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

Abstract

A quantitative multiconfiguration self‐consistent field treatment of the potential curves for the C_2v and C_∞v approach of an He⁺(1 ²S) ion to H₂(X ¹Σ_g⁺) 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 C_2v minimum with the attributes (E_b, R^eHe–H₂, R^eH₂) =(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 C_2v 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 ¹Σ_g⁺) at thermal energies, with or without a third body M He⁺(1 ²S)+H₂(X ¹Σ_g⁺) ^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 ²Σ_g⁺,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.