ROTATIONAL RELAXATION IN FLUID NITROGEN - AN AMBIGUITY IN THE INTERPRETATION OF THE Q-BRANCH COLLAPSE

A molecular dynamics simulation of nitrogen at 295 K and at densities ranging from 400 to 1000 amagat has been carried out. Relaxation times which give the rotational contribution to isotropic Raman Q-branch ha lfwidths have been determined from the simulations. A comparison of th ese simulations with experiment leads to the conclusions that (1) extr eme motional narrowing theory is applicable to the calculation of band widths for densities not less than 700 amagat; (2) the nonlinear densi ty dependence of rotational energy relaxation times should be incorpor ated in the scaling laws to obtain dependable calculations of bandwidt hs; (3) by rescaling molecular dynamics results of Levesque et al. (J. Chem. Phys. 1980, 72, 2744) for liquid nitrogen we show that negative rotational relaxation-vibrational dephasing cross correlation is not negligible for fluid nitrogen at high density and 295 K; and (4) calcu lations of the collision frequency for a model ensemble of hard sphere s with density-dependent diameters (Ben-Amotz, D.; Hershbach, D. R. J. Phys. Chem. 1993, 97, 2295) yield good estimates for the density depe ndence of the simulated relaxation times.