Collision-induced depolarized scattering

Frommhold L, Hong-Hong K, Proffitt MH (1978) Absolute cross sections for collision-induced depolarized scattering of light in krypton and xenon. Mol Phys 35 665-700... 

According to Flory and coworkers [58, 59], experimentally measured J hv contains a contribution i Hv(col) from transient, collision-induced depolarized scattering. At infinite dilution this contribution arises from the collision of individual polymer chains with solvent molecules. Hence it has nothing to do with the intrinsic optical anisotropy of the solute polymer. 

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... 

U. Bafile, L. Ulivi, M. Zoppi, M. Moraldi, and L. Frommhold. The third virial coefficients of collision-induced, depolarized light scattering of hydrogen. Phys. Rev. A, 44 4450, 1991. 

N. Meinander and G. C. Tabisz. Information theory and line shape engineering approaches to spectral profiles of collision induced depolarized light scattering. Chem. Phys. Lett., 110 388, 1984. 

Guillot NC, Chrysos M, Le Duff Y, Rachet F (2000) Depolarized collision-induced light scattering by gaseous helium. J Phys B 33 569-580... 

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. 

B. Oksengorn. Collision induced depolarization of the Rayleigh light scattering by argon at low density. Chem. Phys. Lett., 702 429-432 (1983). 

U. Baffle, R. Magli, F. Barocchi, M. Zoppi, and L. Frommhold. Line shape and moment analysis in depolarized collision induced light scattering. Molec. Phys., 49A149-1166 (1983). 

V. D. Ovsyannikov. Asymptotic line shape for collision induced depolarized light scattering in atomic gases. Phys, Lett. A, 55 275-277 (1981). 

T. Keyes, G. Seeley, P. Weakliem, and T. Ohtsuki. Collision induced light scattering from growing clusters Depolarization by fractals. J. Chem. Soc. Faraday Trans. 2,83 1859-1866 (1987). 

In summary, since the early 1980s or so, collision-induced light scattering experiments on molecular fluids have demonstrated that the study of the collision-induced absolute-unit spectra (depolarized and/or isotropic ones) is an useful tool, and up to now the only one, to measure multipolar polarizabilities of molecules. 

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. 

In the collision-induced Raman experiments the laser light of wavenumber co0 is scattered inelastically by the interacting atoms in the gas. The intensities of the depolarized, D(v), and polarized, P(v), scattered light are given by323 324,... 

Theoretical attempts to deal with complexes of more than two atoms (molecules) are scarce. One notable exception is the intercollisional process [303, 304, 306], which models the existing correlations of subsequent collisions. Intercollisional effects are well known in collision-induced absorption, but in OILS not much experimental evidence seems to exist. Three-body spectral moment expressions have been obtained under the assumptions of pairwise interactions [198, 200, 208, 209, 212, 218, 340, 422] see also references in Part II. Multiple scattering will depolarize light and has been considered in several depolarization studies of simple fluids [273, 274, 290, 376]. 

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). 

T. A. Litovitz, C. J. Montrose, R. A. Stuckart, and T. G. Copeland. Collision induced far infrared absorption and depolarized light scattering in liquids. Molec. Phys., 34 573-578 (1977). 

As noted above, experimental measurements must be very carefully performed. All measurements must be extrapolated to infinite dilution. In addition, care should be taken to subtract collision-induced scattering from the data, especially for the shorter chain molecules (see Chapter 14). For larger molecules extrapolation to zero q must be done in order to avoid intramolecular interference effects. Local field effects must also be considered when relating the experimental depolarized intensities to <(y2)>. 

Much remains to be done in this area. The role of the many-body polarizability has yet to be explored, a simple dynamical model has yet to be presented, and a microscopic justification of Eq. (14.2.5) has yet to be developed. Moreover the important practical question concerning the collision-induced scattering from molecular liquids has yet to be answered. Only after this effect is assessed can it be subtracted from the depolarized scattering in such a manner that the remaining spectrum gives information about molecular tumbling. 

Crand for a scattering angle "Q" = 90° then = 0. Generally this is not the case because either the moTecules or their arrangement are not optically isotropic or the scattering mechanism is not (collision induced scattering). Under these conditions we observe the depolarized spectrum lyu. In this case we have for optically inactive molecules ly = ijjy. 

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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