Quantitative Modelling of the Shifts and Splitting in the IR Spectra of SF₆ in an Ar Matrix

Abstract

An infrared active polyatomic molecule has several vibrational modes, each of which has a characteristic frequency. If the molecule is trapped in a matrix of perturbing atoms, those vibrational frequencies will shift, and if the vibrational mode is degenerate, the perturbation may lift the degeneracy. Such shifts and splitting are due to the dependence of the chromophore/matrix-atom interaction potential on the internal vibrational motion of the chromophore. Applying a previously-developed model for the shifting and splitting of the triply degenerate <em>&nu;</em>₃ mode of SF₆ perturbed by a rare gas atom, we use Monte Carlo simulations to sample the accessible equilibrium configurations of the system and to predict the associated thermally averaged perturbed IR spectra. Since the experimental spectrum has 10 peaks while the triply degenerate <em>&nu;</em>₃ mode of SF₆ in a particular environment could have at most 3 peaks, the observed spectrum must be a combination of spectra for SF₆ trapped in different types of lattice sites. A fit to experiment of simulated spectra generated from a family of lattice sites is then used to identify the peaks in the experimental spectrum, determine the relative importance of the various lattice sites, and semi-quantitatively reproduce the experimental spectrum.