Theoretical and Laboratory Studies on the Interaction of Cosmic-Ray Particles with Interstellar Ices. II. Formation of Atomic and Molecular Hydrogen in Frozen Organic Molecules

摘要

Methane ices are irradiated at 4 × 10-10 mbar at temperatures between 10 and 50 K with 9.0 MeV α-particles and 7.3 MeV protons to elucidate the formation of atomic as well as molecular hydrogen via interaction of Galactic cosmic-ray particles with extraterrestrial organic ices. Theoretical calculations focus on computer simulations of ion-induced collision cascades in irradiated targets. Our data reveal that more than 99% of the energy is transferred via inelastic interactions to the electronic system of the target to form electronically excited CH4 molecules decomposing to a CH3--H radical pair. Two H atoms recombine in a diffusion limited step to H2. Further, secondary dissociation of CH3 to H and CH2 contributes to H production. To a minor amount, implanted ions generate C and H knock-on atoms via elastic encounters which abstract hydrogen atoms or insert into chemical bonds (carbon atoms only). Fourier transform infrared spectroscopy (FTIR) and quadrupole mass spectrometry (QMS) analyses indicate that if these energy-loss processes accumulate up to 6 ± 3% H atoms in the CH4 target, more than 90% of the ice is released in an explosive ejection into the gas phase. This mechanism represents a powerful pathway to supply newly formed molecules from interstellar grains back to the gas phase of the interstellar medium even at temperatures as low as 10 K.