MD SIMULATION OF ROTATIONAL RELAXATION IN LIQUID-NITROGEN - COMPARISON WITH NMR AND RAMAN DATA

A simulation of liquid nitrogen has been performed along the coexistence curve. Angular momentum, rotational energy and second-order orientational correlation times tau(j), tau(E), tau(theta,2) were calculated. The first and the last were compared with the available data. The results for tau(E) were compared with halfwidth data from the isotropic Raman scattering measurements. For the entire coexistence curve, the validity of stochastic perturbation theory was shown: omega(Q) tau(E) < 1, where omega(Q) is an average frequency of the Q branch. The rotational contribution to the halfwidth (estimated from the extreme narrowing formula) decreases with cooling, but does not become less than 25% of the whole width. The ratio tau(E)/tau(J), varies from 1.7 to 1 in the region between the critical and the triple points, which is inconsistent with the Langevin description of the angular momentum relaxation.