DOUBLE EXCITATION IN THE K - SHELL ABSORPTION SPECTRUM OF SULPHUR IN H2S AND D2S

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DOUBLE EXCITATION IN THE K - SHELL ABSORPTION SPECTRUM OF SULPHUR IN H2S

AND D2S

J. Hormes, U. Kuetgens, I. Ruppert

To cite this version:

J. Hormes, U. Kuetgens, I. Ruppert. DOUBLE EXCITATION IN THE K - SHELL ABSORPTION SPECTRUM OF SULPHUR IN H2S AND D2S. Journal de Physique Colloques, 1986, 47 (C8), pp.C8- 569-C8-573. �10.1051/jphyscol:19868106�. �jpa-00226002�

JOURNAL D E P H Y S I Q U E

Colloque C8, supplement au n o 12, Tome 47, dbcembre 1986

DOUBLE EXCITATION IN THE K - SHELL ABSORPTION SPECTRUM OF SULPHUR IN Hz S AND D, S

J. HORMES, U . KUETGENS and I . RUPPERT*

Physikalisches Institut der Universitat Bonn, ~ u s s a l l e e 1 2 , 0-5300 Bonn 1 , F.R.G.

" ~ n s t i t u t fur anorganische Chemie der Universitat Bonn, Gerhard-Domagk-Strasse 1, 0-5300 Bonn 1 , F.R.G.

Abstract

The X- r a y a b s o r p t i o n s p e c t r a o f H S and D S were s t u d i e d n e a r t h e s u l p h u r K - edge a t h i g h r e s o l u t i o n u s i n g s y n c h r o t f o n r a d i i t i o n as a ~ 3 1 ? t l i 1 i l o u s background s o u r c e . I n a d d i t i o n t o t h e well-known s i n g l e e l e c t r o n e x c i t a t ; i o n l i n e s we observe t w o new s t r u c t u r e s i n t h e continuum. On t h e b a s i s o f s i n g l e c o n f i g u ~ ' ~ t . i o i . : U z r t r e e - Fock c a l c u l a t i o n s we a s s i g n t h e s e f e a t u r e s t o t w o - e l e c t r o n t r a n s i t i o r i s l e a d i n g t o l e v e l s o f t h e f i n a l s t a t e c o n f i g u r a t i o n s ?s2s2p63s2p34s np,ns, 4p np a r d 3d n u and t o e x c i t e d s t a t e s o f t h e i o n l s 2 s 2 p 6 3 s 2 p 3 n s , np, nd ~ p .

1. Introduction

P r o g r e s s i n t h e u n d e r s t a n d i n g o f t h e x - r a y a b s o r p t i o n n e a r edge s t r u c t u r e w i t h i n a b o u t 50 eV f r o m t h r e s h o l d - now named XANES o r NEXAFS - was much s l o w e r t h a n t h a t o f t h e e x t e n d e d s t r u c t u r e - now c a l l e d EXAFS - because o f t h e complex p h y s i c a l phenomena o c c u r i n g n e a r t h e i o n i s a t i o n t h r e s h o l d . In m o l e c u l a r s p e c t r a f o r exam- p l e , Rydberg and v a l e n c e s t a t e s , shape resonances due t o m u l t i p l e s c a t t e r i n g and

"shake" peaks have been p o s t u l a t e d t o be o f importance /I/. The l a t t e r m u l t i e l e c t r o n t r a n s i t i o n s , i n v o l v i n g t h e simultaneous change o f s t a t e s o f two o r even more e l e c t r o n s , a r e h a r d l y o b s e r v a b l e i n m o l e c u l a r s p e c t r a because o f t h e much s t r o n g e r s t r u c t u r e o f m o l e c u l a r o r i g i n . Such t r a n s i t i o n s were a l r e a d y p r e d i c t e d by H e i s e n b e r g i n 1925 / 2 / b u t were d e t e c t e d n o t b e f o r e t h e m i d - s i x t i e s i n t h e a b s o r p t i o n / 3 / , t h e shake-up p h o t o e l e c t r o n s p e c t r a o f r a r e gases / 4 / and o n i y r e c e n t l y i n t h e X - r a y a b s o r p t i o n , s p e c t r a o f s o l i d s / 5 / . M u l t i - e l e c t r o n e f f e c t s a r e s t i l l an o b j e c t o f t h e o r e t i c a l s t u d i e s o f ground and f i n a l s t a t e c o r r e l a t i o n phenomena / 6 / as t h e y s u p p o r t and check t h e s e t h e o r i e s .

