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SURFACE EXTENDED X-RAY ABSORPTION FINE STRUCTURE STUDY ON (2x1) OXYGEN ON Ni(110)
U. Döbler, L. Wenzel, D. N. Arvanitis, K. Baberschke
To cite this version:
U. Döbler, L. Wenzel, D. N. Arvanitis, K. Baberschke. SURFACE EXTENDED X-RAY ABSORP-
TION FINE STRUCTURE STUDY ON (2x1) OXYGEN ON Ni(110). Journal de Physique Colloques,
1986, 47 (C8), pp.C8-473-C8-478. �10.1051/jphyscol:1986889�. �jpa-00226220�
JOURNAL DE PHYSIQUE
Colloque C8, supplgment au n o 1 2 , Tome 47, decembre 1986
SURFACE EXTENDED X-RAY ABSORPTION FINE STRUCTURE STUDY ON (2x1) OXYGEN ON Ni(ll0)
U . D ~ B L E R ' ' ) , L. WENZEL, D. ARVANITIS and K. BABERSCHKE Institut fiir Atom- und Festkorperphysik, Freie Universitdt Berlin, Arnimalfee 14, D-1000 Berlin 3 3 , F.R.G.
Resume:
-
La c h e m i s o r p t i o n d e5
1 L d'oxygene d 470 K s u r l a s u r f a c e o u v e r t e ( 1 10) du N i donne une s u r s t r u c t u r e (2x1) en DEL. C e c i e s t admis d ' e t r e du a u n e r e c o n s t r u c t i o n i n d u i t e pour l ' a d s o r b a t . On a t r o u v e que l e s d i s t a n c e s 0-Ni p r e m i e r e t second v o i s i n s s o u t 1 . 8 5 ( 3 ) e t 1 . 9 6 ( 8 )8
r e s p e c t i v e m e n t . La d e t e r m i n a t i o n de s i p e t i t e s d i f f e r e n c e s aux l o n g u e u r s de l i a s o n e s t rendue p o s s i b l e g r a c e d l a p o l a r i s a t i o n l i n e a i r e du champ g l e c t r i q u e8 .
P a r l e r a p p o r t d e s a m p l i t u d e s pour d i f f e r e n t s a n g l e s azimutaux e t p o l a i r e l a g e o m e t r i e d ' a d s o r p t i o n e s t t r o u v e e d ' e t r e s u r un " l o n g b r i d g e " s i t e s u i v a n t [ l o o ] , l ' a t o m e d l 0 s e t r o u v a n t 0.568
au d e s s u s du p l a n d e t e r m i n e p a r l e c e n t r e d e s atomes de N i de l a p r e m i s r e couche. La s u r f a c e de N i s e r e c o n s t r u i t s u i v a n t l e models "saw t o o t h " . Un deusidme maximum de l a t r a n s f o r m e e de F o u r i e r pour E s u i v a n t [OOI] e s t d i s c u t 6 . L e s modeles de r e c o n s t r u c - t i o n s o n t d i s c u t g s e t compargs a v e c l e systeme O(2x1) s u r du C u ( l l 0 ) . Un c a l c u l de d i f f u s i o n m u l t i p l e a u v o i s i n a g e du s e u i l d ' a d s o r p t i o n(NEXAFS), mesur& s u r l e m&me g c h a n t i l l o n e s t t r o u v e an a c c o r d a v e c l e r g s u l t a t de 1' a n a l y s e SEXAFS.
