On the electronic structure of the He+H2 system: Characterization of, and nonadiabatic interactions between, the 1 1A′ and 2 1A′ potential energy surfaces

The 1, 2 1A′ potential energy surfaces (PESs) of the He–H2 system, surfaces which correlate asymptotically with He(1S)+H2(X 1Σ+g, B 1Σ+u) system states, are characterized using MCSCF/CI wave functions. The existence of charge transfer structures of the form (HeH)+–H− on the two PESs is considered as are the electronic structure aspects of the nonadiabatic quenching process He+H2(B 1Σ+u )→He+H2(X 1Σ+g). While this work builds on previously reported theoretical treatments of these PESs, both qualitative and quantitative differences are found. In particular, our predicted entrance channel saddle point corresponds to a barrier of 1.5 kcal/mol on the 2 1A′ PES which is significantly lower than previous work. More significantly an extended region of large nonadiabatic effects characterized by the near degeneracy of the 1 1A′ and 2 1A′ PESs, E(2 1A′)−E(1 1A′)<0.5 kcal/mol, has been located. This region of the 2 1A′ PES, which is exothermic with respect to dissociation to He+H2(B 1Σ+u) and is characterized by general Cs, rather than C2v or C∞v geometries, was not uncovered in previous studies. Analyses based on the molecular dipole moment and the nonadiabatic coupling matrix elements 〈Ψ(2 1A′)‖(∂/∂Rα) Ψ(1 1A′)〉 are used to characterize this region.