On the continued search for an infrared spectroscopic identification of OH radicals in water-ice

The chemistry of hydroxyl (OH) radicals plays a key role in the radiolysis and photolysis of water-rich ices of astrochemical origin. Other radical species such as hydroperoxy (HO2) radicals are also potentially important in oxygen cycles that exist on icy satellite surfaces due to the stoichiometric loss of H from radiation-processed water-ices. Both OH and HO2 are likely key intermediates in the formation of H2O in the ISM. Despite the importance of such species, laboratory measurements of OH and HO2 in water-ices are sparse. This is due to a combination of low abundances and overlapping spectral features that makes detection using infrared spectroscopy difficult. There have been several reports in the literature claiming detection of both OH and HO2 in laboratory water-ices. In the present work OH radicals are formed in the gas-phase prior to deposition with water to attempt to trap them stably in water-ice at 6 K. The present work does not find any IR spectral detection of OH in ices prepared by this method. Chemical reactions are likely the sink of OH that limits detection in additiontospectraloverlapwithH2O.Theimplicationsofthisforastrochemistryarediscussed herein. Additional experiments have been performed using the matrix-isolation technique to determine the intrinsic band strengths of the infrared absorption of the OH radical and the related H2O–HO complex. These could be used to quantify OH radical in non-water-ices where spectral interference from H2O is absent.