Energy density effects in the formation of organic residues in frozen methane by MeV ions

Carbonaceous residues were formed by irradiation of thin layers of frozen methane at 10 to 15 K by 17 MeV protons and 3He2+ ions and successive warming to ambient temperature. Analysis was performed by optical microscopy, scanning electron microscopy (SEM), Rutherford backscattering spectroscopy (RBS), elastic recoil detection analysis (ERDA), infrared spectroscopy (IR) in transmission, hydrogen nuclear magnetic resonance (1H-NMR), high performance liquid chromatography (HPLC) and gas chromatography-mass spectrometry (GC-MS). Long chain aliphatic and olefinic hydrocarbons constituted the main products. The formation of aromatic and polycyclic compounds, PAHs and amorphous carbon increased with the energy deposited in one collision cascade. The linear energy transfer with respect to CH4, LT(CH4), was varied in the systems p/ /Ar/CH4 (12:1), p/ /CH4 and 3He2+/ /CH4 from 160 to 10810 eV μm-1, respectively. A threshold LT for the formation of PAHs and related structures seems to range between 2 and 10 keV μm-1. The experiments give evidence for a fast multicenter combination of intermediate radicals formed by the secondary suprathermal carbon atoms from knock on processes in one collision cascade. The experiments were aimed to simulate the effects of cosmic rays on primordial frozen matter. The results underline the role of heavier ion irradiation (He, etc.) in the prebiotic buildup of complex organic matter in space. (orig.)