Light-Scattering by Longitudinal phonons in Molecular Supercooled Liquids I: Phenomenological Approach

TL;DR

This paper develops a phenomenological model for light scattering in supercooled molecular liquids, accounting for molecular anisotropy, density fluctuations, and retardation effects, revealing complex spectral features influenced by phonons and relaxation times.

Contribution

It introduces a comprehensive expression for Brillouin scattering in supercooled liquids, including new scattering channels and their impact on spectral line-shapes.

Findings

Spectral splitting into rotational and phonon-dependent components.

Identification of two new scattering channels affecting line-shape.

Modification of Brillouin spectra when relaxation time matches phonon period.

Abstract

We derive expressions for the intensity of the Brillouin polarized spectrum of a molecular liquid formed of axially symmetric molecules. These expressions take into account both the molecular dielectric anisotropy and the modulation of the local polarisability by density fluctuations. They also incorporate all the retardation effects which occur in such liquids. We show that the spectrum splits into a q-independent rotational contribution and q-dependent term, which reflects the propagation of longitudinal phonons. In the latter, the two light scattering mechanisms enter on an equal footing and generate three scattering channels. We study the influence of the two new channels and show that they may substantially modify the Brillouin line-shape when the relaxation time of the supercooled liquid and the phonon period are of the same order of magnitude.