UV irradiation of ethanol-containing interstellar ice analogs

Photostability in CH₃CH₂OH:CO mixtures

¹ Laboratory for Astrophysics, Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
² Centre for Interstellar Catalysis, Department of Physics and Astronomy, University of Aarhus, Aarhus 8000, Denmark
³ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁴ Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, PO Box 9502, 2300 RA Leiden, The Netherlands
⁵ Max-Planck Institut für Extraterrestrische Physik (MPE), Giessenbachstr. 1, 85748, Garching, Germany

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

Received: 31 October 2025
Accepted: 11 February 2026

Abstract

Context. Ethanol (CH₃CH₂OH) has been detected in interstellar ices within regions associated with the early stages of star and planet formation. Because of its ability to form through multiple gas- and solid-phase pathways, ethanol is found in a range of astronomical environments, including the solid (ice) phase. These diverse conditions can significantly influence its photostability and photochemistry. While laboratory studies have explored the effects of energetic processing on pure ethanol ices, there is still a gap in understanding how ethanol behaves in mixed ices that more accurately simulate astrophysical conditions.

Aims. This proof-of-principle study aims to quantify how the composition of the ice influences the photostability of ethanol in CH₃CH₂OH and CO ice mixtures, from both physical and chemical perspectives. It also seeks to highlight the importance of balancing constructive and destructive processes in shaping the evolution of molecular complexity within interstellar ices.

Methods. Ice mixtures with ethanol to CO ratios ranging from 1:0 to 1:11 were exposed to ultraviolet irradiation from a microwave discharge hydrogen lamp under ultrahigh vacuum conditions, simulating the secondary UV photons generated by cosmic rays interacting with hydrogen, at 16 K. The evolution of the solid phase was tracked in situ using reflection-absorption infrared spectroscopy (RAIRS), while changes in the gas phase are monitored with a quadrupole mass spectrometer (QMS). Temperature-programmed desorption (TPD) experiments were conducted to aid in the identification of infrared spectral features.

Results. A radiative-transfer model was developed to more accurately account for the influence of the composition of the ice on the effective photon flux experienced by the molecules embedded within the ice. The model reveals that, during later stages of irradiation, photoproducts play a significant role in the absorbing of incident photons, highlighting the complex cascade of processes initiated by single-photon absorption in ethanol-containing ices. By evaluating photodestruction cross sections as a function of the initial ice composition, we find that CO exerts a stabilizing effect on ethanol. For highly dilute ethanol:CO mixtures—representative of conditions in astronomical ices—the photodestruction cross section of ethanol is estimated to be approximately 1.6 × 10⁻¹⁷ cm² photon⁻¹ after correcting for the effective absorbed UV fluence of the studied interstellar ice analogs.