Characterization of the chemical evolution of CH4 ices under processing by cosmic ray analogues with the <scp>procoda</scp> code – I. Effective reaction rate coefficients and chemical equilibrium phase

ABSTRACT Methane (CH$_4$), the simplest alkane, is a fundamental component of astrophysical ices, particularly in the outer Solar system and the interstellar medium. Understanding its chemical evolution under energetic particle irradiation is essential for modelling these environments. In this work, we investigate the chemical evolution of pure methane ice subjected to high-energy ion irradiation until chemical equilibrium is reached. We employ the procoda code to simulate the time-dependent evolution of molecular abundances and to determine effective reaction rate coefficients. The simulations are constrained using experimental data from a previous study, in which pure CH$_4$ ice at 16 K was irradiated, providing the necessary input parameters for the model. Our reaction network comprises 1857 chemical reactions involving 36 molecular species, both observed and unobserved by Fourier-transform infrared spectroscopy during the experiment. The best-fitting model satisfies multiple criteria: a low $\chi ^2$ for observed species, a desorption yield consistent with experimental estimates, similar trends in abundance evolution for observed and unobserved species, and overall mass conservation. At chemical equilibrium, the most abundant species predicted by the model are H$_2$ (38.0 per cent), CH$_4$ (20.8 per cent), H (17.0 per cent), and CH$_3$CH$_2$CH$_3$ (16.9 per cent). The total desorption yield is calculated as $5.4\times 10^{4}$ molecules/ion, and the effective destruction cross-section of CH$_4$ is $7.1\times 10^{-13}$ cm$^2$. The reaction rate coefficients and equilibrium abundances derived from this study provide valuable inputs for astrochemical models, enhancing our understanding of CH$_4$ processing in interstellar ices under cosmic ray irradiation.