Statistical theory of cluster cooling in rare gas
Part II.† The PEMET model
The collisional energy transfer between a palladium cluster and a rarified inert gas medium has been studied by molecular dynamics simulation and interpreted by statistical theory. The cluster is Pd13 and the inert gas may be composed of helium, neon, argon or krypton. The cluster–inert gas collision cross section and the average energy transferred per collision are determined by following the collisions by classical trajectory calculations. Special attention is placed on the development of rigorous sampling techniques such that the average energy transfer vanishes when cluster and gas are at the same initial temperature. The dependence of the energy transfer on impact parameter is found to display an inner region of prevalent excitation and an outer region of prevalent deexcitation of the cluster. The energy transfer efficiencies observed fall a factor of between 0.05 and 0.4 below the ergodic collision limit with the heaviest gas atoms most efficient. The PEMET model explains the observed efficiencies in terms of an increased number of atom–atom encounters with increasing strength of attraction between atom and cluster and with lower temperature.