  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.
