Excited states of gaseous ions. V. The predissociation of the h 2o + ion

Abstract

The B̃ 2 state of H 2O + is predissociated twice. First, by the ã 4B 1 state, giving OH + + H fragments via spinorbit coupling interaction. Secondly, by a 2A state, giving H + OH fragments via spin-orbit coupling and Coriolis interactions. A vibrational analysis of the photoelectron band of the B̃ state of H 2O + and D 2O + is carried out. This provides the vibrational frequencies of the H 2O +, D 2O + and HDO + ions, as well as a vibrational assignment of the peaks. The H 2O + ion in its B̃ 2B 2 state is found to have a OH bond length of 1.12 A and a valence angie of 78°. In order to describe the unimolecular fragmentation process, a distinction is introduced between the totally symmetric, optically active vibrational modes, and the antisymmetric ones which are coupled to the continuum. The former are supplied with photon or electron impact energy, but only the latter are chemically efficient. The dynamics of the dissociation process depends therefore on the couplings among normal modes. This is studied in the framework of two models. In Model 1, it is assumed that, as a result of the anharmonicity of the potential energy surface, only even overtones of the antisymmetric vibration are excited by Fermi resonance. In Model II, excitation of the odd overtones is provided by vibronic coupling. Model II is in better agreement with experiment than Model I. Calculated and experimental results have been compared on the following points: isotopic shift on the appearance potential of OH + and OD + ions, shapes of the photoionization curves, fragmentation pattern with 21 eV photons, presence of a unimolecular metastable transition, production of O + ions. All the vibrational levels situated above the dissociation asymptote are totally predissociated. Autoionization is shown in this case to contribute only to the formation of molecular H 2O + ions, and not to that of the OH + fragments. For 21 eV electrons, the contribution due to direct ionization is calculated to represent about 25% of the total cross section, the rest being due to autoionization.