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 befor e and after exposure to H atoms using X-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS). The foil initially conta ins large amounts of oxygen and carbon at the surface. C is present mo stly as hydrocarbon contamination, and O is present as Sn hydroxide an d surface water in addition to SnO, SnO2 and transitional oxide (possi bly Sn3O4), which has an O content between SnO and SnO2. The C and O c ontents of the near-surface region are significantly reduced by exposu re to the H-atom flux. The large hydroxyl group concentration is reduc ed through formation and desorption of water. A short 1-min exposure r esults 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 e xposure to H atoms due to a chemically induced driving force and then decrease. Migration of subsurface C and O becomes the rate-limiting st ep with regard to further removal of these species at room temperature . (C) 1998 Elsevier Science B.V. All rights reserved.