The initial oxidation of polycrystalline thorium

The initial oxidation of clean, polycrystalline α-Th from background CO/CO2 and saturation of the Th surface by O2 has been examined by angle-resolved Auger electron spectroscopy (ARAES) and time of flight secondary ion mass spectrometry (ToF-SIMS). Following dissociative adsorption of very low doses of background CO/CO2 (<1 L), the carbon surface population was dominant and spontaneously formed thorium carbide. The accompanying oxygen population increased at a rate roughly one-third that of the carbon, suggesting simultaneous oxygen incorporation into the bulk. To further corroborate the surface kinetics of adsorbed oxygen, O2 was admitted, following heating and sputter cleaning of the Th; some oxygen atoms continued to diffuse into the bulk until formation of stoichiometric ThO2 at ∼37 L. ARAES measurements showed an oxygen concentration gradient in the near-surface region confirming rapid oxygen incorporation at low doses; however, once the surface is saturated, virtually no variation in the oxygen intensity is observed. AES and ToF-SIMS depth profiling revealed complete oxide formation to a depth of 2 nm. Copyright © 2010 John Wiley & Sons, Ltd.