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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 M.Sc,  B . S c . M c G i l l U n i v e r s i t y , 1960 U n i v e r s i t y of B r i t i s h C o l u m b i a , 1962.  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n the Department of Chemistry  We accept t h i s t h e s i s as conforming required standard  to  THE UNIVERSITY OF BRITISH COLUMBIA April,  1966.  the  In  presenting  requirements Columbia, for  I  agree  r e f e r e n c e and  extensive granted It  for  is  by  the  gain  Department  Date  in p a r t i a l  an a d v a n c e d  degree  at  that  the L i b r a r y  study.  I  of  thesis  this  that  shall  of  copying  of  British  Canada  April,  1966  shall  for  my D e p a r t m e n t  not  the  or  it  that  of  freely  of  the  British available  permission  for  scholarly  purposes  or  representatives.  by h i s  publication  be a l l o w e d w i t h o u t  Columbia  fulfilment  University  make  f u r t h e r agree  CHEMISTRY  University  V a n c o u v e r 8,  thesis  Head o f  understood  financial  The  copying  this  of  this  may  thesis  my w r i t t e n  be  for  permission  The U n i v e r s i t y  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  M.Sc,  B . S c , M c G i l l U n i v e r s i t y , 1960 The U n i v e r s i t y of B r i t i s h Columbia,  1962  TUESDAY, JULY 19, 1966, AT 3:30 P.M. IN ROOM 261,  CHEMISTRY BUILDING  COMMITTEE IN CHARGE Chairman:  C. V„ Finnegan  A. V. Bree D. C. F r o s t L. G. H a r r i s o n Research S u p e r v i s o r s :  C. A. McDowell R. Nodwell R. Stewart D. C. F r o s t C. A. McDowell  E x t e r n a l Examiner: G. L . W e i s s l e r Department of P h y s i c s U n i v e r s i t y of Southern C a l i f o r n i a U n i v e r s i t y Park Los A n g e l e s , C a l i f o r n i a  THE  PHOTOIONIZATION.AND DISSOCIATION OF MOLECULES ABSTRACT  The 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 o n of m o l e c u l e s was s t u d i e d u s i n g a combination of a vacuum monochromator and a mass spectrometer. The work was performed t o o b t a i n fundamental i n f o r m a t i o n about some simple molecules and t h e i r i o n s , and i t was hoped t h a t t h i s method, would prov i d e a good means f o r the d e t e r m i n a t i o n of a c c u r a t e i o n i zation potentials. 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 sixteen.atoms and molecules, namely: argon, krypton, 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-d^, p r o p y l e n e , a c e t y l e n e , methyl c y a n i d e and methyl a l c o h o l f o r the energy range from e i g h t to twenty-one e l e c t r o n v o l t s were o b t a i n e d . Numerical v a l u e s of i o n i z a t i o n and appearance p o t e n t i a l s were determined 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 c u r v e s , and the i o n i z a t i o n p o t e n t i a l s are d i s c u s s e d and compared w i t h those o b t a i n e d by other investigators. 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 s of these molecules are i n c l o s e agreement w i t h the s p e c t r o s c o p i c v a l u e s and are s u p e r i o r t o those o b t a i n e d by the e l e c t r o n impact method. The shape 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 c u r v e near the t h r e s h o l d g i v e s an i n d i c a t i o n as t o the type of e l e c t r o n removed i n the p h o t o i o n i z a t i o n p r o c e s s , and the c o r r e c t 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 molecule can some-timesbe deduced from the n u m e r i c a l v a l u e s of the i o n i z a t i o n p o t e n t i a l s as demonstrated i n the case of methyl c y a n i d e . The d i s s o c i a t i o n of ammonia, methane, methane-d^, propylene, a c e t y l e n e , methyl c y a n i d e and methyl a l c o h o l was s t u d i e d , and the mechanisms f o r the d i s s o c i a t i o n p r o c e s s e s were d i s c u s s e d . 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 of the fragment i o n s , numberical v a l u e s 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 t e n t i a l s of r a d i c a l s and z e r o - p o i n t d i f f e r e n c e f o r the i s o t o p i c ions are deduced.  1  A u t i o n i z a t i o n p r o c e s s e s were observed i n the study of krypton, xenon, oxygen, n i t r o g e n , carbon monoxide, hydrogen c h l o r i d e and a c e t y l e n e . That the peaks 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 curves of these s p e c i e s are indeed due t o a u t o i o n i z a t i o n has been confirmed by comparison w i t h c o r r e s p o n d i n g peaks i n the o p t i c a l a b s o r p t i o n s p e c t r a . The v i b r a t i o n a l . f r e q u e n c i e s of hydrogen c h l o r i d e and a c e t y l e n e i n the e x c i t e d s t a t e s c o u l d be deduced from the energy s e p a r a t i o n between two a d j a c e n t a u t o i o n i z a t i o n peaks.  GRADUATE STUDIES Field  of Study:  Chemistry  Topics i n Physical  Chemistry  Topics i n Inorganic  T o p i c s i n Organic Spectroscopy Structure  Chemistry  Chemistry  and M o l e c u l a r  Seminar i n Chemistry Quantum Chemistry. S t a t i s t i c a l Mechanics  C. A. McDowell R. F. S n i d e r J . A. R, Coope H. C. C l a r k N. B a r t l e t t W. R. C u l l e n R. Stewart J . P. Kutney R. E. I . P i n c o c k C. Reid L. W. Reeves . K. B. Harvey J . N. B u t l e r R. H o c h s t r a s s e r R. F. S n i d e r  Related Studies: C a l c u l u s and D i f f e r e n t i a l Equations Modern P h y s i c s Elementary Quantum Mechanics  S, A„ W.  Jennings  M. Bloom Opechowski  PUBLICATION  D. C. F r o s t , D. Mak and C. A. McDowell, P h o t o i o n i z a t i o n of N i t r o g e n  Dioxide",  Canadian J o u r n a l of Chemistry, 1064 (1962) .  "The  40y  (ii) ABSTRACT The p r e s e n t work i s concerned w i t h p h o t o i o n i z a t i o n efficiencies  o f gases and v a p o r s determined as a f u n c t i o n o f  photon energy by vacuum s p e c t r o s c o p y and mass a n a l y s i s . 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  The  fundamental  i n f o r m a t i o n about some s i m p l e m o l e c u l e s , and i t was hoped t h a t the r e s u l t s would p r o v i d e a means t o e x p l a i n the apparent d i s c r e p a n c i e s o f 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 s p r e v i o u s l y r e p o r t e d by o t h e r  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 r y p t o n , • xenon, oxygen,  nitrogen,  carbon monoxide, c h l o r i n e , hydrogen c h l o r i d e , ammonia, methane,  methane-d4,  propylene,  acetylene,  methyl c y a n i d e and  m e t h y l a l c o h o l f o r the energy range from e i g h t to e l e c t r o n v o l t s are p r e s e n t e d .  water,  twenty-one  Photoionization efficiency  c u r v e s of these m o l e c u l e s were o b t a i n e d from which n u m e r i c a l values 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 the f i n e s t r u c t u r e  and d i s s o c i a t i o n e n e r g i e s  are  and a u t o i o n i z a t i o n p r o c e s s e s  deduced, are  interpreted. The r e s u l t s o b t a i n e d by o t h e r  are d i s c u s s e d and obmpared w i t h  investigators.  those  The t h r e s h o l d and i n n e r  i o n i z a t i o n p o t e n t i a l s o f these m o l e c u l e s are i n c l o s e agreement w i t h s p e c t r o s c o p i c v a l u e s and are s u p e r i o r to those o b t a i n e d by the e l e c t r o n impact method. A brief  account o f the h i s t o r i c a l developments  i n g to the p r e s e n t work i s d e s c r i b e d ,  lead-  and a few e x i s t i n g methods  f o r the d e t e r m i n a t i o n o f i o n i z a t i o n p o t e n t i a l s w i t h  their  (iii) advantages and limitations are pointed out.  The  components of the instrument and their s p e c i a l  essential characteristics  are briefly discussed, and the major sources of e r r o r included.  The limitations of the a p p a r a t u s  at  are  the  stage are pointed out, and improvements are suggested .  also  present 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 i s also suggested.  (iv)  AC KNOWLEDGEMENTS  I t i s a p l e a s u r e to acknowledge my g r a t i t u d e P r o f e s s o r C . A . McDowell and D r . D. C . F r o s t f o r constant i n t e r e s t this research.  to  their  and encouragement d u r i n g the c o u r s e o f  I w i s h to e x p r e s s my g r a t i t u d e  to  D r . C. E . B r i o n and my c o l l e a g u e s f o r t h e i r many h e l p f u l discussions. of  Thanks are a l s o due t o the t e c h n i c a l s t a f f s  the C h e m i s t r y Department at the U n i v e r s i t y o f B r i t i s h  Columbia f o r t h e i r s k i l f u l  assistance.  (v) CONTENTS PAGE ABSTRACT  " i i  ACKNOWLEDGMENTS I.  INTRODUCTION  1  A.  General  1  B.  Ionization Potentials  3  1.  3  2. 3.  C. II.  III.  iv  Introduction A d i a b a t i c and V e r t i c a l Ionization potentials Determination of  3  Ionization potentials  5  a)  O p t i c a l Spectroscopy  5  b)  Cyclic  6  c)  E l e c t r o n Impact S t u d i e s  d)  Photoelectron  e)  Photon Impact Method  Method  6  Spectroscopy  ....  8 9  H i s t o r i c a l Review o f P h o t o i o n i z a t i o n  11  THEORETICAL  15  A.  Photoionization  15  B.  Autoionization  17  C.  T h r e s h o l d 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 H e l i u m S p e c t r a  24  EXPERIMENTAL A.  Introduction  ;  27.  B.  The Mass S p e c t r o m e t e r  30  1.  Ion Source  30  2.  A n a l y s e r and E l e c t r o m a g n e t  31  3.  Electron Multiplier  33  4.  V i , b r a t i b g Reed E l e c t r o m e t e r  .......  33  (vi) CONTENTS (Continued) C.  D.  E.  F.  PAGE  The Monochromator  35  1.  L i g h t Source  35  2.  G r a t i n g System  36  3.  Photon M o n i t o r . . . ..b  37  4.  Energy C o n v e r s i o n S c a l e  38  The Vacuum System  40  1.  A n a l y s e r Tube  40  2.  Monochromator  40  3.  L i g h t Source  40  4.  Gas H a n d l i n g System  41  5.  Measurement of P r e s s u r e . *  41  E x p e r i m e n t a 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  CSources of E r r o r  ....  44 JJ46  RESULTS AND DISCUSSION IV.  V.  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  P h o t i o n i z a t i o n of D i a t o m i c M o l e c u l e s A.  Oxygen  53  B.  Nitrogen  58  C.  Carbon Monoxide  62  D.  Chlorine  66  E.  Hydrogen C h l o r i d e  68  CONTENTS (Continued) VI.  VII.  (vii)  PAGE  P h o t o i o n i z a t i o n of Polyatomic Molecules, A.  Ammonia  71  B.  Water  76  C.  Methane and Deutero-methane  78  D.  Propylene  84  E.  Acetylene  87  F.  M e t h y l Cyanide  91  G.  Methanol  95  CONCLUSION  BIBLIOGRAPHY  98 102  (viii) LIST OF TABLES PAGE I.  The T h r e s h o l d Laws of Photon  and  E l e c t r o n Impact II. III. IV. V. VI. VII. VIII. IX. X. XI. XII.  21  A u t o i o n i z a t i o n Peaks of K r y p t o n  50  A u t o i o n i z a t i o n Peaks of Xenon  52  Ionization Potential  of Oxygen  54  A u t o i p n i z a t i o n Peaks o f Oxygen  56  '.'Threshold  I . P . of N i t r o g e n  59  A u t o i o n i z a t i o n Peaks of N i t r o g e n  61  A u t o i o n i z a t i o n Peaks o f Carbon Monoxide  63  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  Ionization Potential  73  Ionization P o t e n t i a l Ionization Potential  of Ammonia of Methane,  Deutero-^Methane.  of P r o p y l e n e  XIII.  A u t o i o n i z a t i o n Peaks o f A c e t y l e n e  XIV.  Relative Ionization Probabilities M e t h y l Cyanide I o n i z a t i o n P o t e n t i a l of Methanol  XV.  . ..  79 85 88  of 94 96  (ix) LIST OF FIGURES AFTER PAGE 1.  P o t e n t i a l Energy Curves  2.  Autoionization  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.  H e l i u m Spectrum  25  6.  The Monochromator  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 o f 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 o f 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 o f Carbon Monoxide, I  62  16.  and Mass Spectrometer  3  27  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  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 . . . .  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  62 66  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 o f 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  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 o f M e t h y l Cyanide ....,  91  I o n i z a t i o n E f f i c i e n c y Curves of K r y p t o n and M e t h y i C y a n i d e  91  28.  Mass Spectrum of M e t h y l 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 M e t h a n o l .  95  27.  +  90  CHAPTER ONE INTRODUCTION A.  General This thesis  i s m a i n l y concerned w i t h the r e s u l t s  i o n i z a t i o n and d i s s o c i a t i o n of m o l e c u l e s when s u b j e c t e d impact i n the wavelength r e g i o n 15008 - 500A  of  the  t o photon  (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 c a p a b l e of removing the v a l e n c e e l e c t r o n from the atom or m o l e c u l e , and sometimes e l e c t r o n s which are somewhat more s t r o n g l y bound. is  This r a d i a t i o n  a l s o c a p a b l e o f b r e a k i n g c h e m i c a l bonds. A 1-meter Seya-Namioka type s c a n n i n g vacuum u l t r a v i o l e t  monochromator c o u p l e d w i t h a N i e r - t y p e mass s p e c t r o m e t e r to measure i o n i z a t i o n p o t e n t i a l s p o t e n t i a l s of t h e i r fragment Mass s p e c t r o m e t r i c  of m o l e c u l e s and appearance  ions. 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 o f t e n l e d to a b e t t e r u n d e r s t a n d i n g o f the v a r i o u s which occur when m o l e c u l a r and fragment impact.  was used  reactions  i o n s are formed by photon  C a r e f u l e x a m i n a t i o n o f 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 c u r v e s has i n g e n e r a l enabled i o n i z a t i o n and fragmentation  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 o f the c u r v e s t o be i n t e r p r e t e d and v i b r a t i o n a l energy  i n terms of e l e c t r o n i c  states.  T h i s work i s concerned w i t h t h r e e problems: determine a c c u r a t e l y the photon energy n e c e s s a r y of  (a)  to  f o r the removal  an e l e c t r o n from a m o l e c u l e to form an i o n , t h i s amount of  energy b e i n g e q u a l t o the i o n i z a t i o n p o t e n t i a l o f the m o l e c u l e , (b)  to determine the photon energy r e q u i r e d to form X  +  i o n from a  m o l e c u l e XY, and from t h i s energy t o o b t a i n the d i s s o c i a t i o n energy  o f the bond X - Y ,  and (c)  to determine  photoionization,  i f any, e . g . whether  the p r o d u c t s o f m o l e c u l a r i n methane,  of photon o f a g i v e n energy produces C H ^ or +  CHg  the +  absorption  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 f o r m i n g them.  B.  Ionization 1.  Potentials  Introduction D u r i n g the l a s t f o r t y y e a r s ,  potentials  the study o f i o n i z a t i o n  of gaseous atoms and m o l e c u l e s has o c c u p i e d an  i n g number o f w o r k e r s , a c c u r a t e l y determined literature.  and a c o n s i d e r a b l e number o f ionization potentials  increas-  fairly  have appeared  i n the  These have been made p o s s i b l e by methods o f p p t i c a l  spectroscopy,  photoelectron  spectroscopy,  e m p i r i c a l c a l c u l a t i o n s , charge t r a n s f e r  theoretical  spectra,  and s e m i -  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  is  d e f i n e d as the energy r e q u i r e d to remove an e l e c t r o n c o m p l e t e l y from the ground v i b r a t i o n a l l e v e l o f the l o w e s t e l e c t r o n i c of the m o l e c u l e to the ground v i b r a t i o n a l l e v e l o f the  state  relevant  e l e c t r o n i c s i i a t e o f the m o l e c u l e - i o n . The p r o b a b i l i t y o f i o n i z a t i o n o f a d i a t o m i c m o l e c u l e 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 c o n s i d e r e d be governed by the Born-Oppenheimer  to  approximation:-  where ^v' i s the v i b r a t i o n a l w a v e f u n c t i o n of the l e v e l v ' o f  the  i o n , M'VJ" i s the v i b r a t i o n a l w a v e f u n c t i o n o f the ground s t a t e  (v"=0)  of the n e u t r a l m o l e c u l e , and r i s the i n t e r n u c l e a r  separation.  D i a t o m i c m o l e c u l e s can be t r e a t e d as anharmonic tors,  and the v i b r a t i o n a l w a v e f u n c t i o n s  for d i f f e r e n t  oscilla-  vibrational  l e v e l s o f the m o l e c u l e and i t s i o n have the form shown i n F i g u r e 1  Suppose the minima o f 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.  It  p r o b a b i l i t y , whereas  i s c l e a r t h a t the  (0-1),  (0-2) e t c .  (0-0)  t r a n s i t i o n has h i g h  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 n e g a t i v e p a r t o f the i n t e g r a l  partly  c a n c e 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  in  energy between the ground v i b r a t i o n a l s t a t e o f 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  tances f o r the m o l e c u l e and i t s i o n are the same, p o t e n t i a l s o b t a i n e d by o p t i c a l s p e c t r o s c o p y ,  dis-  the i o n i z a t i o n  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 s h o u l d a l l c o r r e s p o n d to the  adiabatic  value,  enought.  p r o v i d e d t h a t the i n s t r u m e n t s  used are s e n s i t i v e  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 a n c e o f  the  i o n i c s t a t e i s g r e a t e r than t h a t of the m o l e c u l a r ground s t a t e , most p r o b a b l e t r a n s i t i o n i s to a h i g h e r v i b r a t i o n a l l e v e l and  the 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 i g h e r than adiabatic value.  the  The same i s t r u e 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 a n c e o f the i o n i c s t a t e i s l e s s than t h a t o f m o l e c u l a r ground s t a t e ,  the  and the h i g h e r v a l u e f o r 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 v a l u e f o r the i o n i z a t i o n p o t e n t i a l o b t a i n e d by e l e c t r o n and photon impact s h o u l d depend l a r g e l y on the s e n s i t i v i t y f o r d e t e c t i o n of i o n s .  The g e n e r a l p i c t u r e f o r p o l y a t o m i c m o l e c u l e s  s h o u l d be s i m i l a r . In  photoionization studies,  ionization potentials  o b t a i n e d 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 c u r v e s are i n g e n e r a 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 o b t a i n e d by o t h e r methods.  When i o n i z a t i o n i s  caused  by the removal o f a b o n d i n g e l e c t r o n and the minima o f 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 c u r v e u s u a l l y shows c u r v a t u r e the t h r e s h o l d .  near  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 c u r v e at the p o i n t o f s t e e p e s t s l o p e b e f o r e  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 o f a m o l e c u l e w i l l always be e q u a l to or h i g h e r than the a d i a b a t i c v a l u e . e x p e r i m e n t a l e v i d e n c e has shown t h a t the d i f f e r e n c e  In  fact,  is usually  between 0 . 0 2 to 0 . 5 e l e c t r o n v o l t . 3.  Determination of I o n i z a t i o n P o t e n t i a l s Ionization potentials  are among the most  important  p r o p e r t i e s o f a m o l e c u l e , and i t i s d e s i r a b l e to have methods o f d e t e r m i n i n g them a c c u r a t e l y . a) The O p t i c a l S p e c t r o s c o p i c Method One o f the most a c c u r a t e methods f o r the  determination  o f 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 s p e c t r o s c o p y . i n v o l v e s a study of absorption s p e c t r a ,  This  and the f i t t i n g o f  the  d a t a i n t o Rydberg s e r i e s : v  =  I  — —-_ (n + a )  where:  1.2  R  2  I and a are c o n s t a n t s s p e c i f i c to a p a r t i c u l a r m o l e c u l e , v i s the w a v e l e n g t h o f a p a r t i c u l a r m o l e c u l e , R i s the Rydberg c o n s t a n t , n i s an i n t e g r a l v a l u e r e p r e s e n t i n g Rydberg band. Once the Rydberg (0-0)  the  particular  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 a b s o r p t i o n s p e c t r a ,  or by  comparison w i t h the s p e c t r a o f i s o t o p i c m o l e c u l e s , the i o n i z a t i o n  p o t e n t i a l of a m o l e c u l e can r e a d i l y be c a l c u l a t e d .  The wave-  l e n g t h of s p e c t r a l bands i n s p e c t r o s c o p i c work can be measured w i t h a h i g h degree of a c c u r a c y , of  and the u n c e r t a i n t y  i n the v a l u e  the i o n i z a t i o n p o t e n t i a l d e r i v e d from i t i s u s u a l l y o n l y a  few p a r t s o f a thousand.  However, t h i s method cannot be a p p l i e d  to q u i t e a l a r g e number o f m o l e c u l e s which g i v e c o n t i n u o u s or diffused  spectra,  transitions  and an unambiguous assignment of the Rydberg  i s not then p o s s i b l e .  b) The C y c l i c Method When the a b s o r p t i o n spectrum of a m o l e c u l e i s so complex t h a t the Rydberg s e r i e s the i o n cannot be o b t a i n e d , method to determine  l e a d i n g to the ground s t a t e of  some w o r k e r s (84) have used a c y c l i c  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  T h i s method i s based on the f o l l o w i n g  equation:-  I(XY) + D ( X Y ) = I ( X ) + D (XY)  1.3  +  Q  Q  where I ( X Y ) i s the i o n i z a t i o n p o t e n t i a l o f XY, I ( X ) i s i o n i z a t i o n p o t e n t i a l of X,  (XY) and D ( X Y ) +  Q  the  are the d i s s o c i a -  t i o n e n e r g i e s o f the m o l e c u l e and the i o n r e s p e c t i v e l y . I(X),  D (XY) and D ( X Y ) +  Q  Q  are known a c c u r a t e l y ,  Tf  the I . P . ( X Y )  of  the m o l e c u l e can be d e t e r m i n e d . c)  The E l e c t r o n Impact Method The e s s e n t i a l s  f o 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 through the gas under i n v e s t i g a t i o n , and a d e v i c e f o r  passed detecting  the i o n s produced and f o r measuring t h e i r i n t e n s i t y .  Electron  impact s t u d i e s have been h i g h l y d e v e l o p e d ,  f a r more  and are  g e n e r a l l y a p p l i c a b l e because m o l e c u l e s 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 m o l e c u l e s , the impact method p r o v i d e s the o n l y way to determine the  electron  ionization  potential. The defects.  