Predissociation of chlorobenzene, beyond the pseudo-diatomic model

Dissociation of chlorobenzene via the lowest singlet excited state has been investigated by means of pump–probe femtosecond spectroscopy and spin–orbit corrected ab initio quantum chemistry. We have found that the so far accepted model with a 1ππ∗ → 3π/nσ∗ reaction mechanism has to be amended. We suggest that the mechanism goes via a transition from 1ππ∗ to a πσ∗ state that is to 90% a singlet. Further, three nuclear degrees of freedom required to describe the dissociation have been defined.