Scattering of Electromagnetic Waves in Molecular Crystals

The scattering of electromagnetic waves in molecular crystals has been investigated by considering the polarization operator of the system that results from the Dyson equation in the first approximation. The Green's function for the photon field is then calculated and an expression for the frequency‐ and wavevector‐dependent index of refraction of optical waves is developed including nonlinear effects. The excitation spectrum has been studied by evaluating the spectral intensity for the photon field. Expressions are derived for the electronic contribution to the spectral width and energy shift for physical processes: (i) resonance Raman scattering and (ii) resonances occurring when either two excitons are created or one exciton is created and the other is absorbed by a single incident photon. A theory for the dielectric permeability of the crystal is developed and a relation between the dielectric permeability, the polarization operator, and the photon Green's function is established. Contributions to the binding energy of the crystal resulting from the dispersion and scattering of the polarization waves at finite temperatures have been calculated and expressed in terms of the excitation energies and the index of refraction of the medium.