Quick EXAFS on SAMBA

 
May 14^th 2009, SAMBA team celebrating the first Quick EXAFS spectrum with a few colleagues.
From left to right, standing: D. Vantelon (LUCIA beamline), F. Villain (SAMBA), A-M Flank (LUCIA), V. Briois (SAMBA),
S. Chagnot (ODE beamline), L. Barthe, C. La Fontaine and S. Belin (SAMBA), E. Fonda is squatting

SAMBA’s optics hutch has two monochromators. The first classically allows the selection of the energy that will be used for an experiment, with step-by-step energy changes and measurements taken at each energy increment. This monochromator, which has an acceptance of several tens of millimeters, can, among other things, focus the beam on the sample to a sub-millimetric size on the horizontal plane. The time needed to acquire a spectrum of 1000 electron-Volts is between 15 and 30 minutes. This monochromator, associated with collimating and focusing mirrors in the vertical plane, allows it to deliver a monochromatic beam of about 300x300 µm² on the sample. The line is then in a high mode flux configuration set up for experiments requiring a high photon density on the sample, such as for the study of low concentrations of elements in materials (dopants), soil (pollutants) or biological systems (metalloproteins). It is also possible to study surfaces in this mode, either by ReflEXAFS in the case of thin films developed ex-situ, or by Surface EXAFS (or SEXAFS) which can characterize monolayers deposited on clean surfaces under ultra-vacuum conditions.
The second monochromator, much less standard, is the fruit of much thought and endeavour started in 2003. It started with the presentation by Pr R. Frahm (University of Wuppertal) of the principle of a new monochromator, during the 12^th International Conference on X-ray Absorption Spectroscopy (Malmö, June 2003). The device consists of a high precision goniometer, to impose the position of the crystal that is impinged by the incident X-ray beam, permitting the photon energy reflected by the crystal to be selected (Bragg reflection). There was nothing original up to this point. However, a motor was added to this goniometer which performed a small angle balancing movement (from 0.1 to 1°) of the crystal. It is during this to and fro movement of the crystal, and therefore for the whole range of energies selected, that data are recorded. It is simple idea, but someone had to think of it and design the instrument!
The energy field accessible with this monochromator, named Quick-EXAFS, depends both on the nature of the crystal and the degree of crystal balancing required. In the case of Titanium K-edge measurements (4966 eV), a movement of 1 degree permits 300 eV to be covered, i.e. a near-edge or XANES spectrum.

This monochromator was tested in December 2003 at LURE on the D25 beamline of the DCI storage ring during the last series of experiments carried out before the laboratory was closed. The results showed that this idea should be followed up. This is what several groups at SOLEIL have done, around the SAMBA station.

The SAMBA team had a very clear idea of the modifications to be made to the German prototype, and with detailed specifications, Valérie Briois, the beamline manager, contacted her colleagues at SOLEIL’s research and IT departments.
 

May 2009, installation of the Quick EXAFS monochromator in the SAMBA’s optics hutch. Mourad Sebdaoui is in charge of the alignment, Laurent Barthe is cabling the motors.

May 2009, last tunings of the mechanics and motorization, made by Marc Ribbens and Dominique Corruble, in the presence of Camille La Fontaine and Emiliano Fonda.

The first request concerned the treatment of raw data: rather than a number of points, corresponding to all the acquisitions as a function of time, it was necessary to know, at each moment, the angle of the crystal moving to and fro in order to obtain a directly exploitable energy spectrum. This encoding work was carried out by Marc Ribbens (Engineering Conception) and Nelson de Oliveira (researcher on the DESIRS beamline, at that time a post-doc with the Optics group). The fruits of their labour were validated in July 2007 on the German monochromator, which had been installed on SAMBA for this occasion. (Ref : Stötzel, J., Lützenkirchen-Hecht, D., Fonda, E., De Oliveira, N., Briois, V., & Frahm, R. Novel angular encoder for a quick-extended x-ray absorption fine structure monochromator Review of Scientific Instruments, 2008, 79(8): art.n°08310).

A second requirement concerned the range of the crystal’s balancing movement that had to correspond, whatever the energy, to a domain of at least 1000 eV. The maximum range of movement of the balance was thus increased from 1 to 4°.

