Olivier Mathon

Energy-dispersive absorption spectroscopy for hard-X-ray micro-XAS applications.

Originally developed for time-resolved X-ray absorption spectroscopy (XAS), energy-dispersive absorption spectroscopy offers new opportunities for applications such as fluorescence detection and microbeams for scanning probe spectroscopy, thanks to recent developments in both instrumentation and optics. In this context, this paper presents a first example of chemical mapping recorded at ID24, the energy-dispersive XAS beamline at the ESRF. Attributes of this geometry for microanalysis are addressed. Finally, present and future plans are discussed and developed in the light of the evolution of the focal spot on this instrument in the past ten years.

Dynamics of the Magnetic and Structural alpha-epsilon Phase Transition in Iron

We have studied the high-pressure iron bcc to hcp phase transition by simultaneous x-ray magnetic circular dichroism and x-ray absorption spectroscopy with an x-ray energy dispersive spectrometer. The combination of the two techniques allows us to obtain simultaneously information on both the structure and the magnetic state of iron under pressure. The magnetic and structural transitions simultaneously observed are sharp. Both are of first order in agreement with the theoretical prediction. The pressure domain of the transition observed (2.4+/-0.2 GPa) is narrower than that usually cited in the literature (8 GPa). Our data indicate that the magnetic transition slightly precedes the structural one, suggesting that the origin of the instability of the bcc phase in iron with increasing pressure is to be attributed to the effect of pressure on magnetism as predicted by spin-polarized full-potential total energy calculations.

Measurement of femtometre-scale atomic displacements by X-ray absorption spectroscopy

The frequencies of extended X-ray absorption fine-structure (EXAFS) measurements, which are oscillations occurring on the high-energy side of an X-ray absorption edge, can be used to identify interatomic distances in materials. We have used a dispersive X-ray spectrometer, which has no moving components, to make rapid measurements with minimal energy drift of the difference in EXAFS from the Fe K edge in an iron-cobalt thin film undergoing periodic strain through magnetostriction. We show that magnetostriction can be detected by differential X-ray absorption. The magnitude of the recorded signal relative to the noise shows a sensitivity to mean differential atomic motion of one femtometre: a factor of 100 times more sensitive than that normally available.

The time-resolved and extreme conditions XAS (TEXAS) facility at the European Synchrotron Radiation Facility: the general-purpose EXAFS bending-magnet beamline BM23

BM23 is the general-purpose EXAFS bending-magnet beamline at the ESRF, replacing the former BM29 beamline in the framework of the ESRF upgrade. Its mission is to serve the whole XAS user community by providing access to a basic service in addition to the many specialized instruments available at the ESRF. BM23 offers high-signal-to-noise ratio EXAFS in a large energy range (5–75 keV), continuous energy scanning for quick-EXAFS on the second timescale and a micro-XAS station delivering a spot size of 4 µm × 4 µm FWHM.

Energy dispersive XAFS: characterization of electronically excited states of copper(I) complexes.

Energy dispersive X-ray absorption spectroscopy (ED-XAS), in which the whole XAS spectrum is acquired simultaneously, has been applied to reduce the real-time for acquisition of spectra of photoinduced excited states by using a germanium microstrip detector gated around one X-ray bunch of the ESRF (100 ps). Cu K-edge XAS was used to investigate the MLCT states of [Cu(dmp)2]+ (dmp =2,9-dimethyl-1,10-phenanthroline) and [Cu(dbtmp)2]+ (dbtmp =2,9-di-n-butyl-3,4,7,8-tetramethyl-1,10-phenanthroline) with the excited states created by excitation at 450 nm (10 Hz). The decay of the longer lived complex with bulky ligands, was monitored for up to 100 ns. DFT calculations of the longer lived MLCT excited state of [Cu(dbp)2]+ (dbp =2,9-di-n-butyl-1,10-phenanthroline) with the bulkier diimine ligands, indicated that the excited state behaves as a Jahn–Teller distorted Cu(II) site, with the interligand dihedral angle changing from 83 to 60° as the tetrahedral coordination geometry flattens and a reduction in the Cu–N distance of 0.03 Å.

