  The spectral distribution of light scattered by microscopic thermal
fluctuations in binary mixture gases was investigated experimentally and
theoretically. Measurements of Rayleigh-Brillouin spectral profiles were
performed at a wavelength of 532 nm and at room temperature, for mixtures of
SF$_6-$He, SF$_6-$D$_2$ and SF$_6-$H$_2$. In these measurements, the pressure
of the gases with heavy molecular mass (SF$_6$) is set at 1 bar, while the
pressure of the lighter collision partner was varied. In view of the large
polarizability of SF$_6$ and the very small polarizabilities of He, H$_2$ and
D$_2$, under the chosen pressure conditions these low mass species act as
spectators and do not contribute to the light scattering spectrum, while they
influence the motion and relaxation of the heavy SF$_6$ molecules. A
generalized hydrodynamic model was developed that should be applicable for the
particular case of molecules with heavy and light disparate masses, as is the
case for the heavy SF$_6$ molecule, and the lighter collision partners. Based
on the kinetic theory of gases, our model replaces the classical
Navier-Stokes-Fourier relations with constitutive equations having an
exponential memory kernel. The energy exchange between translational and
internal modes of motion is included and quantified with a single parameter $z$
that characterizes the ratio between the mean elastic and inelastic molecular
collision frequencies. The model is compared with the experimental
Rayleigh-Brillouin scattering data, where the value of the parameter $z$ is
determined in a least-squares procedure. Where very good agreement is found
between experiment and the generalized hydrodynamic model, the computations in
the framework of classical hydrodynamics strongly deviate. Only in the
hydrodynamic regime both models are shown to converge.