Finally, a revolutionary concept: in order to avoid having to open the monochromator at each change of balancing angle of the crystal, which is the case for the monochromator constructed the other side of the Rhine, a new mechanical device had to be designed that allowed the degree of oscillation to be remote-controlled by motor from the SAMBA work station.
These mechanical challenges were surmounted with success by Jean-Michel Dubuisson and Marc Ribbens. In September 2008, a strange prototype took pride of place in the SAMBA cabin, ready to be run at high speeds, since the ultimate performance aimed at was 40 to and fro movements of the crystal per second. Position readings of the crystal, synchronized with the X-ray absorption data collection at the rate of one spectrum point acquired every 4 µs for the ultimate performance, would create other challenges that our colleagues in the IT and electronics teams Dominique Corruble, Benjamin Gombert, Florent Langlois, Julien Malik, Arafat Noureddine and Guillaume Renaud would solve brilliantly, by offering today a user-friendly computer interface for controlling the monochromator.

In May 2009, five years after their first meeting, all the members of this work team have been able to celebrate the installation of the Quick-EXAFS monochromator in SAMBA’s optics hutch.

The Quick-EXAFS monochromator of the SAMBA beamline can carry out absorption measurements in the 5-40 keV range, with an acquisition time of 25 ms to 5 s per spectrum (the to-fro swing frequency that corresponds to the measurement of 2 spectra, can vary between 20 and 0.1 Hz), and a 4 mm (in the horizontal direction) x 1 mm beam (in the vertical direction). The greatest number of requests for time-shifts on this Quick-EXAFS station is made by chemists who usually work on timescales of 500-1000 ms.
The monochromator had its scientific baptism with operando catalysis experiments, the aim of which was to understand how a catalyser worked by studying it in real time during the reaction in which it intervened, after having activated it (by raising the temperature under O₂, and then H₂ flows, for example).  
 

Complete setup for the control of the catalytic reaction (including gas distribution and mass spectrometer).

The new high temperature reaction cell in operation on SAMBA beamline.

These experiments, performed in the framework of the SAXO¹ research project funded by ANR were also the spur to try out the new high temperature reaction cell. The cell inlet can be connected to a computer controlled gas distribution and the outlet to a mass spectrometer for the analysis of reaction products. The specific reaction cell is also a “SOLEIL” product, the culmination of a collaboration between the SAMBA group, in particular Camille La Fontaine, a post-doc with the SAXO¹ project, and Laurent Barthe, assistant engineer and the Engineering Design group composed of Christian Basset, Christian Creoff, Carlos De Oliveira, Daniel Lefèbvre, Jean-Louis Marlats and Pascal Prout – who took responsibility for the mechanical aspects. The cell allows EXAFS transmission and fluorescence data to be collected with the possibility of coupling it with Raman spectrometry measurements. This cell is now available to the whole “Catalysis community”.

The aim of his chemical process is to convert carbon monoxide and hydrogen into hydrocarbons in the presence of a catalyst – the most common being iron or cobalt supported catalysts. The point of this conversion, in these periods of reduced hydrocarbon reserves, is to produce synthetic raw fuels from coal or gas, containing no impurities.
Researchers at LRS² in Paris have just studied a Cobalt-based catalytic precursor on SAMBA and analysed the effect of different additives on its degree of oxidation or particle size. The “ultimate” effectiveness of the catalyst, in relation to these additives, will be tested at UCCS³ in Lille.

The “Quick EXFAS” project will not end there: after the monochromator installed in May 2009, a second will complete the equipment by 2011. Placed in the same surroundings as the first, it will contain an equivalent device whose crystal will be able to cover an energy range complementary to that provided by the first crystal. A particularly interesting “plus” for the study of bimetallic catalysts (e.g. Co-Ru) as their follow-up requires two energy levels very far apart.

¹ - SAXO : ANR project for the EXAFS study of catalysts in operando – link to article in ‘Rayon de SOLEIL’

² - Surface Reactivity Laboratory, UMR UPMC/CNRS, (Eric Marceau) 

³ - UCCS : Catalysis and Chemistry of Solids Unit, UMR Lille1/CNRS, directed by Edmond Payen