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Frenkel Excitons--Charge-Transfer Excitons--Phonons Coupling in One-Component Molecular Crystals

In this paper, we simulate the linear absorption spectra of the MePTCDI and PTCDA crystals. The basic Hamiltonian describes the Frenkel excitons and charge-transfer excitons mixing in the molecular stack (point group C_i) and their linear coupling with one vibrational mode of an intramolecular vibration. Using the vibronic approach, we calculate the linear optical susceptibility in the excitonic and one-phonon vibronic regions of the molecular stack and of a crystal with two types of nonequivalent stacks. We put the excitonic and vibrational parameters for the two crystals fitted in the previous studies and analyze some important features of of the linear absorption lineshape in the spectral region of 15 000--23 000 cm^-1 and the virtual positions of many-particle bands. Our study exhibits the necessity of introducing the FE--CTEs mixing in the interpretation of the linear absorption spectra, especially in the MePTCDI crystal.