A dynamic description of surface diffusion was presented for a surface which contained 2 non-equivalent bonding sites with different binding energies. Diffusion proceeded via single hopping events, of 2 types, between adjacent sites. Dynamic coupling was introduced by assuming that the hopping rates into and out of one type of site depended upon whether an adjacent site of the other type was occupied or empty. The derivation was based upon microscopic difference equations which accounted for single jump events for all possible configurations. These were then transformed into differential equations in continuous functions, and were solved by using the finite difference method. The activation energies for individual jumps served as adjustable parameters. It was shown that the commonly observed dependence of the diffusion coefficient upon coverage arose naturally from this model, even though direct particle-particle interactions were excluded. The effects of substrate-adsorbate interaction and surface geometry upon diffusion were analyzed.

Z.Chvoj, H.Conrad, V.Cháb, M.Ondrejcek, A.M.Bradshaw: Surface Science, 1995, 329, 121-34