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X-ray fluorescence spectroscopy with monochromatic photon excitation is presented as a tool for studies of charge-transfer excitations in correlated systems, using CeO 2 and UO 3 as examples. Ce 4f ! 3d and U 5f ! 3d x-ray fluorescence, with excitation near the 3d thresholds, probes states as eigenvalues for the ground state Hamiltonian from the Anderson impurity model. Sweeping the excitation energy across 3d absorption edges enhances contributions of different electronic configurations to fluorescence so that observed resonances indicate the charge-transfer origin of the absorption satellites. [S0031-9007 (96)00691-6] PACS numbers: 78.70.En, 71.27. + a, 71.28. + d, 78.70.DmA degree of localization of the f states which can have both bandlike and localized character is a key point in the description of the electronic structure of rare-earth and actinide systems. While delocalized models based on band theory were proposed to explain ground state [1] and spectroscopic [2] properties, various transport and spectroscopic signs of electron correlation effects are observed even in "itinerant" materials and cannot be fully interpreted within one-electron formalism. As an alternative, localized approach the Anderson impurity model (AIM) is often used [3,4] which treats the f states of a rare-earth or actinide atom as a degenerate impurity level hybridized with the valence band, but neglects the interaction between f levels on different atoms. To describe the ground state, a model Hamiltonian is constructed which includes as parameters the energy of a localized state´f , delocalized states´y, the hybridization strength V , and the on-site f-f Coulomb interaction U ff . The parameters are optimized by fitting both high-energy spectroscopic and low-energy thermodynamic data [3]. However, these parameters may be renormalized in a different way in high-versus lowenergy experiments [5,6]. In this case, those experimental techniques become attractive which can be related to both high-energy and low-energy scale techniques such as valence band resonant x-ray fluorescence spectroscopy (RXFS). In the localized, many-body approach, RXFS is expected to probe the states as eigenvalues of the ground state Hamiltonian via creation or annihilation of a core or hole due to site and symmetry selectivity of RXFS. In this Letter, we explore a potential of RXFS in studies of so-called mixed-valency compounds where the anion 2p ! metal f charge-transfer excitations play an impor-tant role and the ground state can be described as a strong mixture of several electronic configurations. Using CeO 2 [4,7-11] and UO 3 [12-15] as systems with strong metal f O 2p hybridization we demonstrate that the localized, many-body approach is appropriate for the description of the resonant x-ray fluorescence (RXF) process and that RXF spectra can be interpreted within the framework of the AIM. We show that it is essential to include the RXF data in spectroscopic analysis for these systems in order to derive a unique set of model parameters.The measurements on...

We present a computational study of 2p core-level X-ray photoemission spectra of transition metal monoxides MO (M=Ni, Co, Mn) and sesquioxides M2O3 (M=V, Cr, Fe) using a theoretical framework based on the local-density approximation (LDA) + dynamical mean-field theory (DMFT). We find a very good description of the fine spectral features, which improves considerably over the conventional cluster model. We analyze the role of the non-local screening and its relationship to the long-range magnetic order and the lattice geometry. Our results reveal the potential of the present method for the analysis and interpretation of the modern high-energy-resolution experiments.I.

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The available calculated density of states of TiOz does not predict the very first weak peak at the onset of the Ti K-edge absorption spectra indicating that this feature is not simply due to a one-electron (dipole) transition. In this short paper we present a joint experimental and theoretical study of all the three weak pre-peaks at the Ti K-edge of Ti02. Our interpretation takes into account both dipole and quadrupole transitions as well as the influence of the core hole effect.