On the role of inertial effects and dipole-dipole coupling in the theory of the Debye and far-infrared absorption of polar fluids. II. Comparison with experimental observations

Abstract

A general two-friction itinerant-oscillator model is given for molecular motion in dipolar fluids. The polarizability and relevant time-correlation functions are calculated by numerical methods. It is shown how the results of an earlier paper are obtained when the two frictions per unit inertia are equal. The model is compared with recent experimental measurements on hexanone-2 and acetonitrile obtained by using a new quasi-optical technique for frequencies intermediate between the MW and FIR spectral regions. This technique is described briefly; in particular, for a reasonable fitting of the theory to experiment the moment of inertia of the outer cage of molecules is always greater than that of the inner dipole in agreement with the original concept, namely a cage of dipoles surrounding a typical dipole. This is at variance with the earlier concept of the model where the friction on the encaged dipole is zero. There, for a good fit to experiment, the moment of inertia of the cage is always less than that of the dipole. This suggests that the two friction version should be used for such comparison. An analytical formula (analogous to the Rocard equation) for the polarizability derived earlier is shown to agree well with the result of numerical Fourier transformation of the response function. A distinct harmonic peak structure is noted in the FIR region of the spectrum when friction does not act on the inner dipole. The structure is damped out as this friction is increased. Thus only the first harmonic peak is significant when the frictions per unit inertia are approximately equal. Measurements in the frequency region (the intermediate region) lying between the microwave and the FIR for hexanone-2 in dilute solutions of cyclohexane are reported. These show a significant excess absorption over that predicted by the model in this region. It is suggested that the excess absorption arises from jumping of dipoles from well to well of the cosine potential of dipole-dipole interaction indicating that the effects of finite potential well depth must be incorporated in the model to explain the intermediate frequency absorption.