Simulations of mobility and evaporation rate of adsorbate islands on solidsurfaces

We perform kinetic Monte Carlo simulations to examine the kinetic propertie s of one-atom-high islands formed by atoms adsorbed on a single-crystal sur face. At sufficiently high temperature, the atoms can leave the island to m igrate on the substrate. We call this process evaporation. We find that mos t of the evaporation events are described by a Poisson process characterize d by a rate constant k(N, T), where N is the number of atoms in the island and T is the temperature. We also observe correlated evaporation events, wh ich tend to follow each other in rapid succession. This complicated situati on can be described, however, by an effective Poisson process that is defin ed to generate the correct vapor pressure. The dependence of k( N, T) on N follows an equation proposed by Metiu and Rosenfeld, and not a power law ob served in previous work. The random motion of the atoms around the border o f the island causes its center of mass to move along the surface. This isla nd motion is diffusional, except at the shortest times. The dependence of t he diffusion coefficient on N is a power law (if the smallest islands are e xcluded). The exponent is not universal and depends on temperature and the parameters of the model. Theory predicts universal behavior for very large islands and we assume that in our simulations we have not reached this regi me. (C) 1999 American Institute of Physics. [S0021-9606(99)00942-3].