A quantum statistical estimation of the amount of energy dissipated by molecular relaxation processes in liquids

Abstract

The physical conditions concerning the far-infrared (FIR) absorption process in molecular liquids are discussed. It is shown that the well-known first-order perturbational treatment of the fluctuation-dissipation theorem (FDT) cannot be justified within quantum statistics. The basic physical concepts concerning the derivation and validity of the generalized FDT as revealed in an earlier article are discussed. It is shown that dissipation of irradiation energy is mainly due to the coupling V of the molecular system with the thermal bath, as proposed by van Vliet. The following cases are treated: (i) The coupling V leads to a time-dependent density operator. It turns out that nonvanishing dissipation is due to dynamical fluctuations of the density operator. (ii) The operator m(t) which represents the coupling of the system with an external field obeys, for V ≠ 0, a non-Markovian equation of motion. The irradiation energy dissipation is a direct consequence of memory effects concerning m(t). The “anomalous” temperature dependence of the FIR absorption bands of CH3CN is presented and explained by means of the generalized FDT.