DIFFUSION OF ADSORBATES ON RANDOM ALLOY SURFACES

In this work the diffusion of non-interacting adsorbates on a random A B alloy surface is considered. For this purpose a simple cubic (sc), b ody-centered cubic (bcc) or face-centered cubic (fcc) auxiliary metal lattice is introduced. The auxiliary lattice is truncated parallel to its (100) plane in such a way that the fourfold hollow positions of th e metal surface form a regular net of adsorption sites with square sym metry. The adsorption energy of each adsorption site is determined by its own environment, i.e. by the numbers of direct A or B neighbors. T he Monte-Carlo method has been utilized to simulate surface diffusion of adsorbates on such energetically heterogeneous alloy surfaces and t o calculate the tracer, jump and chemical diffusion coefficients. The chemical diffusion coefficient was calculated via two different approa ches: the fluctuation and the Kubo-Green method. The influence of ener getical heterogeneities on the surface diffusion is largely pronounced at low temperatures and low surface coverages, where most of the adat oms are trapped by deep adsorption sites. It was found that at low tem peratures the sequential occupation of the different types of adsorpti on sites can be observed.