THE STRUCTURE OF THE Cu(111) (√3 x √3) R30°-Cl SURFACE : A COMBINED SEXAFS AND PHOTOELECTRON DIFFRACTION STUDY

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THE STRUCTURE OF THE Cu(111) ( √ 3 x √ 3) R30°-Cl SURFACE : A COMBINED SEXAFS AND

PHOTOELECTRON DIFFRACTION STUDY

M. Crapper, C. Riley, P. Sweeney, C. Mcconville, D. Woodruff, R. Jones

To cite this version:

M. Crapper, C. Riley, P. Sweeney, C. Mcconville, D. Woodruff, et al.. THE STRUCTURE OF THE Cu(111) ( √ 3 x √ 3) R30°-Cl SURFACE : A COMBINED SEXAFS AND PHOTOELEC- TRON DIFFRACTION STUDY. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-533-C8-538.

�10.1051/jphyscol:19868100�. �jpa-00225995�

THE STRUCTURE OF THE Cu(ll1)

(G

x G ) R ~ o o - c ~ SURFACE : A COMBINED SEXAFS AND PHOTOELECTRON DIFFRACTION STUDY

M.D. CRAPPER, C.E. R I L E Y ( ' ) , P . J . J . SWEENEY, C.F. McCONVILLE, D.P. WOODRUFF and R . G . JONES*

P h y s i c s Department, U n i v e r s i t y o f Warwick, GB-Coventry CV4 7 A L , G r e a t - B r i t a i n

" c h e m i s t r y Department, U n i v e r s i t y o f Nottingham, GB-Nottingham NG7 ZRD, G r e a t - B r i t a i n

LS6tude de l a surface EKAE'S de C U ( ~ x f?.)~300-C1, en enrigistrant l e spectre du rendement t o t a l electronicpe au d e l a du s e u i l dlabsorption

-

^K de C 1 en u t i l i s a n t SRS Daresbury,, montre que l a distance interatomique Cu-Cl dans c e t t e phase e s t de 0.05

+

^0.024 plus longue que c e l l e de CuCl m s s i f . Ce resultat e s t dorm& a l a fois par l'analyse de Fourier en f i l t r a n t une seule coquille atomique a t par simulation de m l t i c o q u i l l e atomique. Les deux analyses revklent a u s s i laadsorption dans les s i t e s creux de coordination

-

3 , cepedant aucune de ces deux d t h c d e s permet de f a i r e une distinction entre deux s i t e s , creux inequivalents, de coordination

-

3 h l a surface (111) du cubic face centr6e (cfc). Ces dew s i t e s mntrent cpe l a contribution pour llEXAFS des atoms l e s plus proches e s t identique dans toutes l e s directions de polarisation e t differe seulerrent la troisieme ( e t plus) coquille atomique l a plus proche. Par contrast l e s spectres obtenus par d i f f r a c t i o n photoelectronique pour l a photoemission 1s de C 1 , l e long des directions [ I l l ] e t [110], fournissent de c l a i r e s distinctions entre ces s i t e s en mntrant que l e s atoms de C 1 sont adsorb6s dans l e s &mes s i t e s du "cfc" que la prochaine couche atomique de Cu p o u r r a i t occuper, s u r l e s s i t e s creux de coordination

-

3 dans l a couche atomique superieure (ou exterieure) e t l a second du metal. La nature compl6mentaire de ces deux techpiques dans l'enrigistrement du spectre e t de son analyse e s t clairement demontre par c e t t e gtude.

A Surface EXAFS study of the C U ( ~ x n)R300-C1 structure, collecting data above the C 1 K-edge in t o t a l electron yield on th$ SRS a t Daresbury, indicates that the Cu-Cl distance in t h i s phase is 0.05

