FRACTAL APPROACH TO THE EXCITON-PHONON COUPLING AND STRUCTURE OF J-AGGREGATES

We present a study on exciton-phonon coupling in molecular aggregates at low temperature probed by photon-echo and resonance Raman experimen ts. A large peak in the low-frequency Raman spectrum is interpreted as a boson peak. Our results for the J aggregates of the dye 5,5', yl-3, 3'-di(4-sulfobutyl)-benzimidazolocarbocyanine indicate an anomalous vi brational density of states with a spectral (or fracton) dimension of 1.44 at 3.9 K. The spectral dimension increases with temperature, sugg esting that the topological dimension of the aggregates in solution is 1 and that an increase in temperature leads to an increase in effecti ve branching of the aggregate.