Femtosecond Real-Time Probing of Reactions. 20. Dynamics of Twisting, Alignment, and IVR in thetrans-Stilbene Isomerization Reaction

The femtosecond real-time dynamics of the isomerization reaction of trans-stilbene under collisionless conditions are studied using (2+1) resonance-enhanced multiphoton ionization (REMPI) and femtosecond depletion spectroscopy (FDS) in a pump−probe scheme. The observed transients reflect the macroscopic sample anisotropy decay (rotational coherence) and intramolecular vibrational energy redistribution (IVR) as well as the ethylenic twisting isomerization reaction. Polarization-resolved measurements are performed to isolate the influence of rotational dynamics, and the measured anisotropy decay is compared with theoretical calculations of rotational coherence at room temperature. The IVR and nonradiative (isomerization) processes of trans-stilbene are studied as a function of S_1 excess vibrational energy up to ∼6500 cm^(-1). These results are compared with previous measurements of trans-stilbene under collisionless conditions and with predictions of RRK and RRKM theories