e l e c t r o n impact method s u f f e r s  The f i r s t a r i s e s through u s i n g an e l e c t r o n beam e m i t t e d  from a hot f i l a m e n t . tic  from s e v e r a l  i n character  T h i s e l e c t r o n beam w i l l  but w i l l  not be monoenerger  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 m a i n l y M a x w e l l - B o l t z m a n i n n a t u r e , and i s , of c o u r s e ,  governed by the temperature o f the  In a d d i t i p n , a f u r t h e r  filament.  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 l o n g the  filament  due to c o n d u c t i o n of heat through the s u p p o r t i n g l e a d s ,  and by  the v o l t a g e drop a c r o s s the f i l a m e n t . charged p a r t i c l e s ,  Since electrons  are  the e l e c t r i c f i e l d which i s n e c e s s a r y  to  produce an i o n beam i n the i o n s o u r c e u s u a l l y a l s o p e r t u r b s e l e c t r o n energy.  The d i f f i c u l t y  with s u f f i c i e n t l y  low energy spread causes much o f the  the  of o b t a i n i n g an e l e c t r o n beam informat  t i o n o b t a i n e d by the e l e c t r o n impact method t o be o f low p r e c i sion,  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  c u r v e s to be  unresolved. 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 p r e s e n t 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 i n g selectors.  Success i n t h i s f i e l d  under  electrostatic  of study w i l l g r e a t l y improve  the accuracy o f v a l u e s of the i o n i z a t i o n p o t e n t i a l s o b t a i n e d by the e l e c t r o n impact method. Wannier (131) proposed a t h e o r y f o r the i o n i z a t i o n o f m o l e c | i l e by e l e c t r o n impact near the t h r e s h o l d . jfche two slow e l e c t r o n s ,  He s t a t e d  upon emerging from the i o n , remain  that  8. w i t h i n the r e a c t i o n zone at the t h r e s h o l d energy o f the i m p a c t i n g electron.  Only when each slow e l e c t r o n has a p p r e c i a b l e k i n e t i c  energy can i t escape from t h i s r e g i o n and o n l y then does the i o n c u r r e n t s t a r t to grow.  For t h i s reason,  i t i s quite possible that  i o n i z a t i o n c r o s s s e c t i o n s are z e r o or v a n i s h i n g l y s m a l l f o r e l e c t r o n ( i m p a c t i n g ) o f an energy e q u a l t o or j u s t e x c e e d i n g the t h r e s h o l d i o n i z a t i o n energy.  I f t h i s i s s o , i t p l a c e s a r e s t r i c t i o n on the  u l t i m a t e a c c u r a c y o b t a i n a b l e f o r e l e c t r o n impact measurements of ionization potentials,  even w i t h improved methods o f 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) P h o t o e l e c t r o n S p e c t r o s c o p y T h i s i s a r a t h e r new t e c h n i q u e r e p o r t e d by K u r b a t o v , V i l e s o r and T e r e n i n ( 6 6 ) , Schoen (107) and Turner and A l - J o b o u r y (127-129), f o r the d i r e c t measurement o f i o n i z a t i o n p o t e n t i a l s o f a m o l e c u l e l e s s than 21.21 eV.  The gas under study i s  by a beam o f photons of energy 21.21 eV. the e m i s s i o n o f p h o t o e l e c t r o n s ,  illuminated  These photons can cause  and a c y l i n d r i c a l energy a n a l y z e r  i s used to s t u d y the p h o t o e l e c t r o n energy d i s t r i b u t i o n .  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 l e a d 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 o f the m o l e c u l e .  The i o n i z a t i o n p o t e n t i a l s of q u i t e a number o f m o l e -  c u l e s have been r e p o r t e d u s i n g t h i s method and i n some f a v o r a b l e cases v i b r a t i o n a l s t r u c t u r e  can a l s o be seen.  F r o s t , McDowell and Vroom (41) have r e p o r t e d r e c e n t l y the use o f a s p h e r i c a l energy a n a l y z e r o f g r e a t l y improved r e s o l u t i o n to s t u d y the p h o t o e l e c t r o n energy d i s t r i b u t i o n o f hydrogen.  They have  been a b l e t o 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 c u r v e i s known (13, 20) to be obscured c o m p l e t e l y 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 s o l v e d " s t e p s " i n the p h o t o e l e c t r o n r e t a r d i n g c u r v e f o r hydrogen i n d i c a t e 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 p h o t o e l e c t r o n  that  spectros-  copy. e) The Photon Impact Method T h i s method c o n s i s t s o f a c o m b i n a t i o n o f both p h o t o i o n i z a t i o n and mass s p e c t r o m e t r y .  A sample o f gaseous m o l e c u l e s 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 o f monochromatic u l t r a v i o l e t radiation.  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  increased,  s u c c e s s i v e s t a g e s o f e x c i t a t i o n can be reached u n t i l  the  energy of the photon r e a c h e s a c e r t a i n v a l u e when i o n i z a t i o n t a k e s place.  The i o n i z a t i o n p o t e n t i a l o f a m o l e c u l e can be o b t a i n e d  from the p o i n t o f i n i t i a l onset o f i o n i z a t i o n . c r e a s e o f photon energy,  With f u r t h e r  in-  a c u r v e o f 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 o f photon energy can be d e r i v e d .  In f a v o u r a b l e  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 o f the m o l e c u l e and i t s i o n s can b e . s t u d i e d A mass s p e c t r o m e t e r tigation,  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 c u r v e s .  i s used to focus the p a r t i c u l a r i o n under  and to measure the i n t e n s i t y of i o n c u r r e n t s  inves-  produced.  The p h o t o i o n i z a t i o n method i s f a r more g e n e r a l l y a p p l i c a b l e than o p t i c a l s p e c t r o s c o p y ,  as some m o l e c u l e s w i t h complex  s p e c t r a w h i c h do not e x h i b i t w e l l d e f i n e d 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 f o r the s u p e r i o r i t y o f t h i s method threefold.  Firstly,  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 ; gas to c a l i b r a t e  are  secondly, a  the energy s c a l e needs not be i n t r o d u c e d i n t o  the  10.  mass spectrometer i s the r e g u l a r  c o n c u r r e n t l y w i t h the m o l e c u l e to be s t u d i e d  practice  i n the e l e c t r o n impact method;  as  and t h i r d l y ,  the s t e e p r i s e i n 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 at the i o n i z a t i o n t h r e s h o l d p e r m i t s a sharp s e p a r a t i o n o f i o n - f o r m a t i o n The photon impact method i s i n h e r e n t l y more  processes. accurate,  and n u m e r i c a 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 o b t a i n e d by o p t i c a l spectroscopy. for  P r e c i s e d a t a have thus been made a v a i l a b l e  a l a r g e number o f m o l e c u l e s ,  and i n f o r m a t i o n o b t a i n e d 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 c u r v e s can be c o r r e l a t e d w i t h m o l e c u l a r properties  and m o l e c u l a r s t r u c t u r e ,  and to check the t h e o r i e s o f  t h r e s h o l d laws and p r o c e s s e s o f a u t o i o n i z a t i o n . n i s h r e l i a b l e i n f o r m a t i o n f o r some t h e o r e t i c a l  The r e s u l t s  fur-  and s e m i - e m p i r i c a l  m o l e c u l a r quantum m e c h a n i c a l c a l c u l a t i o n s which are now b e i n g 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 i n c e the d i s c o v e r y 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  o f l i g h t on s o l i d s and m e t a l s ,  several  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 o f 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 p e r f o r m i n g experiments on the s p a r k i n g between e l e c t r o d e s ,  observed t h a t u l t r a v i o l e t r a d i a t i o n  c o u l d be used to i o n i z e g a s e s . some s i m i l a r experiments  Lenard (67) i n 1901, c a r r i e d out  and found t h a t a i r was made c o n d u c t i n g  under the a c t i o n of a v e r y a b s o r b a b l e k i n d of u l t r a v i o l e t l i g h t . S t a r k (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 o b t a i n e d r e s u l t s w i t h o r g a n i c compounds i n the vapor s t a t e : d i p h e n y l a m i n e and  anthracene,  certain  diphenylmethane,  oC_-naphthy 1 amine .  D u r i n g the p e r i o d 1920-1930, experiments on p h o t o i o n i z a t i o n were conducted m o s t l y w i t h v a p o r s o f the a l k a l i e s p e c i a l l y caesium, r u b i d i u m and p o t a s s i u m . t e c h n i q u e s were,  metals,  The e x p e r i m e n t a l  as a whole, g r e a t l y improved a f t e r  1920.  However,  l a c k o f b h i g h 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 i n s t r u m e n t s  used has l e f t  i n a somewhat u n c e r t a i n s t a t e .  the d a t a o f many o f these w o r k e r s A c o n s i d e r a b l e amount o f d a t a  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  has  (57,97).  Between 1930 and 1950, v e r y l i t t l e work c o n c e r n i n g phot 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 s t u d y of m o l e -  c u l a r i o n i z a t i o n seemed to be dominated m a i n l y by e l e c t r o n s t u d i e s u s i n g mass s p e c t r o m e t e r s .  impact  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 o f m o l e c u l e s and appearance p o t e n t i a l s o f fragment  ions, to d e t e r -  mine the energy needed to s p l i t up a p o l y a t o m i c m o l e c u l e i n t o  12. specified radicals, radicals. field  and to determine  i o n i z a t i o n p o t e n t i a l s of  Hundreds of papers have a l r e a d y been p u b l i s h e d i n t h i s  and i o n i z a t i o n p o t e n t i a l s o f s e v e r a l hundred m o l e c u l e s have  been s t u d i e d . available  I n many c a s e s ,  t h i s method has p r o v i d e d the o n l y  data. 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  suffi-  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 inaccusrate.  The d5orm o f the i o n i z a t i o n p r o b a b i l i t y c u r v e f o r a s i n g l e  p r o c e s s i s such t h a t when s e v e r a l are superimposed, of s e p a r a t e t h r e s h o l d p o t e n t i a l s After pace,  is  the  resolution  difficult.  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  l a r g e l y due to the development o f i n g e n i o u s d e s i g n s  steady  for  g r a t i n g monochromators and b e t t e r means f o r p r o d u c i n g and measuri n g photon r a d i a t i o n i n the far; . u l t r a v i o l e t . The development o f a l o w - c o s t g r a t i n g monochromator by Seya (108)  and Namioka (89) h e l p e d to f a c i l i t a t e the s t u d i e s o f  photoionization.  It  i s b a s i c a l l y a one-meter  monochromator,  and  the gas p r e s s u r e i n the monochromator i s m a i n t a i n e d at a p r e s s u r e -5 of about 10  mm of Hg. by d i f f e r e n t i a l pumping.  e n t r a n c e and e x i t s l i t system,  I t has a f i x e d  and the wavelength of the monochro-  m a t i c l i g h t p a s s i n g 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  grating.  Johnson e t a l (62) i n 1951, found t h a t c o a t i n g a p h o t o e l e c t r o n 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 rendered  i t satisfactory  radiation intensity.  f o r the measurement  of f a r  material  ultraviolet  Sodium s a l i c y l a t e was found to be b e s t  suited since i t i s stable,  does not evaporate  r e p r o d u c i b l e r e s u l t s up to 850$.  Furthermore,  i n vacuo and g i v e s i t s response  is  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 n e a r l y c o n s t a n t  (62).  In 1953, Watanabe, Marmo and Inn (133) and W a i n f a n , Walker,  and W e i s s l e r (141) r e p o r t e d d a t a 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 o f a m o l e c u l e was measured i n an a b s o r p t i o n  cell,  and the i o n i z a t i o n was found to c o r r e s p o n d to the l o n g wavelength l i m i t of the i o n i z a t i o n continuum.  They showed t h a t the  d e t e r m i n a t i o n of i o n i z a t i o n p o t e n t i a l s  accurate  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 o f 0.001 eV. band w i d t h .  However,  their  methods do not g i v e any i n f o r m a t i o n about the p r o d u c t s which  are  formed by p h o t o i o n i z a t i o n , and no mass a n a l y s e s were i n t r o d u c e d differentiate  between the p a r e n t and fragment  a r i s e from any i m p u r i t i e s i n the sample.  ions.  to  A l s o i o n s may  Because o f the l a c k o f  mass a n a l y s i s , one has to be q u i t e s u r e t h a t the sample b e i n g s t u d i e d i s f r e e 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 . 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 s o f more than a hundred m o l e c u l e s were r e p o r t e d  i n 1959 by Watanabe ( 1 3 9 ) .  are comparable to those o b t a i n e d by s p e c t r o s c o p i c  His results methods.  T e r e n i n and Popov (124) were the f i r s t to use mass analysis  ( 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 f o r the g e n e r a t i o n o f i o n s i n a mass s p e c t r o m e t e r .  source  A krypton 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 e V . ) and 1165A* (10.64 e V . ) p r o v i d e d  enough energy to cause p h o t o i o n i z a t i o n o f acetone, butene, p r o p y l e n e ,  anisole,  butadiene,  a l l y l i o d i d e , d i m e t h y l mercury  but not enough to form i o n i c fragments.  T h e i r experiments  f e r e d from the d e f e c t o f a f i x e d photon energy,  etc., suf-  so the p h o t o i o n i -  z a t i o n y i e l d c o u l d not be s t u d i e d as a f u n c t i o n o f  energy.  14. T e r e n i n 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 c o m b i n a t i o n o f vacuum monochromator mass spectrometer photon i m p a c t .  and  i n a d e t a i l e d study o f the f o r m a t i o n o f i o n s by  The source of l i g h t was a h i g h 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  resi-  d u a l gases i n the light 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 c u t s o f f r a d i a t i o n below 10508 (11.50 e V . ) and m o l e c u l e s 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 r e a t e 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 ( 1 0 ) , u s i n g a l o w - p r e s s u r e r e p e t i t i v e spark s o u r c e and differential  pumping t o r e t a i n a low p r e s s u r e i n t h e i r  apparatus  were a 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 up to about 30 eV. w i t h o u 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  s o u r c e p r o v i d e d a w i d e l y - s p a c e d l i n e spectrum,  light  and so t h e r e were  of c o u r s e "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 c u r v e s . T h i s f r e q u e n t l y made i t i m p o s s i b l e to determine  the o n s e t o f 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 , K r a u s s , Reese and H a r t l e e (19)  have  developed a H i n t e r e g g e r type photon source which p r o v i d e s a H o p f i e l d h e l i u m continuum, and have s t u d i e d normal and ted hydrogen, methane and benzene.  deutera-  Cook and Metzger (12)  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  have  of s e v e r a l hydrogen-  c o n t a i n i n g m o l e c u l e s from i o n i z a t i o n s p e c t r a u s i n g the h e l i u m continuum as a c o n t i n u o u s b a c k g r o u n d - r a d i a t i o n  Hopfield source.  The development o f c o n t i n u o u s photon s p e c t r a marked a major advance i n t h i s  field.  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 m a i n l y w i t h  c o l l i s i o n s o f photons w i t h atoms and m o l e c u l e s .  ionizing  A photon o f  energy hv can be absorbed by an atom or a m o l e c u l e to take  the  system from a s t a t e o f lower energy E " to a s t a t e o f h i g h e r energy E ' , i . e . hv = E ' - E "  2.1  where h i s P l a n c k ' s c o n s t a n t , tion.  v i s the frequency of the r a d i a -  The a b s o r p t i o n o f r a d i a t i o n by the m o l e c u l e XY can be  represented  by:-  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 o f the m o l e c u l e respectively.  This 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 o f a photon o f s u f f i c i e n t l y  h i g h energy t o cause the  XY +.. hv —>XY+ + e  process:  2.3  t h i s p r o c e s s 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 n e c e s s a r y f o r t h i s p r o c e s s i s c a l l e d the t h r e s h o l d  ionization  p o t e n t i a l o f X Y . Beyond the energy o f the i o n i z a t i o n t h e r e w i l l be a r e g i o n o f c o n t i n u o u s a b s o r p t i o n .  threshold,  The measure-  ment of p o s i t i v e i o n c u r r e n t as a f u n c t i o n o f photon energy p r o v i d e s the u s u a l method o f s t u d y i n g t h i s If reaction:  process.  the a b s o r p t i o n o f r a d i a t i o n l e a d s to the  following  16.. XY + hv  >X  +  + Y" + e  2.4  the p r o c e s s 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 n e c e s s a r y f o r t h i s r e a c t i o n i s c a l l e d the p o t e n t i a l of X . +  appearance  The d i s s o c i a t i o n energy or bond s t r e n g t h of  the m o l e c u l e 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  where V ( X ) i s the appearance p o t e n t i a l of X , I ( X ) i s +  2.5 the  i o n i z a t i o n p o t e n t i a l o f 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 s p e c t r o s c o p y , 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 o b t a i n e d by measurement of appearance p o t e n t i a l .  the  O t h e r w i s e , the appearance p o t e n t i a l g i v e s  an upper l i m i t f o r the* energy n e c e s s a r y t o break the X - Y bond p l u s the i o n i z a t i o n energy of X .  B.  Autoionization In i o n i z a t i o n e f f i c i e n c y c u r v e s f o r m o l e c u l a r i o n s  produced by e l e c t r o n impact, breaks are o f t e n observed at  ener-  g i e s which do not c o r r e s p o n d to any known e l e c t r o n i c s t a t e o f the p o s i t i v e i o n s p e c i e s formed. efficiency  Many peaks i n p h o t o i o n i z a t i o n  c u r v e s are a l s o observed at e n e r g i e s  threshold i o n i z a t i o n p o t e n t i a l ,  above the  and the p o s i t i o n s o f these  peaks  c o r r e s p o n d c l o s e l y w i t h some o f those o b t a i n e d from a b s o r p t i o n spectra.  T h i s phenomenon has been e s t a b l i s h e d by s e v e r a l w o r k e r s  (10, 27, 54, 91) as due to a u t o i o n i z a t i o n o f a h i g h l y e x c i t e d s t a t e of the atom or m o l e c u l e c o n c e r n e d . A u t o i o n i z a t i o n phenomenon can be e x p l a i n e d by r e c i p r o c a l i n t e r a c t i o n o f d i s c r e t e s t a t e s w i t h one o r more c o n t i n u a as shown i n F i g u r e 2.  The e x c i t a t i o n o f 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 o f series  1 i s f o l l o w e d 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  continuum at the same energy.  the  The e x c i t e d atom or m o l e c u l e becomes  i o n i z e d by l o s i n g one o f i t s e l e c t r o n s . governed m a i n l y by p h o t o e x c i t a t i o n ,  Since t h i s process  is  and the t h r e s h o l d law f o r  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 s h o u l d 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 f o r the a u t o i o n i zation 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 o f  normal m o l e c u l e i n which two or more e l e c t r o n s atom or m o l e c u l e i n such a c o n d i t i o n w i l l  are e x c i t e d .  Instead  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  which i s absorbed by the o t h e r  An  not u s u a l l y g i v e up i t s  energy o f e x c i t a t i o n by r a d i a t i o n .  one drops to a more f i r m l y bound s t a t e ,  the  i t w i l l break up much electrons,  thereby r e l e a s i n g  energy'  to take i t f r e e o f the atom or  electron exit  energies  photo-ion  series 2 0  series 1  current  Figure 6V  18. molecule a l t o g e t h e r . becomes  The e x c i t e d atom or m o l e c u l e  therefore  ionized. In the work r e p o r t e d  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 i a t o m i c m o l e c u l e s such as k r y p t o n , xenon, oxygen, n i t r o g e n , chloride.  carbon monoxide and hydrogen  For methane and numerous hydrocarbon m o l e c u l e s ,  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 p r e s e n t .  no  The p o s s i b l e  e x c i t a t i o n s i n such m o l e c u l e s appear to r e s u l t o n l y i n c o n t i n u o u s absorption, efficiency  and do not g i v e d i s c r e t e curves.  peaks i n the  photoionization  19. C.  The T h r e s h o l d Laws In o r d e r 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 c u r v e s , necessary  to i n t e r p r e t  near the t h r e s h o l d .  the shape o f the c u r v e s ,  it  especially  The t h r e s h o l d law d e s c r i b e s  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 the  is  above  threshold. The i o n c u r r e n t produced by e l e c t r o n impact  increases  l i n e a r l y as the energy o f the i m p a c t i n g e l e c t r o n  i n c r e a s e d above the t h r e s h o l d . for  is  Wigner (147) has shown t h a t  a s i n g l y charged atom or m o l e c u l e 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 o f the excess energy.  However,  Fox (51) showed t h a t the i o n i z a t i o n e f f i c i e n c y c u r v e f o r h e l i u m s i n g l y charged i s l i n e a r over the f i r s t e i g h t above the  volts  threshold. 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)  t h a t the t h r e s h o l d law f o r d i r e c t  i o n i z a t i o n , when induced by photon i m p a c t , a s t e p f u n c t i o n o f excess photon energy,  i s approximately  and t h a t f o r  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 . z a t i o n process,  induced by photon impact,  single  double  The a u t o i o n i -  i s governed m a i n l y  by the p h o t o e x c i t a t i o n o f an e l e c t r o n i n the normal m o l e c u l e . The appearance o f sharp peaks f o 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 c u r v e i n d i c a t e s t h a t  the  t h r e s h o l d law f o r 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 t h e o r y o f the law f o r m u l t i p l e i o n i z a t i o n . emerging from the i o n a f t e r i m p a c t , remain  threshold  The two slow e l e c t r o n s ,  upon  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  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 o f r a d i u s 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 g r e a t e r than e^/b can i t escape from t h i s r e g i o n . 