Spectroscopic study of molecular hydrogen above its first ionization potential

Volume24A, number 9

PHYSICS

LETTERS

2 4 A p r i l 1967

S P E C T R O S C O P I C STUDY OF MOLECULAR HYDROGEN ABOVE ITS FIRST IONIZATION P O T E N T I A L F . J. C O M E S Institut ]'dr Physikalische Chemic d e r Universitttt Bonn, Bonn. Germany

Received 2 4 M a r c h 1967

Differences in the absorption and ionization c r o s s section of H2 a r e m e a s u r e d with the aid of the Hopfield continuum.

In t h e e n e r g y r e g i o n of t h e i r i o n i z a t i o n c o n t i nua gaseous molecules may be excited, dissociat e d o r i o n i z e d . O f t e n , if t h e e n e r g y of t h e r a d i a tion is high enough, these processes may occur in parallel. Then the total absorption cross section a a b s i s t h e s u m of t h e c o n t r i b u t i o n s f r o m e x c i t a t i o n ~ e x c , d i s s o c i a t i o n Crdiss, a n d i o n i z a t i o n ~ i o n , a c c o r d i n g to t h e r e l a t i o n aabs = ~exc + adiss + ~ion-

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the D' I iiu, D" 1 flu, B' l~u+and B" IS+ states of the molecule. In the energy region above threshold much more states are possible than cited above which makes the spectrum of molecular hydrogen complex. This can easily be seen from its absorption spectrum. The absorption cross section shows a rich structure which can be followed up to about 710 A. Evidence for this behaviour was found earlier in a measurement of the photoionizationyield [2].

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Here dissociative ionization and autoionization p r o c e s s e s b o t h a r e c o m p i l e d in t h e t e r m ~ i o n . F r o m t h e m e a u r e m e n t s of t h e t o t a l a b s o r p t i o n c r o s s s e c t i o n ¢;abs a n d t h e i o n i z a t i o n c r o s s s e c t i o n ~ i o n t h e s u m of t h e c r o s s s e c t i o n s f o r t h e two r e s i d u a l p r o c e s s e s i s g i v e n . T o g e t h e r w i t h a s p e c t r o s c o p i c i d e n t i f i c a t i o n of t h e a b s o r p t i o n curve some interesting results may be drawn from such measurements. This is especially t r u e f o r t h e c a s e of m o l e c u l a r h y d r o g e n . Molecular hydrogen is a diatomic molecule w h o s e a b s o r p t i o n s p e c t r u m in t h e r e g i o n of i t s first ionization potential was thoroughly investig a t e d in r e c e n t t i m e s [1]. T h e r e a r e t h e b a n d s e r i e s of s e v e r a l k n o w n e x c i t e d s t a t e s e x t e n d i n g a b o v e t h e 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 t h e s e t h e X - D b a n d s a r e v e r y p r o n o u n c e d in t h e a b s o r p t i o n spectrum. This is even true above the ionization threshold. Other known series whose rotational s t r u c t u r e w a s t h o r o u g h l y i n v e s t i g a t e d b e l o n g to

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Fig. 1. Difference between absorption and ionization c r o s s section divided by the total absorption c r o s s s e c tion. The a r r o w s indicate vibrational st4~tes of the D s e r i e s of H2 and those of H2 • T h e s t r u c t u r e of t h e a b s o r p t i o n c r o s s s e c t i o n a b o v e t h e f i r s t i o n i z a t i o n p o t e n t i a l i s c a u s e d by a n e x c i t a t i o n of t h e h y d r o g e n m o l e c u l e to h i g h e r electronic states which in part decay into ion p l u s e l e c t r o n by a u t o i o n i z a t i o n [2,3]. A u t o i o n i z a tion is a well known phenomenon in the spectrum of a t o m s a n d m o l e c u l e s n e a r t h e i r i o n i z a t i o n threshold. This can clearly be seen from photo-

