Stimulated Rayleigh-Brillouin spectroscopy for studying low-frequency dynamics in simple liquids

Dynamics of simple liquids are studied in the frequency range from 0 to 200 cm{dollar}sp{lcub}-1{rcub}{dollar} using stimulated gain spectroscopy (SGS). Experimental studies of reorientational and collision-induced molecular dynamics in simple liquids are done using depolarized Rayleigh wing SGS. In addition, SGS is used for the first time to study the effects of high pressure on the Brillouin spectrum of a simple molecular liquid.; We show that SGS is advantageous for use in low-frequency regions where the large elastic scattering intensity interferes with the spontaneous light scattering signal from the material under study. Also, we show how SGS reveals broad weak structure on the wings of the depolarized Rayleigh light scattering spectrum. Precise wavevector selection and instrumental bandwidth dependent only on the laser linewidth make SGS especially useful for Brillouin spectroscopy. SGS is shown to be superior to both spontaneous light scattering and to other forms of coherent spectroscopy for revealing the low-frequency dynamics studied here.; Depolarized spectra of liquids CS{dollar}sb2{dollar}, benzene, pyridine, chlorobenzene, bromobenzene, iodobenzene, methylene chloride and chloroform are investigated in the frequency range from 0 to over 100 cm{dollar}sp{lcub}-1{rcub}{dollar}. Their lineshapes are fit to phenomenological functions consisting of: (1) a low frequency Debye-damped reorientation of half-width between 1 and 4 cm{dollar}sp{lcub}-1{rcub}{dollar}, (2) an intermediate frequency collision-induced decay with half-width between 5 and 20 cm{dollar}sp{lcub}-1{rcub}{dollar}, and (3) an inhomogeneously broadened oscillator with both frequency and width in the range of 20 to 65 cm{dollar}sp{lcub}-1{rcub}{dollar}. Models based on these dynamics provide excellent fits in most, but not in all liquids studied. Because the measured spectra have very good signal-to-noise, they should be compared against theoretical gain spectra based on first principles calculations, when they become available.; As a demonstration of its usefulness, we show that SGS can generate Brillouin signals in a weak scatterer, ethanol, under hydrostatic pressures up to 4 kbar. Given its advantages in wavevector selection and high frequency resolution, SGS promises to be a powerful tool for obtaining high precision Brillouin spectra from a transparent sample in a high pressure cell.