By using the lattice gas model and a transition-type dependent Monte Carlo method, calculations were made of chemical diffusion coefficients on a stepped surface. It was assumed that the step exerted an attraction or repulsion upon adsorbed particles that occupied up or down step sites. However, there were no interactions between the particles. Two kinds of activation energy were used in the calculations. These were calculated from the harmonic potential or from the difference between the saddle-point and single-site energies. The calculated results showed that the perpendicular diffusion decreased markedly with increasing step repulsion or attraction; at all coverages. However, in the case of diffusion parallel to the steps, completely different results were obtained for the two calculation methods. If the energy barrier was calculated by using the harmonic potential, the diffusion parallel to the steps was both coverage-independent and step-independent. If the energy barrier was calculated by using the second method, the diffusion parallel to the steps was greatly enhanced with increasing step repulsion or attraction at middle or high coverages. It decreased slightly at low coverages. The calculated results explained the anisotropy of chemical diffusion on a stepped surface. The results also showed that the harmonic potential method might not be suitable for explaining experiments in which diffusion along step edges could be more rapid than on a flat surface.

M.Qiu, P.L.Cao, J.Ruan: Journal of Physics - Condensed Matter, 1996, 8[27], 4867-79