Kinetic energy dependence of the reactions of Ru+, Rh+, Pd+, and Ag+ with O2

Reactions of Ru+, Rh+, Pd+, and Ag+ with molecular oxygen are studied as a function of kinetic energy by using guided ion beam mass spectrometry. By using a flow tube ion source, it has been possible to create Ru+, Rh+, Pd+, and Ag+ ions in their electronic ground state terms and primarily in the lowest spin–orbit levels. All reactions are observed to be endothermic. The reactivity of ground state Ag+ is found to be particularly inefficient and is believed to occur through an impulsive pairwise mechanism. Excited states of Ag+ are observed to react efficiently at thermal energies. Analyses of the endothermic reaction cross sections yield 0 K bond dissociation energies of D0(Ru+–O)=3.81±0.05 eV, D0(Rh+–O)=3.02±0.06 eV, D0(Pd+–O)=1.46±0.11 eV, and a speculative value of D0(Ag+–O)=1.23±0.05 eV. The reactivity differences among all four metal systems and the electronic states of Ag+ are explained by using simple molecular orbital concepts.