High‐resolution x‐ray analysis with multilayer gratings

Periodic multilayers are nowadays widely used to perform x‐ray analysis in the soft x‐ray range (photon energy lower than 1 keV). However, they do not permit to obtain high‐resolution spectra such as natural or synthetic crystals. Thus, multilayers cannot resolve interferences between close x‐ray lines. It has been shown and demonstrated experimentally that patterning a grating profile within a multilayer structure leads to a diffractive optics with improved resolving power. We illustrate the use of an Mo/B₄C multilayer grating in the Fe L and C K spectral ranges, around 700 and 280 eV, respectively. First, in the Fe L range, the improved spectral resolution enables us to distinguish the Fe Lα and Lβ emissions (separated by 13 eV). In addition, using a sample made of a mix of LiF and an iron ore, we show that it is possible to easily resolve the F K and Fe L emissions. These examples demonstrate that an improved x‐ray analysis can be obtained with multilayer gratings when there is the need to study samples having elements giving rise to close emission lines. Second, in the C K range, by comparing C Kα spectra from B₄C and cellulose, we show that the shape of the emission band is sensitive to the chemical state of the carbon atom. Copyright © 2009 John Wiley & Sons, Ltd.     

Characterization of Pd/Y multilayers with B4C barrier layers using GIXR and X‐ray standing wave enhanced HAXPES

Pd/Y multilayers are high‐reflectance mirrors designed to work in the 7.5–11 nm wavelength range. Samples, prepared by magnetron sputtering, are deposited with or without B₄C barrier layers located at the interfaces of the Pd and Y layers to reduce interdiffusion, which is expected from calculating the mixing enthalpy of Pd and Y. Grazing‐incident X‐ray reflectometry is used to characterize these multilayers. B₄C barrier layers are found to be effective in reducing Pd–Y interdiffusion. Details of the composition of the multilayers are revealed by hard X‐ray photoemission spectroscopy with X‐ray standing wave effects. This consists of measuring the photoemission intensity from the samples by performing an angular scan in the region corresponding to the multilayer period and an incident photon energy according to Bragg's law. The experimental results indicate that Pd does not chemically react with B nor C at the Pd–B₄C interface while Y does react at the Y–B₄C interface. The formation of Y–B or Y–C chemical compounds could be the reason why the interfaces are stabilized. By comparing the experimentally obtained angular variation of the characteristic photoemission with theoretical calculations, the depth distribution of each component element can be interpreted.     

A sagittally focusing double‐multilayer monochromator for ultrafast X‐ray imaging applications

The development of a sagittally focusing double‐multilayer monochromator is reported, which produces a spatially extended wide‐bandpass X‐ray beam from an intense synchrotron bending‐magnet source at the Advanced Photon Source, for ultrafast X‐ray radiography and tomography applications. This monochromator consists of two W/B₄C multilayers with a 25 Å period coated on Si single‐crystal substrates. The second multilayer is mounted on a sagittally focusing bender, which can dynamically change the bending radius of the multilayer in order to condense and focus the beam to various points along the beamline. With this new apparatus, it becomes possible to adjust the X‐ray beam size to best match the area detector size and the object size to facilitate more efficient data collection using ultrafast X‐ray radiography and tomography.     

Comparative study of multilayers used in monochromators for synchrotron‐based coherent hard X‐ray imaging

A systematic study is presented in which multilayers of different composition (W/Si, Mo/Si, Pd/B₄C), periodicity (from 2.5 to 5.5 nm) and number of layers have been characterized. In particular, the intrinsic quality (roughness and reflectivity) as well as the performance (homogeneity and coherence of the outgoing beam) as a monochromator for synchrotron radiation hard X‐ray micro‐imaging are investigated. The results indicate that the material composition is the dominating factor for the performance. By helping scientists and engineers specify the design parameters of multilayer monochromators, these results can contribute to a better exploitation of the advantages of multilayer monochromators over crystal‐based devices; i.e. larger spectral bandwidth and high photon flux density, which are particularly useful for synchrotron‐based micro‐radiography and ‐tomography.     

