Notice of SAMBA

X-ray Absorption Spectroscopy (XAS) allows the experimentalist to characterize either the structural or electronic properties of a material, whatever the state of the target, solid, liquid or gas. This technique is thus widely carried out in a large community of users in the fundamental and applied research fields, including physics, chemistry, Earth and environmental sciences, biology and surface sciences.
 
At LURE, 4 beam lines are fully dedicated to classical XAS, in the range 0.8 to 30 keV with about 550 users per year. On the average over the last four years, the ratio between the number of required runs over the actually allocated ones is about 1.9. On the beam lines dedicated to XAS, BM29 and BM32 at ESRF, this ratio is even larger, about 4.
 
The activity around XAS is thus consistently high, as demonstrated by these numbers and also by the systematic existence of XAS lines on synchrotron facilities over the world. Indeed, XAS now belongs to the conventional tools of structural and electronic characterisation of materials. The measurement is made either i) on already prepared samples with a controlled variation of the parameters of preparation or ii) on samples presenting a controlled evolution of their physical and chemical properties. In this case, one can speak of a static-working mode. XAS can also be carried out to follow sample evolution during structural or electronic transitions, induced by temperature, pressure, applied strains... Here, one can speak of a dynamic working mode. In this dynamic working mode, the experimentalist wants to follow a reaction in situ. Hence Quick EXAFS must be available on a SOLEIL beam line.
 
This proposal concerns a beamline for XAS in the 4-40 keV range, which covers the K edges of elements with 20 = Z = 58 - i.e. from Ca to Ce - and L edges of elements with 50 = Z = 98 - i.e. from Sn to Cf. This concerns the most studied elements in catalysis (Re, Pt, Zr, Mo, Ru, Rh, Pd, Sn and Sb), in environmental sciences (3d-elements, As, Cd, Hg...), in coordination chemistry (3d and 4d-elements), in biology (Ca, transition metals, Pt ...), and in physics (e.g. 3d and 4f elements for magnetic materials...). It is important not only to access to the L edges of heavy elements which offer a reduced energy domain for EXAFS measurements (typically from 150 to 450 eV) but also to the K edges. EXAFS spectroscopy at these K edges allows an accurate determination of structural parameters and their exploitation is very complementary to the one of L edges. Moreover the study of K edges instead of the L edges of the same element is also useful in order to avoid the multi-electronic transitions pitfall, often difficult to handle in the EXAFS treatment. Users ask frequently for the characterisation of the same material at both edges in the same proposal.
The beam line with high spatial and energetic resolutions must cover a large energy range. It will have several detection and acquisition modes and a variety of sample environments. A new and interesting development could be to combine structural techniques such as XAS and XRD, XAS and X-ray scattering, provided that requirements for one technique do not hamper the other one. A specificity of a beam line on SOLEIL could be also the combination of XAS with techniques where a specific property is measured, e.g. UV-Visible absorption or visible scattering (turbidity) measurements, conductivity versus temperature or calorimetric measurements.

From the fundamental and technological point of view, nanosystems are nowadays of great importance. These systems display only short-range order because of their small size, hence, XAS is the required technique to describe their structural and electronic properties but also to determine their average size. The motivation of XAS investigations performed on nanomaterials is the understanding and optimisation of their new magnetic, electrical, optical observed behaviours or of their enhanced chemical reactivity.
For example it has been shown that the optical absorption spectrum of metallic aggregates embedded in dielectric matrices, called nanocermets, is strongly related to the intrinsic electronic properties of the aggregates and to their local surroundings. The development of magneto-optical devices has benefited from relationships between structural and electronic characteristics and physical properties established by XAS on metallic clusters in matrices, metallic thin films and epitaxial multilayers.
Besides the physical techniques (ion implantation, low energy clusters beam deposition, ...) used to prepare the above materials, a lot of chemical techniques allow the researcher to dispose of materials with tailored properties. Among them, Soft Chemistry has largely benefited from XAS to design new molecular precursors used in the elaboration of nanomaterials for catalysis, optical and electrical devices, energy storage, high performance ceramics and so on... and to understand the mechanisms of formation or transformation of such materials. To illustrate this field we can just mention the strong emergence of new anodic and cathodic materials for rechargeable solid state batteries. These materials are based on the chemistry of intercalation and de-intercalation of lithium into host matrices (NixSn, LiNiVO4, ...). The knowledge of oxidation state of the elements of the matrix and of the atomic arrangements of the solid network, generally disordered upon intercalation and de-intercalation processes, provided by XAS is of prime importance for this research field. The community expects from the high photon flux of SOLEIL to be able of studying such batteries in situ during their functioning.

