Photoelectron spectra, electronic structure, coincidence spectra and dissociation mechanisms of the hydrogen cyanide cation Original Research Article

Photoelectron spectra of HCN and DCN have been recorded at a resolution of 4.5 meV or better using HeI and HeII radiation and have been interpreted by comparison with high-level electronic structure calculations. The calculations predict that HCN+ in the B̃ state, previously thought to be linear, is bent at equilibrium. The spectrum for this state shows strong excitation of the C–H stretching mode and the bending mode, both with frequencies much lower than in neutral HCN. A new satellite state with resolved vibrational structure is found at 22.5 eV, and another new state is seen at 33 eV. The dissociation pathways of HCN+ in B̃ and higher states have been determined by photoelectron–photoion coincidence spectroscopy. There is competition between H+ and CN+ production from most of the resolved levels of the B̃ state above the CN+ threshold: the diatomic products have little vibrational energy. Both the sharp vibrational structure and the competition show that predissociation is slow; the calculations indicate the existence of a barrier, preventing direct dissociation.