Legacy pages for the ID24 Dispersive EXAFS Beamline

Description

Energy Dispersive X-ray Absorption Spectroscopy (EDXAS) is a now a well-established method which has been applied to a broad range of applications (for a recent review see reference [1] and references therein). The energy dispersive spectrometer employs a bent crystal to focus and disperse a polychromatic X-ray beam onto the sample. The beam passing through the sample then diverges towards a position sensitive detector, where beam position is correlated to energy. Major advantages of this scheme are: i) an intrinsic stability in focal spot position and in energy scale, since there are no moving components and ii) a high acquisition speed, where all energy points are acquired rigorously in parallel. In general, the spectrometer is installed on a divergent synchrotron source (often a bending magnet). The latter source naturally yields the large horizontal divergence (typically a few mrads) necessary to obtain, with reasonable radii of curvature of the polychromator crystal, an energy dispersion making it possible cover a whole EXAFS spectrum. At the European Synchrotron Radiation Facility, we have adopted a non-conventional optical scheme (Figure l) allowing to take maximum advantage of a high brilliance undulator source. The present performance of ID24 features i) an energy range 5-27 KeV, ii) a focal spot of about 5 x 5 μm² in the energy range 5 – 15 KeV and iii) a time resolution of the order of the msec for non reversible processes.

Figure 1: Schematic view of the optical layout of the Energy Dispersive X-ray Absorption Spectroscopy beamline ID24 at the ESRF.

Applications and examples of recent ID24 publications

I - Time resolved XAS

In EDXAS the time resolution for data acquisition is technically limited only by the readout time of the detection system, since all energy points are collected rigorously in parallel, and advances in time resolution closely followed developments in position-sensitive detectors. Time resolved studies were historically the first to take advantage of EDXAS instruments to track rapid changes in the local and electronic structure of absorber atoms in disordered systems in the fields of biophysics, chemistry and materials science. Presently on ID24 the core of the activities is devoted to the investigation of catalysts under operando conditions, where several methods and techniques are coupled and synchronized to XAS at the millisecond regime. [2,3,13].

II - High pressure XAS studies

EDXAS is also well suited for studies at extreme conditions of pressure using a diamond-anvil cell, where the strongly focusing crystal and the absence of movements provide the necessary small and stable focal spot, respectively. Moreover, from a practical point of view, Bragg reflections from the diamond anvils can be removed quickly from the energy range of interest, because the whole spectral range is observed simultaneously [4,5].

III- X-ray Dichroism studies

The intrinsic stability and speed of EDXAS can be exploited to increase the detection sensitivity to small differences in XAS spectra, by making rapid comparative measurements in samples subject to a periodic external perturbation, such as a temperature gradient, or the rotation of a magnetic field.

1 - XMCD at high pressure

X-ray Magnetic Circular Dichroism (XMCD) is a well known example of "differential XAS" technique, measuring the difference in absorption between two helicity states of the photon, or, with fixed helicity, between two opposite directions of magnetization. An important area of research on ID24 exploits the small focal spot to perform XMCD studies at high pressure using the diamond anvil cell [6,7].

2 - XMCD using pulsed high magnetic fields

Up to now, the combination of synchrotron experiments with pulsed high magnetic fields has led to interesting achievements mainly using X-ray diffraction^. Recent experiments on ID24 using pulsed fields of ~ 30T represent the first attempts to measure XAS and XMCD spectra under extreme magnetic fields, to yield information on the local and electronic structure and on the ordered magnetic moment on the absorber atom. The energy dispersive geometry allows the whole energy range of the XMCD/XAS spectrum to be recorded in parallel at each pulse using a position sensitive detector. This is crucial for measurements under high magnetic fields in pulsed mode as the lifetime of the coil  limits drastically the number of possible measurement cycles [8,9].

3 - XMLD: detection of tiny atomic displacements

The ability of EDXAS to detect very small differential signals has improved the sensitivity of extended X-ray absorption fine structure (EXAFS) measurements to include atomic displacements of the order of 0.00001 Å, which is about 100 times smaller than had previously been assumed. This was made possible by exploiting the stability (no moving parts) and acquisition speed of EDXAS to overcome the major limitations of the conventional scanning EXAFS technique, i.e. the signal-to noise ratio of the measurement and the energy stability between comparative measurements [10,11,14].

IV - MicroXAS 2D chemical mapping

The advantages of an energy dispersive spectrometer, that features no movement of optics during acquisition leading to an enhanced stability of energy scale, spot size and position, combined with a micron sized spot and the option of fluorescence detection [1], have made possible to address 2D mapping with micron resolution on heterogeneous samples, providing full XAS information on each pixel. It is worth noting that due to the absence of mechanical scanning of the monochromator, the spatial resolution is not affected by the energy scan and remains fixed to the dimensions of the probe. Moreover, the dwell time per pixel is short enough to make it practically possible to acquire 100 x 100 pixel images in a few hours in fluorescence (less than 1 hr in transmission) [1,12,15].

