Matteo Porro

Time-resolved X-ray absorption spectroscopy (TRXAS) at the Co L₃-edge was used to identify metal-centered (MC) character in the S₁ excited state of cyanocobalamin (CNCbl). Cobalamins have UV/visible spectra that are dominated by intense corrin-based excitations, but these ligand-centered states energetically overlap with charge transfer and MC excited states that may be populated following photoexcitation. Ultrafast optical and hard X-ray spectroscopy have shown that CNCbl forms a structurally distorted S₁ state, but these probes lack a clear signature of the S₁ electronic identity, which theory has suggested is a ligand-to-metal charge transfer (LMCT) state. Femtosecond soft X-ray TRXAS offers greater state-selectivity than many optical or hard X-ray probes but has, so far, been limited to highly concentrated (≥100 mM) samples. A new experimental setup at the European X-ray Free Electron Laser (EuXFEL) that enables studies of sub-10 mM samples and provides ∼100 fs time-resolution is used to measure the TRXAS of CNCbl at the Co L₃-edge. Comparison of the L₃-edge XAS spectrum measured at 0.8 ps with ligand field multiplet simulations indicates that the S₁ state is primarily an MC excited state. The sub-20 μOD detection sensitivity achieved in this study demonstrates the possibility of applying this method to a wide range of naturally occurring and synthetic transition metal complexes.

Time-resolved X-ray absorption spectroscopy (TRXAS) at the Co L ₃ -edge was used to identify metal-centered (MC) character in the S ₁ excited state of cyanocobalamin (CNCbl). Cobalamins have UV/visible spectra that are dominated by intense corrin-based excitations, but these ligand-centered states energetically overlap with charge transfer and MC excited states that may be populated following photoexcitation. Ultrafast optical and hard X-ray spectroscopy have shown that CNCbl forms a structurally distorted S ₁ state, but these probes lack a clear signature of the S ₁ electronic identity, which theory has suggested is a ligand-to-metal charge transfer (LMCT) state. Femtosecond soft X-ray TRXAS offers greater state-selectivity than many optical or hard X-ray probes, but has, so far, been limited to highly concentrated (≥100 mM) samples. A new experimental setup at the European X-ray Free Electron Laser (EuXFEL) that enables studies of sub-10 mM samples and provides ∼100 fs time-resolution is used to measure the TRXAS of CNCbl at the Co L ₃ -edge. Comparison of the L ₃ -edge XAS spectrum measured at 0.8 ps with ligand field multiplet simulations indicates that the S ₁ state is primarily a MC excited state. The sub-20 µOD detection sensitivity achieved in this study demonstrates the possibility of applying this method to a wide range of naturally-occurring and synthetic transition metal complexes.

Time-resolved X-ray absorption spectroscopy (TRXAS) at the Co L ₃ -edge was used to identify metal-centered (MC) character in the S ₁ excited state of cyanocobalamin (CNCbl). Cobalamins have UV/visible spectra that are dominated by intense corrin-based excitations, but these ligand-centered states energetically overlap with charge transfer and MC excited states that may be populated following photoexcitation. Ultrafast optical and hard X-ray spectroscopy have shown that CNCbl forms a structurally distorted S ₁ state, but these probes lack a clear signature of the S ₁ electronic identity, which theory has suggested is a ligand-to-metal charge transfer (LMCT) state. Femtosecond soft X-ray TRXAS offers greater state-selectivity than many optical or hard X-ray probes, but has, so far, been limited to highly concentrated (≥100 mM) samples. A new experimental setup at the European X-ray Free Electron Laser (EuXFEL) that enables studies of sub-10 mM samples and provides ∼100 fs time-resolution is used to measure the TRXAS of CNCbl at the Co L ₃ -edge. Comparison of the L ₃ -edge XAS spectrum measured at 0.8 ps with ligand field multiplet simulations indicates that the S ₁ state is primarily a MC excited state. The sub-20 µOD detection sensitivity achieved in this study demonstrates the possibility of applying this method to a wide range of naturally-occurring and synthetic transition metal complexes.

The European X-Ray Free Electron Laser (XFEL) represents a significant advancement in photon science, enabling experiments featuring an excellent temporal and spatial resolution. XFEL sources provide extremely intense and ultra-short (in the order of femtoseconds) X-ray pulses. Within this framework, new X-Ray pixel detectors are required to satisfy the challenging requirements from XFEL experiments, including high frame rates, wide dynamic range, and low noise for single-photon sensitivity. In particular, achieving single-photon resolution while maintaining a large dynamic range of up to 10⁴ photons presents substantial challenges in the design of the detector readout electronics. In this article, we present a novel front-end architecture based on a Charge Sensitive Amplifier (CSA) with predictive gain switching and a Flip-Capacitor filter. The CSA is designed for Silicon Pixel Detectors and aims to achieve both a single photon resolution and a high dynamic range up to 10⁴ photons by employing an adaptive control of the gain. The novel predictive gain circuit dynamically sets the proper gain by measuring the time needed by the signal to exceed a fixed threshold. The gain is automatically adjusted by adding feedback capacitances in parallel. When the CSA is in its highest gain configuration, a filter is connected to reduce the electronic noise and achieve the single-photon resolution below 1 keV. The filter is based on the Flip-Capacitor Filter concept and implements a fast trapezoidal weighting function. To couple the CSA with the filter, a voltage-to-current converter has been designed. A first prototype of this front-end has been designed in 65 nm CMOS technology, and the design and simulations are presented here.

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 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.

The first DSSC 1-Mpixel camera became available at the European XFEL (EuXFEL) in the Hamburg area in February 2019. It was successfully tested, installed, and commissioned at the Spectroscopy and Coherent Scattering Instrument. DSSC is a high-speed, large-area, 2-D imaging detector system optimized for photon science applications in the energy range between 0.25 and 6 keV. The camera is based on direct conversion Si sensors and is composed of 1024 ×1024 pixels of hexagonal shape with a side length of 136 μm. The 256 application-specific integrated circuits (ASICs) provide full parallel readout, comprising analog filtering, digitization, and in-pixel data storage. In order to cope with the demanding X-ray pulse time structure of the EuXFEL, the DSSC provides a peak frame rate of 4.5 MHz. The first Mpixel camera is equipped with miniaturized silicon drift detector (MiniSDD) pixel arrays. The intrinsic response of the pixels and the linear readout limit the dynamic range but allow one to achieve noise values of about 60 electrons r.m.s. at the highest frame rate. The challenge of providing high-dynamic range (~10 ⁴ photons/pixel/pulse) and single-photon detection simultaneously requires a nonlinear system front end, which will be obtained with the DEPFET active pixel technology foreseen for the advanced version of the camera. This technology will provide lower noise and a nonlinear response at the sensor level. This article describes the architecture of the whole detector system together with the main experimental results achieved up to now.