Low-Energy Electron Excitation of Methanol@ASW Ices

Low-energy electron-induced processes represent important pathways of excitation and chemical reactivity in the interstellar medium (ISM). These electrons are primarily generated from the interaction of cosmic rays with solid grains in the ISM. When the low-energy electrons interact with grains that are covered by thin layers of amorphous solid water (ASW) with molecules, such as methanol, embedded within these layers, chemical reactivity may take place. Here, we report on an experimental laboratory study within an ultrahigh-vacuum environment, mimicking electron-induced reactivity of the CH3OH (CD3OD)@ASW system in conditions similar to those in the ISM. Electrons at energies of 6.4 and 100 eV bombarded various CH3OH@ASW and CD3OD@ASW sandwich samples followed by temperature-programmed desorption (TPD) analysis. CO, CO2, and CD4 and CD3H were the primary products. From the initial growth of these products, TPD signal vs number of striking electrons, one extracts a cross section for reactivity. The main observation is that the cross sections obtained, for example, CO formation by electrons at 6.4 eV are more than 4 orders of magnitude larger than those of the same energy photons (193 nm) under identical experimental conditions reported before (Akerman et al., Langmuir, 2023, 39, 2838–2849). The possible reasons for the difference in magnitude of reactivity and, in particular, the different diversities of products formed are discussed.