Intermolecular Light Scattering in Liquids

Abstract

The inelastic scattering of light arising from the time-dependent polarizability induced by intermolecular interactions has been observed in the simple classical liquid, argon. The spectrum at 90 °K and 1.5 atm is centered at zero frequency shift, and is exponential in shape, with a characteristic width of 21 ${\mathrm{cm}}^{-1\mathrm{}}$. The depolarization ratio for the scattered light is 0.72 ± 0.02, implying that the anisotropy of the polarizability dominates the scattering. The liquid spectra are compared with previously obtained spectra in gaseous argon and with the low-temperature gas data obtained in the present work. Both the linewidth and intensity for the liquid spectra suggest the inadequacy of the simple collision-induced view of the scattering process when clustering occurs. Calculations of the liquid spectrum, based on a model which is a generalization of the second-order Raman effect, yield for the liquid the correct line shape and a linewidth much closer to that observed than does the binary collision model. The lack of structure in the spectrum is taken as direct evidence against the existence of well-defined collective modes of short wavelength in liquid argon.