Exploring the chemistry induced by energetic processing of the H2-bearing, CO-rich apolar ice layer

Interstellar ice mantles on the surfaces of dust grains are thought to have a bi-layered structure, with a H2O-rich polar layer, covered by a CO-rich apolar layer that probably harbors H2 and other volatiles such as N2. In this work, we explore the chemistry induced by 2 keV electrons and Ly-alpha photons in H2:CO:15N2 ice analogs of the CO-rich layer when exposed to similar fluences to those expected from the cosmic-ray-induced secondary electrons and UV photons during the typical lifetime of dense clouds. Six products were identified upon 2 keV electron irradiation: CO2, C2O (and other carbon chainoxides), CH4, H2CO, H2C2O, and H15NCO. The total product abundances corresponded to 5-10% of the initial CO molecules exposed to the electron irradiation. Ly-alpha photon irradiation delivered 1-2 orders of magnitude lower yields with a similar product branching ratio, which may be due to the low UV-photon absorption cross-section of the ice sample at this wavelength. Formation of additional N-bearing species, namely C215N2 and 15NH3, was only observed in the absence of H2 and CO molecules, respectively, suggesting that reactants derived from H2 and CO molecules preferentially react with each other instead of with 15N2 and its dissociation products. In summary, ice chemistry induced by energetic processing of the CO-rich apolar ice layer provides alternative formation pathways for several species detected in the interstellar medium, including some related to the complex organic molecule chemistry. Further quantification of these pathways will help astrochemical models to constrain their relative contribution to the interstellar budget of, especially, the organic species H2CO and HNCO.