We observed for the first time ultra-fast relaxation process due to internal conversion from S2 -exciton to S1-exciton in porphyrin J-aggregates using the optical Kerr gate method. The S2 fluorescence shows sharp spectra with almost zero Stokes shift. The decay time is less than the instrumental function and cannot be time-resolved by the Kerr gate method. The relaxation time of the S2-exciton was also measured by pump-probe spectroscopy with about 10 fs resolution, and the decay time, 130 ± 10 fs, was consistently obtained with that of the S2 fluorescence.
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