Molecular rotation in some simple fluids

Abstract

Experimental studies of the rotational dynamics in liquid nitrogen, ethane, ethylene, and carbon dioxide are reported for a number of temperatures and densities along the saturated vapor lines. The intensity of depolarized light scattering was measured as a function of frequency shift; after attempting to correct for collision−induced contributions to the total intensity, the spectra were Fourier transformed to yield orientational correlation functions [C2(t*)] and cross angular momentum−orientation correlation functions [𝒢2(t*)]. It is argued that the collision−induced part may be too large relative to the scattering from permanent polarizability anisotropy to allow one to extract reliable rotational correlation functions for ethane, and possibly for ethylene as well. Data for the remaining systems (nitrogen and carbon dioxide) were compared to theoretical curves calculated from the J−diffusion model for random walk in angular momentum space. Values for the frequency of the randomization of angular momentum were derived for the experimental systems studied. The results are compared with calculations of the collision frequency in dense fluids of nonspherical hard−core molecules.