Rayleigh-Brillouin light scattering spectroscopy of nitrous oxide (N$_2$O)

TL;DR

This study measures high-resolution Rayleigh-Brillouin light scattering spectra of nitrous oxide gas and compares them to various models, revealing the effectiveness of macroscopic and kinetic models in different pressure regimes.

Contribution

The paper provides experimental spectra of N₂O and evaluates multiple theoretical models, highlighting their accuracy and limitations in describing collisional dynamics.

Findings

Tenti-S6 and Grad's 6-moment models accurately reproduce spectra within 2% deviation.

Hydrodynamic model agrees well at pressures above 2 bar but not at lower pressures.

Bulk viscosity of approximately 6 x 10^{-5} Pa·s describes internal molecular relaxation.

Abstract

High signal-to-noise and high-resolution light scattering spectra are measured for nitrous oxide (N$_2$O) gas at an incident wavelength of 403.00 nm, at 90$^\circ$ scattering, at room temperature and at gas pressures in the range $0.5-4$ bar. The resulting Rayleigh-Brillouin light scattering spectra are compared to a number of models describing in an approximate manner the collisional dynamics and energy transfer in this gaseous medium of this polyatomic molecular species. The Tenti-S6 model, based on macroscopic gas transport coefficients, reproduces the scattering profiles in the entire pressure range at less than 2\% deviation at a similar level as does the alternative kinetic Grad's 6-moment model, which is based on the internal collisional relaxation as a decisive parameter. A hydrodynamic model fails to reproduce experimental spectra for the low pressures of 0.5-1 bar, but yields very good agreement ($< 1$\%) in the pressure range $2-4$ bar. While these three models have a different physical basis the internal molecular relaxation derived can for all three be described in terms of a bulk viscosity of $\eta_b \sim (6 \pm 2) \times 10^{-5}$ Pa$\cdot$s. A 'rough-sphere' model, previously shown to be effective to describe light scattering in SF$_6$ gas, is not found to be suitable, likely in view of the non-sphericity and asymmetry of the N-N-O structured linear polyatomic molecule.