J. Goulon

The ESRF beamline ID26: X-ray absorption on ultra dilute sample

The ESRF beamline ID26 is dedicated to XAFS studies on Ultra-diluted Samples (XAUS beamline). The spectral range 2.3-30 keV is covered. The aim of the beamline is to extract structural and electronic information on dilute samples for which the concentration of the absorbing element ranges from a few ppm up to 10 000 ppm. A wide range of applications is covered in biology, catalysis, chemistry, environmental sciences, solid state physics, etc. The beamline accepted the first users in November 1997. We give herein a brief description of the beamline and report on recent results.

Instrumentation development for ESRF beamlines

Abstract We report on instrumentation developments concerning a subset of ESRF beamlines: BL6 & 26 (linear, circular dichroism); BL8 (energy dispersive XAFS); BL18 (multipurpose energy scanned XAFS studies). We review in more detail the instrumentation of BL6 & 26 which are now under commissioning. A 1 4 wave plate was successfully used at 2.8 keV in the direct undulator beam. High-quality CMXD spectra have already been recorded in the fluorescence detection mode.

Diamond-based semi-transparent beam-position monitor for synchrotron radiation applications

A soft X-ray beam-position monitor based on a thin CVD diamond photodetector has been developed for use on third-generation synchrotron radiation facilities, and has been tested on beamlines ID26 and ID12 at the European Synchrotron Radiation Facility. Since diamond exhibits a low absorption to low-energy X-ray photons, this device allows beam-position monitoring with very little beam attenuation at photon energies as low as 2u2005keV. The devices presented here find their use essentially on low-energy monochromatic beamlines with narrow beam sizes (typically below 1u2005mm2). Measurements performed on a 20u2005µm-thick device show that a position sensitivity of 2u2005µm can readily be achieved with a photon flux attenuation of 22% at 4u2005keV. This allows permanent in-situ monitoring of beam instabilities during experiments.

Advanced detection systems for X-ray fluorescence excitation spectroscopy

This paper accounts for selected detector developments carried out over the past 15 years within the ESRF X-ray Absorption Spectroscopy group. This includes various types of photodiodes used as integrated current detectors. Special emphasis is put on the long-standing development of a Si drift-diode array suitable for energy-dispersive detection of X-ray fluorescence. This detector, which is now operational, was used to record high-quality XMCD/XAFS spectra on [Fe70Pt30] nanoparticles highly dispersed on a Si wafer. Using numerically deconvoluted spectra, energy resolution was decreased to 82 eV for the Si Kalphabeta line, 126 eV for the Fe Kalpha line and 176 eV for the Pt Lalpha line. A high-vacuum-compatible high-energy-resolution crystal analyzer was also installed on ID12, making it possible to record X-ray fluorescence excitation spectra in the photon-in/photon-out mode over a wide spectral range. Prospects of adapting these methods in order to investigate biological samples are briefly discussed.

Influence of atomic displacements on XAFS

Abstract We propose a new formulation for the effects of topological disorder and small structural distortions on XAFS. Our approach is based on the general curved-wave multiple-scattering formalism. Applications related to both EXAFS and XANES are considered.

A high-resolution silicon drift chamber for X-ray spectroscopy

Abstract Silicon drift chambers are very attractive detectors for X-ray fluorescence excitation spectroscopy (between 1 and 15 keV) due to their ultra low capacitance (typically 200 fF) which is also independent of the detector active area. High-energy resolution can be obtained while preserving a large solid detection angle. We have designed a new silicon drift chamber for X-ray spectroscopy. The active area is 1 cm 2 , the measured energy resolution is 143 eV FWHM for the Kα line of Mn (5.89 keV). This detector has also been used to characterize the spectral line shape of the emission of a standard planar undulator installed on the ESRF 6 GeV storage ring.

Element-selective X-ray detected magnetic resonance: a novel application of synchrotron radiation.

X-ray detected magnetic resonance (XDMR) is a new element-selective spectroscopy in which X-ray magnetic circular dichroism is used to probe the resonant precession of spin and orbital magnetization components when a strong microwave pump field is applied perpendicularly to the static bias field. Experimental configurations suitable for detecting the very weak XDMR signal are compared. XDMR signatures were measured in yttrium iron garnet and related thin films on exciting not only the iron K-edge but also the yttrium at diamagnetic sites. These measurements are shown to yield unique information regarding the wide-angle precession of induced magnetization components involving either orbital p-projected densities of states at the iron sites, or spin polarized d-projected densities of states at the yttrium sites. Extending XDMR measurements into the millimeter wave range would make it possible to study paramagnetic systems routinely and investigate optical modes as well as acoustic modes in ferrimagnetic/antiferromagnetic systems.

X-ray optical activity: Applications of sum rules

Edge-selective sum rules are proposed for a variety of X-ray dichroisms related to natural or nonreciprocal optical activity. Four spherical operators are identified that mix orbitals of different parities in what is assumed to be the ground state. The orbital anapole moment Ω(1) is primarily responsible for the magnetochiral dichroism; the time-even rank-2 tensor N(2)=[L, Ω](2) for natural circular dichroism; the time-odd rank-2 tensor W(2)=[L, n](2) for nonreciprocal magnetic linear dichroisms. At higher orders, the time-odd rank-3 tensor Γ(3)=[L, L, Ω](3) can also contribute to all nonreciprocal dichroisms. The physical content of these operators is analyzed. For every magnetoelectric group, one can predict which dichroic effect can be measured with either a single crystal or a powdered sample. Experimental spectra are produced to illustrate the value of the sum rules and the practical conditions of their application. Regarding nonreciprocal activity, one should be cautious about discussing magnetic symmetry because the deep core hole can couple the true ground state with low-lying excited states.

Use of an asymmetric wiggler to study the magnetic circular x‐ray dichroism of Ni, Ho, and Tb

The asymmetric wiggler, inserted since the end of 1990 on the Super‐Aco storage ring in Orsay, has been built to produce a high rate and high flux of circularly polarized soft x rays in the 100–3000 eV range. This insertion device is particularly well suited to measure x‐ray absorption spectra corresponding to dipole transitions towards electronic orbitals that are directly responsible for magnetism (3d states of first row transition metals, 4f states of rare earths). In this article we report the first magnetic circular x‐ray dichroism spectra obtained with this insertion device. They exemplify the wide prospects of this experimental setup.

X-ray natural circular dichroism and chiral-EXAFS in gyrotropic crystals

X-ray natural circular dichroism (XNCD) has been recently detected in the XANES region for uniaxial and biaxial gyrotropic crystals. Chiral-EXAFS (chi-EXAFS) spectra are reported for the first time over a wider energy range and are analysed in terms of multiple-scattering paths of relevant symmetry. For such heavily absorbing single crystals as lithium iodate, paratellurite or potassium titanyl phosphate, the differential absorption between left-handed and right-handed circularly polarized X-ray photons cannot be measured in transmission but gyrotropy effects can still be detected in fluorescence excitation spectra. Whereas XNCD and fluorescence-detected X-ray natural circular dichroism spectra are strictly identical for uniaxial crystals, it has been established that this was true only to the first order for biaxial crystals such as potassium titanyl phosphate.