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The complex electronic structure of metallic ferromagnets is determined by a balance between exchange interaction, electron hopping leading to band formation, and local Coulomb repulsion. By combining high energy and temporal resolution in femtosecond time-resolved X-ray absorption spectroscopy with ab initio time-dependent density functional theory we analyze the electronic structure in fcc Ni on the time scale of these interactions in a pump-probe experiment. We distinguish transient broadening and energy shifts in the absorption spectra, which we demonstrate to be captured by electron repopulation respectively correlation-induced modifications of the electronic structure, requiring to take the local Coulomb interaction into account.

Abstract The 64k pixel DEPFET module is the key sensitive component of the DEPFET Sensor with Signal Compression (DSSC), a large area 2D hybrid detector for capturing and measuring soft X-rays at the European XFEL. The final 1-megapixel camera has to detect photons with energies between $$250\,\text{eV}$$ 250 eV and $$6\,\text{keV}$$ 6 keV , and must provide a peak frame rate of $$4.5\,\text{MHz}$$ 4.5 MHz to cope with the unique bunch structure of the European XFEL. This work summarizes the functionalities and properties of the first modules assembled with full-format CMOS-DEPFET arrays, featuring $$512\,\times \,128$$ 512 × 128 hexagonally-shaped pixels with a side length of 136 μm. The pixel sensors utilize the DEPFET technology to realize an extremely low input capacitance for excellent energy resolution and, at the same time, an intrinsic capability of signal compression without any gain switching. Each pixel of the readout ASIC includes a DEPFET-bias current cancellation circuitry, a trapezoidal-shaping filter, a 9-bit ADC and a 800-word long digital memory. The trimming, calibration and final characterization were performed in a laboratory test-bench at DESY. All detector features are assessed at $$18\,^{\circ }\text{C}$$ 18 ∘ C . An outstanding equivalent noise charge of $$9.8$$ 9.8 e−rms is achieved at 1.1-MHz frame rate and gain of 26.8 Analog-to-Digital Unit per keV ( $$\,\text {ADU}/\text{keV}$$ ADU / keV ). At $$4.5\,\text{MHz}$$ 4.5 MHz and $$3.1\,\,\text {ADU}/\text{keV}$$ 3.1 ADU / keV , a noise of $$25.5$$ 25.5 e−rms and a dynamic range of $$26\,\text{k}\text {e}^{-}$$ 26 ke - are obtained. The highest dynamic range of $$1.345\,\text{M}\text {e}^{-}$$ 1.345 Me - is reached at $$2.25\,\text{MHz}$$ 2.25 MHz and $$1.6\,\text {ADU}/\text{keV}$$ 1.6 ADU / keV . These values can fulfill the specification of the DSSC project.

Femtosecond transient soft X-ray absorption spectroscopy (XAS) is a very promising technique that can be employed at X-ray free-electron lasers (FELs) to investigate out-of-equilibrium dynamics for material and energy research. Here, a dedicated setup for soft X-rays available at the Spectroscopy and Coherent Scattering (SCS) instrument at the European X-ray Free-Electron Laser (European XFEL) is presented. It consists of a beam-splitting off-axis zone plate (BOZ) used in transmission to create three copies of the incoming beam, which are used to measure the transmitted intensity through the excited and unexcited sample, as well as to monitor the incoming intensity. Since these three intensity signals are detected shot by shot and simultaneously, this setup allows normalized shot-by-shot analysis of the transmission. For photon detection, an imaging detector capable of recording up to 800 images at 4.5 MHz frame rate during the FEL burst is employed, and allows a photon-shot-noise-limited sensitivity to be approached. The setup and its capabilities are reviewed as well as the online and offline analysis tools provided to users.

The European X-ray Free Electron Laser (EuXFEL) is a light source of the 4th generation which provides spatially coherent ultrashort X-ray pulses at high rate. The facility enabled unprecedented advancement in fundamental research, but also needed new detectors to fit the EuXFEL requirements, such as single photon resolution, large dynamic range, and high repetition rate: three 2D megapixel detectors have been developed to cope with the demanding environment. The DEPleted Field Effect Transistors (DEPFET) Sensor with Signal Compression (DSSC) detector is one of them. The parallel readout from each pixel is performed employing a dedicated Application Specific Integrated Circuit (ASIC). The generated data flow is read through a 2-stage Field Programmable Gate Array (FPGA)-based Data AcQuisition (DAQ) chain: the Input Output Board (IOB) controls the primary readout from 16 ASICs and serializes data on high-speed links that are sent to the Patch Panel Transceiver (PPT). It is also responsible for the clock distribution and timing control of the front-end and switched power channels. The PPT reorders and forwards the received data toward the EuXFEL back-end, and allows remote control over the whole DAQ. A first DSSC camera, based on miniaturized silicon drift detectors (mini-SDD) has been available for users’ experiments since 2019, and a second DEPFET-based camera is under construction. The IOB firmware has been thoroughly reviewed and modified to cope with the new DEPFET sensors’ physics and to improve the performance of both the mini-SDD and DEPFET versions. We present an overview of the DSSC, focusing on the DAQ, highlighting the main properties of the environment where the detector operates. We assess the firmware improvements introduced, with a particular focus on the IOB, and present the results obtained in comparison to the original firmware.

The advent of X-ray free-electron lasers (XFELs) has revolutionized fundamental science, from atomic to condensed matter physics, from chemistry to biology, giving researchers access to X-rays with unprecedented brightness, coherence and pulse duration. All XFEL facilities built until recently provided X-ray pulses at a relatively low repetition rate, with limited data statistics. Here, results from the first megahertz-repetition-rate X-ray scattering experiments at the Spectroscopy and Coherent Scattering (SCS) instrument of the European XFEL are presented. The experimental capabilities that the SCS instrument offers, resulting from the operation at megahertz repetition rates and the availability of the novel DSSC 2D imaging detector, are illustrated. Time-resolved magnetic X-ray scattering and holographic imaging experiments in solid state samples were chosen as representative, providing an ideal test-bed for operation at megahertz rates. Our results are relevant and applicable to any other non-destructive XFEL experiments in the soft X-ray range.