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MEASUREMENT OF THE DISPERSION CURVES OF SURFACE VIBRATIONS ON Ni (100) AND Ni
(100) C (2x2) O BY ELECTRON ENERGY LOSS SPECTROSCOPY
S. Szeftel, S. Lehwald, H. Ibach
To cite this version:
S. Szeftel, S. Lehwald, H. Ibach. MEASUREMENT OF THE DISPERSION CURVES OF SURFACE VIBRATIONS ON Ni (100) AND Ni (100) C (2x2) O BY ELECTRON ENERGY LOSS SPECTROSCOPY. Journal de Physique Colloques, 1984, 45 (C5), pp.C5-109-C5-112.
�10.1051/jphyscol:1984515�. �jpa-00224133�
JOURNAL DE PHYSIQUE
Colloque C 5 , suppl6ment au n04, Tome 45, avril 1984 page C5-109
MEASUREMENT OF THE DISPERSION CURVES OF SURFACE VIBRATIONS ON Ni
( 1 0 0 )AND Ni
( 1 0 0 )C
( 2 x 2 ) 0BY ELECTRON ENERGY LOSS SPECTROSCOPY
J. S z e f t e l , S. ~ e h w a l d * and H. ~ b a c h *
CEN - SacZay, S e r v i c e de Physique des Atomes e t des Surfaces, 9 2 191 Gif -sur-Yvette Cedex, Fmnnce
* . K F A - J - I I z ~ ~ ~ , IGV, 517-JitZich, F.R. G .
RSsurni.5- Les courbes de d i s p e r s i o n de p l u s i e u r s v i b r a t i o n s de s u r f a c e ont S t 6 mesurdes s u r t o u t e l a zone d e B r i l l o u i n
B
deux dimensions dans l a d i r e c t i o n(1 10) s u r Ni(100) e t Ni(100) C (2x2) 0. Le phonon de Rayleigh a dtL observd dans l e s deux cas. Sur Ni(100) C(2x2) 0 nous avons de p l u s observe une rGso- nance e t deux v i b r a t i o n s d e l'oxygSne, perpendiculairement e t p a r a l l s l e m e n t 1 l a s u r f ace. L a d i s c u s s i o n d e s r L s u l t a t s dans l e cadre d'un modsle simple de l a dynamique du r e s e a u donne des renseignements s u r l a r e l a x a t i o n de l a s u r f a c e e t l a d i s t a n c e Ni-0.
Abstract- The d i s p e r s i o n curves of s e v e r a l s u r f a c e v i b r a t i o n s have been mea- s u r e d throughout t h e two-dimensional B r i l l o u i n zone i n t h e d i r e c t i o n (1lO)on Ni(lO0) and N i (1 00) C(Zx2)O. The Rayleigh phonon h a s been observed on both systems. Besides a resonance and two 0 - v i b r a t i o n s a s s o c i a t e d w i t h motions p e r p e n d i c u l a r and p a r a l l e l t o t h e s u r f a c e have been observed on Ni(100) C(Zx2)O. Those r e s u l t s a r e d i s c u s s e d w i t h i n t h e framework of a simple model of t h e l a t t i c e dynamics, which y i e l d s i n f o r m a t i o n upon s u r f a c e r e l a x a t i o n and t h e Ni-0 d i s t a n c e .
By c o n t r a s t
t o
most of e l e c t r o n energy l o s s spectroscopy (EELS) measurements c a r r i e d o u t s o f a r a t vanishing wave-number q of t h e d e t e c t e d v i b r a t i o n , we r e p o r t h e r e t h e EELS o b s e r v a t i o n of v i b r a t k o n a l l o s s e s a t l a r g e q on N i (100) and Ni (100)C ( 2 x 2 ) 0 throughout t h e two -dimensional B r i l l o u i n zone (B. 2). The measurement of t h e d i s p e r s i o n curve u (q) (o i s the v i b r a t i o n a l frequency) p r o v i d e s i n f o r m a t i o n about f o r c e c o n s t a n t s and i n t e r a t o m i c d i s t a n c e s a t t h e s u r f ace.
