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 similar to 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 H(2) and D(2) we find that the overall fragmentation probability of the intermediate MgH(2)(+), MgHD(+), or MgD(2)(+) 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.