3, the k i n e t i c are p l o t t e d ,  and T 2 o f the two emerging  energies  electrons  and the r e g i o n i n which i o n i z a t i o n o c c u r s i s  l i m i t e d by two s t r a i g h t of c o n s t a n t  In Figure  l i n e s p a r a l l e l to the axes.  energy excess  '  Now a l i n e  A E i n t h i s diagram i s s t r a i g h t  and  of s l o p e - 1 ; the segment o f 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 .  Clearly,  the l e n g t h o f t h i s segment  is  propor-  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 increases  Fig.  3.  l i n e a r l y w i t h the excess  energy.  R e g i o n i n which i o n i z a t i o n o c c u r s i n the case o f single ionization. and T 2 are the k i n e t i c e n e r g i e s 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 , o n l y one  slow e l e c t r o n emerges from the i o n . k i n e t i c energy T g r e a t e r than e^/b, place.  When the e l e c t r o n has a p h o t o i o n i z a t i o n takes  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 c o n s t a n t  the t h r e s h o l d energy p l u s the k i n e t i c energy T, and the  after excess  energy o f the i n c i d e n t photon i s c a r r i e d away by the slow electron.  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 f o r s i n g l e i o n i z a t i o n  i s therefore  a step function.  21.  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 ( 4 3 ) 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 s h o l d 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 o f excess energy o f the i n c i d e n t p a r t i c l e s  among the s e p a r a t i n g  T a b l e I summarises  laws f o r i o n i z a t i o n and  the t h r e s h o l d  e x c i t a t i o n by photon and e l e c t r o n  particles.  impact.  Table I The T h r e s h o l d Laws  Number of separating particles  Types o f r e a c t i o n s  Ionization Efficiency  A + hv —»A* A + e —>A~  1  delta 1  2  A + hv — * A + + e A + e •A* + e  3  A + hv A + e  A A  + + +  + +  step _y  2e 2e  Thus, we would expect from the efficiency process, process.  Function  linear  photoionization  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  and a s t e p - f u n c t i o n These a r e ,  however,  for a s i n g l e p h o t o i o n i z a t i o n idealized situations,  a c t u a l c u r v e s o b t a i n e d do not always show p e r f e c t delta functions.  Some f i n e s t r u c t u r e and s t e p s  to the h i g h e r 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  cannot be o b t a i n e d because o f the i n t e r f e r e n c e of m u l t i p l e a u t o i o n i z a t i o n p r o c e s s e s .  and  the  steps or corresponding  molecules and c o m p e t i t i o n  22  D.  Theory of the Mass Spectrometer The  term 'mass s p e c t r o m e t e r '  to an i n s t r u m e n t i n which s e p a r a t e d electrically.  i s now u s u a l l y restricdberh  i o n beams are measured  I t w i l l be u s e f u l to w r i t e down the s i m p l e  equations g o v e r n i n g the m o t i o n o f a charged i o n i n a mass spectrometer. If and  the charged p a r t i c l e of mass M ( g ) , charge  v e l o c i t y v (cm per sec)  e(e.s.u.)  i s sent i n t o a magnetic f i e l d o f  f o r c e H ( e . s . u . ) , the e q u a t i o n o f m o t i o n may be d e s c r i b e d as follows. S i n c e the f o r c e i s always at r i g h t a n g l e s t o  the  d i r e c t i o n o f m o t i o n o f the p a r t i c l e , t h e r e i s no l i n e a r , a constant it  angular a c c e l e r a t i o n .  From elementary mechanics,  i s seen t h a t the p a r t i c l e w i l l  force,  hut  experience a c e n t r i f u g a l  and f o r e q u i l i b r i u m t h i s must b a l a n c e the f o r c e due t o  the magnetic f i e l d ,  i.e.  Hev  2.6  R where R i s the r a d i u s o f c u r v a t u r e o f the. i o n beam. If  i t i s assumed now t h a t the charged p a r t i c l e  a c q u i r e s 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 potential difference 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 o f the p a r t i c l e a f t e r ation,  i.e. Mv  2  -g-  If  acceler-  =  eV  equations 2 . 6 and 2 . 7 are combined, e l i m i n a t i n g v ,  ..2.7  then  23.  The e q u a t i o n 2.8 may be termed spectrometer  equation.  the mass  In the mass s p e c t r o m e t e r  used i n the  p r e s e n t work, the r a d i u s o f c u r v a t u r e o f the charged p a r t i c l e is fixed,  and when a n a l y s i s o f the i o n s formed from a g i v e n  molecule i s d e s i r e d , e i t h e r maintained constant,  the i o n a c c e l e r a t i n g v o l t a g e  and the magnetic f i e l d  is  strength varied  c o n t i n u o u s l y (magnetic scanning) , or the a c c e l e r a t i n g v o l t a g e i s v a r i e d k e e p i n g the magnetic f i e l d (voltage scanning).  strength  constant  24 E.  The Hydrogen and Helium S p e c t r a In o r d e r t o o b t a i n a monochromatic photon beam o f  v a r y i n g energy  i t i s n e c e s s a r y t o e x c i t e gaseous m o l e c u l e s  such as hydrogen or h e l i u m i n the l i g h t s o u r c e .  The l i g h t  e m i t t e d from the source i s scanned by a 30,000-lines per i n c h g r a t i n g t o g i v e a spectrum o f photon i n t e n s i t y as a f u n c t i o n of photon e n e r g i e s .  The spectrum o b t a i n e d 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 s o u r c e :  t h a t o f hydrogen i s  u s e f u l from 90oS - 1500& (8 - 14 e V . ) , and t h a t of h e l i u m i s u s e f u l from 500& - 120o8 (10 - 21 e V . ) . b i n a t i o n of both s p e c t r a , zation results a)  By employing a com-  i t i s possible to obtain p h o t o i o n i -  i n the energy range o f 500& - 15008 (8 - 21 e V . ) .  The Hydrogen Spectrum The hydrogen lamp g i v e s an i n t e n s e " m a n y - l i n e "  spectrum i n the 9008 - 1500S r e g i o n , and the s p e c t r a l  charac-  t e r i s t i c o f 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 o f 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 o f t h e p h o t o i o n i z a t i o n efficiency  curve.  The c o n t o u r o f 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 c u r v e f o r almost every m o l e cule studied. fact,  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 t o the  as N i c h o l s o n (92) has i n d i c a t e d , t h a t a s t r o n g i s o l a t e d  l i n e i n a r e g i o n o f the spectrum where the t r u e p h o t o i o n i z a t i o n efficiency  c u r v e 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  duce a f a l s e  pro-  '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 .  J u d g i n g from the shape o f the hydrogen spectrum, i s reasonable  t o assume t h a t the spectrum c o n s i s t s o f peaks  it  PHOTOELECTRIC  CURRENT  25.  superimposed on a hydrogen continuum. efficiency P.E.  (P.E.)  The p h o t o i o n i z a t i o n  at the top o f a peak r e p r e s e n t s the true;  at t h a t photon energy,  and the P . E . at the bottom o f the  v a l l e y between two photon peaks r e p r e s e n t s the c o n t r i b u t i o n s among t h r e e f a c t o r s : hydrogen continuum.  the two n e i g h b o u r i n g peaks and the The P . E . c o n t r i b u t i o n s from the n e i g h -  b o u r i n g peaks are f u n c t i o n s o f the peak h e i g h t s  and the  d i s t a n c e between the v a l l e y and the "peak maximum". 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 , d a t a p o i n t s on the hydrogen  continuum and t h e i r c o r r e s p o n d i n g i o n i n t e n s i t i e s ted,  and the t r u e P . E . c u r v e drawn.  structure b)  Taking  are c a l c u l a ^  I n t h i s way f a l s e  on the P . E . c u r v e can be e l i m i n a t e d . The H e l i u m Spectrum The h e l i u m lamp g i v e s an i n t e n s e continuum i n the  5008 - 12008 r e g i o n ,  and the s p e c t r a l c h a r a c t e r i s t i c o f t h e  h e l i u m 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 o f i n t e n s i t y w i t h photon energy and the c o n t i n u o u s  nature  of t h i s e m i s s i o n spectrum are o f p a r t i c u l a r advantage i n t h i s work, and the problem caused by the " m a n y - l i n e " hydrogen spectrum p r e v i o u s l y d i s c u s s e d i s not s i g n i f i c a n t h e r e .  The  h e l i u m spectrum was used f o r most o f the m o l e c u l e s s t u d i e d i n t h i s work. I t can be seen i n F i g u r e 5 t h a t 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. resonance it  The  l i n e o f atomic h e l i u m at 5848 i s v e r y s m a l l because  i s strongly self-absorbed.  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 m o l e c u l e s as i n d i c a t e d by W e i s s l e r , Samson, Ogawa and Cook  13  _  ENERGY ev 15 16  14 ,  )  17  .  (  /  J  1000  i_  950  I  90CT  ,  850  l  18  ,  i  800 750 WAVELENGTH  j—  19 .  20 ,  21 r  NEON LINES  I  7700 A  i  650  1  600  26.  (146).  The two l i n e s at 736$ and 7448 are resonance l i n e s  o f atomic neon i m p u r i t y w h i c h appear w i t h c o n s i d e r a b l e intensity.  Because o f i t s f a v o u r a b l e p o s i t i o n i n the spectrum  and i t s c o n s i d e r a b l e 7448  intensity,  the i o n i z a t i o n caused by the  resonance l i n e o f the atomic neon i s used f o r the l o c a t i o n  of t h e i o n beam. The h e l i u m e m i s s i o n 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 t o the ground s t a t e o f the helium molecule.  The two main continuum maxima at 8I08 and  6708 are the r e s u l t o f t r a n s i t i o n s A Z 1  X-'-Z  respectively  - X £ 1  g  and D  1  S u  (123).  The h e l i u m continuum i s a f u n c t i o n o f the h e l i u m p r e s s u r e i n the l i g h t s o u r c e .  A t low h e l i u m p r e s s u r e , the  resonance l i n e o f the atomic h e l i u m at 5848 appears w i t h considerable  intensity,  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 p r e s s u r e o f h e l i u m i s i n -  creased,  o f t h e two continuum maxima  the i n t e n s i t i e s  w h i l e the 5848 l i n e d i m i n i s h e s because o f s t r o n g absorption.  increase  self-  CHAPTER THREE EXPERIMENTAL A.  Introduction The work d e s c r i b e d i n t h i s t h e s i s was done i n a  p h o t o i o n i z a t i o n mass s p e c t r o m e t e r which was a c o m b i n a t i o n o f two major components:  the 60 degree N i e r type s i n g l e f o c u s -  s i n g mass s p e c t r o m e t e r  and the Seya-Namioka type 1-meter  s c a n n i n g vacuum monochromator. i s o l a t e d by a two and a h a l f valve, ing.  The two p a r t s c o u l d be  inch  diameter Crane wedge-type  so t h a t e i t h e r s i d e c o u l d be opened t o a i r f o r s e r v i c T h i s v a l v e was f i t t e d w i t h an a p p r o p r i a t e 0 - r i n g . A s i m p l e form o f 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 o f gaseous or  compound t o be s t u d i e d was s t o r e d i n a sample tube, of  liquid a fraction  i t was expanded i n t o t h r e e l a r g e evacuated g l a s s b u l b s .  The gas then moved through a v e r y f i n e l e a k i n t o the i o n s o u r c e of  the mass s p e c t r o m e t e r ,  an i o n i z a t i o n gauge.  where the p r e s s u r e was measured w i t h  T h i s p r e s s u r e was kept c o n s t a n t  at  about  5 x 10 ° mm. of Hg. f o r s e v e r a l h o u r s . 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 i n c h g r a t i n g , and a r e p e t i t i v e , h i g h v o l t a g e spark through gas i n a p y r e x g l a s s c a p i l l a r y s e r v e d as a l i g h t source.  The gases used i n the l i g h t s o u r c e were e i t h e r  pure  hydrogen or pure h e l i u m m a i n t a i n e d at about 50 m i c r o n s pressure.  E n t r a n c e and e x i t s l i t w i d t h s o f 0.020 t o . 0 . 0 4 0  i n c h l i m i t e d the r e s o l u t i o n t o 4 8 or about 0.05 eV. at a photon energy o f 10 eV.  F i g u r e 6:  The Monochromator and Mass S p e c t r o m e t e r  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 e n t r a n c e s l i t a g r a t i n g G, s i t t i n g on a t a b l e T.  to  By t u r n i n g the arm A ,  monochromatic r a d i a t i o n o f d i f f e r e n t wavelengths c o u l d be selected.  The r e f r a c t e d  monochromatic l i g h t t r a v e r s e d  s l i t o f the monochromator,  the  exit  passed through the i o n chamber and  was i n c i d e n t on a p h o t o d e t e c t o r  which had been s e n s i t i z e d  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 o u t s i d e  to with  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 i o n s were formed i n the i o n source by the a b s o r p t i o n o f 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 i o n s formed i n the i o n source were f o r c e d  through a s m a l l s l i t b y r a r e p e l l e r  (with 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 a c c e l e r a t e d 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  o f s l i t s o f 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 t o the  magnetic  analyser. In the magnetic f i e l d and f o l l o w e d a c i r c u l a r p a t h . upon the mass, strength.  r e g i o n , the i o n s were The r a d i u s o f the path  the v e l o c i t y o f the i o n and upon the  By s u i t a b l e adjustment  The s m a l l c u r r e n t  o f the magnetic f i e l d  strength, ratio  slit.  due to the i o n beam was then  a m p l i f i e d by a 1 7 - s t a g e e l e c t r o n m u l t i p l i e r . spectrometer  depended  magnetic  a homogeneous beam o f i o n s o f the same mass per charge c o u l d be f o c u s s e d at the e x i t  deflected  Near the mass  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 o t h e r p o s i t i v e or n e g a t i v e .  at 100 v o l t s  The p o t e n t i a l o f the second p l a t e was  29.  adjusted  so t h a 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 s t a g e o f the e l e c t r o n m u l t i p l i e r . output e l e c t r o n c u r r e n t  The  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 c o r d e r . C u r r e n t s from the p h o t b d e t e c t o r v i b r a t i n g reed were r e c o r d e d s e p a r a t e l y rotated,  and from the as the g r a t i n g was  and a p l o t of i o n s per photon ( a r b i t r a r y  a g a i n s t photon energy c o n s t i t u t e d efficiency  curve.  units)  the p h o t o i o n i z a t i o n  30. B.  The Mass Spectrometer The mass s p e c t r o m e t e r was a N i e r  (93) type  instrument,  u s i n g a 6 0 ° s e c t o r shaped magnetic f i e l d  f o r mass a n a l y s i s ;  p e r m i t t e d r a d i u s o f i o n path was 15 cm.  The i o n s o u r c e used  y i e l d e d an i o n beam n e a r l y homogeneous i n energy.  the  A calibrated  p o t e n t i o m e t e r 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 c o n t i n u o u s l y 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 p r o v i d e d by a s t e e l e l e c t r o m a g n e t ,  and  enabled f o c u s s i n g over the range o f 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 s o u r c e was p l a c e d behind the e x i t s l i t o f  the monochromator as shown i n F i g u r e 6.  A diagram o f the end  and s i d e s e c t i o n o f the i o n s o u r c e 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 steel,  stainless  chosen f o r 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  resistance.  The i o n s o u r c e and i t s a s s o c i a t e d  a l s o made o f s t a i n l e s s s t e e l .  A l l the s p a c e r s ,  members were i n s u l a t o r s and  the sample i n l e t tube were made o f p y r e x g l a s s . At the s e c t i o n through A ( F i g u r e 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 p h o t o e l e c t r o n s which were produced when photons h i t the m e t a l l i c p a r t o f the monochromator e x i t  slit.  A t u n g s t e n f i l a m e n t mounted behind a s l i t p r o v i d e d 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 angle to the d i r e c t i o n of the photon beam.  This filament  p r o v i d e d a means o f 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 comparison.  at r i g h t  for  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 were one or two  gas inlet  V  * photons \  1  \  r  secondary THR0U6H"P ? A deflection plates  SECTION  h  t 0 e l e C t r 0 n  monochromator exit slit  t  MASS SPECTROMETER  ION SOURCE END SECTION  SIDE SECTION  Figure 7  31. o r d e r s of magnitude l e s s than those f o r impact by 7 0 - v o l t electrons,  ( i o n i z a t i o n c r o s s s e c t i o n f o r e l e c t r o n impact i s  — 16 about 10  2 cm ) , but the i o n c u r r e n t s o b t a i n e d were s m a l l e r  by two or t h r e e o r d e r s o f 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 f o r any p r e s e n t l y a v a i l a b l e l i g h t s o u r c e than i n the case o f e l e c t r o n beams ( 2 5 ) . 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 a p p l i e d 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 o f i t t o e l e c t r o d e s 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 o f 3 and 5 c o u l d be adjusted ion source.  the  to a l i g n the i o n beam i n the  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 m o l e c u l e s e n t e r e d i n t o the photon beam.  the i o n chamber and diffused  P o s i t i v e i o n s produced by photon impact  were drawn out o f the i o n chamber by a s m a l l e l e c t r i c 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.  field  The i o n r e p e l l e r  was made of t u n g s t e n mesh and was m a i n t a i n e d at a p o s i t i v e p o t e n t i a l o f a few v o l t s ,  so t h a t n e u t r a l m o l e c u l e s c o u l d  diffuse  through i t , but p o s i t i v e i o n s formed i n the i o n chamber were f o r c e d down through the e l e c t r o d e 1.  A f t e r p a s s i n g through  the  i o n chamber, the i o n beam was a c c e l e r a t e d by e l e c t r o d e 1 and focussed by e l e c t r o d e s 2, 3 and 5, and f i n a l l y passed the e x i t slit  6 i n t o the mass a n a l y s e r . 2.  The A n a l y s e r and the E l e c t r o m a g n e t T h e n a n a l y s e r ' t u b e was made from a 5 cm. diameter  copper tube bent through 9 0 ° on a r a d i u s o f c u r v a t u r e o f 17.2 cm.  I t was f l a t t e n e d over the c e n t r e s e c t i o n to f i t  between the 2 . 2 cm. p o l e gap o f the a n a l y s e r magnet. complete u n i t was r i g i d l y l o c k e d t o the framework of  The the  32. instrument to the  so t h a t i t s p o s i t i o n c o u l d be f i x e d w i t h  respect  magnet. The e l e c t r o m a g n e t  comprised f i v e  10,000-turn  wound on a low carbon s t e e l core of 6" x 3" s e c t i o n .  coils The  machined p o l e p i e c e s were made of the same m a t e r i a l and had a gap of 2 . 2 c m : . The maximum f i e l d was a p p r o x i m a t e l y 5,000 gauss,  strength  i n the p o l e gap  and c o u l d be v a r i e d f o r  the  d e t e c t i o n o f i o n s over t h e . r a n g e 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 v o l t a g e o f 2,500 v o l t s . In the a n a l y s e r ,  a beam o f i o n s p a s s i n g at  right  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 o f the Since,  i n general,  ions.  the beam emerging from the i o n source was  inhomogeneous i n momentum, the s e v e r a l types o f i o n s h a v i n g different  momenta would be d e f l e c t e d by d i f f e r e n t  amounts.  practice,  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 m a i n t a i n e d  and i o n s o f e q u a l charge i n the o r i g i n a l beam were i n energy  In  constant,  homogeneous  the momentum depended o n l y on the mass o f the i o n .  For a f i x e d system of s l i t s , one c o u l d c o l l e c t a homogeneous beam of i o n s o f the same mass at the e x i t s l i t . l y c h a n g i n g the magnetic f i e l d  strength,  By c o n t i n u o u s -  one c o u l d  determine  the mass spectrum of the i o n s formed from a g i v e n compound. On emerging from the magnetic a n a l y s e r ,  the r e s o l v e d  i o n beam passed through the 2 mm. wide s l i t ,  and f e l l  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  and the e l e c t r o n m u l t i p l i e r a s u p p r e s s o r at n e g a t i v e 22^ v o l t s ) suppressed i o n bombardment o f the s l i t .  electrode  on to  (maintained  any secondary e l e c t r o n s  Furthermore,  two p a r a l l e l  were r e c e n t l y i n s t a l l e d to focus the i o n beam.  slit  from plates  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 o r d e r o f 100 v o l t s , and c o u l d be v a r i e d c o n t i n u o u s l y i n o r d e r to o b t a i n the maximum ion  current. 3.  The E l e c t r o n  Multiplier  The 14 s t a g e 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 d e t e c t o r of i o n s .  The p o s i t i v e i o n s from the magnetic a n a l y -  s e r impinged upon the f i r s t p l a t e of the d e t e c t o r t o secondary e l e c t r o n s .  These e l e c t r o n s were i n t u r n caused  to s t r i k e a s u c c e s s i o n of p l a t e s , e l e c t r o n y i e l d g r e a t e r than u n i t y . 2% B e - C u ,  giving rise  each g i v i n g a secondary The p l a t e s were made o f  and were connected to 14 s u c c e s s i v e l y h i g h e r p o s i t i v e  potentials,  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 a c r o s s them. p r i n c i p a l m e r i t s o f the e l e c t r o n m u l t i p l i e r were i t s sensitivity,  The  extreme  f a s t response and l o w - n o i s e , w i d e - b a n d w i d t h  amplification. The f i n a l  e l e c t r o n c u r r e n 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 t u n g s t e n 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 4.  electrometer.  The V i b r a t i n g Reed E l e c t r o m e t e r The main u s e f u l n e s s o f a V i b r a t i n g reed  i s i n the measurement o f s m a l l d . c . c u r r e n t s . c o n s i s t e d o f two p a r t s :  electrometer  The e l e c t r o m e t e r  the head u n i t and the main a m p l i f i e r .  The i n p u t d . c . p o t e n t i a l which arose from the passage o f  the  e l e c t r o n c u r r e n t through a l a r g e r e s i s t o r of the o r d e r of 10 ohms, was c o n v e r t e d t o a . c .  