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: CnH2n+2 (n = 4-8), CnH2n (n = 3-9), CnH2n-2 (n = 3-9), CnH2n-4 (n = 4-9), and CnH2n-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.

Thermal desorption effects on fragment ion production from multi-photon ionized uridine and selected analogues

“…More generally, there are numerous precedents in the literature for different sublimation temperatures leading to different gas-phase isomer populations. [47][48][49] The paragraphs above argue that the thermal desorption dependence of S + production from multi-photon ionized uridine via what we have called process (i) can be traced to changes in isomeric form prior to MPI. Furthermore, process (i) demonstrates wavelength threshold behavior and this is indicative of a critical mechanism in an electronic excited state.…”

On the Formation of the Popcorn Flavorant 2,3‐Butanedione (CH3COCOCH3) in Acetaldehyde‐Containing Interstellar Ices

“…As COMs often have similar infrared absorptions, infrared spectroscopy rarely allows the unambiguous detection of discrete molecules due to overlapping absorptions. [65] Therefore, the subliming molecules were also photoionized (PI) by tunable vacuum ultraviolet (VUV) light and detected according to their m/z ratios in a reflectron time-of-flight mass spectrometer (ReToF-MS). By systematically tuning the photon energy, distinct structural isomers of the products can be exclusively distinguished.…”

Detectability of biosignatures on LHS 1140 b

“…We underestimate the absorption of the C-H stretching mode of aliphatic hydrocarbon chains near 3.4 µm (see Bellucci et al 2009;Maltagliati et al 2015). This discrepancy is likely due to incomplete line lists or cross sections in the HITRAN 2016 database for several hydrocarbons such as the allyl radical (C 3 H 5 ; Uy et al 1998;DeSain & Curl 1999), butane (C 4 H 10 ; Abplanalp et al 2019) and methylacetylene (CH 3 CCH; Abplanalp et al 2019). Further, our chemical network lacks some of the higher hydrocarbons for which absorption cross sections exists such as isoprene (C 5 H 8 ; Brauer et al 2014).…”