Mechanistical Studies on the Irradiation of Methanol in Extraterrestrial Ices

Abstract

Pure ices of amorphous methanol, CH3OH( X(1)A(1)), were irradiated at 11 K by 5 keV electrons at 100 nA for 1 hr. These energetic electrons simulate electronic energy transfer processes that occur as interstellar ices, comets, and icy solar system bodies are subjected to irradiation from MeVions and secondary electrons produced in this process. The results were analyzed quantitatively via absorption-reflection- absorption Fourier transform infrared FTIR) spectroscopy, with the identification of new species aided by high-level electronic structure calculations. The unimolecular decomposition of methanol was found to proceed via the formation of 1) the hydroxymethyl radical, CH2OH( X (2)A(11)), and atomic hydrogen, H( S-2(1/2)), 2) the methoxy radical, CH3O( X (2)A(1)), plus atomic hydrogen, 3) formaldehyde, H2CO( X(1)A(1)) plus molecular hydrogen, H-2( X-1 Sigma(+)(g)), and 4) the formation of methane, CH4( X(1)A(1)), together with atomic oxygen, O(D-1). The accessibility of the last channel indicates that the reverse process, oxygen addition into methane to form methanol, should also be feasible. A kinetic model is presented for the decomposition of methanol into these species, as well as the formyl radical, HCO( X-2 2(1)), and carbon monoxide, CO( X-1 Sigma(1) ). During the subsequent warming up of the sample, radicals previously generated within the matrix were mobilized and found to recombine to form methyl formate, CH3OCHO( X(1)A(1)), glycolaldehyde, CH2OHCHO( X(1)A(1)), and ethylene glycol, HOCH2CH2OH( X (1)A). Upper limits for the production of these species by the recombination of neighboring radicals produced during irradiation as well as during the warm-up procedure are presented. The generation of these molecules by irradiation of ices in the solid state and their subsequent sublimation into the gas phase can help explain their high abundances as observed toward hot molecular cores and underlines their importance in astrobiology.