Ionization rates for cold collisions of metastable antihydrogen atoms

A recent study [Zammit et al, Phys. Rev. A 100, 042709 (2019)] suggested that it may be feasible to produce antihydrogen molecular ions in the near future by laser exciting antihydrogen atoms held in a magnetic trap. The desired antihydrogen molecular ion would then form by associative ionization that may occur during cold collisions of metastable antihydrogen atoms. The proposed experimental scheme relied on low energy extrapolations of thermal ionization rates which suggested associative ionization would be the fastest exothermic process. Our rigorous calculations presented here, however, show that associative ionization is less efficient than Penning ionization at the low temperatures required by the experimental scheme. Any future effort to pursue this formation route will therefore need to account for this trap loss mechanism.