Synthesis of complex organic molecules in simulated methane rich astrophysical ices.

Abstract

It has been proposed that organic molecules required for life on earth may be formed by the radiation processing of molecular ices in space environments, e.g., within our solar system. Such processes can be studied in the laboratory with surface science analytical techniques and by using low-energy electron (LEE) irradiation to simulate the effects of the secondary electrons that are generated in great abundance whenever ionizing radiation interacts with matter. Here we present new measurements of 70 eV LEE irradiation of multilayer films of CH4, 18O2, and CH4/18O2 mixtures (3:1 ratio) at 22 K. The electron stimulated desorption (ESD) yields of cations and anions have been recorded as a function of electron fluence. At low fluence, the prompt desorption of more massive multi-carbon or C-O containing cationic fragments agrees with our earlier measurements. However, new anion ESD signals of C2-, C2H-, and C2H2- from CH4/18O2 mixtures increase with fluence, indicating the gradual synthesis (and subsequent electron-induced fragmentation) of new, more complex species containing several C and possibly O atoms. Comparisons between the temperature programed desorption (TPD) mass spectra of irradiated and unirradiated films show the electron-induced formation of new chemical species, the identities of which are confirmed by reference to the NIST database of electron impact mass spectra and by TPD measurements of films composed of the proposed products. New species observed in the TPD of irradiated mixture films include C3H6, C2H5OH, and C2H6. Furthermore, X-ray photoelectron spectroscopy of irradiated films confirms the formation of C-O, C=O, and O=C-O- bonds of newly formed molecules. Our experiments support the view that secondary LEEs produced by ionizing radiation drive the chemistry in irradiated ices in space, irrespective of the radiation type.