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Interstellar ice mantles on the surfaces of dust grains are thought to have abi-layered structure, with a H2O-rich polar layer, covered by a CO-rich apolarlayer that probably harbors H2 and other volatiles such as N2. In this work, weexplore the chemistry induced by 2 keV electrons and Ly-alpha photons inH2:CO:15N2 ice analogs of the CO-rich layer when exposed to similar fluences tothose expected from the cosmic-ray-induced secondary electrons and UV photonsduring the typical lifetime of dense clouds. Six products were identified upon2 keV electron irradiation: CO2, C2O (and other carbon chainoxides), CH4, H2CO,H2C2O, and H15NCO. The total product abundances corresponded to 5-10% of theinitial CO molecules exposed to the electron irradiation. Ly-alpha photonirradiation delivered 1-2 orders of magnitude lower yields with a similarproduct branching ratio, which may be due to the low UV-photon absorptioncross-section of the ice sample at this wavelength. Formation of additionalN-bearing species, namely C215N2 and 15NH3, was only observed in the absence ofH2 and CO molecules, respectively, suggesting that reactants derived from H2and CO molecules preferentially react with each other instead of with 15N2 andits dissociation products. In summary, ice chemistry induced by energeticprocessing of the CO-rich apolar ice layer provides alternative formationpathways for several species detected in the interstellar medium, includingsome related to the complex organic molecule chemistry. Further quantificationof these pathways will help astrochemical models to constrain their relativecontribution to the interstellar budget of, especially, the organic speciesH2CO and HNCO.

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