EFFECTS OF ADSORBATE LATERAL REPULSION ON DESORPTION AND DIFFUSION KINETICS STUDIED BY MONTE-CARLO SIMULATIONS

The effects of adsorbate lateral interactions on the kinetics of surfa ce diffusion and desorption are studied by means of kinetic and thermo dynamic Monte Carlo simulations. This study is motivated by recent dif fusion and desorption experiments on the NH3/Re(001) system, which sho w that the activation energies of these processes decrease (in differe nt fashions) with increasing surface coverage, the interactions betwee n the adsorbates are thus assumed to be repulsive. A long range dipole -dipole-like potential is used to simulate both the diffusion and deso rption processes. Most calculations are carried out with the interacti on range extending up to fourth-order neighbors. Longer ranges are fou nd to barely affect the kinetic behavior. On the other hand, shorter r anges of interaction result in qualitatively and quantitatively differ ent structural (thermodynamic phase) behaviors and, consequently, in v ery different kinetics of diffusion and desorption. The model used to calculate diffusion kinetics assumes that the activation barrier to pa rticle diffusion depends, simultaneously, on the local environments of both the initial and the final sites involved in the elementary event of particle jumps. The chemical diffusion coefficient is evaluated ba sed on thermodynamic and kinetic Monte Carlo simulations. It is found to increase with surface coverage, reflecting the repulsive nature of the interactions. Yet, unlike the experimental results, the increase i s nonmonotonic but rather, somewhat oscillatory-reflecting the structu ral phase transitions of the adsorbed layer. The activation energy of desorption is found to decrease by about 15 kcal/mole as the coverage increases from 0 to 1, showing steeper slopes around the coverages cor responding to a perfectly ordered adlayer phase. These results are in satisfactory qualitative and quantitative agreement with experiment. F inally, it is shown that the coverage dependence of the activation bar rier to diffusion can be reasonably well evaluated from equilibrium th ermodynamic desorption data. (C) 1996 American Institute of Physics.