Electron-energy-loss spectroscopy of core levels and valence-band region in cerium and cerium dioxide

Electron-energy-loss spectra of sputtered clean cerium metal and of cerium dioxide were measured in the valence-band region and in the vicinities of the 4d, 4p, 4s, 3d, and 3p excitation thresholds. The energies of the primary electrons ranged from 80 to 3000 eV. Bulk and surface plasmons were identified in the valence-band region of the metal. For the dioxide, the surface plasmon vanished and "plasmonlike" peaks were observed. Also, an intra-conduction-band transition which appears in the metal and is highly sensitive to the chemical state of the surface was found. Core-level excitations were identified as transitions to the 4f levels located above, within, and below the conduction band. Transitions of the 4d electrons displayed "giant resonances" due to the higher multiplet splittings of the final states. The intensities of some of the excitation lines were enhanced at low incident electron energies due to the contribution of higher multipole transitions. The core-level excitations were also found in the oxide. Chemical shifts of some of the oxide peaks relative to those of the metal were observed, especially for transitions into final states located above the metallic conduction band.