Photoionization Efficiency Spectrum, Ionization Energy, and Heat of Formation of Br₂O

The photoionization efficiency (PIE) spectrum of Br 2 O was measured over the wavelength range λ = 100−125 nm by using a photoionization mass spectrometer apparatus coupled to a synchrotron radiation source. A value of 10.26 4 ± 0.01 1 eV was obtained for the adiabatic ionization energy of Br 2 O from analysis of the photoionization thresholds. Also, the appearance energy (AE) of BrO + (from the dissociative ionization of Br 2 O) was determined from the PIE spectrum of BrO + over the wavelength range λ = 104−110 nm. Combining the AE 298 value, 11.71 4 ± 0.01 4 eV, with known thermodynamic quantities yields a value for Δ f H ° 298 (Br 2 O) = 107.1 ± 3.5 kJ mol -1 . The value for Δ f H ° 298 (Br 2 O) may be related to Δ f H ° 298 (HOBr) through the equilibrium constant for the reaction Br 2 O + H 2 O ⇌ 2HOBr. This leads to Δ f H ° 298 (HOBr) = −60.0 ± 3.5 kJ mol -1 .