Light Scattering from Shear Waves in Liquids

Abstract

In liquids with molecules having nonspherical polarizability, the spectrum of scattered light contains the usual central and Brillouin components and in addition, a depolarized component known as the Rayleigh “wing.” This depolarized component is centered at zero frequency, is often very broad (sometimes extending as far as 10 or even 100 cm−1), and arises from thermal fluctuations of anisotropy that are believed to originate from molecular rotational motions in liquids. We have studied the depolarized spectra from a number of liquids, under the high resolution available with a He-Ne laser and a pressure-scanned Fabry-Perot interferometer. In some liquids, this spectrum exhibits a doublet with an intensity minimum at the exciting frequency and with a peak separation of 1 to 5 Gc/sec, depending on the liquid. We shall discuss our observations of the polarization characteristics, line shape and intensity of the spectrum, along with measurements of the k-vector dependence of the doublet splitting. These results confirm in detail many of the features predicted in the theory of Rytov for light scattering by shear waves in liquids and thus lend strong support to the explanation that the depolarized doublet spectrum originates from scattering by thermally excited shear waves. It will also be shown that for those liquids for which the doublet splitting can be resolved, detailed analyses of the spectra yield values of the transverse sound velocity, the high-frequency shear modulus, and the shear relaxation time.