Memory functions for angular motion in liquids

Abstract

Experimental memory functions are calculated corresponding to the orientational correlation function g(t)=〈u(0)·u(t)〉 and to the rotational velocity correlation function g_rv(t)=〈ů(0)·ů(t)〉. Analytical expressions are obtained corresponding to the Cole-Glarum and the Fatuzzo-Mason theories, relating the spontaneous fluctuations of orientation to the frequency-dependent absorption.

The properties of these functions are interpreted in the framework of generalized Langevin theory. Particular attention is given to the damped oscillatory features of the first memory function, and to the slowly decaying features of the second. This analysis is achieved by investigating the temperature and density variations in some simple organic liquids (CH₃F, CH₃Cl, CHF₃, CHCl₃ and OCS). The occurrence of a plateau in the rotational velocity memory function is discussed. A tentative interpretation of this new feature is conducted by analogy with molecular dynamics experiments dealing with translational motion.