Electron-Induced Processing of Methanol Ice

The formation of methane (CH4), formaldehyde (H2CO), ethylene glycol (HOCH2CH2OH), methoxymethanol (CH3OCH2OH), dimethyl ether (CH3OCH3), and ethanol (CH3CH2OH) upon electron irradiation of condensed multilayer adsorbates of CH3OH as a model of cosmic CH3OH ice has been monitored by the combined use of electron-stimulated and thermal desorption experiments. The energy-dependent relative yields of all products were measured between 2.5 and 20 eV, and the reaction mechanisms of product formation were deduced. The energy dependences of the yields of HOCH2CH2OH, CH3OCH2OH, CH3OCH3, and CH3CH2OH agree closely with their threshold at the lowest electronic excitation energy of CH3OH. The formation of these products is consequently ascribed to the reactions of radicals formed by the dissociation of neutral electronically excited states and, at higher energy, also by ionization and subsequent proton transfer to an adjacent CH3OH. These electron–molecule interactions also can contribute to the nonresonant formation of H2CO and CH4; these latter products are also produced through resonances around 4 eV reported previously from anion electron-stimulated desorption (ESD) experiments and around 13 eV seen earlier in the energy-dependent yield of carbon monoxide (CO). The present results constitute the most complete data set on the energy dependence of product formation during low-energy electron exposure of condensed CH3OH so far. They provide a comprehensive picture of the reactions triggered by electron impact with energies in the range between 2.5 and 20 eV as representative of low-energy secondary electrons that are released from condensed material, for instance, under the effect of cosmic radiation.