On the chemical processing of hydrocarbon surfaces by fast oxygen ions

Abstract

Solid methane (CH₄), ethane (C₂H₆), and ethylene (C₂H₄) ices (thickness: 120 ± 40 nm; 10 K), as well as high-density polyethylene (HDPE: [C₂H₄]_n) films (thickness: 130 ± 20 nm; 10, 100, and 300 K), were irradiated with mono-energetic oxygen ions (Φ ∼ 6 × 10¹⁵ cm⁻²) of a kinetic energy of 5 keV to simulate the exposure of Solar System hydrocarbon ices and aerospace polymers to oxygen ions sourced from the solar wind and planetary magnetospheres. On-line Fourier-transform infrared spectroscopy (FTIR) was used to identify the following O⁺ induced reaction pathways in the solid-state: (i) ethane formation from methane ice via recombination of methyl (CH₃) radicals, (ii) ethane conversion back to methaneviamethylene (CH₂) retro-insertion, (iii) ethane decomposing to acetyleneviaethylene through successive hydrogen elimination steps, and (iv) ethylene conversion to acetyleneviahydrogen elimination. No changes were observed in the irradiated PE samples viainfrared spectroscopy. In addition, mass spectrometry detected small abundances of methanol (CH₃OH) sublimed from the irradiated methane and ethane condensates during controlled heating. The detection of methanol suggests an implantation and neutralization of the oxygen ions within the surface where atomic oxygen (O) then undergoes insertion into a C–H bond of methane. Atomic hydrogen (H) recombination in forming molecular hydrogen and recombination of implanted oxygen atoms to molecular oxygen (O₂) are also inferred to proceed at high cross-sections. A comparison of the reaction rates and product yields to those obtained from experiments involving 5 keV electrons, suggests that the chemical alteration of the hydrocarbon ice samples is driven primarily by electronic stopping interactions and to a lesser extent by nuclear interactions.