The p u r p o s e o f t h e p r e s e n t l e t t e r i s t o r e p o r t o u r new measurements o f t h e K p t i o t o a b s o r p t i o n spectrum o f s u l p h u r i n gaseous H S and D 2 S where emphasis i s p u t on t h e s t r u c t u r e above t h e f i r s t i o n i s a t i o n l i m i t which was p a r t l y observed by Bodeur and E s t e v a /7/ i n t h e i r r e c e n t s y n c h r o t r o n r a d i a t i o n a b s o r p t i o n e x p e r i m e n t . Because o f much h i g h e r i n t e n s i t y we were a b l e t o measure t h e s t r u c t u r e more c l e a r - l y , t o o b s e r v e more s t r u c t u r e a t h i g h e r e n e r g i e s and t o r e s o l v e even some f i v e

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868106

C8-570 JOURNAL DE PHYSIQUE

s t r u c t u r e i n t h e b r o a d low energy f e a t u r e . Based on a s e r i e s o f s i n g l e c o q f i g u r a . . t i o n H a r t r e e - F o c k c a l c u l a t i o n s t h e observed s t r u c t u r e s a r e a s s i g n e d t o a t o m i c t r a n s i t i o n s showing t h e e x i s t a n c e o f d o u b l e e x c i t a t i o n a l s o i n m o l e c u l a r s p e c t r a .

2. Experimental

Our measurements were c a r r i e d o u t between 2450 eV and 2550 eV a t t h e Bonn 2 . 5 GeV s y n c h r o t r o n r u n n i n g a t 2 . 0 GeV w i t h an e l e c t r o n c u r r e n t o f about 30 mA. The spec- t r a were o b t a i n e d b y s t a n d a r d X-ray a b s o r p t i o n t e c h n i q u e s . The monochrotnatized r a d i a t i o n i s m o n i t o r e d by an i o n i s a t i o n chamber b e f o r e p a s s i n g t h r o u g h t h e sample c e l l . A second i o n i s a t i o n chamber i s used as t h e d e t e c t o r .

A d o u b l e c r y s t a l monochromator s i m i l a r t o t h e one d e s c r i b e d by Lemonier e t a l . /8/ was u s e d and e q u i p p e d w i t h S i ( 1 , l , I ) c r y s t a l s , whereby t h e second o r d e r r a d i a t i o n i s e f f e c t i v e l y suppressed so t h a t h i g h e r o r d e r s c o n t r i b u t e l e s s t h a n 0 . 5 p e r c e n t o f t h e s i g n a l . From comparison of t h e K - s h e l l a b s o r p t i o n o f a t o m i c Ar w i t h p u b l i s h e d d a t a i n t h e l i t e r a t u r e /9/ and from t h e r o c k i n g c u r v e o f o u r cry;- t a l s t h e i n s t r u m e n t a l r e s o l u t i o n was e s t i m a t e d t o be a b o u t 0 . 5 eV i n t h e energy r a n g e o f i n t e r e s t . The e n e r g i e s o f t h e r e p o r t e d s p e c t r a were c a l i b r a t e d i n s,in!ul-.

taneous r u n s a g a i n s t t h e known Ar K-edge / 9 / . The accuracy o f o u r e n e r g y p o s i - t i o n s i s e s t i m a t e d t o be b e t t e r t h a n z 0 . 4 eV. B o t h i o n i s a t i o n chambers were f i l l e d w i t h a i r t o a b o u t 100 T o r r . The sample c e l l had a l e n g t h o f a b o u t 1 2 cv and was s e p a r a t e d f r o m t h e i o n i s a t i o n chambers by vacuum t i g h f 15 mm d i a m e t e r wjn-.

dows c o n s i s t i n g o f a 12 wn t h i c k c a p t o n f i l m . The p r e s s u r e i n t h e c e l l was con- t r o l l e d b y a c a l i b r a t e d p r e s s u r e sensor ( D a t a m e t r i c s ) and was v a r i e d between 1 T o r r and 20 T o r r f o r t h e measurements r e p o r t e d h e r e . The p r e a m p l i f i e d c u r r e n t s

f r o m t h e i o n i s a t i o n chambers were d i g i t i z e d and were r e a d o u t by a Commodore CBM 3032 m i c r o computer a t l e a s t 10 t i m e s b e f o r e t h e mcnochromator was s e t b y t h e com- p u t e r t o t h e n e x t w a v e l e n g t h . Step s i z e was v a r i e d between 0 . 0 2 and 0 . 0 4 eV f o r d i f f e r e n t r u n s .

