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Bimetallic systems respond dynamically to the nature of the gas phase in contact 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 using medium energy ion scattering to obtain such information. The unique shadowing and blocking capabilities of this technique make it possible, in principle, 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 present in the first layer, the top two layers, the top three layers, etc., of the bimetallic surface, thereby giving quantitative compositional information. Using a combination of experiment and simulation, we investigate how the number of layers illuminated depends on the extent of the surface layer relaxation, the adsorbate and the surface Debye temperature. We calculate the layer compositions of the top three layers in the Cu 50 Pd 50 (110)/C 2 H 4-x Cl x system. We show that hydrocarbon adsorption produces a measurable Pd enrichment in the surface layer, while Cu enrichment is produced by the presence of Cl(ads) and C(ads).

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