Collision-induced rotational Raman scattering

The long tails at low and high frequency in the Rayleigh scattering by gases comprising tetrahedral molecules is attributed to rotational Raman scattering. The rotating part of the collision-induced polarizability is due to the dipole-quadrupole polarizability A and varies as the inverse fourth power of the separation of the two interacting molecules. The rotational section rules are Delta J = 0, +/- 1, +/- 2, +/- 3, and quantitative agreement is obtained with the observed frequency dependence of the tail in methane. Contributions due to hyperpolarizability are found to be small. A value for the dipole-quadrupole polarizability A of methane is obtained by attributing the excess scattering over that implied by the dipole-induced-dipole model to A.