Formation of antihydrogen molecular ions by associative ionization

In this work, we present rates for the formation of the antihydrogen molecular anion by associative ionization (AI), as well as for the competing Penning ionization (PI) process, in low-energy collisions between spin-polarized metastable antihydrogen atoms.

These calculations were motivated by a recent study [Zammit et al., Phys. Rev. A. 100, 042709 (2019)] that proposed an experimental scheme by which antihydrogen molecular ions could be produced by laser excitation of metastable antihydrogen atoms held in a magnetic trap. Calculations were performed for temperatures less than 1 K, and high-energy fitting formulas were used to estimate rates at the low energies required for the experiment. It was predicted that at lower energies AI would have a higher rate than PI.

The present work extends the previous study by performing scattering calculations that incorporate fine structure and Lamb shift at lower energies (<10^-7 E_h). The resulting rates are compared with those extrapolated from high-energy fitting formulas. The low-energy behavior differs from what was predicted from the high-energy fits, with PI being the dominant ionization process at all temperatures. The relative efficiency, however, is within a factor of three at temperatures required by the experimental scheme.