Kossel Effect in Periodic Multilayers.

The Kossel effect is the diffraction by a periodically structured medium, of the characteristic X-ray radiation emitted by the atoms of the medium. We show that multilayers designed for X-ray optics applications are convenient periodic systems to use in order to produce the Kossel effect, modulating the intensity emitted by the sample in a narrow angular range defined by the Bragg angle. We also show that excitation can be done by using photons (X-rays), electrons or protons (or charged particles), under near normal or grazing incident geometries, which makes the method relatively easy to implement. The main constraint comes from the angular resolution necessary for the detection of the emitted radiation. This leads to small solid angles of detection and long acquisition times to collect data with sufficient statistical significance. Provided this difficulty is overcome, the comparison or fit of the experimental Kossel curves, i.e., the angular distributions of the intensity of an emitted radiation of one of the element of the periodic stack, with the simulated curves enables getting information on the depth distribution of the elements throughout the multilayer. Thus the same kind of information obtained from the more widespread method of X-ray standing wave induced fluorescence used to characterize stacks of nanometer period, can be obtained using the Kossel effect.

X-ray fluorescence induced by standing waves in the grazing-incidence and grazing-exit modes: study of the Mg-Co-Zr system.

The characterization of Mg-Co-Zr tri-layer stacks using X-ray fluorescence induced by X-ray standing waves, in both the grazing-incidence (GI) and the grazing-exit (GE) modes, is presented. The introduction of a slit in the direction of the detector improves the angular resolution by a factor of two and significantly improves the sensitivity of the technique for the chemical characterization of the buried interfaces. By observing the intensity variations of the Mg Kα and Co Lα characteristic emissions as a function of the incident (GI mode) or detection (GE mode) angle, it is shown that the interfaces of the Si/[Mg/Co/Zr] × 30 multilayer are abrupt, whereas in the Si/[Mg/Zr/Co] × 30 multilayer a strong intermixing occurs at the Co-on-Zr interfaces. An explanation of this opposite behavior of the Co-on-Zr and Zr-on-Co interfaces is given by the calculation of the mixing enthalpies of the Co-Mg, Co-Zr and Mg-Zr systems, which shows that the Co-Zr system presents a negative value and the other two systems present positive values. Together with the difference of the surface free energies of Zr and Co, this leads to the Mg/Zr/Co system being considered as a Mg/CoxZry bi-layer stack, with x/y estimated around 3.5.

High-efficiency B4C/Mo2C alternate multilayer grating for monochromators in the photon energy range from 0.7 to 3.4 keV.

An alternate multilayer (AML) grating has been prepared by coating an ion etched lamellar grating with a B4C/Mo2C multilayer (ML) having a layer thickness close to the groove depth. Such a structure behaves as a 2D synthetic crystal and can reach very high efficiencies when the Bragg condition is satisfied. This AML coated grating has been characterized at the SOLEIL Metrology and Tests Beamline between 0.7 and 1.7 keV and at the four-crystal monochromator beamline of Physikalisch-Technische Bundesanstalt (PTB) at BESSY II between 1.75 and 3.4 keV. A peak diffraction efficiency of nearly 27% was measured at 2.2 keV. The measured efficiencies are well reproduced by numerical simulations made with the electromagnetic propagation code CARPEM. Such AML gratings, paired with a matched ML mirror, constitute efficient monochromators for intermediate energy photons. They will extend the accessible energy for many applications as x-ray absorption spectroscopy or x-ray magnetic circular dichroism experiments.

Polarization-dependent vacuum-ultraviolet reflectometry using elliptically polarized synchrotron radiation.

In the laboratory of the Physikalisch-Technische Bundesanstalt (PTB) at the Berlin electron-storage ring BESSY II, a procedure has been developed to investigate the dependence of vacuum-ultraviolet reflection on polarization. It is based on characterizing the elliptically polarized synchrotron radiation at PTB's normal-incidence monochromator beamline for reflectometry by means of polarization-sensitive photodetectors. For this purpose, the polarization dependency in the detector responsivity was determined at a small, low, solid angle of acceptance for the synchrotron radiation, i.e., within the orbital plane of the storage ring where the degree of linear polarization is known to be almost 100%. Our method allows the polarization dependence of reflection samples to be measured with relative standard uncertainties ranging from 2.4% to 11% in the spectral range between 60 and 160 nm. The method has been applied to the optimization of polarizing mirrors at the Lyman-alpha wavelength of 121.6 nm.

Study of normal incidence of three-component multilayer mirrors in the range 20-40 nm.

We study theoretically and experimentally the increase of normal incidence reflectivity generated by addition of a third material in the period of a standard periodic multilayer, for wavelengths in the range 20 to 40 nm. The nature and thickness of the three materials has been optimized to provide the best enhancement of reflectivity. Theoretical reflectivity of an optimized B4C/Mo/Si multilayer reaches 42% at 32 nm. B4C/Mo/Si multilayers have been deposited with a magnetron sputtering system and a reflectivity of 34% at 32 nm has been measured on a synchrotron radiation source.

X-ray-ultraviolet beam splitters for the Michelson interferometer.

With the aim of realizing a Michelson interferometer working at 13.9 nm, we have developed a symmetrical beam splitter with multilayers deposited on the front and back sides of a silicon nitride membrane. On the basis of the experimental optical properties of the membrane, simulations have been performed to define the multilayer structure that provides the highest reflectivity-transmission product. Optimized Mo-Si multilayers have been successfully deposited on both sides of t he membrane by use of the ion-beam sputtering technique, with a thickness-period reproducibility of 0.1 nm. Measurements by means of synchrotron radiation at 13.9 nm and at an angle of 45 degrees provide a reflectivity of 14.2% and a transmission of 15.2% for a 60% s-polarized light, close to the simulated values. Such a beam splitter has been used for x-ray laser Michelson interferometry at 13.9 nm. The first interferogram is discussed.

X-ray multilayer monochromator with enhanced performance.

An x-ray multilayer monochromator with improved resolution and a low specular background is presented. The monochromator consists of a lamellar multilayer amplitude grating with appropriate parameters used at the zeroth diffraction order. The device is fabricated by means of combining deposition of thin films on a nanometer scale, UV lithography, and reactive ion etching. The performance of this new monochromator at photon energies near 1500 eV is shown.