2014A&A...570A..83C

Gas phase acetylene (C₂H₂) and polyynes (H(-C=C-)_mH) are ubiquitous in the interstellar medium. However, astrochemical models systematically underestimate the observed abundances, supporting the idea that enrichment from the solid state takes place. In this laboratory-based study, we investigate the role C₂H₂ plays in interstellar ice chemistry and we discuss the way its photoproducts may affect gas phase compositions. C₂H₂ ice is investigated under vacuum ultraviolet (VUV) irradiation in its pure form as present in the atmosphere of Titan and in a water-dominated ice as present on grain mantles in molecular clouds and on comets. To disentangle the photochemical network, a unique, complementary combination of infrared and ultraviolet-visible (UV-VIS) spectroscopy is used. From the experimental results, it can be concluded that the VUV-induced solid state C₂H₂ reaction network is dominated by polymerization resulting in the formation of polyynes at least up to C₂₀H₂ and larger polyyne-like molecules. At low temperatures, this process takes place very efficiently and suggests low barriers. When extending this reaction scheme to a water-rich environment, the dominant reaction products are CO and CO₂ but the simultaneous detection of polyyne like molecules is evidence that the reactions as observed in pure C₂H₂ ice persist. From the spectroscopic evidence as presented in this laboratory study, it is concluded that the formation of polyynes upon VUV irradiation of interstellar ices is a process that may contribute to at least part of the observed gas phase enrichment in space.