer of g r e a t l y improved r e s o l u t i o n to study the pho toe l ec t ron energy d i s t r i b u t i o n of hydrogen. They have been able to measure a c c u r a t e l y the f i r s t f i v e v i b r a t i o n a l energy l e v e l s of (3 Z, g)> a n d the 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 to them. The v i b r a t i o n a l s t r u c t u r e of the hydrogen i o n i n the p h o t o i o n i z a t i o n e f f i c i e n c y curve i s known (13, 20) to be obscured comple te ly by a u t o i o n i z a t i o n , and the f i v e c l e a r l y r e so lved "s teps" i n the pho toe lec t ron r e t a r d i n g curve for hydrogen i n d i c a t e tha t a u t o i o n i z a t i o n may be avoided i n s p h e r i c a l pho toe l ec t ron s p e c t r o s -copy. e) The Photon Impact Method Th i s method c o n s i s t s o f a combinat ion of both p h o t o i o n i -z a t i o n and mass spect rometry . A sample of gaseous molecules to be i n v e s t i g a t e d i s i l l u m i n a t e d by a beam of monochromatic u l t r a v i o l e t r a d i a t i o n . I f the photon energy of the r a d i a t i o n i s g r a d u a l l y i nc reased , success ive s tages of e x c i t a t i o n can be reached u n t i l the energy of the photon reaches a c e r t a i n va lue when i o n i z a t i o n takes p l a c e . The i o n i z a t i o n p o t e n t i a l of a molecule can be obta ined from the po in t of i n i t i a l onset of i o n i z a t i o n . With fu r the r i n -crease of photon energy, a curve of p h o t o i o n i z a t i o n e f f i c i e n c y as a f u n c t i o n of photon energy can be d e r i v e d . In favourab le cases the upper energy s t a t e s and the v i b r a t i o n a l l e v e l s of the molecule and i t s ions can b e . s t u d i e d from p h o t o i o n i z a t i o n e f f i c i e n c y cu rves . A mass spectrometer i s used to focus the p a r t i c u l a r i o n under i n v e s -t i g a t i o n , and to measure the i n t e n s i t y of i o n cu r r en t s produced. The p h o t o i o n i z a t i o n method i s fa r more g e n e r a l l y a p p l i c -able than o p t i c a l spec t roscopy, as some molecules w i t h complex s p e c t r a which do not e x h i b i t w e l l def ined Rydberg s e r i e s can r e a d i l y be i n v e s t i g a t e d by the former method. The reasons for the s u p e r i o r i t y of t h i s method are t h r e e f o l d . F i r s t l y , i t i s much e a s i e r to o b t a i n monoenergetic photons than i t i s to o b t a i n monoenergetic e l e c t r o n s ; secondly , a gas to c a l i b r a t e the energy s c a l e needs not be in t roduced i n t o the 10. mass spectrometer c o n c u r r e n t l y w i t h the molecule to be s t ud i ed as i s the r egu la r p r a c t i c e i n the e l e c t r o n impact method; and t h i r d l y , the steep r i s e i n p h o t o i o n i z a t i o n c ross s e c t i o n at the i o n i z a t i o n t h r e sho ld permits a sharp s e p a r a t i o n of i o n - f o r m a t i o n processes . The photon impact method i s i n h e r e n t l y more accura te , and numer ica l vaihues of i o n i z a t i o n and appearance p o t e n t i a l s based on t h i s method are i n e x c e l l e n t agreement w i t h those obta ined by o p t i c a l spec t roscopy . P r e c i s e data have thus been made a v a i l a b l e for a l a rge number of molecules , and i n f o r m a t i o n obta ined from the p h o t o i o n i z a t i o n e f f i c i e n c y curves can be c o r r e l a t e d w i t h molecular p r o p e r t i e s and molecular s t r u c t u r e , and to check the t h e o r i e s of t h r e sho ld laws and processes of a u t o i o n i z a t i o n . The r e s u l t s f u r -n i s h r e l i a b l e i n fo rma t ion for some t h e o r e t i c a l and s e m i - e m p i r i c a l molecular quantum mechanical c a l c u l a t i o n s which are now being made. 11. C- H i s t o r i c a l Review of P h o t o i o n i z a t i o n S ince the d i s cove ry of the i o n i z a t i o n of gases by X - r a y s , and the p h o t o e l e c t r i c e f f e c t of l i g h t on s o l i d s and meta ls , s e v e r a l i n v e s t i g a t i o n s have been made on the i o n i z a t i o n of gases when exposed to u l t r a v i o l e t r a d i a t i o n . H e r t z (46) i n 1887, when performing experiments on the spa rk ing between e l e c t r o d e s , observed that u l t r a v i o l e t r a d i a t i o n cou ld be used to i o n i z e gases. Lenard (67) i n 1901, c a r r i e d out some s i m i l a r experiments and found that a i r was made conduct ing under the a c t i o n of a very absorbable k ind of u l t r a v i o l e t l i g h t . S t a rk (110) i n v e s t i g a t e d the e f f e c t of u l t r a v i o l e t l i g h t on the c o n d u c t i v i t y of gases and obta ined r e s u l t s w i t h c e r t a i n o rgan ic compounds i n the vapor s t a t e : anthracene, diphenylmethane, diphenylamine and oC_-naphthy 1 amine . Dur ing the pe r iod 1920-1930, experiments on p h o t o i o n i z a -t i o n were conducted most ly w i t h vapors of the a l k a l i meta l s , e s p e c i a l l y caesium, rubid ium and potassium. The exper imenta l techniques were, as a whole, g r e a t l y improved a f te r 1920. However, l a c k ofbhigh r e s o l v i n g power and the r e l a t i v e l y low s e n s i t i v i t y of the ins t ruments used has l e f t the data of many of these workers i n a somewhat u n c e r t a i n s t a t e . A cons ide rab l e amount of data has been c o l l e c t e d and reviewed by s e v e r a l w r i t e r s (57 ,97) . Between 1930 and 1950, very l i t t l e work concern ing pho-t o i o n i z a t i o n appeared i n the l i t e r - a t u r e , and the study of mole-c u l a r i o n i z a t i o n seemed to be dominated mainly by e l e c t r o n impact s t u d i e s u s ing mass spect rometers . Many i n v e s t i g a t o r s have used the e l e c t r o n impact meithod to measure i o n i z a t i o n p o t e n t i a l s of molecules and appearance p o t e n t i a l s of fragment i o n s , to d e t e r -mine the energy needed to s p l i t up a polya tomic molecule i n t o 12. s p e c i f i e d r a d i c a l s , and to determine i o n i z a t i o n p o t e n t i a l s of r a d i c a l s . Hundreds of papers have a l ready been pub l i shed i n t h i s f i e l d and i o n i z a t i o n p o t e n t i a l s of s e v e r a l hundred molecules have been s t u d i e d . In many cases , t h i s method has provided the on ly a v a i l a b l e da ta . The d i f f i c u l t y o f o b t a i n i n g an e l e c t r o n beam w i t h s u f f i -c i e n t l y low energy spread and a c c u r a t e l y known energy caused much of the i n f o r m a t i o n from the e l e c t r o n impact method to be inaccus-r a t e . The d5orm of the i o n i z a t i o n p r o b a b i l i t y curve fo r a s i n g l e process i s such that when s e v e r a l are superimposed, the r e s o l u t i o n of separate th re sho ld p o t e n t i a l s i s d i f f i c u l t . A f t e r 1950, p h o t o i o n i z a t i o n s t u d i e s resumed t h e i r steady pace, l a r g e l y due to the development of ingenious designs for g r a t i n g monochromators and be t t e r means for producing and measur-i n g photon rad i a t i o n i n the far; . u l t r a v i o l e t . The development of a low-cos t g r a t i n g monochromator by Seya (108) and Namioka (89) helped to f a c i l i t a t e the s t u d i e s of p h o t o i o n i z a t i o n . I t i s b a s i c a l l y a one-meter monochromator, and the gas pressure i n the monochromator i s mainta ined at a pressure -5 of about 10 mm of Hg. by d i f f e r e n t i a l pumping. I t has a f i x e d entrance and e x i t s l i t system, and the wavelength of the monochro-matic l i g h t pass ing through the e x i t s l i t may be a l t e r e d by r o t a t i n g the g r a t i n g . Johnson et a l (62) i n 1951, found tha t c o a t i n g a pho toe l ec t ron m u l t i p l i e r w i t h a t h i n l a y e r of f l u o r e s c e n t m a t e r i a l rendered i t s a t i s f a c t o r y for the measurement of f a r u l t r a v i o l e t r a d i a t i o n i n t e n s i t y . Sodium s a l i c y l a t e was found to be best s u i t e d s ince i t i s s t a b l e , does not evaporate i n vacuo and g ives r e p r o d u c i b l e r e s u l t s up to 850$. Furthermore, i t s response i s 13. e x c e l l e n t and i t s quantum e f f i c i e n c y i s nea r ly constant (62) . In 1953, Watanabe, Marmo and Inn (133) and Wainfan, Walker , and We i s s l e r (141) repor ted data on p h o t o i o n i z a t i o n meas-urements i n the vacuum u l t r a v i o l e t r e g i o n . The t o t a l absorption-c r o s s - s e c t i o n of a molecule was measured i n an abso rp t ion c e l l , and the i o n i z a t i o n was found to correspond to the long wavelength l i m i t of the i o n i z a t i o n continuum. They showed that the accurate de te rmina t ion of i o n i z a t i o n p o t e n t i a l s i s p o s s i b l e by u t i l i z i n g monochromatic l i g h t of 0.001 eV. band w i d t h . However, t h e i r methods do not g ive any i n f o r m a t i o n about the products which are formed by p h o t o i o n i z a t i o n , and no mass analyses were in t roduced to d i f f e r e n t i a t e between the parent and fragment i o n s . A l s o ions may a r i s e from any i m p u r i t i e s i n the sample. Because of the l a c k of mass a n a l y s i s , one has to be q u i t e sure tha t the sample being s tud i ed i s f ree of i m p u r i t i e s w i t h lower i o n i z a t i o n p o t e n t i a l s . Threshold i o n i z a t i o n p o t e n t i a l s of more than a hundred molecules were repor ted i n 1959 by Watanabe (139). H i s r e s u l t s are comparable to those obta ined by s p e c t r o s c o p i c methods. Te ren in and Popov (124) were the f i r s t to use mass ana-l y s i s ( i n the p h o t o i o n i z a t i o n of t h a l l i u m h a l i d e s ) . More r e c e n t l y , L o s s i n g and Tanaka (70) used a vacuum u l t r a v i o l e t l i g h t source for the genera t ion of ions i n a mass spectrometer . A kryp ton d i s -charge lamp w i t h a l i t h i u m f l u o r i d e window, e m i t t i n g the two resonance l i n e s 12368 (10.03 eV.) and 1165A* (10.64 eV.) provided enough energy to cause p h o t o i o n i z a t i o n of acetone, butadiene, butene, propylene , a n i s o l e , a l l y l i o d i d e , d imethy l mercury e t c . , but not enough to form i o n i c fragments. The i r experiments su f -fered from the defect of a f i x e d photon energy, so the p h o t o i o n i -z a t i o n y i e l d cou ld not be s tud i ed as a f u n c t i o n of energy. 14. Teren in and V i l e s s o v (125) and M o r r i s o n , H u r z e l e r and ,Inghram (58, 83) used a combinat ion of vacuum monochromator and mass spectrometer i n a d e t a i l e d study of the format ion of ions by photon impact . The source of l i g h t was a h igh v o l t a g e hydrogen lamp, and a l i t h i u m f l u o r i d e window was used to i s o l a t e the r e s i -dua l gases i n the l ight source from the i o n i z a t i o n chamber. The l i t h i u m f l u o r i d e window cuts o f f r a d i a t i o n below 10508 (11.50 eV.) and molecules w i t h i o n i z a t i o n p o t e n t i a l s g rea te r than t h i s cannot be s t u d i e d . W e i s s l e r , Samson, Ogawa and Cook (146) and Comes and Lessmann (10) , u s ing a low-pressure r e p e t i t i v e spark source and d i f f e r e n t i a l pumping to r e t a i n a low pressure i n t h e i r apparatus were able to o b t a i n p h o t o i o n i z a t i o n r e s u l t s up to about 30 eV. wi thou t u s i n g a l i t h i u m f l u o r i d e window. However, t h e i r l i g h t source provided a wide ly - spaced l i n e spectrum, and so there were of course "gaps" i n t h e i r p h o t o i o n i z a t i o n e f f i c i e n c y cu rves . Th i s f r equen t ly made i t imposs ib l e to determine the onset of i o n i z a t i o n processes w i t h i n s e v e r a l tenths of a v o l t . R e c e n t l y , D i b e l e r , Krauss , Reese and H a r t l e e (19) have developed a Hinteregger type photon source which p rov ides a H o p f i e l d he l ium continuum, and have s tud i ed normal and deu te ra -ted hydrogen, methane and benzene. Cook and Metzger (12) have computed 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 of s e v e r a l hydrogen-c o n t a i n i n g molecules from i o n i z a t i o n spec t r a u s ing the H o p f i e l d he l ium continuum as a cont inuous backg round- r ad i a t i on source . The development of cont inuous photon spec t r a marked a major advance i n t h i s f i e l d . 15. CHAPTER TWO  THEORETICAL A. P h o t o i o n i z a t i o n and D i s s o c i a t i v e I o n i z a t i o n In t h i s work, we are concerned mainly w i t h i o n i z i n g c o l l i s i o n s of photons w i t h atoms and molecu les . A photon of energy hv can be absorbed by an atom or a molecule to take the system from a s t a t e of lower energy E" to a s t a t e o f h igher energy E ' , i . e . hv = E ' - E" 2.1 where h i s P l a n c k ' s cons tan t , v i s the frequency of the r a d i a -t i o n . The absorp t ion of r a d i a t i o n by the molecule XY can be represented b y : -XY + hv —»XY* 2.2 where XY and XY* are ground and e x c i t e d s t a t e s of the molecule r e s p e c t i v e l y . Th i s process i s c a l l e d p h o t o e x c i t a t i o n . I o n i z a t i o n can be caused by the i n t e r a c t i o n of a photon of s u f f i c i e n t l y h igh energy to cause the process : XY +.. hv —>XY+ + e 2.3 t h i s process i s c a l l e d p h o t o i o n i z a t i o n , and the photon energy necessary for t h i s process i s c a l l e d the t h r e sho ld i o n i z a t i o n p o t e n t i a l of XY. Beyond the energy of the i o n i z a t i o n t h r e s h o l d , there w i l l be a r e g i o n of cont inuous a b s o r p t i o n . The measure-ment of p o s i t i v e i o n cur ren t as a f u n c t i o n of photon energy prov ides the usua l method of s t udy ing t h i s process . I f the abso rp t ion of r a d i a t i o n leads to the f o l l o w i n g r e a c t i o n : XY + hv >X + + Y" + e 2.4 16.. the process i s c a l l e d d i s s o c i a t i v e i o n i z a t i o n , and the photon energy necessary for t h i s r e a c t i o n i s c a l l e d the appearance p o t e n t i a l of X + . The d i s s o c i a t i o n energy or bond s t reng th of the molecule D ( X - Y ) , can be c a l c u l a t e d from the appearance p o t e n t i a l by the f o l l o w i n g r e l a t i o n s h i p : V ( X + ) = I (X) + D(X-Y) + K . E . + E . E 2 .5 where V ( X + ) i s the appearance p o t e n t i a l of X , I(X) i s the i o n i z a t i o n p o t e n t i a l of r a d i c a l X, K . E . i s the k i n e t i c energy w i t h which X + and Y ' may be endowed and E . E . i s any e x c i t a t i o n energy they may have. Most atomic i o n i z a t i o n p o t e n t i a l s are known from o p t i c a l spec t roscopy, and i f K . E . and E . E . are known, the d i s s o c i a t i o n energy can be obta ined by measurement of the appearance p o t e n t i a l . Otherwise , the appearance p o t e n t i a l g ive s an upper l i m i t for the* energy necessary to break the X - Y bond p lus the i o n i z a t i o n energy of X . B . A u t o i o n i z a t i o n In i o n i z a t i o n e f f i c i e n c y curves for molecular ions produced by e l e c t r o n impact, breaks are o f t en observed at ener -g i e s which do not correspond to any known e l e c t r o n i c s t a t e of the p o s i t i v e i o n spec ies formed. Many peaks i n p h o t o i o n i z a t i o n e f f i c i e n c y curves are a l s o observed at energ ies above the t h r e sho ld i o n i z a t i o n p o t e n t i a l , and the p o s i t i o n s of these peaks correspond c l o s e l y w i t h some of those obta ined from absorp t ion s p e c t r a . Th i s phenomenon has been e s t a b l i s h e d by s e v e r a l workers (10, 27, 54, 91) as due to a u t o i o n i z a t i o n of a h i g h l y e x c i t e d s t a t e of the atom or molecule concerned. A u t o i o n i z a t i o n phenomenon can be exp la ined by r e c i p r o c a l i n t e r a c t i o n of d i s c r e t e s t a t e s w i t h one or more con t inua as shown i n F i g u r e 2. The e x c i t a t i o n of an e l e c t r o n i n s e r i e s 2 from the ground s t a t e to 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 continuum of s e r i e s 1 i s fo l lowed r a p i d l y by a n o n - r a d i a t i v e t r a n s i t i o n to the continuum at the same energy. The e x c i t e d atom or molecule becomes i o n i z e d by l o s i n g one of i t s e l e c t r o n s . S ince t h i s process i s governed mainly by p h o t o e x c i t a t i o n , and the t h r e sho ld law for e x c i t a t i o n i s a d e l t a f u n c t i o n , peaks should be observed i n the p h o t o i o n i z a t i o n e f f i c i e n c y curve for a u t o i o n i z a t i o n processes . Massey (78) proposed another mechanism for the a u 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 imagine d i s c r e t e s t a t e s of the normal molecule i n which two or more e l e c t r o n s are e x c i t e d . An atom or molecule i n such a c o n d i t i o n w i l l not u s u a l l y g ive up i t s energy of e x c i t a t i o n by r a d i a t i o n . Instead i t w i l l break up much more q u i c k l y i n the f o l l o w i n g way. Of the two e x c i t e d e l e c t r o n s , one drops to a more f i r m l y bound s t a t e , thereby r e l e a s i n g energy' which i s absorbed by the other to take i t f ree of the atom or 0 series 1 series 2 electron exit energies photo-ion current Figure 6V 18. molecule a l t o g e t h e r . The e x c i t e d atom or molecule the re fore becomes i o n i z e d . In the work repor ted here, a u t o i o n i z a t i o n processes are found to occur s t r o n g l y i n atoms and d ia tomic molecules such as k ryp ton , xenon, oxygen, n i t r o g e n , carbon monoxide and hydrogen c h l o r i d e . For methane and numerous hydrocarbon molecu les , no a u t o i o n i z a t i o n s t r u c t u r e seems to be present . The p o s s i b l e e x c i t a t i o n s i n such molecules appear to r e s u l t on ly i n cont inuous abso rp t ion , and do not g ive d i s c r e t e peaks i n the p h o t o i o n i z a t i o n e f f i c i e n c y cu rves . 19. C. The Threshold Laws In order to determine r e l i a b l e i o n i z a t i o n l i m i t s from observed p h o t o i o n i z a t i o n e f f i c i e n c y curves , i t i s necessary to i n t e r p r e t the shape of the curves , e s p e c i a l l y near the t h r e s h o l d . The t h r e sho ld law desc r ibes the v a r i a t i o n of the i o n i z a t i o n e f f i c i e n c y w i t h energy above the t h r e s h o l d . The i o n cu r ren t produced by e l e c t r o n impact inc reases l i n e a r l y as the energy of the impac t ing e l e c t r o n i s inc reased above the t h r e s h o l d . Wigner (147) has shown that for a s i n g l y charged atom or molecule the i o n i z a t i o n e f f i c i e n c y v a r i e s w i t h the 1.12th power of the excess energy. However, Fox (51) showed that the i o n i z a t i o n e f f i c i e n c y curve for he l ium s i n g l y charged i s l i n e a r over the f i r s t e i gh t v o l t s above the t h r e s h o l d . I t has been e s t a b l i s h e d by p h o t o i o n i z a t i o n s t u d i e s (83,91,136,146) that the t h r e sho ld law for d i r e c t s i n g l e i o n i z a t i o n , when induced by photon impact, i s approximate ly a s tep f u n c t i o n of excess photon energy, and that fo r double i o n i z a t i o n i s approximately a l i n e a r f u n c t i o n . The a u t o i o n i -z a t i o n process , induced by photon impact, i s governed mainly by the p h o t o e x c i t a t i o n of an e l e c t r o n i n the normal molecu le . The appearance of sharp peaks fo r the a u t o i o n i z a t i o n process i n the p h o t o i o n i z a t i o n e f f i c i e n c y curve i n d i c a t e s tha t the t h r e sho ld law for p h o t o e x c i t a t i o n i s a d e l t a f u n c t i o n . Wannier (131) proposed a theory of the t h r e sho ld law for m u l t i p l e i o n i z a t i o n . The two slow e l e c t r o n s , upon emerging from the i o n a f te r the s i n g l e i o n i z a t i o n by: e l e c t r o n impact, remain w i t h i n a s p h e r i c a l r e a c t i o n zone of r ad ius b around the i o n . Only when each e l e c t r o n has a k i n e t i c energy T greater than e^/b can i t escape from t h i s r e g i o n . In F i g u r e 3, the k i n e t i c energ ies and T2 of the two emerging e l e c t r o n s are p l o t t e d , and the r e g i o n i n which i o n i z a t i o n occurs i s ' l i m i t e d by two s t r a i g h t l i n e s p a r a l l e l to the axes. Now a l i n e of constant energy excess A E i n t h i s diagram i s s t r a i g h t and of s lope - 1 ; the segment of i t l e a d i n g to i o n i z a t i o n i s marked i n the f i g u r e . C l e a r l y , the l eng th of t h i s segment i s p ropor -t i o n a l to the energy excess and so the i o n i z a t i o n e f f i c i e n c y inc reases l inear ly w i t h the excess energy. F i g . 