Effect of Coadsorbed Oxygen on the Photochemistry of Methane Embedded in Amorphous Solid Water

Abstract

The photochemistry of methane caged within amorphous solid water (ASW) is interesting as a model for studying interstellar and high-altitude atmospheric pathways for the formation of more complex hydrocarbons. Here, we report on the photoreactivity of clean methane and in the presence of oxygen molecules, known as electron capture species, within two 50 monolayer-thick D2O-ASW films adsorbed on Ru(0001) substrate under ultrahigh vacuum conditions. Irradiation by 248 nm UV photons (5.0 eV), where none of the involved molecules absorb these photons in the gas phase, leads to the formation of diverse hydrocarbons. In all cases, the presence of oxygen results in significantly enhanced reactivity. The dissociative electron attachment mechanism with electrons generated within the metal substrate is thought to largely govern the photoreactivity in this system. Methyl radicals as the primary photoproducts subsequently react with the surrounding water and neighboring methane as well as with the stable O2 – anion. Postirradiation temperature-programmed desorption measurements revealed cross sections for hydrocarbon formation in the range of 10–20 to 10–21 cm2. Possible mechanisms underlying the formation of various hydrocarbons and carbon dioxide as the final oxidation product are discussed.

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

“…51 This process may lead to reducing the most probable product formation pathways. We confirmed from our previous photon-driven experiments 40 that, with 248 nm photon irradiation of the D 2 O|CH 4 + O 2 | D 2 O sample at 25 K, methanol and formaldehyde were formed. Further irradiation with 5 eV incoming electrons may not lead to a partial or complete oxidation to CO and CO 2 , respectively.…”

“…To analyze the nature of these molecules, potential stable product molecules were inserted between two 50 ML of D 2 O and desorbed without any irradiation; a similar comparative study was performed recently following photon irradiation. 40 In general, we should obtain methanol as one of the main products, but from our post-irradiation TPD spectra we did not observe m/z = 33, which confirms that methanol is not produced when our samples are irradiated by electrons. The absence of this mass is attributed to a higher cross-section for further decomposition due to the electrons' impact than in the case of photon irradiation.…”

“…Every molecule has its own desorption pattern from the ASW matrix. To analyze the nature of these molecules, potential stable product molecules were inserted between two 50 ML of D 2 O and desorbed without any irradiation; a similar comparative study was performed recently following photon irradiation Figure shows the comparison of the formed masses 29 and 45 with that of the desorption profile of pure formaldehyde and formic acid molecules from ASW.…”

“…The choice of this electron energy served as complementary research to a study we just completed, in which the same sandwiched system was irradiated by a 248 nm (5 eV photon energy) UV-laser source. 40 The reason for the sandwiched molecular setup was, on the one hand, to avoid electron-induced desorption of the parent methane molecule and to examine the role of the water molecules, as a model for the abundant ASW-covered surfaces of grains in interstellar space and in a high-altitude atmospheric (exosphere) environment. From the TPD plots we can clearly see that the overall yield of the products gradually increases as the number of 5 eV electrons increases with the sample at 25 K. Line shape was analyzed for the low-temperature peaks for m/z = 27, 29, and 30 (shown in Figure 1).…”

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

“…The experiments performed in this study were conducted within an ultrahigh vacuum (UHV) chamber at a base pressure of 2 × 10 –10 mbar, previously described in detail. − All samples were prepared on a Ru(0001) single crystal substrate of 0.5 cm 2 area directly mounted to a cryogenic head with 25 K as the lowest temperature. Prior to sample preparation, the Ru substrate was sputter cleaned using a Ne + ion source operated at 800 eV for 10 min with a constant sample to ground current of 5 μA.…”