Radiolysis of frozen methanol by heavy cosmic ray and energetic solar particle analogues

The chemical and physical effects induced by fast heavy ion irradiation on frozen pure methanol (CH3OH) at 15 K were studied. These energetic ions can simulate the energy transfer processes that occur by cosmic ray irradiation of interstellar ices, comets and icy Solar system bodies. The analysis was made by infrared spectroscopy (Fourier transform infrared) before and after irradiation, with 16-MeV 16O5+, 220-MeV 16O7+, 606-MeV 65Zn20+ and 774-MeV 86Kr31+ ion beams. Integrated values of the absorbance of the main methanol bands were determined. The induced CH3OH dissociation gives rise to the formation of molecular species, particularly H2CO, CH2OH, CH4, CO, CO2, HCO and HCOOCH3. Their formation and dissociation cross-sections were determined. H2CO and CH4 molecules are in general the most abundant new products of the four beams analysed. Except for the HCO and CH2OH species, cross-sections increased with the electronic stopping power, roughly as S3/2e. The G values for CH3OH destruction by fast heavy ion irradiation with Zn and Kr beams were found to be considerably larger than those for oxygen, helium or hydrogen. As an astrophysical implication, the S3/2e power law should be very helpful for predicting the CH3OH formation and dissociation cross-sections for other ion beam projectiles and energies. As astrophysical point of view, the analysis of the predictions reveals the unexpected importance of iron and some other heavy ion constituents of cosmic rays in astro-chemistry. (authors)