Formation of complex molecules in translucent clouds: Acetaldehyde, vinyl alcohol, ketene, and ethanol via nonenergetic processing of C2H2 ice
Abstract
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 C2HnO are still under debate. 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≤AV≤5 and nH103 cm-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 10 K. Ultra-high vacuum (UHV) 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. Reflection absorption infrared spectroscopy (RAIRS) 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 (TPD-QMS) was used as a complementary analytical tool. The investigated 10 K 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 a so far experimentally unreported solid-state route for the formation of these species without the need of energetic input.