A b s t r a c t :
-
Chemisorption of5
1 L oxygen on t h e open N i ( l l 0 ) f a c e a t 470 K r e s u l t s i n a (2x1) LEED p a t t e r n . T h i s i s b e l i e v e d t o b e o r i g i n a - t e d by an a d s o r b a t e i n d u c e s r e c o n s t r u c t i o n of t h e N i s u b s t r a t e . The 0-
N i f i r s t and second n e a r e s t n e i g h b o u r d i s t a n c e s habe been d e t e r - mined t o b e 1 . 8 5 ( 3 )8
and 1 . 9 6 ( 8 )8 ,
r e s p e c t i v e l y . A d e t e r m i n a t i o n of such s m a l l d i f f e r e n c e s i n b o n d l e n g t h i s p o s s i b l e due t o t h e s e a r c h l i g h t c h a r a c t e r of t h e l i n e a r p o l a r i z a t i o n . From t h e a m p l i t u d e r a t i o s f o r d i f f e r e n t a z i m u t h a l and p o l a r a n g l e s o f i n c i d e n c e one d e t e r m i n e s t h e a d s o r p t i o n geometry t o b e a b r i d g e s i t e a l o n g cl00J and t h e 0 atom s i t t i n g 0.562
above t h e m i d p o i n t of t h e f i r s t l a y e r N i atoms. The N i s u b s t r a t e i s r e c o n s t r u s t e d i n t o a saw t o o t h model. A second peak i n t h e F o u r i e r t r a n s f o r m f o r E p a r a l l e l t o [OOI] i s a n a l y s e d . The models f o r r e c o n s t r u c t i o n a r e d i s c u s s e d and compared w i t h O(2x1) on C u f l l O ).
A f u l l m u l t i p l e s c a t t e r i n g c a l c u l a t i o n of t h e n e a r edge 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 (NEXAFS) measured on t h e same sample a g r e e s w i t h t h e SEXAFS a n a l y s i s .I. I n t r o d u c t i o n
The submonolayer c o v e r a g e of oxygen (2x1) on t h e r e a c t i v e s u r f a c e of N i ( 1 10) h a s been i n v e s t i g a t e d by numerous t e c h n i q u e s . however, t h e m i c r o s c o p i c p i c t u r e of t h e s i t e of c h e m i s o r p t i o n remains c o n t r o v e r s a l . S l i g h t l y l e s s t h a n 1 Langmuir (10-6 T o r r - s e c ) i s needed f o r chemisorp- t i o n of h a l f a monolayer and f o r a c h e m i c a l l y induced r e c o n s t r u c t i o n i n t o a (2x1) s u p e r s t r u c t u r e . T h i s system a l l o w s some comparison w i t h
(l)~iemens-~ystentechnik. Nonnendammallee 101. D-1000 Berlin 13. F.R.G.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986889
C8-474 JOURNAL DE PHYSIQUE
O(2x1) on C u ( 1 l C ) . For b o t h , a unique C e t e r m i n a t i o n of t h e a d s o r p t i o n s i t e and of t h e l o c a l s t r u c t u r e i n c l u d i n g v e r t i c a l and h o r i z o n t a l re- l a x a t i o n of t h e s u b s t r a t e s u r f a c e l a y e r i s of i n t e r e s t . On t h e o r e t i c a l a r o u n d s t h e r e s t i l l remain s e v e r a l open q u e s t i o n s : h%at d r i v e s t h e c h e m i c a l l y induced r e c o n s t r u c t i o n ? The f o r c e s w i l l be of s h o r t r a n g e
( f i r s t and second n e i g h b o u r s o n l y ) . Does t h i s imply a change of n.n.