Interface characterization of B4C‐based multilayers by X‐ray grazing‐incidence reflectivity and diffuse scattering

B₄C‐based multilayers have important applications for soft to hard X‐rays. In this paper, X‐ray grazing‐incidence reflectivity and diffuse scattering, combining various analysis methods, were used to characterize the structure of B₄C‐based multilayers including layer thickness, density, interfacial roughness, interdiffusion, correlation length, etc. Quantitative results for W/B₄C, Mo/B₄C and La/B₄C multilayers were compared. W/B₄C multilayers show the sharpest interfaces and most stable structures. The roughness replications of La/B₄C and Mo/B₄C multilayers are not strong, and oxidations and structure expansions are found in the aging process. This work provides guidance for future fabrication and characterization of B₄C‐based multilayers.     

La/B4C small period multilayer interferential mirror for the analysis of boron

An La/B₄C multilayer interferential mirror with small period d (4.8 nm) was produced by diode sputtering for the detection of the boron K emission by wavelength‐dispersive x‐ray spectrometry at a large Bragg angle (close to 45°). The structure of the mirror was characterized by grazing incidence x‐ray reflectometry and its performance at the energy of the boron K emission (183 eV) was evaluated by means of polarized synchrotron radiation. Spectrometric measurements showed that the La/B₄C mirror improved the detection limit of boron using by a factor of 2 with respect to similar Mo/B₄C mirrors and by a factor of 4 with respect to a lead stearate crystal. Copyright © 2005 John Wiley & Sons, Ltd.     

Characterization of EUV periodic multilayers

Nanometric Co/Mg, Co/Mg/B₄C, Al/SiC and Al/Mo/SiC periodic multilayers deposited by magnetron sputtering are studied in order to correlate their optical performances in the extreme ultraviolet (EUV) range to their structural quality. To that purpose, our recently developed methodology based on high‐resolution X‐ray emission spectroscopy (XES) and X‐ray and EUV reflectometry is now extended to nuclear magnetic resonance (NMR) spectroscopy and time‐of‐flight secondary ions mass spectrometry (ToF‐SIMS). The analysis of the Co Lαβ and Mg Kβ emission spectra shows that the Co and Mg atoms within the multilayers are in a chemical state equivalent to that of the atoms in the pure Co and Mg references, respectively. But NMR spectra give evidence for a reaction between Co atoms and B and/or C atoms from B₄C. The Al and Si Kβ emission spectra do not reveal the formation of an interfacial compound in Al/SiC and Al/Mo/SiC. Only the roughness limits the optical quality of Al/SiC. The comparative analysis of the ToF‐SIMS spectra of Al/SiC and Al/Mo/SiC indicates that the structural quality is enhanced when Mo is introduced within the stack. Copyright © 2011 John Wiley & Sons, Ltd.     

Collimating Montel mirror as part of a multi‐crystal analyzer system for resonant inelastic X‐ray scattering

Advances in resonant inelastic X‐ray scattering (RIXS) have come in lockstep with improvements in energy resolution. Currently, the best energy resolution at the Ir L₃‐edge stands at ～25 meV, which is achieved using a diced Si(844) spherical crystal analyzer. However, spherical analyzers are limited by their intrinsic reflection width. A novel analyzer system using multiple flat crystals provides a promising way to overcome this limitation. For the present design, an energy resolution at or below 10 meV was selected. Recognizing that the angular acceptance of flat crystals is severely limited, a collimating element is essential to achieve the necessary solid‐angle acceptance. For this purpose, a laterally graded, parabolic, multilayer Montel mirror was designed for use at the Ir L₃‐absorption edge. It provides an acceptance larger than 10 mrad, collimating the reflected X‐ray beam to smaller than 100 µrad, in both vertical and horizontal directions. The performance of this mirror was studied at beamline 27‐ID at the Advanced Photon Source. X‐rays from a diamond (111) monochromator illuminated a scattering source of diameter 5 µm, generating an incident beam on the mirror with a well determined divergence of 40 mrad. A flat Si(111) crystal after the mirror served as the divergence analyzer. From X‐ray measurements, ray‐tracing simulations and optical metrology results, it was established that the Montel mirror satisfied the specifications of angular acceptance and collimation quality necessary for a high‐resolution RIXS multi‐crystal analyzer system.     

