Ice on Np Oxide Films

The surface reductions of films of neptunium dioxide (NpO2) with adsorbed water ice was studied by ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS, respectively). The films were produced as thin films by reactive sputter deposition. Water was condensed as a thick ice overlayer on the surface at low temperature. Subsequent warming led to desorption of the ice. When warmed up under ultraviolet light (UV light, HeI and HeII radiation), the surface was reduced. NpO2 was reduced to surface neptunium sesquioxide (Np2O3). Surface reduction is explained as a photocatalytic reaction of the surface, triggered by the excitation of electrons from the valence (or impurity) band into the conduction band. While up to now Np2O3 had only be observed during the early oxidation of Np metal, it was now shown to be stable also on NpO2. It is a stable surface compound and not a metastable reaction intermediate.
The machine used for this study was the hot machine.