Gibbs free-energy changes for the growth of argon clusters adsorbed on graphite

Changes in Gibbs free energies for the process Arn−1,(a) + Ar(a) → Arn,(a) are calculated by Monte Carlo simulations for Ar clusters physisorbed on a graphite substrate. Calculations are performed for clusters Ar2 through Ar12 at 10 K. Using a simulated annealing procedure, the minimum-energy configurations at 0 K are obtained. In all cases studied minimal-energy two-dimensional structures are found at a distance above the surface identical to that determined for an argon monomer. Some cluster sizes exhibit isomerization between several low-energy configurations during the simulations. This isomerization leads to sampling difficulties that are significantly reduced by using the J-walking method. Minima in the Gibbs free energy are found for cluster sizes 7, 10, and 12. An explanation for the location of the minima in the free-energy function is given in terms of cluster structure and energetics.