Photodynamics of ethylene:: ab initio studies of conical intersections

We have used extended basis sets and configuration interaction wave functions to systematically characterize the excited state potential energy surfaces of ethylene, including conical intersections between electronic states. The results are consistent with our previous ab initio multiple spawning simulations of ethylene photodynamics and electronic spectra. The C-C bond on the optically accessible V state is extended in planar geometries, suggesting a role for C-C stretching in the electronic absorption spectrum. A cascade of conical intersections connecting the V state in the Franck-Condon region to each of the low-lying Rydberg states has been identified, in addition to intersections connecting the excited state manifold back to the ground state. The D-2d twisted geometry of ethylene is found to be a saddle point, not a local minimum. Pyramidalization of one of the methylene units in twisted ethylene is found to be favorable, leading to a conical intersection. We have identified and characterized eight conical intersections involving the V state which are likely to be relevant in the photochemistry of ethylene. (C) 2000 Published by Elsevier Science B.V.