The photoion yield spectrum of HOBr was measured over the wavelength range λ=108–121 nm by using a discharge flow‐photoionization mass spectrometer apparatus coupled to a synchrotron radiation source. HOBr was generated by the reaction of OH with molecular bromine. A value of (10.62±0.04) eV was obtained for the adiabatic ionization energy (I.E.) of HOBr from photoion thresholds, corresponding to the HOBr+(2A″)←HOBr(1A′) transition. The structure observed in the spectrum is discussed in terms of the available states for HOBr+, which have been determined using multiconfiguration‐self‐consistent field calculations. A new value for ΔH0f 298(HOBr) of −9 kcal mol−1 is derived from I.E.(HOBr) and estimates of ΔHf(HOBr+).

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