Dynamical study of nonadiabatic unimolecular reactions: The conical intersection between the B̃ 2B2 and à 2A1 states of H2O+

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Abstract
The conical intersection connecting the B̃ 2A′ and à 2A′ states of the H2O+ ion is studied. The two potential energy surfaces are calculated ab initio by the SCF/CI method within the CS point group. The nonadiabatic coupling matrix elements 〈Ã‖∂/∂q‖B̃〉 are computed for several cross sections throughout the potential energy surfaces. A transformation to the diabatic representation is performed. The linear model is found to be a good approximation in the region close to the apex of the cone. The global functions t(s) and T(S) governing the nonadiabatic transition probability are calculated; their shapes are those predicted by the Landau–Zener model (in the Nikitin bidimensional version). A dynamical study is undertaken by means of classical trajectory calculations on the upper adiabatic potential energy surface. An averaged transition probability P̄tr is derived. Excitation of rotation or of the bending mode of H2O before photon impact has no influence on P̄tr. Excitation of the symmetrical or antisymmetrical valence modes of H2O lowers P̄tr. The shape of ln (1−P̄tr) as a function of time indicates the existence of two distinct regimes at short and intermediate time ranges, characterized by two different rate constants k1 and k2, respectively. The rate constants are of the order of 1014 s−1. k1 exhibits a maximum as a function of the absorbed energy Eabs, whereas k2 decreases as a function of Eabs.

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