Metastable BrO2+ and NBr2+ molecules in the gas phase.

The doubly positively charged gas-phase molecules BrO2+ and NBr2+ have been produced by prolonged high-current energetic oxygen (17 keV 16O-) ion surface bombardment (ion beam sputtering) of rubidium bromide (RbBr) and of ammonium bromide (NH4Br) powdered ionic salt samples, respectively, pressed into indium foil. These novel species were observed at half-integer m/z values in positive ion mass spectra for ion flight times of roughly ∼12 μs through a magnetic-sector secondary ion mass spectrometer. Here we present these experimental results and combine them with a detailed theoretical investigation using high level ab initio calculations of the ground states of BrO2+ and NBr2+, and a manifold of excited electronic states. NBr2+ and BrO2+, in their ground states, are long-lived metastable gas-phase molecules with well depths of 2.73 × 104 cm-1 (3.38 eV) and 1.62 × 104 cm-1 (2.01 eV); their fragmentation channels into two monocations lie 2.31 × 103 cm-1 (0.29 eV) and 2.14 × 104 cm-1 (2.65 eV) below the ground state minimum. The calculated lifetimes for NBr2+ (v″ < 35) and BrO2+ (v″ < 18) are large enough to be considered stable against tunneling. For NBr2+, we predicted Re = 3.051 a0 and ωe = 984 cm-1; for BrO2+, we obtained 3.033 a0 and 916 cm-1, respectively. The adiabatic double ionization energies of BrO and NBr to form metastable BrO2+ and NBr2+ are calculated to be 30.73 and 29.08 eV, respectively. The effect of spin-orbit interactions on the low-lying (Λ + S) states is also discussed. [ABSTRACT FROM AUTHOR]