Normal to tangential velocity conversion in cluster-surface collisions: Ar-N on graphite

Collisions between large neutral argon clusters and a pyrolytic graphite surface have been investigated at normal incidence by measuring angular distributions of density and angularly resolved time-of-flight distributions of scattered species (essentially monomers). These data have been taken for two surface temperatures (440 and 660 K) with cluster sizes N ranging from 400 to 8200 atoms per cluster, and at an incoming cluster energy of 72 meV/atom. A higher incident kinetic energy (115 meV/atom) has been investigated also for N=900 atoms per cluster. The experimental results have been analyzed by considering two components. First, an ''ejection'' contribution, dominant at large scattering angles, has been assigned to the evaporation of monomers from cluster material having acquired a flow velocity parallel to the surface. The average value of this flow velocity increases sharply with the incident cluster velocity in the investigated range. Second, a thermal contribution, dominant close to the surface normal, has been assigned to the trapping-desorption of monomers on the graphite surface for the smallest values of N, and to direct evaporation from the cluster when N increases. (C) 1996 American Institute of Physics.