RESONANCES IN NEAR-THRESHOLD X-RAY PHOTOABSORPTION OF INNER SHELLS

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RESONANCES IN NEAR-THRESHOLD X-RAY PHOTOABSORPTION OF INNER SHELLS

N. del Grande, K. Tirsell, M. Schneider, R. Garrett, E. Kneedler, S. Manson

To cite this version:

N. del Grande, K. Tirsell, M. Schneider, R. Garrett, E. Kneedler, et al.. RESONANCES IN NEAR-

THRESHOLD X-RAY PHOTOABSORPTION OF INNER SHELLS. Journal de Physique Colloques,

1987, 48 (C9), pp.C9-951-C9-954. �10.1051/jphyscol:19879171�. �jpa-00227286�

JOURNAL DE PHYSIQUE

Colloque C9, suppl6ment au n 0 1 2 , Tome 48, d6cembre 1987

RESONANCES I N NEAR-THRESHOLD X-RAY PHOTOABSORPTION OF INNER SHELLS

N.K. DEL GRANDE, K.G. TIRSELL, M.B. SCHNEIDER, R.F. GARRETT*

E.M. KNEEDLER* and S.T. MANSON*"

Lawrence Livermore National Laboratory, PO Box 808, Livermore, CA 94550, U.S.A.

rookha haven

National Laboratory, Upton, NY 11973, U.S.A.

* * Department of Physics and Astronomy, Georgia State University, Atlanta, GA 30303, U.S.A.

Resume - Nous avons mesure au L a b o r a t o i r e de Brookhaven (NSLS) e t

a\

S t a n f o r d (SSRL) des s e c t i o n s e f f i c a c e s absolues d e p h o t o a b s o r p t i o n au moyen de rayonments de s y n c h r o t r o n p r e s du s e u i l e t pour un l a r g e e v e n t a i l d ' e n e r g i e (20 eV

-

³ keV). Des donnees d e t r a n s m i s s i o n s r e c e u i l l i e s pour des c i b l e s m u l t i - c o u c h e s aux p r o p r i e t e s b i e n d e f i n i e s nous o n t permis de d e t e r m i n e r l e s s e c t i o n s e f f i c a c e s absolues avec une i n c e r t i t u d e g l o b a l e de l ' o r d r e de 10% e t pour une r e s o l u t i o n s u r l ' e n e r g i e m e i l l e u r e que 0.2%. Nous prCsentons quelques e x e m p l a i r e s de nos r 6 s u l t a t s .

A b s t r a c t - Synchrotron r a d i a t i o n measurements o f n e a r - t h r e s h o l d and broad-range (20 eV

-

³ keV) a b s o l u t e p h o t o a b s o r p t i o n c r o s s s e c t i o n s were made a t t h e Brookhaven N a t i o n a l L a b o r a t o r y (NSLS) and a t S t a n f o r d (SSRL). Transmission d a t a f o r w e l l - c h a r a c t e r i z e d m u l t i l a y e r f o i l s p r o v i d e d a b s o l u t e c r o s s s e c t i o n s w i t h 10% o v e r a l l u n c e r t a i n t i e s and b e t t e r t h a n 0.2% energy r e s o l u t i o n . Several examples o f o u r r e s u l t s a r e presented.

The i n t e r a c t i o n o f e l e c t r o m a g n e t i c r a d i a t i o n w i t h m a t t e r i s o f fundamental s i g n i f i c a n c e , as w e l l as o f g r e a t importance i n numerous a p p l i c a t i o n s . I n t h e s o f t x-ray range, p h o t o a b s o r p t i o n i s t h e dominant i n t e r a c t i o n process. Yet o u r knowledge o f t h e p h o t o a b s o r p t i o n c r o s s s e c t i o n s i n t h e s o f t x-ray range, over t h e b u l k o f t h e p e r i o d i c t a b l e , i s demonstrably p o o r [ l ] . Thus, a t t h e Lawrence Livermore N a t i o n a l L a b o r a t o r y , we have embarked on a program t o measure r e l i a b l e a b s o l u t e p h o t o a b s o r p t i o n c r o s s s e c t i o n s f o r a broad range o f elements. I n t h i s paper, we r e p o r t on some d e t a i l s o f o u r experiment, a l o n g w i t h some examples o f o u r r e s u l t s .

