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

Carbonaceous residues were formed hy irradiation of thin layers of frozen methane at 10 to 15 K by 17 MeV protons and ‘Fle“ 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 (‘H-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. PAIls and amorphous carbon increased with the energy deposited in one collision cascade. The linear energy transfer with respect to C‘H,. L ,(CH,). was varied in the systems p//Ar/CH, (12: I). p//CFl, and “He’ ’ //CH, from 160 to 10X10 eVFm_ I3 respectively. A threshold L, for the formation of PAHs and related structures seem\ to range hetween 2 and IO keVkm ‘, The experiments give evidence for a fast multicenter combination of intermediate radicals formed by the secondary auprathermal 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 (Fie. etc.) in the prehiotic buildup of complex organic matter in space.