The electronic states of mixed molecular crystals - III. The general trapping problem

Abstract

A theory of energy levels is given for rigid lattice mixed crystals in which the impurity differs from the host in its intermolecular coupling properties as well as in excitation energy. The influence of secondary trapping sites, induced by the impurity in its neighbour host molecules, is considered. The effects of a vacancy, of an isotopically distinguished impurity, and of a deep trap are treated as special cases. While the band structure is sensitive in details to differences between host–guest and host–host resonance coupling its general features are unaffected except that for a range of values of coupling parameters and trap depths two levels may split away from the band instead of one. The secondary traps cause additional levels to split away from the band and are probably important in the process in which delocalized excitation in the host crystal becomes localized in the impurity, as in fluorescence quenching. Typical results of calculations based on the theory are given for mixed crystals with naphthalene as host.