Maria Brzhezinskaya

A novel monochromator for experiments with ultrashort X‐ray pulses

Aiming at advancing storage-ring-based ultrafast X-ray science, over the past few years many upgrades have been undertaken to continue improving beamline performance and photon flux at the Femtoslicing facility at BESSY II. In this article the particular design upgrade of one of the key optical components, the zone-plate monochromator (ZPM) beamline, is reported. The beamline is devoted to optical pump/soft X-ray probe applications with 100 fs (FWHM) X-ray pulses in the soft X-ray range at variable polarization. A novel approach consisting of an array of nine off-axis reflection zone plates is used for a gapless coverage of the spectral range between 410 and 1333 eV at a designed resolution of E/ΔE = 500 and a pulse elongation of only 30 fs. With the upgrade of the ZPM the following was achieved: a smaller focus, an improved spectral resolution and bandwidth as well as excellent long-term stability. The beamline will enable a new class of ultrafast applications with variable optical excitation wavelength and variable polarization.

Monochromatization of femtosecond XUV light pulses with the use of reflection zone plates.

We report on a newly built laser-based tabletop setup which enables generation of femtosecond light pulses in the XUV range employing the process of high-order harmonic generation (HHG) in a gas medium. The spatial, spectral, and temporal characteristics of the XUV beam are presented. Monochromatization of XUV light with minimum temporal pulse distortion is the central issue of this work. Off-center reflection zone plates are shown to be advantageous when selection of a desired harmonic is carried out with the use of a single optical element. A cross correlation technique was applied to characterize the performance of the zone plates in the time domain. By using laser pulses of 25 fs length to pump the HHG process, a pulse duration of 45 fs for monochromatized harmonics was achieved in the present setup.

X-ray absorption spectroscopy using a self-seeded soft X-ray free-electron laser

X-ray free electron lasers (XFELs) enable unprecedented new ways to study the electronic structure and dynamics of transition metal systems. L-edge absorption spectroscopy is a powerful technique for such studies and the feasibility of this method at XFELs for solutions and solids has been demonstrated. However, the required x-ray bandwidth is an order of magnitude narrower than that of self-amplified spontaneous emission (SASE), and additional monochromatization is needed. Here we compare L-edge x-ray absorption spectroscopy (XAS) of a prototypical transition metal system based on monochromatizing the SASE radiation of the linac coherent light source (LCLS) with a new technique based on self-seeding of LCLS. We demonstrate how L-edge XAS can be performed using the self-seeding scheme without the need of an additional beam line monochromator. We show how the spectral shape and pulse energy depend on the undulator setup and how this affects the x-ray spectroscopy measurements.

Design and optimization of a parallel spectrometer for ultra-fast X-ray science.

In the present work, different varied line space (VLS) and reflection zone plate (RZP) gratings are analyzed for their suitability in low-signal femtosecond soft X-ray spectroscopy. The need for high efficiency suggests a straightened focal line whose sharpness and residual curvature will determine the quality. One- and two-dimensional VLS structures feature an attractive trade-off between a sufficient optical performance and a strongly relaxed fabrication, due to moderate line densities which are easily accessible by e-beam lithography. Based on fanned-out RZP arrays, their continuous limit version is identified to generate an almost perfect focal line however, with an aberration level three orders of magnitude better than for the VLS gratings and well below the diffraction limit over large acceptance angles.

Femtosecond high-resolution hard X-ray spectroscopy using reflection zone plates.

An off-axis total external reflection zone plate is applied to wavelength-dispersive X-ray spectrometry in the range from 7.8 keV to 9.0 keV. The resolving power E/ΔE of up to 1.1 × 10(2), demonstrated in a synchrotron proof-of-concept experiment, competes well with existing energy-dispersive instruments in this spectral range. In conjunction with the detection efficiency of (2.2 ± 0.6)%, providing a fairly constant count rate across the 1.2 keV band, the temporal pulse elongation to no more than 1.5 × 10(-15) s opens the door to wide-range, ultra-fast hard X-ray spectroscopy at free-electron lasers (FELs).

