Vibrational frequency shift of H2 in rare gas clusters and solutions: Comparison of semi-classical theory and experiment

Abstract

A recently developed semi-classical statistical mechanical formulation [de Souza et al., J. Chem. Phys. 99, 9954 (1993)] is combined with accurate H2-rare gas potentials [Le Roy and Hutson, J. Chem. Phys. 86, 837 (1987)] to predict H2 vibrational frequency shifts in rare gas clusters and low density solutions. The results are compared with available experimental measurements as well as with predictions derived assuming a Lennard-Jones (LJ) atom–atom potential. The Le Roy–Hutson potential has a minimum cluster energy and maximum H2 bond softening in the linear atom–diatom geometry, in contrast to the T geometry predicted using the LJ potential. The Le Roy–Hutson potential also yields better agreement with experimental temperature and density dependent H2 frequency shifts. A classical approximation to the ground state frequency of H2-rare gas clusters is suggested which relates the probability density of the cluster configuration to the classical Boltzmann distribution at a temperature equal to the cluster zero point energy.