3. Results and Discussion

The K - s h e l l a b s o r p t i o n of H2S r e c o r d e d a t a p r e s s u r e o f a b o u t 10 T o r r i s shown i n f i g u r e 1. D i s p l a y e d a r e o r i g i n a l d a t a w i t h o u t any p r o c e s s i n g . Up t o t h e f i r s t i o n i s a t i o n l i m i t t h i s spectrum agrees v e r y w e l l w i t h t h o s e r e p o r t e d p r e v i o u s l y by s e v e r a l o t h e r a u t h o r s . The e n e r g i e s o f t h e l i n e s l a b e l l e d I t o 3 i n c h i s f i g u r e (2472.4 eV, 2475.5 eV and 2476.8 eV) agree v e r y w e l l w i t h t h o s e g i v e n b y L a y i l i a

/ l o / and Mazalov e t a l . 1 I n c l u d e d i n t h i s f i g u r e a r e p o s i t i o n s o f sinr;!e e l e c t r o n e x c i t a t i o n s I s -9 np ( n 2 3 ) c a l c u l a t e d u s i n g a H a r t r e e - Fock program /12/. The c a l c u l a t e d energy p o s i t i o n s were a d j u s t e d r e l a t i v e t o t h e measurec i o n i s a t i o n p o t e n t i a l o f 2478.5 eV / 1 3 / . A comparison o f t h e measured ana r a l c ; t - l a t e d l i n e s s t r o n g l y s u p p o r t s t h e assignment o f t h e s e l i n e s as g i v e n b y L a v i l a

/ l o / and and Mazalov e t a l . /14/ which was c o n f i r m e d a l s o by Bodeur and i s t e v a / 7 1 on t h e b a s i s o f u n p u b l i s h e d r e s u l t s o f L a n g h o f f . The f i r s t l i n e i s molecu!ar

i n o r i g i n and i s assigned t o a t r a n s i t i o n t o t h e antibonding o r b i t a l s 3b2 and 6 a l L i n e s No 2 (2475.4 eV) and No 3 (2476.8 eV) may w i t h good confidence be assigned t o t h e second and t h i r d member o f t h e 1s 2s2323p4np (n23) Rydberg s e r i e s .

ENERGY [eVl

F i g . 1 : Sulphur K - s h e l l photoabsorption c r o s s s e c t i o n i n H S

Above t h e f i r s t i o n i s a t i o n p o t e n t i a l we observe some f i n e s t r u c t u r e w i t h small a m p l i t u d e which i s b a r e l y v i s i b l e i n f i g u r e 1. T h i s s t r u c t u r e t u r n e d o u t t o be r e p r o d u c i b l e r e g a r d i n g shape and p o s i t i o n under d i f f e r e n t experimental c o n d i t i o n s and was a l s o observed i n our D 2 S s p e c t r a . I n f i g u r e 2 we show on an e n l a r g e d s c a l e t h e p a r t o f t h e H2S spectrum extending beyond peak No 3. To make t h e f i n e s t r u c t u r e observed i n t h i s r e g i o n stand o u t more c l e a r l y a smooth background was s u b t r a c t e d from t h e spectrum i n f i g u r e 1 where t h e form o f t h e background d i d n o t change e i t h e r t h e shape o r t h e p o s i t i o n of t h e r e p o r t e d bands. We observe a broad f e a t u r e ( 4 - 6 ) between about 2482 eV and 2490eV w i t h some f i n e s t r u c t u r e a t t n e low energy s i d e and a second l e s s broad l i n e ( 7 ) centered a t about 2491.7 eV. As suspected by Bodeur and Esteva who observed t h e f i r s t p a r t o f t h e low energy s t r u c t u r e these s t r u c t u r e s c o u l d be e i t h e r shape resonances o r double e x c i t a t i o n s .

I ^I I

2480 2485 2490 2495

ENERGY 1 eVI

F i g . 2 : Sulphur K - s h e l l photoabsorption i n t h e double e x c i t a t i o n r e g i o n

An assignment t o a molecular shape resonance seems t o be u n l i k e l y as hydrogen i s a v e r y weak b a c k s c a t t e r e r . Therefore we b e l i e v e a l l s t r u c t u r e t o be due t o atomic

(28-5 72 JOURNAL DE PHYSIQUE

d o u b l e e x c i t a t i o n s . To c o n f i r m t h i s assignment we have performed a s e r i e s o f s i n g l e c o n f i g u r a t i o n Hartree-Fock c a l c u l a t i o n s t h e r e s u l t s o f which a r e a l s o i n c l u d e d i n f i g u r e 2. As t h e e x c i t e d c o r e i o n i s a t i o n l i m i t s o f H S a r e n o t known a l l l i n e p o s i t i o n s i n f i g u r e 2 a r e n o t a d j u s t e d b u t g i v e n as c a l c u l a t e d .

