Laboratory Studies on the Irradiation of Methane in Interstellar, Cometary, and Solar System Ices

Abstract

Pure methane ices (CH 4 ) 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 CH x (x ¼ 1Y4) and C 2 H x (x ¼ 2Y6) species and also investigates their formation pathways quantitatively. The primary reaction step is determined to be the cleavage of a carbonhydrogen bond of the methane molecule to form a methyl radical (CH 3 ) 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 (C 2 H 6 ), which either can be stabilized by the surrounding matrix or was found to decompose unimolecularly to the ethyl radical (C 2 H 5 ) plus atomic hydrogen and then to the ethylene molecule (C 2 H 4 ) plus molecular hydrogen. The initially synthesized ethane, ethyl, and ethylene molecules can be radiolyzed subsequently by the impinging electrons to yield the vinyl radical (C 2 H 3 ) and acetylene (C 2 H 2 ) 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.

Cosmic ray impact on astrophysical ices: laboratory studies on heavy ion irradiation of methane

“…The typical virgin CH 4 FTIR spectrum (shown in Fig. 1a) is in good agreement with previous studies Moore & Hudson 1998;Kaiser & Roessler 1998;Bennett et al 2006). The initial column density of the CH 4 before irradiation, N virgin = (2.69 ± 0.50) × 10 18 molecules/cm 2 , was determined by measuring the area of the ν 1 + ν 4 vibration mode band at 4202 cm −1 .…”

“…The total value, which is the sum of all these curves, is labeled ΣC n and the column density of CH 4 was plotted using the expression N 0 − N F . of the band 821 cm −1 adopted by Kaiser et al (1996) and Bennett et al (2006), the produced values C 2 H 6 increase to (4.42 ± 0.50) × 10 17 , meaning that the destruction of CH 4 is less than the creation of C 2 H 6 , and the carbon budget will not be conserved. This remark shows that the judicious choice of coherent A-values is important, because otherwise the abundance ratio may be incorrect, hence the formation cross section values.…”

“…4 and 5). (ii) For the precursor species and its daughter species, the consistency of their A relative values can be checked by the socalled carbon budget test (Bennett et al 2006): if no sputtering of these species occurs, the total number of carbon References.…”

“…(1) Gerakines et al (1996), (2) Kaiser et al (1998), (3) Moore et al (2003), and (4) Bennett et al (2006). atoms must stay constant during irradiation regardless of the chemical reactions induced by radiation.…”

“…The occurrence of molecular synthesis in these astrophysical ices by cosmic rays, solar wind, UV radiation, and electron collision has been the object of research over the past decades (e.g. Gerakines et al 1996;Kaiser & Roessler 1998;Moore & Hudson 1998;Baratta et al 2002;Bennett et al 2006). Heavy galactic cosmic rays (HGCR) are considered as the main agent for keeping a significant fraction of gas phase material in molecular clouds (Leger et al 1985;Hasegawa & Herbst 1993;Willacy & Williams 1993;Bergin et al 1995;Shematovich et al 1997;Willacy & Millar 1998;Nguyen et al 2002).…”