Theoretical interpretation of electro-absorption spectra for intense optical transitions

The interaction of a light wave with a molecular crystal subjected at the same time to the influence of static electric field is analyzed. The coupling of the crystal to the radiation field is described in terms of classical electrodynamics, the molecular transition moments being represented by oscillating dipoles. The molecular parameters that enter the classical equations of motion (transition energy and oscillator strength), modified by the static electric field, are derived from the corresponding zero-field values using quantum-mechanical perturbation theory. Subsequently, the field-induced change of the absorption spectrum [electro-absorption (EA) signal] is calculated as the difference between the absorption spectra at nonzero and at zero modulating field. The approach is valid for any allowed transition, irrespective of its intensity. The results demonstrate that, while the absorption spectra of very intense transitions exhibit substantial peculiarities (such as orientational dispersion and polariton effects), the relationship between the absorption and electro-absorption spectra is always the same, regardless of the oscillator strength; specifically, the EA signal of a nondegenerate Frenkel exciton follows the first derivative of the corresponding absorption band. These conclusions are discussed in the context of recent literature on this subject.