The r a d i a t i o n sources used f o r these measurements were t h e S t a n f o r d Synchrotron R a d i a t i o n L a b o r a t o r y (SSRL) and t h e Brookhaven N a t i o n a l S y n c h r o t r o n L i g h t Source (NSLS) e l e c t r o n s t o r a g e r i n g s which a l l o w e d us t o make measurements f r o m 20 eV t o s e v e r a l keV. U s i n g w e l l - c h a r a c t e r i z e d u l t r a - t h i n t a r g e t s i n t r a n s m i s s i o n , and c o n t i n u o u s l y t u n a b l e monoenergetic beams o f s o f t x-ray r a d i a t i o n , a b s o l u t e c r o s s s e c t i o n measurements were made w i t h e r r o r b a r s o f a p p r o x i m a t e l y +lo%.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19879171

JOURNAL DE PHYSIQUE

Key t o r e d u c i n g c r o s s s e c t i o n u n c e r t a i n t i e s f r o m f a c t o r s o f two o r more t o

lo%,

were t h e developments o f monoenergetic VUV and s o f t x-ray s y n c h r o t r o n beam l i n e s and t h e f a b r i c a t i o n o f e x t r e m e l y s t a b l e , pure, m u l t i l a y e r t a r g e t s . A l s o , s i g n i f i c a n t backgrounds were g r e a t l y reduced by t a k i n g redundant measurements t o v e r i f y t h e s e l f - c o n s i s t e n c y o f o u r d a t a . To t h i s end, we had a v a i l a b l e s e v e r a l samples p e r element ( i n v a r y i n g t h i c k n e s s e s ) , a dozen p r e f i l t e r s , up t o t h r e e m i r r o r / g r a t i n g combinations per monochromator, o v e r l a p p i n g wavelength r e g i o n s , and a t o t a l o f f o u r d i f f e r e n t monochromators a t NSLS and SSRL.

The use o f j u d i c i o u s l y s e l e c t e d p r e f i l t e r s i n c o n j u n c t i o n w i t h v a r i o u s m i r r o r and g r a t i n g combinations a l l o w s us t o g r e a t l y reduce s i g n i f i c a n t backgrounds f r o m h i g h o r d e r harmonics and s c a t t e r e d l i g h t . These backgrounds l i m i t t h e u s e f u l c o n t i n u o u s energy r e g i o n f o r a g i v e n u n f i l t e r e d m i r r o r l g r a t i n g c o m b i n a t i o n . E r r o r s generated by n o t t a k i n g t h i s i n t o account have i n t r o d u c e d d i s c r e p a n c i e s o f a f a c t o r o f two o r more i n p r e v i o u s measurements.

F u r t h e r , we developed e x t r e m e l y s t a b l e , s t r o n g , f r e e - s t a n d i n g f o i l s w i t h carbon s u r f a c e l a y e r s and i n t e r l a y e r s o f v a r y i n g t h i c k n e s s e s f r o m 50

!i

t o 500

a.

The s u r f a c e l a y e r s o f carbon m i n i m i z e o x i d a t i o n which has c o n t r i b u t e d s i g n i f i c a n t e r r o r s f o r p r e v i o u s measurements. The f o i l s were made i n p a i r s , w i t h one f o i l h a v i n g o n l y l a y e r s o f carbon and t h e o t h e r f o i l h a v i n g t h e e l e m e n t a l f o i l i n t h e m u l t i l a y e r c o n f i g u r a t i o n . Layer t h i c k n e s s e s were designed t o m i n i m i z e i n t e r f e r e n c e e f f e c t s f r o m Bragg d i f f r a c t i o n . The carbon-only and m u l t i l a y e r t a r g e t s were made s i m u l t a n e o u s l y and measured s e p a r a t e l y . The f o i l s were c h a r a c t e r i z e d f o r t h e i r p u r i t y , w e i g h t - p e r - u n i t area and e l e m e n t a l t h i c k n e s s u s i n g v e r y a c c u r a t e balances, i o n b a c k - s c a t t e r i n g and p a r t i c l e - i n d u c e d x-ray e m i s s i o n .

