Suboptical Dielectric Losses by Condensed Nonpolar Media

Debye theory applied to the observed microwave and far-infrared losses by nonpolar liquids based on the assumption either of (a)temporary, multipole-induced dipoles or (b) a number of equivalent equilibrium positions for the molecular charges separated by potential barriers is shown to be inadequate. In dense non-polar media absorption at suboptical frequencies, i.e. frequencies below visible and near-infrared will arise from nonharmonic properties of the relative motions of electrons and nuclei. To develop a phenomenological theory of the effect in condensed fluids, we introduce the cell model and assume densities sufficiently high that a nonpolar particle is bound harmonically to the cell center through short-range interaction with its immediate neighbors for times long compared to its period of oscillation. It also interacts with all others through long-range dipolar forces provided by the Lorentz field. Assuming the fluctuating forces lead to velocity damping of the cell, the dielectric constant is shown to have the same form as found in ionic crystals in the harmonic approximation with the residual ray frequency replaced by the infrared normal mode frequency—prescribed by the present model. The medium thus shows a maximum in its absorption coefficient at the infrared mode frequency and all nonpolar media at sufficiently high densities will exhibit infrared losses. As the density of an insulating nonpolar liquid is raised, the infrared mode frequency shifts to lower values and ultimately to zero (the "dielectric catastrophe") at which point the liquid becomes metallic.