Formation of complex organic molecules in molecular clouds: acetaldehyde, vinyl alcohol, ketene, and ethanol via the “energetic” processing of C₂H₂ ice

¹ Laboratory Astrophysics Group of the Max Planck Institute for Astronomy at the Friedrich Schiller University Jena, Institute of Solid State Physics, Helmholtzweg 3, 07743 Jena, Germany
e-mail: chuang@mpia.de
² Laboratory for Astrophysics, Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
³ INAF – Osservatorio Astrofisico di Catania, via Santa Sofia 78, 95123 Catania, Italy
⁴ Research Laboratory for Astrochemistry, Ural Federal University, Kuibysheva St. 48, 620026 Ekaterinburg, Russia
⁵ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany

Received: 10 March 2021
Accepted: 1 April 2021

Abstract

Context. The simultaneous detection of organic molecules of the form C₂H_nO, such as ketene (CH₂CO), acetaldehyde (CH₃CHO), and ethanol (CH₃CH₂OH), toward early star-forming regions offers hints of a shared chemical history. Several reaction routes have been proposed and experimentally verified under various interstellar conditions to explain the formation pathways involved. Most noticeably, the non-energetic processing of C₂H₂ ice with OH-radicals and H-atoms was shown to provide formation routes to ketene, acetaldehyde, ethanol, and vinyl alcohol (CH₂CHOH) along the H₂O formation sequence on grain surfaces in translucent clouds.

Aims. In this work, the non-energetic formation scheme is extended with laboratory measurements focusing on the energetic counterpart, induced by cosmic rays penetrating the H₂O-rich ice mantle. The focus here is on the H⁺ radiolysis of interstellar C₂H₂:H₂O ice analogs at 17 K.

Methods. Ultra-high vacuum experiments were performed to investigate the 200 keV H⁺ radiolysis chemistry of predeposited C₂H₂:H₂O ices, both as mixed and layered geometries. Fourier-transform infrared spectroscopy was used to monitor in situ newly formed species as a function of the accumulated energy dose (or H⁺ fluence). The infrared spectral assignments are further confirmed in isotope labeling experiments using H₂¹⁸O.

Results. The energetic processing of C₂H₂:H₂O ice not only results in the formation of (semi-) saturated hydrocarbons (C₂H₄ and C₂H₆) and polyynes as well as cumulenes (C₄H₂ and C₄H₄), but it also efficiently forms O-bearing COMs, including vinyl alcohol, ketene, acetaldehyde, and ethanol, for which the reaction cross-section and product composition are derived. A clear composition transition of the product, from H-poor to H-rich species, is observed as a function of the accumulated energy dose. Furthermore, the astronomical relevance of the resulting reaction network is discussed.