3. Region i n which i o n i z a t i o n occurs i n the case of s i n g l e i o n i z a t i o n . and T2 are the k i n e t i c energ ies of the two emerging e l e c t r o n s , and E i s the energy excess . In s i n g l e i o n i z a t i o n by p h o t o i o n i z a t i o n , on ly one slow e l e c t r o n emerges from the i o n . When the e l e c t r o n has a k i n e t i c energy T grea te r than e^/b, p h o t o i o n i z a t i o n takes p l a c e . The p h o t o i o n i z a t i o n e f f i c i e n c y remains constant a f t e r the t h r e sho ld energy p lus the k i n e t i c energy T, and the excess energy of the i n c i d e n t photon i s c a r r i e d away by the slow e l e c t r o n . The p h o t o i o n i z a t i o n e f f i c i e n c y fo r s i n g l e i o n i z a t i o n i s the re fore a s tep f u n c t i o n . 2 1 . D e t a i l e d t h e o r e t i c a l c a l c u l a t i o n s by Geltman (43) of i o n i z a t i o n by photon and e l e c t r o n impact have e s t a b l i s h e d that the i o n i z a t i o n e f f i c i e n c y above the t h r e sho ld energy would be determined by the c o n d i t i o n s of the d i s t r i b u t i o n of excess energy of the i n c i d e n t p a r t i c l e s among the s epa ra t i ng p a r t i c l e s . Table I summarises the th re sho ld laws for i o n i z a t i o n and e x c i t a t i o n by photon and e l e c t r o n impact . Table I The Threshold Laws Number of s epa ra t i ng Types of r e a c t i o n s I o n i z a t i o n F u n c t i o n p a r t i c l e s E f f i c i e n c y 1 A + hv —»A* d e l t a A + e —>A~ 1 2 A + hv —*A+ + e s tep A + e • A * + e 3 A + hv A + + + 2e _y l i n e a r A + e A + + 2e Thus, we would expect from the p h o t o i o n i z a t i o n e f f i c i e n c y curves a d e l t a f u n c t i o n for a p h o t o e x c i t a t i o n process , and a s t e p - f u n c t i o n for a s i n g l e p h o t o i o n i z a t i o n process . These are, however, i d e a l i z e d s i t u a t i o n s , and the a c t u a l curves obta ined do not always show pe r fec t s teps or d e l t a f u n c t i o n s . Some f i n e s t r u c t u r e and s teps cor responding to the h igher i o n i z a t i o n p o t e n t i a l s of s e v e r a l molecules cannot be obta ined because of the i n t e r f e r e n c e and compe t i t i on of m u l t i p l e a u t o i o n i z a t i o n processes . 22 D. Theory of the Mass Spectrometer The term 'mass spec t rometer ' i s now u s u a l l y restricdberh to an instrument i n which separated i o n beams are measured e l e c t r i c a l l y . I t w i l l be u s e f u l to w r i t e down the s imple equations governing the motion of a charged i o n i n a mass spectrometer . I f the charged p a r t i c l e of mass M(g) , charge e ( e . s . u . ) and v e l o c i t y v (cm per sec) i s sent i n t o a magnetic f i e l d of fo rce H ( e . s . u . ) , the equat ion of motion may be desc r ibed as f o l l o w s . S ince the force i s always at r i g h t angles to the d i r e c t i o n of motion of the p a r t i c l e , there i s no l i n e a r , hut a constant angular a c c e l e r a t i o n . From elementary mechanics, i t i s seen that the p a r t i c l e w i l l exper ience a c e n t r i f u g a l f o r c e , and fo r e q u i l i b r i u m t h i s must balance the fo rce due to the magnetic f i e l d , i . e . Hev 2 .6 R where R i s the r a d i u s of curva tu re of the. i o n beam. I f i t i s assumed now that the charged p a r t i c l e acqui res i t s v e l o c i t y by f a l l i n g through an e l e c t r o s t a t i c p o t e n t i a l d i f f e r e n c e V ( e . s . u . ) , the p o t e n t i a l energy must be the same as the k i n e t i c energy of the p a r t i c l e a f t e r a c c e l e r -a t i o n , i . e . Mv 2 - g - = eV . . 2 . 7 I f equations 2 .6 and 2 . 7 are combined, e l i m i n a t i n g v , then 23. The equat ion 2.8 may be termed the mass spectrometer equa t ion . In the mass spectrometer used i n the present work, the r ad ius of curva ture of the charged p a r t i c l e i s f i x e d , and when a n a l y s i s of the ions formed from a g iven molecule i s d e s i r e d , e i t h e r the i o n acce lera t ing v o l t a g e i s maintained cons tan t , and the magnetic f i e l d s t r eng th v a r i e d c o n t i n u o u s l y (magnetic scanning) , or the accelera t ing v o l t a g e i s v a r i e d keeping the magnetic f i e l d s t r eng th constant (vo l tage scann ing) . 24 E . The Hydrogen and Hel ium Spec t ra In order to o b t a i n a monochromatic photon beam of v a r y i n g energy i t i s necessary to e x c i t e gaseous molecules such as hydrogen or hel ium i n the l i g h t source . The l i g h t emit ted from the source i s scanned by a 30,000-lines per inch g r a t i n g to g ive a spectrum of photon i n t e n s i t y as a f u n c t i o n of photon ene rg i e s . The spectrum obta ined i s c h a r a c t e r i s t i c of the gas employed i n the l i g h t source: tha t of hydrogen i s u s e f u l from 90oS - 1500& (8 - 14 e V . ) , and that of he l ium i s u s e f u l from 500& - 120o8 (10 - 21 e V . ) . By employing a com-b i n a t i o n of both s p e c t r a , i t i s p o s s i b l e to o b t a i n p h o t o i o n i -z a t i o n r e s u l t s i n the energy range of 500& - 15008 (8 - 21 e V . ) . a) The Hydrogen Spectrum The hydrogen lamp g ives an in tense "many- l ine" spectrum i n the 9008 - 1500S r e g i o n , and the s p e c t r a l cha rac -t e r i s t i c of the hydrogen spectr.um i s i l l u s t r a t e d i n F i g u r e 4. The r a p i d change i n i n t e n s i t y w i t h wavelength of the " l i n e " causes a problem i n the i n t e r p r e t a t i o n of the p h o t o i o n i z a t i o n e f f i c i e n c y curve . The contour of the hydrogen spectrum appears i n the p h o t o i o n i z a t i o n e f f i c i e n c y curve fo r almost every mole-c u l e s t u d i e d . There i s an a d d i t i o n a l c o m p l i c a t i o n due to the f a c t , as N i c h o l s o n (92) has i n d i c a t e d , tha t a s t rong i s o l a t e d l i n e i n a r e g i o n of the spectrum where the t rue p h o t o i o n i z a t i o n e f f i c i e n c y curve i s i n c r e a s i n g l i n e a r l y w i t h energy w i l l p r o -duce a f a l s e 'hump' i n the curve at an energy j u s t below the l i n e and a f a l s e ' d i p ' above i t . Judging from the shape of the hydrogen spectrum, i t i s reasonable to assume that the spectrum c o n s i s t s of peaks PHOTOELECTRIC CURRENT 25. superimposed on a hydrogen continuum. The p h o t o i o n i z a t i o n e f f i c i e n c y ( P . E . ) at the top of a peak represents the true; P . E . at that photon energy, and the P . E . at the bottom of the v a l l e y between two photon peaks represen ts the c o n t r i b u t i o n s among three f a c t o r s : the two neighbour ing peaks and the hydrogen continuum. The P . E . c o n t r i b u t i o n s from the n e i g h -bour ing peaks are func t ions of the peak he igh t s and the d i s t ance between the v a l l e y and the "peak maximum". Taking these f a c t o r s i n t o c o n s i d e r a t i o n , data p o i n t s on the hydrogen continuum and t h e i r cor responding i o n i n t e n s i t i e s are c a l c u l a ^ ted , and the t rue P . E . curve drawn. In t h i s way f a l s e s t r u c t u r e on the P . E . curve can be e l i m i n a t e d . b) The Hel ium Spectrum The he l ium lamp g ives an in tense continuum i n the 5008 - 12008 r eg ion , and the s p e c t r a l c h a r a c t e r i s t i c of the he l ium continuum i s i l l u s t r a t e d i n F i g u r e 5. The slow v a r i a -t i o n of i n t e n s i t y w i t h photon energy and the cont inuous nature of t h i s emiss ion spectrum are of p a r t i c u l a r advantage i n t h i s work, and the problem caused by the "many- l ine" hydrogen spectrum p r e v i o u s l y d i scussed i s not s i g n i f i c a n t here . The hel ium spectrum was used for most of the molecules s t ud i ed i n t h i s work. I t can be seen i n F i g u r e 5 that the continuum has two p r i n c i p a l i n t e n s i t y maxima, at about 8108 and 6708. The resonance l i n e of atomic he l ium at 5848 i s very s m a l l because i t i s s t r o n g l y s e l f - a b s o r b e d . The s e v e r a l l i n e s observed around 9008 are i m p u r i t y l i n e s from n i t r o g e n atoms and oxygen molecules as i n d i c a t e d by W e i s s l e r , Samson, Ogawa and Cook ENERGY ev 13 14 15 16 17 18 19 20 21 _ ) , ( . , j— . , r NEON / LINES J i _ I , l i I i 1 1000 950 90CT 850 800 750 7700 650 600 WAVELENGTH A 26. (146). The two l i n e s at 736$ and 7448 are resonance l i n e s of atomic neon impur i t y which appear w i t h cons ide rab l e i n t e n s i t y . Because of i t s favourable p o s i t i o n i n the spectrum and i t s cons ide rab l e i n t e n s i t y , the i o n i z a t i o n caused by the 7448 resonance l i n e of the atomic neon i s used for the l o c a t i o n of the i o n beam. The he l ium emiss ion continuum i s produced by t r a n s i t i o n from the e x c i t e d s t a t e to the ground s t a t e of the he l ium molecu le . The two main continuum maxima at 8I08 and 6708 are the r e s u l t of t r a n s i t i o n s A 1 Z - X 1 £ g and D 1 S u -X-'-Z r e s p e c t i v e l y (123) . The he l ium continuum i s a f u n c t i o n of the he l ium pressure i n the l i g h t source . At low hel ium pressure , the resonance l i n e of the atomic he l ium at 5848 appears w i t h cons ide r ab l e i n t e n s i t y , w h i l e both maxima at 8 108 and 6708 are at low i n t e n s i t i e s . As the pressure o f he l ium i s i n -creased , the i n t e n s i t i e s of the two continuum maxima inc rease w h i l e the 5848 l i n e d imin i shes because of s t rong s e l f -a b s o r p t i o n . CHAPTER THREE  EXPERIMENTAL A. I n t r o d u c t i o n The work desc r ibed i n t h i s t h e s i s was done i n a p h o t o i o n i z a t i o n mass spectrometer which was a combinat ion of two major components: the 60 degree N i e r type s i n g l e focus -s i n g mass spectrometer and the Seya-Namioka type 1-meter scanning vacuum monochromator. The two par t s cou ld be i s o l a t e d by a two and a h a l f inch diameter Crane wedge-type v a l v e , so tha t e i t h e r s ide cou ld be opened to a i r for s e r v i c -i n g . Th i s v a l v e was f i t t e d w i t h an appropr ia te 0 - r i n g . A s imple form of the apparatus was i l l u s t r a t e d s c h e m a t i c a l l y i n F i g u r e 6. A sample of gaseous or l i q u i d compound to be s t ud i ed was s to red i n a sample tube, a f r a c t i o n of i t was expanded i n t o three l a rge evacuated g l a s s b u l b s . The gas then moved through a very f i n e leak i n t o the i o n source of the mass spectrometer , where the pressure was measured w i t h an i o n i z a t i o n gauge. Th i s pressure was kept constant at about 5 x 10 ° mm. of Hg. fo r s e v e r a l hours . The monochromator u t i l i z e d an a l u m i n i z e d , 30,000 l i n e s per inch g r a t i n g , and a r e p e t i t i v e , h igh v o l t a g e spark through gas i n a pyrex g l a s s c a p i l l a r y served as a l i g h t source . The gases used i n the l i g h t source were e i t h e r pure hydrogen or pure he l ium mainta ined at about 50 microns p ressure . Entrance and e x i t s l i t wid ths of 0.020 to .0 .040 inch l i m i t e d the r e s o l u t i o n to 48 or about 0.05 eV. at a photon energy of 10 eV. F i g u r e 6: The Monochromator and Mass Spectrometer 28. Vacuum u l t r a v i o l e t r a d i a t i o n was generated from the l i g h t source L , and passed through a narrow entrance s l i t to a g r a t i n g G, s i t t i n g on a t ab l e T. By t u r n i n g the arm A, monochromatic r a d i a t i o n of d i f f e r e n t wavelengths cou ld be s e l e c t e d . The r e f r a c t e d monochromatic l i g h t t r ave r sed the e x i t s l i t of the monochromator, passed through the i on chamber and was i n c i d e n t on a photodetector which had been s e n s i t i z e d to vacuum u l t r a v i o l e t r a d i a t i o n by c o a t i n g i t on the ou t s ide w i t h a t h i n l a y e r of sodium s a l i c y l a t e . P o s i t i v e ions were formed i n the i o n source by the absorp t ion of vacuum u l t r a v i o l e t l i g h t i f i t had s u f f i c i e n t energy. The p o s i t i v e ions formed i n the i on source were forced through a s m a l l s l i t byra r e p e l l e r (wi th a p o s i t i v e p o t e n t i a l of a few v o l t s ) , and were then acce l e ra t ed by a p o t e n t i a l d i f f e r e n c e of about 2000 v o l t s . They passed through a system of s l i t s of c o n t r o l l e d v o l t a g e which caused them to be c o l l i m a ^ ted i n t o a narrow beam, and thence down to the magnetic ana ly se r . In the magnetic f i e l d r e g i o n , the ions were d e f l e c t e d and fo l lowed a c i r c u l a r pa th . The r a d i u s of the path depended upon the mass, the v e l o c i t y of the i o n and upon the magnetic s t r e n g t h . By s u i t a b l e adjustment of the magnetic f i e l d s t r eng th , a homogeneous beam of ions of the same mass per charge r a t i o cou ld be focussed at the e x i t s l i t . The s m a l l cu r ren t due to the i o n beam was then a m p l i f i e d by a 17-stage e l e c t r o n m u l t i p l i e r . Near the mass spectrometer e x i t s l i t s , two p a r a l l e l p l a t e s were i n s t a l l e d , one of them at ground p o t e n t i a l and the other at 100 v o l t s p o s i t i v e or nega t ive . The p o t e n t i a l of the second p l a t e was 2 9 . adjusted so tha t the i o n beam through the e x i t s l i t was focussed on the f i r s t stage of the e l e c t r o n m u l t i p l i e r . The output e l e c t r o n cu r r en t from the e l e c t r o n m u l t i p l i e r was fed i n t o a v i b r a t i n g - r e e d a m p l i f i e r and then i n t o a r e co rde r . Cur ren t s from the photbdetector and from the v i b r a t i n g reed were recorded s epa ra t e ly as the g r a t i n g was r o t a t e d , and a p l o t of ions per photon ( a r b i t r a r y u n i t s ) aga ins t photon energy c o n s t i t u t e d the p h o t o i o n i z a t i o n e f f i c i e n c y curve . 30. B . The Mass Spectrometer The mass spectrometer was a N i e r (93) type ins t rument , u s ing a 60° sec to r shaped magnetic f i e l d for mass a n a l y s i s ; the permi t ted r ad ius of i o n path was 15 cm. The i o n source used y i e l d e d an i o n beam nea r ly homogeneous i n energy. A c a l i b r a t e d potent iometer s u p p l i e d the- i o n a c c e l e r a t i n g v o l t a g e which was con t inuous ly v a r i a b l e over the range of 580 to 2800 v o l t s . The magnetic f i e l d was provided by a s t e e l e lectromagnet , and enabled focus s ing over the range of mass numbers 10 to 300 w i t h an i o n a c c e l e r a t i n g v o l t a g e of 2500 v o l t s . 1. The Ion Source The i o n source was p laced behind the e x i t s l i t of the monochromator as shown i n F i g u r e 6. A diagram of the end and s i d e s e c t i o n of the i o n source was i l l u s t r a t e d i n F i g u r e 7. The e l e c t r o d e s 1, 2, 3, 4, 5 and 6 were made of s t a i n l e s s s t e e l , chosen for i t s non-magnetic p r o p e r t i e s and c o r r o s i o n r e s i s t a n c e . The i o n source and i t s a s soc ia t ed members were a l so made of s t a i n l e s s s t e e l . A l l the spacers , i n s u l a t o r s and the sample i n l e t tube were made of pyrex g l a s s . At the s e c t i o n through A (F igure 7) , two d e f l e c t i o n p l a t e s at a p o t e n t i a l of 500 v o l t s were i n s t a l l e d to d e f l e c t secondary pho toe lec t rons which were produced when photons h i t the m e t a l l i c par t of the monochromator e x i t s l i t . A tungsten f i l amen t mounted behind a s l i t p rovided a beam of e l e c t r o n s which passed over the i o n e x i t s l i t at r i g h t angle to the d i r e c t i o n of the photon beam. Th i s f i l amen t provided a means of o b t a i n i n g e l e c t r o n impact r e s u l t s for comparison. P h o t o i o n i z a t i o n c ross s e c t i o n s were one or two END SECTION gas inlet V * 1 \ \ r t photons secondary SECTION T H R 0 U 6 H " P h ? t 0 e l e C t r 0 n A deflection plates monochromator exit slit MASS SPECTROMETER ION SOURCE SIDE SECTION F i g u r e 7 31. orders of magnitude l e s s than those for impact by 7 0 - v o l t e l e c t r o n s , ( i o n i z a t i o n c ross s e c t i o n for e l e c t r o n impact i s — 16 2 about 10 cm ) , but the i o n cu r ren t s obta ined were sma l l e r by two or three orders of magnitude, because the f l u x d e n s i t y i n terms of i o n i z i n g p a r t i c l e s was much l e s s for any p r e s e n t l y a v a i l a b l e l i g h t source than i n the case of e l e c t r o n beams (25) . An i o n a c c e l e r a t i n g p o t e n t i a l of 2000 v o l t s was app l i ed to e l e c t r o d e 1, and a s m a l l f r a c t i o n of i t to the e l ec t rodes 2, 3 and 5. The p o t e n t i a l s on e l e c t r o d e 2 and each h a l f of 3 and 5 cou ld be adjusted to a l i g n the i o n beam i n the i on source . E l e c t r o d e s 4 and 6 were at ground p o t e n t i a l . Gaseous molecules entered the i o n chamber and diffused i n t o the photon beam. P o s i t i v e ions produced by photon impact were drawn out of the i o n chamber by a s m a l l e l e c t r i c f i e l d between the i o n r e p e l l e r and the e l e c t r o d e 1. The i o n r e p e l l e r was made of tungsten mesh and was maintained at a p o s i t i v e po t -e n t i a l of a few v o l t s , so tha t n e u t r a l molecules cou ld d i f f u s e through i t , but p o s i t i v e ions formed i n the i o n chamber were forced down through the e l e c t r o d e 1. A f t e r pass ing through the i o n chamber, the i o n beam was acce le ra t ed by e l e c t r o d e 1 and focussed by e l e c t rode s 2, 3 and 5, and f i n a l l y passed the e x i t s l i t 6 i n t o the mass ana lyse r . 