An approach to the ‘‘channel three’’ phenomenon of benzene

The nonradiative decay of benzene excited within the S1 manifold is studied by assuming an open vibrational potential for the S2 state that leads to spontaneous breaking of hexagonal symmetry along a path for valence isomerization. The S2 state crosses the S1 electronic configuration (at the channel three threshold) as well as the ground state along the reaction path. The rate constants of nonradiative decay of the S1 state (in fact the S1uS2 internal conversion) and the S2 state (S2uS0 IC) are both quantum mechanically calculated by invoking microcanonical averaging over all vibrational levels accessible at a given excess of vibrational energy. The results obtained account for a number of features observed in spectroscopy of benzene at the channel three threshold.