c o o r 2 i n a t i o n number and symr.etry w i t h r e s p e c t t o t h e u n p e r t u r b e d (11C) s u r f a c e of t h e f c c l a t t i c e ? We s t r o n g l y want t o d i s t i n a u i s h between t e m p e r a t u r e d r i v e n r e c o n s t r u c t i o n s (of c l e a n s u r f a c e s , f o r example) and t h e " l o c a l " mechanism o f c h e m i s o r p t i o n . F o r t h e l a t t e r q u e s t i o n t h e c h e n ? i s o r p t i o n b i n d i n g e n e r q i e s f o r a l l t h r e e s u r f a c e s ( I C O )
,
( I l l ) , and ( 1 10) of 0 on N i would b e u s e f u l . U n f o r t u n a t e l y o n l y a p o l y c r i s t a l - l i n e a v e r a a e of 4 t o 5 eV i s known. A t p r e s e n t , t h e o r y i s n o t a b l e t o c a l c u l a t e r e l i a b l y t h e s m a l l changes of t h e b i n d i n a eneruy ( f e w t e n t h s of a n eV f o r d i f f e r e n t f a c e s ),
t h e e f f e c t of a d i f f e r e n t c o o r e i n a t i o n number and i t s c o n s e a u e n c e s f o r t h e l o c a l bonding g e o r e t r y . A f u r t h e r q u e s t i o n remains open: Fow i s t h e d i f f e r e n c e i n b u l k o x i d a t i o n f o r CuO, Cu20 and N i O e x p l a i n e d ? What c a u s e s i n EXFFS t h e l a r a e d i f f e r e n c e i n pahse s h i f t f o r 0-Ni b a c k s c a t t e r e r w i t h r e s p e c t t o C-Cu b a c k s c a t t e - r i n q ?The p r e s e n t SEXAFS work a d d s some more e x p e r i m e n t a l e v i d e n c e f o r t h e bonding g e o m e t r i e s . I n p a r t i c u l a r f o r 0(2x! ) on Ni ( I 10) r e c e n t s c a n n i n g - t u n n e l i n g microscopy (STM) e x p e r i m e n t s / I / i n d i c a t e a "saw t o o t h " t y p e of r e c o n s t r u c t i o n and (2x1) i s l a n d s w i t h
2
100 oxygen atoms. Consequently we have u s e d t h e i d e n t i c a l F i s i n g l e c r y s t a l of t h e IBM-group and f o l l o w e d t h e i r p r o c e d u r e of oxygen-dosing. The d e t a i l s of t h e e x p e r i m e n t a r e discussed. i n Ref. / 2 / . I n Sec. 11. we summarize t h e a n a l y s i s of n . n . c o n f i g u r a t i o n . I n Sec. 111. we d i s c u s s a second peak "B" i n t h e F o u r i e r t r a n s f o r m f o r E [0@1]. T h i s i s a t t r i b u t e d t o t h e b a c k s c a t t e r i n g of a d j a c e n t 0-atoms. A d i f f e r e n t mean f r e e p a t h f o r t h e p h o t o e l e c t r o n wave above t h e s u r f a c e w i l l be used. I n Sec. I V . w e compare O / N i a n a l y s i s w i t h r e c e n t SEXAFS work f o r O ( 2 x I ) on Cu(1 I @ ) by Bader e t a l . / 3 / . F i n a l l y i n Sec. V a summary and a comparison w i t h a f u l l m u l t i p l e s c a t t e r i n g c a l c u l a t i o n and o u r Near Edge X-ray A b s o r p t i o n S t r u c t u r e (REXAFS) w i l l b e g i v e n .11. Local bonding geometry of t h e 0 atoms
F i g . 1 shows t h e EXAFS o s c i l l a t i o n s above t h e 0 K-edge f o r d i f f e r e n t a z i m u t h a l a n g l e @ ( a : ~ / / [ 1 1 0 1 and b: E / / [001]) and f o r a p o l a r a n g l e of i n c i d e n c e of 0=45O. The spectrum i n F i g . I b w i t h an edge jump of approx. 6 % f o r e f f e c t i v e l y 0.35 ML i s c n e w i t h l a r g e s t s i g n a l - t o - n o i s e r a t i o s f o r subnonolayer SEXAFS. T h i s a l s o r e s u l t s i n a s m a l l n o i s e i n t h e F o u r i e r t r a n s f o r m ( F i q . 2 ) . The a m p l i t u d e r e d u c t i o n f o r E / / [ I I Q ] w i t h r e s p e c t t o E / / [0011 i s a f a c t o r of 5. However, t h e d i s t a n c e de- t e r m i n e d by u s i n g t h e e x p e r i m e n t a l phase s h i f t from b u l k N i O i s o n l y s l i g h t l y l a r g e r ( 1 -96