2. The Ana lyse r and the Electromagnet Thenanalyser ' tube was made from a 5 cm. diameter copper tube bent through 90° on a r ad ius of curva tu re of 17.2 cm. I t was f l a t t e n e d over the cent re s e c t i o n to f i t between the 2.2 cm. pole gap of the analyser magnet. The complete u n i t was r i g i d l y locked to the framework of the 32. instrument so that i t s p o s i t i o n cou ld be f i x e d w i t h respect to the magnet. The electromagnet comprised f i v e 10 ,000- turn c o i l s wound on a low carbon s t e e l core of 6" x 3" s e c t i o n . The machined pole p ieces were made of the same m a t e r i a l and had a gap of 2.2 cm:. The maximum f i e l d s t r eng th i n the pole gap was approximately 5,000 gauss, and cou ld be v a r i e d for the d e t e c t i o n of ions over the . range of mass numbers of 10 to 300 w i t h an i o n a c c e l e r a t i n g vo l t age of 2,500 v o l t s . In the ana lyse r , a beam of ions pass ing at r i g h t angle through a homogeneous magnetic f i e l d was d e f l e c t e d by an amount which was determined by the momentum of the i o n s . S ince , i n gene ra l , the beam emerging from the i o n source was inhomogeneous i n momentum, the s e v e r a l types of ions having d i f f e r e n t momenta would be d e f l e c t e d by d i f f e r e n t amounts. In p r a c t i c e , the i o n a c c e l e r a t i n g p o t e n t i a l was maintained constant, and ions of equal charge i n the o r i g i n a l beam were homogeneous i n energy the momentum depended on ly on the mass of the i o n . For a f i x e d system of s l i t s , one cou ld c o l l e c t a homogeneous beam of ions of the same mass at the e x i t s l i t . By con t inuous -l y changing the magnetic f i e l d s t r eng th , one cou ld determine the mass spectrum of the ions formed from a g iven compound. On emerging from the magnetic ana lyse r , the r e so lved i o n beam passed through the 2 mm. wide s l i t , and f e l l on to the f i r s t p l a t e of the e l e c t r o n m u l t i p l i e r . Between the s l i t and the e l e c t r o n m u l t i p l i e r a suppressor e l e c t r o d e (maintained at negat ive 22^ v o l t s ) suppressed any secondary e l e c t r o n s from ion bombardment of the s l i t . Furthermore, two p a r a l l e l p l a t e s were r e c e n t l y i n s t a l l e d to focus the i on beam. The p o t e n t i a l 33. d i f f e r e n c e between the two p l a t e s was of the order of 100 v o l t s , and cou ld be v a r i e d con t inuous ly i n order to o b t a i n the maximum i o n c u r r e n t . 3. The E l e c t r o n M u l t i p l i e r The 14 stage e l e c t r o n m u l t i p l i e r was a s e n s i t i v e de tec tor of i o n s . The p o s i t i v e ions from the magnetic ana ly -ser impinged upon the f i r s t p l a t e of the de tec tor g i v i n g r i s e to secondary e l e c t r o n s . These e l e c t r o n s were i n tu rn caused to s t r i k e a succes s ion of p l a t e s , each g i v i n g a secondary e l e c t r o n y i e l d grea ter than u n i t y . The p l a t e s were made of 2% Be-Cu, and were connected to 14 s u c c e s s i v e l y higher p o s i t i v e p o t e n t i a l s , and the secondary e l e c t r o n s were d i r e c t e d from p l a t e to p l a t e by the p o t e n t i a l d i f f e r e n c e across them. The p r i n c i p a l mer i t s of the e l e c t r o n m u l t i p l i e r were i t s extreme s e n s i t i v i t y , f a s t response and l ow-no i s e , wide-bandwidth a m p l i f i c a t i o n . The f i n a l e l e c t r o n cur ren t from the l a s t p l a t e was c o l l e c t e d by a f i n e tungsten mesh, and then d i r e c t e d to a Cary Model 31 v i b r a t i n g reed e l ec t rome te r . 4. The V i b r a t i n g Reed E lec t romete r The main usefulness of a V i b r a t i n g reed e lec t romete r i s i n the measurement of s m a l l d . c . c u r r e n t s . The e lec t romete r c o n s i s t e d of two p a r t s : the head u n i t and the main a m p l i f i e r . The input d . c . p o t e n t i a l which arose from the passage of the e l e c t r o n cu r ren t through a l a rge r e s i s t o r of the order of 10 ohms, was converted to a . c . p o t e n t i a l by app ly ing i t through a s e r i e s r e s i s t o r to a c a p a c i t o r whose capac i tance was p e r i o d i c a -l l y v a r y i n g w i t h t ime. Th i s a . c . s i g n a l then underwent s e v e r a l 34 stages of a m p l i f i c a t i o n and the r e c t i f i e d output was d i s p l a y e d on a meter s i t u a t e d i n the rack u n i t . The input r e s i s t o r and c a p a c i t o r were mounted i n sma l l i n d i v i d u a l p l u g - i n u n i t s to f a c i l i t a t e changes of range and response t ime. The input c a p a c i t y cou ld a l so be v a r i e d i n t e r n a l l y i n the range of 0 to 10 pf . I n t e r n a l s e n s i t i v i t y c o n t r o l s enabled the a m p l i f i e r ga in to be adjusted i n the range 1 to 30,000. Th i s was u s e f u l for s c a l i n g the output to reduce i o n -i z a t i o n curves to almost equal s e n s i t i v i t y . With an input r e s i s t a n c e of 10 * ohms and capac i tance i a of 5 p f . , cu r r en t s as low as 10 amperes cou ld be measured. The time constant i n t h i s range was about f i v e seconds. The a m p l i f i e r output was fed i n t o a char t recorder which enabled automatic r e c o r d i n g of cu r ren t from the i o n source . 3 5 . C. The Monochromator E s s e n t i a l s for making photon impact experiments are a beam of monochromatic l i g h t of known energy pass ing through the gas under study and a dev ice for d e t e c t i n g and measuring the photon i n t e n s i t y . A 1-meter monochromator based on the Seya-Namioka (89,108) mounting was c o n s t r u c t e d . The body of the monochromator was cons t ruc ted as a compact brass u n i t , w i t h f i x e d entrance and e x i t s l i t s . The instrument was espec-i a l l y good for the vacuum r eg ion because the source and e x i t s l i t s d id not need to be moved as the wavelength was changed. The on ly mechanical motion i n v o l v e d was a s imple r o t a t i o n of the concave g r a t i n g , which g r e a t l y s i m p l i f i e d the vacuum s e a l problems. 1. The L i g h t Source The l i g h t source used i n t h i s work was b a s i c a l l y a r e p e t i t i v e , h igh v o l t a g e spark d i scharge through gas i n a pyrex c a p i l l a r y capable of e m i t t i n g a vacuum u l t r a v i o l e t con-tinuum from 8 to 21 eV. when i t was acanned by the g r a t i n g . The l i g h t source c o n s i s t e d of two major components: a) The McPherson Model 720 h igh v o l t a g e A . C . power supply which was capable of s u p p l y i n g 0 - 10,000 v o l t s at a maximum cu r ren t of 60 m i l l i a m p e r e s , from a c o n t r o l l e d h igh frequency tungsten spark source . S t ab l e o p e r a t i o n was achieved by adjustment of the j e t of a i r b lowing across the spark gap. The a i r j e t removed the i o n i z e d gases and metal vapors r e s u l t i n g from each e l e c t r i c a l breakdown. Fur ther s t a b i l i t y was gained by the use of a mercury vapor i o n i z a t i o n lamp which i l l u m i n a t e d the spark gap. 36. b) The McPherson Model 630 vacuum u l t r a v i o l e t l i g h t source and a diagram of the l i g h t source i s i l l u s t r a t e d i n F i g u r e 8. I t was a c a p i l l a r y d i scharge l i g h t source , c o n s i s t e d of water cooled c a p i l l a r y , a water cooled Anode and an a i r cooled cathode. The cathode was a i r cooled by a b u i l t i n b lower . In o p e r a t i o n , t h i s l i g h t source could produce a l i n e spectrum or continuum, and i t was connected to the monochromator w i t h or wi thout a l i t h i u m - f l u o r i d e window. In the present work a window-v l e s s l i g h t source was used. Therefore i t was necessary to keep gas pressure i n the mass spectrometer as sma l l as p o s s i b l e , and so d i f f e r e n t i a l pumping was employed between the source and monochro-mator i n l e t s l i t . A cam operated s l i d i n g plunger sea led aga ins t the entrance s l i t jaw, and the monochromator was fu r the r evacua-ted by a 6" o i l d i f f u s i o n pump at the g r a t i n g hous ing . A gas r e g u l a t i n g system c o n s i s t i n g of a two-stage tank r e g u l a t o r , t h r o t t l i n g v a l v e and absolute pressure i n d i c a t o r c a l i -bra ted 0 to 100 mm. Hg. was connected between the gas tank and the l i g h t source . Before the gas from the gas tank entered the l i g h t source , i t f i r s t passed through a U=tube c o n t a i n i n g Linde s y n t h e t i c z e o l i t e s (molecular S ieves) cooled to l i q u i d n i t r o g e n temperature i n order to e l i m i n a t e any i m p u r i t i e s from the gas. The gases used i n the l i g h t source were main ly hydrogen or he l ium. A cont inuous spectrum was produced by he l ium from o o 500A to 1200A (10 to 21 e V . ) , and hydrogen was u s e f u l for the wavelength r e g i o n of 900$ to 1500A (8 to 13 e V . ) . The gas pressure was u s u a l l y mainta ined constant at 50 mm. Hg. as i n d i c a t e d i n the absolute pressure i n d i c a t o r . 2 . The G r a t i n g System In the present work, a 30,000 l i n e s per i n c h , 54 cm. f o c a l t to monochromator I anode water capillary gas inlet water P R ^ ^ - = = « cooling fins I ^ £ ' c LIGHT SOURCE F i g u r e 8 37. l eng th concave g r a t i n g w i t h an a c t i v e area of 25 x 30 mm. was used. The g r a t i n g ro ta t ed about a v e r t i c a l ax i s through the cen t re of the o o g r a t i n g which enabled wavelengths between 500A and 1500A (about 8 o to 21 eV.) to be s e l e c t e d w i t h a r e s o l u t i o n of about 4A. The g r a t i n g G, and the e x t e r n a l d r i v i n g system were shown s c h e m a t i c a l l y i n F i g u r e 6. The g r a t i n g was clamped i n a ho lder i n p o s i t i o n on a t a b l e T i n s i d e the g r a t i n g hous ing . The g r a t i n g was ro t a t ed by moving the 12- inch arm A which was connected to a s p i n d l e pass ing through a vacuum s e a l to the g r a t i n g t ab l e base. The arm A bore aga ins t a p r e c i s i o n micrometer screw Z which cou ld e i t h e r be turned manually or by the v a r i a b l e speed motor U. D was a rubbe r - r inged gear wheel which helped to t r ansmi t a smooth d r i v e to the micrometer . 3. The Photon Moni tor A RCA-1P28 g l a s s - e n c l o s e d p h o t o m u l t i p l i e r tube was used to measure the photon i n t e n s i t y . I t was capable of m u l t i p l y i n g the feeb le p h o t o e l e c t r i c cu r ren t produced at the cathode by a mean va lue of 1.0 x 10^ when operated at 100 v o l t s per s tage . The ou t -put cu r ren t of the 1P28 was a l i n e a r f u n c t i o n of the photon i n t e n -s i t y under normal c o n d i t i o n s . The p h o t o m u l t i p l i e r was s e n s i t i z e d to yacuum u l t r a v i o l e t r a d i a t i o n by c o a t i n g i t on the ou t s ide w i t h a t h i n l a y e r of sodium s a l i c y l a t e d i s s o l v e d i n methyl a l c o h o l . The quantum e f f i c i e n c y of f luorespence of t h i s m a t e r i a l has been measured and found to be cons tant i n the wavelength r e g i o n above 1000A\ and i s a l i n e a r f u n c t i o n of the photon energy i n the wavelength r e g i o n below 1000S (134) . The output cu r ren t of the p h o t o m u l t i p l i e r was a m p l i f i e d fu r the r by a K e i t h l e y d . c . e l ec t rome te r , capable of measuring down 38. -12 to 10 amperes. The f i n a l output s i g n a l from the K e i t h l e y e lec t rometer was recorded i n a Speedomax r eco rde r . 4. The Energy Convers ion Sca le The wavelength of a p a r t i c u l a r beam of monochromatic l i g h t depends on the angular p o s i t i o n of the d i f f r a c t i o n g r a t i n g , and i s g iven by the Bragg E q u a t i o n : -n x = 2d s i n <j> 3.1 where n i s the order of the l i n e , d i s the g r a t i n g spac ing , and <fr i s the angle of the d i f f r a c t i o n for wavelength A . The order n, can be roughly determined by spectrum a n a l y s i s . (Only n = 1 was used i n t h i s work ) . The g r a t i n g spac ing d i s an a c c u r a t e l y determined cons tan t . A r e l a t i o n there fore e x i s t s between the wavelength and the angle of d i f f r a c t i o n fo r any wavelength i n the spectrum. The g r a t i n g t ab l e used i n the present work was connec-ted to a 12- inch arm A which bore aga ins t a p r e c i s i o n micrometer screw. The 584A* he l ium resonance l i n e , the 744°i neon i m p u r i t y o l i n e of the he l ium spectrum and the 1215A l i n e of the hydrogen spectrum were focussed s e p a r a t e l y , and the micrometer readings were recorded for each l i n e . Using these three r ead ings , a c a l i -b r a t i o n curve was drawn, which i s a s t r a i g h t l i n e r e l a t i n g wave-length to micrometer r ead ing , from which the wavelength of any other s p e c t r a l l i n e cou ld be determined d i r e c t l y from the m i c r o -meter r e a d i n g . The r e l a t i o n E = he g ives us a means of comparing our r e s u l t s which are i n terms of photon wavelength w i t h the e l e c t r o n impact data which are i n e l e c t r o n v o l t s . E i s the energy of the r a d i a t i o n i n e l e c t r o n v o l t s , h i s P l a n c k ' s cons tan t , c i s the v e l o c i t y of l i g h t , and 7s i s the wavelength of the r a d i a t i o n . 39. A conver s ion t a b l e for wavelengths to e l e c t r o n v o l t s based on t h i s equat ion was pub l i shed i n 1961 by Samson (106) and —1 —8 the convers ion f ac to r used was 1 cm = 12397.8 + 0 .5 x 10 eV. Th i s t ab l e was used throughout t h i s work for a l l energy conve r s ions . 40. D. The Vacuum System A good vacuum i s e s s e n t i a l i n p h o t o i o n i z a t i o n work, because atmospheric spec ies such as oxygen and n i t r o g e n absorb u l t r a v i o l e t r a d i a t i o n s t r o n g l y i n the wavelength r e g i o n below 2000$. A l s o atom-atom and ion-atom r e a c t i o n s w i l l a r i s e i f the i o n source pressure of the mass spectrometer i s not under a good vacuum. The vacuum system used f o l l o w s the conven t iona l l i n e s for mass spect rometers , and can be subd iv ided i n t o four d i f f e r e n t s e c t i o n s . 1. The Ana lyse r Tube: The analyser tube was pumped from the source end by a 2 - inch a l l - m e t a l MCF-60 f r a c t i o n a t i n g o i l d i f f u s i o n pump f i t t e d w i th a c o l d t r ap , and backed by a Welch duo-sea l vacuum pump. _7 The u l t i m a t e vacuum of these pumps was about 5 x 10 mm. H g . , and net pumping speed was between 20 to 30 l i t r e s per second. A NRC-518 i o n i z a t i o n gauge mounted near the i o n source , was used to measure the pressure i n t h i s r e g i o n . 2. The Monochromator The monochromator was pumped near the g r a t i n g mounting by a 6 - inch a l l - m e t a l MCF-700 f r a c t i o n a t i n g o i l d i f f u s i o n pump f i t t e d w i t h c o l d - b a f f l e s , and backed by a l a rge Welch duo-sea l r o t a r y pump. ANRC-501 thermocouple, mounted on the top of the g r a -t i n g hous ing , was used to measure the pressure of t h i s r eg ion before the o i l d i f f u s i o n pump was switched on. 3. The L i g h t Source Two Welch duo-sea l vacuum pumps, one i n f ron t and the other behind the monochromator entrance s l i t s , were employed to 41. minimise the gas pressure i n the l i g h t source r e g i o n . I t i s important to evacuate t h i s r e g i o n , because the photon i n t e n s i t y w i l l be d imin i shed i f the r a d i a t i o n i s a l lowed to cause i o n i z a t i o n the re . 4. The Gas Hand l ing System A Welch duo-sea l type r o t a r y pump was used to evacuate the gas p a r t i c l e s i n the three s torage g l a s s b u l b s . I t was sepa-ra ted from the bulbs by means of a tap before samples were i n t r o -duced i n t o the s torage bulbs and aTso du r ing a run . The pressure -4 i n t h i s r e g i o n was u s u a l l y mainta ined i n the order of 10 mm of Hg. 5. The measurement of Pressure - Gauges  The Thermocouple Gauge Th i s type of gauge c o n s i s t s of a w i r e through which a f i x e d cu r r en t of about 6 mA i s passed. The temperature of the w i re depends upon the r a t e of heat l o s s from the w i r e and, t he re fo re , upon the gas pressure , and i s measured by means of a thermocouple at tached to the w i r e . The output of the thermocouple i s observed on a meter, which i s c a l i b r a t e d d i r e c t l y i n terms of p ressure . I o n i z a t i o n Gauge The NRC-518 i o n i z a t i o n gauge i s s imply a d i scharge tube r e l y i n g on i o n i z a t i o n for i t s p r i n c i p l e of o p e r a t i o n . I t c o n s i s t s of a tungsten f i l a m e n t , a g r i d at constant p o t e n t i a l , and a p l a t e . The f i l amen t emits e l e c t r o n s which i o n i z e , gas molecules i n the gauge, and the p l a t e (negat ive wi th respect to the g r i d ) i s used to c o l l e c t the i o n s . The number of ions h i t t i n g the p l a t e depends on the gas pressure , and the p l a t e cu r ren t i s a m p l i f i e d and observed on a meter c a l i b r a t e d d i r e c t l y i n terms of p ressure . 42. The i o n i z a t i o n gauge i s equipped w i t h a sa fe ty r e l a y which w i l l a u t o m a t i c a l l y tu rn o f f the gauge when the pressure reaches some set v a l u e , say l j t imes the f u l l s c a l e i n d i c a t i o n of the p a r t i c u l a r range s e l e c t e d . 43. E . Exper imenta l Techniques 1. Sampling The water used was d i s t i l l e d and the gaseous compounds used i n t h i s research were pure samples supplied by Matheson of Canada Co. L t d . and were not fu r the r p u r i f i e d . ' Before the study of a molecule by photon impact was c a r r i e d out , a mass spectrum of the sample was f i r s t taken. Th i s was done by f o c u s s i n g the in tense 744$ (about 16.64 eV.) neon i m p u r i t y l i n e of the he l ium spectrum and keeping the i o n a c c e l e r a -t i n g vo l t age at about 2000 v o l t s , the magnetic f i e l d s t r eng th was scanned s l o w l y by a motor, and the i o n cu r ren t was measured and d i s p l a y e d on the r e c o r d e r . In each case, peaks cor responding to the parent and fragment ions were observed on the mass spectrum, but i m p u r i t i e s i f any, were not observed. The reasons for t a k i n g the mass spectrum were t h r e e f o l d , f i r s t l y , to de tec t the a p p r o x i -mate p o s i t i o n s of the parent and i t s fragment ions i n the mass spectrum; secondly , to measure t h e i r r e l a t i v e i o n i z a t i o n p r o b a b i l -i t i e s ' at a c e r t a i n photon energy; and t h i r d l y , to de tec t i m p u r i -t i e s i n the sample. 2. Exper imen ta l Procedure In order to o b t a i n p h o t o i o n i z a t i o n data for v a r i o u s molecular i ons , the f o l l o w i n g procedure was fo l lowed fo r each gas. — 6 The system was pumped down to approximate ly 10 > mm Hg. and the e l e c t r o n i c equipment was a l lowed to warm up fo r a pe r iod of h a l f an' hour. The sample to be s tud ied was in t roduced i n t o the gas h a n d l i n g system and subsequent ly leaked i n t o the i o n chamber of the mass spect rometer . The gas pressure i n the i o n chamber was mainta ined at 3 x 1 0 - ^ mm H g . , and remained constant fo r a pe r iod of at l e a s t three hours . 