Resonance Absorption: A New Absorption Mechanism in Solids

Abstract

Thermal oscillations in molecular solids are of two distinct classes: lattice vibrations or the acoustical branch, and molecular vibrations or the optical branch. These two vibrational classes are weakly coupled; hence the exchange of energy is negligibly small except when these two branches overlap in frequency and “resonate.” This phenomenon is closely analogous to the exchange between two weakly coupled pendula; appreciable energy is transferred only at or near resonance, in which case the rate of transfer is slow. Stated in other terms, the relaxation time in resonant processes is usually lengthy. A long relaxation time is often manifested as anomalously high acoustic absorption. The relaxation time and hence the absorption in molecular solids can be calculated, using the procedures of a time-dependent perturbation calculation in quantum mechanics. Molecular constants for the calculation are obtained from the Lennard-Jones potential. Calculation shows that many molecular solids can be expected to exhibit unusually high acoustic absorption resulting from resonance absorption. As an example, benzene was studied experimentally. A single crystal of average linear dimension 10 cm was grown from the liquid. Absorption measurements using pulse techniques were made in the frequency range from 3 to 10 Mc. At 10 Mc the observed absorption has the excessively high value, 0.2 cm−1; this value is within limits of the theoretically predicted value. Thus resonance absorption in many molecular crystals can be expected to be 10 to 100 times greater than other absorption processes.