I
-
EXPERIMENTALThe N i s i n g l e c r y s t a l was prepared and cleaned according t o s t a n d a r d procedures / I / . The e q o s u r e t o oxygen was done s o t h a t i t produced s h a r p , h a l f i n t e g e r s p o t s c h a r a c t e r i s t i c of the C ( 2 x 2 ) s t r u c t u r e i n t h e e l e c t r o n d i f f r a c t i o n p a t t e r n
1 2 1 .
I n t h e EELS measurement an incoming e l e c t r o n of energy E i and wave-number k impinges on t h e s u r f a c e , emits o r absorbs a phonon (w,q) and emerges with
ds
andk
.
I n our working c o n d i t i o n s , k;, ks and t h e normal d i r e c t i o n t o t h e (100) s c r f a c e l i e i n t h e s c a t t e r i n g p l a n e . Two c o n s e r v a t i o n laws hold which r e a d , t a k i n g i n t o account t h a t hu << E i :E i - E s 2 h u = 0 (1)
where 8; and 8 a r e t h e angles of t h e incoming and outgoing beams r e s p e c t i v e l y measured from ?he normal d i r e c t i o n
.
The + o r-
s i g n i n e q . 1 r e f e r s t o a n n i h i l a - t i o n o r c r e a t i o n of a phonon, which corresponds i n t u r n i n t h e EELS spectrum t o a g a i n o r a l o s s . In a l l experiments 8 was always s e t t o Q 70° and 8 . v a r i e d by s t e p s with E i k e p t c o n s t a n t s o t h a t$
was scanned over t h e whgle B.$. i n t h e d i r e c t i o n (110) from T(q=O) t o t h e zone edge p o i n t X (q=1 .26 A l ) and t h e l o s s spectrum was recorded f o r each B i v a l u e .Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984515
C5-110 JOURNAL
DE
PHYSIQUEI1
-
THE CLEAN SURFACE : N i (100)The measurements were made a t 300 K and a t Ei = 180 and 322 eV. The spectrum recorded a t X shows ( f i g . l c ) an e l a s t i c contribution, t h e l o s s peak on i t s r i g h t s i d e and t h e gain neak l y i n g a t t h e same d i s t a n c e on t h e l e f t s i d e . As q d e c r e a s e s , w goes t o zero and both l o s s and g a i n f e a t u r e s grow a t t h e expense of t h e l o s s
-
l e s s peak which d i s a ~ p e a r s ( f i g . lb) till f i n a l l y l o s s and g a i n merge i n t o a s i n g l e
?eak ( s e e f i g . l a ) .
The frequency w has been p l o t t e d v e r s u s q i n f i g . 2 . By u s i n g symmetry considera- t i o n s and comparing t h e experimental curve with c a l c u l a t e d phonon d i s p e r s i o n d a t a 131 one i d e n t i f i e s the measured branch i n f i g . 2 t o t h e Rayleigh mode 111.
I I
ENERGY LOSS lcm-'1
F i g . 1
-
EELS s p e c t r a taken a t E; = 180 eV on N i (100).Fig. 2
-
Measured frequency w of t h e Rayleigh phonon (open c i r c l e s and squares) versus q on N i (100). The d a t a a r e f i t t e d by t h e s o l i d and dashed l i n e s a s e x p l a i n e d i n the t e x t .Each Ni-atom i s then assumed t o i n t e r a c t only w i t h i t s n e a r e s t neighbours v i a c e n t r a l f o r c e s d e s c r i b e d by t h e f o r c e c o n s t a n t Kb which i s a s s e s s e d independently from bulk phonon d i s p e r s i o n d a t a . With Kb a t hand Rahman and M i l l s
111
have c a l c u l a t e d t h e s o l i d curve. The disagreement occuring a t t h e zone edge i s l i f t e d by i n c r e a s i n g t h e f o r c e c o n s t a n t K between Ni-atoms i n t h e f i r s t and second l a y e r s by 25Z with r e s p e c t t o Kb, whic8 y i e l d s t h e dashed l i n e i n f i g . 2. Such as t i f f e n i n g of K i s consistent w i t h a c o n t r a c t i o n of t h e f i r s t Ni-plane which h a s moreover been r g p o r t e d independently by Frenken, Smeenk, Van d e r Veen from i o n blocking measurements.
