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Fluorescence excitation spectra and fluorescence lifetimes at single vibronic levels in the S 1 state have been observed for jet-cooled pyrene. The fluorescence lifetimes at the zero-vibrational levels of the S 1 1 B 3 u states of pyrene- h 10 and pyrene- d 10 are 1480 and 1470 ns, respectively, and the relaxation is considered to be dominated by the radiative process. For some vibrational levels, however, the lifetimes are remarkably shorter such as 765 ns at the 221 vibronic level of pyrene- h 10 ( ν 22 ( b 1 g ); C–H bending and skeletal deforming mode), indicating that nonradiative transition occurs at a specific vibrational level. In this study, we demonstrate that the main process is internal conversion to the S 0 1 A g state caused by nonadiabatic vibronic interaction via b 3 u promoting modes. |
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