Simulations of the photodynamics of ethylene were carried out by employing the semiempirical direct trajectory with surface hopping method in order to assess quantitatively the importance of different regions of the S_2/S_1 and S_1/S_0 crossing seams.The results show that during the first 50 fs after a vertical photoexcitation to the pipi* state,the nonadiabatic coupling between the S_1 and the S_2 states produces a recurrence pattern of oscillation of the populations in these states.Within the first 100 fs,the S_1 state population spans a limited region of the configuration space between the initial geometries and the twisted-pyramidalized minimum on the crossing seam(MXS).Depending on the way of counting,about 50% of the S_1->S_0transitions occur in the pyramidalized region of the crossing seam,but not necessarily close to the MXS.The remaining 50% occurs in the H-migration and ethylidene regions.Our analysis shows that the ethylidene region becomes more important in later stages of the dynamics when the flux of trajectories that was not effectively converted to the ground state in the pyramidalized region starts to reach this part of the configuration space.The excited-state nonadiabatic dynamics could be employed to generate suitable initial phase space distributions for the hot-ethylene ground-state kinetic studies.
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