collision at energy below 25 meV and isotope effect

At collision energy below 25 meV, our branching ratios of the mutual neutralization in He⁺ + H^- and He⁺ + D^- collisions measured with the single-pass merged beam setup at the Université catholique de Louvain in Belgium benchmark a Landau-Zener model that uses potential energy curves and couplings computed by an anion-centered asymptotic method. In addition, we identified a low collision energy regime (50 meV and below) where these branching ratios, at first order, no longer depend on the collision energy. Moreover, we deduced that the mutual neutralization of heavier isotopes tends to populate higher excited states than lighter isotopes do, as a result of the reduced radial velocity at the avoided crossings.

This work was supported by Fonds De La Recherche Scientifique Grant No. 4.4504.10. The authors thank the Belgian State for the grant allocated by Royal Decree for research in the domain of controlled thermonuclear fusion. A.D. is supported by the Swedish National Infrastructure, DESIREE (Swedish Research Council Contracts No. 2017-00621 and No. 2021-00155). X.U. is a Senior Research Associate of the Fonds de la Recherche Scientifique. This paper is based upon work from COST Action CA18212—Molecular Dynamics in the GAS phase (MD-GAS), supported by COST (European Cooperation in Science and Technology). V.A. is supported by NSF Grant No. PHY-1530944.