The Time-resolved and Extreme-conditions XAS (TEXAS) facility at the European Synchrotron Radiation Facility: the energy-dispersive X-ray absorption spectroscopy beamline ID24

The new energy-dispersive XAS beamline at the European Synchrotron Radiation Facility is presented. A technical description of the beamline (optical scheme, detection, sample environments) is provided and its performance is illustrated with a few recent examples of experiments by different user groups.

XMCD under pressure at the Fe K edge on the energy-dispersive beamline of the ESRF.

The present paper demonstrates the feasibility of X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) at high pressure at the Fe K edge on the ID24 energy-dispersive beamline of the ESRF. In 3d transition metals, performing experiments at the hard X-ray K edge rather than at the magnetically interesting soft X-ray L edges represents the only way to access the high-pressure regime obtainable with diamond anvil cells. The simultaneous availability of a local structure (XAS) and of a magnetic (XMCD) probe on the sample under identical thermodynamical conditions is essential for studying correlations between local structural and magnetic properties. The state-of-the-art theoretical understanding of K-edge XMCD data is briefly summarized, and the set-up of beamline ID24 for high-pressure XMCD experiments is illustrated and the conditions required to perform measurements at the K edges of 3d transition metals are underlined. Finally, two examples of recent high-pressure results at the Fe K edge in pure Fe and Fe3O4 powder are presented.

Quick-EXAFS implementation on the general purpose EXAFS beamline at ESRF

The new implementation of QEXAFS acquisition on the general purpose EXAFS beamline BM29 at the European Synchrotron Radiation Facility is presented.

New opportunities for high pressure X-ray absorption spectroscopy using dispersive optics

Energy dispersive X-ray absorption spectroscopy (XAS) is particularly suited for high pressure studies using diamond anvil cells (DAC). The absence of mechanical movements of the spectrometer during the acquisition of the spectra and the strongly focusing polychromator crystal yield the required spot stability and dimensions, respectively, and offer advantages compared to energy scanning methods, especially for applications at very high pressures and to detect very small signals. This paper describes new developments in the optics of the dispersive XAS beamline at the ESRF (ID24). Although mainly focused on technical aspects, some examples of recent applications are given as an illustration of the potential of this instrument for high pressure applications.

Melting of iron determined by X-ray absorption spectroscopy to 100 GPa

Significance There is a long-standing controversy over the melting curve of Fe at high pressure as determined from static laser heated diamond anvil cell and dynamic compression studies. X-ray absorption spectroscopy measurements are used here as a criterion to detect melting under pressure. Confronted with a diversity of obtained melting curves, this technique, used at such pressure and temperature conditions, is eligible to be at the forefront to probe Earths deep interior. Furthermore, the experiment reported here holds promise for addressing important issues related to the structure and phase diagram of compressed melts, such as the existence of structural complexity (polyamorphism) in the liquid phase or the extent of icosahedral ordering whose investigation has been limited until now to ambient conditions. Temperature, thermal history, and dynamics of Earth rely critically on the knowledge of the melting temperature of iron at the pressure conditions of the inner core boundary (ICB) where the geotherm crosses the melting curve. The literature on this subject is overwhelming, and no consensus has been reached, with a very large disagreement of the order of 2,000 K for the ICB temperature. Here we report new data on the melting temperature of iron in a laser-heated diamond anvil cell to 103 GPa obtained by X-ray absorption spectroscopy, a technique rarely used at such conditions. The modifications of the onset of the absorption spectra are used as a reliable melting criterion regardless of the solid phase from which the solid to liquid transition takes place. Our results show a melting temperature of iron in agreement with most previous studies up to 100 GPa, namely of 3,090 K at 103 GPa.