Complex hydrocarbon chemistry in interstellar and solar system ices revealed: a combined infrared spectroscopy and reflectron time-of-flight mass spectrometry analysis of ethane (C₂H₆) and D6-ethane (C₂D₆) ices exposed to ionizing radiation

The irradiation of pure ethane (C₂H₆/C₂D₆) ices at 5.5 K, under ultrahigh vacuum conditions was conducted to investigate the formation of complex hydrocarbons via interaction with energetic electrons simulating the secondary electrons produced in the track of galactic cosmic rays. The chemical modifications of the ices were monitored in situ using Fourier transform infrared spectroscopy (FTIR) and during temperature-programmed desorption via mass spectrometry exploiting a quadrupole mass spectrometer with electron impact ionization (EI-QMS) as well as a reflectron time-of-flight mass spectrometer coupled to a photoionization source (PI-ReTOF-MS). FTIR confirmed previous ethane studies by detecting six molecules: methane (CH₄), acetylene (C₂H₂), ethylene (C₂H₄), the ethyl radical (C₂H₅), 1-butene (C₄H₈), and n-butane (C₄H₁₀). However, the TPD phase, along with EI-QMS, and most importantly, PI-ReTOF-MS, revealed the formation of at least 23 hydrocarbons, many for the first time in ethane ice, which can be arranged in four groups with an increasing carbon-to-hydrogen ratio: C_nH_2n+2 (n = 3, 4, 6, 8, 10), C_nH_2n (n = 3–10), $C_n{H}_{2n-2}$ (n = 3–10), and $C_n{H}_{2n-4}$ (n = 4–6). The processing of simple ethane ices is relevant to the hydrocarbon chemistry in the interstellar medium, as ethane has been shown to be a major product of methane, as well as in the outer solar system. These data reveal that the processing of ethane ices can synthesize several key hydrocarbons such as C₃H₄ and C₄H₆ isomers, which have been found to synthesize polycyclic aromatic hydrocarbons like indene (C₉H₈) and naphthalene (C₁₀H₈) in the ISM and in hydrocarbon-rich atmospheres of planets and their moons such as Titan.