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THE SURFACE SENSITIVITY OF FLUORESCENCE EXAFS AT REFLECTION CONDITIONS
S. Affrossman, S. Doyle, G. Lamble, M. Morris, K. Roberts, D. Sheen, J.
Sherwood, R. Oldman, D. Hall, R. Davey, et al.
To cite this version:
S. Affrossman, S. Doyle, G. Lamble, M. Morris, K. Roberts, et al.. THE SURFACE SENSITIVITY
OF FLUORESCENCE EXAFS AT REFLECTION CONDITIONS. Journal de Physique Colloques,
1986, 47 (C8), pp.C8-167-C8-171. �10.1051/jphyscol:1986831�. �jpa-00226153�
THE SURFACE SENSITIVITY OF FLUORESCENCE EXAFS AT REFLECTION CONDITIONS
.
AFFROSSMAN, S. DOYLE, G.M. LAMBLE, M.A. MORRIS(",.J. ROBERTS, D.B. SHEEN, J.N. SHERWOOD, R.J. OLDMAN*, D. HALL*, .J. DAVEY* and G.N. GREAVES**
Department of Pure and Applied Chemistry, University of Strathclyde, GB-Glasgow G1 IXL, Great-Britain
* I C I plc, The Heath, GB-Runcorn WA7 4 Q E , Great-Britain
* * S E R C Daresbury Laboratory, Daresbury, GB-Warrington WA4 4AD, Great-Britain
ABSTRACT
The development o f r e f l e c t i o n EXAFS methods h a s l e d t o t h e t e c h n i q u e becoming a p o t e n t i a l l y i m p o r t a n t t o o l f o r t h e s t u d y o f s o l i d s u r f a c e s . Here we p r e s e n t t h e r e s u l t s f o r t h i n f i l m s o f s i l v e r s u p p o r t e d on q u a r t z
s u b s t r a t e s . Two e x p e r i m e n t a l measurements can be used t o c o l l e c t r e f l e c t i o n EXAFS d a t a , i . e . d e t e c t i o n o f 1 ) t h e e m i t t e d f l u o r e s c e n c e o r 2 ) t h e r e f l e c t e d X-ray beam. Both d e t e c t i o n methods have been employed i n t h i s work and, o f che two, t h e l a t t e r method h a s been shown t o b e more s u r f a c e s e n s i t i v e . F l u o r e s c e n c e is l e s s s o because o f c o n t r i b u t i o n s from d e e p l y p e n e t r a t i n g X-rays which a r e n o t i n v o l v e d i n t h e r e f l e c t i o n p r o c e s s . These e f f e c t s can b e u n d e r s t o o d by c o n s i d e r i n g t h e dynamical t h e o r y o f X-ray s c a t t e r i n g which is c o n s i s t e n t w i t h o u r e x p e r i m e n t a l d a t a . The r e s u l t s from Ag f i l m s i n d i c a t e t h a t , under a t m o s p h e r i c c o n d i t i o n s , t h e m e t a l s u r f a c e is c o v e r e d w i t h a t h i n l a y e r o f o x i d e . Additonal d a t a s u g g e s t s t h a t t h e s u r f a c e c o a t i n g is 2 monolayers o f o x i d e , t h e r e f l e c t i o n EXAFS d a t a s u g g e s t s t h a t t h i s c o a t i n g h a s a s t r u c t u r e s i m i l a r t o Ago.
