Probing isotope effects in chemical reactions using single ions.

Isotope effects in reactions between Mg+ in the 3p{2}P{3/2} excited state and molecular hydrogen at thermal energies are studied through single reaction events. From only approximately 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.