Photoemission studies of the light actinides

The surface reactivity of thorium, uranium, neptunium and americium has been investigated under UHV conditions using X-ray photoelectron spectroscopy (XPS) and ultra violet photoelectron spectroscopy (UPS). Oxygen, nitric oxide, nitrogen dioxide and nitrous oxide adsorption on a polycrystalline thorium surface has been investigated at 298 K. 02 dissociatively adsorbs on the surface resulting in the growth of Th02. All three of the nitrogen oxides adsorb dissociatively with both oxygen and nitrogen adsorbed on the thorium surface. The formation of thorium oxynitride (ThOxNy) is proposed. Reaction of NO, NO2 and N20 with a polycrystalline uranium surface has also been investigated at 298 K. N2O adsorbs dissociatively leaving only oxygen adsorbed on the uranium surface. NO and N2O also adsorb dissociatively but in these cases both oxygen and nitrogen remain on the surface. The formation of uranium oxynitride (UOxNy) is proposed. For exposures >350 L the rate of reaction of NO with the oxynitride surface decreases significantly. In contrast, NO2 continues to react with the surface and a further increase in surface oxygen concentration is observed. Adsorption of O2, NO and CO on thin films of neptunium metal has been studied at 80 and 300K. Following exposure to 02 at 80 and 300K, an intermediate surface oxide of Np is formed which is unstable with respect to the formation of Np02. This intermediate oxide is proposed to be associated with the chemisorption phase, NpO(chem). Dissociative adsorption of CO and NO is observed at 80 and 300K. A surface oxidic compound is formed which is not a simple neptunium oxide. NpO(chem) states are proposed to be stabilized by the presence of carbon/nitrogen as neptunium oxycarbide and oxynitride respectively. The interaction of O2 and NO with americium thin films has also been investigated. The formation of A1TI2O3 is proposed and it was found that it is not thermodynamically viable to produce a higher oxidation state of americium via gas dosing (at 300 K under UHV). Adsorption of NO on the surface results in the dissociative adsorption of N and O, and in the formation of a surface compound