Emission of large hydrocarbons from frozen CH₄ by keV proton irradiation

Abstract

Ever since the first description of the synthesis of molecules by impinging protons on ices¹, the possibility of formation of various molecules on cold surfaces by particle radiation has been widely discussed^2–5. Because fluxes of light ions with energies of ∼1 keV AMU⁻¹ exist in a wide variety of astrophysical sites⁶ there has been a dispute over whether molecules in space are formed by such a process or by gaseous reactions⁷. We have recently studied sputtering of species by bombarding simple frozen gases such as water, carbon monoxide and ammonia with light ions². With these substances, only rather simple molecules were found. However, when bombarding methane, we detected much larger molecules in the sputtered beam—hydrocarbons containing up to at least 13 atoms. We propose, therefore, that bombardment of frozen methane by keV protons may produce large molecules in the Solar system.