8)
f o r t h e former, compared t o 7 . 8 52
f o r t h e l a t t e r . Such a s m a l l change of AR ?j 0.18
can o n l y b e d e t e r m i n e d by v i r t u e of t h e a z i m u t h a l a n g u l a r dependence, s e e F i g . 2. T h i s @-depen- dence and t h e two v a l u e s f o r R s t r o n g l y favors a l o n g b r i d g e s i t e w i t h t h e 0-atom s i t t i n g above t h e f i r s t l a y e r of N i / 4 / . I n F i g . 3 , 3 model- s t r u c t u r e s a r e drawn. A l l have i n common t h e b r i d g e s i t e p o s i t i o n . The v e r y s e n s i t i v e p o i n t i n a SEXAFS e x p e r i m e n t i s t h a t f o r t h e saw t o o t h model i n [!lo] d i r e c t i o n oxygen h a s o n l y one b a c k s c a t t e r e r , whereas f o r m i s s i n g row a n d b u c k l e d s u r f a c e i n t h e [0011 and ~ I I O ] d i r e c t i q n t h e r e remain always 2 b a c k s c a t t e r e r s . Without a changg of R o r c o s a ohe e x p e c t s a r e d u c t i o n i n t h e SEXAFS a m p l i t u d e by a f a c t o r of 2 i n going from@=oO
t o @=go0. Taking i n t o a c c o u n t t h e change i n R , i n c o s 2 a and u s i n g an e f f e c t i v e mean f r e e p a t h ofX 2
58 one f i n d s p r e - c i s e l y t h e e x p e r i m e n t a l v a l u e of 0.19, shown i n Tab. 1 .A d e t a i l e d s i m u l a t i o n o f t h e EXAFS o s c i l l a t i o n t a k i n g i n t o c o n s i d e r a - t i o n v e r t i c a l r e l a x a t i o n and t i l t o f t h e oxygen w i t h r e s p e c t t o t h e
F i g . 1: P a r t i a l e l e c t r o n y i e l d ( u s i n g 350 eV r e t a r d i n g v o l t a g e ) a f t e r n o r m a l i z a t i o n . c j = 0 c o r r e s p o n d s t o t h e [OOI] a x i s .
F i g . 2: F o u r i e r t r a n s f o r m of spectrum i n F i g . l a and b. Right hand scale for the dashed line.
Irnl
T
F i g . 3: D i f f e r e n t models t r u c t u r e s f o r (2x1) r e - c o n s t r u c t i o n on a (110) f c c s u r f a c e . . - -
saw tooth missing row buckled surface
s u r f a c e normal ( s e e F i g . 2 i n Ref. 2 ) y i e l d s t h e f o l l o w i n g r e s u l t (Tab. 1 ) : A r i g i d N i - s u b s t r a t e ( n o v e r t i c a l r e l a x a t i o n ) y i e l d s a tilt a n g l e of 6 = 33O. Keepinq 6 =
o0
one n e e d s 23% r e l a x a t i o n between t h e 1 . and 2. N i l a y e r . An i n t e r m e d i a t e f i t w i t h 1 0 % i n v e r t r e l a x a t i o n and 6 = 20° f i t s t h e e x p e r i m e n t a l c u r v e r e a s o n a b l y w e l l . T h i s tilt a n g l e 6 t o w a r d s t h e (100) f a c e t of t h e s a w t o o t h may b e i n t e r e s t i n g , i f one assumes t h a t 0 c h e m i s o r p t i o n p r e f e r s a (100) f a c e .J O U R N A L D E PHYSIQUE
TABLE 1: E x p e r i m e n t a l v a l u e s f o r R , a m p l i t u d e r a t i o s , and c a l c u l a t e d a m p l i t u d e v a l u e s f o r d i f f e r e n t models and o r - i e n t a t i o n s . ST and MR de- n o t e sawtooth and missinq-row models, r e s p e c t i v e l y .
amp1 . r a t i o
e x p t . ( a ) 6=0° (b) 6=20° ( c ) 6=33O
Orien- amplitude r e l a x : 23% r e l a x : 10% r e l a x : 0 %
t a t i o n 0
~ ( 2 )
r a t i o ST MR ST MR ST MR111. Peak "B" i n t h e F o u r i e r transform.