+

^0.02A longer than i n bulk CuC1. This result is given by both single s h e l l Fourier f i l t e r i n g analysis and by a rmltishell s i m l a t i o n treatment. Both analyses also reveal that adsorption is in a 3-fold coordinated hollow s i t e , although neither method is able t o distinguish between the two inequivalent 3-fold hollow s i t e s present on t h i s fcc (111) surface. These two s i t e s show i d e n t i c a l n e a r e s t neighbour c o n t r i b u t i o n s t o t h e EXAFS a t a l l p o l a r i s a t i o n d i r e c t i o n s , and d i f f e r only i n t h e t h i r d (and higher) nearest neighbour s h e l l structure. By contrast photoelectron diffraction data for the C 1 1s photoemission, collected along the [ I l l ] an6 [110] directions, do provide a clear distinction between these s i t e s , showing that the C 1 atoms are adsorbed in the same "fcc" s i t e s as the next Cu atom layer would occupy, above 3-fold hollow s i t e s in both the top and second metal atom layers. The complementary nature of these two techniques in the collection and analysis of data is highlighted by t h i s study.

("present Address : VG S c i e n t i f i c Ltd.. Irnberhorne Lane, E a s t G r i n s t e a d , Great-Britain

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868100

JOURNAL DE PHYSIQUE

Surface MAFS has now become a well-established technique for the rather precise determination of adsorbate-substrate nearest neighbour bondlengths using procedures developed for the ,analysis of bulk EXAE'S. The rather high precision of t h i s measurement (-0.02-0.03A) conpares very favourably with any other mthod of surface structure determination and is also l i k e l y t o be rather accurate due t o the use of conparative, model compounds t o reduce systematic e r r o r s a r i s i n g from non-structural parameters (especially t h e s c a t t e r i n g phase-shifts)

.

^Surface

adsorption systems do, however, present an acute case of a d i l u t e system and the signal-to-noise and data range of SMAFS data are invariably p r corwred with typical MAFS data from bulk system. This mans that it is often only possible t o extract nearest neighbour bondlengths reliably, and rarely possible t o extract more than second nearest neighbour bondlengths. As the nearest neighbour bondlength alone is insufficient t o determine the adsorption

site

it is usual t o obtain t h i s additional information through the polarisation direction dependence of the EXAE'S amplitude obtained from t h e oriented s i n g l e c r y s t a l samples. This method is p a r t i c u l a r l y e f f e c t i v e in photoionisation of i n i t i a l s - s t a t e s f o r which the outgoing photoelectron s t a t e i s a pwave directed along t h e A_ vector of t h e incident X-radiation. For surfaces having rotational symmetry higher than 2-fold, however, no a z i m t h a l variation i n the EXAFS amplitude is seen and the information extracted is the angle of the nearest neighbour bond relative t o the surface normal rather than the absolute, real space direction of t h i s bond. [e .g

.

^{refs. 1 and 21}

Here we present SEXAFS data on an adsorption system for which these methods l i m i t cur a b i l i t y t o s c i f completely the adsorption s i t e The system, chlorine on C u ( 1 l l ) i n a ( r f e x f3)R30° ordered s t r u ~ t u r e , ~ ~ f p r e s e n t s a problem characteristic of adsorption on fcc (111) surfaces. This problem is that although t h i s surface is three-fold symmetric, the top substrate atom layer (and indeed each other layer) is six-fold syrmetric. There are therefore two d i s t i n c t l y different maximally (3-fold) coordinated hollow s i t e s on such a surface. In common with previous studies of atomic adsorption on an fcc (111) surface we find from the SEXAF'S data that C 1 occupies one of the 3-fold hollow s i t e s . The cpestion remains as t o && 3-fold hollow. The s i t e s are distinguished by the stacking sequence of the hexagonal close-packed atom layers below the surface layer. One s i t e , which would be occupied by the next substrate atom layer were it present (the 'fcc s i t e ' ) l i e s above 3-fold hollows in both the top and second substrate layers but d i r e c t l y atop an atom in the third layer. The alternative ('hcp') s i t e l i e s directly atop a substrate atom i n the second layer. These s i t e s therefore d i f f e r in t w o ways; the f i r s t and second nearest neighbour substrate bondlengths (in the top layer) a r e identical but the

third

nearest neighbour substrate bondlengths d i f f e r , while the directions of the nearest neighbour atom also d i f f e r by an azimthal rotation of

€00.

