Rayleigh-Brillouin scattering from [formula omitted] and [formula omitted] mixtures at elevated pressures

This paper presents spectral measurements of Rayleigh-Brillouin scattering (RBS) spectra in binary mixtures of and at a number of mixture ratios at 300 K, and pressures ranging from 4 to 60 atm. To identify trends in lineshape changes associated with addition, measured spectra of pure and at 300 K and similar pressures are included. These measurements were acquired with a custom spectrometer based on an air-spaced virtually-imaged phased array whose resolving power was on the order of 10. Improvements upon previous work to the data processing procedure, including revised implementation of the scalar envelope correction, and the addition of a dispersion correction, are described. Measurements of RBS spectra in pure and are compared to their counterparts modelled by Tenti S6. Good agreement is observed in pressures up to 20 atm, and discrepancies at higher pressure are attributable to known limitations in the measurements and model. Attenuation and broadening of the Brillouin and central Rayleigh peaks, as well as an outward shift of the Brillouin doublet, are observed with addition. These trends are related to the dissipative role of on thermodynamic fluctuations in the fluid medium, enhanced by its mass disparity with the host species. Analysis of spectral energy distribution in mixture spectra at 20+ atm reveals a net transfer of spectral energy from the Brillouin doublet towards the central Rayleigh peak with increasing dilution. Experimental and modelled RBS spectra, and a model I filter, are used to test the validity, at elevated pressure, of approximating a mixture filtered Rayleigh scattering (FRS) signal by a mole fraction-weighted sum of isolated FRS signals from mixture constituents. FRS signals calculated from measured mixture spectra are compared to their approximations calculated from modelled pure gas spectra. Results show limited and inconsistent agreement. Where agreement is strong, further analysis reveals a sensitive counterbalance of factors affecting FRS signal generation that is not reliable for robust FRS signal inversion. The work demonstrates the need for improved spectral models to support the application of FRS diagnostics to fluid mixtures at elevated pressure.