On the formation and the isomer specific detection of methylacetylene (CH3CCH), propene (CH3CHCH2), cyclopropane (c-C3H6), vinylacetylene (CH2CHCCH), and 1,3-butadiene (CH2CHCHCH2) from interstellar methane ice analogues

Abstract

Pure methane (CH₄) ices processed by energetic electrons under ultra-high vacuum conditions to simulate secondary electrons formed via galactic cosmic rays (GCRs) penetrating interstellar ice mantles have been shown to produce an array of complex hydrocarbons with the general formulae: C_nH_2n+2 (n = 4–8), C_nH_2n (n = 3–9), C_nH_2n−2 (n = 3–9), C_nH_2n−4 (n = 4–9), and C_nH_2n−6 (n = 6–7). By monitoring the in situ chemical evolution of the ice combined with temperature programmed desorption (TPD) studies and tunable single photon ionization coupled to a reflectron time-of-flight mass spectrometer, specific isomers of C₃H₄, C₃H₆, C₄H₄, and C₄H₆ were probed. These experiments confirmed the synthesis of methylacetylene (CH₃CCH), propene (CH₃CHCH₂), cyclopropane (c-C₃H₆), vinylacetylene (CH₂CHCCH), 1-butyne (HCCC₂H₅), 2-butyne (CH₃CCCH₃), 1,2-butadiene (H₂CCCH(CH₃)), and 1,3-butadiene (CH₂CHCHCH₂) with yields of 2.17 ± 0.95 × 10⁻⁴, 3.7 ± 1.5 × 10⁻³, 1.23 ± 0.77 × 10⁻⁴, 1.28 ± 0.65 × 10⁻⁴, 4.01 ± 1.98 × 10⁻⁵, 1.97 ± 0.98 × 10⁻⁴, 1.90 ± 0.84 × 10⁻⁵, and 1.41 ± 0.72 × 10⁻⁴ molecules eV⁻¹, respectively. Mechanistic studies exploring the formation routes of methylacetylene, propene, and vinylacetylene were also conducted, and revealed the additional formation of the 1,2,3-butatriene isomer. Several of the above isomers, methylacetylene, propene, vinylacetylene, and 1,3-butadiene, have repeatedly been shown to be important precursors in the formation of polycyclic aromatic hydrocarbons (PAHs), but until now their interstellar synthesis has remained elusive.