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 bilayered 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 explored the chemistry induced by 2 keV electrons and Lyα 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 chain oxides), CH4, H2CO, H2C2O, and H15NCO. The total product abundances corresponded to 5%-10% of the initial CO molecules exposed to electron irradiation. Lyα 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 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. 

Here, we present all the IR spectra and temperature programmed desorption data for the 16 experiments that were used for this publication (Exp. 1-14, A1,A2), along with 7 additional calibration experiments (C1-C7). Please, note that all infrared spectra are taken in reflection-absorption mode, except for those corresponding to Experiments C1-C7, taken in transmittance mode.