Photolysis of interstellar ice analogs containing ethanol: A new approach to forming C₂H₄O₂ isomers

¹ Sorbonne Université, CNRS, De la Molécule aux Nano-Objets: Réactivité, Interactions, Spectroscopies, MONARIS, 75005 Paris, France
² LUX, Observatoire de Paris, PSL University, Sorbonne Universités, CNRS, 75014 Paris, France

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 21 July 2025
Accepted: 8 December 2025

Abstract

Context. The three most common of the eight isomers of C₂H₄O₂ - methyl formate (CH₃OCHO, MF), glycolaldehyde (CH₂OHCHO, GA), and acetic acid (CH₃COOH, AA) - are detected in various interstellar clouds, often with differing abundance ratios. Laboratory experiments simulating astrophysical ice conditions have successfully produced methyl formate and glycolaldehyde from methanolrich ices, but acetic-acid formation remains poorly understood, with only a few experimental successes. Current gas-grain chemical models can reproduce some observed abundances, whereas gas-phase models alone are insufficient.

Aims. The aim of this study is to investigate new formation pathways for C₂H₄O₂ isomers through the vacuum-ultraviolet (VUV) irradiation of ethanol (CH₃CH₂OH) under astrophysically relevant ice conditions. By analyzing the photo-products formed in different environments - ethanol isolated in a neon matrix, in pure form, and mixed with molecular oxygen - we aim to identify mechanisms that could account for the observed abundances of these isomers in interstellar molecular clouds.

Methods. Fourier-transform infrared (FTIR) spectroscopy was performed on the ice analog before and after VUV irradiation at 3 K and at 5 K intervals from 10 to 300 K. The column density of each isomer in the ice was determined through quantitative FTIR analysis. As the temperature increased, desorption of the sample was monitored using a quadrupole mass spectrometer. The temperatureprogrammed desorption (TPD) profiles for each mass were used to corroborate the infrared (IR) results and to hypothesize the presence of specific species.

Results. Under conditions simulating astrophysical ice environments, we successfully formed the three isomers from CH₃CH₂OH:O₂ ice irradiation. At 160 K, the abundance ratio of MF:GA:AA was determined to be 1.3:1:3.9. At this temperature, MF has already largely begun to desorb, and the associated abundance ratio does not represent the total quantity of MF formed. However, its detection at 160 K in the ice indicates that trapping is taking place in our experiment.

Conclusions. Our oxygenated ethanol ice irradiation experiment favors AA formation over GA, a result that can explain some interstellar medium (ISM) cloud observations.