The shadowing and blocking effects exhibited by medium energy ion scatterin
g (MEIS) make it a uniquely effective technique for the analysis of the lay
er composition of bimetallic surfaces. We use MEIS to study adsorbate-induc
ed segregation effects at the Cu50Pd50(110) surface. We show that the clean
surface composition is strongly dependent on sample preparation. In additi
on, the adsorption of chlorohydrocarbons causes substantial surface layer C
u enrichment (up to 100% Cu), whereas hydrocarbon adsorption causes Pd enri
chment. We propose an explanation for the magnitude of these segregation ef
fects in terms of the surface chemistry of these molecules. (C) 1999 Publis
hed by Elsevier Science B.V. All rights reserved.