Characterization of the chemical evolution of CH4 ices under processing by cosmic ray analogues with the PROCODA code – II. Dominant chemical reactions for the formation and consumption of CHCH, C2H6, and C3H8

ABSTRACT Methane-rich ices in the Solar system and in interstellar environments undergo extensive chemical processing under cosmic-ray irradiation, leading to the formation of new species such as CHCH, ${\rm C}_2{\rm H}_6$, and ${\rm C}_3{\rm H}_8$. In Paper I, [MNRAS 544, 855, 2025], we modelled the irradiation of CH$_4$ ice with 15.7 MeV $^{16}{\rm O}^{5+}$ ions at 16 K using the PROCODA kinetic code, incorporating more than 1800 chemical reactions. In this work, we concentrate on the dominant chemical reaction responsible for the production and consumption of the hydrocarbons of interest. We find that ${\rm C}_2{\rm H}_6$ forms primarily through the reaction ${\rm H}_2 + {\rm C}_2{\rm H}_5$, while ${\rm C}_3{\rm H}_8$ originates mainly from ${\rm H}_2 + {\rm C}_3{\rm H}_6$. The radical CHCH is produced predominantly via the reaction ${\rm C}_2{\rm H}_3 + {\rm R} \rightarrow {\rm H} + {\rm CHCH}$. Radiation-driven processes dominate the early stages, while the post-relaxation chemistry is governed by neutral–neutral and hydrogenation reactions that control cumulative production. CHCH behaves as a short-lived intermediate due to its high-effective consumption rates, with its destruction pathways shifting from CH$_4$-dominated to H$_2$-mediated channels over time. In contrast, C$_2$H$_6$ and C$_3$H$_8$ are more stable under energetic processing because of their comparatively lower consumption rates. Finally, effective rate coefficients and reaction enthalpies ($\Delta \rm rH$) are shown for each chemical reaction considered. Our results can provide constraints that can be directly incorporated into astrochemical models of cosmic ray-processed ices.