On experimental attenuation factors of the amplitude of the EXAFS oscillations in absorption, reflectivity and luminescence measurements

This paper is reviewing a number of artefactual limitations in the current EXAFS analysis. In conventional absorption measurements, the major sources of errors are the transmission of harmonics by the monochromator and inevitable leakages of radiation around or through the sample. The consequences are an alteration of the amplitude of the signal but also some harmonic distortion of the EXAFS oscillations and a systematic perturbation of the measured Debye-Waller terms. An efficient correction of the effects of X-ray leakage is suggested whereas the presence or the elimination of harmonics can be assessed by monitoring the reproducibility of the edge jump when calibrated metallic foils of different nature are inserted behind the sample. The basic analysis developed for absorption measurements has been successively extended to the case of REFLEXAFS and various emission techniques : X-ray fluorescence/scattering, optical luminescence and total electron yield. It became possible to rationalize a number of experimental observations and to determine appropriate operating conditions.Cette publication examine un certain nombre d'artefacts expérimentaux qui limitent généralement l'analyse des spectres EXAFS. Dans le cas des mesures conventionnelles par absorption, les principales sources d'erreurs sont d'une part, la transmission d'harmoniques par le monochromateur, et d'autre part, l'existence d'inévitables fuites de rayonnement autour ou au travers de l'échantillon. Il en résulte non seulement une altération de l'amplitude du signal mais encore une distorsion harmonique des oscillations d'EXAFS et une perturbation systématique des termes de Debye-Waller déterminés. Une correction efficace des effets dus plus précisément aux fuites de rayons X est proposec et il est suggéré de contrôler la présence ou l'élimination des harmoniques en suivant la reproductibilité des hauteurs des seuils lorsque l'on insere, derrière l'échantillon, des feuilles métalliques étalons de différentes natures. L'analyse fondamentale développée par les mesures par absorption a été étendue aux cas de la technique REFLEXAFS et d'autres techniques utilisant les émissions secondaires : fluorescence X/diffusion X, luminescence optique et rendement électronique total. Il s'est révélé possible ainsi de rationaliser un certain nombre d'observations expérimentales et de determiner les conditions opératoires les plus appropriées

X-ray dichroism in biaxial gyrotropic media: Differential absorption and fluorescence excitation spectra

PACS. 33.55.Ad Optical activity, optical rotation; circular dichroism - 41.50.+h X-ray beams and X-ray optics - 78.70.Dm X-ray absorption spectra,

Calculation of X-ray natural circular dichroism

A method is presented to calculate the natural circular dichroism recently discovered in the X-ray range (XNCD). The basic formula represents XNCD as an odd second-rank tensor and leads to a sum rule that relates XNCD to the mixing of odd and even orbitals in the ground state. A multiple-scattering theory of XNCD is presented, and calculated spectra for the L-edges of iodine in LiIO3 compare favorably with the experiments

Sulfur K-Edge XAS Study of Sulfidic Crosslinks in Vulcanised Rubbers

This feasibility study allowed us to establish three main results : (i) With high quality XANES spectra, it becomes quite possible to resolve the characteristic σ*(S-S), π*(S-S) and σ*(S-C) resonances; (ii) The "undulator gap scan" technique can yield excellent EXAFS spectra at the sulfur K-edge over more than 1300 eV; (iii) S K-edge EXAFS spectra made it possible to discriminate between vulcanizates with variable amounts of sulfur crosslinks and zinc sulfide

Vertically Focusing Double Mirror Device (VF-2M) for the ESRF Beamline ID12A

A double mirror device (VF-2M) has been inserted in the monochromatic section of the ESRF beamline (BL) ID12A. It is aimed at refocusing the beam vertically very near the sample location and uses two identical bulk CVD SiC bendable mirrors in an antiparallel configuration. The benders are actuated by two UHV compatible digital piezo devices with nanometric precision (Queensgate Instruments Ltd.). The first commissioning tests at BL ID12A revealed an excellent agreement between ray-tracing predictions and experimental data. Vertical focus sizes as small as 22 um FWHM have been obtained. The measured reflectivity is very close to calculated values, too

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 non- reciprocal 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 W(1) is primarily responsible for the magne- tochiral dichroism; the time-even rank-2 tensor N(2) = [L, W](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 G(3) = [L, L, W](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

A Study Using Sulfur K-Edge XAS of Bitumens, Asphaltenes, Maltenes and their Oxidation Products by Comparison with Model Compounds

High resolution Sulfur K-edge XANES spectra have been recorded at the ESRF on "test" samples of bitumen, asphaltene and maltene fractions and various model compounds. All these fractions were found to have a low content of sulfidic species as opposed to thiophenic species and it is therefore not surprising that thermally oxidized samples (RTFOT) had also a low content (1%) of sulfoxides. This is indeed consistent with the absence of any detectable S=0 signature in the EXAFS spectra which were recorded over more than 1300 eV using the "undulator gap scan" technique