A 4 3 0

Surface Science 189/190 (1987) 645-648 645 North-Holland. Amsterdam

SURFACE MELTING B E H A V I O U R OF Au(I l I ) P. C A R N E V A L I

IBM European Center for Scientific and Engineering Computing, Via Giorgione 15~), 00147 Rome, ltalv

F. E R C O L E S S I a n d E. T O S A T Y I

International S(hool for Advanced Studies, Strada Costiera I I, 34014 Trieste. Italy

Received 1 April 1987; accepted for publication 4 May 1987

The melting behaviour of a metal surface is of considerab[e experimental interest, but has not so far been studied theoretically at a quantitative level. We have undertaken a molecular dynamics study of Au(111) based on the empirical but thoroughly tested "glue'" many-body force scheme. Using a slab geometry, we consider both reconstructed and unreconstructed surfaces. The reconstructed surface (which has a lower surface energy) remains stable in its "crystalline" form up to the bulk melting temperature T.,. It further survives as a metastable state for another ~ 15{/ K above T m, Hence, we conclude that microscopic surface melting is absent in this case. The two outermost layers of the unreconstructed surface "melt'" -100 K below T m. This state of the surface remains stable up to T,,~. and it further survives as a metastable state again as high as - 100 K above T m. Within the severe limits imposed by our finite-size fluctuations, we observe no other stable surface melting behaviour. These results for Au( l l l ) are contrasted with those for Lennard-Jones cry'sial surfaces.

Surface Science 189/I90 (19871 649-655 649 North-Holland, Amsterdam

NEW EVIDENCE FOR SEGREGATION OF PI ON P I ~ N i s o ( l l l ) ALLOY SURFACE FROM F O R W A R D SCATI'ERING P H O T O E L E C T R O N DIFFRACTION Y. J U G N E T , N . S . P R A K A S H , G. G R K N E T , T r a n M i n h D U C

lnstitut de Physique Nuel~aire de l~von, UniPersit~ Claude Bernard L~on 1, 43 Boule~,ard du iI noPembre 1918. 69622 Villeurbanne Cedex, France

a n d

H . C . P O O N

The Blackett Laboratory, Imperial College, lxmdon S W7 2BZ, UK

Received 30 March 1987; accepted for publication 15 April 1987

It has been shown thal on PtsoNis{~{lll), PI segregates to the surface and forms the uppermost layer. X-ray photoelectron diffraction experiments appear to be useful for investigating this segregation. We found that Ni 2p azimuthal plots exhibit stronger anisotropy than Pt 4f does. This is new evidence for Pt segregation at the surface. The results are interpreted in terms of multiple forward scattering. The calculations have been done for different Pt surface concentra- tions in order to estimate the degree of segregation.