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

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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.

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