Surface recoiling, XPS and UPS study of chemisorption of H₂, O₂ and H₂O on lanthanum

Chemisorption of H₂, O₂ and H₂O on polycrystalline lanthanum is characterized by X-ray and UV photoelectron spectroscopy (XPS and UPS) and surface recoiling (SR) of adsorbate atoms. The SR technique using a pulsed 3 keV Ar⁺ source and time-of-flight detection of both neutrals and ions is demonstrated to have outer-monolayer sensitivity and the capability of directly detecting adsorbate hydrogen. The high sensitivity of SR (particularly to surface hydrogen) coupled with the chemical state information from XPS and UPS provides an excellent combination for monitoring adsorbates. Reaction of H₂ with La forms the dihydride compound, reaching saturation on the surface after ∼ 50 L exposure and in the outer ∼ 20 Å after ∼ 2000 L. Reaction with O₂ is consistent with the formation of La₂O₃ at exposures ≲ 10 L. The results suggest that during the initial stages of reaction with H₂O, oxide and hydride are formed followed by hydroxide formation beginning at ∼ 35 L. The saturation level exhibits a ratio OH/O ∼ 0.4.