44. When a l l these p repara t ions had been completed, the in tense 7448 (about 16.64 eV.) neon impur i t y l i n e of the he l ium spectrum was focussed. The magnetic f i e l d s t r eng th was adjusted to b r i n g the i o n to be s tud i ed to focus at the c o l l e c t o r w i t h an i o n a c c e l e r a t i n g v o l t a g e at about 2 0 0 0 v o l t s . Sometimes i t was a l so necessary to adjust the i o n a c c e l e r a t i n g v o l t a g e , the poten-t i a l s at the r e p e l l e r e l e c t r o d e , e l e c t r o d e s 2 , 3 and 5 (F igure 7) , and the two e l e c t r o d e s between the i o n e x i t s l i t and the e l e c t r o n m u l t i p l i e r , to g ive the maximum i o n c u r r e n t . The spectrum of hydrogen or he l ium was scanned at 3 8 per minute by r o t a t i n g the g r a t i n g w i t h a m u l t i p l e - s p e e d motor, and l i g h t of v a r i o u s wavelengths conta ined i n the source'/was a l lowed to enter the i o n chamber. The scanning was s t a r t e d at a photon energy w e l l below the t h r e sho ld i o n i z a t i o n p o t e n t i a l of the molecule , and the i o n and photon cu r r en t i n t e n s i t i e s for each wavelength were measured s e p a r a t e l y on two r e c o r d i n g c h a r t s . F i n a l l y , a: p h o t o i o n i z a t i o n e f f i c i e n c y curve was ob ta ined . For each molecule , the experiment was repeated s i x qr more times u n t i l c l o s e agreement between the success ive runs was ob ta ined . When the i o n and photon cu r r en t i n t e n s i t i e s became very low, both the r o t a r y andvd i f fu s ion pumps were s topped, and a i r was in t roduced i n t o the system. The g r a t i n g was taken out , dismounted, and sprayed w i t h pure to luene . The pho toe l ec t ron de tec to r was c leaned w i t h methanol, and coated w i t h a f resh l aye r of sodium s a l i c y l a t e . A f t e r these o p e r a t i o n s , both the i o n and photon c u r -ren t i n t e n s i t y were found to be g r e a t l y improved. 3 . The P h o t o i o n i z a t i o n E f f i c i e n c y Curve The s i g n i f i c a n c e of the d e t a i l e d shape o f t t h e p h o t o i o n i -z a t i o n e f f i c i e n c y curves , i s not on ly for the accurate measurement, of i o n i z a t i o n and appearance p o t e n t i a l s , but a l so for the i d e n t i -f i c a t i o n of the v a r i o u s processes l e a d i n g to the i on and the d e t e c t i o n of h igher energy s t a t e s . These curves were drawn, from the exper imenta l da ta , re la t ing the number of ions of a g iven k ind per number of i n c i d e n t photons, produced by photon impact, agains t the photon energy of the i o n i z i n g r a d i a t i o n . The p h o t o i o n i z a t i o n e f f i c i e n c y fo r a g iven molecule at a c e r t a i n photon energy i s def ined by the e q u a t i o n : -E f f i c i e n c y = number of pr imary ionj-pairs formed number of i n c i d e n t photons absorbed ampli tude of the i o n cur ren t . 3.2 amplitude of the photon cu r ren t These p h o t o i o n i z a t i o n e f f i c i e n c i e s have been found to be r e l a -t i v e l y unaffected by e x t e r n a l p e r t u r b a t i o n s such as gas pressure , app l i ed f i e l d , and the geometry of the i o n i z a t i o n chamber (139). The instrument used i n t h i s work has a r e s o l u t i o n of 4°i, and as a r e s u l t , the a c t u a l ' t h r e s h o l d ' va lue of the i o n i z i n g photon energy w i l l be g rea te r than the va lue obta ined from the p h o t o i o n i z a t i o n e f f i c i e n c y curves by a l h a l f - w i d t h of the r e s o l u -t i o n , namely 2°.. A c o r r e c t i o n has been made for a l l measurements repor ted i n t h i s work. 46. F . Sources of E r r o r One of the major sources of e r r o r was due to the i n t e r -p r e t a t i o n of the p h o t o i o n i z a t i o n e f f i c i e n c y cu rves . The th resho ld i o n i z a t i o n p o t e n t i a l s of a l l the molecules could be determined to • a h igh degree of accuracy. However, the no ise to s i g n a l r a t i o of the i o n r a n d photon in tens i ty .measurements a f te r the t h r e sho ld i n some cases was ra the r h i g h , and i t was d i f f i c u l t to l o c a l i z e the po in t where maximum change of s lope i n d i c a t e d the inner i o n i -z a t i o n p o t e n t i a l s . Only es t imated p o s i t i o n s were obta ined from.;.': the exper imenta l curves i n s p i t e o f the f a c t that the band wid th of the photon beam used was on ly 0.05 eV. at 10 eV. photon energy. The second source of e r r o r was caused by the v a r i a t i o n o of the gas pressure du r ing a run . In the e a r l y phase of the work, a t h r e e - l i t r e s torage bulb was used fo r sample, and the gas was a l lowed to leak s l o w l y i n t o the mass spect rometer . I t was found that a f te r about three hours of o p e r a t i o n , the gas pressure dropped to nea r ly h a l f i t s o r i g i n a l v a l u e , and as a r e s u l t the i o n cu r ren t decreased s i g n i f i c a n t l y w h i l e the photon cu r r en t remained cons tan t . Two f i v e - l i t r e gas bulbs were s i n c e then added to the supply system making a t o t a l c a p a c i t y of t h i r t e e n l i t r e s . The gas pressure drop for the same pe r iod of time was very s m a l l , and the drop i n i on cu r ren t per photon at a p a r t i c u -l a r wavelength du r ing a run was n e g l i g i b l e . The t h r e sho ld i o n i z a t i o n p o t e n t i a l s of a l l the mole-cu l e s s t ud i ed represent the average of s i x or more runs . The p h o t o i o n i z a t i o n e f f i c i e n c y curves of s e v e r a l runs e x h i b i t e d good r e p r o d u c i b i l i t y , and the d i f f e r e n c e of t h i s i o n i z a t i o n po-tential between each run was u s u a l l y l e s s than 0.05 eV. CHAPTER FOUR  PHOTOIONIZATION OF ATOMS The p h o t o i o n i z a t i o n e f f i c i e n c i e s of ' n o b l e ' gaseous atoms are of s p e c i a l i n t e r e s t i n that the i o n i z a t i o n p o t e n t i a l of these atoms i s known; a c c u r a t e l y from s p e c t r o s c o p i c da ta . A comparison of data obta ined by the. two methods p rov ides a means of de te rmin ing the accuracy and r e l i a b i l i t y o f the photo-i o n i z a t i o n r e s u l t s . P h o t o i o n i z a t i o n r e s u l t s on the ' n o b l e ' atoms a l so p rov ide a check for the v a l i d i t y of t h e o r e t i c a l models of e x c i t a t i o n and a u t o i o n i z a t i o n processes . Argon, k ryp ton and xenon each has an outer mp^ s h e l l , where m = 3 for argon, 4 for k ryp ton and 5 for xenon, and the i o n i z a t i o n of these molecules r e f e r s to the removal of one of the s i x p e l e c t r o n s . The ground s t a t e of the i o n formed i s s p l i t i n t o 2 2 two l e v e l s , the P 3 / 2 a n d P l / 2 ' T h e l a t t e r n a s t n e h igher energy B e u t l e r (3) s t ud i ed the absorp t ion s p e c t r a of these 2 2 gases, and obta ined i o n i z a t i o n th resho lds of the P 3 / 2 a n < ^ P i / 2 s t a t e s of the ions from the ah^alysis of the Rydberg s e r i e s . He observed tha t the s e r i e s members converg ing to the h igher energy s u b l e v e l of the ground s t a t e of the i on (^P^/2^ become very d i f f u s e near the i o n i z a t i o n l i m i t , and proposed that t h i s was due to a u t o i o n i z a t i o n , i . e . the ' n o b l e ' atom was f i r s t e x c i t e d to one of i t s / ; e x c i t e d s t a t e s , and subsequent ly i o n i z e d by a r a d i a t i o n l e s s t r a n s i t i o n to the i o n i z a t i o n continuum. A. Argon The absorp t ion c o e f f i c i e n t s for argon have been measured by W e i s s l e r and Lee (142) and Huffman, Tanaka and Larrabee (54) . The p h o t o i o n i z a t i o n e f f i c i e n c y of argon has been 48. measured by Wainfan, Walker and We i s s l e r (143), W e i s s l e r , Samson, Ogawa and Cook (146) and Comes and Lessman (9 ) . A l l t h e i r data were obta ined u s ing a l i n e spectrum as t h e i r l i g h t source . F r o s t and McDowell (35) , Foner and N a i l (29) , Marmet and Kerwin (77) s t ud i ed argon us ing the e l e c t r o n impact method. F i g u r e 9 shows the p h o t o i o n i z a t i o n e f f i c i e n c y for argon as a f u n c t i o n of the photon energy. The t h r e sho ld i o n i -z a t i o n of argon to g ive a P 3 / 2 s t a t e of the i o n i s obta ined from the po in t of i n i t i a l onset of the argon curve at 15.73 ± 0.05 e V . , which i s i n good agreement w i t h the s p e c t r o s c o p i c va lue at 15.76 eV. (1 ) , w i t h the p h o t o i o n i z a t i o n r e s u l t s at 15.7 eV. by W e i s s l e r et a l . (146) and w i t h the e l e c t r o n impact data at 15.76 eV. by D i b e l e r et a l . (17) , at 15.77 eV. by M o r r i s o n (81) and at 15.74 eV. by Foner and N a i l (29) . From t h i s t h r e s h o l d , the i o n i z a t i o n e f f i c i e n c y i s observed to r i s e s t e e p l y as would be expected i n the case of a 0 monatomic gas. The s p e c t r o s c o p i c s e p a r a t i o n between the P 3 / 2 and 2 P i / 2 s t a t e of Ar+ i s 0.18 e V . , and w i t h our present mono-chromator r e s o l u t i o n we do not expect to r e s o l v e any s t r u c t u r e between these two s t a t e s . That a u t o i o n i z a t i o n peaks appear i n t h i s r e g i o n has been demonstrated by the t o t a l abso rp t ion ex-periment of Huffman, Tanaka and Larrabee (54) , and Cook and Ching (14) . The p h o t o i o n i z a t i o n e f f i c i e n c y of Ar 4" remains f a i r l y constant at h igher energ ies up to about 16.5 eV. However, Comes and Lessman (9) found s e v e r a l peaks i n t h e i r A r + curve up to 18 e V . , and they c o r r e l a t e d these w i t h breaks found by v a r i o u s workers u s ing e l e c t r o n impact methods. We would expect to be able to r e s o l v e most of the peaks appearing ARGON 6 17 18 19 20 21 PHOTON ENERGY (eV) Figure 9 49. i n the Comes and Lessman curve , but we f i n d no evidence for them. Huffman, Tanaka and Larrabee (54) u s ing a much sma l l e r energy bandwidth (0.02 eV. at 60o8) found no evidence for these t r a n s i t i o n s e i t h e r , and i t seems u n l i k e l y tha t photo-i o n i z a t i o n e f f i c i e n c y curves should be very d i f f e r e n t from t o t a l abso rp t ion curves for t h i s s imple case . Of course , there are d i f f e r e n t s e l e c t i o n r u l e s for e l e c t r o n impact ex-c i t a t i o n , and t h e i r observed t r a n s i t i o n s may be o p t i c a l l y fo rb idden . Th i s would e x p l a i n t h e i r absence i n the r e s u l t s repor ted here but not i n the Comes and Lessman o b s e r v a t i o n s . The main d i f f e r e n c e between the two se t s of data l i e s i n the l i g h t source - the Comes and Lessman data was obta ined us ing a l i n e spectrum, and ours u s ing a he l ium continuum. B . Krypton Krypton has been s tud i ed by B e u t l e r (3 ) , Huffman, Tanaka and Larrabee (53) u s ing the s p e c t r o s c o p i c method. F r o s t and McDowell (35) u s ing . t h e e l e c t r o n impact method s tud i ed K r + i o n and found that the ground s t a t e of K r + con -2 9 s i s t s of two components, P3/2 a n d p l / 2 s t a ' t e s ) whose s e p a r a t i o n i s 0.68 eV. The p h o t o i o n i z a t i o n e f f i c i e n c y curve of k ryp ton i s shown i n the lower h a l f of F i g u r e 10, and the top curve i s the absorp t ion c o e f f . curve of krypton by Huffman, Tanaka and Larrabee (53) fo r comparison. The t h r e sho ld i o n i z a t i o n p o t e n t i a l of k rypton obta ined at the po in t of i n i t i a l onset of the former curve i s found to be 13.99 ± 0.05 eV. which i s i n good agreement w i t h the s p e c t r o s c o p i c i o n i z a t i o n p o t e n t i a l of 50. k ryp ton at 14.009 eV. (1) and the e l e c t r o n impact data of D i b e l e r (17) at 13.96 eV. A f t e r the t h r e sho ld energy, the curve of K r + i s observed to r i s e s t e e p l y as would be expected i n the case of a monatomic gas. A s t rong p h o t o i o n i z a t i o n peak i s observed at 14.09 eV. which p a r t i a l l y obscures the i o n i z a t i o n continuum 2 . of the P 3 / 2 s t a t e of K r T . Seve ra l l e s s in tense i o n i z a t i o n peaks are a l so observed at h igher ene rg i e s . These peaks have been found p r e v i o u s l y by B e u t l e r (3) and Huffman (53) as au to ion ized i n t h e i r abso rp t ion s p e c t r a . The p o s i t i o n of the peaks i n F i g u r e 10 are compared w i t h those found by Huffman (53) i n the absorp t ion spec t r a of Kr i n Table I I . Table I I Peak Energ ies for Krypton Ion (eV.) T h i s work 13.9 9 ( I P . ) 14.08 14.26 "14.37 14.47 Huffman (53) 14.09 14.26 14.36 14.47 The good agreement between absorp t ion spect roscopy and the p h o t o i o n i z a t i o n data s t r o n g l y suggested that the peaks determined here a r i s e from a u t o i o n i z a t i o n . At 14.75 eV. another s m a l l i o n i z a t i o n continuum i s observed from the curve fo r K r + (F igure 10), t h i s continuum corresponds to the i o n i z a t i o n of k ryp ton to the 2 P ] y 2 s t a t e . The 2 P 3 / 2 a n < 3 2 p l / 2 ground s t a t e s e p a r a t i o n i n K r + i s found to be 0.78 eV. which i s h igher than the va lue of 0.666 eV. Wavelength, X(A) 850 900 T 1 i — i — i — r *6 >.r£/2 4P6-4P5(2P l /2)nd' 1513 1311 12109 8 lllll I I 4P6-4^(2F>/2)ns' 1 1 I I I I I I I L _ I _ U F i g u r e 10 14.5 14 Photon energy , eV 61. obta ined by the spec t ro scop ic method (1 ) . The l a rge d i f f e r e n c e i n energy i s probably due to the f a c t tha t the i o n i z a t i o n continuum for the S ta te of : K r + i s very, s m a l l , and the accurate de te rmina t ion of the i o n i z a t i o n p o t e n t i a l r e s p o n s i b l e for t h i s process i s d i f f i c u l t w i t h the r e s o l u t i o n at present a t t a i n a b l e w i t h the apparatus. C. Xenon Xenon has been s t ud i ed by B e u t l e r (3 ) , Huffman, Tanaka and Larrabee (53) (Absorp t ion Spec t roscopy) , by N i c h o l s o n (91), Matsunage, Jackson and Watanabe ( 7 9 ) ( P h o t o i o n i z a t i o n ) and by D i b e l e r , Mohler and Reese (17). and Foner and N a i l (29) ( E l e c t r o n Impact) . The p h o t o i o n i z a t i o n e f f i c i e n c y curve of xenon i s shown i n F i g u r e 11. The t h r e sho ld i o n i z a t i o n p o t e n t i a l of xenon measured at the po in t of s teepest s lope i s found to be 12.13 +• 0.05 eV. which i s i n e x c e l l e n t agreement w i t h the spec t ro scop i c va lue 12.129 eV. ( 1 ) . A f t e r the th re sho ld energy, s e v e r a l peaks appear on the e f f i c i e n c y curve for xenon. These peaks have been found p r e v i o u s l y by B e u t l e r (3) as au to ion i zed i n h i s abso rp t ion spectrum, and that they are indeed due to t h i s type of process has been r e c e n t l y confirmed by N i c h o l s o n (91) i n a t o t a l p h o t o i o n i z a t i o n apparatus. The present p h o t o i o n i z a t i o n work was done w i t h mass a n a l y s i s and w i t h much lower sample pressure (10 mm. of Hg.) than those used by N i c h o l s o n (20 - 100 ) . Atom-atom and i o n -atom r e a c t i o n s are the re fo re v i r t u a l l y e l i m i n a t e d . In Table I I I below the p o s i t i o n of peaks i n F i g u r e 11 are compared w i t h those found by B e u t l e r (3 ) , Huffman (53) and I O N S / P H O T O N (arbitrary units) 52. Watanabe (79) i n the absorp t ion spectrum of xenon, and a l so by N i c h o l s o n (91) u s ing the t o t a l p h o t o i o n i z a t i o n apparatus. Table I I I Peak Energ ies for X e + ( eV) . T h i s work B e u t l e r ( 3 ) Huffman(53) Watanabe:(79) Nicholson(91) 12.12 ( IP . ) 12.43 12. 81 13.03 13.18 13.23 13.45 13.63 12.45 12.82 13.02 13.14 13.21 12.46 12.83 13.02 13. 15 13. 21 12.15 ( IP . ) 12.46 12.83 13.00 13.48 12.129 ( I P . ) 12.48 12.86 13.05 13. 16 13. 25 The c loseness of the agreement between the abso rp t ion spect roscopy and the p h o t o i o n i z a t i o n data prove beyond doubt tha t the peaks determined here a r i s e from the process of a u t o i o n i z a t i o n . 2 2 The P - P ground s t a t e s e p a r a t i o n i n X e + i s O/ di J./ found to be 1.32 e V . , which i s i n good agreement w i t h the separa-t i o n of 1.31 eV. obta ined by the s p e c t r o s c o p i c method (1 ) . 53. CHAPTER FIVE  PHOTOIONIZATION OF DIATOMIC MOLECULES A. Oxygen Oxygen i s a major component of the atmosphere, and the study of i t s p h o t o i o n i z a t i o n i s of fundamental importance i n molecular spect roscopy and ionosphe r i c p h y s i c s . Oxygen has been the sub jec t of many f a i r l y d e t a i l e d s t u d i e s . P r i c e and C o l l i n s (09) s tud i ed the molecule by the o p t i -c a l s p e c t r o s c o p i c method, and M u l l i k e n and Stevens (84) u s ing the c y c l i c method have obta ined the f i r s t i o n i z a t i o n p o t e n t i a l of oxygen as 12.2 eV. P h o t o i o n i z a t i o n s t u d i e s of oxygen have been done by Inn (60) , Watanabe (138), N i c h o l s o n (91) and Samson and C a i r n s (105), Tate and Smith (122), Hagstrum (44) , F r o s t and McDowell (38) , C l a r k e (5) and B r i o n (4) have s t ud i ed oxygen by the e l e c t r o n impact method. The ground s t a t e of the oxygen molecule has the e l e c t r o -n i c s t r u c t u r e : (CT g 2 s ) 2 ( ( T u 2 s ) 2 ( ( r g 2 p ) 2 ( T T u 2 p ) 4 ( ^ g 2 p ) 2 ; 3 £ - . . . . 5 . 1 The molecular o r b i t a l s are l i s t e d i n the order of dec reas ing b i n d i n g energy, o m i t t i n g the inner ones. The f i r s t i o n i z a t i o n p o t e n t i a l r e f e r s to the removal of an e l e c t r o n from the outer 4- 2 an t ibonding (TT _2p) o r b i t a l l e a v i n g the 0^ i o n i n i t s X TT ground s t a t e . The p h o t o i o n i z a t i o n e f f i c i e n c y curves fo r oxygen are shown i n F i g u r e s 12 and 13. The i o n i z a t i o n p o t e n t i a l o f oxygen measured from the po in t of i n i t i a l onset of the curve i s 12.06 + 0.05 eV. which i s i n e x c e l l e n t agreement w i t h other p h o t o i o n i z a -t i o n va lues at 12.065 eV. by N i c h o l s o n (91) , 12.063 eV. by 850 900 950 1000 I050A — | 1 1 » c 1 1 1 1 1 1 1 1 : 1 1 • ' * • J K IONIZATION EFFICIENCY 54. Samson (105) and 12.07 eV. by Watanabe (138). The c l o s e agree-ment between the p h o t o i o n i z a t i o n r e s u l t s u s ing d i f f e r e n t t e c h -niques proves beyond doubt that the p h o t o i o n i z a t i o n methods of o b t a i n i n g i o n i z a t i o n p o t e n t i a l s are r e l i a b l e . Values obta ined by e l e c t r o n impact methods are s l i g h t l y h i g h e r . The v a r i o u s e l e c t r o n impact , spec t ro scop i c and 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 of oxygen found by d i f f e r e n t workers are summarised i n Table IV. Tab le r IV Threshold I o n i z a t i o n P o t e n t i a l of Oxygen I . P . ( e V . ) WORKERS METHODS YEAR 12.06 + 0.05 Present r e s u l t P h o t o i o n i z a t i o n 1966 12.04 + 0.01 Inn (60) P h o t o i o n i z a t i o n 1953 12.07 + 0.01 Watanabe (138) P h o t o i o n i z a t i o n 1957 12.065 N i c h o l s o n (91) P h o t o i o n i z a t i o n 1963 12.063 Samson (105) P h o t o i o n i z a t i o n 1965 12.2 M u l l i k e n (84) C y c l i c Method 1933 12.2 + 0.1 P r i c e (99) Spectroscopy 1934 12.5 + 0.1 Tate (122) E l e c t r o n Impact 1941 12.1 + 0 .2 Hagstrum (44) E l e c t r o n Impact 1951 12.21 + 0.04 F r o s t (38) E l e c t r o n Impact 1958 12.04 + 0.02 C l a r k e (5) E l e c t r o n Impact 1964 12.20 + 0.05 B r i o n (4) E l e c t r o n Impact 1964 A f t e r the t h r e sho ld i o n i z a t i o n p o t e n t i a l of oxygen, the p h o t o i o n i z a t i o n e f f i c i e n c y curve e x h i b i t s many peaks. The whole r eg ion a f te r the th re sho ld energy i s known to be o v e r l a i d w i t h many a u t o i o n i z a t i o n s t a t e s (130), and a comparison of the absorp-t i o n spectrum and the p h o t o i o n i z a t i o n e f f i c i e n c y curve shows that the peaks i n the p h o t o i o n i z a t i o n spectrum correspond w e l l w i t h 55. peaks i n the absorp t ion s p e c t r a . Table V shows a comparison of peaks i n p h o t o i o n i z a t i o n e f f i c i e n c y curves obta ined i n t h i s work w i t h those obta ined i n the work of N i c h o l s o n (91) and Watanabe (138), and i n the absorp t ion s p e c t r a of Huffman (55) , Cook (11) and P r i c e and C o l l i n s (99) . . The agreement between p h o t o i o n i z a -t i o n and abso rp t ion peaks e s t a b l i s h w i t h c e r t a i n t y that c e r t a i n of the oxygen peaks a r i s e from a u t o i o n i z i n g l e v e l s . The l e t t e r i n g scheme of f i g u r e 12 and 13 i s such that bands i n the groups H , . I , . J , , K H ' , I ' , M,N, 0 . . . . . . M ' , N ' b e l o n g to Rydberg s e r i e s ' approaching the l i m i t c h a r a c t e r i s t i c of each p a r t i c u l a r group (99) . o The w e l l e s t a b l i s h e d s t a t e s of oxygen ions are X TT^ a 4 * T U , A 2 T l u and b 4 2 g - . The energy s t a t e of C-2+ 2 H g i s obta ined by the removal of a ("TT" 2p) e l e c t r o n , the a ^ TT and A 2 * T U s t a t e s are obta ined by the removal of (TT u 2p) e l e c t r o n s , and b ^ S e ~ s t a t e i s obta ined by the removal of an (G~ 2p) e l e c -& g t r o n . The present p h o t o i o n i z a t i o n data fo r oxygen g ives evidence for the e x c i t a t i o n of on ly two types , namely the t r a n s i t i o n s to 2 4 — X: TT g and b £ g s t a t e s . In the r e g i o n of the TT u< e l e c t r o n i c t r a n s i t i o n s , a u t o i p n i z i n g t r a n s i t i o n s e s p e c i a l l y the neon i m p u r i -ty l i n e s are so in tense tha t the e l e c t r o n i c l e v e l s of the ions are obscured by. the r e s o l u t i o n s of the monochromator. The i o n i z a t i o n p o t e n t i a l of oxygen l e a d i n g to the 1 b 4 S ~ s t a t e i s obta ined from the break of the curve at 18.20 eV. g Th i s va lue i s i n c l o s e agreement w i t h those at 18.16 eV. by P r i c e and C o l l i n s (99) and 18.17 eV. by Huffman, Larrabee and Tanaka (55) . The e l e c t r o n impact work of F r o s t and McDowell (38) g ive s a va lue of 18.42 eV. 56. Table V Comparison of Peaks i n P h o t o i o n i z a t i o n E f f i c i e n c y  Curves and Abso rp t i on Spectrum of Oxygen Des igna t i on Th i s Work Huffman Cook N i c h o l s o n P r i c e Watanabe (56) (55) (11) (91) (99) (138) - 12. 