Anomalous fluorescence from the second excited singlet state upon excitation to the S4 state of pyrene in a supersonic jet

The electronic spectra of pyrene-h10 and pyrene-d10 were measured in supersonic jets to investigate dynamics in the higher electronic states. The dispersed fluorescence of pyrene-h10 obtained by excitation into the S2 state shows structured bands in the 380 nm region. The 380 nm fluorescence is assigned to the S1 fluorescence, which indicates that fast internal conversion occurs even jet-condition. By excitation into the S4 state, an anomalous S2 fluorescence is observed in addition to the S1 fluorescence. The intensity ratio between the S2 fluorescence and the S1 fluorescence (S2/S1) was calculated. The S2/S1 ratios under jet-cooled conditions is 0.2 which is larger than that in static gas reported by Baba [H. Baba, A. Nakajima, M. Aoi, K. Chihara, J. Chem. Phys. 55 (1971) 2433]. On the other hand, the S2/S1 is 0.1 for pyrene-d10, which indicates that the anomalous S2 fluorescence of pyrene-d10 is weaker than that of pyrene-h10. This result supports that the anomalous S2 fluorescence occurs through a mechanism involving reverse internal conversion from the S1 to the S2 state.