Photoionization efficiency spectrum and ionization energy of OBrO

The photoionization efficiency (PIE) spectrum of OBrO was measured over the wavelength range lambda = 86-126 nm using a discharge flow-photoionization mass spectrometer (DF-PIMS) apparatus coupled to a VUV synchrotron radiati on source. Bromine dioxide was generated in a flow tube reactor by first fo rming BrO via the reaction O(P-3) + Br-2 and then allowing the BrO to react on the cold flow tube wall. The PIE spectrum of the BrO reactant was obtai ned 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 displaye d steplike behavior near threshold. A value of 10.29 +/- 0.03 eV was obtain ed for the adiabatic ionization energy (IE) of OBrO from analysis of the ph otoionization threshold at lambda = 120.5 nn. An ab initio determination of the IE of OBrO using the CCSD(T)/6-311+G(3dF)//CCD/6-311+G(3df) level of t heory gives a value of 10.26 +/- 0.06 eV, which is in excellent agreement w ith the experimental result. The results are compared with previous determi nations of the IE for both halogen monoxides, XO, and halogen dioxides, OXO (X = Cl, Br, I).