Characterization of the Chemical Evolution of CH4 Ices under Processing by Cosmic Ray Analogs with the PROCODA code - I. Effective Rate Coefficients and Chemical Equilibrium Phase

Methane (CH4), 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 modeling 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 CH4 ice at 16 K was irradiated, providing the necessary input parameters for the model. Our reaction network comprises 1,857 chemical reactions involving 36 molecular species, both observed and unobserved by FTIR spectroscopy during the experiment. The best-fit model satisfies multiple criteria: a low χ2 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 H2 (38.0 %), CH4 (20.8 %), H (17.0 %), and CH3CH2CH3 (16.9 %). The total desorption yield is calculated as 5.4 × 104 molecules/ion, and the effective destruction cross-section of CH4 is 7.1 × 10−13 cm2. The reaction rate coefficients and equilibrium abundances derived from this study provide valuable inputs for astrochemical models, enhancing our understanding of CH4 processing in interstellar ices under cosmic ray irradiation.