INTRODUCTION
Extended X-ray a b s o r p t i o n F i n e S t r u c t u r e (EXAFS) h a s been v e r y f r e q u e n t l y shown t o b e a most u s e f u l t e c h n i q u e i n m a t e r i a l s c i e n c e [ I ] . There h a s been an e v e r i n c r e a s i n g i n t e r e s t i n t h e s u r f a c e s o f m a t e r i a l s a l t h o u g h t h e
t e c h n i q u e s a p p l i c a b l e t o t h e i r s t u d y a r e g r e a t l y l i m i t e d . I n 1976 i t was p o i n t e d o u t [ 2 ] t h a t t h e t e c h n i q u e o f EXAFS c o u l d b e a p p l i e d t o t h e s t u d y o f
(')to whom correspondence should be addressed
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986831
JOURNAL
DE
PHYSIQUEs u r f a c e s . T h i s l e d t o t h e d e v e l o p m e n t o f t h e s u r f a c e a n a l o g u e t e c h n i q u e , SEXAFS. M e a s u r e m e n t s a r e t a k e n by c o l l e c t i n g t h e ' b y p r o d u c t s ' o f t h e p r o c e s s o f a b s o r p t i o n ( i . e . t h e e l e c t r o n o r f l u o r e s c e n c e y i e l d s ) a n d s u r f a c e s e n s i t i v i t y i n t r o d u c e d by m o n i t o r i n g t h e ' a d s o r b a t e ' e d g e . G e n e r a l l y s p e a k i n g t h i s ' c o n v e n t i o n a l ' a p p l i c a t i o n o f t h e t e c h n i q u e t o s u r f a c e s is l i m i t e d t o s t u d i e s o f t h e g a s / s o l i d i n t e r f a c e . An a l t e r n a t i v e t o t h i s h a s b e e n d e v e l o p e d w h e r e a d v a n t a g e i s t a k e n o f t h e s u r f a c e s e n s i t i v i t y o f ' t o t a l l y e x t e r n a l l y r e f l e c t e d ' X-rays C31. T h i s o f f e r s e n o r m o u s a d v a n t a g e s i n s o f a r as i t c a n be a p p l i e d t o l i q u i d / s o l i d and s o l i d / s o l i d interfaces, i n a d d i t i o n t o t h e g a s / s o l i d i n t e r f a c e [4].
EXPERIMENTAL
The e x p e r i m e n t s were p e r f o r m e d o n w i g g l e r beam l i n e a t SERC D a r e s b u r y L a b o r a t o r y . A s c h e m a t i c o f t h e e x p e r i m e n t a l s e t - u p 1s shown i n f i g u r e 1 . U s i n g s l i t s Si a n d S g t o d e f i n e a c o l l i m a t e d beam (3Upm w i d t h ) i n c i d e n t a t t h e s a m p l e , t h e t o t a l l y r e f l e c t e d X-ray beam was d e t e c t e d . The c r i t i c a l a n g l e a t t h e s i l v e r K-edge was f o u n d t c b e 0.125O. B o t h t h e r e f l e c t e d beam
( m e a s u r e d a t i o n chamber 12) a n d t h e f l u o r e s c e n t wave ( m e a s u r e d a t i o n chamber 11) c a r r y EXAFS i n f o r m a t i o n .
RESULTS AND DISCUSSION
A t y p i c a l r e f l e c t i o n EXAFS (REFLEXAFS) s p e c t r u m is shown i n f i g u r e 2.
T h i s was r e c o r d e d a t B/Bc = 0 . 7 , w h e r e B i s t h e i n c i d e n t a n g l e a n d Bc t h e c r i t i c a l a n g l e . A l l d a t a w e r e t a k e n i n a t m o s p h e r e f r o m e v a p o r a t e d s i l v e r f i l m s . X-ray 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 (XPS) r e v e a l e d a n o x i d e c o v e r i n g o f a b o u t 2 m o n o l a y e r s a t t h e s u r f a c e . F i g u r e 3 shows a F o u r i e r t r a n s f o r m o f t h i s s p e c t r u m , t o g e t h e r w i t h t h o s e o f d a t a t a k e n i n f l u o r e s c e n c e g e o m e t r i e s w i t h v a r i o u s p h o t o n i n c i d e n c e a n g l e s . A l l t h r e e t r a n s f o r m s a r e v e r y s i m i l a r . Above t h e c r i t i c a l a n g l e , 8/6, = 2 , p e n e t r a t i o n o f t h e s a m p l e is l a r g e a n d t h e t h i n o x i d e c o v e r i n g is n o t d e t e c t e d . The e x p e c t e d i n c r e a s e i n s u r f a c e sensitivity a s t h e c r i t i c a l a n g l e i s a p p r o a c h e d is s t i l l n o t enough t o a l l o w s i g n i f i c a n t o x i d e c o n t r i b u t i o n ; t h e d a t a shows l i t t l e d i f f e r e n c e . However, when compared w i t h f i g u r e 3 a ) , d i f f e r e n c e s c a n b e o b s e r v e d , t h e m o s t s t r i k i n g f e a t u r e b e i n g t h e a p p e a r a n c e o f a s h o u l d e r a t a r o u n d 3 . 1 8 . T h i s p e a k
c o r r e s p o n d s c l o s e l y w i t h t h e ' n o n - p h a s e s h i f t c o r r e c t e d ' Ag-Ag d i s t a n c e i n Ago a n d t h u s i n d i c a t e s t h e g r e a t e r s u r f a c e s e n s i t i v i t y on d e c r e a s i n g t h e i n c i d e n t a n g l e b e l o w t h e c r i t i c a l a n g l e . The t h e o r e t i c a l F o u r i e r t r a n s f o r m i s shown w i t h t h a t o f t h e e x p e r i m e n t i n f i g u r e 4.
