Depolarized Light Scattering: Theory of the Sharp and Broad Rayleigh Lines

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Abstract
A statistical theory of depolarized laser light scattering is developed, based upon the projection operator transport theory of Mori. It is assumed that the autocorrelation function of the dielectric tensor is proportional to the autocorrelation function of a ``primary'' variable, and the time evolution of this autocorrelation function is calculated by means of a pair of coupled linear transport equations in which the primary variable is coupled to a ``secondary'' variable. The theory ignores some of the k−dependent fine structure of the spectra and concentrates upon explaining the sharp and broad depolarized Rayleigh lines which dominate the spectra. It is concluded, in contradiction to numerous existing theories, that the observed spectra cannot be explained in terms of a single primary variable. It is believed that the sharp line arises from molecular reorientations in the role of primary variable and that a separate mechanism, dependent upon a distinct primary variable such as the one describing local field fluctuations, gives rise in large part to the broad line. The theory is also used to obtain autocorrelation functions for reorientation in an ``extended diffusion model limit.'' The relationships between one‐ and two‐particle orientational correlation functions are discussed, and it is shown how the discrepancies between reorientation times determined by NMR and those determined by light scattering might possibly be understood on this basis.

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