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Photoelectron experiments and studies of X-ray absorption near edge structure in alkaline-earth and rare-earth flurides Gao, Yuan


Alkaline-earth fluorides and rare-earth trifluorides possess technological importance for applications in multi-layer electronic device structures and opto-electronic devices. Interfaces between thin films of YbF₃ and Si(111) substrates were studied by photoelectron spectroscopy and x-ray absorption spectroscopy using synchrotron radiation. Results of YbF₃/Si(111) were compared with those of TmF₃Si(111). While electrons in the Si valence band are prevented from occupying the empty 4f levels in TmF₃ at the interface by the on-site Coulomb repulsion energy, the charge transfer from Si to YbF₃ is possible because the totally filled 4f states in Yb still lie below the Si valence band maximum. The theory of x-ray absorption near edge structure (XANES) is incomplete except for a few particularly simple special cases. A Bragg reflection model was developed to qualitatively explain the oscillations in XANES, in terms of the scattering of the photoelectron wave between families of lattice planes as set out by the Bragg condition for backscattering. The model was found to represent the data for systems with nearly free electron like conduction bands reasonably well. High resolution CaF₂ fluorine K edge XANES was used as a prototype to understand XANES in more depth on systems with strong core hole effects. Unlike previous work which involved multiple scattering cluster calculations that include only short range order effects, both the long range order and the symmetry breaking core holes are included in a new bandstructure approach in which the core hole is treated with a supercell technique. A first principles calculation with the use of pseudopotentials successfully reproduced all the main features of the first 15 eV of the fluorine K edge in CaF₂ which had not been explained with the cluster calculations. A comparison of the theoretical and experimental fluorine K edges in CaF₂ and BaF₂ was used to identify the structure related features. The possibility of multi-electron excitations being responsible for higher energy features in the XANES was investigated by comparing the energy loss satellites in the fluorine is x-ray photoelectron spectra with features at corresponding energies in the fluorine K edge absorption spectra. Finally the fluorine K edges in the rare-earth trifluorides LaF₃, CeF₃, NdF₃, SmF₃, EuF₃, DyF₃ and YbF₃ were explored for the first time with the high resolution x-ray absorption spectroscopy. The near edge part of the fluorine K edges in all seven rare-earth trifluorides was found not to be dominated by the Bragg peaks because of the short life time of the photoelectron and the low crystal symmetry.

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