PNNL

Abstract

Electron-stimulated desorption (ESD) of H2, O2 and H2O from 0 - 60 ML films of amorphous solid water (ASW) adsorbed on TiO2(110) are investigated as function of film thickness and isotopic composition. For 100 eV incident electrons, both the H2 and O2 ESD yields have maxima when the ASW coverage is ~ 20 monolayer (ML), while the H2O ESD yield increases monotonically with water coverage. All the products reach a coverage-independent yield above 40 - 50 ML. Experiments using isotopically layered films of H2O and D2O demonstrate that the molecular hydrogen is produced in reactions that occur preferentially at or near both the ASW/TiO2 interface and the ASW/vacuum interface. However, electronic excitations or ionic defects created within the interior of the ASW films by the energetic electrons can subsequently migrate to the interfaces where they initiate reactions. Electron irradiation of ASW films results in the formation of bridge-bonded hydroxyls on the TiO2(110). These hydroxyls do not contribute to the H2 produced near the ASW/TiO2 interface. Instead, the results suggest that this H2 is produced from a stable precursor, trapped near the substrate. The proposed mechanism for the H2 production near the ASW/TiO2(110) interface is supported by a kinetic model that semi-quantitatively reproduces the main features of the non-thermal reactions.