EVIDENCE FOR BIEXCITONS AND DYNAMIC STARK-EFFECT IN J-AGGREGATES FROM FEMTOSECOND SPECTROSCOPY

J-aggregates in aqueous solution are investigated at room temperature with tunable fs pulses using near-resonant excitation at 565 nm. The coherent response of the J-band is observed at short times leading to a precursor of the dynamic Stark effect similar to recent findings for excitonic transitions in semiconductors. Approximately 150 fs after the excitation a blue-shifted absorption peak is measured that presents direct evidence for the formation of biexcitons. Intensity-dependent decay constants in the range < 10(13) s-1 give experimental support to the excitonic annihilation process discussed in previous work. Our analysis shows that hopping models for the incoherent exciton transfer via nearest-neighbour interaction are not consistent with the experimental findings.