Exploring the Chemistry Induced by Energetic Processing of the H2-bearing, CO-rich Apolar Ice Layer

Abstract

Interstellar ice mantles on the surfaces of dust grains are thought to have a bilayered structure, with a H 2 O-rich polar layer, covered by a CO-rich apolar layer that probably harbors H 2 and other volatiles such as N 2 . In this work, we explore the chemistry induced by 2 keV electrons and Ly α photons in H 2 :CO: 15 N 2 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: CO 2 , C 2 O (and other carbon chain oxides), CH 4 , H 2 CO, H 2 C 2 O, and H 15 NCO. The total product abundances corresponded to 5%−10% of the initial CO molecules exposed to electron irradiation. Ly α photon irradiation delivered one to two 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 C 2 15 N 2 and 15 NH 3 , was only observed in the absence of H 2 and CO molecules, respectively, suggesting that reactants derived from H 2 and CO molecules preferentially react with each other instead of with 15 N 2 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 constrain their relative contribution to the interstellar budget of, especially, the organic species H 2 CO and HNCO.