A guided ion beam investigation of UO2+ thermodynamics and f orbital participation: Reactions of U+ + CO2, UO+ + O2, and UO+ + CO

Abstract

A guided ion beam tandem mass spectrometer was employed to study the reactions of U+ + CO2, UO+ + O2, and the reverse of the former, UO+ + CO. Reaction cross sections as a function of kinetic energy over about a three order of magnitude range were studied for all systems. The reaction of U+ + CO2 proceeds to form UO+ + CO with an efficiency of 118% ± 24% as well as generating UO2+ + C and UCO+ + O. The reaction of UO+ + O2 forms UO2+ in an exothermic, barrierless process and also results in the collision-induced dissociation of UO+ to yield U+. In the UO+ + CO reaction, the formation of UO2+ in an endothermic process is the dominant reaction, but minor products of UCO+ + O and U+ + (O + CO) are also observed. Analysis of the kinetic energy dependences observed provides the bond energies, D0(U+–O) = 7.98 ± 0.22 and 8.05 ± 0.14 eV, D0(U+–CO) = 0.73 ± 0.13 eV, and D0(OU+–O) = 7.56 ± 0.12 eV. The values obtained for D0(U+–O) and D0(OU+–O) agree well with the previously reported literature values. To our knowledge, this is the first experimental measurement of D0(U+–CO). An analysis of the oxide bond energies shows that participation of 5f orbitals leads to a substantial increase in the thermodynamic stability of UO2+ relative to ThO2+ and especially transition metal dioxide cations.