Pure rotational Raman scattering by symmetric top molecules

Abstract

The distribution of intensities between the branches of the pure rotational Raman spectrum of a symmetric top molecule is calculated according to the quantum mechanical method of Placzek and Teller. The total intensities are obtained by summing over the individual line intensities associated with transitions from rotational state J,K to state J′,K′. The exact distribution depends on the temperature of the system and the detailed geometry of the symmetric top molecule. In the classical (i.e., high temperature) limit, the fraction of intensity due to transitions for which Δ J=0 is found to be 0.25 for a linear molecule, to decrease to a minimum of approximately 0.167 for a prolate symmetric top, and to increase to a maximum of 0.254 for the limit of a disk. This fraction for an oblate symmetric top is in the range 0.20–0.254. Thus, measurements of the distribution of intensities of the branches of the pure rotational Raman spectrum can be used to obtain structural information for symmetric top molecules.