Anisotropy collision-induced light scattering

Several types of collision-induced light scattering spectra are known. We have already mentioned the depolarized translational spectra of rare gas pairs and bigger complexes which arise from the anisotropy of the diatom polarizability. Contrary to the infrared inactivity of like pairs, e.g., Ar-Ar like pairs are Raman active. Furthermore, polarized translational spectra... 

In general, the polarizability is a tensor whose invariants, trace and anisotropy, give rise to polarized and fully depolarized light scattering, respectively. Collision-induced light scattering is caused by the excess polarizability of a collisional pair (or a larger complex of atoms or molecules) that arises from the intermolecular interactions. In Section I.l, we are concerned with the definition, measurement, and computation of interaction-induced polarizabilities and their invariants. 

Collision-induced light scattering spectra vary with temperature. These variations are of interest because at high temperatures the interactions are probed at smaller separations than at the lower temperatures. An accurate knowledge of the induction operators (i.e., induced trace and anisotropy) over a maximal range of separations is most desirable. Such temperature-controlled studies are just now emerging [197]. 

Here, a. and a L are the polarizabilities of the diatom parallel and perpendicular to the internuclear separation, R12. The electrostatic theory accounts for the distortions of the local field by the proximity of a point dipole (the polarized collisional partner) and suggests that the anisotropy is given by ft Rn) 6intermolecular interactions). This is the so-called dipole-induced dipole (DID) model, which approximates the induced anisotropy of such diatoms often fairly well. It gives rise to pressure-induced depolarization of scattered light, and to depolarized, collision-induced Raman spectra in general. 

D. A. Dunmur, M. R. Manterfield, and D. J. Robinson. Depolarized light scattering studies of the collision induced polarizability anisotropy of atoms and spherical top molecules. Molec. Phys., 50 573-583 (1983). 

N. Meinander, A. R. Penner, U. Bafile, F. Barocchi, M. Zoppi, D. P. Shelton, and G. C. Tabisz. The spectral profile of the collision induced translational light scattering by gaseous CH4 Determination of the pair polarizability anisotropy. Molec. Phys., 54 493-503 (1985). 

Consequently, the light scattered from spherical molecules is not expected to be de polarized.5 Nevertheless as we see in Chapter 14, even inert gas atoms depolarize th< light (i.e., Ivh 0) this arises from the anisotropy induced by collisions, an effec which is discussed in Section 10.1 and Chapter 14. 

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