Line shape of a molecular resonance

In this paper we consider some implications of intramolecular electronic relaxation on the optical line shape of large molecules in the statistical limit. General expressions for the line shape were derived utilizing the Green’s function formalism, which account both for interference with background absorption and for interference between resonances. The energy dependence of the line-width functions was elucidated. We have demonstrated that the line shape for an ‘isolated’ resonance is a fanian; however, as the line profile index is determined by the ratio of the level spacing to the non-radiative width, the corrections to the lorentzian line shape are small in this case. We have established the equivalence of the physical description utilizing a three-level crude adiabatic basis and a two-level adiabatic basis. In the case of overlapping resonances the line shape may be recast in terms of a modified Fano-type formula where the line profile index is energy dependent. The nature of interference effects in this case is determined by the ratio and the relative signs of projections of the transition moments on the polarization vector.