Hard x-ray photon-in-photon-out spectroscopy with lifetime resolution – of XAS, XES, RIXSS and HERFD

Spectroscopic techniques that aim to resolve the electronic configuration and local coordination of a central atom by detecting inner-shell radiative decays following photoexcitation using hard X-rays are presented. The experimental setup requires an X-ray spectrometer based on perfect crystal Bragg optics. The possibilities arising from non-resonant (X-Ray Emission Spectroscopy - XES) and resonant excitation (Resonant Inelastic X-Ray Scattering Spectroscopy – RIXSS, High-Energy-Resolution Fluorescence Detected (HERFD) XAS) are discussed when the instrumental energy broadenings of the primary (beamline) monochromator and the crystal spectrometer for x-ray emission detection are on the order of the core hole lifetimes of the intermediate and final electronic states. The small energy bandwidth in the emission detection yields line-sharpened absorption features. In transition metal compounds, electron-electron interactions as well as orbital splittings and fractional population can be revealed. Combination with EXAFS spectroscopy enables to extent the k-range beyond unwanted absorption edges in the sample that limit the EXAFS range in conventional absorption spectroscopy

Sulfur-Metal orbital hybridization in sulfur-bearing compounds studied by X-ray Emission Spectroscopy

The electronic structure and ligand environment of sulfur was investigated in various sulfur-containing compounds with different structures and chemical states by using X-ray emission spectroscopy (XES). Calculations were performed using density functional theory (DFT) as implemented in the StoBe code. The sulfur chemical state and atomic environment is discussed in terms of the molecular orbitals and partial charges that are obtained from the calculations. The main spectral features can be modeled using our calculational approach. The sensitivity of the Kβ emission to the cation and the local symmetry is discussed

The oxidation state of vanadium in titanomagnetite from layered basic intrusions

The redox conditions prevailing during the formation of vanadiferous titanomagnetites from three layered intrusions (Bushveld; Koillismaa; Skaergaard) have been estimated from the valence state of vanadium using synchrotron X-ray absorption near edge structure spectroscopy (XANES). Using a high energy-resolution X-ray emission spectrometer, we show that vanadium occurs mostly as V3+, with minor V4+. The most concentrated samples (up to 2.4 Wt% V2O3) contain approximately 10% of vanadium as V4+. Both V3+ and V4+ occur in the octahedral site of the spinel structure. Considering the low magnetite/melt V4+ partition coefficients, this suggests that vanadium ores crystallized under specific oxidizing conditions

Tektites and microtektites Fe oxidation state and water content

Asteroid or cometary impacts onto the Earth surface are known to have played an important role in modifying the composition of the earth crust. Impact glasses, resulting from the rapid cooling of the molten target rock, are clues of the complex melting and metamorphic processes taking place during an impact. Tektites and micro-tektites are a sub class of impact glasses formed during the very first stages of the cratering process by high temperature melting of the target rock. They usually display rounded shapes and can be found over wide areas called strewn fields. As Fe oxidation state could be a useful probe to obtain information on the formation conditions of tektites, it has been the focus of many studies. However, the difficulties in analysing samples with small dimensions and high Fe dilution have so far hindered the possibility to systematically study the Fe oxidation state in these glasses. To this aim, XANES is an ideal technique as it allows to determine the Fe oxidation state also in small samples even at very high dilution without deteriorating the error in the Fe3+/(Fe2++Fe3+) ratio. Fe K-edge XANES spectra have been collected in fluorescence mode at the ID26 beamline of ESRF using a Si (311) monochromator focusing the X-ray beam down to about 50 x 200 μm. The excellent energy reproducibility (±0.03 eV) allowed to obtain a small error in the determination of the Fe oxidation state. Micro-IR data have been collected in transmission mode at the LNF (Frascati, Italy). Areal analyses (50 x 50 μm) have been collected for 16 moldavites, 7 North American tektites and 5 microtektites. Tektite glasses display Fe3+/(Fe2++Fe3+) ratios close to 0.05 (±0.03). With few exceptions (moldavites from the Moravian area), no significant variations have been found in the Fe oxidation state of tektite samples according to impact age or target rock composition [1]. Even for very large impacts events, where the target rock presumably displayed a wide range of Fe oxidation states, tektites have been homogeneously reduced to almost exclusively Fe2+ [2]. Similar behaviour has been observed in molten rock from the first atomic bomb test (Alamogordo, USA) [3]. Contrary to tektites, North American microtektites show a wide variation in the Fe oxidation state raising the issue of a possible difference with the formation mechanism of tektites [4]. Water content of all the tektites and microtektites are in the range of already published tektite water content data and display no correlation with the Fe oxidation state. The low water content of North American microtektites studied suggests that there has been no sea-water induced alteration in these samples, thus strongly suggesting that the variation of the Fe oxidation state in the North American microtektite samples studied here is not due to secondary alteration. We maintain that the mechanism responsible for NA microtektite oxidation is not sea-water alteration, nor oxidation in air. [1] Giuli, G. et al. (2002) Geochim. Cosmochim. Ac., 66, 4347- 4353. [2] Giuli, G. et al. (in press) Geol. S. Am. S. [3] Giuli, G. et al. (in press) Geol. S. Am. S. [4] Giuli, G. et al. (in prep)