Characterization of H2O: N2 ice under bombardment by cosmic rays: Part II - Evolution of dominant reaction pathways for selected species

Abstract This study investigates the chemical evolution of H₂O: N₂ (1:5) ice at 15 K under cosmic ray analogue irradiation using (40 MeV Ni¹¹⁺ ions). The present study constitutes the second part of a series, with the initial study reporting the effective reaction rate coefficients for a network of 930 coupled reactions involving 28 chemical species derived from kinetic modelling of the same experimental dataset. Here, we identify and analyze the dominant reactions responsible for the production and consumption of six species, including H₂O, N₂, H₂O₂, O₃, N₃, and HNO₃, and to characterize how these pathways evolve as irradiated ice progresses from initial radiolysis towards a chemical equilibrium regime. We show that the dominant reactions at the onset of irradiation, which are largely driven by direct interaction with the ionizing radiation, are progressively replaced by bimolecular reactions between accumulated species as the system approaches chemical equilibrium. The effective rate coefficient for the dominant hydrogen peroxide formation pathway, via O₂ + H₂O recombination, is 1.07 × 10⁻²⁴ cm³ molecule⁻¹ s⁻¹, consistent within an order of magnitude with values reported for different ice compositions and radiation sources at 15 K. The reactive intermediate and nitrogen-bearing species produced and also desorbed from the ice, including HNO and the N3 radical, represent potential precursors to more complex molecules if combined with carbon-bearing species under astrophysical conditions.