Dissociation of the COS+ ion by photoionisation: experiment and ab initio calculations The

The predissociation or dissociation of selected vibronic levels of the X2Π, A2Π, andB2 Σ+ states of the COS+ ions has been studied experimentally. The product state distribution has been probed by photoabsorption of the synchrotron radiation in the 13.5–16.5 eV range and threshold photoelectron-photoion coincidence spectroscopy. Ab initio calculations have been performed to model the potential energy surfaces of the X2Π, A2Π, and B2Σ+ electronic states and also of the lowest energy 4Σ− state. This latter is coupled by spin-orbit to X2Π and is confirmed to be responsible of the predissociation in the A−X Franck-Condon gap. In the A−X Franck-Condon energy gap the predissociation mechanism by 4Σ− is not statistical and favours the production of vibrationally excited CO (v″) with maximum probability for the highest energy accessible levels. It involves also a highly effective transfer into rotational motion. For the A2Π state, there is internal conversion to X2Π followed by predissociation to the lowest limit and, for the levels above the second limit, the predissociation by 4Σ− is in competition with direct dissociation of X2Π to the second limit. The B2Σ+ state also undergoes internal conversion to X2Π followed by dissociation to the second limit and predissociation by 4Σ− to the first limit.