Collision-induced rotational Raman scattering by tetrahedral and octahedral molecules

Abstract

The long tails at high Stokes and anti-Stokes frequencies observed in Rayleigh scattering from compressed methane, carbon tetrafluoride and sulphur hexafluoride are attributed to induced rotational transitions. The longest-range contribution to the collision-induced polarizability, the dipoleinduced-dipole term in R ^-3, where R is the separation of a pair of molecules, does not fluctuate with molecular rotation. However, the dipole-quadrupole polarizability A of the tetrahedron leads to a rotating contribution in R ^-4. The dipole-octopole polarizability E of an octahedron and a tetrahedron leads to a contribution in R ^-5 which also rotates with the molecule. The rotational transitions associated with these polarizabilities can explain both the intensity and the shape of the long tails. Approximate values of A for CH₄ and CF₄ and of E for SF₆ are deduced. As in the monatomic case, there is a simple relationship between the total scattered intensity and the second Kerr virial coefficient.