Perturber and Temperature Dependence of Collision - Light Scattering for Spherical Top Molecules.

Collision-induced light scattering (CILS) refers to the Rayleigh and Raman spectral features forbidden by the symmetry of a free molecule, but which appear in the scattering from dense media through molecular interactions. In order to avoid complications from allowed rotational Raman lines, molecules of high symmetry (spherical top) were chosen for the present study. The two- and three-body (CILS) spectra of both pure octahedral molecules (SF_6), and tetrahedral molecules (CF_4) at 295 K have been determined experimentally. As well, the two-body spectra of these systems perturbed by inert gases have been measured. Classical line shape calculations were performed for the free-free interactions (translational spectrum), and a model was used to calculate the effects of bound dimers. Comparison between the two-body experimental and theoretical spectra showed a very good agreement at low frequency shifts where the collision-induced rotational Raman scattering has a negligible effect. This agreement was best in the cases of SF_6 mixtures with inert gases because of the availability of good intermolecular potentials. At higher frequency shifts, higher order polarizabilities played a role, the dipole-octopole polarizability E in the case of octahedral molecules, and both the dipole-quadrupole A and the dipole-octopole E polarizabilities in the case of tetrahedral molecules. The intensity and the shape of the rotational spectra were accounted for well by theory. The values of the polarizabilities found were E = 10.7 +/- 2.5 A^5 for SF _6, and A = 2.5 +/- 0.3 A^4, E = 2.5 +/- 0.3 A^5 for CF_4 . The CILS spectrum of SF_6 -Xe, together with both the values of the diffusion coefficient and the second virial coefficient, were used in developing a new M2SV intermolecular potential for SF_6 -Xe. A temperature study of the CILS of CH _4 was performed in the range 130-295 K. The model for the pair polarizability anisotropy with no adjustable parameters still holds at lower temperatures. Calculations for both the induced translational and rotational scattering worked well over this wide temperature range. Values of the dipole-quadrupole polarizability A of 0.88 +/- 0.03 A^4 and the dipole-octopole polarizability E of 2.5 +/- 0.26 A^5 were found.