Brillouin Scattering and Relaxation in Liquids

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Abstract
The spectrum of light inelastically scattered from density fluctuations is considered both for nonrelaxing and relaxing fluids. When the thermal conductivity of the liquid is small, the functions describing these spectra take on particularly simple forms. In addition to the unshifted Rayleigh line, caused by scattering from isobaric density fluctuations, the spectrum for a nonrelaxing liquid is just the response of a resonant system driven by thermal energy. For a relaxing liquid, the spectral distribution function has a somewhat more complicated form, but its interpretation is still quite straightforward. The use of electric circuit equivalents illustrates the principal features of the response spectrum. These results are formal ones, but while they are useful in understanding the scattering process, they are not particularly suited for the analysis of experimental results. For this purpose, approximations are developed that enable one to relate the out‐standing characteristics of the spectrum (for example, the positions and half‐widths of shifted lines) to fundamental relaxation parameters, such as the longitudinal storage and loss moduli.

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