[en] The conical intersection conecting the B 2A' and A 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 elements <A|d/dq|B> 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 average transition probability Ptr is derived. Excitation of rotation or of the bending mode of H2O before photon impact has no influence on Ptr. Excitation of the symmetrical or antisymmetrical valence mode of H2O lowers Ptr. The shape of ln(1-Ptr) 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 10exp(14) s-1. k1 exhibits a maximum as a function of the absorbed energy Eabs whereas k2 decreases as a function of Eabs.
Fonds pour la formation à la Recherche dans l'Industrie et dans l'Agriculture (Communauté française de Belgique) - FRIA ; Fonds de la Recherche Fondamentale Collective d’Initiative des Chercheurs - FRFC ; Politique Scientifique Fédérale (Belgique) = Belgian Federal Science Policy ; Centre National de la recherche Scientifique (CNRS)(France)