Diffusion of adsorbates on a square lattice containing steps every fourth row was investigated by Monte Carlo simulations. For the case where steps were attractive, i.e. step sites bind more tightly than terrace sites, there was very little diffusion anisotropy at low coverage, since steps acted as traps about equally for diffusion parallel and perpendicular to steps. As coverage increased steps became highly populated and then acted as barriers for diffusion across them so that anisotropy increases, particularly as temperature was lowered. Diffusion parallel to steps then became temperature independent and diffusion perpendicular to steps remains activated. For diffusion coefficients obtained by the fluctuation autocorrelation method the inability to leave terraces at high θ and low T introduces artifacts for measurements perpendicular to steps. For repulsive steps, constituting diffusion barriers but not serving as adsorption sites anisotropy was pronounced at all temperatures; diffusion parallel to steps was temperature independent, and diffusion perpendicular to steps was activated at all coverages.

Monte Carlo Study of Diffusion on Stepped Surfaces. I. Step Attraction Or Repulsion Only. C.Uebing, R.Gomer: Surface Science, 1994, 306[3], 419-26