The effects of collision mass and potential on the energy transfer in thermal collisions of gas phase clusters

Abstract

We have studied the collisional energy transfer between molybdenum clusters and the rare-gas atoms Ne, Ar, and Xe. We have chosen these systems as nontrivial models of the thermalization process of metal clusters in a background gas. The mean energy transfer cross sections and energy transfer rate constants for collisions of molybdenum clusters (Mo5) with rare gas atoms are computed as functions of relative collision energy (gas temperature) and mass. The dynamics of gas phase molybdenum clusters are simulated by classical trajectories whose initial conditions are sampled from a distribution appropriate to thermal collisions. For the interaction of the molybdenum cluster atoms with the background gas, a Buckingham-type potential for unlike atoms was fitted to energies obtained using standard quantum chemistry techniques. The molybdenum cluster atoms interact among themselves by a Lennard-Jones potential. The simulation shows that the energy transfer rate constants are dominated by the characteristic collision velocity, i.e., within the domain of internal cluster temperature and background gas temperature investigated here, the mass and the interaction force do not change the energy transfer rate constants very much. The mean energy transfer cross sections, however, are coupled to the collision mass as well as to the actual interaction force. The coupling is nonlinear, and there is some evidence that in the energy transfer, for small clusters, complex collisions with more than one cluster atom are involved.