Exciton–Phonon Interactions in Molecular Crystals

The interaction of Frankel excitons with optical phonons in molecular crystals is studied by means of a Green's function method in which the use of a canonical transformation allows most of the exciton–phonon interaction to be treated nonperturbatively. The self‐energy of the Green's function is then expanded in terms of the intermolecular potential. An expression is obtained for the absorptive part of the dielectric function of the exciton–phonon system. Comparison is made with experiment and also with earlier theories, some of the limitations of which are pointed out.