For t r a n s m i s s i o n measurements, t h e NSLS U14A p l a n e g r a t i n g monochromator was used, a l o n g w i t h t o r o i d a l g r a t i n g , d o u b l e m u l t i l a y e r c r y s t a l , and s p h e r i c a l g r a t i n g monochromators a t SSRL. I n a d d i t i o n , w,e designed and i n s t r u m e n t a t i o n t h a t enabled us t o c o n t i n u o u s l y m o n i t o r t h e i n c i d e n t bea.byig i n t e r c h a n g e up t o s i x p r e f i l t e r s , and t o t r a n s l a t e each o f s e v e r a l samples i n t o t h e t a r g e t p o s i t i o n . To measure t h e t r a n s m i t t e d s o f t x-ray photons a t NSLS, we designed a t h r e e element photod i o d e t h a t p e r m i t t e d s e l e c t i o n o f p u r e m e t a l cathodes o f v a p o r - d e p o s i t e d g o l d , and diamond-turned A1 o r Cr, i n o r d e r t o t a k e advantage o f t h e m e t a l l i c s u r f a c e w i t h t h e optimum response i n each s p e c t r a l r e g i o n . Photodiode c u r r e n t s were measured w i t h e l e c t r o m e t e r s . The energy s c a l e was c a l i b r a t e d u s i n g photoemission a l o n g w i t h known a b s o r p t i o n edges; o u r energy r e s o l u t i o n was b e t t e r t h a n 0.2%. A more d e t a i l e d d e s c r i p t i o n o f t h e experiment w i l l be p r e s e n t e d elsewhere [2].

As a f i r s t example, o u r measured p h o t o a b s o r p t i o n c r o s s s e c t i o n f o r amorphous carbon, near t h e K-edge, i s shown i n F i g u r e 1, a l o n g w i t h a c o m p a r i s i o n w i t h t h e t h e o r e t i c a l r e l a t i v i s t i c H a r t r e e - S l a t e r (RHS) r e s u l t o f S c o f i e l d [3] and t h e c o m p i l a t i o n o f Henke [4] as w e l l as an LLNL e x t e n s i o n and upgrade o f Ref. 4 (CSLOW). From F i g u r e 1 i t i s c l e a r t h a t t h e e x p e r i m e n t a l r e s u l t s show a broad t h r e s h o l d f e a t u r e which i s n o t p r e d i c t e d by RHS t h e o r y o r i n c l u d e d i n t h e l a t e s t c o m p i l a t i o n s . E x c e l l e n t agreement, however, i s found b o t h above and below t h i s t h r e s h o l d enhancement. Our t e n t a t i v e e x p l a n a t i o n i s t h a t t h e enhancement i s due t o a m o l e c u l a r shape resonance [ 5 ] , a l t h o u g h a u t o i o n i z i n g t r a n s i t i o n s f r o m t h e 1s s t a t e t o unoccupied valence bands i n t h e s o l i d a r e a l s o p o s s i b l e . T h i s p o i n t must be i n v e s t i g a t e d f u r t h e r i n o r d e r t o understand t h e mechanism f o r t h i s phenomenon. It i s i n t e r e s t i n g t o n o t e , however, t h a t even f o r K - s h e l l p h o t o a b s o r p t i o n f o r so l i g h t an element, as y e t u n e x p l a i n e d e f f e c t s occur. Note f u r t h e r , t h a t t h i s r e p r e s e n t s t h e f i r s t a b s o l u t e measurement o f t h e p h o t o a b s o r p t i o n c r o s s s e c t i o n o f carbon i n t h e v i c i n i t y o f t h e K-edge.

As a n e x t example, t h e p h o t o a b s o r p t i o n cross s e c t i o n f o r uranium, i n t h e r e g i o n o f t h e 5d-5f g i a n t resonance, i s shown i n F i g u r e 2 a l o n g w i t h RHS t h e o r y [3], t h e Henke c o m p i l a t i o n [4], and t h e l o c a l d e n s i t y random phase a p p r o x i m a t i o n (LDRPA) c a l c u l a t i o n o f Wendin [GI; t h e Henke c o m p i l a t i o n i n t h i s r e g i o n i s based on t h e e x p e r i m e n t a l work o f C u k i e r e t a1 [7]. The o u t s t a n d i n g f e a t u r e o f t h e e x p e r i m e n t a l r e s u l t i s t h e s t r o n g , broad a b s o r p t i o n f e a t u r e m a x i m i z i n g a t about 115 eV, a l o n g w i t h t h e s m a l l e r one a t about 101 eV. Agreement w i t h t h e LDRPA

Region of molecular shape resonances or other unexplained effects

Region of 1s

-

2p autoionizing resonances

200 240 280 320 360 400

Photon energy (eV)

Fig.

1.

Experimental photoabsorption cross section for amorphous carbon in the vicinity of the K-edge compared with theoretical and compiled results. See text for details of the latter curves.

