Gas-phase UV/visible absorption spectra of HOBr and Br₂O

The gas-phase UV/visible absorption spectrum of HOBr, an important atmospheric trace species, is reported for the first time. The HOBr spectrum, measured over the range 200-420 nm, consists of two absorption bands peaking near 280 nm (σ_max = (3.1 ± 0.4) × 10^-19 cm² molecule^-1) and near 350 nm (σ_max = (6.1 ± 1.0) × 10^-20 cm² molecule^-1). Atmospheric photolysis rates for HOBr calculated from these data are found to be significantly slower than previously assumed. The UV/visible absorption spectrum of Br₂O, which was used as the HOBr precursor, was also recorded for the first time. The Br₂O spectrum has maxima near 200 nm (σ_max = (2.0 ± 0.2) × 10^-17 cm² molecule^-1) and 314 nm (σ_max = (2.3 ± 0.3) × 10^-18 cm² molecule^-1). These spectra of Br₂O and HOBr are compared with those of the analogous chlorine species. In addition, some thermodynamic properties for Br₂O and HOBr have been obtained. The equilibrium constant for the reaction Br₂O + H₂O ↔ 2 HOBr (10, -10), was determined to be 0.02, yielding ΔG°_r(298 K) = 2.33 kcal/ mol for the forward reaction. This equilibrium constant is evaluated in terms of recent reports of ΔH_f(298 K) for HOBr.