Although there are a nunter of surface structural probes other than SEXAFS which have a capability t o resolve t h i s issue, we show here that one technique which can be used, photoelectron diffraction, is naturally conplementary t o SEXAE'S.

In particular t h i s technique uses the s m range of photon energies £ram the same source t o collect the data (the measurements could even be made simltaneously) while the method of analysis and the underlying physical assumptions are also very similar.

The experiments e r e performed a t the Synchrotron Radiation Source (SRS) a t Daresbury i n Cheshire using the SEXAFS focussed beamline equipped with a two c r y s t a l (Ge (111) ) ultrahigh vacuum monochromtor and standard surface science chanber. The Cu(ll1) sanple was prepared and cleaned i n the usual way with characterisation by LEED and Auger electron spectroscopy. Chlorine dosing was from an electrochemical ce11[4] f i t t e d t o t h i s chamber. SEWS n-easurements were mde i n t h e t o t a l e l e c t r o n y i e l d mode above t h e c h l o r i n e K-edge (-2820eV).

Photoelectron diffraction measurements were mde in the sarre photon energy range by

3. SMAFS Results

SEXAE'S measurements were m d e at two different grazing incidence angles, 60°

and 35O, fro12 the chemisorbed phase, and from a CuCl &el wnpund. They have been analysed by two different approaches. The first is the usual, hell-established, method of Fourier filtering the nearest neighbour contribution to the EXAE'S and comparing filtered backtransforms of the model and unknown structures to obtain the bondlength difference. The experimental data, presented as fine structure functions, are shown in fig. 1 and seem to be well-represented by their filtered backtransforms (smooth line). The Cu-Cl nearest neighbour bondlength for the chemisorbed structure deduced from this procedure is 2.291, 0.05bi longer than in the nodel compound; the error is estimated to be 0.02

.

The 3-fold hollow adsorption site assignment mentioned earlier is established from the polarisation dependence of the EXAFS amplitudes from the chemisorption system. The ratio of these amplitudes for the 350 (grazing) incidence data relative to that at 60°

incidence was found to be 1.25 40.10 while the values predicted for hollow, bridge and atop sites are 1.19, 1.81 and 2.67 respectively.

Fig. 1 Experimental fine structure functions,

X

(k) k, obtained in total electron yield n ~ ~ ~ ~ e m e n t s £ran bulk CuCl and from the Cu(ll1)

(n

x

n)R3@-c1

structure at tw grazing incidence angles of 35O and 600. The smDoth lhs passing through the experimentdl data are backtransforms of the windowed nearest neigbour contributions t o their Fourier transforms.

JOURNAL DE PHYSIQUE

I I

l o o m 3 0 0 100 200 300 Electron energy, E (eV)

Fig. 2 Conparison of expe~iumtal SB[AFS f o r the chemisorbed phase at 600 grazing incidence with nulti-shell sirmlations for £cur d i f f e r e n t adsorption sites.

A second analysis procedure used the 'EXCURVE' rmlti-shell sirmlation program available a t Daresbury Laboratory. [5r61 We have discussed the use of t h i s approach t o surface s t r u c t u r e analysis i n m r e d e t a i l elsewhere.

L7

^r8] An important feature of t h e use of t h i s m l t i s h e l l sirmlation approach is t h a t c e r t a i n adsorption s i t e s c a n o f t e n b e e x c l u d e d b e c a u s e t h e y c a n b e shown t o l e a d t o s t r o n g non-nearest-neighbour contributions t o t h e EXAFS not seen in the experiment. I n the present case, howver, t h e r e s u l t s only confirm those of the s i n g l e s h e l l Fourier f i l t e r i n g approach i n both bondlength and s i t e , but cannot distinguish t h e two 3-fold hollow s i t e s . A t y p i c a l conparison, of t h e raw MAFS data a t 60°

incidence with sirmlations for four d i f f e r e n t s i t e s , is shorn in f i g . 2. These and other r e s u l t s c l e a r l y exclude the bridge and atop s i t e s , but the differences in the t h i r d nearest neighbour s h e l l s (which have low effective coordination nurrlsers)

,

^{a r e}

i n s u f f i c i e n t t o distinguish t h e fcc and hcp hollow s i t e s reliably.