Reflection zone plate concept for resonant inelastic x-ray scattering spectrometry

We simulate a proof-of-principle design of a wavelength dispersive, parallel spectrometer for use in resonant inelastic x-ray scattering (RIXS). The instrument relies on a multiple-channel reflection zone plate (RZP) array, enabling the recording of fluorescence spectra from an acceptance angle of 18  arc min×19  arc min with a mainly source-size-limited resolving power of (0.2-2.6)×104 over an energy range of 21 eV at the L-edge of Fe around 715 eV. An optimal two-dimensional signal readout preserves the spectral resolution to a large extent for widely open exit apertures of ≳50  mm2. The geometrical parameters are matched to the PEAXIS end station at the BESSY II synchrotron facility, and relaxed RZP line densities of <9×102  mm-1 assure the technical feasibility. An error budget estimation with respect to fabrication and alignment tolerances provides the link to real, RZP-based RIXS experiments in the future.

Morphology, Atomic and Electronic Structure of Metal Oxide (CuOx, SnOx) Nanocomposites and Thin Films

We investigated the surface morphology , oxidation state , local and electronic structure of a number of metal oxide (CuOx, SnOx) nanoparticles , nanocomposites and thin films based on them, synthesized by electrochemical deposition and sol-gel techniques. Surface morphological studies were carried out using a SEM (scanning electron microscopy). The bonding structure with element sensitivity to Cu and Sn, local environment and electronic structure were studied by the XPS (X-ray photoelectron spectroscopy) and the XANES (X-ray absorption near edge structure ) spectroscopy. The changes in local and electronic structure , oxidation state and the surface morphology of nanocomposites in the interaction of metal oxide nanoparticles with CNT (carbon nanotubes) and SiO2 (silicon oxide) matrices were observed. We discovered that the shape and size of metal oxide nanoparticles depend on metal oxidation state as well as parameters of synthesis and, moreover, kind of matrix affect in direction of the nano-unit nucleation and subsequent organization of metal oxide nanoparticles /microcrystals.

Dedicated software for diffractive optics design and simulation

An efficient software package for the structure design and simulation of imaging properties of diffraction optical elements has been developed. It operates with point source and consists of: the ZON software, to calculate the structure of an optical element in transmission and reflection; the KRGF software, to simulate the diffraction properties of an ideal optical element with point source; the DS software, to calculate the diffraction properties by taking into consideration material and shadowing effects. Optional software allows simulation with a real non-point source. Zone plate thickness profile, source shape as well as substrate curvature are considered in this calculation. This is especially important for the diffractive focusing elements and gratings at a total external reflection, given that the lateral size of the structure can be up to 1 m. The program package can be used in combination with the Nanomaker software to prepare data for ion and e-beam surface modifications and corrections.

Copper-oxide metalorganic nanocomposite: morphological and X-ray spectroscopy studies

Oxidation state, local and electronic structure as well as the surface morphology of a CuOx nanocomposites were investigated. The research is focused on studying the relationships between surface morphology and structure of the inorganic component of the nanocomposites. The physico-chemical mechanisms of change in local and electronic structure, oxidation state and the surface morphology of nanocomposites in the interaction of metal oxide nanoparticles with different types of matrices were determined.

Influence of the surface morphology and structure on the gas-sorption properties of SiO2CuOx nanocomposite materials: X-ray spectroscopy investigations

Thin films of SiO2CuOx nanocomposite materials were synthesized by the sol-gel method upon deposition of solutions containing 1, 3, 5, and 7 wt % Cu. The scanning electron microscopy examination of the surface morphology revealed that a change in the copper concentration in the initial solution has an influence on the size and amount of crater-like pores formed in the amorphous silicon dioxide matrix and on the localization of copper crystallites on the surface of the films. X-ray absorption near-edge structure (XANES) spectroscopy and X-ray photoelectron spectroscopy (XPS) investigations showed that the structure of crystallites is predominantly formed by divalent copper oxide (CuO). However, an increase in the copper concentration in the initial solution leads to a systematic increase in the content of the Cu2O phase. At copper concentrations of 1 and 7 wt %, the surface layers are most likely characterized by the formation of several divalent copper oxides (Cu(OH)2, CuO, CuSiO3), which results in the deterioration of the gas-sensitive characteristics of the material. It was established that the optimum set of parameters (the presence of pores, localization of crystallites, copper phase composition in the crystallites) for the best gas-sensitive characteristics of SiO2CuOx composite films is observed at copper concentrations of 3 and 5 wt % in the initial solution.