16 .12. . 16 — — — — H l 12. 30 12. ,30 - 12.33 12.33 12.33 H 2 12. 45 12. .47 - 12.47 12.48 12.48 H 3 12. 53 12, .53 12.56 12.61 12.61 12.61 12. 68 12. .69 12. 68 - - -H 4 12. 76 12, , 75 12. 75 12. 75 12. 75 12. 76 H 3 ' 12. 87 12, .88 12.88 12.84 12.84 12.84 V 12. 97 12, .97 12.97 12.97 12.97 12.98 13. 10 13 .08 13 .08 13.09 13.08 13.09 13. 31 13 .31 13 .31 13.31 13.31 13.31 M s ' 13. 51 13 .52 13.52 13.52 13.52 13. 53 13. 63 13 .62 13.63 13.62 13.62 13.62 - 13. 77 13 . 76 13. 77 13. 75 - 13. 77 - 13. 94 13, .95 13.95 13.96 - -- 13. 99 13, .98 13.99 13.99 - 13.98 J 14. 14 14, . 11 14.13 - - -J 14. 25 14, .25 14.25 - - -J 14. 34 14, .33 14.34 - - -K 14. 49 14, ,48 14.48 - - -- 14. 57 14, .56 14.57 - - -I 14. 67 14, .66 14.66 - - -I 14. 79 14, .79 14. 79 - - - -I , K 14. 92 14, .91 14.91 - - -I ' 15. 07 15, .07 15.06 — - -I ' 15. 16 15, , 17 15.17 - - -N 15. 32 15, .33 15.33 - - -P 15. 45 15, .45 15.45 - - -- 15. 56 15, . 55 15.55 - - -P 15. 60 15 .60 15.60 - - -P 15. 77 15 . 77 15. 77 - - -_ 15. 80 15 .80 15.80 _ _ 57. The in t e r a tomic d i s t a n c e , accord ing to F r o s t and McDowell (38) , for the v a r i o u s s t a t e s of oxygen and i t s molecular ions are: 0 2 1,204A (X 3 £ ") ; 0 2 + : 1.12278' (X 27T ) , 1.3813A (a 4TT ) , 1.4038$ (A 2 H .0 and 1.2795$ (b 4 £ ~ ) . The u u g most probable t r a n s i t i o n s i n p h o t o i o n i z a t i o n accord ing to the Franck-Condon P r i n c i p l e (and w i t h the i n t e r a t o m i c d i s t ances between the molecule and the i o n i n mind) , w i l l be those near the v e r t i c a l l i n e r i s i n g from the ground s t a t e of the oxygen o molecule to one of the low l y i n g v i b r a t i o n a l l e v e l s of X TT g 4 4 and B £ g ~ s t a t e s of the oxygen i o n . The t r a n s i t i o n s to a ^TT U 2 _ and A Tv s t a t e s of the oxygen i o n are l e s s probable s i n c e they l i e at a g rea te r i n t e r a t o m i c d i s t a n c e . 58. B . N i t r o g e n N i t r o g e n has been s tud i ed by many workers . The absorp t ion c o e f f i c i e n t s of n i t r o g e n have been measured by Huffman, Tanaka and Larrabee (55) , Cook and Metzger (11) , Ogawa and Tanaka (95) , Watanabe and Marmo (136), Itamoto and M c A l l i s t e r (61) and C l a r k e (5 ) . Only a few papers on the photo-i o n i z a t i o n of n i t r o g e n , p a r t i c u l a r l y those measuring p h o t o i o n i z a -t i o n e f f i c i e n c i e s , have been p u b l i s h e d . E a r l i e r work was done by Wainfan, Walker and W e i s s l e r (141), Cook and Metzger (11), Samson and C a i r n s (103) and Comes and Lessman (8 ) . E a r l i e r workers , however, measured abso rp t ion c o e f f i -c i e n t s u s ing a w e l l - r e s o l v e d l i n e source . In some cases , the fca . source remission l i n e used by these authors d i d not c o i n c i d e w i t h \ an a c t u a l n i t r o g e n absorp t ion maximum or minimum so that the c o e f f i c i e n t s l i s t e d for comparison are those at the two nearest maxima or minima. The present data i s based on a continuum source , and such d i f f i c u l t i e s are not expected. The e l e c t r o n c o n f i g u r a t i o n of n i t r o g e n p r e d i c t e d from elementary molecular o r b i t a l theory i s : -KK ( C T g 2 s) 2((r u > 2 s ) 2 ( T T u 2 p ) 4 ( t r g 2 p ) 2 ; 1 £ g + 5.2 The molecular o r b i t a l s are l i s t e d i n the order of dec reas ing b i n d i n g energy, o m i t t i n g the inner o r b i t a l s . The p h o t o i o n i z a t i o n e f f i c i e n c y of n i t r o g e n as a func-t i o n o f the photon energy i s shown i n F i g u r e 14. The th re sho ld i o n i z a t i o n p o t e n t i a l of n i t r o g e n measured from the po in t of i n i t i a l onset of the curve i s at 15.55 + 0.05 eV. Th i s va lue r e f e r s to the removal of an e l e c t r o n from the outer (KT„2p) o o r b i t a l , to leave the N 9 i o n i n i t s X 2 ground s t a t e . The 650 700 750 800A 1 1 1 1 1 1 1 1 1 1 1 1 ' ' » 1 PHOTON ENERGY 59. va lue of 15.55 eV. i s i n good agreement w i t h the s p e c t r o s c o p i c va lue of 15.576 e V . ( 4 8 ) , and the pboiroioniza t ion i o n i z a t i o n p o t e n t i a l of n i t r o g e n at 15.580 eV. by Watanabe and Marmo (136), and by Ogawa and Tanaka (95) . Numerous workers have ob ta ined e l e c t r o n impact va lue for the f i r s t i o n i z a t i o n p o t e n t i a l of n i t r o g e n . F r o s t and McDowell (38) , Fox and Hickman (30) and C l o u t i e r and S c h i f f (6) have used a R . P . D . method and< Obtained va lues of 15.63 e V . , 15.60 eV. and 15.58 eV. r e s p e c t i v e l y for the t h r e sho ld i o n i z a t i o n p o t e n t i a l of n i t r o g e n . The v a r i o u s e l e c t r o n impact, spec t ro scop i c and p h o t o i o n i z i t i o n i o n i z a t i o n p o t e n t i a l s of n i t r o g e n are summarised i n Table V I . Tabla VI Threshold I o n i z a t i o n P o t e n t i a l of N i t r o g e n I . P . (e. V . ) WORKERS . METHODS YEAR 15 .55 + 0.05 Present r e s u l t P h o t o i o n i z a t i o n 1966 15 .6 + 0.1 W e i s s l e r (146) P h o t o i o n i z a t i o n 1956 15 .580 Watanabe (136) P h o t 6 i o n i z a t i o n 1956 15 .580 Ogawa (95) P h o t o i o n i z a t i o n 1962 15 .58 Huffman (55) P h o t o i o n i z a t i o n 1963 15 .576 Herzberg (48) Spectroscopy 1950 15 .65 Tate (121) E l e c t r o n Impact 1936 15 .60 Fox (52) E l e c t r o n Impact 1954 15 .63 + 0.02 F r o s t (35) E l e c t r o n Impact 1955 15 .58 + 0.02 C l o u t i e r (6) E l e c t r o n Impact 1959 The p h o t o i o n i z a t i o n e f f i c i e n c y curve of n i t r o g e n (F igu re 14) e x h i b i t s a s l i g h t curva tu re near the t h r e sho ld energy, and t h i s conf i rms the f ac t tha t there i s a s m a l l d i f f e r e n c e i n 60. e q u i l i b r i u m in t e r a tomic d i s t ance between the ground s t a t e s of the n i t r o g e n molecule and i t s i o n . The e q u i l i b r i u m i n t e r a t o m i c d i s -tances of the ground s t a t e s of the molecule and i t s ions are 1 . 0 9 4 8 and 1.116A r e s p e c t i v e l y (48) . There has been some question i n the e l e c t r o n impact "f data concern ing the shape of the i o n i z a t i o n e f f i c i e n c y durve of n i t r o g e n i n the r e g i o n of a few v o l t s above the t h r e sho ld energy. C l a r k e ' s data (5) u s ing an e l e c t r o s t a t i c s e l e c t o r shows a non-l i n e a r i t y i n t h i s r e g i o n of h i s Ng"1" curve , which was not observed by other i n v e s t i g a t o r s u s ing c o n v e n t i o n a l e l e c t r o n impact methods (30) . Fox (32) u s ing a R . P . D . method confirmed the n o n l i n e a r i t y i n the same r e g i o n of the N 2 + CUr.vfe, arid attStlbil.tdQ th i i s to an a d d i t i o n i o n i z a t i o n p p r o c e s s such as a u t o i o n i z a t i o n . The p h o t o i o n i z a t i o n e f f i c i e n c y curve for n i t r o g e n e x h i -b i t s a s e r i e s o f sharp peaks above the t h r e s h o l d . Table V I I shows I a comparison of these peaks w i t h the abso rp t ion s p e c t r a of Huffman Tanaka and Larrabee (55) and Cook and Metzger (11) . The c l o s e agreement i n peak: energ ies found by the two techniques proves beyond doubt tha t they a r i s e from the process o f a u t o i o n i z a t i o n and that the r e g i o n above the t h r e sho ld energy i s o v e r l a i d w i t h au to ion i zed s t a t e s . More peaks appear i n the absorp t ion spectrum of n i t r o g e n than i n the p h o t o i o n i z a t i o n spectrum i n the same r e g i o n . Th i s may be due to two reasons: f i r s t l y , the r e s o l u t i o n of the p h o t o i o n i z a -tdityh monochromator i s very much l e s s than that i n the absorp t ion o spec t roscopy , so tha t a s e p a r a t i o n of l e s s than 4A between two peaks cannot be r e s o l v e d i n the p h o t o i o n i z a t i o n spectrum but are e a s i l y r e so lved i n the absorp t ion spectrum; and secondly , the a u t o i o n i z a t i o n process i s governed by c e r t a i n s e l e c t i o n r u l e s 61. Table V I I Comparison of Peaks i n the P h o t o i o n i z a t i o n Curve  and Abso rp t i on Spectrum Of N i t r o g e n D e s i g n a t i o n Th i s work Huffman (56) C o o k n ( l l ) 15.55 ( I P . ) R x 8 .1 15.65 15.65 15. .65 R a 3 .3 15. 79 15.81 15. .80 P 0 .0 15.97 15.98 15, .98 R a 4 . 1 16.06 16.06 16. ,06 P. 1. .0 16.18 16.19 16. , 19 R a 5 . 1 16.38 16.39 16. 39 R a 4. .3 16.53 16.52 16. 52 R a 6. .2 16.75 16. 76 16. ,76 P 3. .0 16.65 16.65 16. 65 R b 3. .0 17.14 17.14 17. 13 17.41 -R b 4, ,0 17.84 17.84 17. ,84 R b 5, .0 18.18 18.18 18. 18 R b 7, .0 18.46 18.46 18. ,46 which determined the r e c i p r o c a l i n t e r a c t i o n between the e x c i t e d s t a t e s of molecule and the i o n i z a t i o n continuum. Not a l l e x c i -t a t i o n of e l e c t r o n s to the e x c i t e d s t a t e s of the molecule r e s u l t s i n i o n i z a t i o n , and t h i s e x p l a i n s why peaks observed i n the absorp t ion spectrum are absent i n the p h o t o i o n i z a t i o n spectrum. 62 C. Carbon Monoxide The absorp t ion spectrum of carbon monoxide i n the vacuum u n t r a v i o l e t has been s tud i ed by many workers . The most recent work on p h o t o i o n i z a t i o n i s by Huffman, Tanaka and Larrabee; (56) , Watanabe, Z e l i k o f f and Inn (132) Sun and W e i s s l e r (145), Tanaka, J u r s a and LeBlanc (120) and '-.W.atanabe, Nakayama and M o t t l (140). Fox and Hickam (30) and Hagstrum (44) have s tud i ed carbon monoxide by the e l e c t r o n impact method. The carbon monoxide molecule has 14 e l e c t r o n s , and the ground s t a t e has the f o l l o w i n g e l e c t r o n c o n f i g u r a t i o n : ( o - g 2 s ) 2 ( < T u 2 s ) 2 ( ¥ u 2 p ) 4 ( ( T g 2 p ) 2 ; 1 £ g + . . . 5.3 where the molecular o r b i t a l s are arranged i n the order of de-c r e a s i n g b i n d i n g energy, o m i t t i n g the inner o r b i t a l s . The p h o t o i o n i z a t i o n e f f i c i e n c y curve fo r carbon monoxide i s shown i n F igu re s 15 and 16. The th re sho ld i o n i z a -t i o n p o t e n t i a l measured from the po in t of i n i t i a l onset of the curve i s 13.98 ± 0.05 eV. which r e f e r s to the removal of an (<T 2p) e l e c t r o n to g ive an i on i n the X 2 £ + ground s t a t e . The va lue of 13.98 eV. i s i n c l o s e agreement w i t h the spec t ro s - . cop ic va lue of 14.01 eV. by Watanabe (140) and the e l e c t r o n impact va lue of 13.98 eV. by Fox and Hickam (30) and 14.1 eV. by Hagstrum (44) . The e f f i c i e n c y curve of carbon monoxide r i s e s sha rp ly a f te r the th re sho ld onset as p r e d i c t e d from the Franck-Condon p r i n c i p l e s i n c e ( t h e e q u i l i b r i u m i n t e r a t o m i c d i s t ance of the normal molecule i s almost the same as the C 0 + i o n i n the : X 2 < E + s t a t e . IONIZATION EFFICIENCY O CJ» O i 1 — • 1 — • 1 — i 1 '—•—n r~~ IONIZATION EFFICIENCY O Ol o : 1 1 1 1 1 1 1 1 1 1 r 63. Fox (32) observed a d e v i a t i o n from l i n e a r i t y i n h i s e l e c t r o n impact data po in t s i n the v i c i n i t y of the X 2 £L+ t h r e s h o l d , and Wal lace (130) has measured the carbon monoxide a u t o i o n i z a t i o n bands i n h i s absorp t ion s p e c t r a . The p h o t o i o n i -z a t i o n e f f i c i e n c y curve of carbon monoxide a l so e x h i b i t s many peaks above the th re sho ld i o n i z a t i o n p o t e n t i a l . Table V I I I shows a comparison of the p o s i t i o n of peaks i n the p h o t o i o n i z a -t i o n spectrum and the absorp t ion s p e c t r a of Huffman (56) and Cook (14) . Table V I I I Comparison of Peaks i n P h o t o i o n i z a t i o n Curves and A b s o r p t i o n Spectrum of Carbon Monoxide D e s i g n a t i o n T h i s work Huffman (56) Cook (14) R x 7.1 R x 10.1 Hi H l H i R a 3.0 H 2 R a 3 . 2 , ? 1 R a 3 . 3 , R a 4 . 1 , Po 13. 98 ( I P . ) 14. ,01 14. ,01 14. ,01 14. , 17 14. , 17 14. . 17 14. 26 14. .25 14. 26 14. 31 14. ,31 14. .31 14. .57 14. .56 14. .56 14. . 77 14, . 78 14. . 78 14. ,98 14. .98 14. .9/8 15. .08 15, .09 15 .08 15. . 16 15. . 16 15. . 16 15, .29 15, .29 15. .29 15, .54 15, . 55 15. .54 15, .66 15, .66 15, .66 15, ,84 15, .84 15. .84 15. . 99 15, .99 15. 99 64. Table V I I I (cont inued) D e s i g n a t i o n Th i s work Huffman (56) Cook (14) P 2 16.33 16.33 16.33 P 3 16.54 16.54 16.54 P 2 , P 3 16.71 16. 71 16. 72 - 16.87 16.88 16.88 B 17.04 17.07 17.04 P 4 17.96 17.96 17.96 A comparison of the p h o t o i o n i z a t i o n e f f i c i e n c y curve and the absorp t ion s p e c t r a shows that the peaks i n the p h o t o i o n i z a t i o n spectrum agree w e l l w i t h the cor responding peaks i n the absorp-t i o n s p e c t r a . C lo se agreement between the two shows that the peaks ( f i g . 1 5 and 16) i n our p h o t o i o n i z a t i o n e f f i c i e n c y curve are from a u t o i o n i z i n g l e v e l s . The second i o n i z a t i o n p o t e n t i a l of carbon monoxide r e f e r s to the removal of e l e c t r o n to form C 0 + i n the e x c i t e d 2 A Tf i s t a t e , and the s p e c t r o s c o p i c va lue for t h i s l i m i t i s 16.536 eV. (119) as shown i n F i g u r e 16 by broken arrow. Th i s i o n i z a t i o n p o t e n t i a l was not observed fo r the f o l l o w i n g reason. The e q u i l i b r i u m i n t e r a t o m i c d i s t ances as g iven by Herzberg (48) are: 1.128l8, X 1 £ g + of CO; 1.1150$, X 2 £ + , and 1.2436$, 2 + A T T . o f CO . The l a rge d i f f e r e n c e i n e q u i l i b r i u m i n t e r a t o m i c d i s t ance for the C0(X 1 £ + ) and C 0 + ( A 2 T T i ) s t a t e s r e s u l t s i n the p r o b a b i l i t y fo r t h i s t r a n s i t i o n be ing d i s t r i b u t e d f a i r l y e q u a l l y over s e v e r a l v i b r a t i o n a l l e v e l s , and the 0-0 t r a n s i -t i o n between the X *£Ia; + of the molecule and the A 2 T\ ± s t a t e 65. of the i on i s s m a l l . Th i s means that there are probably s e v e r a l s teps i n the continuum th re sho ld for t h i s s t a t e , and apparent ly the compe t i t i on of the a u t o i o n i z a t i o n process i n the same r eg ion i s so in tense tha t the s teps are obscured at the r e s o l u t i o n of the experiment . 66. D. C h l o r i n e The on ly s p e c t r o s c o p i c i o n i z a t i o n p o t e n t i a l of the c h l o r i n e molecule has been repor ted by Gaydon (42) . Watanabe (138), u s ing the p h o t o i o n i z a t i o n method has obta ined the t h r e sho ld i o n i z a t i o n p o t e n t i a l , and M o r r i s o n and N i c h o l s o n (90) and Thorburn (126) have measured the e l e c t r o n impact i o n i z a -t i o n p o t e n t i a l s . F r o s t and McDowell (39) u s ing the R . P . D . e l e c t r o n impact method have been able to measure f i r s t and inner i o n i z a t i o n p o t e n t i a l s of the molecu le . Acco rd ing to M u l l i k e n (86)', the e l e c t r o n i c s t r u c t u r e of c h l o r i n e may be represented by the formula: ( 0 - g 3 s ) 2 ( c r u 3 s ) 2 ( c r g 3 p ) 2 ( T T u 3p) 4 (TT g 3 p ) 4 , 1 E g + . . . 5.4 The inner e l e c t r o n s are omit ted and the o r b i t a l s are l i s t e d i n order of decreas ing b i n d i n g energy. The p h o t o i o n i z a t i o n e f f i c i e n c y curve fo r the c h l o r i n e molecule i s shown i n F i g u r e 17. The t h r e s h o l d i o n i z a t i o n po t -e n t i a l (obta ined from the po in t of i n i t i a l onset of the curve) at 11.47 i 0.05 eV. r e f e r s to the removal of an e l e c t r o n from the an t i -bond ing (TT g 3p) o r b i t a l . Th i s va lue i s h igher than the s p e c t r o s c o p i c va lue at 11.32 eV. found by Gaydon (42) , but i t i s i n e x c e l l e n t agreement w i t h the va lue at 11.48 eV. found by Watanabe (138). The e l e c t r o n impact method g ives the t h r e s -ho ld i o n i z a t i o n p o t e n t i a l of the c h l o r i n e molecule at 11.8 eV. (Morr i son and N i c h o l s o n (90) ) , 11.8 eV. (Thorburn (126)) and 11.63 eV. (Fros t and McDowell (39) ) . I t i s noted that the e l e c t r o n impact f i g u r e s are c o n s i d e r a b l y h igher than the va lue found i n t h i s work, and t h i s i s probably due to the d i f f e r e n c e i n the i o n i z a t i o n c r o s s - s e c t i o n at the t h r e s h o l d , and i n the degree of s e n s i t i v i t y i n the i on cur ren t measurement. 67. The i o n i z a t i o n e f f i c i e n c y shows a s l i g h t cu rva tu re near the t h r e s h o l d , and t h i s i n d i c a t e s a change of 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 ance between the molecule (normal) and C l 2 + 2 _ i o n ( TV ) , and confi rms that the primary i o n i z a t i o n i n v o l v e s the removal of an an t ibonding e l e c t r o n . The outer (TT 3p) and (TT u 3p) o r b i t a l s are mainly atomic i n cha rac t e r , and the ions formed by the removal of 2 9 e l e c t r o n s from these o r b i t a l s w i l l be i n TT and ^ TT s t a t e s , g u Q and each of these TV s t a t e s would be a doub le t . F r o s t and McDowell (39) i n d i c a t e d that there appear to be s e v e r a l i o n i c energy l e v e l s w i t h i n about 0.6 eV. of the i o n i z a t i o n t h r e s h o l d , but we have not been able to r e s o l v e these i o n i c energy l e v e l s . Samce the curve fo r CI24" above the t h r e sho ld energy appears abnormal i n shape, t h i s suggests the o v e r l a p p i n g of s e v e r a l i o n i z a t i o n processes . 