Formation of complex molecules in translucent clouds: acetaldehyde, vinyl alcohol, ketene, and ethanol via “nonenergetic” processing of C<sub>2</sub>H<sub>2</sub>ice
K.-J. Chuang, G. Fedoseev, D. Qasim, S. Ioppolo, C. Jäger, Th. Henning, M. E. Palumbo, E. F. van Dishoeck, H. Linnartz
Abstract
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 C 2 H n O, such as acetaldehyde (CH 3 CHO), vinyl alcohol (CH 2 CHOH), ketene (CH 2 CO), and ethanol (CH 3 CH 2 OH), 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 H 2 O ice under translucent cloud conditions (1 ≤ A V ≤ 5 and n H ~ 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. Methods. Ultra-high vacuum 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. These C 2 H 2 experiments were extended by a set of similar experiments involving acetaldehyde (CH 3 CHO) – an abundant product of C 2 H 2 + O 2 + 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 18 O 2 . Results. 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 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.