Collision-induced scattering in simple liquids investigated with simulated gain spectroscopy at ambient and high pressures

Abstract

Stimulated gain spectroscopy (SGS) provides an excellent tool for the investigation of low frequency dynamics of molecular liquids, with sufficient resolution and signal-to-noise ratio to reflect intermolecular interactions. Liquid CCl4 was selected for investigation because its isotropic polarizability ensures that depolarized scattering is due only to intermolecular interactions, thus providing a good test of various models for collision-induced (CI) anisotropy based on different interaction distances, r. The depolarized spectrum for CCl4 was decomposed phenomenologically into two modeled components: a narrow Lorentzian function with a damping rate, Γ, representing diffusive reorientation and a broad weighted exponential function of the form ωsexp(–ω)/ωc) corresponding to collision-induced scattering with a characteristic frequency, ωc. Different values for s correspond to different sources for the induced anisotropy and provide distinct models for fitting to the measured spectrum. A model with s = 1.571, indicating a collision-induced electronic repulsive force with a r-9 dependence provided the best fit to the CCl4 spectrum. The best fitted values are Γ = 2.2 ± 0.5 cm-1 and ωc = 17.1 ± 0.1 cm-1, in general agreement with earlier results from spontaneous light scattering and coherent time-domain techniques, which were usually unable to determine all three parameters from a single measured spectrum. These results in CCl4 enable comparison with liquids comprised of nearly spherical, linear and planar molecules. While a three-component model which decomposes the CI component into an intermediate frequency term and a librational term is generally best for less isotropic molecules, the application of the two-component model for diffusive reorientation and CI scattering fit many molecules surprisingly well, indicating that, for many molecular liquids, binary collisions dominate depolarized scattering even at liquid densities. In a second experiment, high pressure is incorporated into the SGS experiment and benzene spectra at atmospheric pressure and at 350 bars are analyzed using a curve-fitting model comprised of a low frequency diffusive reorientational term, a high frequency librational term, and an intermediate frequency term. The 3% increase in liquid density resulted in a 17% decrease in the rate of reorientational diffusion and a librational component with a 12% higher center frequency and a 31% reduction in inhomogeneous broadening. The intermediate frequency term exhibited the strongest pressure sensitivity, resulting in a broader, higher intensity component with a 32% higher characteristic frequency. The effects of pressure on the Brillouin spectrum of liquid benzene is also investigated with stimulated Brillouin gain measurements made at 1,540, and 880 bars of pressure. Results indicate a Brillouin shift with pressure of 1.92 GHz/kbar.