The crystal spectra of very weak transitions

Abstract The theory of intermolecular resonance effects in the spectra of molecular crystals is extended to very weak systems in which the pure electronic transitions are either forbidden (as in the benzene 2600 å system) or comparable in intensity with accompanying vibrationally induced transitions (as in the naphthalene 3200 å system). For the pure electronic transitions the intermolecular coupling is through transition octupole moments, giving splittings and intensity transfers of the same type, though smaller, as those familiar in the stronger, dipole-coupled, systems. Values of octupole-octupole and octupole-dipole interactions sums are reported for naphthalene and the crystal spectrum calculated. For vibration-induced transitions it is shown that the intermolecular effects are due only to the small component of ‘stolen’ character, thus allowing the lack of splitting and intensity transfer to be understood. The nature of such bands as arising from localized excitation can thus be explained in the framework of the simple rigid-lattice excition theory.