Cooperative Exciton States in Molecular Crystals

Abstract

In this paper we calculate the transition probability for excitation of double-exciton states in absorption and emission processes in molecular crystals. The formalism is based on the tight-binding approximation, treating the intermolecular electrostatic and exchange interactions and the interaction with the radiation field by perturbation theory. It is found that: (1) A major contribution to the transition probability for cooperative excitation to electronic states arises from intermolecular Coulomb interactions, while the contribution of intermolecular electron-exchange interactions is relatively small. (2) Contributions to the transition probability from high-order-transition multipole interactions are of considerable importance. (3) Approximate selection rules for cooperative electronic excitation imply that for one component the initial and final states are of the same parity, while for the second component the transition is symmetry allowed. (4) Theoretical evidence is obtained for the appearance of double-excitation bands in the infrared spectra of solids in the overtone region. (5) A major contribution to the intensity of the double-vibrational-exciton states arises from an intermolecular Fermi resonance effect. (6) The direct detection of the radiative annihilation of a double-exciton state is not likely to be experimentally feasible.