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

Abstract

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