Femtosecond four-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 their influences on pump-probe difference absorption measurements. Remarkable changes of the signal spectra around the zero time delay arising from the two-photon resonance transitions to the two-exciton states were discovered. Time-and frequency-resolved pump-probe measurements in the vicinity of zero delay times were shown to be sensitive to the manifestation of two-exciton states and, consequently, may be used for the analysis of the transition dipole orientation of the individual molecules as well as the extent of exciton localization in the circular aggregates.