Guided ion beam studies of reactions of alkaline earth ions with O_2

The endothermic reactions of Mg^+ and Sr^+ with O_2 are studied in a guided ion beam mass spectrometer. Analysis of the reaction thresholds are used to obtain 0 K bond dissociation energies of MgO^+ (2.50 ± 0.10 eV) and SrO^+ (3.47 ± 0.06 eV), which are compared with other literature values. Analysis of these reaction cross sections and those previously published for the reaction of Ca^+ with O_2 [E.R. Fisher, J.L. Elkind, D.E. Clemmer, R. Georgiadis, S.K. Loh, N. Aristov, L.S. Sunderlin, and P.B. Armentrout, J. Chem. Phys., 93 (1990) 2676] suggests that there are both thermodynamic and impulsive reaction pathways. The latter play an increasingly important role in the reaction dynamics as the mass of the alkaline earth ion increases