The hydroxyl free radical, OH, is one of the most important radicals in atmospheric and
interstellar chemistry, and its cation plays a role in the reactions leading to H 2 O formation.
Knowledge of the photoionization efficiency of the OH radical is crucial to properly model the
water photochemical cycle of atmospheres and astrophysical objects. Using a gas-phase
radical source based on a single H-abstraction reaction combined with a photoelectron/
photoion imaging coincidence spectrometer coupled with synchrotron radiation, we …