Micrometre-sized polymer layers synthesized by MeV ions impinging on frozen methane

Abstract

There has recently been much experimental and theoretical interest among astrophysicists in the synthesis of complex organic materials (polymers) as a consequence of the energy deposition by external agents on carbon-containing molecular solids such as CH₄, C₂H₆, C₃H₈, C₆H₆ and mixtures with CO₂, CO, NH₃ and H₂O. Solid residues have been obtained by bombardment of hydrocarbon rich frozen layers with UV photons¹ and energetic particles^2–4. We present here the first quantitative results on the rate of production of solid residues, obtained by bombarding thick (≍l0¹⁹–10²⁰C atoms cm⁻²) frozen layers (T≍4K) of methane with 1.5-MeV protons produced at the Catania University van de Graaff accelerator. This process converts the methane molecules to a polymer-like residue which is stable at room temperature and above. The thickness of the solid residues has been measured both in situ by elastic proton scattering (in C atoms cm⁻²) and remotely, by talysurf measurements (in μm). Thicknesses of up to 15 μm or 3.1 × 10¹⁹ C atoms cm⁻² have been measured and the density of the residues is ρ ≥ 0.5 g cm⁻³. The process appears to occur along the entire path of the incoming protons.