Finite‐element modelling of multilayer X‐ray optics

Multilayer optical elements for hard X‐rays are an attractive alternative to crystals whenever high photon flux and moderate energy resolution are required. Prediction of the temperature, strain and stress distribution in the multilayer optics is essential in designing the cooling scheme and optimizing geometrical parameters for multilayer optics. The finite‐element analysis (FEA) model of the multilayer optics is a well established tool for doing so. Multilayers used in X‐ray optics typically consist of hundreds of periods of two types of materials. The thickness of one period is a few nanometers. Most multilayers are coated on silicon substrates of typical size 60 mm × 60 mm × 100–300 mm. The high aspect ratio between the size of the optics and the thickness of the multilayer (10⁷) can lead to a huge number of elements for the finite‐element model. For instance, meshing by the size of the layers will require more than 10¹⁶ elements, which is an impossible task for present‐day computers. Conversely, meshing by the size of the substrate will produce a too high element shape ratio (element geometry width/height > 10⁶), which causes low solution accuracy; and the number of elements is still very large (10⁶). In this work, by use of ANSYS layer‐functioned elements, a thermal‐structural FEA model has been implemented for multilayer X‐ray optics. The possible number of layers that can be computed by presently available computers is increased considerably.     

Pressure study of monoclinic ReO2 up to 1.2 GPa using X‐ray absorption spectroscopy and X‐ray diffraction

The crystal and local atomic structure of monoclinic ReO₂ (α‐ReO₂) under hydrostatic pressure up to 1.2 GPa was investigated for the first time using both X‐ray absorption spectroscopy and high‐resolution synchrotron X‐ray powder diffraction and a home‐built B₄C anvil pressure cell developed for this purpose. Extended X‐ray absorption fine‐structure (EXAFS) data analysis at pressures from ambient up to 1.2 GPa indicates that there are two distinct Re—Re distances and a distorted ReO₆ octahedron in the α‐ReO₂ structure. X‐ray diffraction analysis at ambient pressure revealed an unambiguous solution for the crystal structure of the α‐phase, demonstrating a modulation of the Re—Re distances. The relatively small portion of the diffraction pattern accessed in the pressure‐dependent measurements does not allow for a detailed study of the crystal structure of α‐ReO₂ under pressure. Nonetheless, a shift and reduction in the (011) Bragg peak intensity between 0.4 and 1.2 GPa is observed, with correlation to a decrease in Re—Re distance modulation, as confirmed by EXAFS analysis in the same pressure range. This behavior reveals that α‐ReO₂ is a possible inner pressure gauge for future experiments up to 1.2 GPa.     

A dedicated small‐angle X‐ray scattering beamline with a superconducting wiggler source at the NSRRC

At the National Synchrotron Radiation Research Center (NSRRC), which operates a 1.5 GeV storage ring, a dedicated small‐angle X‐ray scattering (SAXS) beamline has been installed with an in‐achromat superconducting wiggler insertion device of peak magnetic field 3.1 T. The vertical beam divergence from the X‐ray source is reduced significantly by a collimating mirror. Subsequently the beam is selectively monochromated by a double Si(111) crystal monochromator with high energy resolution (ΔE/E? 2 × 10^?4) in the energy range 5–23 keV, or by a double Mo/B₄C multilayer monochromator for 10–30 times higher flux (～10¹¹ photons s^?1) in the 6–15 keV range. These two monochromators are incorporated into one rotating cradle for fast exchange. The monochromated beam is focused by a toroidal mirror with 1:1 focusing for a small beam divergence and a beam size of ～0.9 mm × 0.3 mm (horizontal × vertical) at the focus point located 26.5 m from the radiation source. A plane mirror installed after the toroidal mirror is selectively used to deflect the beam downwards for grazing‐incidence SAXS (GISAXS) from liquid surfaces. Two online beam‐position monitors separated by 8 m provide an efficient feedback control for an overall beam‐position stability in the 10 µm range. The beam features measured, including the flux density, energy resolution, size and divergence, are consistent with those calculated using the ray‐tracing program SHADOW. With the deflectable beam of relatively high energy resolution and high flux, the new beamline meets the requirements for a wide range of SAXS applications, including anomalous SAXS for multiphase nanoparticles (e.g. semiconductor core‐shell quantum dots) and GISAXS from liquid surfaces.     

