Photoionization Efficiency Spectrum and Ionization Energy of OBrO

The photoionization efficiency (PIE) spectrum of OBrO was measured over the wavelength range λ = 86−126 nm using a discharge flow-photoionization mass spectrometer (DF−PIMS) apparatus coupled to a VUV synchrotron radiation source. Bromine dioxide was generated in a flow tube reactor by first forming BrO via the reaction O(3P) + Br2 and then allowing the BrO to react on the cold flow tube wall. The PIE spectrum of the BrO reactant was obtained and the photoionization threshold evaluated from the half-rise point of the step at threshold. This remeasurement yields IE(BrO) = 10.48 ± 0.02 eV, which supersedes our previous result. The PIE spectrum of OBrO displayed steplike behavior near threshold. A value of 10.29 ± 0.03 eV was obtained for the adiabatic ionization energy (IE) of OBrO from analysis of the photoionization threshold at λ = 120.5 nm. An ab initio determination of the IE of OBrO using the CCSD(T)/6-311+G(3df)//CCD/6-311+G(3df) level of theory gives a value of 10.26 ± 0.06 eV, which is in excellent agreement with the experimental result. The results are compared with previous determinations of the IE for both halogen monoxides, XO, and halogen dioxides, OXO (X = Cl, Br, I).