Laboratory studies on the irradiation of methane in interstellar, cometary, and solar system ices

Pure methane ices (CH4) were irradiated at 10 K with energetic electrons to mimic the energy transfer processes that occur in the track of the trajectories of MeV cosmic-ray particles. The experiments were monitored via an FTIR spectrometer (solid state) and a quadrupole mass spectrometer (gas phase). Combined with electronic structure calculations, this paper focuses on the identification of CHx (x = 1-4) and C2Hx (x = 2-6) species and also investigates their formation pathways quantitatively. The primary reaction step is determined to be the cleavage of a carbon-hydrogen bond of the methane molecule to form a methyl radical (CH3) plus a hydrogen atom. Hydrogen atoms recombined to form molecular hydrogen, the sole species detected in the gas phase during the irradiation exposure. In the matrix two neighboring methyl radicals can recombine to form an internally excited ethane molecule (C2H6), which either can be stabilized by the surrounding matrix or was found to decompose unimolecularly to the ethyl radical (C2H5) plus atomic hydrogen and then to the ethylene molecule (C2H4) plus molecular hydrogen. The initially synthesized ethane, ethyl, and ethylene molecules can be radiolyzed subsequently by the impinging electrons to yield the vinyl radical (C2H3) and acetylene (C2H2) as degradation products. Upon warming the ice sample after the irradiation, the new species are released into the gas phase, simulating the sublimation processes interstellar ices undergo during the hot core phase or comets approaching perihelion. Our investigations also aid the understanding of the synthesis of hydrocarbons likely to be formed in the aerosol particles and organic haze layers of hydrocarbon-rich atmospheres of planets and their moons such as Titan.