111
-
THE 0-COVERED SURFACE : N i (100) C ( 2 x 2) 0A l l s p e c t r a have been taken a t 150K and a t s e v e r a l impact e n e r g i e s E because of the l a r g e q- and E i deoendence of t h e l o s s e s / Z / i l l u s t r a t e d i n f i g .
3.
The f o u r observed v i b r a t i o n a l modes ( t h e i r d i s p e r s i o n i s r e p r e s e n t e d i n f i g . 4) comprise two modes of oxygen(Oil
and 0~ i n f i g . 3, 4) and two resonances ( R and R i n f i g . 3 , 4 ) . I t has been shown i 2 / using s e l e c t i o n r u l e s t h a t t h e oxygen atom moves a tr
p a r a l l e l t o t h e s u r f a c e f o r the 0 and R modes and p e r p e n d i c u l a r t o i t f o r t h e OL-one. A t l a s t Rr has been i d e n t i i j e d t o t h e Rayleigh phonon 121.F i g . 3 - EELS s p e c t r a t a k e n o n N i (100) C ( 2 x 2) 0 a t s e v e r a l E i
-
a n d q-values.r
0.5 1.0x
q
(A-')
F i g . 4 - D i s p e r s i o n d a t a (open c i r c l e s ) o f s u r f a c e v i b r a t i o n s on N i (100) C (2 x 2) 0 . S o l i d and dashed l i n e s a r e t h e o r e t i c a l d i s p e r s i o n c u r v e s , c a l c u l a t e d a s e x p l a i n e d i n t h e t e x t . The d o t t e d l i n e r e p r e s e n t s t h e Rayleigh b r a n c h on N i ( 100)
.
Rahman, Black and M i l l s have extended t h e model used i n t h e p r e c e d i n g s e c t i o n t o f i t t h e d a t a . Two e x t r a f o r c e - c o n s t a n t s K N e O and KO0. have been i n t r o d u c e d t o a c c o u n t f o r t h e c e n t r a l i n t e r a c t i o n of e a c h oxygen w l t h i t s Ni-and 0- n e a r e s t n e i g h b o u r s r e s p e c t i v e l y . The d i s t a n c e d between t h e 0 - l a y e r and t h e f i r s t N i p l a n e mugt a l s o b e i n s e r t e d i n t o t h e c a l c u l a t i o n and h a s been a s s i g n e d t o a v a l u e of 0.88 A p r o v i d e d by SEXAFS measurements 141. The b e s t f i t d e t e r m i n e s KN. and KO0 and g i v e s t h e s o l i d l i n e s i n f i g . 4 . Then K i s matched t o reproduce
*&
Raylkigh curve and i s found s m a l l e r t h a n K
,
~h:ch i s i n d i c a t i v e o f an outwards e x p a n s i o n of t h e f i r s t N i l a y e r . consisten! w i t h a l r e a d y q u o t e d measurements by Frenken e t a l . The agreement of t h e s o l i d c u r v e s w i t h t h e d a t a i s good a p a r t from t h e o v e r a l l d i s p e r s i o n o f O_L which i s s m a l l e r t h a n t h e measured one. Upton and Goddard / 5 / have claimed t h a t d = 0.26 A and Rahman, Black and M i l l s have t r i e d t o f i t t h e d i s p e r s i o n d a t a w i t h t h a t assumption. I t i s t h e n r e q u i r e d t o i n t r o d u c e bond bending f o r c e s t o f i t t h e OL and Of/ b r a n c h e s a t T. The r e s u l t o f t h e c a l c u l a t i o n i s i n d i c a t e d by t h e dashed l i n e s i n f i g . 4 and t h e l a r g e d i s c r e p a n c y a l l o w s t o d i s c a r d t h e d-value of Upton and Goddard.
I V
-
CONCLUSIONThe measurement of s u r f a c e phonon d i s p e r s i o n on N i (100) and N i (100) C (2 x 2) 0 r e v e a l s a r e l a x a t i o n o f t h e N i s u r f a c e d i r e c t e d inwards on N i (100) and outwards on N i (100) C ( 2 x 2) 0 and s u p p o r t s t h e SEXAFS d e t e r m i n a t i o n of t h e d i s t a n c e s e p a r a t i n g t h e 0 - l a y e r from t h e s u r f a c e .
JOURNAL DE PHYSIQUE
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