DEPOPULATION OF METASTABLE HELIUM BY RADIATIVE ASSOCIATION WITH HYDROGEN AND LITHIUM IONS

Abstract

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.