Surface characterization study of the chemical alteration of an air-exposed polycrystalline tin foil during H-atom exposures

An air-exposed polycrystalline Sn foil surface has been examined before and after exposure to H atoms using X-ray photoelectron spectroscopy XPS.and ion scattering spectroscopy ISS.. The foil initially contains large amounts of oxygen and carbon at the surface. C is present mostly as hydrocarbon contamination, and O is present as Sn hydroxide and surface water in addition to SnO, SnO2and transitional oxide possibly Sn3O4., which has an O content between SnO and SnO2. The C and O contents of the near-surface region are significantly reduced by exposure to the H-atom flux. The large hydroxyl group concentration is reduced through formation and desorption of water. A short 1-min exposure results in reduction of some of the SnO2 or transitional oxide to SnO. Longer reduction periods result in the formation of a large amount of transitional oxide and some metallic Sn. ISS data indicate that the O and C contents of the outermost atomic layer initially increase upon exposure to H atoms due to a chemically induced driving force and then decrease. Migration of subsurface C and O becomes the rate-limiting step with regard to further removal of these species at room temperature. q1998 Elsevier Science B.V. All rights reserved.