Molecular motions in liquids: comparison of light scattering and infra-red absorption

Abstract

A comparison of infra-red absorption and Rayleigh light-scattering spectra show that whereas the low-frequency (Lorentzian) regions of each yields information on the orientational relaxation times, consideration of the high-frequency “wings” of the spectra can yield information on the details of the diffusive steps involved in orientational motion. We show how i.-r. and light-scattering (LS) spectra can yield the angular velocity correlation function from which one can determine the time between collisions. In addition to orientational effects, collision-induced aspects of i.-r. and LS spectra are considered. It is shown why polar liquids exhibit a high-frequency maximum. This frequency decreases with increasing temperature; the collision-induced i.-r. maximum varies in the opposite fashion.