p o t e n t i a l by a p p l y i n g 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 c a p a c i t a n c e was p e r i o d i c a l l y varying with time.  This a.c.  s i g n a l then underwent s e v e r a l  34 s t a g e s o f 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 r a c k u n i t . The i n p u t r e s i s t o r  and c a p a c i t o r were mounted i n s m a 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 o f range and response t i m e .  The i n p u t c a p a c i t y c o u l d a l s o 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 p f . enabled the a m p l i f i e r 30,000.  Internal s e n s i t i v i t y controls  g a i n to be adjusted  i n the range 1 to  T h i s was u s e f u l f o r s c a l i n g the output to reduce  ion-  i z a t i o n c u r v e s to almost e q u a l s e n s i t i v i t y . With an i n p u t r e s i s t a n c e o f 10 * ohms and c a p a c i t a n c e i a  of 5 p f . ,  c u r r e n t s as low as 10  The time c o n s t a n t  amperes c o u l d be measured.  i n t h i s range was about f i v e seconds.  The  a m p l i f i e r o u t p u t was fed i n t o a c h a r t r e c o r d e r which enabled automatic r e c o r d i n g o f c u r r e n t from the i o n s o u r c e .  35.  C.  The Monochromator E s s e n t i a l s f o r making photon impact experiments  a beam o f monochromatic l i g h t o f known energy p a s s i n g  are  through  the gas under study and a d e v i c e f o r 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 monochromator was c o n s t r u c t e d  The body o f  as a compact b r a s s u n i t ,  w i t h f i x e d e n t r a n c e and e x i t s l i t s .  The i n s t r u m e n t was  espec-  i a l l y good f o r the vacuum r e g i o n because the source and e x i t s l i t s d i d not need to be moved as the wavelength was changed. The o n l y m e c h a n i c a l motion i n v o l v e d was a s i m p l e r o t a t i o n o f 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  repetitive,  h i g h v o l t a g e s p a r k d i s c h a r g e through gas i n a  pyrex c a p i l l a r y c a p a b l e o f e m i t t i n g a vacuum u l t r a v i o l e t c o n tinuum from 8 to 21 eV. when i t was acanned by the The l i g h t source c o n s i s t e d of two major a)  grating.  components:  The McPherson Model 720 h i g h v o l t a g e A . C . power  s u p p l y which was c a p a b l e of s u p p l y i n g 0 - 10,000 v o l t s at a maximum c u r r e n t o f 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 i g h frequency t u n g s t e n spark s o u r c e .  S t a b l e o p e r a t i o n was  a c h i e v e d by adjustment o f the j e t o f a i r b l o w i n g a c r o s s s p a r k gap.  The a i r j e t  the  removed the i o n i z e d gases and m e t a l  vapors r e s u l t i n g from each e l e c t r i c a l breakdown.  Further  s t a b i l i t y was g a i n e d by the use o f 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 s o u r c e and a diagram of the l i g h t s o u r c e F i g u r e 8.  is illustrated in  I t was a c a p i l l a r y d i s c h a r g e l i g h t s o u r c e ,  of water c o o l e d c a p i l l a r y , cooled cathode. In o p e r a t i o n , continuum,  consisted  a water c o o l e d Anode and an a i r  The cathode was a i r c o o l e d by a b u i l t i n b l o w e r .  t h i s l i g h t s o u r c e c o u l d produce a l i n e spectrum or  and i t was connected to the monochromator w i t h or  w i t h o u t a l i t h i u m - f l u o r i d e window. l e s s l i g h t s o u r c e was used.  T h e r e f o r e i t was n e c e s s a r y  pressure i n the mass s p e c t r o m e t e r differential  In the p r e s e n t work a window-v to keep gas  as s m a l l as p o s s i b l e , and so  pumping was employed between the s o u r c e and monochro-  mator i n l e t s l i t .  A cam o p e r a t e d s l i d i n g p l u n g e r s e a l e d  against  the e n t r a n c e s l i t  jaw, and the monochromator was f u r t h e 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 h o u s i n g .  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 regulator,  t h r o t t l i n g v a l v e and a b s o l u t e p r e s s u r e  b r a t e d 0 t o 100 mm. Hg. the l i g h t s o u r c e . l i g h t source,  indicator  cali-  was connected between the gas tank and  B e f o r e the gas from the gas tank e n t e r e d  the  i t f i r s t passed through a U=tube c o n t a i n i n g L i n d e  s y n t h e t i c z e o l i t e s (molecular Sieves) cooled to l i q u i d temperature  tank  nitrogen  i n o r d e r t o 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 s o u r c e were m a i n l y hydrogen or h e l i u m . o  A c o n t i n u o u s spectrum was produced by h e l i u m from o  500A to 1200A (10 to 21 e V . ) , and hydrogen was u s e f u l f o r wavelength r e g i o n o f 900$ to 1500A (8 to 13 e V . ) . was u s u a l l y m a i n t a i n e d c o n s t a n t absolute pressure  The gas  the pressure  at 50 mm. Hg. as i n d i c a t e d i n the  indicator.  2 . The G r a t i n g System In the p r e s e n t 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  gas inlet  anode  water  water P  R  capillary I  c  ^ ^^ -£ = = «' cooling fins  LIGHT SOURCE Figure 8  37. l e n g t h 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 r o t a t e d  about a v e r t i c a l a x i s through the c e n t r e o f the o o g r a t i n g which enabled wavelengths between 500A and 1500A (about 8 o to 21 e V . ) 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 o f 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  h o l d e r 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 h o u s i n g . g r a t i n g was r o t a t e d  by moving the 1 2 - i n c h arm A which was connected  to a s p i n d l e p a s s i n g through a vacuum s e a l to the g r a t i n g base.  The  The arm A bore a g a i n s t  table  a p r e c i s i o n micrometer screw Z which  c o u l d 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 r u b b e r - r i n g e d gear wheel which h e l p e d to t r a n s m i t d r i v e to the  a smooth  micrometer.  3. The Photon M o n i t o r 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 c a p a b l e o f m u l t i p l y i n g the  f e e b l e p h o t o e l e c t r i c c u r r e n t produced at the cathode by a mean v a l u e o f 1.0 x 10^ when o p e r a t e d  at 100 v o l t s per s t a g e .  The o u t -  put c u r r e n t o f the 1P28 was a l i n e a r f u n c t i o n o f 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 o u t s i d e 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 l u o r e s p e n c e o f t h i s m a t e r i a l has been measured constant  i n the wavelength r e g i o n above 1000A\  and found to be and i s a l i n e a r  f u n c t i o n o f the photon energy i n the wavelength r e g i o n below 1000S (134) . The output c u r r e n t o f 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 further  by a K e i t h l e y d . c .  electrometer,  c a p a b l e of measuring down  38. to 10  -12  amperes.  The f i n a l output s i g n a l from the  Keithley  e l e c t r o m e t e r was r e c o r d e d i n a Speedomax r e c o r d e r . 4. The Energy C o n v e r s i o n S c a l e The wavelength o f a p a r t i c u l a r beam o f monochromatic l i g h t depends on the angular p o s i t i o n o f the d i f f r a c t i o n g r a t i n g , and i s g i v e n 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 o r d e r o f the l i n e ,  d i s the g r a t i n g s p a c i n g , and <fr  i s the angle of the d i f f r a c t i o n f o r wavelength A . can be r o u g h l y determined by spectrum a n a l y s i s . used i n t h i s w o r k ) . determined c o n s t a n t .  The o r d e r n,  (Only n = 1 was  The g r a t i n g s p a c i n g d i s an a c c u r a t e l y A r e l a t i o n therefore  e x i s t s between  the  wavelength and the angle of d i f f r a c t i o n f o r any wavelength i n the spectrum.  The g r a t i n g t a b l e used i n the p r e s e n t work was connec-  ted to a 1 2 - i n c h arm A which bore a g a i n s t screw.  The 584A* h e l i u m resonance  line,  a p r e c i s i o n micrometer  the 744°i neon i m p u r i t y o  l i n e o f the h e l i u m spectrum and the 1215A spectrum were f o c u s s e d s e p a r a t e l y , were r e c o r d e d f o r each l i n e .  l i n e of the hydrogen  and the micrometer  readings  U s i n g these t h r e e r e a d i n g s ,  b r a t i o n c u r v e was drawn, which i s a s t r a i g h t  a cali-  l i n e r e l a t i n g wave-  l e n g t h to micrometer r e a d i n g , from which the w a v e l e n g t h o f any o t h e r s p e c t r a l l i n e c o u l d be determined d i r e c t l y from the m i c r o meter  reading. The r e l a t i o n E = he g i v e s us a means o f comparing our  r e s u l t s which are i n terms o f photon wavelength w i t h the e l e c t r o n impact d a t a which are i n e l e c t r o n v o l t s . E i s the energy of 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 constant,  c is  the  the  v e l o c i t y o f l i g h t , and 7s i s the wavelength o f the r a d i a t i o n .  39. A c o n v e r s i o n t a b l e f o r wavelengths to e l e c t r o n v o l t s based on t h i s e q u a t i o n was p u b l i s h e d i n 1961 by Samson (106) and —1 —8 the c o n v e r s i o n f a c t o r used was 1 cm = 12397.8 + 0 . 5 x 10 eV. T h i s t a b l e was used throughout t h i s work f o r a l l energy c o n v e r s i o n s .  40. D.  The Vacuum System A good vacuum i s e s s e n t i a l  because atmospheric  i n p h o t o i o n i z a t i o n work,  s p e c i e s 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 i o n - a t o m r e a c t i o n s w i l l  i o n source p r e s s u r e of the mass spectrometer vacuum.  arise  i s not under  The vacuum system used f o l l o w s the c o n v e n t i o n a l  f o r mass s p e c t r o m e t e r s ,  if  and can be s u b d i v i d e d i n t o four  the  a good lines different  sections. 1. The A n a l y s e r Tube: The a n a l y s e r  tube was pumped from the source end by a  2 - i n c h 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 with a cold trap,  and backed by a Welch d u o - s e a l vacuum pump. _7  The u l t i m a t e vacuum o f 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 s o u r c e ,  was used to measure the p r e s s u r e i n t h i s  region.  2. The Monochromator The monochromator was pumped near the g r a t i n g mounting by a 6 - i n c h 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 fitted with cold-baffles, rotary  and backed by a l a r g e Welch  pump. A N R C - 5 0 1 thermocouple,  mounted on the top o f the  t i n g h o u s i n g , was used to measure the p r e s s u r e o f t h i s before  duo-seal  gra-  region  the o i l d i f f u s i o n pump was s w i t c h e d o n . 3. The L i g h t  Source  Two Welch d u o - s e a l vacuum pumps, one i n f r o n t o t h e r behind the monochromator  and  the  e n t r a n c e s l i t s , were employed to  41. m i n i m i s e the gas p r e s s u r e  i n the l i g h t s o u r c e r e g i o n .  It  is  i m p o r t a n t 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 i m i n i s h e d i f the r a d i a t i o n i s a l l o w e d to cause  ionization  there. 4 . The Gas H a n d l i n g System A Welch d u o - s e a l type r o t a r y pump was used to the gas p a r t i c l e s i n the t h r e e s t o r a g e g l a s s b u l b s .  evacuate  I t was s e p a -  r a t e d from the b u l b s by means o f a tap b e f o r e samples were i n t r o duced i n t o the s t o r a g e b u l b s and aTso d u r i n g a r u n .  The p r e s s u r e -4  i n t h i s r e g i o n was u s u a l l y m a i n t a i n e d i n the o r d e r of 10  mm of  Hg. 5. The measurement o f P r e s s u r e - Gauges The Thermocouple Gauge T h i s type o f gauge c o n s i s t s o f a w i r e through which a f i x e d c u r r e n t o f about 6 mA i s p a s s e d .  The temperature  depends upon the r a t e o f heat l o s s from the w i r e and,  o f the w i r e therefore,  upon the gas p r e s s u r e ,  and i s measured by means o f a thermocouple  a t t a c h e d t o the w i r e .  The o u t p u t o f 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  pressure.  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 i m p l y a d i s c h a r g e r e l y i n g on i o n i z a t i o n f o r i t s p r i n c i p l e o f o p e r a t i o n .  It  tube  consists  o f a t u n g s t e n f i l a m e n t , a g r i d at c o n s t a n t p o t e n t i a l , and a p l a t e . The f i l a m e n t e m i t s e l e c t r o n s which i o n i z e , gauge,  and the p l a t e  gas m o l e c u l e s i n the  (negative with 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 o f i o n s h i t t i n g the p l a t e  on the gas p r e s s u r e ,  and the p l a t e c u r r e n 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 o f  depends  pressure.  42. The i o n i z a t i o n gauge i s equipped w i t h a s a f e t y which w i l l reaches  a u t o m a t i c a l l y t u r n o f f the gauge when the  some s e t v a l u e , say l j times the f u l l  of the p a r t i c u l a r range  selected.  relay  pressure  scale indication  43. E.  E x p e r i m e n t a 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 r e s e a r c h were pure samples supplied by Matheson of Canada Co. L t d . and were not f u r t h e r  purified.'  B e f o r e the study o f a m o l e c u l e by photon impact was c a r r i e d out,  a mass spectrum o f the sample  was f i r s t t a k e n .  This  was done by f o c u s s i n g the i n t e n s e 744$ (about 16.64 e V . ) neon i m p u r i t y l i n e of the h e l i u m spectrum and k e e p i n g the i o n a c c e l e r a t i n g v o l t a g e at about 2000 v o l t s , scanned s l o w l y by a motor,  and fragment  but i m p u r i t i e s  s t r e n g t h was  and the i o n c u r r e n t was measured and  d i s p l a y e d on the r e c o r d e r . the parent  the magnetic f i e l d  I n each c a s e ,  peaks c o r r e s p o n d i n g to  i o n s were observed on the mass spectrum,  i f any, were not o b s e r v e d .  the mass spectrum were t h r e e f o l d , f i r s t l y , mate p o s i t i o n s o f the p a r e n t  The reasons f o r t a k i n g to d e t e c t  and i t s fragment  the a p p r o x i -  i o n s i n the mass  spectrum; s e c o n d l y , t o measure t h e i r r e l a t i v e i o n i z a t i o n  probabil-  i t i e s ' at a c e r t a i n photon energy;  impuri-  and t h i r d l y ,  to detect  t i e s i n the sample. 2. E x p e r i m e n t a l P r o c e d u r e In o r d e r to o b t a i n p h o t o i o n i z a t i o n d a t a f o r v a r i o u s m o l e c u l a r i o n s , the f o l l o w i n g procedure was f o l l o w e d f o r each gas. —6  The system was pumped down to a p p r o x i m a t e l y 10 > mm Hg. and the e l e c t r o n i c equipment was a l l o w e d to warm up f o r a p e r i o d of h a l f  an' h o u r .  The sample t o be s t u d i e d was i n t r o d u c e d i n t o  the  gas h a n d l i n g system and s u b s e q u e n t l y l e a k e d i n t o the i o n chamber of the mass s p e c t r o m e t e r .  The gas p r e s s u r e  i n the i o n chamber was  m a i n t a i n e d at 3 x 1 0 ^ mm H g . , and remained c o n s t a n t f o r a p e r i o d -  of at l e a s t t h r e e h o u r s .  44. When a l l these p r e p a r a t i o n s  had been c o m p l e t e d ,  the  i n t e n s e 7448 (about 16.64 e V . ) neon i m p u r i t y l i n e of the h e l i u m spectrum was f o c u s s e d .  The magnetic f i e l d  s t r e n g t h was  adjusted  to b r i n g the i o n to be s t u d i e d 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 . also necessary  to a d j u s t  Sometimes i t was  the i o n a c c e l e r a t i n g v o l t a g e ,  t i a l s at the r e p e l l e r e l e c t r o d e ,  poten-  e l e c t r o d e s 2 , 3 and 5 ( F i g u r e 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 multiplier,  the  and the e l e c t r o n  to g i v e the maximum i o n c u r r e n t .  The spectrum o f hydrogen or h e l i u m 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 o f v a r i o u s wavelengths c o n t a i n e d i n the source'/was a l l o w e d t o e n t e r the i o n chamber.  The s c a n n i n g was s t a r t e d  at a photon  energy w e l l below 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 molecule,  and the i o n and photon c u r r e n t  the  i n t e n s i t i e s f o r 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 . Finally,  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  c u r v e was o b t a i n e d .  For  each m o l e c u l e , the experiment was r e p e a t e d s i x qr more times  until  c l o s e agreement between the s u c c e s s i v e runs was o b t a i n e d . When the i o n and photon c u r r e n t  i n t e n s i t i e s became v e r y  low, both the r o t a r y a n d v d i f f u s i o n pumps were s t o p p e d , i n t r o d u c e d i n t o the system.  The g r a t i n g was taken o u t ,  and sprayed w i t h pure t o l u e n e . c l e a n e d w i t h methanol, salicylate.  After  and a i r was dismounted,  The p h o t o e l e c t r o n d e t e c t o r  and c o a t e d w i t h a f r e s h  these o p e r a t i o n s ,  was  l a y e r o f sodium  both the i o n and photon c u r -  r e n t i n t e n s i t y were found to be g r e a t l y i m p r o v e d . 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 o f 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 -  zation efficiency  curves,  i s not o n l y f o r the a c c u r a t e  measurement,  of i o n i z a t i o n and appearance p o t e n t i a l s , f i c a t i o n o f the v a r i o u s p r o c e s s e s  l e a d i n g to the i o n and  d e t e c t i o n o f h i g h e r energy s t a t e s . the e x p e r i m e n t a l d a t a ,  but a l s o f o r the i d e n t i the  These c u r v e s were drawn, from  r e l a t i n g the number of i o n s of a g i v e n k i n d  per number o f i n c i d e n t photons,  produced by photon i m p a c t ,  against  the photon energy o f 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 f o r a g i v e n m o l e c u l e at a c e r t a i n photon energy i s d e f i n e d by the Efficiency  =  equation:-  number of p r i m a r y i o n j - p a i r s formed number of i n c i d e n t photons absorbed a m p l i t u d e o f the i o n c u r r e n t a m p l i t u d e o f the photon c u r r e n t  . 3.2  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 applied f i e l d ,  by e x t e r n a l p e r t u r b a t i o n s  pressure,  and the geometry of the i o n i z a t i o n chamber (139).  The i n s t r u m e n t 4°i,  such as gas  and as a r e s u l t ,  used i n t h i s work has a r e s o l u t i o n o f  the a c t u a l  ' t h r e s h o l d ' v a l u e o f the  ionizing  photon energy w i l l be g r e a t e r than the v a l u e o b t a i n e d 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 c u r v e s by a l h a l f - w i d t h of the tion,  namely 2°..  reported  resolu-  A c o r r e c t i o n has been made f o r a l l measurements  i n t h i s work.  46. F.  Sources of E r r o r One of the major s o u r c e s of e r r o r was due t o the  p r e t a t i o n o f 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.  inter-  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 s of a l l the m o l e c u l e s c o u l d be determined to • a h i g h degree o f a c c u r a c y .  However, the n o i s e to s i g n a l r a t i o  o f the i o n a n d photon i n t e n s i t y . m e a s u r e m e n t s r  i n some c a s e s was r a t h e r  high,  after  and i t was d i f f i c u l t  the  threshold  to  localize  the p o i n t where maximum change o f s l o p e i n d i c a t e d the i n n e r z a t i o n p o t e n t i a l s . Only e s t i m a t e d p o s i t i o n s were o b t a i n e d  ioni-  from.;.':  the e x p e r i m e n t a l c u r v e s i n s p i t e o f the f a c t t h a t the band w i d t h of the photon beam used was o n l y 0.05 eV. at 10 eV. photon energy. The second s o u r c e o f e r r o r was caused by the v a r i a t i o n o of the gas p r e s s u r e d u r i n g a r u n .  In the e a r l y phase o f  work, a t h r e e - l i t r e s t o r a g e b u l b was used f o r sample,  and the  was a l l o w e d t o l e a k s l o w l y i n t o the mass s p e c t r o m e t e r . found t h a t a f t e r  about t h r e e hours o f o p e r a t i o n ,  dropped t o n e a r l y h a l f  the gas  I t was  the gas  pressure  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 c u r r e n t decreased s i g n i f i c a n t l y w h i l e the photon c u r r e n t remained c o n s t a n t .  Two f i v e - l i t r e  gas b u l b s were s i n c e  added t o the s u p p l y system making a t o t a l c a p a c i t y o f litres.  The gas p r e s s u r e  very small,  then  thirteen  drop f o r the same p e r i o d o f time was  and the drop i n i o n c u r r e n t per photon at a p a r t i c u -  l a r wavelength d u r i n g a r u n was n e g l i g i b l e . 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 s o f a l l the m o l e cules studied represent  the average o f s i x or more r u n s .  photoionization efficiency  The  c u r v e s 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 o f t h i s i o n i z a t i o n p o tential between each r u n 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 atoms are of s p e c i a l i n t e r e s t of t h e s e atoms  is  'noble'  gaseous  i n t h a t the i o n i z a t i o n p o t e n t i a l  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  A comparison o f d a t a o b t a i n e d by the.  data.  two methods p r o v i d e s a  means o f d e t e r m i n i n g the accuracy and r e l i a b i l i t y o f the ionization results.  Photoionization results  on the  photo-  'noble'  atoms  a l s o p r o v i d e a check f o r the v a l i d i t y of t h e o r e t i c a l models o f 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.  A r g o n , k r y p t o n and xenon each has an o u t e r mp^ s h e l l , where m = 3 f o r argon, 4 f o r k r y p t o n and 5 f o r xenon, and i o n i z a t i o n o f these m o l e c u l e s r e f e r s six p electrons.  the  to the removal of one of  The ground s t a t e o f the i o n formed i s s p l i t 2 2 P3/2  two l e v e l s , the Beutler  a  n  d  P  l/2'  T  h  e  l  a  t  t  e  r  n  a  s  t  n  e  higher  the into  energy  (3) s t u d i e d the a b s o r p t i o n s p e c t r a of these 2 2  g a s e s , and o b t a i n e d i o n i z a t i o n t h r e s h o l d s  of the  P3/2  a n <  ^  P  i/2  s t a t e s o f the i o n s from the ah^alysis o f the Rydberg s e r i e s .  