2020ApJ...889....3A

The processing of the simple hydrocarbon ice, acetylene (C₂H₂/C₂D₂), via energetic electrons, thus simulating the processes in the track of galactic cosmic-ray particles penetrating solid matter, was carried out in an ultrahigh vacuum surface apparatus. The chemical evolution of the ices was monitored online and in situ utilizing Fourier transform infrared spectroscopy (FTIR) and ultraviolet-visible spectroscopy and, during temperature programmed desorption, via a quadrupole mass spectrometer with an electron impact ionization source (EI-QMS) and a reflectron time-of-flight mass spectrometer utilizing single-photon photoionization (SPI-ReTOF-MS) along with resonance-enhanced multiphoton photoionization (REMPI-ReTOF-MS). The confirmation of previous in situ studies of ethylene ice irradiation using FTIR was accomplished with the detection of five products: ethane (C₂H₆/C₂D₆), ethylene (C₂H₄/C₂D₄), diacetylene (C₄H₂/C₄D₂), vinylacetylene (C₄H₄/C₄D₄), and benzene (C₆H₆/C₆D₆). Alternatively to previous gas-phase analytical studies, the sensitive SPI-ReTOF-MS analysis detected 10 hydrocarbon groups of distinct degrees of saturation: C_nH_2n+2 (n = 4, 6-16), C_nH_2n (n = 2, 3, 6, 8-16), C_nH_2n–2 (n = 3, 4, 6, 8, 10-15), C_nH_2n–4 (n = 4-8, 10-14), C_nH_2n–6 (n = 4-10, 12-16), C_nH_2n–8 (n = 6-10, 12, 14-17), C_nH_2n–10 (n = 6-12, 14-17), C_nH_2n–12 (n = 8-14), C_nH_2n–14 (n = 8-16), and C_nH_2n–16 (n = 10-16). From these detected groups the REMPI-ReTOF-MS method was able to assign the isomer-specific production of five aromatic hydrocarbons: benzene (C₆H₆), phenylacetylene (C₈H₆), styrene (C₈H₈), naphthalene (C₁₀H₈), and phenanthrene (C₁₄H₁₀).