Theoretical and laboratory studies on the interaction of cosmic-ray particles with interstellar ices. III. Superthermal chemistry-induced formation of hydrocarbon molecules in solid methane (CH4), ethylene (C2H4), and acetylene (C2H2)

Methane, ethylene, and acetylene ices are irradiated in a ultra high vacuum vessel at 10 K with 9.0 MeV alpha-particles and 7.3 MeV protons to elucida te mechanisms to form hydrocarbon molecules upon interaction of Galactic co smic-ray particles with extraterrestrial, organic ices. Theoretical calcula tions focus on computer simulations of ion-induced collision cascades in ir radiated targets. Our experimental and computational investigations reveal that each MeV particle transfers its kinetic energy predominantly through i nelastic encounters to the target leading to electronic excitation and ioni zation of the target molecules. Here electronically excited CH4 species can fragment to mobile H atoms and non-mobile CH3 radicals. The potential ener gy stored in Coulomb Interaction of the CH4+ ions release energetic H and C atoms not in thermal equilibrium with the 10 K target (suprathermal specie s). Moderated to 1-10 eV kinetic energy, these carbon atoms and those trigg ered by the elastic energy transfer of the MeV projectile to the target are found to abstract up to two H atoms to yield suprathermal CH and CH2 speci es. C and CH, as well as CH2, can insert into a C-H bond of a CH4 molecule to form methylcarbene (HCCH3), the ethyl radical (C2H5), and ethane (C2H6). HCCH3 either loses H-2/2H to form acetylene, C2H2, rearranges to ethylene, C2H4, or adds two H atoms to form ethane, C2H6. C2H5 can abstract or lose an H atom, giving ethane and ethylene, respectively. C2H2 and C2H4 are foun d to react with suprathermal H atoms to form C2H3 and C2H5, respectively. O verlapping cascades and an increasing MeV ion exposure transforms C2Hx (x = 2, ..., 6) to even more complex alkanes up to C14H30. These elementary rea ctions of suprathermal species to insert, abstract, and add in/to bonds sup ply a powerful pathway to form new molecules in icy grain mantles condensed on interstellar grains or in hydrocarbon rich bodies in our solar system e ven at temperatures as low as 10 K.