The effects of molecular geometry on the depolarized stimulated gain spectra of simple liquids

Abstract

Depolarized Rayleigh-wing spectra of simple liquids, including CS2, chlorinated methanes, benzene and benzene derivatives have been recorded using stimulated gain spectroscopy. To adequately interpret these spectra of simple liquids, a dephasing dynamical contribution in an intermediate time scale is needed in addition to a slower Debye reorientation relaxation and a faster broad inhomogeneous oscillator. The curve-fit dynamical characteristic parameters of these simple liquids are tabulated for comparison. The microscopic molecular properties, shape anisotropy, and dominating moment of inertia are found to play an important role in determining the rates of interaction-induced dynamics; smaller shape anisotropy and smaller moment of inertia give rise to faster collision rate in the intermediate frequency regime and faster oscillation in the higher frequency regime, respectively. In the low frequency region, the shear viscosity (a macroscopic property) of the liquid and the general molecular shapes dictate reorientation diffusion; for molecules within a family, the Debye relaxation rate is faster for liquids with smaller viscosity and shape anisotropy. Halogenation of benzene breaks the symmetry of the benzene ring giving two distinct oscillator frequencies. In the case of iodobenzene, three principle molecular axes are clearly distinct; it is necessary to use two Debye relaxation rates and two broad inhomogeneous oscillators to fit its stimulated gain spectrum.