He  observed t h a t the s e r i e s members c o n v e r g i n g to the h i g h e r  energy  s u b l e v e l of the ground s t a t e o f the i o n (^P^/2^ become v e r y diffuse  near the i o n i z a t i o n l i m i t ,  to a u t o i o n i z a t i o n , i . e . of i t s / ; e x c i t e d s t a t e s ,  the  'noble'  and proposed t h a t t h i s was due atom was f i r s t e x c i t e d to one  and s u b s e q u e n t l y  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 a b s o r p t i o n c o e f f i c i e n t s f o r argon have been  measured by W e i s s l e r and Lee (142) Larrabee (54).  and Huffman, Tanaka 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 of argon has  been  48. measured by Wainfan, Walker and W e 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 ) .  All  t h e i r d a t a were o b t a i n e d u s i n g a l i n e spectrum as t h e i r source.  F r o s t and McDowell ( 3 5 ) , Foner and N a i l  light  ( 2 9 ) , Marmet  and K e r w i n (77) s t u d i e d argon u s i n g 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 f o r argon as a f u n c t i o n o f the photon energy. z a t i o n o f argon to g i v e a  The t h r e s h o l d  ioni-  P 3 / 2 s t a t e o f the i o n i s o b t a i n e d  from the p o i n t o f i n i t i a l onset o f the argon c u r v e at 15.73 ± 0.05 e V . , which i s i n good agreement w i t h the  spectroscopic  v a l u e 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 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  d a t a at 15.76 eV. by D i b e l e r et a l . M o r r i s o n (81)  (17),  at 15.77 eV. by  and at 15.74 eV. by Foner and N a i l  From t h i s t h r e s h o l d ,  at  (29).  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. and  2  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  P i / 2 s t a t e of Ar+ i s 0.18 e V . , and w i t h our p r e s e n t 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 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 a b s o r p t i o n e x -  periment o f Huffman, Tanaka and L a r r a b e e Ching  structure  (54),  and Cook and  (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 o f Ar " remains 4  fairly  constant  at h i g h e r e n e r g i e s  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 Ar  +  c u r v e up to 18 e V . , and they c o r r e l a t e d these w i t h  found by v a r i o u s w o r k e r s u s i n g e l e c t r o n impact methods. would expect to be a b l e to r e s o l v e most o f the peaks  breaks We  appearing  ARGON  6  17  18  19  20  PHOTON ENERGY (eV) Figure 9  21  49. i n the Comes and Lessman c u r v e , but we f i n d no e v i d e n c e them.  for  Huffman, Tanaka and L a r r a b e e (54) u s i n g a much s m a l l e r  energy bandwidth (0.02  eV. at 60o8) found no e v i d e n c e f o r  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 t h a t  photo-  i o n i z a t i o n e f f i c i e n c y c u r v e s s h o u l d be v e r y d i f f e r e n t t o t a l absorption curves for t h i s simple case. t h e r e are d i f f e r e n t citation,  from  Of c o u r s e ,  s e l e c t i o n r u l e s f o r e l e c t r o n impact e x -  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  forbidden.  T h i s would e x p l a i n t h e i r  absence i n the  results  r e p o r t e d 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 s e t s o f d a t a l i e s i n the l i g h t s o u r c e - the Comes and Lessman d a t a was o b t a i n e d u s i n g a l i n e spectrum,  B.  and ours u s i n g a h e l i u m continuum.  Krypton K r y p t o n has been s t u d i e d by B e u t l e r ( 3 ) , Huffman,  Tanaka and L a r r a b e e (53) u s i n g the s p e c t r o s c o p i c F r o s t and McDowell (35) u s i n g . t h e studied K r  e l e c t r o n impact method  i o n and found t h a t the ground s t a t e o f K r 2 9  +  s i s t s o f two components, separation  method.  P3/2  a  n  d  p  l/2  s t a  '  t e s  )  +  con-  whose  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 c u r v e of k r y p t o n  i s shown i n the lower h a l f o f F i g u r e 10, and the top c u r v e i s the a b s o r p t i o n c o e f f . and L a r r a b e e  c u r v e o f k r y p t o n by Huffman, Tanaka  (53) f o r c o m p a r i s o n .  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 o f k r y p t o n o b t a i n e d at the p o i n t of i n i t i a l onset of the former c u r v e i s found to be 13.99 ± 0.05 eV. w h i c h 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 o f  50. k r y p t o n at 14.009 eV. (1) Dibeler  and the e l e c t r o n impact d a t a o f  (17) at 13.96 eV. After  the t h r e s h o l d energy,  the c u r v e o f K r  +  is  observed to r i s e s t e e p l y as would be expected i n the case o f a monatomic gas.  A s t r o n g p h o t o i o n i z a t i o n peak i s observed  14.09 eV. which p a r t i a l l y obscures o f the  2  .  P 3 / 2 s t a t e of K r . T  the i o n i z a t i o n continuum  Several less intense i o n i z a t i o n  peaks are a l s o observed at h i g h e r e n e r g i e s . been found p r e v i o u s l y by B e u t l e r (3) autoionized i n their  at  absorption  These peaks have  and Huffman (53)  as  spectra.  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 a b s o r p t i o n s p e c t r a o f Kr i n T a b l e I I .  Table  II  Peak E n e r g i e s f o r K r y p t o n Ion ( e V . )  T h i s work Huffman  13.9 9 ( I P . )  (53)  14.08  14.26  "14.37  14.47  14.09  14.26  14.36  14.47  The good agreement between a b s o r p t i o n s p e c t r o s c o p y and p h o t o i o n i z a t i o n d a t a s t r o n g l y suggested  t h a t the  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 observed from the c u r v e f o r K r corresponds The P 3 / 2 2  an<  s m a l l i o n i z a t i o n continuum i s +  ( F i g u r e 1 0 ) , t h i s continuum  to the i o n i z a t i o n o f k r y p t o n to the 3  2 p  l / 2 ground s t a t e s e p a r a t i o n  2  in Kr  P]y2 +  state.  i s found  to be 0.78 eV. which i s h i g h e r than the v a l u e o f 0.666 eV.  Wavelength, X ( A ) 850 900 T  1  i — i — i — r  *6 >.r£/2  4P -4P ( P )nd' 6  5  2  l/2  1513  1311 12109 8  lllll I I 4P -4^( F> )ns' 1 6  2  /2  F i g u r e 10  1  I  I  I  I  I  I  I  L_I_U  14.5 14 Photon energy , eV  61. o b t a i n e d by the s p e c t r o s c o p i c method ( 1 ) .  The l a r g e d i f f e r e n c e  in  energy i s p r o b a b l y due to the f a c t t h a t the i o n i z a t i o n continuum f o r the  State of : K r i s very, s m a l l , +  and the  accurate  d e t e r m i n a t i o n o f 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 f o r process  is difficult  w i t h the  apparatus.  C.  w i t h the r e s o l u t i o n at p r e s e n t  this  attainable  Xenon Xenon has been s t u d i e d by B e u t l e r ( 3 ) , Huffman, Tanaka  and L a r r a b e e ( 5 3 ) ( A b s o r p t i o n S p e c t r o s c o p y ) , by N i c h o l s o n Matsunage, J a c k s o n and Watanabe D i b e l e r , Mohler and Reese ( 1 7 ) .  (79)(Photoionization) and Foner and N a i l  (29)  (91), and by (Electron  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 c u r v e of xenon i s shown i n F i g u r e 11.  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 o f xenon measured  at the p o i n t of s t e e p e s t s l o p e 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 s p e c t r o s c o p i c v a l u e 12.129 eV.  (1). After  efficiency  the t h r e s h o l d energy,  c u r v e f o r xenon.  s e v e r a l peaks appear on the  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 a u t o i o n i z e d i n h i s a b s o r p t i o n spectrum,  and t h a t  they are indeed due to t h i s type o f p r o c e s s 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 p r e s e n t 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 p r e s s u r e (10 than those used by N i c h o l s o n (20 - 100 atom r e a c t i o n s  are t h e r e f o r e  I n T a b l e I I I below  ).  mm. of Hg.)  Atom-atom and i o n -  v i r t u a l l y eliminated. 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 a b s o r p t i o n spectrum o f xenon, and a l s o by  N i c h o l s o n (91) u s i n g the t o t a l p h o t o i o n i z a t i o n a p p a r a t u s . Table  III  Peak E n e r g i e s f o r X e T h i s work 12.12  Beutler(3)  Huffman(53)  (IP.)  +  (eV).  Watanabe:(79) 12.15  Nicholson(91)  (IP.)  12.129  12.43  12.45  12.46  12.46  12.48  12. 81  12.82  12.83  12.83  12.86  13.03  13.02  13.02  13.00  13.05  13.18  13.14  13. 15  13. 16  13.23  13.21  13. 21  13. 25  13.45  (IP.)  13.48  13.63  The c l o s e n e s s o f the agreement between the a b s o r p t i o n  spectroscopy  and the p h o t o i o n i z a t i o n d a t a prove beyond doubt t h a t the determined here 2 The P  peaks  a r i s e from the p r o c e s s o f a u t o i o n i z a t i o n . 2 - P ground s t a t e s e p a r a t i o n i n X e i s J./ +  O/  di  found to be 1.32 e V . , which i s i n good agreement w i t h the t i o n of 1.31 eV. o b t a i n e d by the s p e c t r o s c o p i c method  (1).  separa-  53. CHAPTER FIVE PHOTOIONIZATION OF DIATOMIC MOLECULES A.  Oxygen Oxygen i s a major component of the atmosphere,  study o f i t s p h o t o i o n i z a t i o n i s o f fundamental molecular spectroscopy  P r i c e and C o l l i n s  c a l s p e c t r o s c o p i c method,  o f many f a i r l y  in  and M u l l i k e n and Stevens  (84) u s i n g  the  i o n i z a t i o n p o t e n t i a l of  Photoionization studies  done by Inn ( 6 0 ) , Watanabe  detailed  (09) s t u d i e d the m o l e c u l e by the o p t i -  c y c l i c method have o b t a i n e d the f i r s t oxygen as 12.2 eV.  importance  the  and i o n o s p h e r i c p h y s i c s .  Oxygen has been the s u b j e c t studies.  and  o f oxygen have been  (138), N i c h o l s o n (91)  and Samson and  C a i r n s (105), Tate and Smith ( 1 2 2 ) , Hagstrum ( 4 4 ) , F r o s t and McDowell ( 3 8 ) , C l a r k e (5) and B r i o n (4) have s t u d i e d oxygen by  the  e l e c t r o n impact method. The ground s t a t e o f the oxygen m o l e c u l e has the nic  electro-  structure: (CT 2 s ) ( ( T 2 s ) ( ( r 2 p ) ( T T 2 p ) ( ^ 2 p ) ; 2  2  g  u  4  2  g  2  u  g  The m o l e c u l a r o r b i t a l s are l i s t e d i n the o r d e r o f b i n d i n g energy, potential refers  o m i t t i n g the i n n e r ones.  3£  ionization  to the removal of an e l e c t r o n from the  4ground  ....5.1  decreasing  The f i r s t  a n t i b o n d i n g (TT _2p) o r b i t a l l e a v i n g the 0^  -  outer 2  i o n i n i t s X TT  state. 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 c u r v e s f o r oxygen  shown i n F i g u r e s 12 and 13. measured 0.05 eV.  are  The i o n i z a t i o n p o t e n t i a l o f oxygen  from the p o i n t o f i n i t i a l onset o f the curve i s 12.06 + which i s i n e x c e l l e n t  agreement w i t h o t h e r  t i o n v a l u e s at 12.065 eV. by N i c h o l s o n ( 9 1 ) ,  photoioniza-  12.063 eV. by  850 —|  K  1  900 1  »  c  1  950 1  1  1  1  1  1000 1  1  :  1  1  I050A •  '  *  •  J  IONIZATION  EFFICIENCY  54. Samson (105)  and 12.07 eV. by Watanabe  (138).  The c l o s e a g r e e -  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 i n g d i f f e r e n t  tech-  niques proves beyond doubt t h a t the p h o t o i o n i z a t i o n methods o f obtaining ionization potentials  are r e l i a b l e .  Values obtained  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 i m p a c t , s p e c t r o s c o p 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 w o r k e r s are summarised i n Table IV. TablerIV 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 o f Oxygen I.P.(eV.) 12.06 + 0 . 0 5  WORKERS Present r e s u l t  Photoionization  1966  12.04 + 0.01  Inn  Photoionization  1953  12.07 + 0.01  Watanabe  (138)  Photoionization  1957  12.065  Nicholson  (91)  Photoionization  1963  12.063  Samson (105)  Photoionization  1965  12.2  Mulliken  C y c l i c Method  1933  12.2 + 0 . 1  Price  (99)  Spectroscopy  1934  12.5 + 0 . 1  Tate  (122)  Electron  Impact  1941  12.1 + 0 . 2  Hagstrum (44)  Electron  Impact  1951  12.21 + 0.04  Frost  (38)  Electron  Impact  1958  12.04 + 0.02  Clarke  (5)  Electron  Impact  1964  12.20 + 0.05  Brion  Electron  Impact  1964  After  (60)  (84)  (4)  METHODS  YEAR  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 o f oxygen,  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 c u r v e e x h i b i t s many peaks. region after  the  The whole  the t h r e s h o l d 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 o f 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 c u r v e shows t h a t the peaks i n the p h o t o i o n i z a t i o n spectrum c o r r e s p o n d w e l l  with  55. peaks i n the a b s o r p t i o n s p e c t r a .  T a b l e 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 c u r v e s o b t a i n e d i n t h i s work w i t h those o b t a i n e d i n the work o f N i c h o l s o n (91) (138),  and Watanabe  and i n the a b s o r p t i o n s p e c t r a o f Huffman ( 5 5 ) , Cook  and P r i c e and C o l l i n s  ( 9 9 ) . . The agreement between p h o t o i o n i z a -  t i o n and a b s o r p t i o n peaks e s t a b l i s h w i t h c e r t a i n t y of  (11)  that  certain  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 o f f i g u r e 12 and 13 i s such t h a t  bands i n the groups H , . I , . J , , K M', N ' b e l o n g  H', I',  to Rydberg s e r i e s '  M,N, 0 . . . . . .  approaching the  c h a r a c t e r i s t i c o f each p a r t i c u l a r group  limit  (99). o  The w e l l e s t a b l i s h e d s t a t e s o f oxygen i o n s are X TT^ a *T , 4  U  A Tl 2  u  and b  4  2  g  - .  The energy s t a t e o f C-2  o b t a i n e d by the removal o f a ("TT" 2p) e l e c t r o n ,  +  2  H  the a ^ TT  is  g  and  A * T s t a t e s are o b t a i n e d by the removal o f (TT 2 p ) e l e c t r o n s , and b ^ S ~ s t a t e i s o b t a i n e d by the removal o f an (G~ 2p) e l e c & g tron. The p r e s e n t p h o t o i o n i z a t i o n d a t a f o r oxygen g i v e s e v i d e n c e 2  U  u  e  f o r the e x c i t a t i o n o f o n l y two t y p e s , 2 4— X:  TT  g  and b  transitions,  £  g  states.  namely the t r a n s i t i o n s  I n the r e g i o n o f the  TT < e l e c t r o n i c u  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 -  t y l i n e s are so i n t e n s e  t h a t the e l e c t r o n i c l e v e l s of the  are obscured by. the r e s o l u t i o n s o f the  ions  monochromator.  The i o n i z a t i o n p o t e n t i a l o f oxygen l e a d i n g to t h e S ~ s t a t e i s o b t a i n e d from the break o f the c u r v e at 18.20 eV. g 1  b  4  to  T h i s v a l u e 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 Tanaka ( 5 5 ) .  (99) and 18.17 eV. by Huffman, L a r r a b e e and  The e l e c t r o n impact work o f F r o s t and McDowell  (38) g i v e s a v a l u e o f 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 A b s o r p t i o n Spectrum of Oxygen Designation (56)  T h i s Work  Huffman (55)  -  12. 16  l 2  12. 30 12. 45  .12.. 16 12.,30 12..47  Nicholson (91)  Price (99)  —  —  —  —  -  -  12.33 12.47  12.33 12.48  12.33 12.48  3  12. 53  12,.53  12.56  12.61  12.61  12.61  12. 68  12..69  12. 68  -  -  -  12. 76  12,, 75  12. 75  12. 75  12. 75  12. 76  12. 87  12,.88  12.88  12.84  12.84  12.84  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  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 14. 14  13,.98 14,. 11  13.99 14.13  -  14,.25 14,.33 14,,48 14,.56  14.25 14.34 14.48 14.57  13.98 -  -  14. 25 14. 34 14. 49 14. 57  13.99 -  I I  14. 67 14. 79  14,.66 14,.79  14.66 14. 79  I, K I '  14. 92 15. 07 15. 16  14,.91 15,.07  14.91 15.06  15,, 17  N P -  15. 32 15. 45 15. 56  P P _  H  H  H  H H  4  3'  V  M  s'  J J J K  I '  Cook (11)  -  -  Watanabe (138)  -  -  -  - -  -  —  -  -  15.17  -  -  15,.33 15,.45  15.33 15.45  -  -  -  -  15.55 15.60 15. 77  -  -  -  15. 60 15. 77  15,. 55 15 .60 15 . 77  -  -  -  15. 80  15 .80  15.80  _  _  -  -  -  -  57.  The i n t e r a t o m i c d i s t a n c e , McDowell ( 3 8 ) ,  f o r the v a r i o u s s t a t e s o f oxygen and  m o l e c u l a r i o n s are: 1.3813A (a TT u ) , 4  a c c o r d i n g to F r o s t and  0  2  1,204A (X  1.4038$ (A  most p r o b a b l e t r a n s i t i o n s Franck-Condon P r i n c i p l e  2  3  £  ") ;  0  + 2  :  its  1.12278' (X 7T ) ,  H .0u and 1.2795$ (b  2  4  £  g~ ) .  The  i n p h o t o i o n i z a t i o n a c c o r d i n g to  (and w i t h the i n t e r a t o m i c  the  distances  between the m o l e c u l e and the i o n i n m i n d ) , 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 o f the oxygen o  m o l e c u l e to one o f the low l y i n g v i b r a t i o n a l l e v e l s o f X TT 4 4 and B £ g ~ s t a t e s o f the oxygen i o n . The t r a n s i t i o n s to a ^ T T 2 _ g  and A  Tv  s t a t e s of the oxygen i o n are l e s s p r o b a b l e s i n c e  l i e at a g r e a t e r i n t e r a t o m i c  distance.  U  they  58. B.  Nitrogen N i t r o g e n has been s t u d i e d by many w o r k e r s .  The  a b s o r p t i o n c o e f f i c i e n t s o f n i t r o g e n have been measured Huffman,  Tanaka and L a r r a b e e  (55) , Cook and Metzger ( 1 1 ) , Ogawa  and Tanaka (95) , Watanabe and Marmo ( 1 3 6 ) , McAllister  (61)  and C l a r k e ( 5 ) .  i o n i z a t i o n of n i t r o g e n ,  by  Itamoto and  Only a few papers on the  photo-  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 Samson and C a i r n s (103)  (11),  and Comes and Lessman ( 8 ) .  E a r l i e r w o r k e r s , however, measured cients using a w e l l - r e s o l v e d l i n e source.  absorption  coeffi-  In some c a s e s , the  fca.  source remission l i n e used by t h e s e a u t h o r s 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 a b s o r p t i o n maximum or minimum so t h a t coefficients  the  l i s t e d f o r comparison are those at the two n e a r e s t  maxima or minima.  The p r e s e n t d a t a i s based on a continuum s o u r c e ,  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 o f n i t r o g e n p r e d i c t e d elementary m o l e c u l a r o r b i t a l t h e o r y KK  is:-  (CT 2s) ((r 2s) (TT 2 p ) ( t r 2 p ) ; 2  2  g  u >  4  u  2  £  1  g  The m o l e c u l a r o r b i t a l s are l i s t e d i n the o r d e r o f b i n d i n g energy,  o m i t t i n g the i n n e r  from  + g  5.2  decreasing  orbitals.  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 o f n i t r o g e n as a f u n c t i o n o f the photon energy i s shown i n F i g u r e 14.  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 o f n i t r o g e n measured from the p o i n t o f i n i t i a l onset o f the c u r v e i s at 15.55 + 0.05 eV. refers  to the removal of an e l e c t r o n from the o u t e r  orbital,  to l e a v e the N  9  ion i n i t s X  2  This value  (KT„2p) o ground s t a t e . The  650  1  1  1  1  1  700 1  1  PHOTON  1  1  1  ENERGY  750 1  1  '  '  »  800A 1  59. v a l u e o f 15.55 eV. i s i n good agreement w i t h the  spectroscopic  v a l u e o f 15.576 e V . ( 4 8 ) , and the p b o i r 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 o f n i t r o g e n at 15.580 eV. by Watanabe and Marmo ( 1 3 6 ) , and by Ogawa and Tanaka ( 9 5 ) . Numerous workers have o b t a i n e d e l e c t r o n impact v a l u e f o r the f i r s t n i t r o g e n . F r o s t and McDowell C l o u t i e r and S c h i f f  i o n i z a t i o n p o t e n t i a l of  ( 3 8 ) , Fox and Hickman (30)  (6) have used a R . P . D .  and  method and< Obtained  v a l u e s o f 15.63 e V . , 15.60 eV. and 15.58 eV. r e s p e c t i v e l y 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 n i t r o g e n . e l e c t r o n impact, potentials  spectroscopic  for  The v a r i o u s  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  of n i t r o g e n are summarised i n T a b l e V I .  Tabla VI 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 N i t r o g e n I.P.  (e. V . )  WORKERS  15 .55 + 0.05  Present  result  . METHODS Photoionization  15 .6 + 0 . 1  Weissler  (146)  Photoionization  1956  15 .580  Watanabe  (136)  Phot6ionization  1956  15 .580  Ogawa (95)  Photoionization  1962  15 .58  Huffman  Photoionization  1963  15 .576  Herzberg  Spectroscopy  1950  15 .65  Tate  Electron  Impact  1936  15 .60  Fox  Electron  Impact  1954  15 .63 + 0 . 0 2  Frost  Electron  Impact  1955  15 .58 + 0 . 0 2  Cloutier  Electron  Impact  1959  (55) (48)  (121) (52) (35) (6)  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 c u r v e o f ( F i g u r e 14) e x h i b i t s a s l i g h t c u r v a t u r e and t h i s c o n f i r m s the f a c t  YEAR 1966  nitrogen  near the t h r e s h o l d  that there i s a small difference  energy, in  60. equilibrium interatomic  distance  n i t r o g e n m o l e c u l e and i t s i o n .  between the ground s t a t e s o f  The e q u i l i b r i u m i n t e r a t o m i c  dis-  t a n c e s o f the ground s t a t e s o f the m o l e c u l e and i t s i o n s 1 . 0 9 4 8 and 1.116A r e s p e c t i v e l y  the  are  (48) .  There has been some question i n the e l e c t r o n  impact "f  d a t a c o n c e r n i n g the shape o f the i o n i z a t i o n e f f i c i e n c y durve o f 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 s h o l d C l a r k e ' s d a t a (5) u s i n g an e l e c t r o s t a t i c  selector  shows a non-  l i n e a r i t y i n t h i s r e g i o n o f h i s Ng" " c u r v e , which was not 1  by o t h e r i n v e s t i g a t o r s (30).  energy.  observed  u s i n g c o n v e n t i o n a l e l e c t r o n impact methods  Fox (32) u s i n g a R . P . D . method confirmed the  i n the same r e g i o n of the N addition ionizationpprocess  + 2  nonlinearity  CUr.vfe, arid attStlbil.tdQ t h i i s to an  such as  autoionization.  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 c u r v e f o r n i t r o g e n 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 .  exhi-  T a b l e V I I shows I  a comparison o f these peaks w i t h the a b s o r p t i o n s p e c t r a o f Huffman Tanaka and L a r r a b e e  (55)  and Cook and Metzger ( 1 1 ) .  The c l o s e  agreement i n peak: e n e r g i e s found by the two t e c h n i q u e s beyond doubt t h a t they a r i s e from the p r o c e s s o f and t h a t the r e g i o n above the t h r e s h o l d energy autoionized  proves  autoionization  i s overlaid with  states.  More peaks appear i n the a b s o r p t i o n spectrum o f 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 . be due to two r e a s o n s : f i r s t l y , tdityh monochromator spectroscopy,  the r e s o l u t i o n o f the  i s v e r y much l e s s than t h a t i n the o  so t h a t a s e p a r a t i o n  nitrogen T h i s may  photoionizaabsorption  o f l e s s than 4 A 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 e a s i l y r e s o l v e d i n the a b s o r p t i o n spectrum; a u t o i o n i z a t i o n p r o c e s s i s governed by  and s e c o n d l y ,  certain selection  are  the rules  61. Table VII Comparison o f Peaks i n the P h o t o i o n i z a t i o n Curve and A b s o r p t i o n Spectrum Of N i t r o g e n Designation  T h i s work 15.55  Huffman  (56)  Cookn(ll)  (IP.)  