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 O-16(7+) 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.