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Volume 24A. n u m b e r 9

PHYSICS

ionization experiments. Besides existing selection rules not all of these states decay by autoionization which can be deduced from this experiment. This is in qualitative agreement with an earlier remark [4]. With the background of a helium continuum light source part of the absorption- and ionization spectrum of H2 was measured with a resolution of about 0.4 A. Though the rotational structure could not be resolved, some of the bands of the D Iii u series can be clearly identified. As the result the experiment shows that within the experimental accuracy those states of the D series with vibrational quantum numbers v' = 6, 7, 8, 9, I0 and 11 have no or at least only a very small contribution to the ionization yield (fig. I). Contributions from higher states (v' > Ii) have not yet been investigated on account of the strong overlapping of higher states and the existing resolution of the monochromator. In fig. 1 t h e r a t i o of t h e e x c e s s a b s o r p t i o n c r o s s s e c t i o n to t h e t o t a l a b s o r p t i o n c r o s s s e c t i o n (Crabs - eion)/Crabs i s p l o t t e d in u n i t s of 1 0 - 1 8 c m 2 . T h e s e v a l u e s a r e c e r t a i n l y not e x a c t due to t h e u n s u f f i c i e n t r e s o l u t i o n of t h e a p p a r a t u s f o r w h a t w a s r e a l l y m e a s u r e d in t h e c a s e of u n resolved structure is not the mean absorption c r o s s s e c t i o n ~ but a v a l u e w h i c h i s p r o p o r t i o n a l to t h e m e a n a b s o r p t i o n (Io - I ) . T h e d i f f e r e n c e b e t w e e n (~ a n d ( I ~ - / ) i s a l w a y s p o s i t i v e , s o t h a t t h e r e a l c r o s s s e c t i o n s in t h e c a s e of u n r e s o l v e d structure are higher. Although there is a rich s t r u c t u r e b o t h in t h e a b s o r p t i o n a n d t h e i o n i z a t i o n c u r v e in t h a t e n e r g y r e g i o n d u e to b a n d s f r o m several electronic states which partially overlap, t h e r e p r e s e n t a t i o n of t h e r a t i o of t h e e x c e s s a b sorption to the total absorption clearly demonstrates the competition between autoionization and nonionizing processes for this molecule. The s p e c t r a l r e s o l u t i o n i s n o t h i g h e n o u g h to r e s o l v e t h e r o t a t i o n a l s t r u c t u r e of t h e b a n d s b u t t h e p e a k s in fig. 1 p o i n t to t h e f a c t t h a t a t l e a s t t h e m a i n p a r t of t h e a b s o r p t i o n i n t o t h e D b a n d s d o e s n o t l e a d to i o n i z a t i o n . T h e f i r s t two p e a k s in fig. 1 (v' = 6 a n d 7) r e p r e s e n t n e a r l y 90% of t h e t o t a l a b sorption at these wavelengths. Those states lying b e t w e e n t h e s e two l e v e l s s h o w n e a r l y e q u a l i o n i z a t i o n p r o b a b i l i t y . T h i s i s n o t t r u e f o r s o m e of the states between the higher vibrational levels. In t h i s r e g i o n h i g h e r e l e c t r o n i c s t a t e s a r e p r e s e n t which are more unstable against autoionization. This latter statement agrees with a recent c a l c u l a t i o n on t h e p r o b a b i l i t y of a u t o i o n i z a t i o n in H2 n e a r i t s i o n i z a t i o n t h r e s h o l d [5]. T h e t h e o r y w a s d e v e l o p e d f o r t h e a u t o i o n i z a t i o n of m o l e c u l e s a t e n e r g i e s w i t h i n a f e w e l e c t r o n v o l t s o r l e s s of 466

LETTERS

2 4 A p r i l 1967

threshold, specifically to include effects of the interaction of an excited Rydberg electron with the rotating, vibrating molecule ion to which it is bound. It is expected that the dominant coupling mechanism involves conversion of vibrational energy to electronic energy and that electronically induced autoionization should play no part in the autoionization of H2 near threshold. The calculated rates show a great variety of values for the various excited states treated, ranging from 2.21 × 1013 sec -I for the 7pn(v' = 1) state to 0.304 x 107 sec -I for the 4p(~(v' = 5) state. It is suggested from this calculations that the lower Rydberg states 3p% 4pcr, 3p~, and 4p~ do not play a significant role in the autoionization of H 2. The lifetimes of the 4p states shall be even long compared with spontaneous emission lives of allowed transitions. The calculated autoionization rates for these states are so low that there should be no contribution to direct autoionization. Most of the ionization observed within one eV above threshold is infered to come from autoionization of higher electronic states for which a significant line broadening is predicted. Whereas rotational structure is expected for the 5p and 6p states, the 7pg(v' = 2) state shall show only a broad peak. The transitions from the 7p~(v' = i, 2) state and all the vibrationally excited 8p states shall be very fast and should not contribute vibrational structure to the ionization cross section. Since the band width of the monochromator used was about 0.4A the rotational structure could not be resolved. Therefore the discussion of the finer details of the photoionization cross section of H 2 has to be postponed. I am much indepted to Mr. Wellern for his help in the performance of the measurements. Furthermore I thank the Deutsche Forschungsgemeinschaft and the Landesamt fur Forschung in DUsseldorf for their financial support of the experiments.

RcfcTcnccs 1. G . H e r z b e r g a n d A . M o n f i l s ,

J. Mol. Speetr. 5(1960)

482;

2. 3. 4. 5.

A.Morffils, J. Mot. Speetr. 15 (1965) 265; T. Namioka, J. Chem. Phys. 40 (1964) 3154; T. Namioka, J. Chem. Phys. 41 (1964)2141. F , J . Comes and W. L e s s m a n n , Z. Naturforseh. 19a (1964) 508. V . H . D i b e l e r , R . M . R e e s e and M . K r a u s s , J. Chem. Phys. 42 (1965) 2045. G . R . C o o k and P . H . M e t z g e r , J. Opt. Soc. Am. 54 (1964) 968. R . S . B e r r y , J. Chem. Phys. 45 (1966) 1228.