Stochastic trajectory studies of small argon cluster scattering from Pt(111)

Abstract

We have carried out stochastic trajectory calculations of the scattering of small argon clusters, n=1,2,3, and 5, from a Pt(111) surface at incident energies of 0.1 and 0.5 eV per atom. We employed a 6 by 6 by 2 layer slab of platinum atoms with periodic boundary conditions imposed in the x and y (surface plane) directions. We applied local friction and white random forces in the z (surface normal) direction to the bottom surface layer to maintain the proper temperature and to account for energy transfer with the bulk. We assumed Lennard-Jones interactions for Ar–Ar and Ar–Pt with realistic parameters. We have found that the scattering of small clusters from surfaces exhibits a very different behavior from the scattering of individual atoms or molecules. At the collision energies considered, most clusters fragment into atoms upon impact with the surface, but a surprising number survive either partially or totally intact. Angular distributions of the fragmented monomers are much broader than those of surviving clusters. The average energy of the fragmented monomers increases with scattering angle, the reverse of the ‘‘hard-cube’’ trend for atoms scattering from surfaces. In addition, cluster scattering is associated with an enhanced trapping probability and enhanced initial lateral mobility of the trapped species in comparison to individual atom scattering. A sequential binary collision model invoking gas–surface and gas–gas collisions is suggested to account for these results.