Integration method for directly analyzing interface statistics of periodic multilayers from X‐ray scattering

An integration method is demonstrated for directly determining the average interface statistics of periodic multilayers from the X‐ray scattering diagram. By measuring the X‐ray scattering diagram in the out‐of‐plane geometry and integrating the scattered intensity along the vertical momentum transfer q_z in an interval, which is decided by the thickness ratio Γ (ratio of sublayer's thickness to periodic thickness), the cross‐correlations between different interfaces are canceled and only the autocorrelations are reserved. Then the multilayer can be treated as a `single interface' and the average power spectral density can be obtained without assuming any vertical correlation model. This method has been employed to study the interface morphology of sputter‐deposited W/Si multilayers grown at an Ar pressure of 1–7 mTorr. The results show an increase in vertical correlation length and a decrease in lateral correlation length with increased Ar pressure. The static roughness exponent α = 0 and dynamic growth exponent z = 2 indicate the Edwards–Wilkinson growth model at an Ar pressure of 1–5 mTorr. At an Ar pressure of 7 mTorr, α = 0.35 and z = 1.65 indicate the Kardar–Parisi–Zhang growth model.     

Implementation of ultrafast X‐ray diffraction at the 1W2B wiggler beamline of Beijing Synchrotron Radiation Facility

The implementation of a laser pump/X‐ray probe scheme for performing picosecond‐resolution X‐ray diffraction at the 1W2B wiggler beamline at Beijing Synchrotron Radiation Facility is reported. With the hybrid fill pattern in top‐up mode, a pixel array X‐ray detector was optimized to gate out the signal from the singlet bunch with interval 85 ns from the bunch train. The singlet pulse intensity is ～2.5 × 10⁶ photons pulse^?1 at 10 keV. The laser pulse is synchronized to this singlet bunch at a 1 kHz repetition rate. A polycapillary X‐ray lens was used for secondary focusing to obtain a 72 µm (FWHM) X‐ray spot. Transient photo‐induced strain in BiFeO₃ film was observed at a ～150 ps time resolution for demonstration.     

Highly efficient beamline and spectrometer for inelastic soft X‐ray scattering at high resolution

The design, construction and commissioning of a beamline and spectrometer for inelastic soft X‐ray scattering at high resolution in a highly efficient system are presented. Based on the energy‐compensation principle of grating dispersion, the design of the monochromator–spectrometer system greatly enhances the efficiency of measurement of inelastic soft X‐rays scattering. Comprising two bendable gratings, the set‐up effectively diminishes the defocus and coma aberrations. At commissioning, this system showed results of spin‐flip, d–d and charge‐transfer excitations of NiO. These results are consistent with published results but exhibit improved spectral resolution and increased efficiency of measurement. The best energy resolution of the set‐up in terms of full width at half‐maximum is 108 meV at an incident photon energy tuned about the Ni L₃‐edge.     

A miniature X‐ray emission spectrometer (miniXES) for high‐pressure studies in a diamond anvil cell

Core–shell X‐ray emission spectroscopy (XES) is a valuable complement to X‐ray absorption spectroscopy (XAS) techniques. However, XES in the hard X‐ray regime is much less frequently employed than XAS, often as a consequence of the relative scarcity of XES instrumentation having energy resolutions comparable with the relevant core‐hole lifetimes. To address this, a family of inexpensive and easily operated short‐working‐distance X‐ray emission spectrometers has been developed. The use of computer‐aided design and rapid prototype machining of plastics allows customization for various emission lines having energies from ～3 keV to ～10 keV. The specific instrument described here, based on a coarsely diced approximant of the Johansson optic, is intended to study volume collapse in Pr metal and compounds by observing the pressure dependence of the Pr Lα emission spectrum. The collection solid angle is ～50 msr, roughly equivalent to that of six traditional spherically bent crystal analyzers. The miniature X‐ray emission spectrometer (miniXES) methodology will help encourage the adoption and broad application of high‐resolution XES capabilities at hard X‐ray synchrotron facilities.     

