Radiolysis versus photolysis of nitrile ices: Laboratory constraints for interstellar chemistry

Interstellar ice chemistry is a fundamental driver of the molecular complexity observed within the interstellar medium. Moreover, the chemical evolution of icy mantles is predominantly influenced by the energetic processes to which they are exposed. Differences between radiolytic and photolytic pathways for the formation of complex organic molecules (COMs) can lead to significant variations in the resulting molecular abundances. Application of Chirped Pulse ICE enables the energetic processing of astrophysical ices through both vacuum ultraviolet photolysis and high-energy electron bombardment, facilitating direct comparison of the chemical outcomes produced by each radiation source. Within a single apparatus, ices are characterized in the condensed phase using reflection-absorption infrared spectroscopy, and in the gas phase directly following temperature-programmed desorption via chirped-pulse millimeter-wave rotational spectroscopy coupled with buffer gas cooling. Comparative energetic processing of two parent ices, methyl and ethyl cyanide ( and reveals several key findings. Following radiolysis, the formation of ketenimine ( from methyl cyanide increases by a factor of 3.9-5.9 relative to hydrogen cyanide (HCN) production, compared to photolysis, with detection limited to the condensed phase. This enhancement increases to 7.0-8.3 for methyl ketenimine ( formation from ethyl cyanide. In contrast, the abundance of the corresponding isonitrile relative to HCN remains consistent across both radiation sources, as evidenced by its detection in both the condensed and gas phase. Our results suggest that observed imine-to-HCN ratios may be diagnostic tracers of the prevailing energetic processing and chemical pathways to COM formation in specific interstellar environments.