Optical absorption spectra of the Holstein molecular crystal for weak and intermediate electronic coupling

We investigate the optical absorption spectrum in a Holstein model for a molecular chain with Frenkel excitons and linear coupling to one internal vibration. The model is extended for nearest-neighbor charge-transfer excitons that mix with the Frenkel excitons. We represent the Hamiltonian in a displaced oscillator (Lang-Firsov) basis and employ a problem-adapted scheme for the truncation of the phonon basis. For weak and intermediate electronic coupling, the complete absorption spectrum and the structure of the relevant eigenstates become accessible by direct numerical diagonalization. We discuss the structure of the phonon clouds and the applicability of the molecular vibron model, in which only joint exciton-phonon configurations are included. As examples, we model absorption spectra of PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) and MePTCDI (N-N-'-dimethylperylene-3,4,9,10-dicarboximide).