Electron-stimulated reactions in nanoscale water films adsorbed on α-Al2O3(0001)

Abstract

The radiation-induced decomposition and desorption of nanoscale amorphous solid water (D₂O) films adsorbed on an α-Al₂O₃(0001) surface was studied at low temperature in ultrahigh vacuum using temperature programmed desorption (TPD) and electron stimulated desorption (ESD) with a mono-energetic, low energy electron source. ESD yields of molecular products (D₂, O₂ and D₂O) and the total sputtering yield increased with increasing D₂O coverage up to ∼15 water monolayers (i.e. ∼15 × 10¹⁵ cm⁻²) to a coverage-independent level for thicker water films. Experiments with isotopically-layered water films (D₂O and H₂O) demonstrated that the highest water decomposition yields occurred at the interfaces of the nanoscale water films with the alumina substrate and vacuum. However, the increased reactivity of the water/alumina interface is relatively small compared to the enhancements in the non-thermal reactions previously observed at the water/Pt(111) and water/TiO₂(110) interfaces. We propose that the relatively low activity of Al₂O₃(0001) for the radiation-induced production of molecular hydrogen is associated with lower reactivity of this surface with hydrogen atoms, which are likely precursors for the formation of molecular hydrogen.