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

Context. The simultaneous detection of organic molecules of the form C 2 H n O, such as ketene (CH 2 CO), acetaldehyde (CH 3 CHO), and ethanol (CH 3 CH 2 OH), 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 C 2 H 2 ice with OH-radicals and H-atoms was shown to provide formation routes to ketene, acetaldehyde, ethanol, and vinyl alcohol (CH 2 CHOH) along the H 2 O 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 H 2 O-rich ice mantle. The focus here is on the H + radiolysis of interstellar C 2 H 2 :H 2 O ice analogs at 17 K. Methods. Ultra-high vacuum experiments were performed to investigate the 200 keV H + radiolysis chemistry of predeposited C 2 H 2 :H 2 O 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 H 2 18 O. Results. The energetic processing of C 2 H 2 :H 2 O ice not only results in the formation of (semi-) saturated hydrocarbons (C 2 H 4 and C 2 H 6 ) and polyynes as well as cumulenes (C 4 H 2 and C 4 H 4 ), 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.