Rotational–translational coupling and asymmetric line shapes in the high resolution stimulated Brillouin gain spectra of liquid carbon disulfide

We have measured high resolution stimulated Brillouin gain spectra of liquid carbon disulfide at a number of temperatures between 158 and 301 K. As the temperature decreases the shape of the Brillouin peak becomes increasingly asymmetric, with the intensity skewed away from the center of the spectrum. We show that this unusual asymmetry results from the coupling between rotational and translational motions. Rotational–translational coupling is well known to give rise to line shape distortions in the depolarized light scattering spectra of nonspherical molecules (e.g., the ‘‘Rytov dip’’), but the observation of line shape distortions (the asymmetry) in the polarized Brillouin lines has not been noted before. In the latter case, the asymmetry in the Brillouin peak can be traced to a cross correlation between the collective molecular orientation variable and the number density fluctuation and can be viewed as an acoustically induced birefringence. A calculation using the theory of rotational–translational coupling and experimental values for the relevant parameters reproduces quantitatively the observed asymmetry in the Brillouin lines over the entire temperature range.