Influence of Exciton Phonon Interaction on Metallic Reflection from Molecular Crystals

Abstract

Reflection spectra of a number of polymethine dyes are presented and analysed. Characteristic features of these spectra appear to be outside the framework of the Lorentz-Lorenz theory. An alternate approach based on an appropriate linear response function for surface excitation is developed. The linear response at a particular frequency involves a sum over all k-dependent Green's functions. Damping is brought in through the exciton-phonon interaction. A model Hamiltonian is introduced which can be diagonalized exactly for the single molecule. For the crystal, this permits the treatment of single phonon effects in the strong coupling limit almost as if they were multiple phonon effects. Structure, mainly associated with damping, in the reflection spectrum is expected and found in those regions of the exciton dispersion curve where the density of momentum states is large. Reasonably good agreement is found between the theoretical and experimental reflection curves.