2011MNRAS.418.1363D

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 CH₃ OH 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. H₂ CO and CH₄molecules are in general the most abundant new products of the four beams analysed. Except for the HCO and CH₂ OH species, cross-sections increased with the electronic stopping power, roughly as σ~S^3/2_e. The G values for CH₃ OH 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 S^3/2_e power law should be very helpful for predicting the CH₃ OH 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 astrochemistry.