2017ApJ...836..195A

The processing of the hydrocarbon ice, ethylene (C₂H₄/C₂D₄), via energetic electrons, thus simulating the processes in the track of galactic cosmic-ray particles, was carried out in an ultrahigh vacuum apparatus. The chemical evolution of the ices was monitored online and in situ utilizing Fourier transform infrared spectroscopy (FTIR) and during temperature programmed desorption, via a quadrupole mass spectrometer utilizing electron impact ionization (EI-QMS) and a reflectron time-of-flight mass spectrometer utilizing a photoionization source (PI-ReTOF-MS). Several previous in situ studies of ethylene ice irradiation using FTIR were substantiated with the detection of six products: [CH₄ (CD₄)], acetylene [C₂H₂ (C₂D₂)], the ethyl radical [C₂H₅ (C₂D₅)], ethane [C₂H₆ (C₂D₆)], 1-butene [C₄H₈ (C₄D₈)], and n-butane [C₄H₁₀ (C₄D₁₀)]. Contrary to previous gas phase studies, the PI-ReTOF-MS detected several groups of hydrocarbon with varying degrees of saturation: C_nH_2n+2 (n = 4-10), C_nH_2n (n = 2-12, 14, 16), C_nH_2n–2 (n = 3-12, 14, 16), C_nH_2n–4 (n = 4-12, 14, 16), C_nH_2n–6 (n = 4-10, 12), C_nH_2n–8 (n = 6-10), and C_nH_2n–10 (n = 6-10). Multiple laboratory studies have shown the facile production of ethylene from methane, which is a known ice constituent in the interstellar medium. Various astrophysically interesting molecules can be associated with the groups detected here, such as allene/methylacetylene (C₃H₄) or 1, 3-butadiene (C₄H₆) and its isomers, which have been shown to lead to polycyclic aromatic hydrocarbons. Finally, several hydrocarbon groups detected here are unique to ethylene ice versus ethane ice and may provide understanding of how complex hydrocarbons form in astrophysical environments.