One of the well known applications of nanomaterials, in particular nanoparticles lies in the heterogeneous catalysis field. Supported noble metal catalysts are used in a number of important industrial processes (e.g. Fisher Tropsch reaction (production of long chain paraffins from syngas CO + H2) uses for the most part cobalt catalysts supported on different oxides) but also to solve environmental challenges like the emission control of toxic gases (e. g. use of Pt-based catalysts supported on oxides or zeolithes for the reduction of NOx emissions from Diesel car exhaust gases). Research at the cutting-edges requires the capability to perform in situ characterisation of the reactivity of these catalysts under experimental conditions.

In coordination chemistry, prussian blue analogues have recently attracted great interest because their use as molecular magnets. In particular the understanding of magnetic phase transitions of these molecular systems upon light irradiation is nowadays the subject of intense XAS and XMCD investigations.

The understanding of the mechanism of hydrogen adsorption in metals and intermetallics motivates a lot of XAS studies. Metal-hydrogen systems are used in a variety of technological applications including hydrogen storage materials and metal hybride batteries.

The synthesis of glasses with optimized optical characteristics (eg high non linear refractive index in tellurite glasses) or ion conduction properties suitable for applications in circuits, semiconductors devices, optical fibers, wave guides ... motivates also XAS studies. Besides these technological motivations, a lot of structural XAS studies are connected to Earth Science since the structure of glasses is considered to be analogous to that of magmatic liquids.

Finally the access of compressibility factors around minor elements in various compounds (impurity in oxides, magnetic dopants in semi-conductors, minor elements in metallic alloys...) by performing XAS experiments under high pressure and high temperature is nowadays a challenge in Materials Science which could be taken up on a classical XAS beam line at SOLEIL.

The application of XAS to biology, biomaterials and pharmacology is growing, in connection with methodological and theoretical advances.
Genome programmes have given access to sequences of various organisms including the entire human genome. The next step is now the structural characterization of a very large number of proteins. Only a fraction of proteins are amenable to a state where X-ray crystallography and NMR can be applied. Together with small angle X-ray scattering, XAS is complementary to these key techniques and can play a significant role to in structural genomics. XAS allows to determine the metal site structure at atomic resolution of metalloproteins, which are estimated to make up 25-30% of all proteins. It does not require extreme protein purity ; it avoids the requirement to grow crystals as proteins could be in solution ; it is not limited by protein size ; and metal sites are described at atomic resolution. Although XAS approach is a local structural method, the access to the structure of the metal site, which often corresponds to the catalytic site, will help to understand catalytic processes and probe biological functions in greater depth.
XAS, coupled with cyclic voltammetry, allows to follow changes of the oxidation state and of the local environment of selected atoms, for instance transition metals in metalloproteins, especially enzymes. This is important to understand catalytic mechanisms. The same procedure can also be applied to biomimetic chemistry, which tries to construct new catalysts using the same procedure as the natural catalyst, but easier to produce and that can be used under different physical conditions (pH, temperature).

