Quantifying the photoionization cross section of the hydroxyl radical

International audienceThe 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 recorded the OH + photoion yield over the 12.6-15 eV energy range, and we set it to an absolute cross section scale using an absolute point measurement performed at 13.8 eV: σ ion OH = 9.0 ± 2.7 Mb. The resulting cross section values differ by approximately a factor 2 from the recent measurement of Dodson et al. performed with a different radical source is somewhat greater than the combined uncertainties of the measurements. This finding underlines the need for further investigations of this cross section. The hydroxyl radical, OH, is one of the most important free radicals in atmospheric and astrophysical chemistry. It is involved in atmospheric cycles, e.g. as an oxidizing "detergent" on Earth, 1,2 but also in complex photophysical processes in a wide variety of astrophysi-cal media (planetary atmospheres, 3-5 comets, 6 interstel-lar clouds, 7,8 etc.). In these media, OH plays a key role in the water photochemical cycle. The OH + H 2 → H 2 O + H reaction is endothermic, 9 thus most of the reactions involving OH at low temperature occur with O, N, and C atoms and lead to O 2 , NO, and CO compounds rather than H 2 O. On the other hand, the cationic form OH + reacts fast with H 2 , 10 to produce H 2 O + which in turn reacts with H 2 to generate H 3 O +. H 2 O can then be formed through dissociative recombination of H 3 O +. 11 Several theoretical 12 and experimental 13-17 studies have been carried out on the lowest electronic states of neutral OH. However, absorption studies in the Vacuum UltraViolet (VUV) range (λ < 200 nm) are scarce and absolute measurements in this region are even more so. 18,19 For the ionization process, only three experimental works have reported the relative photoionization yield (or constant-ionic-state spectra) of the hydroxyl radical in the VUV range, to our knowledge. 20-23 Dehmer's work covered the photon energy range between 13.0 and 16.5 eV (≈ 95 − 75 nm), at a resolution of 1-3 meV (0.007 − 0.023 nm). 20 Autoionization features were observed in the ion yield and were assigned to a Rydberg series converging to the a + 1 ∆ state of the OH + ion (lo-cated 2.16 eV above the cationic ground state). Later, Cutler et al. carried on Dehmer's work and studied the photoionization of both OH and OD isotopologues between 13.1 and 18.2 eV (94.64-68.12 nm) at a resolution of 1 meV (0.007 nm). 21 They assigned two new Rydberg series converging to the OH + b + 1 Σ + and A + 3 Π ionic states. In 2018, Dodson et al. published the first experimental measurement of the absolute photoionization cross section for OH, where they deduced the cross section from the analysis of time-resolved radical-kinetics measurements of a multi-reaction network in which OH is produced in the reaction of O(1 D) with H 2 O. The absolute cross section of OH was determined relative to that of O(3 P). Their work was supported by new theoretical calculations of the OH cross section using equation-of-motion coupled-cluster Dyson orbitals and a Coulomb photoelectron wave function. 24 The absolute photoionization cross section of the OH radical is important for reliably describing the abundances of OH and OH + and their involvement in the photochemical networks of interstellar media. However, before the work of Dodson et al., modelers could only use the results from theoretical calculations, 12,25,26 or ignore the cross section completely. For instance, the photoion-ization of OH is not present in the Leiden Database 27 or the Meudon PDR code. 28 The new value is expected to find considerable applications in the modeling of atmospheric and interstellar chemistry. Given the importance of this cross section, a complementary experimental determination would also be valuable. We have recently determined the absolute photoio