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 half-widths have been determined from the simulations. A comparison of these simulations with experiment leads to the conclusions that (1) extreme motional narrowing theory is applicable to the calculation of bandwidths for densities not less than 700 amagat; (2) the nonlinear density dependence of rotational energy relaxation times should be incorporated in the scaling laws to obtain dependable calculations of bandwidths; (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) calculations of the collision frequency for a model ensemble of hard spheres with density-dependent diameters (Ben-Amotz, D.; Hershbach, D. R. J. Phys. Chem. 1993, 97, 2295) yield good estimates for the density dependence of the simulated relaxation times.