2020A&A...635A.199C

Context. Complex organic molecules (COMs) have been identified toward high- and low-mass protostars as well as molecular clouds, suggesting that these interstellar species originate from the early stage(s) of starformation. The reaction pathways resulting in COMs described by the formula C₂H_nO, such as acetaldehyde (CH₃CHO), vinyl alcohol (CH₂CHOH), ketene (CH₂CO), and ethanol (CH₃CH₂OH), are still under debate. Several of these species have been detected in both translucent and dense clouds, where chemical processes are dominated by (ground-state) atom and radical surface reactions. Therefore, efficient formation pathways are needed to account for their appearance well before the so-called catastrophic CO freeze-out stage starts.
Aims. In this work, we investigate the laboratory possible solid-state reactions that involve simple hydrocarbons and OH-radicals along with H₂O ice under translucent cloud conditions (1≤A_V≤5 and n_H∼10³cm^–3). We focus on the interactions of C₂H₂ with H-atoms and OH-radicals, which are produced along the H₂O formation sequence on grain surfaces at 10K.
Methods. Ultra-high vacuum experiments were performed to study the surface chemistry observed during C₂H₂+O₂+H codeposition, where O₂ was used for the in situ generation of OH-radicals. These C₂H₂ experiments were extended by a set of similar experiments involving acetaldehyde (CH₃CHO) - an abundant product of C₂H₂+O₂+H codeposition. Reflection absorption infrared spectroscopy was applied to in situ monitor the initial and newly formed species. After that, a temperature-programmed desorption experiment combined with a quadrupole mass spectrometer was used as a complementary analytical tool. The IR and QMS spectral assignments were further confirmed in isotope labeling experiments using ¹⁸O₂.
Results. The investigated 10K surface chemistry of C₂H₂ with H-atoms and OH-radicals not only results in semi and fully saturated hydrocarbons, such as ethylene (C₂H₄) and ethane (C₂H₆), but it also leads to the formation of COMs, such as vinyl alcohol, acetaldehyde, ketene, ethanol, and possibly acetic acid. It is concluded that OH-radical addition reactions to C₂H₂, acting as a molecular backbone, followed by isomerization (i.e., keto-enol tautomerization) via an intermolecular pathway and successive hydrogenation provides so far an experimentally unreported solid-state route for the formation of these species without the need of energetic input. The kinetics of acetaldehyde reacting with impacting H-atoms leading to ketene and ethanol is found to have a preference for the saturated product. The astronomical relevance of the reaction network introduced here is discussed.