  The influence of nearest-neighbor diffusion on the decay of a metastable
low-coverage phase (monolayer adsorption) in a square lattice-gas model of
electrochemical metal deposition is investigated by kinetic Monte Carlo
simulations. The phase-transformation dynamics are compared to the
well-established Kolmogorov-Johnson-Mehl-Avrami theory. The phase
transformation is accelerated by diffusion, but remains in accord with the
theory for continuous nucleation up to moderate diffusion rates. At very high
diffusion rates the phase-transformation kinetic shows a crossover to
instantaneous nucleation. Then, the probability of medium-sized clusters is
reduced in favor of large clusters. Upon reversal of the supersaturation, the
adsorbate desorbs, but large clusters still tend to grow during the initial
stages of desorption. Calculation of the free energy of subcritical clusters by
enumeration of lattice animals yields a quasi-equilibrium distribution which is
in reasonable agreement with the simulation results. This is an improvement
relative to classical droplet theory, which fails to describe the
distributions, since the macroscopic surface tension is a bad approximation for
small clusters.
