ELECTRONIC STATES OF MIXED MOLECULAR CRYSTALS - SINGLET EXCITED STATES OF ISOTOPIC IMPURITIES IN CRYSTALLINE BENZENE AND NAPHTHALENE

In this paper the one-particle Green's-function method has been applied for the study of singlet excited states of isotopically mixed crystals of benzene and naphthalene. Calculations of energy levels, of excitation amplitudes, and of line-shape functions were performed, considering both bound and virtual impurity energy levels. The present treatment relates experimentally observable properties for the dilute mixed crystals with the exciton density-of-states function of the pure crystal. The available experimental data for mixed crystals can be adequately accounted for using empirical data for the exciton density-of-states functions derived from hot-band spectroscopy. The theoretical density of states for naphthalene based on the transition-octupole coupling terms provide only qualitative agreement with the experimental mixed-crystal data. We consider the applicability of simple perturbation expressions by using a moments-expansion method of the Green's function. Finally, we discuss the information on the intermolecular interactions derived from the moments of the density-of-states functions.