2014ApJ...782...46A

Depopulation of metastable He(2³S) by radiative association with hydrogen and lithium ions is investigated using a fully quantal approach. Rate coefficients for spontaneous and stimulated radiative association of the HeH⁺, HeD⁺, and LiHe⁺ molecular ions on the spin-triplet manifold are presented as functions of temperature considering the association to rotational-vibrational states of the lowest triplet electronic states a ³Σ⁺ and b ³Σ⁺ from the continuum states of the b ³Σ⁺ electronic state. Evaluation of the rate coefficients is based on highly accurate quantum calculations, taking into account all possible state-to-state transitions at thermal energies (for spontaneous association) or at higher background energies (stimulated association). As expected, calculations show that the rate coefficients for radiative association to the a state are several orders of magnitude larger than the one for the b state formation. A noticeable effect by blackbody background radiation on the radiative association is only obtained for the b ⟶ b process. Aspects of the formation and abundance of the metastable HeH⁺(a ³Σ⁺) in astrophysical environments are briefly discussed.