In situ characterization of volatile and refractory hydrocarbons produced by UV photolysis of interstellar C 2 H 2 ice

Context . Acetylene (C 2 H 2 ) 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 C 2 H 2 on dust grains. Aims . In this work, we aim to experimentally study the chemical complexity induced by the UV irradiation of pure C 2 H 2 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 MATRI 2 CES, an ultra-high vacuum, cryogenic setup to investigate the C 2 H 2 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 C 2 H 2 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 C 2 H 2 ice with a fluence of 3 × 10 17 photons cm −2 (10 6 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.