Oxygen as an Electron Scavenger: Its Role in Electron-Induced Activation of Coadsorbed Methane Embedded in Amorphous Solid Water

Abstract

Low-energy electrons are known to play a fundamental role in activating small molecules in interstellar chemistry. Here we illustrate the electron-induced activation of the inert molecule methane while sandwiched between two 50 monolayers thick layers of amorphous solid water (ASW) on ruthenium substrate at 25 K by employing externally supplied low-energy electrons (5 eV) under ultrahigh vacuum conditions. We demonstrate how electron transmission through ASW layers under cryogenic conditions is strongly affected in the presence of cosandwiched oxygen molecules. We conclude that the resonant nature and direct electron attachment process leads to a higher degree of conversion in the presence of embedded oxygen molecules along with methane. Cross sections ranging from 1 × 10–18 to 1 × 10–19 cm2/electrons were obtained from the post-irradiation temperature-programmed desorption spectra.

Same-Energy UV Photons and Low-Energy Electrons Activating Methane and Ammonia Frozen in Amorphous Solid Water

“…This procedure resulted in cross sections more than 3 orders of magnitude larger for formation of similar products by the 6.4 eV electrons compared to those formed by photons of the same energy (193 nm = 6.4 eV). Similar results were reported in previous studies from our group. , …”

“…Finally, the presence of oxygen molecules within the main sample leads to a very rapid drop in the transmission current (curve number 7, violet), suggesting that oxygen molecules act as electron scavengers. A similar observation of the electron trapping propensity of oxygen was reported when oxygen molecules were frozen/embedded with pure methane in ASW . These decay profiles of the transmitted current, which occur when stable molecules are embedded inside ASW, is attributed to the generation of more defect sites in ASW (probably due to strain in the ASW matrix).…”

“…A similar observation of the electron trapping propensity of oxygen was reported when oxygen molecules were frozen/ embedded with pure methane in ASW. 44 These decay profiles of the transmitted current, which occur when stable molecules are embedded inside ASW, is attributed to the generation of more defect sites in ASW (probably due to strain in the ASW matrix). These defect sites lead to more electron trapping and eventually to retardation of further incoming electrons, resulting in the eventual decay to practically zero current (see curve number 7 in Figure 6).…”

“…The absence of these masses may be attributed to higher excitation cross sections of these molecules (fragments) that have further dissociated to form other complex molecules. The same results were obtained using 5 eV electrons …”

“…In addition, the integrated areas under the peaks resulting from the photons and electrons irradiation of the same sample and their corresponding formation cross sections are given in Figure S5 and Tables TS1 and TS2 of the Supporting Information. Similar results were previously reported for the case of 248 nm photons 36 and the same-energy 5.0 eV electrons 44 irradiating samples of pure methane within the ASW matrix.…”