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Rayleigh-Brillouin scattering spectra of CO$_2$ were measured at pressuresranging from 0.5 to 4~bar, and temperatures from 257 to 355~K using green laserlight (wavelength 532~nm, scattering angle of 55.7$^\circ$). These spectra werecompared to two lineshape models, which take the bulk viscosity as a parameter.One model applies to the kinetic regime, i.e. low pressures, while the secondmodel uses the continuum, hydrodynamic approach and takes the rotationalrelaxation time as a parameter, which translates into the bulk viscosity. We donot find a significant dependence of the bulk viscosity with pressure ortemperature. At pressures where both models apply we find a consistent value ofthe ratio of bulk viscosity over shear viscosity $\eta_b/\eta_s = 0.41 \pm0.10$. This value is four orders of magnitude smaller than the common valuethat is based on the damping of ultrasound, and signifies that in lightscattering only relaxation of rotational modes matters, while vibrational modesremain 'frozen'.

Bulk viscosity of CO$_2$ from Rayleigh-Brillouin lightscattering spectroscopy at 532 nm