Rayleigh and Brillouin Scattering in Liquids: The Landau—Placzek Ratio

The classical theory of light scattering in liquids leading to the Landau—Placzek ratio for the intensities of the Rayleigh and Brillouin components is reviewed. A simple modification is proposed in which the effect of dispersion is included by defining a hypersonic compressibility βshs=1/(ρvhs2), where vhs is found from the experimentally measured Brillouin shifts. Further modification is presented to account for dispersion in the electrostriction. A modification of the Landau—Placzek equation due to Fabelinskii is discussed and shown to be essentially equivalent to our equation. The predictions of the ratios are evaluated for 11 common liquids and compared with new experimental values reported in this paper and with other values taken from the literature. It is found that in most cases the dispersion modification leads to ratios in considerably better agreement with the experimental values than does the simple Landau—Placzek equation. We present our measurements of the ratio of the Rayleigh and Brillouin intensities obtained with an He–Ne laser light source and photoelectric detection. We discuss the experimental technique and the data-reduction procedure. The detection time constant was sufficiently short that contributions to the Rayleigh component from dust were distinguishable from true molecular scattering. The anomalously large central component for water which has been frequently reported in the literature was thus traced to residual dust contamination and was found to be less than 2% of the Brillouin-component intensity when dust was absent.