Laboratory Studies on the Formation of Three C₂H₄O Isomers—Acetaldehyde (CH₃CHO), Ethylene Oxide (c-C₂H₄O), and Vinyl Alcohol (CH₂CHOH)—in Interstellar and Cometary Ices

Laboratory experiments were conducted to unravel synthetic routes to form three C₂H₄O isomers—acetaldehyde (CH₃CHO), ethylene oxide (c-C₂H₄O), and vinyl alcohol (CH₂CHOH)—in extraterrestrial ices via electronic energy transfer processes initiated by electrons in the track of MeV ion trajectories. Here we present the results of electron irradiation on a 2 : 1 mixture of carbon dioxide (CO₂) and ethylene (C₂H₄). Our studies suggest that suprathermal oxygen atoms can add to the carbon-carbon π bond of an ethylene molecule to form initially an oxirene diradical (addition to one carbon atom) and the cyclic ethylene oxide molecule (addition to two carbon atoms) at 10 K. The oxirene diradical can undergo a [1, 2]-H shift to the acetaldehyde molecule. Both the ethylene oxide and the acetaldehyde isomers can be stabilized in the surrounding ice matrix. To a minor amount, suprathermal oxygen atoms can insert into a carbon-hydrogen bond of the ethylene molecule, forming vinyl alcohol. Once these isomers have been synthesized inside the ice layers of the coated grains in cold molecular clouds, the newly formed molecules can sublime as the cloud reaches the hot molecular core stage. These laboratory investigations help to explain astronomical observations by Nummelin et al. and Ikeda et al. toward massive star-forming regions and hot cores, where observed fractional abundances of these isomers are higher than can be accounted for by gas-phase reactions alone. Similar synthetic routes could help explain the formation of acetaldehyde and ethylene oxide in comet C/1995 O1 (Hale-Bopp) and also suggest a presence of both isomers in Titan's atmosphere.