Gas-Phase Reactions of U+ and U2+ with O2 and H2O in a Quadrupole Ion Trap

Reaction pathways and rate constants of gas-phase uranium and uranium oxide ions with O2 and H2O have been investigated using a quadrupole ion trap mass spectrometer (QIT-MS). A new reaction pathway is identified for the reaction between U2+ and H2O, which leads to the formation of UO+ via the intermediate UOH2+. Reaction rate constants are determined for several reactions by measuring the reaction rate at different partial pressures of the reagent gas and are found to be in reasonable agreement with the literature. These rate constants include the first known measurement for the reaction of U2+ with H2O (∼0.4 kADO). New limits on thermochemical values are also provided for certain species. These include ∆Hf (UO2+) e 1742 kJ mol-1 and 1614 e ∆Hf (UOH2+) e 1818 kJ mol-1 and are based on the assumption that only exothermic or thermoneutral reactions are possible under the conditions used. This assumption is supported by simulations of the root-mean-square (RMS) ion kinetic energy of stored uranium ions in the QIT. Only a slight increase in the RMS ion kinetic energies, from 0.1 to 0.2 eV, is predicted over the range of trapping conditions studied (0.05 e qz e 0.75) corresponding to a theoretical reaction temperature of ∼384 K. The simulations also compare helium and neon as bath gases and show that the RMS kinetic energies are found to be very similar at long trapping times (>20 ms), although neon establishes steady state conditions in approximately half the time.