Cosmic ray impact on astrophysical ices: laboratory studies on heavy ion irradiation of methane

Laboratory data of CH₄ ice radiolysis promoted by fast heavy ions are obtained by infrared spectroscopy (FTIR). CH₄ molecules are condensed on a CsI substrate at 15 K, and the ice layer is bombarded by 220 MeV ¹⁶O⁷⁺ ion beam. The ice thickness is thin enough to be traversed by projectiles at constant velocity close to the equilibrium charge state. The induced CH₄ dissociation gives rise to the formation of molecular species CH₃, C₂H₂, C₂H₄, C₂H₆, and C₃H₈. Their formation and dissociation cross sections are determined. C₂H₆ represent the most abundant daughter molecules. The carbon budget analysis of CH₄ and its radiolysis products shows that the column density of carbon atoms contained in the methane destroyed during ion irradiation is 30 - 50% greater than the sum for the column densities of the newly formed species. As an astrophysical implication, the current results allow estimation of chemical reaction rates in ices covering interstellar grains.