VIBRATIONAL FREQUENCY-SHIFT OF H-2 IN RARE-GAS CLUSTERS AND SOLUTIONS- COMPARISON OF SEMICLASSICAL THEORY AND EXPERIMENT

A recently developed semi-classical statistical mechanical formulation [de Souza et al., J. Chem. Phys. 99, 9954 (1993)] is combined with ac curate H-2-rare gas potentials [Le Roy and Hutson, J. Chem. Phys. 86, 837 (1987)] to predict H-2 vibrational frequency shifts in rare gas cl usters and low density solutions. The results are compared with availa ble experimental measurements as well as with predictions derived assu ming a Lennard-Jones (LJ) atom-atom potential. The Le Roy-Hutson poten tial has a minimum cluster energy and maximum H-2 bond softening in th e linear atom-diatom geometry, in contrast to the T geometry predicted using the LJ potential. The Le Roy-Hutson potential also yields bette r agreement with experimental temperature and density dependent H-2 fr equency shifts. A classical approximation to the ground state frequenc y of H-2-rare gas clusters is suggested which relates the probability density of the cluster configuration to the classical Boltzmann distri bution at a temperature equal to the cluster zero point energy. (C) 19 96 American Institute of Physics.