C. Goulon-Ginet

X-ray natural circular dichroism in a uniaxial gyrotropic single crystal of LiIO3

We produce the first experimental evidence of x-ray natural circular dichroism (XNCD) in a uniaxial gyrotropic crystal of α-LiIO3 that is known to crystallize with space group P63 and to exhibit a very strong nonlinear susceptibility. For the sake of simplicity, the optical axis was set colinear to the direction of the exciting x-ray beam in order to get rid of undesirable birefringence and linear dichroism effects. The nicely structured XNCD spectra recorded at the LI, LII, and LIII edges of iodine are assigned to the electric dipole–electric quadrupole (E1.E2) interference terms which do not vanish in gyrotropic crystals but contribute to a second order polarizability. Our interpretation is consistent with a band structure calculation and is fully supported by ab initio multiple scattering simulations. The signatures recorded at the LII and LIII edges are similar and have the same sign. This is in contrast to x-ray magnetic circular dichroism (XMCD) spectra which usually exhibit opposite signs at the LI...

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.

Instrumentation Developments for X-ray Linear and Circular Dichroism at the ESRF Beamline ID12A

This paper reports on the performance of the instrumentation developed for the ESRF beamline ID12A, which is dedicated to spectroscopic applications requiring full control of the polarization at energies >/=2.0 keV. Emphasis is placed on the characterization of various optical components of the beamline and on problems associated with either the control or the conversion of the polarization state. A few examples have been selected to illustrate what sort of new spectroscopic information has already been obtained at the beamline. These include the comparison of X-ray magnetic dichroism spectra recorded with linear or circular polarization and the very first detection of X-ray natural circular dichroism in single crystals known to exhibit a very large non-linear susceptibility at optical wavelengths.

Instrumentation Developments for Polarization Dependent X-ray Spectroscopies

This paper reports on advanced instrumentation developed for the ESRF beam line ID12A which is dedicated to polarization dependent x-ray absorption, excitation and emission spectroscopies at photon energies hν ≥ 2.0 keV. Emphasis is laid on problems associated with either the control or the conversion of the polarization state of the x-ray photons. Several examples have been selected to illustrate the performances of the beam line and show what type of novel spectroscopic information can be obtained.

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.

Investigation of uranium edges in by x-ray magnetic circular dichroism

We report a high statistic x-ray magnetic circular dichroism study of carried out at the uranium edges. Using the sum rules, the detailed analysis of our data yields orbital and spin moments of the 5f uranium electrons in reasonable agreement with values extracted from a localized moment approach.

Structural characterization by X-ray absorption spectroscopy (EXAFS/XANES) of the vanadium chemical environment in Boscan asphaltenes

X-Ray absorption spectroscopies, EXAFS (extended X-ray absorption fine structure) and XANES (X-ray absorption near-edge structure), were used for probing the structural and chemical environment of vanadium in two different asphaltenic fractions of a Boscan crude oil. Although the level of porphyrins detectable by u.v.–visible spectrophotometry was substantially reduced (i.e. <15% from the 4 000 p.p.m. of metal) due to successive solvent extractions, the EXAFS spectra of these fractions were still largely dominated by the typical pattern of vanadyl porphyrins, whereas the intense prepeak observed in the XANES spectra also clearly supported the presence of quite large amounts of porphyrins in these fractions. We thus suggest that the u.v.–visible spectrophotometric analyses tend systematically to underestimate the porphyrinic content in these heavy fractions, probably due to some hardly detectable microheterogeneity of the test solutions. E.s.r. spectra are produced which clearly support these various conclusions. Much more elaborate separation procedures are required in order to isolate and identify any ‘non-porphyrinic vanadium fraction’ from Boscan crudes.

Vector part of optical activity probed with x-rays in hexagonal ZnO

We discuss how measurements of x-ray circular intensity differentials (XCIDs) could be used to probe the Voigt-Fedorov vector part of optical activity (OA) due to electric dipole-electric quadrupole (E 1E 2) interferences. The first experiment was carried out on a single crystal of zincite (hexagonal ZnO: class 6mm). XCID spectra were recorded in the x-ray resonant diffraction regime near the Zn K edge using the (300) reflection at Bragg angles near 45°. In the x-ray range, the effective operator responsible for the vector part of OA can be assigned to the vector product \mathbf {L} \times {\bOmega }_{L}-{\bOmega }_{L} \times {\mathbf {L}} \propto {\mathbf {n}} , in which L and ΩL are time-reversal odd operators associated with the orbital angular momentum and the orbital anapole respectively, whereas n is a true time-reversal even electric dipole. This is consistent with the pyroelectric properties of zincite crystals.

Optical activity probed with x-rays

Optical activity (OA) was only recently discovered in the x-ray range. It is caused mainly by the electric dipole–electric quadrupole E1E2 interference terms which mix multipole moments of opposite parity and exist only in structures with odd space parity. OA related phenomena can be either even or odd with respect to time reversal: natural OA is concerned with time-reversal even effects whereas non-reciprocal OA refers to time-reversal odd contributions. Various types of x-ray dichroisms related to either natural or non-reciprocal OA have been detected and are reviewed in the present paper. Extra emphasis is put on two different non-reciprocal effects that were observed in selected magnetoelectric materials: (i) x-ray magnetochiral dichroism (XMχ D) and (ii) the non-reciprocal x-ray magnetic linear dichroism. Edge-selective OA sum rules have recently been established which show that the effective operator of XMχ D is the orbital anapole moment at the absorbing centre.

X-ray natural circular dichroism of gyrotropic crystals.

X-ray natural circular dichroism of gyrotropic crystals crystals due to the substantial contribution in the X-ray range of the Electric Dipole-Electric Quadrupole (El.E2) interference terms first investigated by Barron at optical wavelengths (Barron, 1971). We will discuss first which crystal class is appropriate to detect XNCD and we will give a general formulation of the XNCD cross sections o (El .E2).