4. Photoelectron Diffraction Results

Photoelectron d i f f r a c t i o n [9] involves t h e coherent i n t e r f e r e n c e of t h e d i r e c t l y emitted and e l a s t i c a l l y scattered photoelectron wavefields outside the c r y s t a l . The e f f e c t is q u a l i t a t i v e l y similar t o MAFS but involves mdulations t y p i c a l l y 1 0 tims larger than i n EXAFS, due t o t h e use of the non-local angle resolving d e t e c t o r . This angle resolving measurement a l s o allows real-space d i r e c t i o n a l information t o be obtained which is l o s t i n SEXAFS. Fig. 3 shows the i n t e n s i t y of t h e C 1 1s photoemission peak as a function of photoelectron k i n e t i c energy for two collection directions, normal t o t h e surface along [ l l l ] , and 35O t o t h e s u r f a c e normal i n a d i r e c t i o n corresponding t o [110]. It is now ell-

calculated curves based on a plane w a v e single scattering desorption;

197

t h e f u l l l i n e s correspond to calculations based on a large (converged) c l u s t e r , the dashed l i n e s t o c a l c u l a t i ~ n s of t h e s c a t t e r i n g from t h e n e a r e s t n e i g h b u r , Cu along 'behind' the emitter ( r e l a t i v e t o t h e collector) only. For normal emission t h e main features of t h e experimental data a r e reproduced by the nearest neighbour scattering alone which, along this symmetry a x i s , is identical for both fcc and hcp hollow s i t e s . Not surprisingly, therefore, t h e f u l l c l u s t e r calculations do not resolve these s i t e s s a t i s f a c t o r i l y i n comparison t o the data, as the differences a r e due only t o d i s t s n t neighbour s t r u c t u r a l differences as in SEXAFS. Off nornlal einission, however, the s i t u a t i o n is quite d i f f e r e n t . Along [110] a single Cu nearest neighbour &om l i e s d i r e c t l y behind a C 1 atom adsorbed in the fcc s i t e , but not in t h e hcp s i t e . Comparison of theory and experiment for the [110] data (which have a data 'gapr due t o t h e interference of an X-ray d i f f r a c t i o n feature i n t h i s energy range) show t h a t the adsorption s i t e is c l e a r l y the f c c s i t e . ,

loo 260 300 460

Kinetic energy (ev)

Kinetic energy (eV)

Fig. 3 Omparison of experimental C l Is photoelectron diffracticn spectra taken along [lll] and I1101 (central curves) w i t h the results of theoretical c a l a l a t i o n s for the fcc and hcp sites. Dashed lines correspond t o theoretical calculations w h i c h incorporate only the single nust irrportant Cu scatterer atom.

JOURNAL DE _PHYSIQUE

SEXAFS is a b l e t o d e f i n e t h e Cu-Cl nearest n e i g b o u r bondlength and t o i d e n t i f y t h e adsorption s i t e as a 3-fold hollow for t h e Cu(ll1) ( 3 x 3)R30°-C1 surface b u t , i n c o m n with previous SMAFS s t u d i e s of similar systems, [l5116]

unable t o distinguish t h e two threefold hollow s i t e s . Photoelectron d i f f r a c t i o n measurements, n a t u r a l l y complementary t o t h e SMAFS measurements, allow t h e d i s t i n c t i o n t o be made c l e a r l y i n a fashion h i c h is rather insensitive t o t h e l e v e l of conplexity of the t h e o r e t i c a l approach used. The preference of C1 for the fcc s i t e is t y p i c a l of other strong chemisorption s y s t e m although it has not been found a t a l l coverages for a l l chemisorption s y s t e m . ( e .g

.

refs. 17-21)

The authors a r e pleased t o acknowledge t h e f i n a n c i a l support of the Science and Engineering Research Council of the United Kingdom in p1;oviding studentships, research a s s i s t a n t s h i p s and other funds as w l l as t h e l i g h t from t h e Synchrotron Radiation Source a t t h e i r Daresbury Laboratory. The a s s i s t a n c e of s t a f f of Daresbury is a l s o appreciated.

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