Present experiment

--+ LDRPA theory (Wendin) -+ 1982 compilation (Henke)

---b RHS theory (Scofield)

80 100 120 140 160 180

Photon energy (eV)

Fig. 2. Experimental photoabsorption cross section for uranium in the region of

the 5d-5f resonance compared with theoretical and compiled results. See text

for details o f the latter curves.

J O U R N A L DE PHYSIQUE

c a l c u l a t i o n [63 and t h e e x p e r i m e n t o f Ref. [ 7 ] i s good q u a l i t a t i v e l ~ , i n d i c a t i n g our qua1 i t a t i v e u n d e r s t a n d i n g o f t h e process, b u t i s s t i l l l a c k i n g q u a n t i t a t i v e l y ; i t i s , however, c l e a r f r o m F i g u r e 2 t h a t t h e c a l c u l a t i o n [6] i s

<n s i g n i f i c a n t l y b e t t e r agreement w i t h t h e p r e s e n t e x p e r i m e n t a l v a l u e s t h a n w i t h t h e e a r l i e r measured values [7]. On t h e o t h e r hand, t h e s i n g l e p a r t i c l e RHS c a l c u l a t i o n , which does n o t i n c l u d e a u t o i o n i z a t i o n o r o t h e r many-body e f f e c t s , c o m p l e t e l y f a i l s t o r e p r o d u c e t h i s resonance even q u a l i t a t i v e l y .

P h y s i c a l l y , t h i s s t r u c t u r e i s due t o t h e / v e r y s t r o n g An = 0 t r a n s i t i o n s t o an u n f i l l e d f-subs'hell; s i m i l a r resonances a r e i n e v i d e n c e i n t h e l a n t h a n i d e s , w i t h u n f i l l e d 4 f - s u b s h e l l s , and o t h e r a c t i n i d e s w i t h u n f i l l e d 5 f ' s [I]. The phenomenon was f i r s t e x p l a i n e d i n lanthanum [8]. B a s i c a l l y , we b e l i e v e t h a t t h e process h i n g e s on t h e s t r e n g t h o f t h e 5d-5f t r a n s i t i o n s , a l t h o u g h shape resonance e f f e c t s c o u l d c o n t r i b u t e a l s o . The process c o u l d proceed as a 5d-5f a b s o r p t i o n , f o l l o w e d by an a u t o i o n i z i n g decay; o r as a 5 f - ~ g a b s o r p t i o n , s t r o n g l y m o d i f i e d by i n t e r c h a n n e l c o u p l i n g w i t h t h e 5d-5f t r a n s i t i o n s . Owingg t g t h e open f - s u b s h e l l , t h e r e a r e a m y r i a d o f p o s s i b l e o p t i c a l l y a l l o w e d 5d 5f f i n a l m u l t i p l e t s ; t h e s t r e n g t h o f t h e i n t e r a c t i o n s between t h e s e m u l t i p l e t s makes t h e c a l c u l a t i o n s v e r y d i f f i c u l t because many-body e f f e c t s a r e s o i m p o r t a n t [6]. Furthermore, t h e f a c t t h a t t h e experiment was performed on a s o l i d , w h i l e t h e c a l c u l a t i o n was done f o r a f r e e atom, c o u l d l e a d t o some d i s c r e p a n c i e s . However, t h i s s h o u l d n o t be a l a r g e e f f e c t s i n c e b o t h t h e 5d and most o f t h e 5 f wave f u n c t i o n s a r e l o c a l i z e d and n o t i n v o l v e d i n t h e b i n d i n g o f t h e s o l i d .

F i n a l l y , we emphasize t h a t o n l y a s m a l l s e l e c t i o n o f our r e c e n t p h o t o a b s o r p t i o n measurements u s i n g s y n c h r o t r o n r a d i a t i o n a r e p r e s e n t e d h e r e . These r e s u l t s , which i n c l u d e elements as l i g h t as Be (Z = 4) and as heavy as Th (Z = g o ) , show numerous new s t r u c t u r e s and enhancements i n t h e v i c i n i t y o f i n n e r - s h e l l t h r e s h o l d s . F u r t h e r r e p o r t s w i l l be p r e s e n t e d elsewhere.

T h i s work was s u p p o r t e d by t h e Lawrence Livermore N a t i o n a l Laboratory under t h e auspices o f t h e U. S. Department o f Energy C o n t r a c t No. W-7405-Eng-48, by U. S.

Department o f Energy ( O f f i c e o f Basic Energy Sciences) C o n t r a c t No. DE-AC02- 76CH00016 and by U. S. Army Research O f f i c e C o n t r a c t No. DAAL 03-86-K-0085.

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