Light scattering in binary gas mixtures : evidence of fast and slow sound modes

Abstract

In this thesis Brillouin scattering data are presented for three different binary gas mixtures composed of species with widely different masses, i.e. Ar + H₂(mAr/mH₂ ≃20), SF₆ + CH₄ (mSF₆/mCH₄ ≃ 9), and SF₆ + H₂(mSF₆/mH₂ ≃73). Under the experimental conditions predicted by the theory a fast sound mode contribution to the light scattering spectra of Ar + H₂ mixtures has been detected. Unlike the ordinary sound mode the fast sound mode propagates only in the light component with a velocity higher than that obtained using hydrodynamic theory. However, the attempts to observe the same effect in mixtures of SF₆ + CH₄ under similar conditions were unsuccessful. An analogous slow sound mode contribution to the spectra of SF₆ + H₂ mixtures has been clearly identified and it was found this mode was, in general, much easier to detect. -- A new parameter called the effective mean free path was also introduced in order to obtain a more consistent characterization of the dynamical behavior in binary mixtures with disparate masses.