  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 additional 65 reactions
with H2 and D2 we find that the overall decay 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.