R  x 8 .1  15.65  15.65  15..65  R  a 3 .3  15. 79  15.81  15..80  0 .0  15.97  15.98  15,.98  4.1 a P. 1..0  16.06  16.06  16.,06  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  a 6..2 P 3..0  16.75  16. 76  16.,76  16.65  16.65  16. 65  b 3..0  17.14  17.14  17. 13  17.41  -  4,,0  17.84  17.84  17.,84  5,.0  18.18  18.18  18. 18  b 7,.0  18.46  18.46  18.,46  P R  R  R  R  b  R  b  R  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  s t a t e s o f m o l e c u l e and the i o n i z a t i o n continuum.  excited  Not a l l e x c i -  t a t i o n o f e l e c t r o n s to the e x c i t e d s t a t e s o f the m o l e c u l e 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 absorption  spectrum  in  are absent i n the p h o t o i o n i z a t i o n  results  the spectrum.  62 C.  Carbon Monoxide The a b s o r p t i o n spectrum of carbon monoxide i n the  vacuum u n t r a v i o l e t has been s t u d i e d by many w o r k e r s .  The most  r e c e n t work on p h o t o i o n i z a t i o n i s by Huffman, Tanaka and Larrabee; ( 5 6 ) , Watanabe,  Zelikoff  and Inn (132)  Tanaka, J u r s a and L e B l a n c (120) Mottl  (140).  and  Sun and W e i s s l e r  (145),  '-.W.atanabe, Nakayama and  Fox and Hickam (30) and Hagstrum (44) have s t u d i e d  carbon monoxide by the e l e c t r o n impact method. The carbon monoxide m o l e c u l e has 14 e l e c t r o n s , 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 (o- 2s) (<T 2s) (¥ 2p) ((T 2p) ; 2  2  g  u  4  u  2  where the m o l e c u l a r o r b i t a l s are arranged c r e a s i n g b i n d i n g energy,  configuration: £  1  g  ...  + g  de-  orbitals.  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 c u r v e f o r  t i o n p o t e n t i a l measured  5.3  i n the o r d e r of  o m i t t i n g the i n n e r  monoxide i s shown i n F i g u r e s 15 and 16.  and  carbon  The t h r e s h o l d  ioniza-  from the p o i n t o f i n i t i a l onset o f  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 i v e an i o n i n the X  2  £  ground s t a t e .  +  The v a l u e o f 13.98 eV. i s i n c l o s e agreement w i t h the c o p i c v a l u e o f 14.01 eV. by Watanabe  (140)  and the  impact v a l u e o f 13.98 eV. by Fox and Hickam (30) by Hagstrum  spectros-.  electron  and 14.1 eV.  (44).  The e f f i c i e n c y curve of carbon monoxide r i s e s after  the t h r e s h o l d onset as p r e d i c t e d from the Franck-Condon  principle  since the (  e q u i l i b r i u m i n t e r a t o m i c distance of  normal m o l e c u l e i s almost the same as the C 0 X  2 <  E  sharply  +  state.  +  i o n i n the  the :  IONIZATION  O  i  1—•  1—•  CJ»  1—i  1  EFFICIENCY '—•—n  O  r~~  IONIZATION  O :  EFFICIENCY  Ol  1  1  1  1  1  1  1  1  1  1  o r  63. Fox (32) observed electron  impact d a t a p o i n t s  threshold,  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 i n the v i c i n i t y of the X  and W a l l a c e (130)  has measured the carbon  a u t o i o n i z a t i o n bands i n h i s a b s o r p t i o n  spectra.  2  £L  +  monoxide  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 c u r v e o f carbon monoxide a l s o e x h i b i t s many peaks above the t h r e s h o l d  ionization potential.  Table V I I I  shows a comparison o f the p o s i t i o n o f peaks i n the t i o n spectrum Cook  and the a b s o r p t i o n  photoioniza-  s p e c t r a o f Huffman (56)  and  (14). Table V I I I Comparison o f 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 o f Carbon Monoxide  Designation  T h i s work 13. 98  Huffman (56)  Cook (14)  (IP.)  R  x  7.1  14.,01  14.,01  14.,01  R  x  10.1  14., 17  14., 17  14.. 17  14. 26  14..25  14. 26  14. 31  14.,31  14..31  14..57  14..56  14..56  Hi  14.. 77  14,. 78  14.. 78  l  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  H  Hi R H  a  3.0  2  R R R  a  a  a  3.2,  ?  1  3.3, 4 . 1 , Po  64. Table V I I I Designation  (continued)  Huffman  T h i s work  (56)  Cook (14)  P  2  16.33  16.33  16.33  P  3  16.54  16.54  16.54  16.71  16. 71  16. 72  16.87  16.88  16.88  17.04  17.07  17.04  17.96  17.96  17.96  P ,  P  2  3  B P  4  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 c u r v e and  the  a b s o r p t i o n s p e c t r a shows t h a t 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 c o r r e s p o n d i n g peaks i n the tion spectra.  absorp-  C l o s e agreement between the two shows t h a t  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 o f carbon monoxide refers  to the removal o f 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 v a l u e f o r t h i s l i m i t  is  16.536 eV. (119) as shown i n F i g u r e 16 by broken arrow.  This  i o n i z a t i o n p o t e n t i a l was not observed f o r the f o l l o w i n g 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 a n c e s  are:  2 A  1.128l8, X £ 1  as g i v e n by H e r z b e r g (48)  o f CO; 1.1150$, X £  +  2  g  reason.  +  , and 1.2436$,  + T T . o f CO .  The l a r g e d i f f e r e n c e  d i s t a n c e f o r the C0(X £ 1  +  )  and C 0 ( A +  i n equilibrium interatomic 2  TT ) i  states results  the p r o b a b i l i t y f o r t h i s t r a n s i t i o n b e i n g d i s t r i b u t e d 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 i o n between the X *£Ia;  +  o f the m o l e c u l e and the A  2  in  fairly transi-  T\ ± s t a t e  65. o f the i o n i s s m a l l .  T h i s means t h a t t h e r e are  probably  s e v e r a l s t e p s i n the continuum t h r e s h o l d f o r t h i s s t a t e ,  and  apparently  in  the c o m p e t i t i o n o f the a u t o i o n i z a t i o n p r o c e s s  the same r e g i o n i s so i n t e n s e the r e s o l u t i o n o f the  t h a t the s t e p s are obscured  experiment.  at  66. D. C h l o r i n e The o n l y 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 o f  the  c h l o r i n e m o l e c u l e has been r e p o r t e d by Gaydon ( 4 2 ) .  Watanabe  ( 1 3 8 ) , u s i n g the p h o t o i o n i z a t i o n method has o b t a i n e d  the  threshold i o n i z a t i o n potential,  and M o r r i s o n and N i c h o l s o n  and Thorburn (126) have measured the e l e c t r o n impact tion potentials.  (90)  ioniza-  F r o s t and McDowell (39) u s i n g the R . P . D .  e l e c t r o n impact method have been a b l e to measure f i r s t  and  i n n e r i o n i z a t i o n p o t e n t i a l s of the m o l e c u l e . A c c o r d i n g to M u l l i k e n (86)', the e l e c t r o n i c  structure  of c h l o r i n e may be r e p r e s e n t e d by the f o r m u l a : ( 0 - 3 s ) ( c r 3 s ) ( c r 3 p ) ( T T 3p) (TT 3 p ) , 2  g  2  2  u  4  g  The i n n e r e l e c t r o n s  4  u  1  g  E  ...  + g  5.4  are o m i t t e d and the o r b i t a l s are l i s t e d i n  o r d e r of d e c r e a s i n g 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 c u r v e f o r the c h l o r i n e m o l e c u l e i s shown i n F i g u r e 17. ential  The t h r e s h o l d i o n i z a t i o n p o t -  ( o b t a i n e d from the p o i n t of i n i t i a l onset o f the  at 11.47 i 0.05 eV. r e f e r s  curve)  to the removal o f an e l e c t r o n from  the a n t i - b o n d i n g (TT 3 p ) o r b i t a l . g  This value i s higher  than  the s p e c t r o s c o p i c v a l u e at 11.32 eV. found by Gaydon ( 4 2 ) , it  but  i s i n e x c e l l e n t agreement w i t h the v a l u e at 11.48 eV. found  by Watanabe  (138).  The e l e c t r o n impact method g i v e s the  thres-  h o l d i o n i z a t i o n p o t e n t i a l o f the c h l o r i n e m o l e c u l e at 11.8 eV. ( M o r r i s o n and N i c h o l s o n ( 9 0 ) ) ,  11.8 eV. (Thorburn (126))  11.63 eV. ( F r o s t and McDowell ( 3 9 ) ) .  It  i s noted t h a t  and 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 i g h e r than the v a l u e found i n t h i s work, and t h i s i s p r o b a b l y due to the 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 , degree o f s e n s i t i v i t y i n the i o n c u r r e n t  difference  and i n the  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 near the t h r e s h o l d ,  curvature  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 a n c e between the m o l e c u l e (normal)  2_  and  Cl  + 2  i o n ( TV ) , and c o n f i r m s t h a t the p r i m a r y i o n i z a t i o n i n v o l v e s the removal of an a n t i b o n d i n g e l e c t r o n . The o u t e r  (TT 3p) and (TT 3 p ) o r b i t a l s are m a i n l y u  atomic i n c h a r a c t e r , electrons  and the i o n s formed by the removal of  from these o r b i t a l s w i l l be i n Q  and each o f these  2  TT  9  g  and ^ TT  TV s t a t e s would be a d o u b l e t .  u  states,  F r o s t and  McDowell (39) i n d i c a t e d t h a t t h e r e appear to be s e v e r a l  ionic  energy l e v e l s w i t h i n about 0 . 6 eV. o f 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 a b l e to r e s o l v e these i o n i c energy l e v e l s . Samce the c u r v e f o r CI2 " above the t h r e s h o l d energy 4  abnormal i n shape, ionization  appears  t h i s s u g g e s t s the o v e r l a p p i n g o f s e v e r a l  processes.  68. E.  Hydrogen C h l o r i d e Hydrogen c h l o r i d e has been s t u d i e d e x t e n s i v e l y by  infrared  and Raman s p e c t r o s c o p y ,  v i o l e t r e g i o n are s c a r c e .  but d a t a i n the f a r  ultra-  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  c u r v e o f hydrogen c h l o r i d e i s shown i n F i g u r e 18.  From the  c u r v e the t h r e s h o l d o f i o n i z a t i o n i s 12.56 ± 0.05 e V . , which refers  to the removal o f a non-bonding e l e c t r o n l o c a l i z e d i n  the h a l o g e n atom as d i s c u s s e d 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 TT g / 2 ground s t a t e f o r the m o l e c u l a r i o n .  This  2  v a l u e i s i n good agreement w i t h the v a l u e o f 12.56 eV. found by Fox (31) u s i n g the R . P . D . method,  and 12.53 eV. by M o r r i s o n  (81)  u s i n g the e l e c t r o n impact method.  However, i t i s lower than  the v a l u e at 12.74 eV. by Watanabe  (138) u s i n g the  t r i c measurement, and at 12.90 eV. by P r i c e (100) spectroscopic  photoelecusing  means.  The c u r v e f o r hydrogen c h l o r i d e r i s e s s h a r p l y the t h r e s h o l d energy,  a g r e e i n g w i t h the f a c t  p h o t o i o n i z a t i o n process a further  t h a t the  i s indeed a s t e p f u n c t i o n .  after  single  There  is  sharp i n c r e a s e 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 t e e p e s t ascent o f the curve i s s e l e c t e d onset o f 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  hydrogen c h l o r i d e i o n at 14.04 eV. and i t i s 1.48 the i o n i c ground s t a t e . 1.6 eV. by Fox (31) sistent  ;  eV-.above  and H I  +  at  and i s c o n -  w i t h the v a l u e s o f 1.14 e V . . a n d 2.83 eV. f o r +  the  the  T h i s v a l u e agrees w i t h the v a l u e s  and 1.5 eV. by M o r r i s o n ( 8 1 ) ,  similar states i n HF  as  the  r e s p e c t i v e l y by F r o s t and  McDowell ( 3 7 ) ,  i f one assumes t h a t the energy s e p a r a t i o n o f 2 2 + the X ^ 3 / 2 ~ ^ ^ s t a t e s should increase 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 s p e c t r o s c o p i c  value  l  \\  12  i  1  1  1  i  1 — — i  •  i  1  1  13  14  15  16  17  Photon energy, eV F i g u r e 18  i  18  r  1  '  19  20  69. o f 3.48 eV. o b t a i n e d by N o r l i n g The A £ 2  +  (94).  s t a t e f o r the hydrogen h a l i d e  results  from the removal o f an e l e c t r o n from a 2pcr type b o n d i n g o r b i t a l , and thus one might expect the b i n d i n g energy o f the  electron  to v a r y as the d i s s o c i a t i o n energy o f the H - X bond. the d i s s o c i a t i o n energy d e c r e a s e s p r o g r e s s i v e l y f o r  Since  these  m o l e c u l e s (5.83 eV. f o r HF, 4 . 4 3 eV. f o r HC1, 3.75 eV. f o r HBr and 3.06 eV. f o r H I ) , one might expect the v a l u e of 2  2  +  X ^ 3 / 2 ~ A E, manner.  the  energy s e p a r a t i o n  to v a r y i n a p r o g r e s s i v e  I f t h i s i s s o , the 1;48 eV. v a l u e f o r the  energy  s e p a r a t i o n o f these two s t a t e s i s c o n s i s t e n t whereas the eV. v a l u e by N o r l i n g i s  3.48  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 c u r v e e x h i b i t s many peaks o f n e a r l y e q u a l energy s e p a r a t i o n 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 H 2  ion.  after +  the second  s t a t e of the H C 1  +  These peaks can be e x p l a i n e d as the a u t o i o n i z e d peaks  which are caused by the e x c i t a t i o n o f an e l e c t r o n to an e x c i t e d s t a t e o f the m o l e c u l e which has an energy g r e a t e r than the ond i o n i z a t i o n p o t e n t i a l ,  and f o l l o w e d 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 of an i o n .  formation  The photon energy at the top o f each peak i s g i v e n  i n T a b l e IX\ (A v)  The energy s e p a r a t i o n between two adjacent  peaks  d i f f e r s by the o r d e r o f 100 cm~l which c o r r e s p o n d s  o n l y 0.008 eV. i s 1450 c m  sec-,  - 1  The average energy s e p a r a t i o n between the  which g i v e s the v i b r a t i o n a l frequency f o r  e x c i t e d s t a t e o f the hydrogen c h l o r i d e m o l e c u l e .  to peaks  the  S i n c e the  i n t e r n u c l e a r d i s t a n c e f o r HC1 (normal ) i s n e a r l y e q q a l  to  t h a t f or HC 1^.(2 £ ) , the v i b r a t i o n a l frequency f o r HC1 ( e x c i t e d ) s h o u l d be the same o r d e r o f magnitude as t h a t f o r H C 1 ( 2 £ ) . +  70. T a b l e IX A u t o i o n i z e d Peaks of Hydrogen C h l o r i d e Transitions  E (eV.)  v (cm 1)  0-0  14..55  117371  0-1  14.. 74  118906  0-2  14.,92  120337  0-3  15.. 10  121803  0-4  15.,26  123076  0-5  15..40  124224  0-6  15,,60  125825  0-7  15., 78  127307  0-8  16.,00  129067  0-9  16., 18  130503  0-10  16..34  131792  0-11  16.,54  133422  Av  (cm ) - 1  1535 1431 1466 1273 1148 1601 1472  Average: 1450 cm -1  1760 1436 1189 1630  2  4-  4-  The v i b r a t i o n a l frequency f o r the A £  s t a t e of HC1  g i v e n by H e r z b e r g (47) as 1526.5 c m * .  Therefore i t  -  reasonable to i n t e r p r e t  ion i s is  the a u t o i o n i z e d peaks as b e i n g due  t o the v i b r a t i o n a l s t r u c t u r e o f the e x c i t e d s t a t e of hydrogen c h l o r i d e m o l e c u l e .  the  71. CHAPTER SIX P h o t o i o n i z a t i o n of Polyatomic M o l e c u l e s . A.  Ammonia The ammonia m o l e c u l e i s known (47) to have p y r a m i d a l  symmetry C g , and i t s m o l e c u l a r o r b i t a l f o r m u l a i s : v  (la ) (2a ) (le) (3a ) ; 2  2  1  4  1  6.1  A  2  1  1  1  The m o l e c u l a r o r b i t a l s are l i s t e d i n the o r d e r of energy.  increasing  The e l e c t r o n i c s t r u c t u r e o f the ground s t a t e o f  the  ammonia m o l e c u l e can be compared w i t h t h a t o f the n i t r o g e n which has a c o n f i g u r a t i o n of I s ,  2s ,  2  2  2p^.  atom,  When a n i t r o g e n r  atom and t h r e e hydrogen atoms combine to form an ammonia m o l e cule,  the two I s e l e c t r o n s  o f the n i t r o g e n atom occupy  the  ( l a ^ ) o r b i t a l , the innermost o r b i t a l o f the ammonia m o l e c u l e . The two (2a^) o r b i t a l e l e c t r o n s c e r t a i n extent,  and the e l e c t r o n s  the 2s e l e c t r o n s of  are bonding e l e c t r o n s  occupying t h i s o r b i t a l  o f the n i t r o g e n atom.  the n i t r o g e n atom,  atom form the four  One o f the 2p  and the t h r e e e l e c t r o n s  ( l e ) o r b i t a l s i n ammonia.  l y bonding o r b i t a l s .  to a  o f the  electrons hydrogen  These are  The two r e m a i n i n g 2p e l e c t r o n s  are  of  strongthe  n i t r o g e n atom form the non-bonding o r b i t a l o f ( 3 a ^ ) , 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 o f unshared  electrons  and  is  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 o f 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 c u r v e f o r the ammonia m o l e c u l e i s shown i n F i g u r e 19.  The photon energy at  the  p o i n t o f i n i t i a l onset o f i o n i z a t i o n at 10.12 ± 0.05 eV. o b viously refers  to the energy r e q u i r e d 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  +  ion i n  2 A-^ ground s t a t e , the i o n .  assuming the C 3  V  symmetry i s r e t a i n e d  in  its  72. 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 ammonia at  10.12  eV. i s i n good agreement w i t h the r e s u l t s o b t a i n e d by o t h e r w o r k e r s u s i n g d i f f e r e n t methods. (137)  Inn (60) i n 1953 and Watanabe  i n 1959, measured the a b s o r p t i o n and p h o t o i o n i z a t i o n c o e f f -  i c i e n t s o f ammonia u s i n g f a r 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 s h o l d i o n i z a t i o n p o t e n t i a l o f 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 r o s s s e c t i o n o f 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 w o r k e r s used p h o t o i o n i z a t i o n i n s t r u m e n t s and used d i f f e r e n t zation  of d i f f e r e n t  methods f o r the i n t e r p r e t a t i o n  design,  of the photoionii-%  data. The v a l u e s f o 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 o b t a i n e d by the e l e c t r o n impact method are i n a l l c a s e s s l i g h t l y h i g h e r than these o b t a i n e d 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 i n g 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 o f 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 i n g a m o d i f i e d and much more s e n s i t i v e mass s p e c -  t r o m e t e r , 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 o f ammonia to  be 10.40 eV. and 10.42 eV. r e s p e c t i v e l y .  The f a c t  that  the  e l e c t r o n impact method tends to measure the v e r t i c a l p r o c e s s  and  not n e c e s s a r i l y the minimum energy r e q u i r e d f o r i o n i z a t i o n , accounts f o r the i o n i z a t i o h p o t e n t i a l s o b t a i n e d by t h e s e methods being generally larger  (by 0 . 0 2 t o 0 . 5 e V . ) than the  adiabatic  values. 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 e n e r g i e s o f ammonia was undertaken by Duncan (22) first  i n 1957, and he found  the  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 r e p o r t e d e x p e r i m e n t a l  values.  Tab/le X summarises the r e p o r t e d v a l u e s o f the  threshold  i o n i z a t i o n p o t e n t i a l s o f ammonia:Table X 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 o f Ammonia I.P.  Workers  Methods  10 . 12 + 0,.05  P r e s e n t work  Photoionization  Year 1965  10 . 13 + 0..02  Inn (60)  Photoionization  1953  10 .07 + 0,.05  Weissler  (144)  Photoionization  1955  10 . 15 + 0,.02  Watanabe  (139)  Photoionization  1959  10 . 15 • + 0 .02  Cook (12)  Photoionization  1964  Tate  Electron  Impact  1940  Electron  Impact  1952  Electron  Impact  1958  10 . 5  (eV.)  + 0,. 1  (76)  10 .42 + 0 .05  Morrison  10 .40 + 0 ,02  Frost  9.!94  Duncan (22)  (82)  (37)  1957  Theoretical  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  c u r v e o f ammonia  ( F i g u r e 19) shows c u r v a t u r e near the t h r e s h o l d , the f i n d i n g of Watanabe (139)  and t h i s c o n f i r m s  t h a t the 0-0 t r a n s i t i o n o f ammonia  from the ground s t a t e o f the m o l e c u l e to the ground s t a t e o f  the  ion i s rather 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 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 .  an i n n e r or second  after  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 c o n s t a n t . energy,  14 eV.  . . A t the  latter  ionization potential is indicated,  s i n c e the c u r v e b e g i n s to r i s e a g a i n .  The photon energy at  p o i n t o f s t e e p e s t ascent i s 15.30 eV. c o r r e s p o n d s vertical ionization potential.  to the  the  second  T h i s 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 b e i n g due to the f o r m a t i o n of the f i r s t  excited  74. s t a t e o f the NH of  +  ion, i . e .  an e l e c t r o n from the  molecule.  2  the  (le)  E state,  formed by the  degenerate o r b i t a l o f the  removal ammonia  T h i s ('le) o r b i t a l o f ammonia i s the main bonding  o r b i t a l which spans the t h r e e 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 w i t h the f o r m a t i o n o f the N H c i e n c y was observed  ion.  + 2  molecule  A maximum i o n i z a t i o n  at 15.70 eV. on the  effi-  curve.  Walker and W e i s s l e r (144) have found e v i d e n c e o f 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  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  electron  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 f o r the N H is  shown i n F i g u r e 19.  + 0 . 0 5 eV.  The appearance p o t e n t i a l  is  +  T h i s v a l u e i s s l i g h t l y lower than t h a t o b t a i n e d  W i l k i n s o n and Johnson  (148)  can be formed from the  at 15.8 eV. i n 1950.  A s t a t e of the NH 1 n  ion.  3  ion  of NH2  + 2  i s assumed  to have the symmetry C >  2  by ion 0  configu-  probably:-  (le ) (2a ) (le) (3a ) ; 1  + 2  15.55  I f the N H 2  the e l e c t r o n i c  2v  r a t i o n of the i o n i s  The N H  ion  2  2  1  2  1  ^  6.2  The c o m b i n a t i o n o f N H + ( 1A-^) + H (2s) c o r r e l a t e s w i t h NH^+ 2 + 2 + ( A ) , and t h u s the f o r m a t i o n o f NH from the A, s t a t e o f NH 1 & 1 3 2  Q  is  a l l o w e d by group  theory.  Recent s t u d i e s of the p h o t o c h e m i c a l d e c o m p o s i t i o n o f ammonia (21) g i v e a mechanism f o r the N H requires  + 2  i o n f o r m a t i o n which  a p r i m a r y 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  + hv = N H + H  3  6.3  2  NH  + hv = NH * + e  .  In the I n i t i a t i o n s t e p ,  6.4  the ammonia m o l e c u l e 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 N H and a hydrogen atom.  In the  2  the N H r a d i c a l i s i o n i z e d to g i v e an NH^+ i o n .  step,  2  i o n w i l l be predominent,  second The N H + 2  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 t h a t o f the hydrogen atom. An a l t e r n a t i v e NH  3  + hv —?