and d i f f r a c t e d ( i n t h i s c a s e r e f l e c t e d ) waves, r e s u l t i n g i n a s t a n d l n g wave o f v e r y low p e n e t r a t i o n d e p t h s . Thus b o t h f l u o r e s c e n c e and r e f l e c t e d S i g n a l s s h o u l d c o n t a i n s u r f a c e i n f o r m a t i o n ( t h e o r e t i c a l models a r e c u r r e n t l y b e i n g developed a t S t r a t h c l y d e U n i v e r s i t y ) . For r e a l s a m p l e s , where p e r f e c t r e f l e c t i v i t i e s a r e n o t o b s e r v e d , f l u o r e s c e n c e i s a l s o g e n e r a t e d by d e e p l y
p e n e t r a t i n g i n c i d e n t X-rays. For t h e i n c i d e n t wave, s u r f a c e s e n s i t i v i t y can be e s t i m a t e d from, I/I, = ex;[-*t/sin81 where t h i n B i s t h e e f f e c ' i v e e s c a p e d e p t h i n t h e sample ( 8 b e i n g t h e a n g l e o f i n c i d e n c e w i t h t h e s u r f a c e p l a n e ) a n d - i s t h e l i n e a r a b s o r p t i o n c o e f f i c i e n t . Because o f t h e i n t e r f e r e n c e between r e f l e c t e d and i n c i d e n t r a d i a t i o n a t a n g l e s c l o s e t o t h e c r i t i c a l a n g l e , & i s around t w i c e t h e v a l u e normally r e c o r d e d . Because o f t h e l / e a t t e n u a t i o n o f s i g n a l , t can be c a l c u l a t e d from t = sind/.x. U s i n g d = 238.5, we o b t a i n an e f f e c t i v e p e n e t r a t i o n d e p t h o f a b o u t 9008. Because a t t e n u a t i o n o f t h e e v a n e s c e n t wave a l s o o c c u r s , t h e a c t u a l p e n e t r a t i o n d e p t h i s somewhat l e s s t h a n t h i s ; a 10x o x i d e f i l m would t h u s produce n e g l i g i b l e c o n t r i b u t i o n t o t h e s i g n a l , a l t h o u g h some may be e x p e c t e d from f l u o r e s c e n c e c r e a t e d a t r e f l e c t i o n . For t h e r e f l e c t e d wave t h e a t t e n u a t i o n d e p t h a t @/Be = 0.7 i s p r e d i c t e d by dynamical t h e o r y [ 5 ] and g i v e n by t h e e x t i n c t i o n f a c t o r < ,
Where
3
i s t h e d i s t a n c e i n t h e s a m p l e , r = Y e pV ,
K is t h e i n c i d e n t wave v e c t o r and F a s c a t t e r i n g term. The t e r m s f o r a r e V,
t h e u n i t c e l l volume andre
t h e c l a s s i c a l e l e c t r o n r a d i u s . A t t h e Ag e d g e X = 0.568. The a t t e n u a t i o n o f t h e l / e s i g n a l t h e n r e s u l t s i n a n e s c a p e d e p t h o f around 108.We might t h e n s u s p e c t a l a r g e c o n t r i b u t i o n from t h e o x i d e l a y e r .
