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.

中文翻译：

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探索由含 H2、富含 CO 的非极地冰层的能量处理引起的化学反应

尘埃颗粒表面的星际冰幔被认为具有双层结构，具有富含 H2O 的极性层，被富含 CO 的非极性层覆盖，该层可能含有 H2 和其他挥发物，如 N2。在这项工作中，我们探索了由 2 keV 电子和 Ly-α 光子在富 CO 层的 H2:CO:15N2 冰类似物中引起的化学反应，当暴露于与宇宙射线诱导的二次电子和密集云的典型生命周期中的紫外线光子。在 2 keV 电子辐射下鉴定出六种产物：CO2、C2O（和其他碳链氧化物）、CH4、H2CO、H2C2O 和 H15NCO。总产物丰度对应于暴露于电子辐射的初始 CO 分子的 5-10%。Ly-α 光子照射产生 1-2 个数量级的低产量，产品分支比相似，这可能是由于在该波长下冰样品的紫外光子吸收截面低。仅在分别不存在 H2 和 CO 分子的情况下才观察到额外的含氮物质，即 C215N2 和 15NH3，这表明源自 H2 和 CO 分子的反应物优先相互反应，而不是与 15N2 及其解离产物反应。总之，由富含 CO 的非极地冰层的能量处理引起的冰化学为星际介质中检测到的几种物种提供了替代的形成途径，包括一些与复杂有机分子化学相关的物种。