So f a r , w e have a n a l y s e d peak "A" o n l y , i n F i g s . 4 and 1. E v i d e n t l y a second o s c i l l a t i o n o r peak c a n be deduced from t h e e x p e r i m e n t . The back- t r a n s f o r m a t i o n ~ ( k ) .k i n F i g . 4b was c a r r i e d o u t f o r 3 windowfunctions, o n l y peak "A" (See S e c . I I ) , o n l y peak "B" y i e l d s t h e h i g h f r e q u e n c y o s c i l l a t i o n i n k , and t h e sum of t h e two ( d a s h e d d o t t e d l i n e ) i n F i g . 4b. It i s e v i d e n t t h a t t h i s f o l l o w s t h e e x p e r i m e n t a l c u r v e s a l m o s t p e r f e c t l y . The s m a l l asymmetry i n t h e e x p e r i m e n t a l o s c i l l a t i o n i s p e r f e c t l y reproduced. A p r i o r i 0 and N i a r e a s b a c k s c a t t e r e r p o s s i b l e . Using t h e e x p e r i m e n t a l l y d e t e r m i n e d 0-0 phase of +(k)=1.91-0.56.k-B /11/, t h e d i s t a n c e i s d e t e r p i n e d t o b e 3 . 3 7 ( 1 2 ) a . T h i s i s c l o s e t o t h e
l a t t i c e c o n s t a n t a = 3.52
8
and would s u p p o r t t h e p i c t u r e t h a t oxygen i s a l l i g n e d ? l o n u t h e [0011 d i r e c t i o n w i t h p e r i o d i c i t y " 1 " . Such an a n a l y s i s h a s t o b e t a k e n w i t h c a u t i o n , s i n c e t h e o u t - o f - p l a n e a n g l e f o r t h e 0-Ni-0 t r i a n s l e i s o n l y 17O. F o r such a s m a l l v a l u e a m u l t i p l e s c a t t e r i n g v i a t h e N i i s v e r y l i k e l y and l i m i t e s t h e a c c u r a c y of t h e d e t e r m i n a t i o n f o r t h e 0-0 d i s t a n c e . The 0-Ni b a c k s c a t t e r i n g phase y i e l d s a d i s t a n c e of 3 . 4 8 ( 1 2 ) 2 . T h i s i s c l o s e r t o t h e l a t t i c e c o n s t a n t , b u t it i s i n none of t h e models ( F i q . 3 ) i n agreement t o an e x p e c t e d N i p o s i t i o n . F o r t h e ~ ~ ~ n . n . one c a l c u l a t e s a d i s t a n c e of 4.16W. As i m i l a r a n a l y s i s h a s been c a r r i e d o u t by A. Puschmann f o r 0 ( 2 x l ) C u ( l l O ) '
/5/.
I f we f o l l o w t h e p i c t u r e of 0-0 s c a t t e r i n g , we now have t o c a l c u l a t e t h e a m p l i t u d e r a t i o f o r peak B w i t h r e s p e c t t o peak A . At k = 5.2 R-I
,
t h e maximum of x ( k ) - k , F ( k ) = 0 . 3 9 0 5 ( B ) and 0.4151 (A) /12/. The R - ~ term y i e l d s a r a t i o of 0.298 f o r B/A. F i n a l l y a mean f r e e p a t h o f A = 5
2
( a s i n t h e b u l k ! ) a g a i n a i v e s a r e d u c t i o n of 0.542. I n t o t a l , t h e B/A a m p l i t u d e r a t i o i s d e t e r m i n e d t o b e 0.13. The e x p e r i m e n t ( F i g . 2) y i e l d s 0.25, a f a c t o r of two t o l a r g e . However, i t i s u n r e a l i s t i c t o choose such a s h o r t mean f r e e p a t h f o r t h e p h o t o - e l e c t r o n wave above t h e s u r f a c e l a y e r . Very l i t t l e i s known a b o u t r e a l A-values f o r a pho- t o e l e c t r o n t r a v e l l i n g p a r a l l e l t o t h e s u b s t r a t e s u r f a c e . I£ one d o u b l e s t h e p a t h l e n g t h ( A = 10
8 ,
s t i l l i s a s h o r t p a t h ) , one g a i n s t h e f a c - t o r of two, i n a c c o r d w i t h t h e e x p e r i m e n t .I n c o n c l u s i o n : I t i s v e r y l i k e l y , t h a t peak "B" i s due t o t h e back- s c a t t e r i n g of t h e oxygen n . n . a l o n g [OOI]. But t h i s s e c t i o n demonstra- t e s some l i m i t a t i o n s of SEXAFS : m u l t i p l e s c a t t e r i n g , phase t r a n s f e r a b i - l i t y , and d i f f e r e n t 1 - v a l u e s a s f u n c t i o n of s h e l l , o r i e n t a t i o n and k- v e c t o r . Moreover, r e c e n t l y Foubin e t a l . / 6 / have shown t h e e f f e c t o f a n a n i s o t r o p i c s u r f a c e Debye-Waller f a c t o r . T h i s c o m p l i c a t e s t h e compari- son of r e l a t i v e a m p l i t u d e s , t o o .
