Lattice Scattering of Exciton in Molecular Crystals

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Abstract
The general formulas of scattering probabilities by acoustic- and optical-mode vibrations are computed for the Frenkel exciton in molecular crystals. Two cases, fast and slow excitons, are distinguished in the formulation, and the different dependencies on temperature and wave vector are shown. At low temperature both acoustic and optical modes have about the same effect on the fast exciton, while the acoustic mode has a predominant effect on the slow exciton. The dependence of scattering probability on the direction of exciton wave vector relative to the crystal axis is also shown. The mean free path and the diffusion coefficient are calculated for the thermal equilibrium state attained after a multiple scattering. Comparison with theoretical and experimental results of other authors is discussed.