Dynamics of Frenkel excitons in 2-dimensional molecular crystals

We investigate theoretically the linear low-temperature excitonic and vibronic absorption spectrum of Frenkel excitons in a 2-dimensional lattice with two identical, but differently oriented molecules in the unit cell. The calculation of the spectrum is based on Green's function formalism using the dynamic and dispersive approximations according to Rashba et al. and Lalov et al., respectively. With the experimental data for anthracene we find that the linear coupling between excitons and intramolecular vibrations allows for a quasi-bound state between the exciton and a single phonon, but is not able to couple an exciton and more than one phonon. The coupling is strengthened by the contribution of quadratic exciton-phonon coupling.