  Rayleigh-Brillouin spectral profiles are measured with a laser-based
scatterometry setup for a 90 degrees scattering angle at a high signal-to-noise
ratio (r.m.s. noise below 0.15 \% w.r.t. peak intensity) in
sulphur-hexafluoride gas for pressures in the range 0.2 -- 5 bar and for a
wavelength of $\lambda=403.0$ nm. The high quality data are compared to a
number of light scattering models in order to address the effects of rotational
and vibrational relaxation. While the vibrational relaxation rate is so slow
that vibration degrees of freedom remain frozen, rotations relax on time scales
comparable to those of the density fluctuations. Therefore, the heat capacity,
the thermal conductivity and the bulk viscosity are all frequency-dependent
transport coefficients. This is relevant for the Tenti model that depends on
the values chosen for these transport coefficients. This is not the case for
the other two models considered: a kinetic model based on rough-sphere
interactions, and a model based on fluctuating hydrodynamics. The deviations
with the experiment are similar between the three different models, except for
the hydrodynamic model at pressures $p \lesssim 2\;{\rm bar}$. As all models
are in line with the ideal gas law, we hypothesize the presence of real gas
effects in the measured spectra.
