Abstract
The linear absorption spectra in the excitonic and vibronic regions in the case of mixing of Frenkel excitons (FEs) and charge-transfer excitons (CTEs) have been theoretically studied for the exciton parameters of the crystals of MePTCDI and PTCDA. Two coupling parameters for the exciton–phonon coupling are introduced: the FE–phonon coupling and the CTE–phonon coupling. The main features of the vibronics in the absorption spectra are the following: (a) the existence of a doublet structure in the vibronic spectra of CTEs; (b) the vibronic levels of the FE at intermediate values of both coupling parameters are located in the continuum of the many-particle exciton–phonon states which makes its absorption line wide and flat; (c) in the case of strong coupling (coupling constants larger than 1) a doublet of bound states appears above this continuum; (d) in the case of vanishing CTE–phonon coupling the vibronics of the charge transfer excitons practically disappear in the absorption spectra.
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