Diffraction of short X-ray pulses in the general asymmetric Laue case - an analytic treatment.

After briefly describing the concept of short X-ray pulses (delta-function), the diffraction of such a short pulse by a crystal in the asymmetric Laue case is given. The results of the dynamical theory are adopted and an analytic result for the intensity distribution behind the crystal in the diffracted direction as well as in the forward direction is given and discussed in detail. The incoming delta pulse is no longer infinitely short but shows a pronounced structure over a limited temporal or spatial region which is connected to the well known Pendellösung effect. Also the limitations of these findings are critically inspected.

Changes in fatty acid composition of Pseudomonas aeruginosa ATCC 15442 induced by growth conditions: consequences of resistance to quaternary ammonium compounds.

The effects of changes in the fatty acid composition of Pseudomonas aeruginosa induced by growth conditions on its resistance to two quaternary ammonium compounds (QAC) were investigated. The temperature and growth phase were the most influential parameters affecting the fatty acid composition of this bacterium. Furthermore, the formation of saturated fatty acids and cyclopropane fatty acids was stimulated by increasing the temperature, whereas the proportion of unsaturated fatty acids fell. The degree of saturation and the proportion of cyclopropane fatty acids increased in the course of the exponential and stationary phases. These modifications mostly concerned the inner membrane of the bacterium. Resistance of P. aeruginosa to both QAC tested was not significantly influenced by temperature and growth phase variations. Thus, resistance to the two QAC did not seem to be dependent on modifications of the fatty acid composition of the inner membrane.

First observation of nonreciprocal X-Ray gyrotropy

We report the first observation of a nonreciprocal x-ray linear dichroism caused by the time-reversal odd, real part zeta of the complex gyrotropy tensor zeta(*) which is dominated by electric dipole-electric quadrupole E1E2 interference terms. A nonreciprocal transverse anisotropy was observed in the low temperature insulating phase of a Cr doped V2O3 Mott crystal when a single antiferromagnetic domain was grown by magnetoelectric annealing along the hexagonal c axis. This new element (edge) specific spectroscopy could nicely complement x-ray magnetic circular dichroism which is silent for antiferromagnetic materials.

Effect of temperature and physiological state on the fatty acid composition of Pseudomonas aeruginosa.

The influence of temperature and physiological state on fatty acid profiles of cell membranes of a gram-negative bacteria was studied in this work. It has been shown that fatty acid composition is largely modified by these two parameters. Lipids play an important role in the composition and the function of cell membranes. These modifications of membrane structures are very important to understand because of their consequences on cell viability.

Diffraction of 100 to 200 keV X-rays from an Si1-xGex gradient crystal: comparison with results from dynamical theory.

In a large Czochralski-grown Si(1-x)Ge(x) (0.02

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.

Nanosecond-resolved XMCD on ID24 at the ESRF to investigate the element-selective dynamics of magnetization switching of Gd-Co amorphous thin film.

The one-bunch filling mode of the ESRF is combined with a microcoil to generate a pulsed-magnetic-field pump phased with respect to the probe that is given by the bunch of photons emitted each turn (357 kHz). Nanosecond-resolved X-ray magnetic circular dichroism (XMCD) is carried out. Besides the microcoil, the two other key-elements are the energy-dispersive XAS spectrometer, which yields parallel data acquisition, and the diamond-based quarter-wave plate, which tunes the helicity of the photon alternatively left and right.

Quarter-wave plates and X-ray magnetic circular dichroism on ID24 at the ESRF.

The first XMCD measurements carried out on the ID24 energy-dispersive XAS beamline at the ESRF are reported. Circular-polarized X-rays are obtained using perfect diamond crystals as quarter-wave plates. The very small source divergence allows circular polarizations close to unity to be obtained.

Design of an X-ray Phase-Plate Analyzer to Measure the Circular Polarization Rate of a Helical Undulator Source.

A quarter-wave plate made of a ca 16 mum-thick silicon single-crystal was used at energies as low as 2.8 keV to convert circularly polarized photons into linearly polarized photons. Coupled to a linear polarimeter, this quarter-wave plate enabled the characterization of the circular polarization rate of the radiation emitted by one of the ESRF helical undulators, Helios-I. The measured value (ca 97%) is in good agreement with theoretical predictions. Special attention was paid to the alignment procedures of all relevant optical components of the beamline.