Implications for Extraterrestrial Hydrocarbon Chemistry: Analysis of Acetylene (C2H2) and D2-acetylene (C2D2) Ices Exposed to Ionizing Radiation via Ultraviolet-Visible Spectroscopy, Infrared Spectroscopy, and Reflectron Time-of-flight Mass Spectrometry

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: CnH2n+2 (n = 4, 6-16), CnH2n (n = 2, 3, 6, 8-16), CnH2n-2 (n = 3, 4, 6, 8, 10-15), CnH2n-4 (n = 4-8, 10-14), CnH2n-6 (n = 4-10, 12-16), CnH2n-8 (n = 6-10, 12, 14-17), CnH2n-10 (n = 6-12, 14-17), CnH2n-12 (n = 8-14), CnH2n-14 (n = 8-16), and CnH2n-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).