68. E . Hydrogen C h l o r i d e Hydrogen c h l o r i d e has been s tud i ed e x t e n s i v e l y by i n f r a r e d and Raman spec t roscopy, but data i n the fa r u l t r a -v i o l e t r eg ion are s c a r c e . The p h o t o i o n i z a t i o n e f f i c i e n c y curve of hydrogen c h l o r i d e i s shown i n F i g u r e 18. From the curve the th resho ld of i o n i z a t i o n i s 12.56 ± 0.05 e V . , which r e f e r s to the removal of a non-bonding e l e c t r o n l o c a l i z e d i n the halogen atom as d i scussed by M u l l i k e n (88) and P r i c e (100) l e a d i n g to a X 2TT g/2 ground s t a t e for the molecular i o n . Th i s va lue i s i n good agreement w i t h the va lue of 12.56 eV. found by Fox (31) u s ing the R . P . D . method, and 12.53 eV. by Mor r i son (81) u s ing the e l e c t r o n impact method. However, i t i s lower than ; the va lue at 12.74 eV. by Watanabe (138) u s ing the pho toe l ec -t r i c measurement, and at 12.90 eV. by P r i c e (100) u s i n g s p e c t r o s c o p i c means. The curve fo r hydrogen c h l o r i d e r i s e s sha rp ly a f t e r the th re sho ld energy, agreeing w i t h the f ac t tha t the s i n g l e p h o t o i o n i z a t i o n process i s indeed a s tep f u n c t i o n . There i s a fu r the r sharp inc rease of p h o t o i o n i z a t i o n e f f i c i e n c y around 14 eV. and the s teepes t ascent of the curve i s s e l e c t e d as the onset of the i o n i z a t i o n to the A 2 £ + e x c i t e d s t a t e of the hydrogen c h l o r i d e i o n at 14.04 eV. and i t i s 1.48 eV-.above the i o n i c ground s t a t e . Th i s va lue agrees w i t h the va lues at 1.6 eV. by Fox (31) and 1.5 eV. by M o r r i s o n (81) , and i s con-s i s t e n t w i t h the va lues of 1.14 eV. . and 2.83 eV. for the s i m i l a r s t a t e s i n HF + and H I + r e s p e c t i v e l y by F r o s t and McDowell (37) , i f one assumes that the energy s epa ra t i on of 2 2 + the X ^ 3 / 2 ~ ^ ^ s t a t e s should inc rease p r o g r e s s i v e l y . However, i t i s not i n agreement w i t h the spec t ro scop ic va lue l 1 1 1 i i r \\ i • i 1 1 1 — — i 1 ' 12 13 14 15 16 17 18 19 20 Photon energy, eV Figure 18 69. of 3.48 eV. obta ined by N o r l i n g (94) . The A 2 £ + s t a t e for the hydrogen h a l i d e r e s u l t s from the removal of an e l e c t r o n from a 2pcr type bonding o rb i t a l , and thus one might expect the b i n d i n g energy of the e l e c t r o n to vary as the d i s s o c i a t i o n energy of the H - X bond. S ince the d i s s o c i a t i o n energy decreases p r o g r e s s i v e l y for these molecules (5.83 eV. for HF, 4.43 eV. for HC1, 3.75 eV. for HBr and 3.06 eV. fo r H I ) , one might expect the va lue of the 2 2 + X ^ 3 / 2 ~ A E, energy s epa ra t i on to vary i n a p rog re s s ive manner. I f t h i s i s so, the 1;48 eV. va lue fo r the energy s e p a r a t i o n of these two s t a t e s i s c o n s i s t e n t whereas the 3.48 eV. va lue by N o r l i n g i s not . The p h o t o i o n i z a t i o n e f f i c i e n c y curve e x h i b i t s many peaks of nea r ly equal energy s e p a r a t i o n a f te r the second i o n i z a t i o n p o t e n t i a l l e a d i n g to the A 2 H + s t a t e of the HC1 + i o n . These peaks can be exp la ined as the au to ion i zed peaks which are caused by the e x c i t a t i o n of an e l e c t r o n to an e x c i t e d s t a t e of the molecule which has an energy grea te r than the s e c - , ond i o n i z a t i o n p o t e n t i a l , and fo l lowed r a p i d l y by a r a d i a t i o n -l e s s t r a n s i t i o n to the i o n i z a t i o n continuum w i t h the format ion of an i o n . The photon energy at the top of each peak i s g iven i n Table IX\ The energy s e p a r a t i o n between two adjacent peaks ( A v ) d i f f e r s by the order of 100 cm~l which corresponds to on ly 0.008 eV. The average energy s e p a r a t i o n between the peaks i s 1450 c m - 1 which g ives the v i b r a t i o n a l frequency for the e x c i t e d s t a t e of the hydrogen c h l o r i d e molecu le . S ince the i n t e r n u c l e a r d i s t ance fo r HC1 (normal ) i s n e a r l y eqqal to that f or HC 1^.(2 £ ) , the v i b r a t i o n a l frequency for HC1 (exc i t ed ) should be the same order of magnitude as that for HC1 + ( 2 £ ) . 70. Table IX A u t o i o n i z e d Peaks of Hydrogen C h l o r i d e T r a n s i t i o n s E (eV.) v (cm 1) Av ( c m - 1 ) 0-0 14. .55 117371 1535 0-1 14. . 74 118906 1431 0-2 14. ,92 120337 1466 0-3 15. . 10 121803 1273 0-4 15. ,26 123076 1148 0-5 15. .40 124224 1601 0-6 15, ,60 125825 1472 0-7 15. , 78 127307 1760 0-8 16. ,00 129067 1436 0-9 16. , 18 130503 1189 0-10 16. .34 131792 1630 0-11 16. ,54 133422 Average: 1450 cm -1 2 4- 4-The v i b r a t i o n a l frequency for the A £ s t a t e of HC1 i o n i s g iven by Herzberg (47) as 1526.5 c m - * . Therefore i t i s reasonable to i n t e r p r e t the au to ion i zed peaks as be ing due to the v i b r a t i o n a l s t r u c t u r e of the e x c i t e d s t a t e of the hydrogen c h l o r i d e molecu le . 71. CHAPTER SIX  P h o t o i o n i z a t i o n of Polya tomic M o l e c u l e s . A. Ammonia The ammonia molecule i s known (47) to have pyramidal symmetry C g v , and i t s molecular o r b i t a l formula i s : -( l a 1 ) 2 ( 2 a 1 ) 2 ( l e ) 4 ( 3 a 1 ) 2 ; 1A1 6.1 The molecular o r b i t a l s are l i s t e d i n the order of i n c r e a s i n g energy. The e l e c t r o n i c s t r u c t u r e of the ground s t a t e of the ammonia molecule can be compared w i t h that of the n i t r o g e n atom, which has a c o n f i g u r a t i o n of I s 2 , 2 s 2 , 2p^. When a n i t r o g e n r atom and three hydrogen atoms combine to form an ammonia mole-c u l e , the two Is e l e c t r o n s of the n i t r o g e n atom occupy the ( la^) o r b i t a l , the innermost o r b i t a l of the ammonia molecule . The two (2a^) o r b i t a l e l e c t r o n s are bonding e l e c t r o n s to a c e r t a i n ex ten t , and the e l e c t r o n s occupying t h i s o r b i t a l are the 2s e l e c t r o n s of the n i t r o g e n atom. One of the 2p e l e c t r o n s of the n i t r o g e n atom, and the three e l e c t r o n s of the hydrogen atom form the four ( l e ) o r b i t a l s i n ammonia. These are s t r o n g -l y bonding o r b i t a l s . The two remaining 2p e l e c t r o n s of the n i t r o g e n atom form the non-bonding o r b i t a l of (3a^) , which i s an o r b i t a l c o n s i s t i n g a p a i r of unshared e l e c t r o n s and i s l o c a l i z e d l a r g e l y on the n i t r o g e n atom of ammonia. The p h o t o i o n i z a t i o n e f f i c i e n c y curve for the ammonia molecule i s shown i n F i g u r e 19. The photon energy at the po in t of i n i t i a l onset of i o n i z a t i o n at 10.12 ± 0.05 eV. ob-v i o u s l y r e f e r s to the energy r equ i r ed to remove an e l e c t r o n from the non-bonding (3a^) o r b i t a l to form an NHg + i o n i n i t s 2 A-^  ground s t a t e , assuming the C 3 V symmetry i s r e t a i n e d i n the i o n . 72. The t h r e sho ld i o n i z a t i o n p o t e n t i a l of ammonia at 10.12 eV. i s i n good agreement w i t h the r e s u l t s obta ined by other workers u s ing d i f f e r e n t methods. Inn (60) i n 1953 and Watanabe (137) i n 1959, measured the absorp t ion and p h o t o i o n i z a t i o n c o e f f -i c i e n t s of ammonia us ing far u l t r a v i o l e t r a d i a t i o n , and determined the t h r e sho ld i o n i z a t i o n p o t e n t i a l of ammonia as 10.13 eV. and 10.15 eV. r e s p e c t i v e l y . Walker and W e i s s l e r (144) i n 1959 mea-sured the p h o t o i o n i z a t i o n e f f i c i e n c y and c ross s e c t i o n of ammonia and found the f i r s t i o n i z a t i o n p o t e n t i a l to be 10.07 eV. A l l these workers used p h o t o i o n i z a t i o n ins t ruments of d i f f e r e n t des ign , and used d i f f e r e n t methods for the i n t e r p r e t a t i o n of the photoionii-% z a t i o n da ta . The va lues fo r the f i r s t i o n i z a t i o n p o t e n t i a l of ammonia obta ined by the e l e c t r o n impact method are i n a l l cases s l i g h t l y h igher than these obta ined by p h o t o i o n i z a t i o n . Mann, H u s t r u l i d and Tate (76) i n 1940 u s ing the e l e c t r o n impact method, found the f i r s t i o n i z a t i o n p o t e n t i a l of ammonia to be 10.5 eV. L a t e r , F r o s t and McDowell (37) i n 1958, and M o r r i s o n and N i c h o l s o n (82) i n 1952, u s ing a modi f ied and much more s e n s i t i v e mass spec-t rometer , found , the f i r s t i o n i z a t i o n p o t e n t i a l of ammonia to be 10.40 eV. and 10.42 eV. r e s p e c t i v e l y . The f ac t tha t the e l e c t r o n impact method tends to measure the v e r t i c a l process and not n e c e s s a r i l y the minimum energy r e q u i r e d for i o n i z a t i o n , accounts for the i o n i z a t i o h p o t e n t i a l s obta ined by these methods be ing g e n e r a l l y l a r g e r (by 0.02 to 0 .5 eV.) than the a d i a b a t i c v a l u e s . A t h e o r e t i c a l c a l c u l a t i o n on the o r b i t a l energies of ammonia was undertaken by Duncan (22) i n 1957, and he found the f i r s t i o n i z a t i o n p o t e n t i a l of ammonia to be 9.94 e V . , which i s 73. c o n s i d e r a b l y lower than a l l the repor ted exper imenta l v a l u e s . Tab/le X summarises the repor ted va lues of the t h r e sho ld i o n i z a t i o n p o t e n t i a l s of ammonia:-Table X Threshold I o n i z a t i o n P o t e n t i a l of Ammonia I . P . (eV.) Workers Methods Year 10 . 12 + 0, .05 Present work P h o t o i o n i z a t i o n 1965 10 . 13 + 0. .02 Inn (60) P h o t o i o n i z a t i o n 1953 10 .07 + 0, .05 We i s s l e r (144) P h o t o i o n i z a t i o n 1955 10 . 15 + 0, .02 Watanabe (139) P h o t o i o n i z a t i o n 1959 10 . 15 • + 0 .02 Cook (12) P h o t o i o n i z a t i o n 1964 10 . 5 + 0, . 1 Tate (76) E l e c t r o n Impact 1940 10 .42 + 0 .05 M o r r i s o n (82) E l e c t r o n Impact 1952 10 .40 + 0 ,02 F r o s t (37) E l e c t r o n Impact 1958 9.! 94 Duncan (22) T h e o r e t i c a l 1957 The p h o t o i o n i z a t i o n e f f i c i e n c y curve of ammonia (F igu re 19) shows curva tu re near the t h r e s h o l d , and t h i s conf i rms the f i n d i n g of Watanabe (139) that the 0-0 t r a n s i t i o n of ammonia from the ground s t a t e of the molecule to the ground s t a t e of the i o n i s r a the r n o n - v e r t i c a l . The i o n i z a t i o n e f f i c i e n c y r i s e s : s t e a d i l y w i t h i n c r e a s i n g photon energy up to about 14 eV. af ter the t h r e sho ld i o n i z a t i o n p o t e n t i a l . Between 14 and 15 e V . , the i o n i z a t i o n e f f i c i e n c y remains f a i r l y cons tan t . . . A t the l a t t e r energy, an inner or second i o n i z a t i o n p o t e n t i a l i s i n d i c a t e d , s i n c e the curve begins to r i s e aga in . The photon energy at the po in t of s teepes t ascent i s 15.30 eV. corresponds to the second 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 . This second i o n i z a t i o n p o t e n t i a l i s i d e n t i f i e d as be ing due to the format ion of the f i r s t e x c i t e d 74. + 2 s t a t e of the NH i o n , i . e . the E s t a t e , formed by the removal of an e l e c t r o n from the ( l e ) degenerate o r b i t a l of the ammonia molecule . Th i s ('le) o r b i t a l of ammonia i s the main bonding o r b i t a l which spans the three N-H bonds, and the removal of an e l e c t r o n from here may cause the d i s s o c i a t i o n of the molecule w i t h the format ion of the N H 2 + i o n . A maximum i o n i z a t i o n e f f i -c i ency was observed at 15.70 eV. on the cu rve . Walker and W e i s s l e r (144) have found evidence of a second i o n i z a t i o n p o t e n t i a l of ammonia a l i t t l e over 15 eV. Duncan (22) c a l c u l a t e d the second i o n i z a t i o n p o t e n t i a l of ammonia to be at 16.20 eV< , and F r o s t and McDowell (37) used the e l e c t r o n impact method f i n d i n g the second i o n i z a t i o n p o t e n t i a l to be 15.31 eV. The D i s s o c i a t i o n of Ammonia The p h o t o i o n i z a t i o n e f f i c i e n c y curve fo r the N H 2 + i on i s shown i n F igu re 19. The appearance p o t e n t i a l of NH2 + i s 15.55 + 0.05 eV. Th i s va lue i s s l i g h t l y lower than that obta ined by W i l k i n s o n and Johnson (148) at 15.8 eV. i n 1950. The N H 2 + i o n can be formed from the A n s t a t e of the NH i o n . I f the NH 0 1 3 2 i o n i s assumed to have the symmetry C 2 v > the e l e c t r o n i c c o n f i g u -r a t i o n of the i o n i s p r o b a b l y : -( l e 1 ) 2 ( 2 a 1 ) 2 ( l e ) 2 ( 3 a 1 ) 2 ; ^ 6.2 + 1 + The combinat ion o f NH 2 ( A-^) + H (2s) c o r r e l a t e s w i t h NH^ 2 + 2 + ( A ) , and thus the format ion of NH Q from the A, s t a t e of NH 1 & 1 3 i s a l lowed by group theory . Recent s t u d i e s of the photochemical decomposi t ion of ammonia (21) g ive a mechanism for the N H 2 + i o n format ion which r equ i r e s a pr imary d i s s o c i a t i o n and i o n i z a t i o n process : 75. NH 3 + hv = NH 2 + H 6.3 NH + hv = NH * + e . 6.4 In the I n i t i a t i o n s tep , the ammonia molecule i s d i s s o -c i a t e d i n t o a r a d i c a l NH 2 and a hydrogen atom. In the second + + s tep , the NH 2 r a d i c a l i s i o n i z e d to g ive an NH^ i o n . The NH 2 i o n w i l l be predominent, s i n c e the i o n i z a t i o n p o t e n t i a l of NH Q i s much l e s s than that of the hydrogen atom. An a l t e r n a t i v e and e q u a l l y probable mechanism i s : -NH 3 + hv —?• N H 3 + ( 2 E) + e 6.5 N H ^ E ) — * N H 2 + + H 6.6 A l l these mechanisms w i l l be ope ra t i ve i n the format ion of N H 2 + ions at the expense of the ammonia parent i o n s . They he lp to e x p l a i n the d e c l i n e of the p h o t o i o n i z a t i o n e f f i c i e n c y for the ammonia pareirt ions at about 15.70 eV. , i f the peak i s not caused by a u t o i o n i z a t i o n . The energy r e q u i r e d to remove the f i r s t hydrogen atom, i . e . D (NH 2 -H) , i s known (117) to be 104 K c a l . (4.52 e V . ) . The appearance p o t e n t i a l of the NH^ + i o n , or V ( N H 2 + ) , has been o b t a i n -ed from F i g u r e 19 as 15.55 eV. Hence, u s ing the f o l l o w i n g equa-t i o n : -V ( N H 2 + ) = D(NH 2 -H) + I (NH 2 ) + K . E . + E . E 6.7 (where K . E . and E . E . are the k i n e t i c and e x c i t a t i o n energies of the N H 2 + : i o n ) , and s i n c e the N H 2 + i o n i s known to have l i t t l e excess k i n e t i c and e x c i t a t i o n energy (72), the i o n i z a t i o n p o t e n t i a l of NH 9 i s equal to or l e s s than 11.03 eV. B . Water Water vapor has been e x t e n s i v e l y s tud ied by absorp t ion spectroscopy below the th re sho ld i o n i z a t i o n p o t e n t i a l . However above t h i s energy data i s s c a r c e . P h o t o i o n i z a t i o n of water vapor i s important i n upper atmospheric s t u d i e s , because the extreme u l t r a v i o l e t l i g h t from the sun can i o n i z e the molecu le . Acco rd ing to M u l l i k e n (87) , the e l e c t r o n i c ground s t a t e of water has^A^ symmetry and i s de r ived from the e l e c t r o n i c con-f i g u r a t i o n : -( l a 1 ) 2 ( 2 a 1 ) 2 ( l b 2 ) 2 ( 3 a 1 ) 2 ( l b 1 ) 2 ; \ 6.9 Recent s e l f - c o n s i s t e n t molecular o r b i t a l c a l c u l a t i o n s by E l l i s o n and S h u l l (24) ha/e shown that t h i s i s the c o r r e c t order of the o r b i t a l s and t h a i the (lb-^) i s a pure 2 p x o r b i t a l of oxygen, the main bonding o r b i t a l s be ing (3a^) and ( l b 2 ) • The p h o t o i o n i z a t i o n e f f i c i e n c y curve of water i s shown i n F igu re 20. The i n i t i a l onset of the curve at 12.56 + 0.05 eV. g ives the energy for the removal of an e l e c t r o n from the ( lb^) npn-banding o r b i t a l . Th i s va lue i s i n e x c e l l e n t agreement w i th the s p e c t r o s c o p i c work by P r i c e , Teegen and Walsh (102) who were able to arrange the bands i n t o four Rydberg s e r i e s w i t h a common l i m i t at 12.56 e V . , and i s a l so i n good agreement w i t h the va lue of 12.59 eV. found by Watanabe, Nakayama and M o t t l (140), and Cook and Metzger (12) u s ing the p h o t o i o n i z a t i o n method, and 12.60 eV. by F r o s t and McDowell (37) , and Foner and Hudson (28) u s ing the e l e c t r o n impact t echnique . The p h o t o i o n i z a t i o n e f f i c i e n c y curve of water r i s e s g r a d u a l l y a f te r the t h r e sho ld energy and reaches a maximum about 16 eV. W i t h i n t h i s range, no e x c i t e d s t a t e s or a u t o i o n i z a t i o n 77. peaks were observed, and t h i s conf i rms the f i n d i n g s of Cook and Metzger (12) . Sugden and P r i c e (116) repor ted i n 1948 the f i n d -i n g of e x c i t e d s t a t e s of water ions at 16.2 and 18.0 e V . , but F i e l d and F r a n k l i n :(33) cons idered these e x c i t e d s t a t e s were doub t fu l probably on the ground that P r i c e ' s work was done w i t h -out mass a n a l y s i s , and the e x c i t e d s t a t e s observed might be due to i m p u r i t i e s . However, F r o s t and McDowell (37) repor ted e x c i t e d s t a t e s of the water i o n at 14.35 and 16.34 eV. by the e l e c t r o n impact method. The f a i l u r e to observe the e x c i t e d s t a t e s on the p h o t o i o n i z a t i o n e f f i c i e n c y curves i s probably due to the d i f f e r e n c e i n i o n i z a t i o n c ross s e c t i o n s i n the p h o t o i o n i z a t i o n and e l e c t r o n impact methods. The e x c i t e d s t a t e s of the water ions were probably formed by the removal of e l e c t r o n s from the main bonding o r b i t a l s ( l b 2 ) and Oa-^) , and the ions formed w i l l be i n ^ B ^ , and 2 B 2 s t a t e s . The removal of an e l e c t r o n from the main bonding o r b i t a l s may cause the b reak ing of the 0-H bond and d i s s o c i a t i o n of the water molecule w i th the format ion of H + or 0 H + . A survey of the l i t e r a t u r e found that the appearance p o t e n t i a l of 0 H + i s at about 18 eV. and that for H + i s at about 19.6 eV. 78. C. Methane and Deutero-methane Methane,deutero-methane and t h e i r fragment ions have been subjec ted to cons ide r ab l e study by many workers u s i n g p h o t o i o n i z a t i o n and e l e c t r o n impact methods. Seve ra l d i s c u s -s ions have been repor ted about p o s s i b l e i n c o n s i s t e n c i e s i n the e l e c t r o n impact r e s u l t s , and the accuracy of the de r ived d i s -s o c i a t i o n energy of the CH3 -H bond have been ques t ioned . The appearance of ions i n the p h o t o i o n i z a t i o n e f f i c i e n c y curves of methane and deutero-methane and t h e i r fragment ions has been f a i r l y sharp and t h i s permi ts qu i t e accurate de te rmina t ion of the appearance p o t e n t i a l s of the ions and the d i s s o c i a t i o n energ ies of the parent molecu les . There are ten e l e c t r o n s i n methane, and these can be put i n t o f i v e doubly occupied o r b i t a l s . The e l e c t r o n i c s t r u c t u r e of methane can be represented by: ( l s c ) 2 ( s a 1 ) 2 ( p t 2 ) 6 , 1 A 1 6.10 M u l l i k e n (85) has 1 shown that the t r i p l y degenerate ( p t 2 ) o r b i t a l has the lowest b i n d i n g energy, and the i o n i z a t i o n p o t e n t i a l of methane obta ined from the p h o t o i o n i z a t i o n e f f i c i e n c y curve (F igure 21) at 12.87 eV. can be r e f e r r e d to the removal of an e l e c t r o n from the ( p t 2 ) o r b i t a l to leave a C H ^ + i o n w i t h the e l e c t r o n i c s t r u c t u r e : ( l s c ) 2 ( s a i ) 2 ( p t 2 ) 5 , 2 T 2 6.11 Th i s t h r e sho ld i o n i z a t i o n p o t e n t i a l of methane at 12.87 eV. agrees w i t h the va lue at 12.8 eV. found by W e i s s l e r (143), at 12.98 eV. by Watanabe (140) and at 12.71 eV. by D i b e l e r , Krauss , Reese and H a r t l e e (19) u s ing the p h o t o i o n i z a t i o n method. The 79. repor ted va lues by e l e c t r o n impact methods g ive a somewhat higher v a l u e . Table XI summarizes the i o n i z a t i o n and appearance p o t e n t i a l s of methane, deutero-methane and the fragment i p n s . Table XI I o n i z a t i o n P o t e n t i a l s of Methane, Deutero-methane and Fragment Ions . I . P . 