Probing Isotope Effects in Chemical Reactions Using Single Ions

Isotope effects in reactions between Mg⁺ in the $3p$ ²P3/2 excited state and molecular hydrogen at thermal energies are studied through single reaction events. From only ~250 reactions with HD, the branching ratio between formation of MgD⁺ and MgH⁺ is found to be larger than 5. From an additional 65 reactions with H₂ and D₂ we find that the overall fragmentation probability of the intermediate MgH₂⁺, MgHD⁺, or MgD₂⁺ complexes is the same. Our study shows that few single ion reactions can provide quantitative information on ion-neutral reactions. Hence, the method is well suited for reaction studies involving rare species, e.g., rare isotopes or short-lived unstable elements.