Notice of ODE

X-ray-absorption spectroscopy has developed since more than twenty years to become one of the most used probe of local environment in material science. The modulated structures in absorption spectra just around the absorption edge of a specific atom (XANES) provides information about the geometry of the absorbing atom environment and the nature of its bonding with its neighbouring atoms. EXAFS is the analysis of the modulations of the absorption in a large energy domain, typically 1000 eV above the absorption edge. It gives precise information about the chemical nature, the number and the distance of the atoms in the neighbouring shells around the excited atom. The progresses in the data analysis have made EXAFS and XANES to become powerful tools for the study of local order in condensed matter.
More recently the sensitivity of the absorption spectroscopy to the magnetic properties of solids has led to the development of the X-ray Magnetic Circular Dichroism (XMCD). Using different X-ray energy domains, it is possible to extract information on the spin and orbital polarization of the conduction band or localized orbitals, which are responsible for the itinerant or localized magnetism respectively. In the soft X-ray range, atomic calculations reproduce quantitatively the XMCD signal, giving a powerful selective magnetometry. In the hard X-ray domain, the theoretical description based on band calculations is more difficult to handle, but progresses have been made since the first experiments and magnetic information on the magnetic structure of the conduction bands are currently extracted from XMCD signals.
The classical method of recording absorption spectra is the step by step measurement of the absorption coefficient for each photon energy point. An other way was first proposed by Matsuchita (1981) : the use of a bent monochromator which disperses the X ray beam and allows to measure the whole XAS spectrum at one time. This method was early developped in LURE (1983) by A. Fontaine and E. Dartyge. The main advantages of Dispersive XAFS are the focussing optics, the short acquisition time ( few ms) and the great stability during the measurements due to the absence of any mechanical movement. They allow the study of small samples (500m m at DCI, 40m m in the future at SOLEIL pushing the limits of studies under extreme conditions of pressure and temperaturee), the following of kinetics at the ms time scale (valuable in catalysis), and experiments needing a high signal to noise ratio (typically 105 at LURE), giving access to the measurement of local magnetic moment on weakly polarized atoms.

In magnetic solids, one can distinguish two kinds of magnetic electron states: those carrying a magnetic moment ( 4f of rare earth, 3d of transition metals) and those whose magnetization arises from hybridization and/or coupling with magnetic electrons and which propagate the magnetic interaction from one site to the other (delocalized 4p states in transition metals, 5d states in rare earth). In magnetic compounds, the knowledge of these two kind of polarization is mandatory to improve
X-ray Magnetic Circular Dichroism is a selective technique where you measure <M>, the mean value of the magnetic moment on a given atomic site; the selectivity of the technique concerns both the atomic species and the orbital symmetry of the shell probed. It is obtained by the difference between left and right circularly polarized X-ray absorption cross section of a ferromagnetic or ferrimagnetic material above an edge. While the 4f states of rare earth and 3d states of transition metals are excited with soft X-rays, the delocalized orbitals, whose polarization is very weak, are probed by hard X-rays. These magnetic properties are of fundamental interest to understand the complete magnetization of alloys, and require band structure theory.
The XMCD studies that can be planned on the dispersive EXAFS beamline at SOLEIL concern characterization of nano objects (multilayers or clusters imbedded in a matrix), magneto-volumic effects with the combined applications of high pressure and magnetic field, high spin-low spin transitions or molecule-based magnets. The high flux of SOLEIL will allow to study phenomena of fundamental interest but with very small magnetic moment, such as Kondo systems, paramagnetic or super-paramagnetic samples.

The main effect of high pressure is to reduce the interatomic distances and therefore to modify the interaction between atoms. This can lead to phase transformations with large changes in the bonds: breaking of sp3 hybridization, delocalisation of f electrons, metallization of semiconductors, charge transfer between intramolecular and intermolecular bonds…. High pressure allows also to reproduce the inner planetary conditions, including the Earth. A lot of compounds exhibit polymorphism under pressure and sometime it is possible to recover them in metastable state under ambient conditions (diamond or cubic BN for example). Some chemical reactions need high pressure to occur or are more efficient under pressure and for instance, large O2 pressure allows to produce unusual oxidation states.
High pressure, possibly combined with high or low temperature, is experimentally produced using diamond anvil cells. In these cells, the sample size is typically 300 mm in diameter and 20 mm in thickness. Thanks to its focusing optic, the X-ray dispersive set-up is particularly well adapted to high pressure XAS experiments. Moreover, the real time visualization of the XAS spectra allows to eliminate the parasitic anvil cell diamond Bragg reflections (glitches) by an appropriate alignment. The dispersive EXAFS beamline at LURE is one of the very few places where this type of measurements is routinely performed. Recently, combination of high temperature (800K) and high pressure has been achieved. This opens the domain of geophysical studies.

