Direct Radiative Exciton—Exciton Annihilation

Abstract

Several experiments on organic crystals have demonstrated the existence of triplet—triplet exciton annihilation in terms of fluoresence from the resulting singlet‐exciton state. We point out the possibility of direct emission of a photon from a pair of excitons and estimate the corresponding lifetime and bimolecular rate constant. While this decay channel for triplet—triplet collisions is very weak compared with decay into a singlet exciton in a crystal such as anthracene, the process is observable in principle as a characteristic emission with a transition probability of ≲ 10⁵ sec⁻¹ at an energy equal to that of two interacting excitons. Its rate depends strongly on the relative energies of all the molecular states involved. Conditions for observation of this emission band are discussed.