2025A&A...700A.115K

Context. The irradiation of condensed hydrocarbons at low temperatures may result in the production of larger, more complex carbon-rich species relevant to biology. This chemistry may therefore influence the prebiotic potential of astronomical environments.
Aims. We have sought to use vacuum-ultraviolet (VUV) photoabsorption spectroscopy to characterise the production of complex carbon-bearing molecules (particularly polyynes and cyanopolyynes) as a result of the 1 keV electron irradiation of neat CH₄, C₂H₄, C₂H₆, C₃H₈, C₆H₁₄, and C₈H₁₈ astrophysical ice analogues as well as their binary mixtures with N₂ and NH₃.
Methods. Ices were nominally prepared and irradiated at low temperatures in an ultra-high-vacuum astrochemistry chamber and the ensuing radiation chemistry was followed via VUV photoabsorption spectroscopy. Following their irradiation, the ices were warmed to allow volatile species to sublimate. Residual refractory species were spectroscopically analysed.
Results. The irradiation of neat hydrocarbon ices promotes the synthesis of a number of polyynes (i.e. C₈H₂ and C₁₀H₂, and possibly C₁₂H₂, C₁₄H₂, and C₁₆H₂) that were identified through the emergence of new features in acquired VUV spectra and that largely remain in the solid phase during post-irradiative warming. When hydrocarbons were mixed with N₂ or NH₃ and irradiated, mixtures of polyynes and cyanopolyynes were formed (i.e. C₈H₂, HC₇N, HC₉N, and C₆N₂); with product yields being greater in irradiated binary ices containing NH₃ compared to ones containing N₂. Such results suggest that the radiolytic synthesis of (cyano)polyynes in hydrocarbon-and nitrogen-rich icy space environments (such as dense interstellar clouds or the Pluto-Charon system) is plausible.