A model description of surface diffusion in the presence of two non-equivalent lattice sites

A dynamical description of surface diffusion is presented for a surface containing two non-equivalent bonding sites with different binding energies. Diffusion proceeds via single hopping events between adjacent sites which can be of both kinds. Dynamical coupling is introduced by assuming that the hopping rates into and out of one type of site depend on whether the adjacent site of the other kind is occupied or empty. The derivation starts out from the microscopic difference equations accounting for the single jump events for all possible configurations. These are then transformed into differential equations with continuous functions and solved by the method of finite differences. The activation energies for individual jumps serve as adjustable parameters. It is shown that the widely observed dependence of the diffusion coefficient on coverage also follows naturally from this model, although direct particle-particle interactions in the usual sense are specifically excluded. The influence of the substrate-adsorbate interaction and of the surface geometry on the diffusion process is also discussed.