THE EFFECTS OF COLLISION MASS AND POTENTIAL ON THE ENERGY-TRANSFER INTHERMAL COLLISIONS OF GAS-PHASE CLUSTERS

We have studied the collisional energy transfer between molybdenum clu sters and the rare-gas atoms Ne, Ar, and Xe. We have chosen these syst ems as nontrivial models of the thermalization process of metal cluste rs in a background gas. The mean energy transfer cross sections and en ergy transfer rate constants for collisions of molybdenum clusters (Mo 5) with rare gas atoms are computed as functions of relative collision energy (gas temperature) and mass. The dynamics of gas phase molybden um clusters are simulated by classical trajectories whose initial cond itions are sampled from a distribution appropriate to thermal collisio ns. For the interaction of the molybdenum cluster atoms with the backg round gas, a Buckingham-type potential for unlike atoms was fitted to energies obtained using standard quantum chemistry techniques. The mol ybdenum cluster atoms interact among themselves by a Lennard-Jones pot ential. The simulation shows that the energy transfer rate constants a re dominated by the characteristic collision velocity, i.e., within th e domain of internal cluster temperature and background gas temperatur e 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, com plex collisions with more than one cluster atom are involved.