Photodynamics of ethylene: ab initio studies of conical intersections

We have used extended basis sets and configuration interaction wave functio ns to systematically characterize the excited state potential energy surfac es of ethylene, including conical intersections between electronic states. The results are consistent with our previous ab initio multiple spawning si mulations of ethylene photodynamics and electronic spectra. The C-C bond on the optically accessible V state is extended in planar geometries, suggest ing a role for C-C stretching in the electronic absorption spectrum. A casc ade of conical intersections connecting the V state in the Franck-Condon re gion to each of the low-lying Rydberg states has been identified, in additi on to intersections connecting the excited state manifold back to the groun d state. The D-2d twisted geometry of ethylene is found to be a saddle poin t, not a local minimum. Pyramidalization of one of the methylene units in t wisted ethylene is found to be favorable, leading to a conical intersection . We have identified and characterized eight conical intersections involvin g the V state which are likely to be relevant in the photochemistry of ethy lene. (C) 2000 Published by Elsevier Science B.V.