Excitonic and vibronic structure of absorption spectra of Me-PTCDI and PTCDA crystals

The excitonic and vibronic spectra (exciton + one quantum of intramolecular vibration) of Me-PTCDI and PTCDA crystals are studied in the case of strong mixing of a Frenkel exciton (FE) and charge-transfer excitons (CTEs). The linear optical susceptibility is calculated in the framework of dynamical theory of vibronic spectra. The absorption spectra of both crystals have been modelled. The positions, maximal values and integral intensity of the absorption peaks which correspond to the bound exciton-phonon states and to unbound (many-particle) states have been calculated using FE and CTEs's parameters of Me-PTCDI and PTCDA crystals. The calculated spectra show: (i) the possible recovering of excitonic and vibronic regions; (ii) bigger integral intensity of many-particle states in the case of weak exciton-phonon coupling; (iii) the dominant role of the bound states in the case of intermediate and strong exciton-phonon coupling.