• N H  NH^E) All of N H  and e q u a l l y p r o b a b l e mechanism i s : -  — * NH  + 3  + 2  ( E) + e  6.5  + H  6.6  2  these mechanisms w i l l be o p e r a t i v e i n the  formation  i o n s at the expense o f the ammonia p a r e n t i o n s .  + 2  They  h e l p to e x p l a i n the d e c l i n e o f 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 f o r the ammonia pareirt i o n s 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 -H),  i s known (117) to be 104 K c a l .  2  (4.52 e V . ) .  appearance p o t e n t i a l o f the N H ^ i o n , or V ( N H ) , +  +  2  ed from F i g u r e 19 as 15.55 eV.  The  has been o b t a i n -  Hence, u s i n g the f o l l o w i n g  equa-  tion: V ( N H ) = D(NH -H) + I ( N H ) + K . E . + E . E +  2  2  2  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 e n e r g i e s o f the N H : i o n ) , and s i n c e the N H +  2  + 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 N H i s e q u a l to or l e s s than 11.03 eV. 9  B.  Water Water vapor has been e x t e n s i v e l y s t u d i e d by  s p e c t r o s c o p y below the t h r e s h o l d  ionization potential.  above t h i s energy d a t a i s s c a r c e . i s important  i n upper  absorption However  P h o t o i o n i z a t i o n of water  atmospheric  studies,  because the  vapor  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 m o l e c u l e . A c c o r d i n g to M u l l i k e n ( 8 7 ) ,  the e l e c t r o n i c ground  state  of water has^A^ symmetry and i s d e r i v e d from the e l e c t r o n i c c o n figuration: (la ) (2a ) (lb ) (3a ) (lb ) ; 2  2  1  1  Recent s e l f - c o n s i s t e n t and S h u l l  2  2  2  1  \  2  1  6.9  m o l e c u l a r 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  (24) ha/e shown t h a t t h i s i s the c o r r e c t o r d e r o f  o r b i t a l s and t h a i the  (lb-^) i s a pure 2 p  main bonding o r b i t a l s b e i n g (3a^)  and  o r b i t a l o f oxygen,  x  2  i s shown  The i n i t i a l onset o f the c u r v e at 12.56 + 0.05 eV.  g i v e s the energy f o r the removal o f an e l e c t r o n from the npn-banding  the  (lb )•  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 o f water i n F i g u r e 20.  the  orbital.  (lb^)  T h i s v a l u e i s i n e x c e l l e n t agreement w i t h  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 limit  at 12.56 e V . , and i s a l s o i n good agreement w i t h the  of 12.59 eV. found by Watanabe,  Nakayama and M o t t l  (140),  Cook and Metzger (12) u s i n g the p h o t o i o n i z a t i o n method, eV. by F r o s t and McDowell ( 3 7 ) , the e l e c t r o n impact  16 eV.  and  and 12.60  and Foner and Hudson (28)  using  technique.  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 c u r v e o f water gradually after  value  rises  the t h r e s h o l d energy and reaches a maximum about  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  autoionization  77. peaks were o b s e r v e d , Metzger ( 1 2 ) .  and t h i s c o n f i r m s the f i n d i n g s o f Cook and  Sugden and P r i c e (116) r e p o r t e d  i n g of e x c i t e d s t a t e s o f water  i n 1948 the  find-  i o n s at 1 6 . 2 and 18.0 e V . , but  F i e l d and F r a n k l i n (33) c o n s i d e r e d these e x c i t e d s t a t e s were :  d o u b t f u l p r o b a b l y on the ground t h a t P r i c e ' s work was done w i t h out mass a n a l y s i s , to i m p u r i t i e s .  However, F r o s t and McDowell (37) r e p o r t e d  s t a t e s o f the water impact method.  and the e x c i t e d s t a t e s observed might be due  i o n at 14.35 and 16.34 eV. by the  excited  electron  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 c u r v e s i s p r o b a b l y due to the i n i o n i z a t i o n cross sections impact  difference  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  methods. The e x c i t e d s t a t e s o f the water i o n s were p r o b a b l y  formed by the removal of e l e c t r o n s (lb ) 2  from the main bonding o r b i t a l s  and Oa-^) , and the i o n s formed w i l l be i n ^ B ^ ,  states.  and  2  B  2  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 r e a k i n g o f the 0-H bond and d i s s o c i a t i o n o f water m o l e c u l e w i t h the f o r m a t i o n o f H literature  +  or 0 H . +  A survey o f  found t h a t the appearance p o t e n t i a l of 0 H  18 eV. and t h a t f o r H  +  i s at about 19.6 eV.  the  +  i s at  the about  78. C.  Methane and  Deutero-methane  Methane,deutero-methane been s u b j e c t e d  and t h e i r fragment  i o n s have  to c o n s i d e r a b 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.  Several discus-  s i o n s have been r e p o r t e d  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 a c c u r a c y of the d e r i v e d d i s -  s o c i a t i o n energy of the CH3-H bond have been q u e s t i o n e d .  The  appearance o f i o n s 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 c u r v e s o f methane and deutero-methane and t h e i r fragment fairly  sharp and t h i s p e r m i t s q u i t e a c c u r a t e  i o n s has  been  d e t e r m i n a t i o n of  the  appearance p o t e n t i a l s of the i o n s and the d i s s o c i a t i o n e n e r g i e s of the p a r e n t m o l e c u l e s . There are t e n e l e c t r o n s  i n methane,  put i n t o f i v e doubly o c c u p i e d o r b i t a l s . of methane can be r e p r e s e n t e d 2  2  Mulliken  6  1  The e l e c t r o n i c  structure  by:  (ls ) (sa ) (pt ) , c  and these can be  1  2  A  6.10  1  (85) has shown t h a t the t r i p l y degenerate ( p t ) 1  2  has the l o w e s t b i n d i n g energy,  orbital  and the i o n i z a t i o n p o t e n t i a l o f  methane o b t a i n e d 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 c u r v e ( F i g u r e 21) at 12.87 eV. can be r e f e r r e d e l e c t r o n from the electronic  (pt )  t o the removal o f an  o r b i t a l to l e a v e a C H ^ i o n w i t h +  2  the  structure: (ls ) (s 2  c  ) (pt ) , 2  a i  5  2  2  T  2  6.11  T h i s 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 o f methane at 12.87 eV. agrees w i t h the v a l u e at 12.8 eV. found by W e i s s l e r ( 1 4 3 ) , 12.98 eV. by Watanabe  (140)  at  and at 12.71 eV. by D i b e l e r , K r a u s s ,  Reese and H a r t l e e (19) u s i n g the p h o t o i o n i z a t i o n method.  The  79. r e p o r t e d v a l u e s by e l e c t r o n impact methods g i v e a somewhat higher v a l u e .  T a b l e 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  ipns.  T a b l e XI Ionization  P o t e n t i a l s o f Methane,  Deutero-methane  and Fragment Ions . Workers  Method  Year  T h i s work W e i s s l e r (143) Watanabe (140) D i b e l e r (19) Smith (110) K o f f e l (64) Honig (52) M i t c h e l l (80) McDowell (71)  Photoionization Photoionization Photoionization Photoionization E l e c t r o n Impact E l e c t r o n Impact E l e c t r o n Impact E l e c t r o n Impact E l e c t r o n Impact  1966 1955 1962 1965 1937 1948 1948 1949 1951  14. 25 ± 0. 05 14.25 ± 0. 05  T h i s work D i b e l e r (19)  Photoionization Photoionization  1966 1965  14.5 14.4 14. 5 14.39 ±  Smith (109) K o f f e l (64) M i t c h e l l (80) McDowell (71)  Electron Electron Electron Electron  1937 1948 1949 1951  T h i s work  Photoionization  1966  Dibeler  (19)  Photoionization  1965  T h i s work D i b e l e r (19)  Photoionization Photoionization  1966 1965  I . P . 1(eV.) CH  + 4  12.87 12.8 12.98 12. 71 13.2 13.0 13.04 13.04 13.12 CH  CD  -fc  ± ± ± -L  ± ± ±  0. 0. 0. 0. 0. 0. 0. 0. 0.  05 2 05 02 4 2 03 02 03  + 3  0. 0. 0. 0.  4 3 05 02  + 4  13.00  0. 05  12.87 ± 0. 02 CD  Impact Impact Impact Impact  + 3  14.46 ± 0. 05 14.38 ± 0. 03  The i o n i z a t i o n p o t e n t i a l of C D at 13.00 ± 0.05 eV. 4  80. is  a l i t t l e h i g h e r than t h a t o b t a i n e d by D i b e l e r e t a l .  at 12.87 eV.  However, a study of t h e i r c u r v e near the onset  showed l o n g t a i l i n g e x t e n d i n g more than 0 . 5 eV. polated values, nificance. higher:  as they a d m i t t e d ,  have no fundamental  sig-  L o s s i n g , Turner and B r y c e (69) o b t a i n e d a v a l u e o f a v a l u e of 13.30 eV.  A difference  i o n i z a t i o n p o t e n t i a l s between CH^ and C D i s a l s o o b s e r v e d , 4  and the  difference: KCD ) 4  is  The e x t r a -  The v a l u e s o b t a i n e d by e l e c t r o n impact are as u s u a l  13.21 e V . , and Honig ( 5 2 ) , of  (19)  - I ( C H ) = 0.13 eV  6.12  4  a l s o i n good agreement w i t h those o b t a i n e d by D i b e l e r et  al.  (19) of 0.16 eV. and by L o s s i n g et a l . (69) o f 0.18 eV. McDowell (72) p o i n t e d out t h a t the the methane m o l e c u l e w i l l  (pt ) 2  o r b i t a l of  be d i v i d e d i n t o two o r b i t a l s :  of  symmetry E and ( z b )  of  the p o t e n t i a l minimum can l e a d to a change i n the  2  o f symmetry B2,  p o p u l a t i o n s o f the d i f f e r e n t  (TT e)  and the r e s u l t a n t  shift  relative  v i b r a t i o n a l l e v e l s i n the i o n s o f  CH^ and CD^ as the amount o f 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 o v e r l a p 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 a p p r e c i a b l y g r e a t e r CH4 than f o r CD4.  for  T h i s may p r o v i d e a p o s s i b l e e x p l a n a t i o n f o r  the l a r g e 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 c u r v e s of C H CD^t  rise  after  the t h r e s h o l d energy u n t i l about  they s t a r t l e v e l l i n g o f f .  + 4  and  14 eV. where  No a u t o i o n i z a t i o n peaks have been  observed i n the c u r v e s f o r the m o l e c u l a r or fragment t h i s i s c o n s i s t e n t w i t h the observed f a c t  that for  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 o n l y i n c o n t i n u o u s  ions,  and  hydrocarbons, absorption,  81. and do not g i v e 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 efficiency  curves. The appearance p o t e n t i a l o f CHg  i s o b t a i n e d from the  +  i n i t i a l onset o f the c u r v e at 14.25 eV. and i s i n good agreement 1  with that  o b t a i n e d by D i b e l e r et a l .  about 0 . 2 eV. lower than t h a t  (19)  at 14.25 e V . , but  o b t a i n e d by the e l e c t r o n  impact  method. The f o r m a t i o n o f the the  CH3 " 4  i o n can be r e p r e s e n t e d  by  process: CH4 + hv  =  CH  + H + e  + 3  McDowell (74) i n d i c a t e d t h a t at  a  6.13  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 c o n s e q u e n t l y d i s s o c i a t i o n to y i e l d the methyl i o n and h y d r o gen atom c o u l d take p l a c e .  The l e v e l l i n g o f f o f the C H ^ c u r v e +  at t h i s energy i n d i c a t e s t h a t the f o r m a t i o n o f 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 o f C U ^ p a r e n t 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 o f the c u r v e at 14.46~eV. i s a l i t t l e h i g h e r that  o b t a i n e d by D i b e l e r e t a l . (19)  at 14.38 eV.  than  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 l o n g t a i l i n g at the o n s e t on t h e i r c u r v e s .  The d i f f e r e n c e between the appearance p o t e n -  t i a l s o f CH3 and CD3 o f 0.21 eV. i n d i c a t e s t h a t t h e r e  is  c o n s i d e r a b l e d i s p l a c e m e n t o f the minima o f the ground s t a t e of the p a r e n t i o n r e l a t i v e to t h a t o f the m o l e c u l a r ground s t a t e . The d i s s o c i a t i o n energy of C H 3 - H can be o b t a i n e d from the f o l l o w i n g A(CH ) 3  +  =  equation:  D(CH -H) + I ( C H ) + K . E . + E . E 3  3  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 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 o f  82. the m e t h y l r a d i c a l ,  I(CH ), 3  i s 9.843 eV. ( 5 0 ) .  p o i n t e d out t h a t the f o r m a t i o n of  McDowell (72)  i o n s at t h i s energy does  CH3 " 4  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 e n e r g i e s ,  and t h i s i s i n  agreement w i t h the f i n d i n g o f B e r r y (2) from d i s c r i m i n a t i o n experiments t h a t the k i n e t i c and e x c i t a t i o n e n e r g i e s o f C H ^  +  i o n are 0.032 eV. i n excess o f the t h e r m a l energy. Therefore,  the d i s s o c i a t i o n energy o f C H 3 - H can be  o b t a i n e d from e q u a t i o n 6.14 as 4 . 4 1 eV. which agrees w i t h  the  v a l u e o f 4 . 4 1 eV. found by D i b e l e r e t a l . ( 1 9 ) , 4 . 4 2 eV. by Stevenson (113) and 4 . 4 2 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 o f 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 m e t h y l - d ^ as 9.832  eV.. ( 4 9 ) , the d i s s o c i a t i o n energy o f C D 3 - D can be o b t a i n e d 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 t o d e t e r m i n e the z e r o - p o i n t energy difference  f o r the C H  ions (19).  + 3  =  E + Z  C H 3 +  - Z  C H 4 +  6.15  D(CD -D)  =  E +: Z  C D 3 +  - Z  C D 4 +  6.16  +  +  3  V  and C D  D(CH -H) 3  where Z  + 3  denotes the z e r o - p o i n t energy of the i o n s p e c i e s x .  Also: D(CH -H)  =  A(CH )  - I(CH )  6.17  D(CH -D)  =  A(CD )  - X(CD )  6.18  +  3  +  3  +  3  4  +  3  4  E q u a t i n g 6.15 and 6 . 1 7 ; 6.16 and 6 . 1 8 , we have E  +  Z  CH + " CH + Z  3  4  "  A  <  C H  3 ) +  " KCH ) 4  6.19  83 E  +  Z  CD  " CD  *  Z  3+  4+  V>  A(C  " >«D )  6.20  4  S u b s t r a c t i n g 6.20 from 6 . 1 9 , we have, The z e r o - p o i n t energy d i f f e r e n c e ,  =  ( Z  CH + " CD + Z  4  "  )  4  I ( C H  3  )  +  I  Z  - Z  ( C D ) + A ( C H ) - A(CD ) 3  3  3  = 0.23 eV. where,  (Z - Z„„ ) CH + CD + 4  =  0.31 eV.  (47).  4  The v a l u e at 0.23 eV. i s i n good agreement w i t h the v a l u e at eV. found by D i b e l e r et a l ( 1 9 ) . the z e r o - p o i n t d i f f e r e n c e  0.18  For c o m p a r i s o n , we note t h a t  f o r the i s o t o p i c ammonias, N H and ND 3  3  i s 0.22 eV. (47) . The mass s p e c 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. i o n i z a t i o n p r o b a b i l i t i e s o f these m o l e c u l e s  CD  4  100  CH 96  4  CH 80  3  CD 69  3  are:  The r e l a t i v e  RELATIVE  IONIZATION^  CJ1  O  O  CJi  W  +  > O  — (/I  ' I H  >  o -n  —  H en era P H CD C ro Z H _ 00  o i  O O  o o  84. D.  Propylene Because o f the g r e a t importance to o r g a n i c c h e m i s t r y  o f C=C bonds and the resonance  effects  which a r i s e from the c o n j u -  g a t i o n o f them, the s t u d y o f p r o p y l e n e has been undertaken by many w o r k e r s .  P r i c e and T u t t l e (101) have s t u d i e d the  molecule's  a b s o r p t i o n spectrum i n the f a r u l t r a v i o l e t r e g i o n ; Watanabe measured the i o n i z a t i o n p o t e n t i a l o f p r o p y l e n e u s i n g the e l e c t r i c method, mass spectrometer  and S t e i n e r ,  r  (139)  photo-  G i e s e and Inghram (114) combined  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.  U s i n g the e l e c t r o n  method, Fox and Hickam ( 3 0 ) , Stevenson and H i p p i e  (111),  and Coleman (80)"and Honig (52) have o b t a i n e d a f a i r l y  impact Mitchell  accurate  i o n i z a t i o n p o t e n t i a l o f the m o l e c u l 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 c u r v e s o f p a r e n t and fragment  i o n s o f p r o p y l e n e are shown i n F i g u r e 23.  It is  t h a t the c u r v e , u n l i k e those o b t a i n e d by e l e c t r o n i m p a c t , a sharp onset near the t h r e s h o l d energy.  seen exhibits  The i n i t i a l onset of  the  c u r v e 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 o f 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 v a l u e at 9.70 eV. by P r i c e and T u t t l e (101), eV. by Watanabe  and the p h o t o i o n i z a t i o n v a l u e s at  (139) and S t e i n e r ,  G i e s e and Inghram ( 1 1 4 ) .  i o n i z a t i o n p o t e n t i a l o f the m o l e c u l e o b t a i n e d by the impact d a t a i s a l i t t l e h i g h e r .  9.73 The  electron  T a b l e X I I summarises the  ioniza-  t i o n p o t e n t i a l s of p r o p y l e n e o b t a i n e d i n t h i s work and by o t h e r workers. At a photon energy below 11 eV. the p h o t o i o n i z a t i o n efficiency  f o r p r o p y l e n e shows a..maximum.  Why t h i s s h o u l d be  so i s not c l e a r , u n l e s s 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 f o r The i o n i n t e n s i t y b e g i n s to r i s e a f t e r  it.  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 p o i n 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 . , s h o u l d :  c o r r e s p o n d 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 refers  to the removal of a 0" e l e c t r o n .  T h i s v a l u e at 11.1 eV.  i s h i g h e r than those at 10.54 eV. by P r i c e and T u t t l e ( l O l ) a n d 10.54 eV. by Fox and Hickam ( 3 0 ) . 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  3 ; 6 (Threshold 1"I.P.) Present 9. 70  Method  Year  Photoionization  1966  Spectroscopic  1940  Photoionization  1956  Photoionization  1957  H  C  result  9. 70  Price  9. 73  Watanabe  9. 73  Inghram  9. 84  Honig (52)  Electron  Impact  1948  Mitchell  Electron  Impact  1949  Electron  Impact  1942  Electron  Impact  1954  Photoionization  1966  Spectroscopy  1940  Electron  Impact  1954  Photoionization  1966  Electron  1942  1 0 . 05  (101) (139) (114)  (80)  9. 77  Stevenson  9. 78  Fox (30)  C  3 ; 6 (second H  I. P.)  11. 1  Present  10. 54  Price  10. 54  Fox (30)  C  3  (111)  iV (appearance  result (101)  potential)  1 1 . 95  Present  1 1 . 96  Stevenson  result (111)  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 c u r v e o f p r o p y l e n e 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 p a r e n t i o n w i t h the f o r m a t i o n o f a  +  fragment.  The appearance p o t e n t i a l o f the  C3H5" " 1  C3H5  ion i s  86. ;•• T  '-.. d  o b t a i n e d from the i n i t i a l onset o f 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 c u r v e ( F i g u r e 23) at 11.95 e V . , which i s i n good agreement w i t h the v a l u e at 11.96 eV. by Stevenson and H i p p i e ( 1 1 1 ) .  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 o f p r o p y l e n e and  the  appearance p o t e n t i a l o f CgHg" ". i o n o b t a i n e d by d i f f e r e n t  workers.  1  B a r r i n g m o l e c u l a r 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 p r o p y l e n e s h o u l d a r i s e from the f o l l o w i n g mechanism:CH =CH-CH 2  because the  3  + hv  =  CH^CH-CH* + H + e  ^ ...6.20  (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 a t t a c h e d  to u n s a t u r a t e d  carbon  atoms. The bond d i s s o c i a t i o n energy o f p r o p y l e n e  D(allyl-H),  can be o b t a i n e d from the appearance p o t e n t i a l o f the a l l y l by t h e f o l l o w i n g  equation:-  V(C H +) 3  where V ( C H g ) +  3  ion  =  5  D(C H -H) + I(C H ) + K . E . + E.E 3  5  3  i s t h e appearance p o t e n t i a l o f the a l l y l  (11.95 e V . ) and K . E . and E . E . - k i n e t i c o f the d i s s o c i a t i o n p r o d u c t s . allyl radical, KC Hg), 3  ion  and-excitation:energies  The i o n i z a t i o n p o t e n t i a l o f  the  i s 8.16 + 0.03 eV. o b t a i n e d by L o s s i n g ,  I n g o l d and Henderson ( 6 8 ) . to have l i t t l e o r no  6.21  5  S i n c e the a l l y l i o n formed i s known  excess k i n e t i c and e x c i t a t i o n  the bond d i s s o c i a t i o n energy D ( a l l y l - H )  energies,  i s e q u a l or l e s s  than  3.79 eV. i n good agreement w i t h the v a l u e at 79 +_ 6 k c a l . / m o l e or 3.43 +_ 0.4 eV. o b t a i n e d by McDowell, L o s s i n g , Henderson and Farmer (75) i n a study o f the i o n i z a t i o n p o t e n t i a l s o f m e t h y l substituted  allyl  radicals.  87. E.  Acetylene A g r e a t d e a l of d a t a has been accumulated on the  tylene molecule.  It  and f i v e f r e q u e n c i e s unexcited states.  ace-  i s known t h a t i t i s l i n e a r and s y m m e t r i c a l , have been i d e n t i f i e d i n the e x c i t e d and  A c e t y l e n e has been s t u d i e d by D i b e l e r and  Reese (18) u s i n g 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 i n g a Lyman continuum (who o b t a i n e d an e x t e n s i v e system o f 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 v a l u e f o r 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 ' v a l u e f o r the C-C bond).  triple  A c e t y l e n e has a l s o been s t u d i e d by Turner (129)  photoelectron spectroscopy,  using  and by L o s s i n g , T i c k n e r and B r y c e  (69) u s i n g the e l e c t r o n impact method. A c e t y l e n e i s a l i n e a r m o l e c u l e which has: f o u r t e e n electrons.  The ground s t a t e c o n f i g u r a t i o n o f the m o l e c u l e can  be r e p r e s e n t e d by: 6. 21 The m o l e c u l a r o r b i t a l s are l i s t e d i n the o r d e r of b i n d i n g energy,  o m i t t i n g the i n n e r  decreasing  electrons.  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 c u r v e f o r the m o l e c u l e i s shown i n F i g u r e 24. t u r e at the i n i t i a l o n s e t , internuclear  distances  acetylene  The c u r v e shows a s l i g h t c u r v a -  and t h i s means t h a t the e q u i l i b r i u m  are s u b s t a n t i a l l y changed i n the i o n s as  compared to the n e u t r a l m o l e c u l e , 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 p r o c e s s 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 o f the a c e t y l e n e c u r v e at 11.40 eV. i s  selected  as 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 which i s a s s o c i a t e d the energy f o r the removal o f an e l e c t r o n from the orbital.  (.TT  with  ) bonding  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 i s i n good agreement  i  1  1  1  Photon  energy  F i g u r e 24-  1  ,eV  1  r  88. w i t h the v a l u e at 11.41 eV. from the s p e c t r o s c o p i c by P r i c e ( 9 8 ) ,  determination  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 ( 1 8 ) .  Turner (129) o b t a i n e d a v a l u e of 11.36  eV. u s i n g p h o t o e l e c t r o n s p e c t r o s c o p y ,  and a v a l u e o f 11.40 eV.  was o b t a i n e d by L o s s i n g , T i c k n e r and B r y c e (69) u s i n g the  elec-  t r o n impact method. After  the t h r e s h o l d energy,  t h e r e 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 c u r v e f o r a c e t y l e n e ted i n F i g u r e 24.  