Methanol Formation through Reaction of Low-energy CH3 + Ions with an Amorphous Solid Water Surface at Low Temperature

Abstract

We have performed experimental investigations of methanol formation via the reactions of low-energy CH3 + ions with an amorphous solid water (ASW) surface at ∼10 K. A newly developed experimental apparatus enabled irradiation of the ASW surface by several eV ions and detection of trace amounts of reaction products on the surface. It was found that methanol molecules were produced by low-energy CH3 + irradiation of the ASW surface and that hydroxy groups in the produced methanol originated from water molecules in the ASW, as predicted in a previous theoretical study. Little temperature dependence of the observed methanol intensity is apparent in the temperature range 12–60 K. Ab initio molecular dynamics simulations under constant-temperature conditions of 10 K suggested that this reaction spontaneously produced a methanol molecule and an H3O+ ion, regardless of the contact point of CH3 + on the ASW surface. We have performed a simulation with an astrochemical model under molecular-cloud conditions, where the reaction between CH3 + and H2O ice, leading to methanol formation, was included. We found that the impact of the reaction on methanol abundance was limited only at the edge of the molecular cloud (<1 mag) because of the low abundance of CH3 + in the gas phase, whereas the reaction between the abundant molecular ion (HCO+) and H2O ice, which has not yet been confirmed experimentally, can considerably affect the abundance of a complex organic molecule. This work sheds light on a new type of reaction between molecular ions and ice surfaces that should be included in astrochemical models.