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The distribution of valence electrons between transition metals (TM) and oxygen has been studied via X-ray absorption near-edge fine structure (XANES) at the TM-L2,3 and O-K thresholds for La2 Li½ Cu½ O4 , Nd2 Li½ Ni½ O4 , and La2 Li½ Co½ O4 . Simulations of the TM-L2,3 XANES by charge-transfer multiplet calculations result in 3d-hole concentrations that increase from ~30% 3d^8 in the Cu(III) to ~57% 3d^7 in the Ni(III) to ~72% 3d^6 in the Co(III) compound. Concomitantly, the O-K XANES spectra reflect an increase of bond ionicity in the same sequence. The results show that the Li-doped holes in Nd2 Li½ Ni½ O4 are distributed between Ni-3d and O-2p orbitals.

The electronic structure of copper and nickel in the oxidation state +III was studied for La2 Li½ Cu½ O4 , Cs2 KCuF6 , and Nd2 Li½ Ni½ O4 by X-ray absorption near-edge structure (XANES) spectroscopy at the L2,3 -thresholds of Cu and Ni as well as the K-thresholds of O and F. The observed multiplet structures are well described by cluster configuration-interac-tion calculations, which result in a high-spin ground state for Cs2 KCuF6 and low-spin ground states for La2 Li½ Cu½ O4 and Nd2 Li½ Ni½ O4 . The simulations yielded 30% and 40% 3d^8 contributions for La2 Li½ Cu½ O4 and Cs2 KCuF6 , respectively, and 57% 3d^7 for Nd2 Li½ Ni½ O4 in the ground state. The O--K and F--K XANES spectra exhibit pronounced preedge peaks that monitor ligand--2p holes induced by covalency. These preedge peaks move to lower energies with increasing oxidation state of Cu and Ni as well as when going from nickelates to cuprates. B.V. All rights reserved.

We report the measurement of the local Ni 3d spin polarization, not only below but also above the Curie temperature (T-C), using the newly developed spin-resolved circularly polarized 2p (L-3) resonant photoemission technique. The experiment identifies the presence of 3d(8) singlets at high energies and 3d(8) triplets at low energies extending all the way to the Fermi energy, both below and above T-C, showing that it is the orbital degeneracy of the 3d band and the Hund's rule splitting which is of utmost importance to understand Ni and other 3d ferromagnets.

The 2p3d and 2p4d x-ray emission spectral shapes have been calculated using a theoretical description of spin-polarized 2p photoemission and atomic multiplet calculations of the 2p3d and 2p4d radiative decay. Emphasis is given to the use of circular-polarized x rays for the excitation process. Good agreement with experiment is found and all visible experimental structures can be explained. It is shown that because of weak multiplet effects in the intermediate state, it is possible to use the incoherent, two-step model. The angle between the emitted x rays and the magnetization is able to affect the magnetic circular dichroism (MCD) spectral shape observed, while the angle between the incident x ray and the magnetization only affects the intensity of the MCD.

This paper demonstrates that spin-polarized x-ray-excitation spectra can be obtained using K a emission as well as K ß lines. A spin-polarized analysis of K a x-ray emission and the excitation spectra by K a detection on a Ni compound is reported. A systematic analysis of the first-row transition-metal ions using the ligand-field multiplet calculation is presented for K a and K ß emission spectra.

We present temperature-dependent V-2p and O-1s x-ray-absorption spectra of LiVO2. The aim of this study is to monitor changes in electronic structure on going through the phase transition. The spectral changes turn out to be very small: the V-3d-O-2p hybridization does not change considerably, and the symmetry of the V-ion ground state (high-spin T-3(1)) is retained. To explain our results, together with the anomalously low magnetic susceptibility below the transition temperature, we propose a model in which a three-orbital sublattice is formed in the V (111) planes, which results in a total singlet state.

The molecular based magnet Cs^(I)[Ni^(II)Cr^(III)(CN)6]. 2H2O is a ferromagnet with a Curie temperature Tc = 90 K. Its structure consists of face centred cubic lattice of Ni^(II) ions connected by Cr(CN)6 entities. We have recorded X-ray Magnetic Circular Dichroism (XMCD) at nickel L2,3 edges. It clearly evidences that nickel II is in a high spin configuration and ferromagnetically coupled to the surrounding Cr^(III). Through Crystal Field Multiplet calculations, we have determined the precise ground state of Ni^(II). Special attention has been paid to the magnetic anisotropy that complicates the calculation of the cross section for a powder. By using sum rulesderived for XMCD, it has been possible to extract the orbital and spin contributions to the total magnetic moment. A too small magnetic moment is found in nickel. A complete calculation taking into account the multiplet coupling effect and the covalent hybridisation showed that hybridisation cannot be responsible for the experimental low nickel magnetic moment. The origin of this effect is discussed.

We have shown from ligand field multiplet calculations that the shape of X-ray magnetic circular dichroism (XMCD) spectra changes drastically with the distortion parameter Ds. The temperature dependence study of XMCD makes it possible to determine both Ds and spin-orbit coupling.

A theoretical description is given of the 2p3p3p (= L2,3M2,3M2,3) resonant Auger spectra in CaF2. Using a charge-transfer multiplet analysis, it is shown that the 2p3p3p Auger spectra excited at the 2p x-ray absorption edge show a large variation if one scans through the edge and excites the different symetry states. A detailed comparison is made with experiment. The origins of the symmetry effects on the resonance are analyzed and the full charge-transfer multiplet theory is approximated using crystal-field and atomic multiplet calculations. The analysis shows that the Auger spectra of CaF2 are dominated by the resonance channel, with little contribution of the normal Auger channel. The differences with other recently used analysis are discussed.

The 2p X-ray absorption spectra of 3d transition metal compounds measured with fluorescene yield (FY) appear disorted. It is shown that this disortion is intrinsic to FY detection and not due to self-absorption. It results from the final-state dependent variation of the fluorescene decay, which is more than 400%. The Auger decay is also final-state dependent but the variation is only 25%. In the approximation that the final states do not interfere with each other (which is shown to be approximately correct for Ni^2+), the FY spectral shape identifies with the X-ray absorption spectrum multiplied with its symmetry-dependent fluorescence decay. Results are given for the nickel 2p edge of Cs[NiCr(CN)6] * 2H2O measured synchronously with FY and electron yield.

To explain the low-energy features of magnetic circular X-ray dichroism (MCXD) at L edges of rare earth compounds, quadrupolar electric transitions (E2) from 2p to 4f levels have been invoked. Such transitions should be distinguished from dipolar electric (E1) ones by looking at the different behaviour of the E1 and E2 MCXD cross sections as a function of the temperature and the angle between the quantization axis and the photon wave vector. However, convincing experimental proof of such transitions has not yet been given. A good candidate for these key tests is Yb, where the E2 transitions are allowed at the L3 edge and forbidden at the L2 edge. In YbFe2, we observe an important change in the shape of the spectrum with temperature at the L3 adge of Yb.

The Kß x-ray emission spectra of divalent nickel compounds are analysed with both the crystal field multiplet model and the charge transfer multiplet model. The spin-dependent Kß x-ray emission spectrum is calculated and the charge transfer effect is rather different in nature for spin-up (²A2) and spin-down (4A2) symmetry states. In case the of a spin-up symmetry state it is shown that multiples-induced mixing strongly affects the spectral shape. It is argued that renormalized crystal field multiplet calculations remain a useful approach to describe the Kß x-ray emission spectral shape.