CONCLUSIONS AND SUMMARY
Use o f EXAFS under r e f l e c t i o n c o n d i t i o n s h a s been shown t o be s u r f a c e s e n s i E i v e . By r e c o r d i n g i n t e n s i t i e s o f r e f l e c t e d X-rays, t h e r e i s
c o n s i d e r a b l e enhancement o f d e p t h s e n s i t i v i t y when compared t o f l u o r e s c e n c e X-ray measurements. For Ag f i l m s , a two monolayer c o v e r i n g o f t h e s i l v e r o x i d e can be observed a t t h e s u r f a c e . The s t r u c t u r e o f t h i s f i l m a p p e a r s t o be v e r y s i m i l a r t o t h a t o f Ago, a s can b e s e e n from t h e F o u r i e r t r a n s f o r m shown i n f i g u r e 4 .
We have shown t h a t t h i s t e c h n i q u e i s a p p l i c a b l e t o t h e d i r e c t o b s e r v a t i o n o f t h e s o l i d / g a s i n t e r f a c e . Not o n l y d o e s t h e t e c h n i q u e a v o i d t h e e x p e r i m e n t c o m p l i c a t i o n s o f t h e SEXAFS t e c h n i q u e b u t i t a l s o surmounts t h e s o - c a l l e d
JOURNAL DE PHYSIQUE
" p r e s s u r e - g a p " between Ultra-High-Vacuum s u r f a c e a n a l y s i s and t h e t r u e r e a c t i o n c o n d i t i o n s o f a c a t a l y t i c p r o c e s s .
REFERENCES
1 . S e e f o r e x a m p l e 'EXAFS S p e c t r o s c o p y ' e d s : B. K . Teo a n d D. C. J o y (Plenum P r e s s , New York 1 9 8 2 ) .
2. P. A. Lee, Phys. Rev. B 72, 5261 (19761.
3 . P. Rabe a n d R . H a e n s e l , Advances i n S o l i d S t a t e P h y s i c s 2 0 , 4 3 ( 1 9 8 0 ) .
4. T h i s is d i s c u s s e d by t h e a u t h o r s i n a p a p e r t o be p r e s e n t e d a t t h e 8 t h I n t e r n a t i o n a l C o n f e r e n c e o f C r y s t a l Growth ( Y o r k , J u l y 1 9 8 6 ) .
5 . B. W. B a t t e r m a n and H. C o l e , Rev. Mod. P h y s i c s 3 6 , 681 ( 1 9 6 4 ) .
from synchrotron
Y
F i g . 1 E x p e r i m e n t a l a r r a n g e m e n t f o r r e f l e x a f s e x p e r i m e n t s . C i s t h e s a m p l e a n d J1 and J2 t o p r e s e n t c o l l e c t i o n o f t h e s t r a i g h t t h r o u g h beam. Not shown i s a n i o n chamber i m m e d i a t e l y a f t e r Sc t o measure i n c i d e n t r a d i a t i o n i n t e n s i t y .
7
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F i g 2 -
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T y p i c a l r e f l e x a f s s p e c t r u m ( r e f l e c t i o n g e o m e t r y ) . The v e r t i c a l s c a l e is a n a r b i t r a r y l i n e a r i n t e n s i t y s c a l e , t h e h o r i z o n t a l s c a l e i n e l e c t r o n v o l t s .
25500 26000 26500
F ~ i e r t r a n s f o r m s o f r e f l e x a f s d a t a ( a ) i n r e f l e c t i o n a t @ / Q ~ = 0.7 ( b ) ( c ) ( d ) i n f l u o r e s c e n c e a t @/@, = .7, 1 , 2 r e s p e c t i v e l y . The h o r i z o n t a l s c a l e i s i n A.
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i s t h e e x p e r i m e n t a l d a t a a n d t h e t h e o r e t i c a l f i t .2 4 6 8
F i g . 4
