Evaporation model of cluster scattering from surfaces

Abstract

We present classical trajectory calculations of ArnNem (n+m=111, 859) clusters scattering from a rigid surface. The dynamics of energy transfer and cluster decomposition during surface scattering is investigated for incident velocities of 100–700 m/s. The initial translational energy is at impact effectively transferred into internal degrees of freedom of the cluster. The overall energy transfer efficiency is very high but not complete, leaving too much energy in translation. No fragmentation takes place below 200 m/s. At incident velocities below 450 m/s, evaporation of small fragments from the heated cluster takes place in thermal equilibrium with the vibrational degrees of the cluster. This thermal evaporation is also the dominating ejection channel up to 700 m/s. Above 450 m/s, the formation of a compressed zone at impact opens up a new channel with ejection of fast fragments parallel to the surface plane. This effect becomes increasingly important at higher velocities. An evaporation model where fragmentation of the heated cluster takes place as isotropic and thermal ejection of small fragments is concluded to account for the major fragmentation processes observed.