Insight into structure, stability and reactivity of metallic compounds used as drugs can be accessed by XAS methods. Before administration of a drug, there is a need for structural characterization, stability determination and reactivity studies. After administration, there is a need to trace and characterize the metabolites of drugs: besides elemental analysis, the metallic speciation in tissues has to be determined, being often related to toxicity.

Researchs in Environmental Sciences at the molecular level concern research groups from various disciplines such as earth sciences, chemistry, biology, catalysis, and material sciences. They provides some bases for the understanding of polluted site reclamation, improvement of water quality, waste management...which are crucial for the conservation of the environment. In the frame of the formations of rocks and magmas, the sensitivity of XAS to the presence of redox states is fruitful to estimate the oxidizing conditions prevailing at the Earth surface during magmatic eruptions.
 
Nowadays, FAME beamline at the ESRF, which is mostly dedicated to Environmental Sciences, opens large possibilities for the study of dilute systems in the 4-40 KeV energy range by XAS, and soon by micro-XAS thanks to the development of micro-focalizing optics.
 
However, the beamtime available on this beamline is not sufficient to satisfy the increasing needs in Geosciences, and another XAS beamline, complementary to FAME, i.e., allowing the study of less dilute samples, at high and low temperature, in variable redox conditions and states, in the 4-40 keV energy range, is essential for the Environmental Sciences community to produce high quality research. Moreover, the beamtime demand on FAME will be greatly increased consecutively to the closure of LURE.

BLP13-XAS proposal concerns well identified, strong and varied scientific communities. The scientific cases are broad and sound. The focus put on the sample environment to offer useful tools to the various communities is certainly a strong point of the proposal. The proposition of combining XAS with different techniques (DSC, electronic or Raman spectroscopy) is original and adds value to the project. The option of performing routinely QuickExafs is appreciated. Since two LURE beamlines offering XAS facilities are to be transferred to SOLEIL, SAC recommends that rather than duplicating experiments, care is taken to be complementary to other available possibilities (time scale of kinetics experiments with dispersive EXAFS, combination of diffraction and XAS with H10 Line).
SAC recommends that the versatility of the line, necessary to fulfil the needs of the different communities, does not restrict the easy use of the set-up by a wide community of users. Therefore, the SAC draws the attention of the proponents on the necessity :

• to provide the line with up-to-date tools of diagnosis on the beam and to push to the limits the automatic control of the experiment ;
• to consider carefully the energy range demanded for the beam line in connection with the constraints of using mirrors at high energy (above 25 KeV), with the scientific needs of the communities and the possibilities offered elsewhere;
• to take care of various unsolved and delicate technical problems and to find "users friendly" solutions (to take only one example, the necessity of cooling the first monochromator crystal and bending the second one makes it rather difficult to change quickly the pair of crystals).
• to compare in details the potentialities of the present state of the art of XAS stations in Europe and in the world before to decide the final design of the line and try to foresee what will be available in five years;
• to have at users’ disposal a support laboratory close to the beam line complementing the facilities offered by larger and better equipped physical chemistry and biology laboratories common to all beam lines at SOLEIL.

In conclusion, SAC is in favour of building a "conventional" and Quick-XAS beamline on a bending magnet at SOLEIL. Moreover, SAC recommends the promoters to continue and extend their endeavour to attract around this line, which will offer unique specific possibilities, new scientific laboratories, particularly in biology and solid state physics. SAC recommends to build BL13-XAS with the highest priority.

La direction de SOLEIL demande l’aval du Conseil pour le lancement de la ligne d’absorption X. En effet, on conçoit mal un centre de rayonnement synchrotron dépourvu de ligne d’absorption X, tant cette technique a d’applications, qui concernent des communautés diverses, fortes et bien identifiées. La demande cumulée en temps de faisceau sera d’emblée très importante.
Compte tenu de la diversité des objectifs scientifiques, il est important que les besoins particuliers de chacune des communautés concernées soient bien pris en compte dans la conception de l’optique de la ligne, dans le choix des détecteurs et dans la définition et la construction des divers environnements.