An MCSCF characterization of the HeH2+ 2 2A1(2Σ+) polarization state. A radiative mechanism for the production of H2+ from thermal He++H2 collisions

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Abstract
A quantitative multiconfiguration self‐consistent field treatment of the potential curves for the C2v and C∞v approach of an He+(1 2S) ion to H2(X 1Σg+) has been made. Second‐root same‐symmetry orbital optimization techniques were employed in order to treat the 2 2A1(2 2A′,2 2Σ+) state involved. These computations have established that the shallow He+–H2 ion–molecule polarization well is characterized by a C2v minimum with the attributes (Eb, ReHe–H2, ReH2) =(0.091 eV, 2.345 Å, 0.714 Å). The fundamental vibrational frequency for the pseudodiatomic He+–H2 is 284 cm−1. As RHe–H2 decreases further the difference between the C∞v and C2v cut narrows as the system approaches the high interaction region. The availability of this polarization state suggests a collisional–radiative mechanism for the production of H2+ from the reaction of He+(1 2S) with H2(X 1Σg+) at thermal energies, with or without a third body M He+(1 2S)+H2(X 1Σg+) M→ He+–H2(2 2A1) He+–H2(2 2A1)→[He–H2+(1 2A1)]°+hν(153 nm) [He–H+2(1 2A1)]°→He(1 1S)+H+2 (X 2Σg+,v)+KE. The polarization state also provides a thermal pathway for vibrational deactivation of hydrogen by helium ions. These thermal mechanisms are discussed in relation to the well‐known tunneling mechanism yielding H+ and with regard to planetary and interstellar chemistry.