Reactions of alkaline earth ions with H_2, D_2, and HD

Guided ion beam mass spectrometry is used to study the kinetic energy dependence of the reactions of ground-state Mg^+ and Sr^+ with molecular hydrogen and its isotopomers. The results for reactions of Sr^+ are analogous to those previously observed for reactions of Ca^+ and Ba^+ with H_2, D_2, and HD. We observe qualitatively different cross sections for the reactions of Mg^+ with H_2, D_2, and HD, even though the monopositive ions in this group all have an ns valence electron configuration above completely filled core orbitals. The different pattern of reactivity can be explained with molecular orbital concepts, focusing on the interaction of the empty orbitals of the reactant ion with those of the hydrogen molecule. The threshold behavior of these reactions is analyzed to yield 0 K bond dissociation energies for SrH^+ of 2.17 ± 0.06 eV (50.0±f 1.3 kcal/mol) and an estimate for MgH^+ of 1.94 ± 0.06 eV (44.7 ± 1.5 kcal/mol).