The bond energy of UN+: Guided ion beam studies of the reactions of U+ with N2 and NO

Abstract

A guided ion beam tandem mass spectrometer was used to study the reactions of U+ with N2 and NO. Reaction cross sections were measured over a wide range of energy for both systems. In each reaction, UN+ is formed by an endothermic process, thereby enabling the direct measurement of the threshold energy and determination of the UN+ bond dissociation energy. For the reaction of U+ + N2, a threshold energy (E0) of 4.02 ± 0.11 eV was measured, leading to D0 (UN+) = 5.73 ± 0.11 eV. The reaction of U+ + NO yields UO+ through an exothermic, barrierless process that proceeds with 94 ± 23% efficiency at the lowest energy. Analysis of the endothermic UN+ cross section in this reaction provides E0 = 0.72 ± 0.11 eV and, therefore, D0 (UN+) = 5.78 ± 0.11 eV. Averaging the values obtained from both reactions, we report D0 (UN+) = 5.76 ± 0.13 eV as our best value (uncertainty of two standard deviations). Combined with precise literature values for the ionization energies of U and UN, we also derive D0 (UN) = 5.86 ± 0.13 eV. Both bond dissociation energies agree well with high-level theoretical treatments in the literature. The formation of UN+ in reaction of U+ with NO also exhibits a considerable increase in reaction probability above ∼3 eV. Theory suggests that this may be consistent with the formation of UN+ in excited quintet spin states, which we hypothesize are dynamically favored because the number of 5f electrons in reactants and products is conserved.