Cj. Baddeley et al., Quantitative analysis of adsorbate induced segregation at bimetallic surfaces: Improving the accuracy of medium energy ion scattering results, J PHYS CH B, 105(14), 2001, pp. 2766-2772
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).