Hydrogen--oxygen interrelations on a niobium surface

Combined time-of-flight spectrometry of direct surface recoil (TOF-DR) and Auger electron spectroscopy (AES) measurements were utilized to study hydrogen--oxygen interrelations on a polycrystalline niobium surface. It was found that oxygen is energetically favored in the surface region with a hydrogen-rich layer located closely beneath it. Such a double-layered structure is obtained either when the niobium sample is heated in vacuum, or when a sputter-cleaned surface is exposed at room temperature to oxygen. The migration of oxygen atoms from the bulk to the surface and the formation of a surface oxygen enriched region prevents the desorption of H2 even at elevated temperatures. Exposing a sputter-cleaned hydrogen-rich surface to oxygen at room temperature, results in its rapid accumulation replacing topmost hydrogen atoms. A monolayer oxygen coverage is obtained at about 20 L. Subsequent exposures result in accommodation of oxygen in the subsurface region