Far-infrared resonance spectroscopy and the effective intermolecular potential.

Abstract

The Kramers equation is solved numerically for circular diffusion in an arbitrary periodic potential V(\ensuremath{\theta}), where \ensuremath{\theta} is the angular coordinate. In the limit \ensuremath{\beta}\ensuremath{\rightarrow}0, where \ensuremath{\beta} becomes proportional to the rate of escape from the potential wells, the far-infrared power absorption coefficient shows many different peaks due to resonance in highly nonlinear systems as described by Bogoliubov and Mitropolsky for ordinary, nonlinear, differential equations. The far-infrared resonance spectrum (FIRRS) is sensitive to the details of V. In structured (associated or hydrogen-bonded) molecular liquids such as water or acetonitrile, or in liquid crystals, the FIRRS peaks can be resolved experimentally. These peaks provide, in principle, detailed information on the effective intermolecular potential in systems of interest such as nematogens. The FIRRS spectrum is sensitive to an applied, uniaxial, external, static, electric or magnetic field.