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Abstract:
Ultraviolet photoemission spectra excited with He I and He II resonance radiation are presented for polycrystalline Er exposed to oxygen, water and hydrogen at room temperature. At UV photon energies, the clean Er substrate emission is generally weak and pronounced changes in the photoemission spectra are observed after the interaction with oxygen, water and hydrogen. Upon oxygen exposure strong bands are detected in the 4–9 eV region below EF together with a depletion of Er 5d6s derived conduction band states near EF. The oxygen induced structure between 4–9 eV is associated with mainly O 2p derived states and the changes of this band with O2 exposure are discussed in terms of oxide formation with corresponding O 2p derived valence bands. H2O induced photoemission features in the 4–9 eV region are similar to those of oxygen, but an additional peak is observed at 11.8 eV. This feature can be interpreted as due to emission from Orbitals with largely O-H σ bonding character, and dissociative adsorption of H2O followed by oxide nucleation and OH surface species formation is suggested. Hydroxyl formation is also observed after exposing an oxidized Er surface to hydrogen. Adsorbed hydrogen shows two weak emission features at 3.8 eV and 6.1 eV below EF and only little reduction of Er conduction band emission is observed. The hydrogen induced structure between 3–8 eV is discussed in analogy to the bonding hydrogen bands observed in rare earth hydrides, although a chemisorbed hydrogen phase is suggested for the low pressure interaction of this study.
