Rayleigh-Brillouin scattering in SF$_6$ in the kinetic regime

TL;DR

This study measures Rayleigh-Brillouin spectral profiles in SF6 gas across various pressures, comparing experimental data with different models to understand the effects of rotational and vibrational relaxation on spectral features.

Contribution

The paper provides high-precision spectral measurements in SF6 and evaluates the accuracy of multiple light scattering models, highlighting the importance of frequency-dependent transport coefficients.

Findings

Vibrational degrees of freedom remain frozen during scattering.

Rotational relaxation occurs on similar timescales as density fluctuations.

Models show similar deviations from experiment, except at low pressures.

Abstract

Rayleigh-Brillouin spectral profiles are measured with a laser-based scatterometry setup for a 90 degrees scattering angle at a high signal-to-noise ratio (r.m.s. noise below 0.15 \% w.r.t. peak intensity) in sulphur-hexafluoride gas for pressures in the range 0.2 -- 5 bar and for a wavelength of $\lambda=403.0$ nm. The high quality data are compared to a number of light scattering models in order to address the effects of rotational and vibrational relaxation. While the vibrational relaxation rate is so slow that vibration degrees of freedom remain frozen, rotations relax on time scales comparable to those of the density fluctuations. Therefore, the heat capacity, the thermal conductivity and the bulk viscosity are all frequency-dependent transport coefficients. This is relevant for the Tenti model that depends on the values chosen for these transport coefficients. This is not the case for the other two models considered: a kinetic model based on rough-sphere interactions, and a model based on fluctuating hydrodynamics. The deviations with the experiment are similar between the three different models, except for the hydrodynamic model at pressures $p \lesssim 2\;{\rm bar}$. As all models are in line with the ideal gas law, we hypothesize the presence of real gas effects in the measured spectra.