We study Rayleigh-Brillouin scattering in gases of N2, O2, and SF6 molecules, Kr atoms, and He-Xe and He-CO2 mixtures at pressures ranging from 1 to 3 bar and using two different experimental setups. In one setup, we measure spectra of light scattered by thermal density fluctuations (spontaneous Rayleigh-Brillouin scattering); in the second setup density waves are induced in the overlap region of two counterpropagating laser beams (coherent Rayleigh-Brillouin scattering). We compare measured spectra to the Tenti models and to a recent model for mixtures. We find new values of the bulk viscosity, which is a parameter in line-shape models that allows for internal degrees of freedom. Both experiments agree on the value of the bulk viscosity. Our results indicate a need for new line-shape models for mixtures of molecules with internal degrees of freedom.
We investigate coherent Rayleigh-Brillouin spectroscopy as an efficient process to measure the bulk viscosity of gases at gigahertz frequencies. Scattered spectral distributions are measured using a Fizeau spectrometer. We discuss the statistical error due to the fluctuating mode structure of the used pump laser. Experiments were done for both polar and nonpolar gases and the bulk viscosity was obtained from the spectra using the Tenti S6 model. Results are compared to simple classical kinetic models of molecules with internal degrees of freedom. At the extremely high ͑gigahertz͒ frequencies of our experiment, most internal vibrational modes remain frozen and the bulk viscosity is dominated by the rotational degrees of freedom. Our measurements show that the molecular dipole moments have unexpectedly little influence on the bulk viscosity at room temperature and moderate pressure.
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