Radiolytic Processing of CH4:CO2:NH3 Ices by Swift Heavy Ions: Coupled C–H–O-N Chemistry

Abstract

The chemical evolution of CH4:CO2:NH3 astrophysical ice analogs was investigated under irradiation by 538 MeV 64Ni24+ ions at 14 K, simulating the effects of heavy cosmic rays in dense interstellar environments. Molecular changes were monitored in situ using Fourier transform infrared (FTIR) spectroscopy. A progressive depletion of the precursor species was observed with increasing ion fluence, accompanied by the formation of a diverse set of products, including CO, H2O, C2H6, C2H4, CH3OH, H2CO, HCOOH, HCOO–, OCN–, and N2O. The evolution of column densities revealed an initial rapid growth phase followed by saturation, reflecting the competition between formation and destruction processes. Destruction and formation cross sections were derived assuming first-order kinetics, and the corresponding radiochemical yields were determined. CO exhibited the highest formation yield, indicating that CO2 dissociation is one of the dominant irradiation-induced processes, while the efficient production of CH3OH, H2CO, OCN–, and N2O demonstrated the simultaneous operation of carbon-, oxygen-, and nitrogen-bearing reaction pathways. An atomic budget analysis indicated that a significant fraction of dissociated atoms was not accounted for in the observed infrared-active species, suggesting the formation of more complex or refractory compounds. These findings demonstrate the key role of heavy-ion processing in driving chemical complexity in interstellar ices, promoting the formation of oxygen- and nitrogen-bearing organic species of astrochemical and prebiotic relevance.