2018ApJ...862..140B

This work investigates the synthesis of complex organic molecules with special focus on acetic acid (CH₃COOH) via experiments involving the processing of astrophysical model ices of carbon dioxide-methane (CO₂-CH₄) by low doses of ionizing radiation, exposing the initial bond-breaking processes and successive reactions initiated by energetic electrons generated in the track of galactic cosmic-ray particles penetrating ice-coated interstellar grains, deep inside molecular clouds in their early stages of evolution. The key results were obtained through single photoionization reflectron time-of-flight mass spectrometry (PI-ReTOF-MS) and exploiting isotopically labeled samples (C¹⁸O₂-CH₄; CO₂-CD₄). Not only acetic acid (CH₃COOH), along with fragments of acetic acid dimers (CH₃COOH)₂, but also the hitherto elusive interstellar methyl hydroperoxide (CH₃OOH) and the hydrocarbons ethane (C₂H₆) and butane (C₄H₁₀), along with species belonging to C₂H₄O, C₂H₆O, and C₃H₆O₂ isomers, are swiftly formed via suprathermal reactions at doses of only 0.88 ± 0.12 eV per molecule of carbon dioxide and 0.32 ± 0.04 eV per molecule of methane, which is equivalent to doses deposited in just (2.0 ± 0.5) x 10⁶ yr in a typical molecular cloud. The results suggest further that the search for acetic acid dimers (CH₃COOH)₂ toward star-forming regions has a significant potential to be successful. Finally, methyl hydroperoxide (CH₃OOH) and dimethyl peroxide (CH₃OOCH₃), as identified previously in our laboratory, are predicted to be present in the interstellar medium, thus providing a homologous series of peroxides-HOOH, CH₃OOH, and CH₃OOCH₃-to shed light on the interstellar oxygen chemistry.