Non-Critical Rayleigh Scattering from Pure Liquids

Abstract

The spectrum of light scattered by a non-critical pure liquid contains both Brillouin components, shifted from the centre frequency, and a non-shifted Rayleigh component. Acoustic phonon scattering gives rise to the Brillouin lines which are typically shifted by a few GHz from the laser line. Isobaric entropy fluctuations give rise to the non-shifted Rayleigh component whose width depends on the thermal diffusivity of the liquid. The half-width at half-height of the Rayleigh line is given by

$$\Gamma \left( {{\rm{radiance/sec}}} \right) = \frac{\Lambda }{{{\rm{C}}_{\rm{p}}^{\rm{*}}}}{\left( {{\rm{2n}}\,{{\rm{k}}_{\rm{o}}}\,{\rm{Sin}}\frac{{\rm{\theta }}}{{\rm{2}}}} \right)^2},$$

((1))

where k_o is the magnitude of the incident wave vector,θ the scatter­ing angle, n the refractive index, C ^*_p the specific heat per unit volume and Λ the thermal conductivity. The thermal diffusivity de­pends on the specific heat and the thermal conductivity and is given by Λ/C ^*_p .