1 (eV.) Workers Method Year C H 4 + 12.87 -fc 0. 05 Th i s work P h o t o i o n i z a t i o n 1966 12.8 0. 2 W e i s s l e r (143) P h o t o i o n i z a t i o n 1955 12.98 ± 0. 05 Watanabe (140) P h o t o i o n i z a t i o n 1962 12. 71 ± 0. 02 D i b e l e r (19) P h o t o i o n i z a t i o n 1965 13.2 ± 0. 4 Smith (110) E l e c t r o n Impact 1937 13.0 -L 0. 2 K o f f e l (64) E l e c t r o n Impact 1948 13.04 ± 0. 03 Honig (52) E l e c t r o n Impact 1948 13.04 ± 0. 02 M i t c h e l l (80) E l e c t r o n Impact 1949 13.12 ± 0. 03 McDowell (71) E l e c t r o n Impact 1951 C H 3 + 14. 25 ± 0. 05 Th i s work P h o t o i o n i z a t i o n 1966 14.25 ± 0. 05 D i b e l e r (19) P h o t o i o n i z a t i o n 1965 14.5 0. 4 Smith (109) E l e c t r o n Impact 1937 14.4 0. 3 K o f f e l (64) E l e c t r o n Impact 1948 14. 5 0. 05 M i t c h e l l (80) E l e c t r o n Impact 1949 14.39 ± 0. 02 McDowell (71) E l e c t r o n Impact 1951 C D 4 + 13.00 0. 05 Th i s work P h o t o i o n i z a t i o n 1966 12.87 ± 0. 02 D i b e l e r (19) P h o t o i o n i z a t i o n 1965 C D 3 + 14.46 ± 0. 05 Th i s work P h o t o i o n i z a t i o n 1966 14.38 ± 0. 03 D i b e l e r (19) P h o t o i o n i z a t i o n 1965 The i o n i z a t i o n p o t e n t i a l of C D 4 at 13.00 ± 0.05 eV. 80. i s a l i t t l e h igher than that obta ined by D i b e l e r et a l . (19) at 12.87 eV. However, a study of t h e i r curve near the onset showed long t a i l i n g extending more than 0.5 eV. The e x t r a -po la ted v a l u e s , as they admit ted, have no fundamental s i g -n i f i c a n c e . The va lues obta ined by e l e c t r o n impact are as u sua l h ighe r : L o s s i n g , Turner and Bryce (69) obta ined a va lue of 13.21 e V . , and Honig (52) , a va lue of 13.30 eV. A d i f f e r e n c e of i o n i z a t i o n p o t e n t i a l s between CH^ and CD 4 i s a l so observed, and the d i f f e r e n c e : K C D 4 ) - I (CH 4 ) = 0.13 eV 6.12 i s a l s o i n good agreement w i th those obta ined by D i b e l e r et a l . (19) of 0.16 eV. and by L o s s i n g et a l . (69) of 0.18 eV. McDowell (72) po in ted out tha t the ( p t 2 ) o r b i t a l of the methane molecule w i l l be d i v i d e d i n t o two o r b i t a l s : (TT e) of symmetry E and ( zb 2 ) of symmetry B2, and the r e s u l t a n t s h i f t of the p o t e n t i a l minimum can l e a d to a change i n the r e l a t i v e popu la t ions of the 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 i n the ions of CH^ and CD^ as the amount of such v i b r a t i o n a l e x c i t a t i o n w i l l depend upon the ove r l ap i n t e g r a l between the ground v i b r a t i o n a l s t a t e and the upper s t a t e s , which i s app rec i ab ly grea te r for CH4 than for CD4 . Th i s may prov ide a p o s s i b l e e x p l a n a t i o n for the l a rge d i f f e r e n c e observed i n the i o n i z a t i o n p o t e n t i a l s of CH4 and C D 4 . The p h o t o i o n i z a t i o n e f f i c i e n c y curves of C H 4 + and CD^t r i s e a f te r the t h r e sho ld energy u n t i l about 14 eV. where they s t a r t l e v e l l i n g o f f . No a u t o i o n i z a t i o n peaks have been observed i n the curves fo r the molecular or fragment i o n s , and t h i s i s c o n s i s t e n t w i t h the observed f ac t tha t for hydrocarbons, the p o s s i b l e e x c i t a t i o n s r e s u l t on ly i n cont inuous abso rp t ion , 81. and do not g ive d i s c r e t e peaks i n the p h o t o i o n i z a t i o n e f f i c i e n c y cu rves . The appearance p o t e n t i a l of CHg + i s obta ined from the i n i t i a l 1 onset of the curve at 14.25 eV. and i s i n good agreement w i t h tha t obta ined by D i b e l e r et a l . (19) at 14.25 e V . , but about 0 .2 eV. lower than tha t obta ined by the e l e c t r o n impact method. The format ion of the CH3 4" i on can be represented by the process : CH4 + hv = C H 3 + + H + e 6.13 McDowell (74) i n d i c a t e d that at a photon energy of about 14 eV the C H ^ + i o n w i l l be formed i n a v i b r a t i o n a l l y e x c i t e d s t a t e , and consequent ly d i s s o c i a t i o n to y i e l d the methyl i o n and hydro-gen atom cou ld take p l a c e . The l e v e l l i n g o f f of the C H ^ + curve at t h i s energy i n d i c a t e s tha t the format ion of the CHg + i o n can indeed a r i s e from the d i s s o c i a t i o n of C U ^ + parent i o n s . The appearance p o t e n t i a l of CDg + measured from the i n i t i a l onset of the curve at 14.46~eV. i s a l i t t l e h igher than tha t obta ined by D i b e l e r et a l . (19) at 14.38 eV. The d i f f e r -ence i s q u i t e s m a l l c o n s i d e r i n g the long t a i l i n g at the onset on t h e i r cu rves . The d i f f e r e n c e between the appearance poten-t i a l s of CH3 and CD3 of 0.21 eV. i n d i c a t e s that there i s cons ide r ab l e displacement of the minima of the ground s t a t e of the parent i o n r e l a t i v e to that of the molecular ground s t a t e . The d i s s o c i a t i o n energy of CH3 - H can be obta ined from the f o l l o w i n g equa t ion : A ( C H 3 ) + = D(CH 3 -H) + I (CH 3 ) + K . E . + E . E 6.14 where the appearance p o t e n t i a l of CR^i" A(CHg)~t has been found to be 14.25 e V . , and the spec t ro scop i c i o n i z a t i o n p o t e n t i a l of 82. the methyl r a d i c a l , I ( C H 3 ) , i s 9.843 eV. (50) . McDowell (72) po in ted out that the format ion of CH3 4" ions at t h i s energy does not i n v o l v e k i n e t i c or e x c i t a t i o n ene rg ie s , and t h i s i s i n agreement w i t h the f i n d i n g of B e r r y (2) from d i s c r i m i n a t i o n experiments tha t the k i n e t i c and e x c i t a t i o n energies of C H ^ + i o n are 0.032 eV. i n excess of the thermal energy. Therefore , the d i s s o c i a t i o n energy of CH3 - H can be obta ined from equat ion 6.14 as 4 .41 eV. which agrees w i t h the va lue o f 4.41 eV. found by D i b e l e r et a l . (19) , 4 .42 eV. by Stevenson (113) and 4.42 eV. by E y r i n g (26) . S i m i l a r l y , w i t h the appearance p o t e n t i a l of CD3 as 14.46 eV. and the i o n i z a t i o n p o t e n t i a l of methyl -d^ as 9.832 eV.. (49) , the d i s s o c i a t i o n energy of CD3 -D can be obta ined as 4.63 eV. The d i f f e r e n c e i n the appearance p o t e n t i a l s between CH3 and CD3 can be used to determine the z e r o - p o i n t energy d i f f e r e n c e fo r the C H 3 + and C D 3 + ions (19) . D ( C H 3 + - H ) = E + Z C H 3 + - Z C H 4 + 6.15 D ( C D 3 + - D ) = E +: Z C D 3 + - Z C D 4 + 6.16 where ZV denotes the z e r o - p o i n t energy of the i o n spec ies x . A l s o : D ( C H 3 + - H ) = A ( C H 3 + ) - I (CH 4 ) 6.17 D ( C H 3 + - D ) = A ( C D 3 + ) - X(CD 4 ) 6.18 Equa t ing 6.15 and 6.17; 6.16 and 6.18, we have E + Z CH 3 + " Z CH 4 + " A < C H 3 + ) " K C H 4 ) 6.19 83 E + Z CD 3 + " Z CD 4 + * A(CV> " >«D 4 ) 6.20 S u b s t r a c t i n g 6.20 from 6.19, we have, The z e r o - p o i n t energy d i f f e r e n c e , Z - Z = ( Z C H 4 + " Z C D 4 + ) " I ( C H 3 ) + I ( C D 3 ) + A ( C H 3 ) - A(CD 3 ) = 0.23 eV. where, (Z - Z„„ ) = 0.31 eV. (47) . CH4+ CD4+ The va lue at 0.23 eV. i s i n good agreement w i t h the va lue at 0.18 eV. found by D i b e l e r et a l (19) . For comparison, we note tha t the z e r o - p o i n t d i f f e r e n c e fo r the i s o t o p i c ammonias, NH 3 and ND 3 i s 0.22 eV. (47) . The mass spec t r a of methane and deutero-methane at a : photon energy of 16.66 eV. i s shown i n F i g u r e 22. The r e l a t i v e i o n i z a t i o n p r o b a b i l i t i e s of these molecules are: C D 4 C H 4 C H 3 C D 3 100 96 80 69 RELATIVE IONIZATION^ CJ1 O CJi > — en H era P H CD ro C Z H _ 00 O W + O ' I > — (/I H O O o -n o i o o 84. D. Propylene Because of the great importance to o rgan ic chemis t ry of C=C bonds and the resonance e f f e c t s which a r i s e from the c o n j u -g a t i o n of them, the study of propylene has been undertaken by r many workers . P r i c e and T u t t l e (101) have s t ud i ed the m o l e c u l e ' s absorp t ion spectrum i n the fa r u l t r a v i o l e t r e g i o n ; Watanabe (139) measured the i o n i z a t i o n p o t e n t i a l of propylene u s ing the photo-e l e c t r i c method, and S t e i n e r , Giese and Inghram (114) combined mass spectrometer w i t h monochromator and measured the p h o t o i o r i i -o z a t i o n e f f i c i e n c y between 1050-1300A. Us ing the e l e c t r o n impact method, Fox and Hickam (30) , Stevenson and H i p p i e (111), M i t c h e l l and Coleman (80)"and Honig (52) have obta ined a f a i r l y accurate i o n i z a t i o n p o t e n t i a l of the molecule . The p h o t o i o n i z a t i o n e f f i c i e n c y curves of parent and fragment ions of propylene are shown i n F i g u r e 23. I t i s seen tha t the curve , u n l i k e those obta ined by e l e c t r o n impact, e x h i b i t s a sharp onset near the t h r e sho ld energy. The i n i t i a l onset of the curve y i e l d s the t h r e s h o l d i o n i z a t i o n p o t e n t i a l of 9.70 + 0.05 eV. which i s i n good" agreement w i t h the s p e c t r o s c o p i c va lue at 9.70 eV. by P r i c e and T u t t l e (101), and the p h o t o i o n i z a t i o n va lues at 9.73 eV. by Watanabe (139) and S t e i n e r , Giese and Inghram (114). The i o n i z a t i o n p o t e n t i a l o f the molecule obta ined by the e l e c t r o n impact data i s a l i t t l e h ighe r . Table X I I summarises the i o n i z a -t i o n p o t e n t i a l s of propylene obta ined i n t h i s work and by other workers . At a photon energy below 11 eV. the p h o t o i o n i z a t i o n e f f i c i e n c y fo r propylene shows a..maximum. Why t h i s should be so i s not c l e a r , un less a u t o i o n i z a t i o n i s r e s p o n s i b l e fo r i t . The i o n i n t e n s i t y begins to r i s e a f te r 11 eV. i n d i c a t i n g an 85, inner i o n i z a t i o n p o t e n t i a l . The photon energy at the po in t of t h e : s t e e p e s t ascent i n t h i s r e g i o n , about 11.1 e V . , should correspond to the second 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 which r e f e r s to the removal of a 0 " e l e c t r o n . Th i s va lue at 11.1 eV. i s h igher than those at 10.54 eV. by P r i c e and T u t t l e ( lO l ) and 10.54 eV. by Fox and Hickam (30) . Table X I I I o n i z a t i o n P o t e n t i a l s of Propylene I . P. ( e . V . ) Workers Method Year C 3 ;H6 (Threshold 1"I.P.) 9. 70 Present r e s u l t P h o t o i o n i z a t i o n 1966 9. 70 P r i c e (101) Spec t ro scop ic 1940 9. 73 Watanabe (139) P h o t o i o n i z a t i o n 1956 9. 73 Inghram (114) P h o t o i o n i z a t i o n 1957 9. 84 Honig (52) E l e c t r o n Impact 1948 10. 05 M i t c h e l l (80) E l e c t r o n Impact 1949 9. 77 Stevenson (111) E l e c t r o n Impact 1942 9. 78 Fox (30) E l e c t r o n Impact 1954 C 3 ;H6 (second I . P . ) 11 . 1 Present r e s u l t P h o t o i o n i z a t i o n 1966 10. 54 P r i c e (101) Spectroscopy 1940 10. 54 Fox (30) E l e c t r o n Impact 1954 C 3 iV (appearance p o t e n t i a l ) 11 . 95 Present r e s u l t P h o t o i o n i z a t i o n 1966 11 . 96 Stevenson (111) E l e c t r o n Impact 1942 The p h o t o i o n i z a t i o n e f f i c i e n c y curve of propylene shows a decrease at about 11.8 e V . , and t h i s may be due to the d i s s o c i a t i o n of the parent i o n w i t h the format ion of a C 3 H 5 + fragment. The appearance p o t e n t i a l of the C3H5"1" i o n i s 86. ;•• T '-.. d obta ined from the i n i t i a l onset of the p h o t o i o n i z a t i o n e f f i c i e n c y curve (F igure 23) at 11.95 e V . , which i s i n good agreement w i t h the va lue at 11.96 eV. by Stevenson and H i p p i e (111) . Table X I I summarises the i o n i z a t i o n p o t e n t i a l s of propylene and the appearance p o t e n t i a l of CgHg"1". i o n obta ined by d i f f e r e n t workers . B a r r i n g molecular rearrangement, the C _ H _ + i o n formed by the p h o t o - d i s s o c i a t i o n of propylene should a r i s e from the f o l l o w i n g mechanism:-CH 2=CH-CH 3 + hv = C H ^ C H - C H * + H + e ^ . . . 6 . 2 0 because the (C-H) bonds of the methyl group are g e n e r a l l y weaker (more r e a c t i v e ) than the bonds at tached to unsatura ted carbon atoms. The bond d i s s o c i a t i o n energy of propylene D ( a l l y l - H ) , can be obta ined from the appearance p o t e n t i a l of the a l l y l i o n by the f o l l o w i n g e q u a t i o n : -V(C 3 H 5 +) = D ( C 3 H 5 - H ) + I ( C 3 H 5 ) + K . E . + E . E 6.21 where V ( C 3 H g + ) i s the appearance p o t e n t i a l of the a l l y l i o n (11.95 eV.) and K . E . and E . E . - k i n e t i c a n d - e x c i t a t i o n : e n e r g i e s of the d i s s o c i a t i o n p roduc t s . The i o n i z a t i o n p o t e n t i a l of the a l l y l r a d i c a l , K C 3 H g ) , i s 8.16 + 0.03 eV. obta ined by L o s s i n g , Ingold and Henderson (68) . S ince the a l l y l i o n formed i s known to have l i t t l e or no excess k i n e t i c and e x c i t a t i o n energ ies , the bond d i s s o c i a t i o n energy D ( a l l y l - H ) i s equal or l e s s than 3.79 eV. i n good agreement w i t h the va lue at 79 +_ 6 k c a l . / m o l e or 3.43 +_ 0.4 eV. obta ined by McDowell , L o s s i n g , Henderson and Farmer (75) i n a study of the i o n i z a t i o n p o t e n t i a l s of methyl s u b s t i t u t e d a l l y l r a d i c a l s . 87. E . Ace ty lene A great dea l of data has been accumulated on the ace-ty l ene molecu le . I t i s known that i t i s l i n e a r and symmet r i ca l , and f i v e f requencies have been i d e n t i f i e d i n the e x c i t e d and unexc i ted s t a t e s . Ace ty lene has been s tud i ed by D i b e l e r and Reese (18) u s ing the p h o t o i o n i z a t i o n method, and by P r i c e (98) u s ing a Lyman continuum (who obta ined an ex tens ive system of o bands from 1520-1050A which y i e l d e d a s p e c t r o s c o p i c va lue for the i o n i z a t i o n p o t e n t i a l and a p l a u s i b l e 'value for the t r i p l e C-C bond). Ace ty lene has a l so been s tud ied by Turner (129) u s ing pho toe lec t ron spec t roscopy, and by L o s s i n g , T ickne r and Bryce (69) u s ing the e l e c t r o n impact method. Ace ty lene i s a l i n e a r molecule which has: four teen e l e c t r o n s . The ground s t a t e c o n f i g u r a t i o n of the molecule can be represented by: 6. 21 The molecular o r b i t a l s are l i s t e d i n the order of dec reas ing b i n d i n g energy, o m i t t i n g the inner e l e c t r o n s . The p h o t o i o n i z a t i o n e f f i c i e n c y curve fo r the ace ty lene molecule i s shown i n F i g u r e 24. The curve shows a s l i g h t c u r v a -ture at the i n i t i a l onset , and t h i s means that 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 ances are s u b s t a n t i a l l y changed i n the ions as compared to the n e u t r a l molecule , and the e l e c t r o n i n v o l v e d i n '; the i o n i z a t i o n process i s a bonding or a n t i - b o n d i n g e l e c t r o n . The i n i t i a l onset of the ace ty lene curve at 11.40 eV. i s s e l e c t e d as the t h r e sho ld i o n i z a t i o n p o t e n t i a l which i s a s soc ia t ed w i t h the energy fo r the removal of an e l e c t r o n from the (.TT ) bonding o r b i t a l . The th re sho ld i o n i z a t i o n p o t e n t i a l i s i n good agreement i 1 1 1 1 1 r P h o t o n e n e r g y , e V Figure 24-88. w i t h the va lue at 11.41 eV. from the spec t ro scop i c de te rmina t ion by P r i c e (98) , and the p h o t o i o n i z a t i o n measurements made by D i b e l e r and Reese (18) . Turner (129) obta ined a va lue of 11.36 eV. u s ing pho toe lec t ron spec t roscopy, and a va lue of 11.40 eV. was obta ined by L o s s i n g , T i ckne r and Bryce (69) u s ing the e l e c -t ron impact method. A f t e r the th re sho ld energy, there are many peaks on the p h o t o i o n i z a t i o n e f f i c i e n c y curve for ace ty lene as i l l u s t r a -ted i n F i g u r e 24. The energies cor responding to the tops of each peak on the p h o t o i o n i z a t i o n e f f i c i e n c y curve are recorded i n Table X I I I . Table X I I I Peaks due to V i b r a t i o n a l S t r u c t u r e of Ace ty lene T r a n s i t i o n s E (eV.) v (cm ) &v (cm ) 0-0 11, .40 91952 1856 0-1 11. ,63 93808 1854 0-2 11 ,86 95662 1830 0-3 12. ,09 97492 1858 0-4 12 .28 99050 1613 0-5 12. .48 100663 1774 0-6 12. , 70 102437 2098 0-7 12, .96 104535 1754 0-8 18. . 19 106389 1659 0-9 13 ,40 108084 1550 0-10 13 . 58 109634 1918 0-11 13 .83 111552 1855 0-12 14 .06 113407 1780 cm -1 From Table X I I I , the energy s e p a r a t i o n between [two 89. adjacent peaks, ( & v ) , d i f f e r s by the order of 100 cm~l which c-r-r:--TT--:ic. .r 'r: ; V:\\J ">. corresponds to on ly 0.008 eV. I t i s reasonable to say that i each peak i s separated from one another by an equal amount of energy, and the aver age energy s e p a r a t i o n between two adjacent peaks i s 1780 c m - 1 . The va lue of 1780 c m - 1 i s very c l o s e to the carbon-carbon s t r e t c h i n g f requencies of 1849 cm--'- for the 3R s t a t e s of ace ty lene repor ted by W i l k i n s o n (149). The photon energy for the f i r s t four peaks as shown i n the p h o t o i o n i z a t i o n e f f i c i e n c y curve for ace ty lene (F igure 24) are i n e x c e l l e n t agreement w i t h the four peaks i n the p h o t o i o n i z a t i o n e f f i c i e n c y curve obta ined by D i b e l e r and Reese (18) , who termed : them v i b r a t i o n a l t r a n s i t i o n s , 0-0 , 0 -1 , 0-2 , and 0-3 , of the ( 1 t i u ) s t a t e . Turner (129) i n a study of the pho toe lec t ron apectrum of ace ty lene , a l s o repor ted v i b r a t i o n a l s t r u c t u r e fo r ace ty lene , but he showed on ly two peaks at almost the same energy as the f i r s t two p e a k s . i n the curve repor ted here . From the evidence of W i l k i n s o n , D i b e l e r and Reese, and Turner , the peaks observed from the p h o t o i o n i z a t i o n e f -f i c i e n c y curve fo r ace ty lene are most probably due to the v i b r a t i o n a l t r a n s i t i o n s of the ( ^ ^ u ) s t a t e . D i s s o c i a t i o n of Ace ty lene The p h o t o i o n i z a t i o n e f f i c i e n c y curve for the C2H?" fragment i o n i s i l l u s t r a t e d i n F i g u r e 25. The appearance p o t e n t i a l of t h i s i o n i s obta ined from the po in t of i n i t i a l onset at 17.76 ± 0 . 0 5 eV. Th i s va lue agrees w e l l w i t h tha t at 17.8 eV. by Coats and Anderson (7) and Tate , Smith and Vaughan (121), and at 17.9 eV. by Kusch, H u s t r u l i d and Tate 90. (65) , a l l u s ing the e l e c t r o n impact method. There were no p h o t o i o n i z a t i o n and s p e c t r o s c o p i c data for comparison. The bond d i s s o c i a t i o n energy D(HC 2~H) can be obta ined from the f o l l o w i n g e q u a t i o n : -D(HC 2 -H) = V ( C 2 H ) + - I (C 2 H) - K . E . - E . E . . . . 6 . 