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The ultrafast laser-induced isomerization dynamics of gas phase stilbene is studied in detail using quasiclassical molecular dynamics methods. To model the dynamics, empirical potential surfaces for both the ground and the first excited electronic states of stilbene are constructed using molecular modeling-type potentials which are fit to available spectroscopic measurements, ultrafast dynamics observations, and theoretical structural information. An algorithm for creating quasiclassical initial conditions that simulate the nonstationary state prepared by an ultrashort (100 fs) laser pulse is presented. This algorithm ultilizes a quantum mechancial formulation of the excitation based on harmonic approximations for the potential surfaces which is then adapted to give initial conditions for an ensemble of trajectories. Using these methods, we recently (Chem. Phys. Lett. 1993,215,306) found evidence for the existence of a small barrier along the cis-trans isomerization (ethylenic torsion) coordinate from the cis-stilbene Franck-Condon region of the excited state and here we improve the estimate of that barrier to between 260 and 400 cm-I. Detailed examination of the excited state dynamics of cis-stilbene reveals that the isomerization process proceeds via multiple pathways to a twisted potential minimum on the excited state from which internal conversion to the ground state occurs. All three photoproducts of cis-stilbene cross to the ground state from the same general region of the excited state and are differentiated during the radiationless transition and subsequent dynamics on the ground state. Nonadiabatic coupling between the electronic states is included in our studies using a semiclassical technique, and this allows the identification of correlations between the excited state dynamics and the branching between final products. The photoexcitation of trans-stilbene is also examined, and our calculations suggest that this isomerization process proceeds through a different minimum on the excited state surface which also corresponds to a 90" twisted configuration. This is a departure from the standard view that both cisand trans-stilbene proceed through the same intermediate in their respective cis-trans photoisomerization dynamics.

Quasiclassical Molecular Dynamics Simulation of the Photoisomerization of Stilbene