F i g . 4 : F o u r i e r t r a n s f o r m ( s a m e a s f u l l l i n e i n F i g . 2 ) and back- t r a n s f o r m f o r 3 d i f f e r e n t window f u n c t i o n s .
I V . The second peak i n t h e F o u r i e r t r a n s f o r m i n O / N i and O/Cu
I n t h e i n t r o d u c t i o n we have d i s c u s s e d t h e s i m u l a r i t i e s between t h e two s y s t e m s . Both systems form a (2x1) s u p e r s t r u c t u r e under s l i g h t l y d i f f e r e n t c o n d i t i o n s on t h e ( 1 10) s u r f a c e . Both systems have been s t u d i e d by t h e SEXAFS t e c h n i q u e / 2 , 7 / . I n a r e c e n t experiment improofed SEXAFS d a t a on O(2x1) on C u ( l l 0 ) a t T = 100 K and under b e t t e r c o n d i - t i o n s of t h e s t o r a g e r i n p BESSY (compared w i t h / 7 / ) w e r e publishedl3/.
I n F i g . 5 we r e p r o d u c e t h e s e r e s u l t s : A second peak B* i n t h e FFT i s i d e n t i f i e d f o r E// [ I I O ] . For c o n s i s t e n c y of t h e SEXAFS t e c h n i q u e and t h e q u e s t i o n of t h e c h e m i s o r p t i o n f o r 1 / 2 ML on N i l 1 10) and Cu ( 11 0 ) we have t o f a c e t h e q u e s t i o n s : ( i ) F o r O / N i t h e second b a c k s c a t t e r e r a l o n q t h e [OOI] i s oxygen, c a n it be i d e n t i f i e d f o r O/Cu i n F i g .
5?
dii'
F o r O/Cu a second peak B* i s due t o t h e 4th n .n. of Cu. The 3r n.n.
of Cu i s m i s s i n g , t h e a u t h o r s p r o p o s e a m i s s i n g row model f o r 0 ( 2 x 1 ) / C u ( l l 0 ) . I s t h e peak B* d e t e c t a b l e f o r t h e O / N i system?
( i ) We compare t h e peak B i n F i g . 4 w i t h t h e peak i n d i c a t e d by an a r r o w i n F i g . 5 . We f e e l t h a t t h e y b o t h a g r e e . The oxygen b a c k s c a t t e - r i n g i s a l s o s e e n f o r O/Cu, however t h e s i g n a l - t o - n o i s e r a t i o i s s m a l l e r by a f a c t o r of two. ( i i ) Peak B* i n F i q . 5 i s due t o t h e back- s c a t t e r i n g of t h e 4th n.n. i n t h e m i s s i n g row model. T h i s i s t h e s e - cond n e a r e s t hashed b a l l s e e n from oxygen i n F i g . 3 ( m i s s i n g r o w ) . L e t u s assume t h a t t h e a n a l y s i s of Sec. I1 i s c o r r e c t , namely a saw t o o t h model f o r O / N i . From F i g . 3 we l e a r n , t h a t t h e r e e x i s t s o n l y one i n s t e a d of two Ni-atoms i n t h e 4 t h s h e l l . Consequently t h e d i s a p p e a r - e n c e o f t h e second peak "B*" f o r 0 = 90°, @ = 90°, i n F i g . 2 i s a v e r y s t r o n g e v i d e n c e t h a t t h e t y p e o f r e c o n b t r u c t i o n f o r O / N i and O/Cu i s d i f f e r e n t . We i n d i c a t e d t h e B* p o s i t i o n i n F i g . 2 by an arrow.
