Vacuum ultraviolet photochemistry of solid acetylene: a multispectral approach

Aims. 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. Methods. 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. Results. 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. Conclusions. 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.