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Advances in merged-beams instruments have allowed experimental studies of themutual neutralisation (MN) processes in collisions of both Li$^+$ and Na$^+$ions with D$^-$ at energies below 1 eV. These experimental results placeconstraints on theoretical predictions of MN processes of Li$^+$ and Na$^+$with H$^-$, important for non-LTE modelling of Li and Na spectra in late-typestars. We compare experimental results with calculations for methods typicallyused to calculate MN processes, namely the full quantum (FQ) approach, andasymptotic model approaches based on the linear combination of atomic orbitals(LCAO) and semi-empirical (SE) methods for deriving couplings. It is found thatFQ calculations compare best overall with the experiments, followed by theLCAO, and the SE approaches. The experimental results together with thetheoretical calculations, allow us to investigate the effects on modelledspectra and derived abundances and their uncertainties arising fromuncertainties in the MN rates. Numerical experiments in a large grid of 1Dmodel atmospheres, and a smaller set of 3D models, indicate that neglect of MNcan lead to abundance errors of up to 0.1 dex (26\%) for Li at low metallicity,and 0.2 dex (58\%) for Na at high metallicity, while the uncertainties in therelevant MN rates as constrained by experiments correspond to uncertainties inabundances of much less than 0.01~dex (2\%). This agreement for simple atomsgives confidence in the FQ, LCAO and SE model approaches to be able to predictMN with the accuracy required for non-LTE modelling in stellar atmospheres.

astrophysical journal/the astrophysical journal

Mutual neutralisation in Li$^+$+H$^-$/D$^-$ and Na$^+$+H$^-$/D$^-$  collisions: Implications of experimental results for non-LTE modelling of  stellar spectra