Oxidation of cadmium: An ultraviolet-photoemission and electron-energy-loss investigation

The first results on the oxidation of metallic Cd performed using the surface-spectroscopic techniques of ultraviolet photoemission (UPS) and electron-energy loss (EELS) are reported. The valence-band UPS spectra of CdO are given for the first time. Both UPS and EELS indicate a clear transition from a chemisorption regime ( 2000 Langmuirs) to an oxide regime. In UPS, the 2p resonance due to isolated oxygen adatoms is clearly seen at 3.5 eV below the Fermi level. At increasing oxygen exposure, the 2p line shape becomes increasingly asymmetric, and a satellite structure appears at higher binding energy. The fingerprint of the oxide at the surface is the reduction of binding energy of this satellite, which we attribute to a narrowing of the total valence-band width of the Cd-O system as the cubic coordination of the oxide is attained. The EELS results support the general trends observed using UPS. In addition, new insight into loss structure previously observed in CdO at 23 eV is given.