Picosecond excitation and TRANS-CIS isomerization of stilbene in a supersonic jet: Dynamics and spectra

New details of the dynamics and spectroscopy of trans-stilbene are revealed by picosecond excitation of jet-cooled and collisin-free molecules. The fluorescence spectra clearly show the low-frequency torsional modes of the ground state. The time-resolved fluorescence following S_1 excitation exhibit exponential decay times ranging from 2.7 ns to ≈190 ps for excitation wavelengths varying from 3102 Å (0 excess energy) to 2850 Å (2851 cm^(−1) excess energy). The results indicate that an energy threshold for isomerization occurs at ≈1200 cm^(−1). The redistribution of vibrational energy from the optically excited modes to the ethylene bond torsional modes leading to isomerization is discussed.