I n c o n c l u s i o n : Both SEXAFS e x p e r i m e n t s on O/Cu and O / N i a r e c o n s i s t e n t , i f one assumes d i f f e r e n t t y p e s of r e c o n s t r u c t i o n , namely saw t o o t h f o r O / N i and m i s s i n g row f o r O/Cu.
V. Summary
F o r t h e two model systems of 1 / 2 ML O / N i and O/Cu t h e SEXAFS t e c h n i q u e h a s proven t o g i v e a u s e f u l a d d i t i o n a l i n f o r m a t i o n a b o u t t h e l o c a l bon- d i n 9 geometry. The l i m i t a t i o n s f o r t h e p r e s e n t a n a l y s i s have been d i s c u s s e d i n Sec. 111. The q u e s t i o n , why O(2x1) on N i ( l l 0 ) and C u ( l l 0 )
C8-478 JOURNAL DE PHYSIQUE
i n d u c e s a d i f f e r e n t t y p e of r e c o n s t r u c t i o n g o e s b e h i n d t h e p r e s e n t work. D i f f e r e n t mechanisms c o n t r i b u t e t o t h e b o n d i n g , d i r e c t i n t e r - a c t i o n of t h e N i 3d-band and t h e oxygen 2 p - o r b i t a l s , h y b r i d i z a t i o n be- tween t h e N i 4s4p a n d oxygen 2p o r b i t a l s , e t ~ . The C u - 3 d - o r b i t a l s a r e f u l l y o c c u p i e d i n c o n t r a s t t o t h e N i - 3 d - o r b i t a l s . I t s e e m s t o b e commonly a c c e p t e d t h a t t h e c h e m i s o r p t i o n f o r O / N i i s s t r o n g e r t h a n f o r O/Cu. T h i s may b e a n e x p l a n a t i o n f o r t h e f a c t t h a t f o r 0 on N i ( l l 0 ) i n a saw t o o t h model t h e l o c a l c o o r d i n a t i o n i s i n d e e d changed. Whereas f o r Q on C u ( l l 0 ) i n a m i s s i n ? row model / 3 / t h e l o c a l e n v i r o n m e n t r e m a i n s t h e same a s on a n u n r e c o n s t r u c t e d s u r f a c e .
I I I I I
tf10011 F i g . 5: F o u r i e r t r a n s f o r m f o r two o r l e n t a t i o n s on ( 2 x 1 ) 0 on Cu ( I 10) t a k e n from Ref. 3 .
I I 4 0 - I I
R e c e n t l y a f u l l m u l t i p l e s c a t t e r i n g (MS) c a l c u l a t i o n f o r t h e n e a r edge s t r u c t u r e o f
~ ( 2 x 2 ) O/Cu (100) /8/ and of ( 2 x 1 ) O/Cu(llO) / 9 / have b e e n u s e d t o d e t e r m i n e t h e l o c a l bonding geometry. The good agreement be- tween SEXAFS a n a l y s i s and NEXAFS-MS r e s u l t l o o k s p r o m i s i n g f o r t h i s new t o o l of l o c a l s t r u c t u r e a n a l y s i s . I n t h e same way t h e NEXAFS r e g i m e s o f o u r d a t a ( F i g . 1) were a n a l y s e d . I t t u r n s o u t t h a t t h e p o l a r and a z i m u t h a l a n g u l a r dependence i s v e r y s e n - s i t i v e on t h e t i l t of oxygen i n t h e b r i d g e p o s i t i o n . New e x p e r i m e n t s w i t h h i g h e r e n e r g y r e s o l u t i o n and b e t t e r s t a t i s t i c s would b e needed f o r a f i r m a n s w e r , however
t h e p r e s e n t a n a l y s i s
/ l o /
a g r e e s w i t h o u r SEXAFS i n t e r p r e t a t i o n u s i n g a saw t o o t h model w i t h t i l t e d oxygen atoms.8 The work was s u p p o r t e d by BMFT g r a n d n r . 05233 BB.
R e f e r e n c e s
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