2 2 The appearance p o t e n t i a l , V ( C 2 H + ) , i s found to be 17.76 e V . , and the i o n i z a t i o n p o t e n t i a l of the C 2 H r a d i c a l i s 11.3 eV. obta ined by E leh ton (23) . I f the k i n e t i c and e x c i t a t i o n ener-g ies are sma l l and can be neg lec t ed , the bond d i s s o c i a t i o n energy D(HC 2 -H) i s equal to or l e s s than 6.46 eV. i n good agree-ment w i t h the va lue of 6.5 eV. found by Coats and Anderson (7 ) . The p h o t o i o n i z a t i o n e f f i c i e n c y curve for the C 2 H + i o n remains f a i r l y constant between 18.4 and 18.7 eV. as shown i n F i g u r e 25. An inner appearance p o t e n t i a l of C H + i o n i s i n d i c a t e d as the curve s t a r t s to r i s e af ter the l a t t e r energy. The second appearance p o t e n t i a l of the C 2 H + i o n i s found from the po in t of s teepest ascent to be 18.96 eV. The p h o t o i o n i z a -t i o n e f f i c i e n c y curve remains constant a f t e r about 19.2 eV. 91. F . Methyl Cyanide Very l i t t l e work concerned w i t h the i o n i z a t i o n of methyl cyanide has been repor ted i n the l i t e r a t u r e even though methyl h a l i d e s and"hydrogen cyanide have been s tud i ed by many workers . McDowell (74) has desc r ibed the e l e c t r o n i c s t r u c t u r e of methyl cyanide as f o l l o w s : ( < r c + C T c , a 1 ) 2 ( T T N + TT C J e ) 4 (TT e ) 4 6.23 w i t h the molecular o r b i t a l s l i s t e d i n the order of decreas ing b i n d i n g energy, o m i t t i n g the inner o r b i t a l s . ( ( T Q + < J ~ C > I S the main bonding C - C o r b i t a l , the C H " N + T T Q > e) represents the two mutua l ly pe rpend icu la r degenerate bonding o r b i t a l s of the CN group, and the (TT e) o r b i t a l s are l a r g e l y l o c a l i s e d i n the CHg group. The p h o t o i o n i z a t i o n e f f i c i e n c y curve of methyl cyanide i s shown i n F i g u r e 26. The th resho ld i o n i z a t i o n p o t e n t i a l of methyl cyanide measured from the po in t of i n i t i a l onset of the curve i s 12.33 eV. which i s i n good agreement w i t h the e l e c t r o n impact data of 12.39 eV. by M o r r i s o n (82) and 12.52 eV. by McDowell and Warren (73) . A f t e r the t h r e sho ld energy, the photo-i o n i z a t i o n e f f i c i e n c y shows three d i s t i n c t s t eps . The second and t h i r d i o n i z a t i o n p o t e n t i a l s measured at the po in t s of s teepest ascent of the curve are 13.01 eV. and 13.80 eV. r e s p e c t i v e l y as i n d i c a t e d i n F i g u r e 26 by arrows. The i o n i z a t i o n e f f i c i e n c y curves of methyl cyanide and kryp ton by e l e c t r o n impact are shown i n F i g . 2 7 ( 4 0 ) . Three d i s t i n c t breaks are observed on the methyl cyanide curve , and the energ ies of these breaks correspond to the energies fo r three processes r e s p o n s i b l e fo r the CHgCN+ i o n fo rmat ion . These va lues are O LO o AON3IOIdd3 N0I1VZIN0I 92. 1 2 . 3 4 + 0 . 1 e V . , 1 2 . 9 6 + 0 . 1 eV. and 1 3 . 9 2 + 0 . 1 eV. which are i n good agreement w i t h the va lues obta ined by p h o t o i o n i z a t i o n . The three i o n i z a t i o n p o t e n t i a l s from p h o t o i o n i z a t i o n and e l e c t r o n impact s t u d i e s should be r e l a t e d to the outer three o r b i t a l s , s i n c e the next innermost o r b i t a l i s l a r g e l y l o c a l i s e d i n the C H ^ group, and the s i m i l a r c o r b i t a l s i n methane and the methyl h a l i d e s are bound by about 19 eV. The energy r equ i r ed fo r the removal of an e l e c t r o n from the (TT e) o r b i t a l of the methyl cyanide should be c l o s e to that from the s i m i l a r o r b i t a l o f methane, which i s 12.87 e V . , and one would expect to r e q u i r e about 13.7 eV. ( i o n i z a t i o n p o t e n t i a l of hydrogen cyanide) i n order to remove an e l e c t r o n from the C H ( T f ^ +TTQ> e ) o r b i t a l of the methyl cyan ide . I n t e r a c t i o n between the TT o r b i t a l s of the CHg and CN groups w i l l produce two new o r b i t a l s of the type (TT + TT Cng CN and ("^ Q J J ~ ~ ^ C N ^ ' a n t * S ° ""~S s u £ S e s t e d here tha t the second 3 i o n i z a t i o n p o t e n t i a l at 13.01 eV. r e f e r s to the removal of an e l e c t r o n from the (TT CE^ TT ^^.) o r b i t a l , and the t h i r d i o n i z a t i o n p o t e n t i a l at 13.80 eV. a r i s e s from the removal o f a (TT ^ ~ ^ C N ^ 3 predominant ly bonding e l e c t r o n . The methyl cyanide i o n w i l l be o formed i n a E s t a t e " i n each case . The th re sho ld i o n i z a t i o n p o t e n t i a l of methyl cyanide at 12.33 eV. apparent ly r e f e r s to the removal of a (<r + <T„, a,) c i bonding e l e c t r o n , as the s l i g h t cu rva tu re near the onset of the p h o t o i o n i z a t i o n e f f i c i e n c y curve of methyl cyanide i n d i c a t e s tha t the bonding e l e c t r o n may be i n v o l v e d i n the t h r e sho ld i o n i z a t i o n . The i o n i z a t i o n p o t e n t i a l of ethane, C 2 H g , i s 11.8 e V . , and t h i s r e f e r s to the removal of an e l e c t r o n from the C-C bonding (CT a^) o r b i t a l . The d i s s o c i a t i o n energy of ethane, D(HgC-CHg), i s 93. 3.68 e V . , and the d i s s o c i a t i o n of methyl cyan ide , D(H3C-CN), i s 5.8 eV. (40) . S ince d i s s o c i a t i o n energy may be cons idered to g ive a f a i r i n d i c a t i o n of the f i rmness w i t h which e l e c t r o n s are he ld i n bonding o r b i t a l s , we expect to f i n d the i o n i z a t i o n p o t e n t i a l for methyl cyanide somewhat h igher than 11.8 eV. Th i s t h r e sho ld i o n i z a t i o n p o t e n t i a l of methyl cyanide cou ld w e l l l i e at 12.33 e V . , which i s the energy at the i n i t i a l onset i n the methyl cyanide p h o t i o n i z a t i o n e f f i c i e n c y curve . F u r t h e r -more, i f the assignments r ega rd ing the two h igher i o n i z a t i o n p o t e n t i a l s of methyl cyanide are c o r r e c t , the remaining one -the lowest of a l l - should be a s soc ia t ed w i t h the <T bonding o r b i t a l s i n c e the next h igher unassigned o r b i t a l /should have an energy of about 19 eV. as mentioned above. Thus, the e l e c t r o n i c s t r u c t u r e of methyl cyanide should be w r i t t e n as f o l l o w s : (TT C H 3 - T T e N ) 4 ( T i C H ; a + T t C N ) 4 ( ( r c + C r c , a i ) 2 . . . 6.24 l ' . P . ( e V . ) 13.80 13.01 12.33 D i s s o c i a t i o n of Methyl Cyanide The p h o t o i o n i z a t i o n e f f i c i e n c y of the CH2CN+ fragment i o n i s shown i n F i g u r e 26. The appearance p o t e n t i a l of CH2CN+ i o n as measured from the po in t of i n i t i a l onset of the curve i s 13.86 eV. which i s lower than the e l e c t r o n impact data at 14.30 eV. by McDowell and Warren (73) . For the f i r s t h a l f of a v o l t a f t e r the th resho ld energy, the i o n i n t e n s i t y inc reases f a i r l y c o n s t a n t l y , and a change of s lope i s observed at 14.25 eV. which may correspond to the v e r t i c a l t r a n s i t i o n of the e l e c t r o n impact data at 14.30 eV. Three s teps are not so w e l l def ined as those i n the parent i o n curve because of the sma l l e r i o n i z a t i o n p r o b a b i l i t y . The process observed here for the format ion of the CH^CN^ fragment i on at 13.86 eV. can be represented as f o l l o w s : CH 3CN + hv = CH2CN+ + H + e 6.25 and the ene rge t i c s of the fragment i o n format ion are g iven by: V(CH 2 CN + ) = D(H-CH 2CN) + I(CH 2 CN) 6.26 S ince I (CH 2 CN) , the i o n i z a t i o n p o t e n t i a l of the fragment i o n , i s unknown, D(H -CH2CN) , the d i s s o c i a t i o n energy of the (H-CH2CN) bond cannot be ob ta ined . I f we assume DCH-CH^CN) = D ( H -CH3) , i . e . 4 .41 e V . , the i o n i z a t i o n p o t e n t i a l of the CH2CN fragment i o n i s approximately 9.4 eV. Table XIV R e l a t i v e I o n i z a t i o n P r o b a b i l i t i e s of Methyl Cyanide Ion Th i s work McDowel l (73)(a t 50V.) C H 3 C N + 100 100 CH 2CN 71.5 50.0 CHCN + 20.0 14. 7 C H 2 + 6.5 8.6 The mass spectrum of methyl cyanide i s shown i n F igu re 28. Table XIV shows the r e l a t i v e i o n i z a t i o n p r o b a b i l i -t i e s of the parent and fragment ions at a photon energy of 16.66 eV. obta ined i n t h i s work, and those of McDowel l ' s (73) at an e l e c t r o n energy of 50 V . o RELATIVE CJI IONIZATION T n o o 2 > IS CO OQ CO p H • c 85 O o X to + 0 o o o 1 I X I N i o to w 2 O O ro o cn o T o en 01 2 > CO CO CO m o> o 0) < H 30 c o X o o o X o + o X to o o X z + 95. G. Methyl A l c o h o l There are no spec t ro scop i c i o n i z a t i o n p o t e n t i a l va lues for t h i s molecule , because the vacuum u l t r a v i o l e t spectrum of methanol does not e x h i b i t w e l l def ined Rydberg s e r i e s . P h o t o i o n i z a t i o n of t h i s molecule has been s tud i ed by Inn (60) and Watanabe (135). Us ing the e l e c t r o n impact method, M o r r i s o n and N i c h o l s o n (82) , Stevenson (112), Cummings and Bleakney (16), Cox (15), Omura, Baba and H i g a s i (96) and Friedman, Long and Wolfsberg (34) , were able to o b t a i n an approximat ion to 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 . Accord ing to M u l l i k e n (87) , the ground s t a t e of methanol should be ^A s t a t e , and t h i s has the f o l l o w i n g e l e c t r o n i c c o n f i g u r a t i o n : -C H 3 0H: ( z ) 2 ( y ) 2 ( x ) 2 + 2 ( x ' ) 2 , X A 6.27 w i t h the molecular o r b i t a l s l i s t e d i n order of decreas ing b i n d i n g energy, o m i t t i n g the inner o r b i t a l s . The e l e c t r o n s i n the ( x ' ) o r b i t a l are l o c a l i z e d i n the oxygen atom, and are p r a c t i c a l l y f ree from m i x i n g . They have the lowest energy of any e l e c t r o n i n the molecu le . The (x) o r b i t a l i s s p l i t i n t o two o r b i t a l s d i f f e r i n g s l i g h t l y i n energy, be long ing essen-t i a l l y to the CH3 group. The (z) and (y) o r b i t a l s together g ive the 0 - H and 0 - C bonding r e s p e c t i v e l y , but the a c t u a l o r b i t a l s must, however, be mixtures of these extreme forms. The p h o t o i o n i z a t i o n e f f i c i e n c y curve for methanol i s shown i n F i g u r e 29. The th re sho ld i o n i z a t i o n p o t e n t i a l , at the po in t of i n i t i a l onset of i o n i z a t i o n , i s 10.53 eV. which r e f e r s to the removal of an e l e c t r o n from a non-bonding o r b i t a l ( x ' ) l o c a l i z e d mainly on the oxygen atom. Th i s va lue IONIZATION EFFICIENCY^ O Oi o 96. i s i n e x c e l l e n t agreement w i t h the p h o t o i o n i z a t i o n va lue at 10.52 eV. by Inn (60) . However, Watanabe (135) obta ined a h igher va lue at 10.35 eV. The curve g iven by Watanabe does not show a d i s t i n c t break near the i o n i z a t i o n t h r e s h o l d , but a weak, long t a i l i n the y i e l d curve which was a sc r ibed to i m p u r i t y . The p h o t o i o n i z a t i o n e f f i c i e n c y curve i n F i g u r e 29 shows a s l i g h t curva tu re near the t h r e sho ld energy, and t h i s i n d i c a t e s that 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 ances of the molecule (neutral.) and i o n are d i f f e r e n t . Th i s might be one of the reasons that no w e l l - d e f i n e d Rydberg s e r i e s are obta ined from the absorp t ion s p e c t r a of methanol. The e l e c t r o n impact data fo r the i o n i z a t i o n p o t e n t i a l range from 10.8 to 10.97 eV. and these are supposed to represent 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 . Table XV summarizes the t h r e sho ld i o n i z a t i o n p o t e n t i a l s of methanol: Table XV Threshold I o n i z a t i o n P o t e n t i a l of Methanol I . P . (eV. ) Workers Method Year 10. 53 ± 0 .05 Present work P h o t o i o n i z a t i o n 1966 10. 52 ± 0 .03 Inn (61) P h o t o i o n i z a t i o n 1953 10. 85 0 .05 Watanabe (142) P h o t o i o n i z a t i o n 1954 10. 8 i 0 .2 Bleakney (15) E l e c t r o n Impact 1940 10. 95 0 . 1 M o r r i s o n (84) E l e c t r o n Impact 1952 10. 86 ± 0 .05 Cqx (14) E l e c t r o n Impact 1954 10. 97 t 0, .05 Omura (100) E l e c t r o n Impact 1956 10. 9 ± 0. . 1 Friedman (34) E l e c t r o n Impact 1957 The second i o n i z a t i o n p o t e n t i a l of methanol (obtained from the po in t of s teepest ascent of the c u r v e ) , i s found to be 97. 12 . 90 eV. and r e f e r s to the removal of an e l e c t r o n from the (x) o r b i t a l , be long ing e s s e n t i a l l y to the CHg group. Three maxima were observed from the p h o t o i o n i z a t i o n e f f i c i e n c y curve at 13 .5 , 15.75 and 17.4 e V . , and the reason for those maxima i s not c l e a r . H a r r i s o n (45) has a l so repor ted two maxima i n h i s abso rp t ion s p e c t r a of methanol. D i s s o c i a t i o n of Methanol The p h o t o i o n i z a t i o n e f f i c i e n c y curve fo r the C H 2 0 H + fragment i o n i s i l l u s t r a t e d i n F i g u r e 29. The appearance p o t e n t i a l of t h i s i o n i s obta ined from the po in t of i n i t i a l onset of the curve at 11 . 5 2 ± 0.05 eV. which i s s l i g h t l y lower than the va lue at 11 . 8 eV. obta ined by Cummings and Bleakney (16) . The d i s s o c i a t i o n of the methanol molecule i n v o l v e s the b reak ing of a C-H bond, and CHg-O-H can become C H 2 = 0 + - H a f t e r d i s s o c i a t i o n . Furthermore, the change from s i n g l e to double bond ' g i v e s back' some of the energy o r d i n a r i l y r e -qu i r ed to break a C-H bond. Thecbond d i s s o c i a t i o n energy D(H-CH 2 0H) can be obta ined from the appearance p o t e n t i a l of the C H 2 0 H + i o n by the f o l l o w i n g e q u a t i o n : -D(H-CH 20H) - V ( C H 2 0H + ) + A E - I (CH3O) - K . E . - E . E . 6.28 where the appearance p o t e n t i a l , V ( C H 2 0 H + ) , i s 11 . 5 2 e V . , A E i s the energy d i f f e r e n c e between C - 0 and C=0 bonds which i s equal to 3.65 eV. (16) , and the i o n i z a t i o n p o t e n t i a l of the (CH 3 0) r a d i c a l , I ( C H 3 O ) , i s 1 0 . 7 eV. (63) . I f the k i n e t i c and e x c i t a t i o n energies are s m a l l and can be neg lec ted , the bond d i s s o c i a t i o n energy D(H-CH3) i s 4.41 eV. CHAPTER SEVEN 98. C o n c l u s i o n In the present work, we have been concerned w i t h the r e s u l t s of i o n i z a t i o n and d i s s o c i a t i o n of v a r i o u s spec ie s produced by p h o t o i o n i z a t i o n i n : a mass spect rometer . We have d i scussed i n some d e t a i l the i n t e r p r e t a t i o n of the p h o t o i o n i z a -t i o n e f f i c i e n c y curves and the measurement of i o n i z a t i o n and appearance p o t e n t i a l s , and the use of such data i n the evaluat ion of b o n d - d i s s o c i a t i o n energ ies and z e r o - p o i n t energy d i f f e r e n c e s between i s o t o p i c i o n s . We have a l so attempted to understand the nature of the t h r e sho ld i o n i z a t i o n law fo r p h o t o i o n i z a t i o n and the mechanisms fo r d i f f e r e n t types of processes such as exci ta- ; : t i o n , a u t o i o n i z a t i o n and p h o t o d i s s o c i a t i o n . In a d d i t i o n , we have been concerned w i t h theiinaghti which these s t u d i e s can provide on problems concern ing the e l e c t r o n i c s t r u c t u r e s of molecules and i o n s . P h o t o i o n i z a t i o n measurements w i t h mass a n a l y s i s p rov ide a powerful method for the de te rmina t ion of i o n i z a t i o n and appearance p o t e n t i a l s . Th i s method ( together w i t h the l e s s accurate e l e c t r o n impact one) i s o f ten the on ly a v a i l a b l e method of s t udy ing those molecules for which the Rydberg s e r i e s near the i o n i z a t i o n p o t e n t i a l i s complex, and for which the i o n i z a -t i o n l i m i t i s not a v a i l a b l e . In f avorab le cases , t h i s method can a l so de tec t v i b r a t i o n a l s t r u c t u r e s such as those found i n ace ty lene and hydrogen c h l o r i d e . P rev ious work on p h o t o i o n i z a t i o n i n t h i s l a b o r a t o r y u t i l i z e d a many- l ine l i g h t source , and t h i s l i m i t e d the number of data p o i n t s on the p h o t o i o n i z a t i o n e f f i c i e n c y cu rves . The 99. gap between two data points was often large and t h i s p r e v e n t e d certain fine features from being observed. In t h i s w o r k , a McPherson spark source produced a hydrogen or helium c o n t i n u u m of f a i r l y strong intensity, and a c o n t i n u o u s p h o t o i o n i z a t i o n efficiency curve could be obtained. Reproducible results on the p h o t o i o n i z a t i o n of A r , Kr. Xe, 0 2 , N 2 , C O , Cl2> H C 1 , N H 3 , N 0 2 , H p , C H 4 , C D 4 , C^EQ, G J H ^ CHgCNand CHgOH have been obtained in the e n e r g y region between 8 a n d 21 e V . The accuracy of ionization and appearance potential in this iwork is comparable to that of the s p e c t r o s c o p -ic method, and superior to that of the electron impact method. The atoms and. molecules investigated are of c o n s i d e r -able importance in many fields. They are rathax simple m o l e -c u l e s . T h e ^electronic structures and the fine features of the photoionization efficiency curves of these molecules s e e m c o m -paratively easy to explain. A l s o , the i o n i z a t i o n a n d appearance potentials of both the parent and fragment ions of these molecules are within our working range of 8 to 21 electron volts. Some of these molecules have been studied by many workers using different approaches, however the reported literature values were often inconsistent. The wish was to present new photoionization measurements, and try to explain the earlier inconsistencies. The main difficulty in this work is that the p h o t o n flux is quite low, and the ionization c r o s s s e c t i o n i s m u c h smaller than that obtained by the electron impact m e t h o d . In 100. order to secure workable pho to - ion c u r r e n t s , the mass s p e c t r o -meter and monochromator s l i t widths have to inc rease at the expense of r e s o l u t i o n . Th i s a f f e c t s the accuracy of the numeri-c a l va lues obta ined from the e f f i c i e n c y curves , and a l so prevents c e r t a i n f i n e s t r u c t u r e from be ing r e s o l v e d . Watanabe has repor ted the a c c i d e n t a l discovery that a s m a l l amount of p la t inum vapor depos i ted on the sur face of a g r a t i n g has an e f f e c t that g r e a t l y inc reases the l i g h t i n t e n s i t y , and he g ives c o n v i n c i n g evidence for t h i s i n l a t e r exper iments . Thus , : :f ur ther research p o i n t i n g i n t h i s d i r e c t i o n should not be de layed . However, a g r a t i n g u s u a l l y con ta ins about 30,000 l i n e s per i n c h , and the space between two adjacent l i n e s i s exceed ing ly s m a l l . The method of c o a t i n g a t h i n l a y e r of p la t inum wi thout a f f e c t i n g the f u n c t i o n i n g of the g r a t i n g i s another problem one w i l l have to face . The other d i f f i c u l t y i s tha t for some molecules such as n i t r o g e n , carbon monoxide, and oxygen, the p h o t o i o n i z a t i o n p r o b a b i l i t i e s l e a d i n g to h igher i o n i z a t i o n p o t e n t i a l s are s m a l l i n comparison w i t h the compe t i t i on of those processes such as a u t o i o n i z a t i o n . Theopno to ion iza t ion e f f i c i e n c y curve o f ten e x h i b i t s s t rong au to ion i zed peaks, and the inner i o n i z a t i o n con t inua of those molecules cannot be c l e a r l y seen. P h o t o i o n i z a t i o n i s a powerful method fo r s t u d y i n g the e l e c t r o n i c s t r u c t u r e of molecu les , and i n recent years i t has been extended to f ree r a d i c a l s t u d i e s . The r e a c t i o n s of atoms and free r a d i c a l s i n the gas phase are of cons ide r ab l e importance i n chemis t ry , and they have provided subjec t matter fo r a great many i n v e s t i g a t i o n s . Most of the data concern ing the i o n i z a t i o n of f re s p e c t r o s c o p i c and e l e c t r o n impac p h o t o i o n i z a t i o n of f ree r a d i c a l s i n recent years (59) . 101. e r a d i c a l s are g iven by the t method i n the past , and . has shown s i g n i f i c a n t progress 102. BIBLIOGRAPHY 1. Becker E . W.,and Goudsmit S. A . , "Atomic Energy S t a t e s " , McGraw H i l l , New York (1932). 2. Be r ry C. E . , Phys . Rev. 7_8, 597 (1950). 3. B e u t l e r H . , Z. Phys. 93 177 (1935). 4 . B r i o n C. E . , J . Chem. 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