High frequency dielectric response of dipolar liquids

Abstract

The dynamical dielectric response of a condensed system of molecules with permanent electric dipole moments is studied. We start from the dynamical extension of the Onsager theory developed by Nee and Zwanzig, but generalized so as to be applicable at high frequencies. Specifically, we retain inertial terms and introduce relaxation effects in a way valid in a high frequency limit. The dielectric function obtained incorporates a diversity of both collective and single-particle behavior, consistently, over the entire range of frequencies below those of intramolecular excitation. It is noteworthy that not only is a collective mode, or dipolar plasmon, exhibited, but that its frequency is also a resonance of the single-dipole motion. The properties of the dielectric function are analyzed and illustrated in detail, and specific applications are made to water, to hydrogen chloride, and to chloroform.