Brillouin scattering in liquid sulfur dioxide

Abstract Liquid sulfur dioxide, SO 2 , shows a clearly resolved double vibrational relaxation process. The first relaxation step is associated with the two stretching modes (at 1152 cm − 1 and at 1362 cm − 1 ), with a relaxation frequency of f 1  = 22.5 MHz. The second relaxation step is associated with the bending mode (at 518 cm − 1 ), with a relaxation frequency in the GHz region. The interpolated/extrapolated ultrasound absorption data α / f 2 , taken from the literature (3 MHz to 90 MHz), were combined with our hypersonic absorption data derived from the half-width at half height of the Brillouin peaks to obtain the complete relaxation behavior of orthobaric liquid sulfur dioxide. We have measured the frequency dependence (1.9 GHz to 4.9 GHz) of the Brillouin shift and the half-width of the Brillouin peaks in orthobaric liquid sulfur dioxide at 293.15 K. For the relaxation frequency of the second relaxation step we obtained f 2  = 1.6 GHz, and as corresponding nonrelaxing bulk viscosity η v∞  = 5.0 × 10 − 4  Pa s ≈ 2 η s was found, where η s denotes the shear viscosity.