FEMTOSECOND NONLINEAR-OPTICAL DYNAMICS OF EXCITONS IN J-AGGREGATES

Time-resolved difference absorption spectra of J-aggregates were measu red by femtosecond pump-probe spectroscopy. The induced absorption nea r the J-band at 20 K was assigned to the transitions from n-exciton st ates to (n = 1)-exciton states (n equal to or greater than 1). The dec ay time of the n(equal to or greater than 2)-exciton states is determi ned to be about 200 fs. The ultrafast dynamics of the exciton states i s explained by a confined exciton model in individual coherent aggrega tes, which incoherently constitute the whole structural aggregate. The average size of the exciton system defined for each coherent aggregat e is estimated to be about 20 molecules.