The coupling between electromagnetic waves and excitation waves in a molecular crystal

Abstract

Analysis is given of some of the approximations involved in the electrostatic calculation of the dipole-dipole forces between molecules in an excited state of a molecular crystal. It is shown that there is a close analogy between the quantum mechanical theory of these excited states and the classical theory of the vibrations of an ionic crystal. The theory of long waves developed by Born & Huang for an ionic crystal is adapted for a molecular crystal which has one molecule in each unit cell. This enables the effects due to the coupling of the transverse excitation waves and the electromagnetic waves, the retardation of the forces, the molecular polarizability and the non-analytic behaviour of the energy bands to be included in the theory. A further modification of the theory allows for the finite life time of the excited state and produces changes in the energy bands, the extinction coefficient and the reflectivity.