Double-scattering correction for the critical dynamics of a classical fluid

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Abstract
It is found that light undergoing double scattering in a fluid introduces a spurious curvature in the semilog plot of the time-dependent correlation function versus time. The amount of curvature to be expected is calculated as a function of the parameters of the experiment. This permits the correction of experimental data by the subtraction of the double-scattering contribution. Alternatively, we express the results of our calculation in frequency space by exhibiting the deviation to be expected from a Lorentzian spectrum. A further description of the double scattering is given in terms of a continuous distribution of relaxation rates. Also included, as a by-product, is a brief treatment of the double-scattering correction for the equal-time correlation function. Small-angle approximations, valid in the extreme critical region, lead to analytic results which are found to be in good agreement with numerical computations of Bray and Chang. As in their work, our calculations are limited to the 90° scattering geometry.