F.M.F. de Groot

X-ray absorption and dichroism of transition metals and their compounds

Abstract This review presents an overview of the X-ray absorption spectra of 3d transition metals and their compounds. The emphasis is on the description of the X-ray absorption process and the various routes to interpret the results within the framework of their electronic structure. This also includes the use of polarization dependent measurements which are particularly used for the study of the magnetic structure. Emphasis will be given to the metal 2p spectra for which the obtainable resolution has been greatly improved over the last 10 years. The interpretation of 2p core spectra, photo-emission as well as absorption, is dominated by short range models, such as the Anderson impurity model. It has been shown that 2p X-ray absorption is relatively insensitive to charge transfer effects which simplifies the analysis. The interpretation with a ligand field multiplet model accounts well for the observed spectra and due to its simplicity this model yields accurate and well defined electronic structure parameters. For the 1s X-ray absorption spectra, of the metals as well as of the ligands, it has been shown that they correspond closely to the unoccupied density of states as determined from single particle schemes using either band structure methods or real space multiple scattering. A number of potentially important effects beyond this interpretation will be discussed. Overviews will be given of the published X-ray absorption results for the metal 2p, the metal 1s and the ligand 1s spectra.

Evolution of Fe species during the synthesis of over-exchanged Fe/ZSM5 obtained by chemical vapor deposition of FeCl3

The evolution of iron in over-exchanged Fe/ZSM5 prepared via chemical vapor deposition of FeCl3 was studied at each stage of the synthesis. Different characterization techniques (EXAFS, HR-XANES, 57 Fe Mossbauer spectroscopy, 27 Al NMR, EELS, HR-TEM, XRD, N2 physisorption, and FTIR spectroscopy) were applied in order to correlate the changes occurring in the local environment of the Fe atoms with migration and aggregation phenomena of iron at micro- and macroscopic scale. Mononuclear isolated Fe-species are formed upon FeCl3 sublimation, which are transformed into binuclear Fe-complexes during washing. During calcination, iron detached from the Bronsted sites migrates to the external surface of the zeolite, finally leading to significant agglomeration. Nevertheless, agglomeration of Fe can be strongly suppressed by adequately tuning the conditions of the calcination.  2002 Elsevier Science (USA). All rights reserved.

Orbital-specific mapping of the ligand exchange dynamics of Fe(CO)(5) in solution

Transition-metal complexes have long attracted interest for fundamental chemical reactivity studies and possible use in solar energy conversion. Electronic excitation, ligand loss from the metal centre, or a combination of both, creates changes in charge and spin density at the metal site that need to be controlled to optimize complexes for photocatalytic hydrogen production and selective carbon–hydrogen bond activation. An understanding at the molecular level of how transition-metal complexes catalyse reactions, and in particular of the role of the short-lived and reactive intermediate states involved, will be critical for such optimization. However, suitable methods for detailed characterization of electronic excited states have been lacking. Here we show, with the use of X-ray laser-based femtosecond-resolution spectroscopy and advanced quantum chemical theory to probe the reaction dynamics of the benchmark transition-metal complex Fe(CO)5 in solution, that the photo-induced removal of CO generates the 16-electron Fe(CO)4 species, a homogeneous catalyst with an electron deficiency at the Fe centre, in a hitherto unreported excited singlet state that either converts to the triplet ground state or combines with a CO or solvent molecule to regenerate a penta-coordinated Fe species on a sub-picosecond timescale. This finding, which resolves the debate about the relative importance of different spin channels in the photochemistry of Fe(CO)5 (refs 4, 16,17,18,19 and 20), was made possible by the ability of femtosecond X-ray spectroscopy to probe frontier-orbital interactions with atom specificity. We expect the method to be broadly applicable in the chemical sciences, and to complement approaches that probe structural dynamics in ultrafast processes.

2p X-ray absorption of titanium in minerals

Ti 2p X-ray absorption spectra for a series of minerals have been measured. Crystal field multiplet calculations can explain the spectral shape. The asymmetry of the eg, peak is shown to be related to distortions of the TiIV octahedron. It is found, theoretically as well as experimentally, that the absorption spectra are more sensitive to tetragonal distortions than to trigonal distortions. A number of silicate minerals and metamict minerals containing titanium are measured and TiIIIhas not been observed in any of these minerals. A comparison is made to the 1s X-ray absorption, and the potential of both for the study of minerals is discussed.

