An experimental and theoretical study of the HNCO+ ion

The dissociations of energy-selected NNCO+ ions have been examined at ionis ation energies up to 40 eV using photoelectron-photoion coincidence spectro scopy. The slow metastable dissociation to HCO+ is shown to occur from init ial population of low vibrational levels within the doublet states correspo nding to the third photoelectron band. Rate constants for the dissociation from several levels have been measured and the existence of an optical emis sion is predicted. High level calculations identify the third band in the photoelectron spectr um as an overlay of almost degenerate states arising from ionisation of the in-plane and out-of-plane bonding x-orbitals. The calculations suggest tha t at energies between 15.5 and 16 eV, the dominant pathway for dissociation involves slow internal conversion to the ground doublet state without surf ace crossing, followed by intersystem crossing to the quartet surface. At e nergies over 16 eV, two mechanisms are possible; intersystem crossing from the second excited doublet state to the lowest quarter surface in a cis-ben t configuration, or internal conversion to the first excited doublet state via a surface crossing in the same region, followed by a second nonradiativ e transition to the doublet ground state and intersystem crossing to the qu artet surface. Tn each case, the initial step is expected to be slow, consi stent with the existence of an optical emission, and H-atom transfer occurs on the quartet surface via a 'loose' transition state leading to the direc t formation of HCO+ and N(S-4(u)). (C) 2000 Published by Elsevier Science B .V. All rights reserved.