State-resolved mutual neutralization of ${}_{16}O_{+}$ with ${}_{1}H_{-}$ and ${}_{2}H_{-}$ at collision energies below 100 meV

We measured the product-state distribution and its dependence on the hydrogen isotope for the mutual neutralization between ${}_{16}O_{+}$ and ${}_{1,2}H_{-}$ at the double electrostatic ion-beam storage ring DESIREE for center-of-mass collision energies below 100 meV. We find at least six product channels into ground-state hydrogen and oxygen in different excited states. The majority of oxygen products populate terms corresponding to $2s^2{2p}^3{(}^4{S}^{\circ })4s$ with ${}_{5}S_{\circ }$ as the main reaction product. We also observe product channels into terms corresponding to $2s^2{2p}^3{(}^{4}S)3p$. Collisions with the heavier hydrogen isotope yield a branching into these lower excited states smaller than collisions with ${}_{1}H_{-}$. The observed reaction products agree with the theoretical predictions. The detailed branching fractions, however, differ between the theoretical results, and none of them fully agree with the experiment.