The e n e r g i e s  as  illustra-  c o r r e s p o n d i n g to the tops o f  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 c u r v e 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 o f A c e t y l e n e  Transitions  E (eV.)  0-0  11,.40  91952  0-1  11.,63  93808  0-2  11 ,86  95662  0-3  12.,09  97492  0-4  12 .28  99050  0-5  12..48  100663  0-6  12., 70  102437  0-7  12,.96  104535  0-8  18.. 19  106389  0-9  13 ,40  108084  0-10  13 . 58  109634  0-11  13 .83  111552  0-12  14 .06  113407  From T a b l e X I I I ,  v (cm  )  & v (cm  )  1856 1854 1830 1858 1613 1774 2098  1780 cm -1  1754 1659 1550 1918 1855  the energy s e p a r a t i o n between [two  89. adjacent  peaks,  c-r-r:--TT--:ic. . r ' r :  ;  ( & v ) , d i f f e r s by the o r d e r o f 100 cm~l which V:\\J  ">.  c o r r e s p o n d s to o n l y 0.008 eV. i each peak i s s e p a r a t e d energy,  It  i s reasonable  to say t h a t  from one another by an e q u a l amount o f  and the aver age energy s e p a r a t i o n between two  peaks i s 1780 c m .  The v a l u e of 1780 c m  - 1  adjacent  i s very c l o s e to  - 1  the c a r b o n - c a r b o n s t r e t c h i n g f r e q u e n c i e s o f 1849 cm -'- f o r -  3R s t a t e s of a c e t y l e n e r e p o r t e d by W i l k i n s o n The  the  (149).  photon energy f o r the f i r s t f o u r 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 acetylene  (Figure  24) are i n e x c e l l e n t agreement w i t h the f o u r peaks i n the photoionization efficiency (18),  c u r v e o b t a i n e d by D i b e l e r and Reese  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 , :  and 0 - 3 , of the  (  1 t i u  )  state.  Turner (129) i n a s t u d y of  p h o t o e l e c t r o n apectrum o f a c e t y l e n e , structure  for acetylene,  0-0, 0-1, 0-2, the  also reported v i b r a t i o n a l  but he showed o n l y two peaks at  almost  the same energy as the f i r s t two p e a k s . i n the c u r v e r e p o r t e d here. From the e v i d e n c e o f W i l k i n s o n , D i b e l e r and Reese, and T u r n e r , 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 c u r v e f o r a c e t y l e n e are most p r o b a b l y due to v i b r a t i o n a l t r a n s i t i o n s o f the  (^^ ) u  the  state.  D i s s o c i a t i o n of Acetylene The fragment  photoionization efficiency  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.  c u r v e f o r the  The appearance  p o t e n t i a l o f t h i s i o n i s o b t a i n e d from the p o i n t o f onset at 17.76 ± 0 . 0 5 eV.  C2H?"  initial  T h i s v a l u e agrees w e l l w i t h  that  at 17.8 eV. by C o a t s and Anderson (7) and T a t e , 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 i n g 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 d a t a f o r c o m p a r i s o n . The bond d i s s o c i a t i o n energy D(HC ~H) can be o b t a i n e d 2  from the f o l l o w i n g D(HC -H) 2  equation:=  V(C H)  - I(C H)  +  2  2  The appearance p o t e n t i a l , V ( C H ) , +  2  - K.E. - E.E.  ...6.22  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 H r a d i c a l i s 11.3 eV. 2  o b t a i n e d by E l e h t o n ( 2 3 ) .  I f the k i n e t i c and e x c i t a t i o n e n e r -  g i e s are s m a l l and can be n e g l e c t e d , energy D(HC -H) 2  the bond d i s s o c i a t i o n  i s e q u a l to or l e s s than 6.46 eV. i n good a g r e e -  ment w i t h the v a l u e of 6.5 eV. found by C o a t s 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 c u r v e f o r the C H  +  2  i o n remains f a i r l y c o n s t a n t i n F i g u r e 25.  between 18.4 and 18.7 eV. as shown  An i n n e r appearance p o t e n t i a l of C H  i n d i c a t e d as the c u r v e s t a r t s to r i s e a f t e r The second appearance p o t e n t i a l of the C H  ion is  the l a t t e r +  2  the p o i n t o f s t e e p e s t ascent to be 18.96 eV. t i o n e f f i c i e n c y c u r v e remains c o n s t a n t  +  after  energy.  i o n i s found from The p h o t o i o n i z a about 19.2 eV.  91. F.  M e t h y l Cyanide V e r y 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 c y a n i d e has been r e p o r t e d  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 c y a n i d e have been s t u d i e d by many workers.  McDowell (74) has d e s c r i b e d the e l e c t r o n i c  of methyl c y a n i d e as (<r +CT , c  c  follows: a ) (TT 2  1  N  + TT  ) (TT e ) 4  CJ  e  6.23  4  w i t h the m o l e c u l a r o r b i t a l s l i s t e d i n the o r d e r o f b i n d i n g energy,  o m i t t i n g the i n n e r o r b i t a l s .  the main bonding C - C o r b i t a l , the two m u t u a l l y p e r p e n d i c u l a r CN group,  and the  structure  C H "  degenerate  N  ((T Q  +TTQ>  decreasing +  <J~C>  I  e) r e p r e s e n t s  bonding o r b i t a l s of  (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  S  the  the 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 c u r v e o f m e t h y l c y a n i d e i s shown i n F i g u r e 26.  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  methyl c y a n i d e measured  from the p o i n t of i n i t i a l onset of  c u r v e i s 12.33 eV. which i s i n good agreement w i t h the impact d a t a o f 12.39 eV. by M o r r i s o n (82) McDowell and Warren ( 7 3 ) .  After  electron  and 12.52 eV. by  the t h r e s h o l d energy,  i o n i z a t i o n e f f i c i e n c y shows t h r e e d i s t i n c t  the  the  photo-  steps.  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 p o i n t s of s t e e p e s t ascent o f the c u r v e 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 c u r v e s o f m e t h y l c y a n i d e and k r y p t o n by e l e c t r o n impact are shown i n F i g . 2 7 ( 4 0 ) . Three breaks of  are observed on the methyl c y a n i d e c u r v e ,  these breaks c o r r e s p o n d  r e s p o n s i b l e f o r t h e CHgCN  +  and the  distinct energies  to the e n e r g i e s  for three processes  ion formation.  These v a l u e s  are  O  AON3IOIdd3  LO  N0I1VZIN0I  o  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 v a l u e s o b t a i n e d by p h o t o i o n i z a t i o n . The t h r e e i o n i z a t i o n p o t e n t i a l s and e l e c t r o n impact s t u d i e s orbitals,  from  s h o u l d be r e l a t e d  photoionization to the o u t e r  three  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 e q u i r e d f o r the removal of an e l e c t r o n from the  (TT e) o r b i t a l o f the methyl c y a n i d e s h o u l d be c l o s e  to  t h a t 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 c y a n i d e ) CH(Tf^  +TTQ>  e  )  i n order  to remove an e l e c t r o n from  the  o r b i t a l of the methyl c y a n i d e .  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 o f the type and ("^ Q J J  3  ~~^CN^'  a n t  ionization potential e l e c t r o n from the  *  S  °  ""~  S s u  £S  e s  TT ^^.) o r b i t a l ,  p o t e n t i a l at 13.80 eV. a r i s e s  + TT  Cng CN t e d here t h a t the second  at 13.01 eV. r e f e r s  (TT CE^  (TT  t o the removal o f an and the t h i r d i o n i z a t i o n  from the removal o f a (TT ^  ~^CN^  3 The m e t h y l c y a n i d e i o n w i l l  p r e d o m i n a n t l y bonding e l e c t r o n . be o formed i n a E s t a t e " i n each c a s e . 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 o f m e t h y l c y a n i d e at 12.33 eV. a p p a r e n t l y r e f e r s to the removal o f a (<r + <T„, a,) c i bonding e l e c t r o n ,  as the s l i g h t c u r v a t u r e  near the o n s e t 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 o f m e t h y l c y a n i d e i n d i c a t e s  that  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 s h o l d 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 o f ethane, C H g , 2  refers  i s 11.8 e V . , and  this  to the removal o f an e l e c t r o n from the C-C bonding (CT a^)  orbital.  The d i s s o c i a t i o n energy of ethane, D(HgC-CHg),  is  93. 3.68 e V . , and the d i s s o c i a t i o n of m e t h y l c y a n i d e , D(H3C-CN), is  5.8 eV. ( 4 0 ) .  to g i v e a f a i r  S i n c e d i s s o c i a t i o n energy may be c o n s i d e r e d  i n d i c a t i o n o f the f i r m n e s s w i t h which  are h e l d i n bonding o r b i t a l s , we expect to f i n d  electrons  the i o n i z a t i o n  p o t e n t i a l f o r methyl c y a n i d e somewhat h i g h e r than 11.8 eV. This threshold i o n i z a t i o n p o t e n t i a l of methyl cyanide could 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 i n the methyl c y a n i d e p h o t i o n i z a t i o n e f f i c i e n c y c u r v e . more, i f the assignments  onset  Further-  r e g a r d i n g the two h i g h e r i o n i z a t i o n  p o t e n t i a l s of methyl c y a n i d e are c o r r e c t ,  the r e m a i n i n g one -  the l o w e s t o f a l l - s h o u l d be a s s o c i a t e d w i t h the  <T bonding  o r b i t a l s i n c e the next h i g h e r unassigned o r b i t a l /should have an energy o f 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 s h o u l d be w r i t t e n as (TT l'.P.(eV.)  CH3  of m e t h y l c y a n i d e  follows: -TT  ) (Ti 4  e N  13.80  C H ; a +  Tt  ) ((r 4  C N  13.01  c +  Cr , c  ) ... 2  a i  6.24  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 o f the CH2CN+  fragment  ion  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  ion  as measured from the p o i n t o f i n i t i a l onset o f the c u r v e  13.86 eV. which i s lower than the e l e c t r o n impact d a t a  at  14.30 eV. by McDowell and Warren ( 7 3 ) . For energy,  the f i r s t h a l f o f a v o l t a f t e r  the i o n i n t e n s i t y i n c r e a s e s  fairly  the  threshold  constantly,  and a  change o f s l o p e i s observed at 14.25 eV. which may c o r r e s p o n d to the v e r t i c a l t r a n s i t i o n o f the e l e c t r o n impact d a t a  at  +  is  14.30 eV.  Three s t e p s are not so w e l l d e f i n e d as those i n  the p a r e n t i o n c u r v e because of the s m a l l e r i o n i z a t i o n probability. The p r o c e s s observed here f o r the f o r m a t i o n of CH^CN^ fragment  i o n at 13.86 eV. can be r e p r e s e n t e d  CH CN + hv  =  3  and the e n e r g e t i c s +  2  Since I(CH CN), 2  =  6.25  2  i o n f o r m a t i o n are g i v e n by:  D(H-CH CN) + I(CH CN) 2  6.26  2  the i o n i z a t i o n p o t e n t i a l o f the fragment  i s unknown, D ( H - C H 2 C N ) , the d i s s o c i a t i o n energy o f bond cannot be o b t a i n e d .  D(H-CH3),  i . e . 4 . 4 1 e V . , the i o n i z a t i o n p o t e n t i a l of  fragment  ion,  the  (H-CH2CN)  CH2CN  follows:  CH CN+ + H + e  o f the fragment  V(CH CN )  as  the  I f we assume DCH-CH^CN) = the  i o n i s a p p r o x i m a t e l y 9.4 eV.  T a b l e 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  T h i s work  CH CN 3  +  100  M c D o w e l l ( 7 3 ) ( a t 50V.) 100  CH CN  71.5  50.0  CHCN  20.0  14. 7  6.5  8.6  2  CH  + 2  +  The mass spectrum o f m e t h y l c y a n i d e i s shown i n F i g u r e 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 o f the p a r e n t and fragment  i o n s at a photon energy o f  16.66 eV. o b t a i n e d i n t h i s work, and those o f M c D o w e l l ' s at an e l e c t r o n energy o f 50 V .  (73)  IONIZATION  RELATIVE CJI  o  T  o X to  +  2  0 1  Ni  no  o  >  o o o I X I o to w O O  CO CO CO  2  m o> o 0) H 30  2  >  cn en  IS CO OQ CO p H • c  ro o  T  01  o o  <  o X o  85O o X o  o  c  +  o X to o  o X z  +  95. G.  Methyl Alcohol There are no  spectroscopic  ionization potential  v a l u e s f o r t h i s m o l e c u l e , because the vacuum u l t r a v i o l e t spectrum o f methanol does not e x h i b i t w e l l d e f i n e d Rydberg series.  P h o t o i o n i z a t i o n o f t h i s m o l e c u l e has been s t u d i e d by  Inn (60)  and Watanabe  (135).  M o r r i s o n and N i c h o l s o n ( 8 2 ) ,  U s i n g the e l e c t r o n impact method, Stevenson  ( 1 1 2 ) , Cummings and  B l e a k n e y ( 1 6 ) , Cox ( 1 5 ) , Omura, Baba and H i g a s i  (96)  and  Friedman, Long and W o l f s b e r g ( 3 4 ) , were a b l e to o b t a i n an a p p r o x i m a t i o n 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 . A c c o r d i n g to M u l l i k e n ( 8 7 ) , methanol s h o u l d be ^A s t a t e , electronic  the ground s t a t e o f  and t h i s has the  configuration:CH 0H: 3  (z) (y) (x) 2  2  2 + 2  (x') , 2  X  A  6.27  w i t h the m o l e c u l a r o r b i t a l s l i s t e d i n o r d e r o f b i n d i n g energy, i n the  following  (x')  o m i t t i n g the i n n e r o r b i t a l s .  decreasing The  electrons  o r b i t a l are l o c a l i z e d i n the oxygen atom,  p r a c t i c a l l y free  from m i x i n g .  any e l e c t r o n i n the m o l e c u l e .  The (x) o r b i t a l i s s p l i t  but the  together actual  however, be m i x t u r e s o f these extreme  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 o r i s shown i n F i g u r e 29.  The t h r e s h o l d  into  belonging essen-  The (z) and (y) o r b i t a l s  g i v e the 0 - H and 0 - C bonding r e s p e c t i v e l y , o r b i t a l s must,  are  They have the lowest energy o f  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, t i a l l y to the CH3 group.  and  forms.  methanol  ionization potential,  at the p o i n t o f i n i t i a l onset o f 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 o f an e l e c t r o n from a non-bonding  orbital  l o c a l i z e d m a i n l y on the oxygen atom.  (x')  This value  O  IONIZATION Oi  EFFICIENCY^  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 v a l u e 10.52 eV. by Inn ( 6 0 ) .  However, Watanabe  h i g h e r v a l u e at 10.35 eV.  (135) o b t a i n e d a  The c u r v e g i v e n 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 , a weak,  long t a i l  at  i n the y i e l d c u r v e which was a s c r i b e d  but to  impurity. 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 c u r v e i n F i g u r e 29 shows a s l i g h t c u r v a t u r e near the t h r e s h o l d energy,  and  this  i n d i c a t e s t h a t the e q u i l i b r i u m i n t e r n u c l e a r d i s t a n c e s o f molecule  (neutral.)  and i o n are d i f f e r e n t .  T h i s might be one  of the reasons t h a t no w e l l - d e f i n e d Rydberg s e r i e s from the a b s o r p t i o n s p e c t r a o f m e t h a n o l .  the  are  obtained  The e l e c t r o n impact  d a t a f o 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 r e p r e s e n t the v e r t i c a l i o n i z a t i o n potential. T a b l e XV summarizes 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 s o f methanol: T a b l e XV 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 Methanol I . P . (eV. )  Workers  Method  Year  10. 53 ± 0 .05 10. 52 ± 0 .03 0 .05 10. 85  P r e s e n t work  Photoionization  1966  Inn (61) Watanabe  (142)  Photoionization Photoionization  1953 1954  Bleakney  (15)  E l e c t r o n Impact  Morrison Cqx (14)  (84)  Electron Electron  Impact Impact  1940 1952 1954  Omura (100)  Electron  Impact  1956  Friedman  Electron  Impact  1957  10. 8 i 0 .2 10. 95 0.1 10. 86 ± 0 .05 10. 97 t 0,.05 10. 9 ± 0.. 1  (34)  The second i o n i z a t i o n p o t e n t i a l o f methanol from the p o i n t o f s t e e p e s t ascent o f the c u r v e ) ,  (obtained  i s found to be  97. 1 2 . 9 0 eV. and r e f e r s  to the removal o f an e l e c t r o n from the  (x) o r b i t a l , b e l o n g i n g 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 c u r v e at 1 3 . 5 , 15.75 and 17.4 e V . , and the r e a s o n f o r those maxima i s not c l e a r .  H a r r i s o n (45) has a l s o r e p o r t e d  two maxima i n  h i s a b s o r p t i o n s p e c t r a o f 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 c u r v e f o r the C H 0 H  +  2  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 o f t h i s i o n i s o b t a i n e d from the p o i n t of  initial  onset o f the c u r v e at 1 1 . 5 2 ± 0.05 eV. which i s s l i g h t l y lower than the v a l u e at 1 1 . 8 eV. o b t a i n e d by Cummings and B l e a k n e y (16). The d i s s o c i a t i o n o f the methanol m o l e c u l e i n v o l v e s the b r e a k i n g o f a C-H bond, and CHg-O-H can become C H 2 = 0 - H +  after  dissociation.  Furthermore,  the change from s i n g l e  to  double bond ' g i v e s b a c k ' some o f the energy o r d i n a r i l y r e q u i r e d to break a C-H bond. Thecbond d i s s o c i a t i o n energy D(H-CH 0H) can be 2  o b t a i n e d from the appearance p o t e n t i a l of the C H 0 H  +  2  the f o l l o w i n g  equation:-  D(H-CH 0H) 2  i o n by  -  V(CH 0H ) + +  2  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 11.52 eV., A E  i s the energy d i f f e r e n c e between C - 0 and C=0 bonds which e q u a l to 3.65 eV. ( 1 6 ) , (CH 0) 3  r a d i c a l , I(CH3O),  and the i o n i z a t i o n p o t e n t i a l o f i s 1 0 . 7 eV. ( 6 3 ) .  is the  I f the k i n e t i c  and e x c i t a t i o n e n e r g i e s are s m a l l and can be n e g l e c t e d , bond d i s s o c i a t i o n energy D(H-CH3) i s 4.41 eV.  the  98. CHAPTER SEVEN Conclusion In the p r e s e n t work, we have been concerned w i t h r e s u l t s o f i o n i z a t i o n and d i s s o c i a t i o n o f v a r i o u s  species  produced by p h o t o i o n i z a t i o n i n : a mass s p e c t r o m e t e r . discussed  i n some d e t a i l the i n t e r p r e t a t i o n  the  o f the  We have photoioniza-  t i o n e f f i c i e n c y c u r v e s 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 d a t a i n the  of b o n d - d i s s o c i a t i o n e n e r g i e s and z e r o - p o i n t  energy  evaluation  differences  between i s o t o p i c i o n s .  We have a l s o attempted to understand  n a t u r e o f the t h r e s h o l d  i o n i z a t i o n law f o r p h o t o i o n i z a t i o n and  the mechanisms f o r d i f f e r e n t tion,  the  t y p e s o f p r o c e s s e s such as e x c i t a - ; :  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 c o n c e r n i n g the e l e c t r o n i c s t r u c t u r e s o f m o l e c u l e s 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  analysis  p r o v i d e a p o w e r f u l method f o r the d e t e r m i n a t i o n o f i o n i z a t i o n and appearance p o t e n t i a l s .  T h i s method  a c c u r a t e e l e c t r o n impact one)  i s often  ( t o g e t h e r w i t h the  less  the o n l y a v a i l a b l e method  of s t u d y i n g those m o l e c u l e s f o r 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 f o r 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 a v o r a b l e c a s e s ,  this  method  can a l s o d e t e c 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 acetylene  and hydrogen c h l o r i d e . P r e v i o u s work on p h o t o i o n i z a t i o n i n t h i s  utilized  a many-line l i g h t source,  and t h i s  laboratory  l i m i t e d the  of d a t a 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 c u r v e s .  number The  99. gap between two data points was often large and t h i s certain fine features from being observed.  In t h i s  prevented  work,  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  photoionization  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 , N , C O , Cl > H C 1 , N H , N 0 , H p , C H , C D , C^EQ, 2  2  2  3  2  4  4  G J H ^ CHgCNand CHgOH have been obtained in the e n e r g y between 8 a n d  21 e V .  region  The accuracy of ionization and appearance  potential i n this iwork is comparable to that of the  spectroscop-  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 cules.  T h e ^electronic structures and the fine features o f  photoionization efficiency curves of these molecules s e e m paratively easy to explain.  Also,  the  ionization  the  com-  and  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. measurements,  The wish was to present new photoionization  and try to explain the earlier inconsistencies.  The main difficulty in this work is that t h e flux i s quite low, and the ionization c r o s s  section  photon is  smaller than that obtained by the electron impact m e t h o d .  much In  100. o r d e r to s e c u r e w o r k a b l e p h o t o - i o n c u r r e n t s ,  the mass  meter and monochromator s l i t w i d t h s have to i n c r e a s e expense of r e s o l u t i o n .  This affects  spectroat  the  the a c c u r a c y o f the  c a l v a l u e s o b t a i n e d from the e f f i c i e n c y c u r v e s ,  numeri-  and a l s o  p r e v e n t s c e r t a i n f i n e s t r u c t u r e from b e i n g r e s o l v e d . Watanabe has r e p o r t e d  the a c c i d e n t a l discovery t h a t  a s m a l l amount o f p l a t i n u m vapor d e p o s i t e d on the s u r f a c e of a g r a t i n g has an e f f e c t intensity,  that greatly increases  the  light  and he g i v e s c o n v i n c i n g e v i d e n c e f o r t h i s i n  experiments.  Thus , ::f ur t h e r r e s e a r c h  s h o u l d not be d e l a y e d .  pointing in this  However, a g r a t i n g u s u a l l y  later  direction  contains  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 exceedingly s m a l l .  The method o f c o a t i n g a t h i n l a y e r  of p l a t i n u m w i t h o u t a f f e c t i n g the f u n c t i o n i n g o f the g r a t i n g another problem one w i l l have to The o t h e r as n i t r o g e n ,  difficulty  is  face.  i s t h a t f o r some m o l e c u l e s such  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 i g h e r 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 c o m p e t i t i o n o f those p r o c e s s e s such autoionization.  Theopnotoionization e f f i c i e n c y curve  e x h i b i t s s t r o n g a u t o i o n i z e d peaks,  seen.  P h o t o i o n i z a t i o n i s a powerful method f o r the e l e c t r o n i c s t r u c t u r e of m o l e c u l e s ,  atoms and f r e e  it  The r e a c t i o n s o f  r a d i c a l s i n the gas phase are o f and they have p r o v i d e d  matter f o r a g r e a t many i n v e s t i g a t i o n s .  studying  and i n r e c e n t y e a r s  to f r e e r a d i c a l s t u d i e s .  importance i n c h e m i s t r y ,  often  and the i n n e r i o n i z a t i o n  c o n t i n u a o f those m o l e c u l e s cannot be c l e a r l y  has been extended  as  considerable subject  Most of the  data  101. c o n c e r n i n g the i o n i z a t i o n of  f r e e r a d i c a l s are g i v e n by the  s p e c t r o s c o p i c and e l e c t r o n impac t method i n the p a s t , p h o t o i o n i z a t i o n of f r e e i n recent  years  (59).  r a d i c a l s has shown s i g n i f i c a n t  and  .  progress  102. BIBLIOGRAPHY 1.  Becker E . W.,and Goudsmit S. 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