Theoretical and Laboratory Studies on the Interaction of Cosmic‐Ray Particles with Interstellar Ices. III. Suprathermal Chemistry–Induced Formation of Hydrocarbon Molecules in Solid Methane (CH4), Ethylene (C2H4), and Acetylene (C2H2)

Abstract

Methane, ethylene, and acetylene ices are irradiated in a ultra high vacuum vessel at 10 K with 9.0 MeV α-particles and 7.3 MeV protons to elucidate mechanisms to form hydrocarbon molecules upon 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 experimental and computational investigations reveal that each MeV particle transfers its kinetic energy predominantly through inelastic encounters to the target leading to electronic excitation and ionization of the target molecules. Here electronically excited CH₄ species can fragment to mobile H atoms and nonmobile CH₃ radicals. The potential energy stored in Coulomb interaction of the CH⁺₄ ions release energetic H and C atoms not in thermal equilibrium with the 10 K target (suprathermal species). Moderated to 1-10 eV kinetic energy, these carbon atoms and those triggered 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 species. C and CH, as well as CH₂, can insert into a CH bond of a CH₄ molecule to form methylcarbene (HCCH₃), the ethyl radical (C₂H₅), and ethane (C₂H₆). HCCH₃ either loses H₂/2H 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 found to react with suprathermal H atoms to form C₂H₃ and C₂H₅, respectively. Overlapping cascades and an increasing MeV ion exposure transforms C₂H_x (x = 2, ..., 6) to even more complex alkanes up to C₁₄H₃₀. These elementary reactions of suprathermal species to insert, abstract, and add in/to bonds supply a powerful pathway to form new molecules in icy grain mantles condensed on interstellar grains or in hydrocarbon rich bodies in our solar system even at temperatures as low as 10 K.