Interactions of water vapor with polycrystalline uranium surfaces

The initial room-temperature interactions of water vapor with polycrystalline bulk annealed uranium surfaces were studied by combined measurements utilizing direct recoil spectrometry (DRS) and X-ray photoelectron spectroscopy (XPS). It was found that the water goes through a complete dissociation into oxidic oxygen and two neutral H atoms throughout the whole exposure range. The process proceeds by two consecutive stages: (i) below about 80% monolayer coverage, the dissociation products chemisorb mainly on the remaining non-reacted metallic surface by a simple Langmuir-type process; (ii) between about 80% and full coverage, three-dimensional oxide islands (that start to form at 50-60% coverage) cover most of the surface and full dissociation continues on top of them. It seems that on top of the oxide the dissociation consists of a two-step process: first partial dissociation into OH^- + H⁰, where the neutral hydrogen atom chemisorbs on the oxide and the hydroxyl group migrates into the subsurface region and then undergoes a reductive dissociation at the oxide-metal interface, producing a second hydrogen atom, located beneath the surface.