Formation of complex molecules in translucent clouds: Acetaldehyde, vinyl alcohol, ketene, and ethanol via nonenergetic processing of C2H2 ice

K. -J. Chuang; G. Fedoseev; D. Qasim; S. Ioppolo; C. Jäger; Th. Henning; M. E. Palumbo; E. F. van Dishoeck; H. Linnartz

doi:10.1051/0004-6361/201937302

paper

Formation of complex molecules in translucent clouds: Acetaldehyde, vinyl alcohol, ketene, and ethanol via nonenergetic processing of C2H2 ice

arXiv:2002.06971 · doi:10.1051/0004-6361/201937302

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 C $_{2}$ H $_{n}$ O are still under debate. In this work, we investigate the laboratory possible solid-state reactions that involve simple hydrocarbons and OH-radicals along with H $_{2}$ O ice under translucent cloud conditions (1 $\leq$ A $_{V}$ $\leq$ 5 and \textit{n} $_{H}$ $\sim$ 10 $^{3}$ cm $^{- 3}$ ). We focus on the interactions of C $_{2}$ H $_{2}$ with H-atoms and OH-radicals, which are produced along the H $_{2}$ O formation sequence on grain surfaces at 10 K. Ultra-high vacuum (UHV) experiments were performed to study the surface chemistry observed during C $_{2}$ H $_{2}$ + O $_{2}$ + H codeposition, where O $_{2}$ 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 C $_{2}$ H $_{2}$ with H-atoms and OH-radicals not only results in semi and fully saturated hydrocarbons, such as ethylene (C $_{2}$ H $_{4}$ ) and ethane (C $_{2}$ H $_{6}$ ), 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 C $_{2}$ H $_{2}$ , 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.

17 pages, 12 figures, 2 tables