A comparison of the spectral features of the collision-induced light scattering by the molecular gases CH4 and CF4 and by argon

Abstract

The collision-induced light scattering from compressed gaseous CH4 and CF4 has been observed at room temperature at densities up to 100 and 166 amagats, respectively. Absolute intensity measurements have been made and the two- and three-body components to the spectra have been identified. A detailed comparison is made with similar data on argon and significant differences emerge between the atomic and molecular cases. The two-body molecular spectra contain an excess of intensity over predictions of the dipole–induced-dipole (DID) model; the argon intensity is less than predicted. At low frequency, the two- and three-body spectra of all species are dominated by the DID interaction. At high frequency the two-body molecular spectra decay much more slowly than the profile due to argon. A moment analysis has been performed wherein the pair polarizability is assumed to be of the form, β(r) = 6α2/r3 + B/rp. The unknown parameters B and p are determined from the experimental zeroth and second moments of the spectra. For the molecules B is found to be greater than unity and p is 7.2 for CH4 and 8.5 for CF4. The difference in the values of B and p, together with the fact the shape of the high frequency tail is not consistent with pure translational broadening, lead to the suggestion that the internal degrees of freedom of the molecules are playing a role in the scattering process.