Statistical theory of cluster cooling in rare gas. I. Energy transfer analysis for palladium clusters in helium

The cooling and heating of palladium clusters Pd-13 and Pd-55 by binary col lisions with atoms of a surrounding helium gas are studied by means of mole cular dynamics simulation. The efficiency of the collisional energy transfe r is determined as a function of cluster and gas temperature and of cluster phase, the cluster being in either a solid or a liquid phase. A simple sta tistical analysis is presented for the energy transfer between a cluster an d a rare gas atom. The analysis is based on an ergodic collision assumption of microcanonical relaxation in each collision. The deviation from this li miting law is collected in a collision efficiency factor which reflects inc omplete energy redistribution during the lifetime of the collision complex. The thermal energy and change in heat capacity observed for the clusters a t the freezing (melting) transition is accounted for by a parametrized dens ity of states reflecting separate contributions from a solid and a molten s tructure. The same density of states Is then used in the ergodic collision theory for the analysis of energy transfer. (C) 1998 American Institute of Physics. [S0021-9606(98)00446-2].