A spectroscopic study of CH3F isolated in rare gas matrices

Abstract

The hindered rotational/librational motion of symmetric top molecules isolated at substitutional sites in rare gas matrices is considered. It is shown that the dominant perturbation of the free rotor states is due to the reorientational forces of the cage atoms. An electrostatic model for symmetric top molecules trapped in octahedral fields that hinder the molecular tumbling motion is developed. The model is successfully applied to the qualitative interpretation of experimentally observed line shapes, linewidths, and spectral shifts of the ν3 mode of CH3F isolated in rare gas matrices. Exact quantitative predictions are not made due to the many model parameters, however, the spectra are compatible with a minimum barrier to tumbling of approximately 80 cm−1 and are compatible with free spinning motion. A complete analysis of the spectra requires the consideration of dynamic perturbations as well as possible sources of dynamic line broadening. The role of rotation-translation coupling is also explored. Experimental studies have provided a direct measure of the ν3 intergrated absorption coefficient from which a radiative lifetime of τ = 82±10 ms is obtained. Direct observation of 2ν3 yields the first anharmonicity constant ωeχe = 8.05±0.05 cm−1. Studies of matrices ’’capped’’ with pure rare gas have provided an insight as to the dynamics of aggregation with the conclusion that aggregation occurs at surfaces and grain boundaries. An absorption at 1034 cm−1 is assigned to molecules trapped in these locations. Implications with respect to energy transfer processes in these systems are also discussed.