Formation of antihydrogen molecular ions by associative ionization

Abstract

Recent calculations have suggested that it may be feasible to produce antihydrogen molecular ions in the near future [Zammit et al., Phys. Rev. A 100, 042709 (2019)]. The proposed scheme would laser excite cold antihydrogen $\overline{\text{H}}(1s)$ atoms held in a magnetic-field minimum trap and the resulting metastable $\overline{\text{H}}(2s)$ atoms would interact to form the antihydrogen molecular ion, ${\overline{\text{H}}}_{2}{}^{\ensuremath{-}}$, by associative ionization. Estimates for the efficiencies of the proposed formation and depletion processes relied on low-energy extrapolations of rate coefficients that were computed at thermal energies. In the present work, we provide quantum calculations of the rate coefficients for associative ionization and for the competing Penning ionization process, which would deplete the population of trapped $\overline{\text{H}}$. In contrast to the low-energy extrapolations, it is found that Penning ionization is the dominant ionization process at all temperatures and the relative efficiency of associative ionization is within a factor of 3 at the low temperatures required by the experimental scheme. Rate constants for single and double excitation transfer are computed as possible loss mechanisms and all rates are found to be small compared to spin-flip decay rates from Stark-induced mixing with $\overline{\text{H}}(2p$) states.