Conical Intersections and the Mechanism of Singlet Photoreactions

摘要

The reaction pathway of a nonadiabatic photochemical reaction begins on the potential energy surface of the excited state (S_1) in the Franck- Condon region or at a spectroscopic minimum of the reactant and ends at the product minimum on the ground-state surface (S_0). The two stages on the S-1 and S_0 surface are separated by a funnel at which radiationless decay from the excited state to the ground state becomes possible (Fig. 1). For a long time it was believed that the funnels mostly correspond to surface crossings (or touchings) that are at least weakly avoided, but recent work has shown that for a great number of inorganic and organic systems the crossings are actually unavoided and correspond to true conical intersections. Nowadays it appearsthat conical intersections are a common feature in most nonadiabatic singlet photoreactions.