In situ characterization of volatile and refractory hydrocarbons produced by UV photolysis of interstellar C₂H₂ ice

¹ Laboratory for Astrophysics, Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
² Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
³ Xinjiang Astronomical Observatory, Chinese Academy of Sciences, Urumqi 830011, China
⁴ Center for Interstellar Catalysis, Department of Physics and Astronomy, Aarhus University, Aarhus 8000, Denmark

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

Received: 30 July 2025
Accepted: 30 November 2025

Abstract

Context. Acetylene (C₂H₂) has been commonly observed in various astronomical objects, including star-forming regions, young stellar objects, and our Solar System. Theoretical and laboratory studies have proposed multiple mechanisms that link this simplest alkyne to volatile hydrocarbons and polycyclic aromatic hydrocarbons through UV- or cosmic-ray-induced energetic processes. However, it is still unclear whether refractory material can be efficiently formed through solid-state reactions involving C₂H₂ on dust grains.

Aims. In this work, we aim to experimentally study the chemical complexity induced by the UV irradiation of pure C₂H₂ ice and characterize both volatile and nonvolatile photoproducts to better understand the evolution of simple hydrocarbons under astronomically relevant conditions.

Methods. Experiments were performed using MATRI²CES, an ultra-high vacuum, cryogenic setup to investigate the C₂H₂ ice chemistry induced by UV photons between 7.2 and 10.2 eV at 15 K. The UV-processed ice samples were monitored in situ by laser desorption post-ionization reflection time-of-flight mass spectrometry (LDPI ReTOF-MS) in combination with the pulsed ion deflection (PID) technique. The mass spectrometric data of volatiles and refractory residues produced upon VUV photolysis of C₂H₂ ice were collected in situ at 15 and 300 K, respectively, minimizing uncertainties associated with external analytical methods used in previous studies.

Results. The experimental results obtained after photolysis of pure C₂H₂ ice with a fluence of 3 × 10¹⁷ photons cm⁻² (10⁶ years in dense clouds, show the formation of large saturated and unsaturated hydrocarbons containing up to 13 carbon atoms, including molecules identified in previous similar studies. After the sublimation of these volatile products, measurements of the residue at 300 K revealed a rich and distinct mass spectrum suggesting the synthesis of refractories composed of conjugated triple bonds (-C≡C-) and double (-C=C−) bonds. The astrochemical implications and the possible connection of the produced residues with unidentified infrared emission bands are discussed.