At the present time, dispersive XAFS is the unique way to collect EXAFS spectra in a sufficient photon energy range in a few ms, This is a valuable characteristic of the technique which allows to follow the kinetic of chemical reactions, mainly in inorganic systems, and in catalysis. Electrochemical problems, linked with catalysis studies have been undertaken in the past and should develop in the future. In-situ studies of corrosion or chemical reaction in super-critical water present theoretical and industrial interests. One can also mention the heterogeneous catalysis studies, with in-situ characterization, and sol-gel processes, where parallel measurements with small angle X-ray scattering give important information on the chemical species involved as well as the size of the clusters. Research at the cutting-edge requires the capability to perform in situ characterisation, i.e. carried out under real conditions Specific reaction cells (1bar, 800°C) and a device with various gas cylinders (such for example H₂, N₂+O₂, CO, NO, H₂S, Ar and other gases) and flow controllers with large flow rate ranges will be developed to reproduce the catalytic conditions as closely as possible.

The Scientific Advisory Committee has decided to postpone its decision concerning the transfer of an optimised dispersive EXAFS beamline until some scientific and technical points of the proposal are clarified.
Meanwhile SAC made the following comments :

• The dispersive EXAFS experiment is operative at LURE and has been, and still is, very successful in high pressure absorption spectroscopy and XMCD spectroscopy. This is due mainly to the high beam stability, good focussing, and adequate photon energy range covering the L edges of rare earth and the K edges of transition metals.
• The possibility to perform time resolved experiments on the one hand, and to work with small samples conditions on the other hand are the bonus of this set up when compared to conventional EXAFS or XMCD beamlines. The foreseen flux increase of two orders of magnitude and a spot size of 50 microns make the SOLEIL version of the beamline very attractive.
• Emphasis should be in performing new types of high pressure experiments ; the example of dichroism spectra obtained very recently at LURE on invar under high pressure and low temperature is very promising.
• It is also advised to develop vigorously challenging time resolved experiments for catalytic studies with appropriate reaction cells.
• Three communities of users have been identified in the project dealing with magnetism, earth science and chemistry. In the first two fields different regular users groups have performed high quality, competitive experiments in the past and are expected to guarantee a continuing input of interesting problems to be solved at SOLEIL. The community of chemists is still very limited at LURE but efforts should be made to make them active in the future.
• From a technical point of view, some questions should be addressed more precisely before an approval is given for the transfer of the beamline. They concern
  •  the quality and size of the different optical elements (first mirror mainly)
  • a precise definition of the photon energy range of the beamline, including the monochromator crystals, their shape and energy resolution
  • the comparison of the expected performances of this beamline on a bending magnet with existing ones on undulators, since the beamline must stay at the highest international level
  • a precision of the pressure range accessible on the samples

SAC also wishes a better presentation of the document. Scientific and technical opportunities and challenges that will accompany a move to SOLEIL should be clearly identified. SAC asks for a new written version of the proposal for the next April SAC meeting.

A revised version of the proposal has been submitted for the 4^th SAC meeting.
SAC has removed the reservations he expressed at his second meeting concerning this project. The recommendation of the 4^th meeting is "SAC congratulates the promoters of the dispersive EXAFS project. The case is well defended, all the questions raised are satisfactorily answered and sufficient arguments on the beamline performance are given to grant a positive answer to proceed with the development of this beamline"

La direction de SOLEIL demande l’aval du Conseil pour le tranfert de la ligne d’EXAFS dispersif sur un aimant de courbure. La brillance de SOLEIL permet d’élargir les performances de la ligne en terme de taille de tache focale, d’étendue spectrale et de vitesse d’acquisition des données en équipant la ligne de dispositifs optiques et détecteurs appropriés. Cette ligne permettra de continuer à doter la communauté française d’un équipement du meilleur niveau international pour réaliser le projet scientifique décrit.