Oxygen 1s and cobalt 2p X-ray absorption of cobalt oxides

The oxygen 1s and cobalt 2p X-ray absorption spectra of CoO, Li-doped CoO and LiCoO2 have been measured with 0.1 eV resolution. The cobalt 2p spectra are analysed with a ligand-field multiplet model and the inclusion of chase-transfer effects is discussed. The oxygen 1s spectra are interpreted as transitions to empty oxygen p states and it is concluded that the effects of correlations in the 3d band possibly are too small to be detectable. The symmetries and the electronic configurations of the cobalt ions in the oxides are determined. It is concluded that, in contrast to for example NiO and La2CuO4, the doping-induced states are possibly of 1A1 symmetry, which would imply that the quasi-particles have spin 3/2 and are most likely trapped.

Fluorescence yield detection: Why it does not measure the X-ray absorption cross section

Abstract It is shown that fluorescence yield (FY) detection does not measure a pure X-ray absorption spectrum if multiplet effects are important. The reason is that while the Auger decay is approximately constant, the fluorescence decay shows a strong dependence on the final state. With a coherent description of the X-ray absorption and X-ray emission processes, the observed differences between TEY and FY detected nickel 2 p edges can be reproduced accurately. In the, approximate, incoherent description the FY spectral shape identifies with the X-ray absorption spectrum multiplied with its state dependent fluorescence decay. Self-absorption effects are calculated and the implications for X-ray Magnetic Circular Dichroism (X-MCD) and generalisations to other systems are discussed.

Differences between L3 and L2 x‐ray absorption spectra of transition metal compounds

The differences between L3 and L2 edges of 3d and 4d transition metal complexes and compounds in octahedral symmetry are discussed. The main origin of these differences are the multiplet effects due to the coupling of the 2p core wave function and the 3d and 4d valence wave functions. The 3d and 4d spin–orbit coupling is a second origin of difference. For 3d systems the multiplet effects dominate all other interactions and the L3 and L2 edge are completely mixed and reordered. For 4d systems the core hole spin–orbit coupling is large and the L3 and L2 are separated by about 100 eV with a ratio close to 2:1. The differences between the L3 and L2 edge originate from the weight transfer between the t2g and eg peaks due to the multiplet effect. This weight transfer is about 25% for the L3 edge and about 5% for the L2 edge, which implies that for a comparison to single‐particle calculations the L2 edge is preferable to use. Partly filled 4d systems are low‐spin and the occupation of the t2g states implies a de...

Soft X-ray absorption spectroscopy of vanadium oxides

Abstract We present the V2p and O1s X-ray absorption spectra of V 2 O 3 , VO 2 and V 2 O 5 . The V2p spectra show strong multiplet effects caused by large 2p-3d and 3d-3d Coulomb and exchange interactions. The spectra exhibit noticeable chemical shifts and completely different multiplets. The shapes of the multiplet are related to the symmetry and spin of the ground state. The V2p spectrum of V 2 O 3 can be simulated by an atomic-multiplet calculation projected in the appropriate crystal field. The O1s spectra are related to unoccupied Op character mixed in the conduction band. The spectra reflect V3d bands near the edge and V4sp bands at higher energies. The V3d bands are split by crystal field effects. Changes in the relative intensities are related to differences in the metal-ligand hybridization. The extra dispersion of the V3d bands in V 2 O 3 is attributed to larger metal-metal interactions. The O1s spectrum of VO 2 compares favorably to a symmetry-projected band structure calculation. Finally, the O1s spectra of VO 2 taken at room temperature (insulator phase) and at T ≈ 120°C (metallic phase) show clearly the splitting of the V3d | band across the metal-insulator transition.

Spin-polarized x-ray emission of 3d transition-metal ions: A comparison via K a and K ß detection

This paper demonstrates that spin-polarized x-ray-excitation spectra can be obtained using

On the electronic structure of Cu(III) and Ni(III) in La2Li1/2Cu1/2O4, Nd2Li1/2Ni1/2O4, and Cs2KCuF6

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