Energy transfer during van der Waals cluster-surface collisions

Experimental results together with a thermokinetic and a dynamic zone structure (DZS) model are presented for the scattering of argon clusters off a graphite surface. Density and time-of-flight angular distributions for a large range of surface temperatures, angles of incidence, and large incident cluster sizes (up to 8700 atoms) exhibit three different outcoming channels: two direct inelastic ones and an enhanced trapping-desorption one. The first inelastic process has been identified and modelized as a thermal evaporation of monomers from clusters gliding along the surface. The second one is characterised by a grazing exit angle component attributed to large cluster fragments leaving the surface before total evaporation and quantitatively reproduced by the DZS model. Finally, the trapping-desorption process results in a significant velocity thermalization with an absolute atom trapping probability enhanced by a factor as large as three for incident clusters over that for incident individual atoms and concerns up to 25% of the incoming cluster atoms.