Distance-Dependent Radiation Chemistry: Oxidation versus Hydrogenation of CO in Electron-Irradiated H₂O/CO/H₂O Ices

Abstract

Electron-stimulated oxidation of CO in layered H 2 O/CO/H 2 O ices was investigated with infrared reflection–absorption spectroscopy (IRAS) as a function of the distance of the CO layer from the water/vacuum interface. The results show that while both oxidation and reduction reactions occur within the irradiated water films, there are distinct regions where either oxidation or reduction reactions are dominant. At depths less than ∼15 ML from the vacuum interface, CO oxidation to CO 2 dominates over the sequential hydrogenation of CO to methanol (CH 3 OH), consistent with previous observations. At its highest yield, CO 2 accounts for ∼45% of all the reacted CO. Another oxidation product is identified as the formate anion (HCO 2 – ). In contrast, for CO buried more than ∼35 ML below the water/vacuum interface, the CO-to-methanol conversion efficiency is close to 100%. Production of CO 2 and formate is not observed for the more deeply buried CO layers, where hydrogenation dominates. Experiments with CO dosed on preirradiated ASW samples suggest that OH radicals are primarily responsible for the oxidation reactions. Possible mechanisms of CO oxidation, involving primary and secondary processes of water radiolysis at low temperature, are discussed. The observed distance-dependent radiation chemistry results from the higher mobility of hydrogen atoms that are created by the interaction of the 100 eV electrons with the water films. These hydrogen atoms, which are primarily created at or near the water/vacuum interface, can desorb from or diffuse into the water films, while the less-mobile OH radicals remain in the near-surface zone, resulting in preferential oxidation reactions there. The diffusing hydrogen atoms are responsible for the hydrogenation reactions that are dominant for the more deeply buried CO layers.