Spectrum of Light Scattering from Thermal Shear Waves in Liquids

Abstract

High-resolution studies of the depolarized Brillouin spectra of many molecular liquids have revealed a prominent doublet centered at the exciting frequency. The observed line shapes and intensities of the $I_H^V$ and $I_H^H$ polarized spectra, as well as the $\overrightarrow{\mathrm{k}}$-vector dependence of the doublet splitting on scattering angle, are all in quantitative agreement with the theories of Leontovich, Rytov, and Volterra on light scattering from orientational motions caused by heavily damped shear waves. Analyses of the spectra on the basis of these theories have given values of the shear-wave frequencies ${\nu }_T$ and relaxation times $\tau$. Values of $\tau$ in the range 3 × ${10}^{-10\mathrm{}}$ -1 × ${10}^{-11\mathrm{}}$ sec were obtained and $\tau$ was found to decrease with increasing temperature. Values of the shear modulus determined from ${\nu }_T$ and its dependence on $\theta$ were found to range from 0.6×${10}^8$ to 9×${10}^8$ ${\mathrm{d}\mathrm{y}\mathrm{n}/\mathrm{c}\mathrm{m}}^2$, and to be essentially independent of temperature. Of the 27 liquids investigated, only those composed of geometrically and electrically anisotropic molecules exhibited depolarized-doublet spectra, and the resolution of the intensity minima was limited by the magnitude of the relaxation time and by the instrumental linewidth.