Heike Löchel

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.

Integration of moth-eye structures into a poly(dimethylsiloxane) stamp for the replication of functionalized microlenses using UV-nanoimprint lithography

The increasing demand for low cost camera modules for mobile devices requires technological solutions for the manufacturing process. One of the most promising fabrication processes for microlenses for camera modules is UV-nanoimprint lithography. In a typical fabrication process, an elastomer stamp is used to replicate microlenses. In this work, a method is presented to integrate moth-eye structures as an antireflective layer into a poly(dimethylsiloxane) (PDMS) stamp containing a microlens array. The integration of these structures is done by a thermoforming process. Due to the integration of the moth-eye structures into the PDMS stamp, the optical performance of the replicated microlenses can be improved and no additional processing steps are necessary after the replication process.

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).

Soft x-ray absorption spectroscopy of metalloproteins and high-valent metal-complexes at room temperature using free-electron lasers

X-ray absorption spectroscopy at the L-edge of 3d transition metals provides unique information on the local metal charge and spin states by directly probing 3d-derived molecular orbitals through 2p-3d transitions. However, this soft x-ray technique has been rarely used at synchrotron facilities for mechanistic studies of metalloenzymes due to the difficulties of x-ray-induced sample damage and strong background signals from light elements that can dominate the low metal signal. Here, we combine femtosecond soft x-ray pulses from a free-electron laser with a novel x-ray fluorescence-yield spectrometer to overcome these difficulties. We present L-edge absorption spectra of inorganic high-valent Mn complexes (Mn ∼ 6–15 mmol/l) with no visible effects of radiation damage. We also present the first L-edge absorption spectra of the oxygen evolving complex (Mn4CaO5) in Photosystem II (Mn < 1 mmol/l) at room temperature, measured under similar conditions. Our approach opens new ways to study metalloenzymes under functional conditions.

Reflection zone plate wavelength-dispersive spectrometer for ultra-light elements measurements.

We have developed an electron beam excitation ultra-soft X-ray add-on device for a scanning electron microscope with a reflective zone plate mulichannel spectrometer in order to analyse ultra-light elements such as Li and B. This spectrometer has high (λ/Δλ~100) resolving power in the energy range of 45 eV - 1120 eV. Metallic Li samples were examined and fluorescence spectra successfully measured. Energy resolution of 0.49 eV was measured in the ultra-low energy range using the Al L(2,3) line at 71 eV. High sensitivity of Boron detection was demonstrated on a B(4)C sample with layer thicknesses of 1-50 nm, detecting an amount of metallic Boron as small as ~0.57 fg.

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.

Probing the oxidation state of transition metal complexes: a case study on how charge and spin densities determine Mn L-edge X-ray absorption energies

A combined experimental and theoretical approach reveals correlations of metal L-edge X-ray absorption energies to local charge and spin densities.

Highly efficient soft X-ray spectrometer based on a reflection zone plate for resonant inelastic X-ray scattering measurements

We present a newly designed compact and flexible soft X-ray spectrometer for resonant inelastic X-ray scattering (RIXS) studies within an energy range from 380 eV to 410 eV, which would include the K alpha emission lines of vital elements like nitrogen. We utilized an off-axis reflection zone plate (RZP) as the wavelength selective element with a maximum line density of 10000 l/mm. A higher energy resolution over a broader range of ± 15 eV around the designed energy was achieved by displacing the RZP. Additionally, for the first time, an actual optical side effect, the so-called comatic aberration was exploited to increase the energy resolution. First results show a resolving power in the order of 1300 for photon energy of 395 eV, which is comparable to a commercial varied line spacing grating (VLS).