I n our c a l c u l a t i o n s we have i n c l u d e d t h e lowest p o s s i b j e shake-up t r a n s i t i o n s as w e l l as some c o n j u g a t e d shake-up s e r i e s . As t h e s t i c k i n g diagrams i n f i g u r e ; show, a l l t r a n s i t i o n s i n t o d o u b l y e x c i t e d s t a t e s o f t h e atom i n c l u d i n g t h e e x c i t a - t i o n o f a 3p e l e c t r o n s e v e r e l y o v e r l a p i n t h e r e g i o n o f t h e broad s t r u c t u r e ( 5 - 6 ) . F e a t u r e No 7 i s p r o b a b l y due t o t h e l o w e s t t r a n s i t i o n s t o e x c i t e d s t a t e s o f t h e i o n though t h e correspondence i s n o t as c l e a r as f o r t h e s t r u c t u r e ( 4 - 6 ) . One has t o keep i n mind though, t h a t t h e e n e r g i e s g i v e n i n t h e s t i c k i n g diagram a r e n o t a d j u s t e d and c a r r y an u n c e r t a i n t y o f s e v e r a l eV as e s t i m a t e d from t h e d i f f e r - ence between e x p e r i m e n t and o u r c a l c u l a t i o n o f t h e f i r s t i o n i s a t i o n l i m i t . I n v i e w o f t h i s u n c e r t a i n t y t h e assigned t r a n s i t i o n s q u i t e s a t i s f a c t o r i l y f i t t h e

observed f e a t u r e s . It even appears p o s s i b l e t h a t t h e t r a n s i t i o n due t o t h e d o u b l y e x c i t e d s t a t e ls2s23s3p44s4p c o n t r i b u t e s t o t h e second s t r u c t u r e . A more s p e c i f i c assignment i s n o t w a r r a n t e d s i n c e severe o v e r l a p i s expected i n v i e w o f th,e c a l c u - - l a t e d l i n e d e n s i t y and t h e n a t u r a l w i d t h s o f t h e l i n e s which i n most cases exceed t h e l i n e s p a c i n g w i t h i n a Rydberg s e r i e s . The s i t u a t i o n i s even worse t h a n i n d i - c a t e d as o u r s t i c k i n g diagrams g i v e o n l y t h e c e n t e r s o f g r a v i t y o f t h e ccnf.ig!:ra- t i o n s , whereas most o f t h e c o n f i g u r a t i o n s , h a v i n g up t o f o u r open s h e l l s , c o n s i s t o f numerous l e v e l s spread sometimes over s e v e r a l eV and more. Thus t h e a p p l i c a - b i l i t y o f t h e s i n g l e c o n f i g u r a t i o n c a l c u l a t i o n s used h e r e i s a t l e a s t q u e s t i o n - a b l e . We b e l i e v e t h i s t o be an example where t h e method o f u n r e s o l v e d t r a n s i t i o n a r r a y s ( U T A ' s l developped by Bauche-Arnoult e t a l . /15/ f o r t r a n s i t i o n s o f h i g h l y i o n i z e d i o n s c o u l d be f a v o u r a b l y a p p l i e d .

4. Conclusion

I n t h e r e g i o n above t h e f i r s t i o n i s a t i o n p o t e n t i a l we have observed a weak b u t r e p r o d u c i b l e f i n e s t r u c t u r e i n t h e s p e c t r a o f H2S and D2S. The d o u b l e e x c i t a t i o n Rydberg s e r i e s w h i c h we have c a l c u l a t e d w i t h a non r e l a t i v i s t i c Hartree-Fock p r o - gram c o i n c i d e f a v o u r a b l y w i t h t h e s e f e a t u r e s . From t h i s f i n d i n g we f e e l compelled t o a s s i g n a l l observed s t r u c t u r e t q doubly e x c i t e d s t a t e s . Since t h e v a l u e s f o r t h e e x c i t e d c o r e i o n i s a t i o n l i m i t s a r e unknown and v a r i o u s s e r i e s s e v e r e l y o v e r l a p each o t h e r a more d e t a i l e d assignment i s n o t a p p r o p r i a t e .

We t h a n k D r . J.M.Esteva f o r f r u i t f u l a d v i c e and P r o f .F.von Busch f o r h i s a s s i s t a n c e i n p r e p a r i n g the m a n u s c r i p t . T h i s work was supported by t h e Bundesministurium f u r Forschung und T e c h n o l o g i e w h i c h i s g r a t e f u l l y acknowledged.

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