Phase-dependent vacuum ultraviolet photochemistry of diacetylene ices: implications for Titan chemistry

The photochemical processing of organic ices is a key driver of molecular complexity in planetary atmospheres and surfaces. On Titan, diacetylene (C₄H₂) has been detected in the atmosphere and is expected to condense at lower altitudes, yet its subsequent solid-phase photochemistry remains poorly understood. This study investigates the vacuum ultraviolet photolysis of C₄H₂ ices to understand how distinct solid phases influence photoreactivity and product formation under simulated Titan conditions. Thin films of C₄H₂ ice in amorphous (20 K) and crystalline (20 K and 70 K) phases were irradiated for 48 hours, with chemical evolution monitored via infrared spectroscopy and temperature-programmed desorption mass spectrometry. Photolysis cross-sections were found to be phase and temperature-dependent, with the amorphous ice exhibiting the highest reactivity, while infrared spectroscopy showed evidence of crystalline phase amorphisation only in the ice irradiated at 70 K. The presence of volatile hydrocarbons including C₆H₂ and C₈H₆ was detected across all phases, while crystalline ice irradiated at 70 K yielded the most complex suite of products, with ions detected up to m/z 200. Non-volatile refractory residues with saturated and unsaturated spectral signatures remained at 300 K for all ices. The results suggest that VUV photons can drive the formation of larger, more complex organic molecules in C₄H₂ ices, with the solid-phase structure modulating the reaction pathways and product distribution. This solid-phase chemistry may contribute to the inventory of complex organics on Titan's surface, providing potential targets for future in situ exploration by missions such as Dragonfly.