INVESTIGATION OF SINGLE-VIBRONIC-LEVEL FLUORESCENCE LIFETIMES OF JET-COOLED S-1 TRANS-STILBENE ABOVE THE ISOMERIZATION BARRIER

The fluorescence lifetimes of jet-cooled trans-stilbene were measured for twenty eight Si vibronic levels in the 1300-1700 cm(-1) region abo ve the 0-0 transition. All fluorescence decay curves were well fitted to single exponential functions in the 0-3 ns time region. It was foun d that the fluorescence lifetime decreases monotonically with the incr ease of the excess vibrational energy. No anomalous lifetime dependenc e on the vibronic level was observed. The (time-integrated) dispersed fluorescence from the corresponding vibronic levels was also measured. The excitation of all twenty eight levels gave indistinguishable fluo rescence spectra exhibiting only broad features, indicating that the I VR rate is much larger than the radiative decay rate regardless of the fundamental, overtone or combination levels. No direct evidence suppo rting the existence of a mode-selective isomerization channel was obta ined.