Alexander Eisfeld, MPIPKS Dresden.
Many organic molecules can form crystal like aggregates. In these aggregates the individual molecules interact strongly via transition dipole-dipole coupling. This interaction often leads to efficient coherent transport of electronic excitation along the aggregate, similar to processes that occur in natural photosynthesis. For strong interaction between the monomers the absorption-spectrum of the aggregate differs strongly from that of the uncoupled molecules. While the uncoupled monomers have a spectrum which is broadened by coupling to vibrations and disorder of the environment, the aggregate spectrum is either red-shifted (J-band) or blue-shifted (H-band). In the case of the J-band the spectrum becomes very narrow without vibrational structure. The H-band, however, remains broad and exhibits a complicated vibronic structure. In the talk I will discuss the origin of these two different cases. As examples I will consider PTCDA aggregates embedded in helium nano-droplets and organic dye molecules in solution.