Rayleigh–Brillouin scattering from methane

The Brillouin displacements and the spectra of light scattered from CH4 in the density range from 2 to 20 amagat, y=2.1–21, have been measured and compared with hydrodynamic and kinetic theories. Using hydrodynamic theory it was found necessary to include both Burnett corrections in the equations describing translational motion and the effects of diffusion on the rotational energy relaxation time. An extended hydrodynamic theory is proposed which includes not only these corrections but, in addition, makes provision for a second internal energy relaxation. Comparisons of experimental results with this extended theory over the entire density range, with a kinetic description at the lower densities, and with translational hydrodynamic theory at higher density are presented. Owing to the success of the extended theory, it is suggested that the use of frequency dependent thermodynamic and transport coefficients is not required. Further, re-examinations of the kinetic and translational hydrodynamic formulations appear to be in order to extend their predicting ability.