FEMTOSECOND 4-WAVE-MIXING SPECTROSCOPY OF MOLECULAR AGGREGATES

Two-photon resonance-enhanced four-wave mixing processes taking place in circular aggregates are considered theoretically with respect to th eir influences on pump-probe difference absorption measurements. Remar kable changes of the signal spectra around the zero time delay arising from the two-photon resonance transitions to the two-exciton states w ere discovered. Time-and frequency-resolved pump-probe measurements in the vicinity of zero delay times were shown to be sensitive to the ma nifestation of two-exciton states and, consequently, may be used for t he analysis of the transition dipole orientation of the individual mol ecules as well as the extent of exciton localization in the circular a ggregates.