Raman scattering from colliding molecules and Van der Waals dimers in gaseous methane

Raman spectra of low-pressure methane gas both at room temperature and near condensation have been studied in the frequency range from 0 to 50 ${\mathrm{cm}}^{-1\mathrm{}}$. The spectra include two contributions: One is due to collision-induced scattering (CIS) and the other to an unresolved dimer rotational band. The latter component is most prominent at low temperatures. Both the dimer band and CIS spectrum have been computed for a Lennard-Jones interaction and point-dipole polarizability. At large frequency shifts (> 20 ${\mathrm{cm}}^{-1\mathrm{}}$) it is sufficient to use classical collision dynamics to compute the CIS spectrum, but at lower shifts it is necessary to use quantum theory and add the dimer contribution to obtain good agreement with the experimental spectrum. From analysis of the spectra we determine that the fractional depolarized scattering from dimers is 10% at 300°K and 43% at 115°K. The shape of the far wing (20-50 ${\mathrm{cm}}^{-1\mathrm{}}$) provides a sensitive means of testing the form of the induced polarizability anisotropy. For methane, excellent agreement with experiment is obtained for the point-dipole model.