A STUDY OF CHEMICAL DIFFUSION ON A STEPPED SURFACE BY THE TRANSITION-TYPE-DEPENDENT MONTE-CARLO METHOD

Using the lattice gas model and the transition-type-dependent Monte Ca rlo method, we calculated the chemical surface diffusion coefficients on a stepped surface. We assume that the step exerts an attraction or repulsion on adsorbed particles (adparticles) that occupy the up or do wn step sites, but no interactions between adparticles. Two kinds of a ctivation energy, calculated from the harmonic potential and from the difference between the saddlepoint and single-site energy, are used in our calculation. The calculated results show that perpendicular diffu sion decreases greatly with increase in step repulsion and attraction at all coverages. However, for diffusion parallel to the steps, comple tely different results are obtained for these two calculation methods. If the energy barrier is calculated by the harmonic potential, diffus ion parallel to the steps is both coverage and step independent. If th e energy barrier is calculated by the second method, diffusion paralle l to the steps is greatly enhanced with increase in the step repulsion and attraction at middle or high coverages but decreases slightly at low coverages. The calculated results explain the chemical diffusion a nisotropy on a stepped surface. The results also show that the popular harmonic potential method may be not suitable for explaining the expe riments where diffusion along step edges may be more rapid than on a f lat surface.