Recent ID24 publications

1. “Energy Dispersive Absorption Spectroscopy for Hard X-ray micro-XAS applications”
S. Pascarelli, O. Mathon, M. Munoz, T. Mairs and J. Susini
Journal of  Synchrotron Radiation 13, 351 (2006)

2. Dynamic in situ observation of rapid size and shape change of supported Pd nanoparticles during CO/NO cycling”
M.A. Newton, C. Belver-Coldeira , A. Martinez-Arias, M. Fernández-García
Nature Materials 6 (7) 528-532 (2007)

3. “Rhodium dispersion during NO/CO conversions"
A.J. Dent , J. Evans , S.G. Fiddy, B. Jyoti, M. A. Newton, M. Tromp
Angewandte Chemie- International Edition 46 (28), 5356 (2007) 

4. “New Phase Transition of Solid Bromine under High Pressure”
A. San Miguel, H. Libotte, M. Gauthier, G. Aquilanti, S. Pascarelli, and J.P. Gaspard
Physical Review Letters 99, 015501 (2007) 

5. “Electronic topological transition in zinc under pressure: An x-ray absorption spectroscopy study”
G. Aquilanti, A. Trapananti, M. Minicucci, F. Liscio, A. Twaróg, E. Principi, and S. Pascarelli
Physical Review B 76, 144102, (2007)

6. “Fe magnetic transition under high pressure”
O. Mathon, F. Baudelet, J. P. Itié, A. Polian, M. D'Astuto, J.C. Chervin, S. Pascarelli
Physical Review Letters 93, 255503 (2004) 

7. “Magnetovolume instabilities in the pressure dependence of the K-edge circular dichroism of Fe₃C Invar particles”
E. Duman, M. Acet , E.F. Wassermann, J. P. Itié, F. Baudelet, O. Mathon, S. Pascarelli
Physical Review Letters 94, 075502 (2005) 

8. “ XAS and XMCD under high magnetic field and low temperature on the energy-dispersive beamline of the ESRF”
O. Mathon , P. van der Linden, T. Neisius, M. Sikora, J. M. Michalik, C. Ponchut, J. M. De Teresa and S. Pascarelli
Journal of Synchrotron Radiation 14, 409 (2007) 

9. “Miniature pulsed magnet system for synchrotron X-ray measurements” 
P. van der Linden, O. Mathon, C. Strohm and M. Sikora
Review of Scientific Instruments 79, 075104 (2008) 

10. “Measurement of femtometre-scale atomic displacements by X-ray absorption spectroscopy”
R. Pettifer, O. Mathon, S. Pascarelli, M. Cooke and M. Gibbs
Nature 435, 78 (2005) 

11. “Effect of Pressure on Magnetoelastic Coupling in 3d Metal Alloys Studied with X-Ray Absorption Spectroscopy”
S. Pascarelli, M. P. Ruffoni, A. Trapananti, O. Mathon, G. Aquilanti, S. Ostanin, J. B. Staunton, and R. F. Pettifer
Physical Review Letters  99, 237204 (2007)

12. “Redox and speciation micro-mapping using dispersive X-ray absorption spectroscopy: Application to iron in chlorite mineral of a metamorphic rock thin section”
M. Munoz, V. De Andrade, O. Vidal, E. Lewin, S. Pascarelli and J. Susini
Geochemistry Geophysics Geosystems, 7, Q11020 (2006) 

13. “In Situ Redispersion of Pt Autoexhaust Catalysts; An On-line Approach to Increasing Catalyst Lifetimes?”
Y. Nagai, K. Dohmae, Y. Ikeda, N. Takagi, T. Tanabe, N. Hara, G. Guilera, S. Pascarelli, M. A. Newton, O. Kuno, H. Jiang, H. Shinjoh and S. Matsumoto
Angewandte Chemie Intl. Ed. (in press, 2008)

14. “Direct Measurement of Intrinsic Atomic Scale Magnetostriction”
M. P. Ruffoni, S. Pascarelli, R. Grossinger and R. Sato Turtelli, C. Bormio-Nunes, R. F. Pettifer
Physical Review Letters  101, 147202 (2008)

15. “Occurrence, composition and growth of polyhedral serpentine”
M. Andreani, O. Grauby, A. Baronnet, and M. Munoz
European Journal of Mineralogy 20, 159-171 (2008)