Direct nano‐scale patterning of Ag films using hard X‐ray induced oxidation

The morphological change of silver nano‐particles (AgNPs) exposed to an intense synchrotron X‐ray beam was investigated for the purpose of direct nano‐scale patterning of metal thin films. AgNPs irradiated by hard X‐rays in oxygen ambient were oxidized and migrated out of the illuminated region. The observed X‐ray induced oxidation was utilized to fabricate nano‐scale metal line patterns using sectioned WSi₂/Si multilayers as masks. Lines with a width as small as 21 nm were successfully fabricated on Ag films on silicon nitride. Au/Ag nano‐lines were also fabricated using the proposed method.     

Inference of the electron temperature in inertial confinement fusion implosions from the hard X‐ray spectral continuum

Using the free‐free continuum self‐emission spectrum at photon energies above 15 keV is one of the most promising concepts for assessing the electron temperature in inertial confinement fusion (ICF) experiments. However, these photons are due to suprathermal electrons whose mean free path is much larger than the thermal one, making their distribution deviate from Maxwellian in a finite‐size hotspot. The first study of the free‐free X‐ray emission from an ICF implosion is conducted, accounting for the kinetic modifications to the electron distribution. These modifications are found to result in qualitatively new features in the hard X‐ray spectral continuum. Inference of the electron temperature as if the emitting electrons are Maxwellian is shown to give a lower value than the actual one.     

Performance of a four‐element Si drift detector for X‐ray absorption fine‐structure spectroscopy: resolution, maximum count rate,and dead‐time correction with incorporation into the ATHENA data analysis software

The performance of a four‐element Si drift detector for energy‐dispersive fluorescence‐yield X‐ray absorption fine‐structure measurements is reported, operating at the National Institute of Standards and Technology beamline X23A2 at the National Synchrotron Light Source. The detector can acquire X‐ray absorption fine‐structure spectra with a throughput exceeding 4 × 10⁵ counts per second per detector element (>1.6 × 10⁶ total counts per second summed over all four channels). At this count rate the resolution at 6 keV is approximately 220 eV, which adequately resolves the Mn Kα and Kβ fluorescence lines. Accurate dead‐time correction is demonstrated, and it has been incorporated into the ATHENA data analysis program. To maintain counting efficiency and high signal to background, it is suggested that the incoming count rate should not exceed ～70% of the maximum throughput.     

Adaption of a diffractometer for time‐resolved X‐ray resonant magnetic scattering

A new set‐up is presented to measure element‐selective magnetization dynamics using the ALICE chamber [Grabis et al. (2003), Rev. Sci. Instrum. 74 , 4048–4051] at the BESSY II synchrotron at the Helmholtz‐Zentrum Berlin. A magnetic‐field pulse serves as excitation, and the magnetization precession is probed by element‐selective X‐ray resonant magnetic scattering. With the use of single‐bunch‐generated X‐rays a temporal resolution well below 100 ps is reached. The ALICE diffractometer environment enables investigations of thin films, described here, multilayers and laterally structured samples in reflection or diffuse scattering geometry. The combination of the time‐resolved set‐up with a cryostat in the ALICE chamber will allow temperature‐dependent studies of precessional magnetization dynamics and of damping constants to be conducted over a large temperature range and for a large variety of systems in reflection geometry.     

Fluorinated Eu‐Doped SnO2 Nanostructures with Simultaneous Phase and Shape Control and Improved Photoluminescence

Fluorinated Eu‐doped SnO₂ nanostructures with tunable morphology (shuttle‐like and ring‐like) are prepared by a hydrothermal method, using NaF as the morphology controlling agent. X‐ray diffraction, field‐emission scanning electron microscopy, high‐resolution transmission electron microscopy, X‐ray photoelectron spectroscopy, and energy dispersive spectroscopy are used to characterize their phase, shape, lattice structure, composition, and element distribution. The data suggest that Eu³⁺ ions are uniformly embedded into SnO₂ nanocrystallites either through substitution of Sn⁴⁺ ions or through formation of Eu‐F bonds, allowing for high‐level Eu³⁺ doping. Photoluminescence features such as transition intensity ratios and Stark splitting indicate diverse localization of Eu³⁺ ions in the SnO₂ nanoparticles, either in the crystalline lattice or in the grain boundaries. Due to formation of Eu‐F and Sn‐F bonds, the fluorinated surface of SnO₂ nanocrystallites efficiently inhibits the hydroxyl quenching effect, which accounts for their improved photoluminescence intensity.