Depopulation of metastable helium He(21S) by radiative association with hydrogen and lithium cations

- Radiative association of molecular ions HeH+ and HeLi+ is studied.
- Cross sections for radiative association are calculated for three processes.
- Rate coefficients are determined as functions of temperature.
- Two-state processes exhibit the largest cross sections and rate coefficients.
- Stimulated radiative association is relevant only for one-state processes.

Depopulation of metastable He(21S) by radiative association with hydrogen and lithium ions resulting in the formation of the HeH+ and HeLi+ molecular ions is investigated. Energy dependent cross-sections for spontaneous and stimulated processes on the spin-singlet manifold are calculated using a fully quantal approach and considering the association to rotational-vibrational states of the lowest singlet electronic states XΣ+1, AΣ+1, and BΣ+1 from the continuum states of the BΣ+1 electronic state. Evaluation of the cross-sections 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 temperatures (stimulated association). The corresponding rate coefficients are then presented as functions of temperature. As expected calculations show that the rate coefficients for radiative association to the X and A states are several orders of magnitude larger than the one for the B state formation. On the other hand, stimulation of the radiative association by black-body background radiation has a noticeable effect only on the B→B process.