Ultrafast exciton dynamics in molecular systems

The theory of subpicosecond Frenkel exciton dynamics in molecular systems is reviewed with emphasis on a stepwise improved description of the coupling to intra- and intermolecular vibrations. After introducing the concept of multiexciton states the motion of electronic Frenkel excitons as they appear in light harvesting antennae of photosynthetic organisms is discussed. The description is based on a multiexciton density matrix theory which accounts for the exciton-vibrational coupling in a perturbative manner. Some improvements of this density matrix theory as suggested in literature are shortly mentioned. Afterwards, vibrational Frenkel excitons as found in polypeptides are considered. By utilizing the multiconfiguration time-dependent Hartree method an exact description of the coupling to longitudinal vibrations of the peptide chain becomes possible. The discussion of the computed transient infrared absorption spectra is supported by the introduction of adiabatic single- and two-exciton states.