Light Scattering and Depolarization in Dense Gases

Abstract

AbstractThe principal features of the theory of light scattering in a dilute (ideal) gas of isotropic molecules were established by Lord Rayleigh in a series of classical papers, the first of which appeared in 1871. The intensity of the scattered light depends on the inverse fourth power of the wavelength and is obtained by simply summing the contribution of independent molecules.In a dense medium of interacting particles the situation is more complicated as the interference between the waves scattered by the particles must be taken into account. A simple, but elegant technique of dealing with this situation is the “thermodynamic method”, initiated by Smoluchowsky and Einstein (1910). It relates light scattering directly to density fluctuations in the medium without actually referring to the molecules and it yields a scattering formula in terms of the isothermal compressibility of the medium. In 1926 Zernike and Ornstein showed that in the vicinity of the critical point the spatial correlations of fluctuations become important, while Cabannes (1929) worked out the treatment of light scattering and depolarization in a system of anisotropic molecules.The modern molecular theory deals with the problem of interference in a rather direct way. The scattered waves from different molecules are added together, taking into account the varying phases and an average over all positions is performed by means of the radial distribution function. In a first order treatment the familiar Smoluchowsky‐Einstein formula is recovered (Zimm 1945), in higher order the theory permits to take into account the effect of induced dipole scattering and it predicts the occurrence of depolarization even in systems of isotropic particles. At high density, however, the theory in its present form is unsatisfactory in that it is not capable of dealing adequately with structural effects in the medium.The molecular theory of scattering by anisotropic molecules (Benoît and Stockmayer) extends the classical Cabannes treatment by taking into account orientational correlations. A short survey will be given of recent improvements of this theory by including induced dipolar scattering. When the depolarization of scattered light as function of density is calculated for a system of anisotropic molecules it shows a minimum in the critical region but it remains finite. This is in accordance with recent measurements of depolarization in CO2 in the critical region, which will be briefly described.