PHOTOELECTRON-SPECTRA, ELECTRONIC-STRUCTURE, COINCIDENCE SPECTRA AND DISSOCIATION MECHANISMS OF THE HYDROGEN-CYANIDE CATION

Photoelectron spectra of HCN and DCN have been recorded at a resolutio n of 4.5 meV or better using HeI and HeII radiation;Ind have been inte rpreted by comparison with high-level electronic structure calculation s. The calculations predict that HCN+ in the (B) over tilde state, pre viously 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 2 2.5 eV, and another new state is seen at 33 eV. The dissociation pathw ays of HCN+ in (B) over tilde and higher states have been determined b y photoelectron-photoion coincidence spectroscopy. There is competitio n between H+ and CN+ production from most of the resolved levels of th e (B) over tilde state above the CN+ threshold: the diatomic products have little vibrational energy. Both the sharp vibrational structure a nd the competition show that predissociation is slow; the calculations indicate the existence of a barrier, preventing direct dissociation. (C) 1998 Elsevier Science B.V.