Unimolecular decay paths of electronically excited species. I. The H2CO+ ion

Abstract

Potential energy surfaces of the lowest electronic states of H₂CO⁺ have been calculated ab initio and the dissociation paths determined. The most important feature is a conical intersection between the surfaces of the first two excited states, à ²B₁ and ²A₁(1). This conical intersection provides a decay path characterized by a very low activation energy. If the path is followed adiabatically, CO⁺ ions are obtained. The path is, however, crossed by the potential energy surface of the ground state, X̃ ²B₂, and, if a transition to this state takes place, HCO⁺ ions are produced. The coupling mechanisms are investigated, and a possible explanation of the observed isotopic effects is offered. There is a whole distribution of rate constants for the transfer of population from state à ²B₁ to state X̃ ²B₂; the distribution is found to span several orders of magnitude because of an interference effect between two predissociation mechanisms. The applicability of the quasiequilibrium theory of mass spectra is discussed.