Molecular dynamics simulations using the Lennard-Jones potential were performed for Kr on Ar(111) in order to investigate the nature of high-temperature surface heterodiffusion (table 31). This suggested that the high activation energies and pre-exponential factors which were observed experimentally above about 0.70Tm arose from adatom-vacancy pair formation, which sharply increased the number of mobile species. Evidence was also obtained that, at intermediate temperatures (0.45Tm to 0.66Tm), mass transfer diffusivity could depend upon both coverage and adsorbate interactions. For adsorbates which exhibited islanding, low coverage diffusion was characterized by an activation energy which was equal to the enthalpy of migration whereas, at moderate coverages, the activation energy contained an additional term which was due to the enthalpy of formation for mobile adatoms from immobile islands.

The Mechanism of Surface Heterodiffusion at Elevated Temperatures. Suni, I.I.: Surface Science, 1996, 349[3], L179-83