CORRELATED CHEMISORPTION - THE EFFECT OF COALESCENCE AND DIFFUSION ONCHEMISORPTION ISLANDS

The theory of island formation in chemisorption [Becker and Ben-Shaul, Phys. Rev. Lett. 61, 2859 (1988)], which treats chemisorption as a co mbination of direct adsorption and cluster formation, is generalized t o account for the effects of cluster-cluster coalescence and of surfac e diffusion. The effect of cluster-cluster coalescence is approximated by series expansion in increasing "overlaps." Exact calculation of th e second order term, which describes the two-cluster overlaps, yields a good agreement with Monte Carlo simulation results. By incorporating the surface diffusion process it is shown that upon increasing the di ffusion rate (i.e., raising the surface temperature) the system change s its behavior from correlated chemisorption (cluster formation) to ra ndom chemisorption. The extended formalism includes, as specific reali zations, both the case of immobile particles and the cases of rapid di ffusing particles (Langmuir's and Kisliuk's models).