Quantitative analysis of adsorbate induced segregation at bimetallic surfaces: Improving the accuracy of medium energy ion scattering results

Bimetallic systems respond dynamically to the nature of the gas phase in co ntact with the surface. The development of a surface analytical tool which probes the layer by layer composition of a bimetallic surface covered by an adsorbate would be of great benefit in understanding complex catalytic and corrosion processes. This paper examines critically the possibility of usi ng medium energy ion scattering to obtain such information. The unique shad owing and blocking capabilities of this technique make it possible, in prin ciple, to selectively illuminate integer numbers of surface layers. As the physics of the incident ion-surface atom collision is well established, it should then be possible to effectively count the number of each atom type p resent in the first layer, the top two layers, the top three layers, etc., of the bimetallic surface, thereby giving quantitative compositional inform ation. Using a combination of experiment and simulation, we investigate how the number of lavers illuminated depends on the extent of the surface laye r relaxation, the adsorbate and the surface Debye temperature. We calculate the layer compositions of the top three layers in the Cu50Pd50(110)/C2H4-x Clx system. We show that hydrocarbon adsorption produces a measurable Pd en richment in the surface layer, while Cu enrichment is produced by the prese nce of Cl(ads) and C(ads).