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Extreme-ultraviolet to x-ray free-electron lasers (FELs) in operation for scientific applications are up to now single-user facilities. While most FELs generate around 100 photon pulses per second, FLASH at DESY can deliver almost two orders of magnitude more pulses in this time span due to its superconducting accelerator technology. This makes the facility a prime candidate to realize the next step in FELs-dividing the electron pulse trains into several FEL lines and delivering photon pulses to several users at the same time. Hence, FLASH has been extended with a second undulator line and self-amplified spontaneous emission (SASE) is demonstrated in both FELs simultaneously. FLASH can now deliver MHz pulse trains to two user experiments in parallel with individually selected photon beam characteristics. First results of the capabilities of this extension are shown with emphasis on independent variation of wavelength, repetition rate, and photon pulse length.

A 1D imaging soft X‐ray spectrometer installed on the small quantum systems (SQS) scientific instrument of the European XFEL is described. It uses movable cylindrical constant‐line‐spacing gratings in the Rowland configuration for energy dispersion in the vertical plane, and Wolter optics for simultaneous 1D imaging of the source in the horizontal plane. The soft X‐ray fluorescence spectro‐imaging capability will be exploited in pump–probe measurements and in investigations of propagation effects and other nonlinear phenomena. A 1D imaging soft X‐ray spectrometer, installed and commissioned on the SQS instrument of the European XFEL, is described.

The PERCIVAL detector is a CMOS imager designed for the soft X‐ray regime at photon sources. Although still in its final development phase, it has recently seen its first user experiments: ptychography at a free‐electron laser, holographic imaging at a storage ring and preliminary tests on X‐ray photon correlation spectroscopy. The detector performed remarkably well in terms of spatial resolution achievable in the sample plane, owing to its small pixel size, large active area and very large dynamic range; but also in terms of its frame rate, which is significantly faster than traditional CCDs. In particular, it is the combination of these features which makes PERCIVAL an attractive option for soft X‐ray science. PERCIVAL is a detector system specifically designed for the soft X‐ray regime. Although still in a development phase, it has already served its first user experiments at both a storage ring and also a free‐electron laser. The device performed remarkably well in all the different techniques tested: ptychography, holography and also X‐ray photon correlation spectroscopy. The results of these tests are presented.

Soft X-ray absorption spectroscopy coupled with nano-scale microscopy has been widely used in material science, environmental science, and physical sciences. In this work, the advantages of soft X-ray absorption spectromicroscopy for plant biopolymer research were demonstrated by determining the chemical sensitivity of the technique to identify common plant biopolymers and to map the distributions of biopolymers in plant samples. The chemical sensitivity of soft X-ray spectroscopy to study biopolymers was determined by recording the spectra of common plant biopolymers using soft X-ray and Fourier Transform mid Infrared (FT-IR) spectroscopy techniques. The soft X-ray spectra of lignin, cellulose, and polygalacturonic acid have distinct spectral features. However, there were no distinct differences between cellulose and hemicellulose spectra. Mid infrared spectra of all biopolymers were unique and there were differences between the spectra of water soluble and insoluble xylans. The advantage of nano-scale spatial resolution exploited using soft X-ray spectromicroscopy for plant biopolymer research was demonstrated by mapping plant cell wall biopolymers in a lentil stem section and compared with the FT-IR spectromicroscopy data from the same sample. The soft X-ray spectromicroscopy enables mapping of biopolymers at the sub-cellular (similar to 30 nm) resolution whereas, the limited spatial resolution in the micron scale range in the FT-IR spectromicroscopy made it difficult to identify the localized distribution of biopolymers. The advantages and limitations of soft X-ray and FT-IR spectromicroscopy techniques for biopolymer research are also discussed.

Two‐dimensional dopant layers (δ‐layers) in semiconductors provide the high‐mobility electron liquids (2DELs) needed for nanoscale quantum‐electronic devices. Key parameters such as carrier densities, effective masses, and confinement thicknesses for 2DELs have traditionally been extracted from quantum magnetotransport. In principle, the parameters are immediately readable from the one‐electron spectral function that can be measured by angle‐resolved photoemission spectroscopy (ARPES). Here, buried 2DEL δ‐layers in silicon are measured with soft X‐ray (SX) ARPES to obtain detailed information about their filled conduction bands and extract device‐relevant properties. This study takes advantage of the larger probing depth and photon energy range of SX‐ARPES relative to vacuum ultraviolet (VUV) ARPES to accurately measure the δ‐layer electronic confinement. The measurements are made on ambient‐exposed samples and yield extremely thin (< 1 nm) and dense (≈10 14  cm −2 ) 2DELs. Critically, this method is used to show that δ‐layers of arsenic exhibit better electronic confinement than δ‐layers of phosphorus fabricated under identical conditions.

We have investigated the photoionization and photofragmentation yields of gas-phase multiply protonated melittin cations for photon energies at the K-shell absorption edges of carbon, nitrogen, and oxygen. Two similar experimental approaches were employed. In both experiments, mass selected [melittin+qH] q + ( q =2–4) ions were accumulated in radiofrequency ion traps. The trap content was exposed to intense beams of monochromatic soft X-ray photons from synchrotron beamlines and photoproducts were analyzed by means of time-of-flight mass spectrometry. Mass spectra were recorded for fixed photon energies, and partial ion yield spectra were recorded as a function of photon energy. The combination of mass spectrometry and soft X-ray spectroscopy allows for a direct correlation of protein electronic structure with various photoionization channels. Non-dissociative single and double ionization are used as a reference. The contribution of both channels to various backbone scission channels is quantified and related to activation energies and protonation sites. Soft X-ray absorption mass spectrometry combines fast energy deposition with single and double ionization and could complement established activation techniques. Graphical Abstract ᅟ

The ultrafast light-activated electrocyclic ring-opening reaction of 1,3-cyclohexadiene is a fundamental prototype of photochemical pericyclic reactions. Generally, these reactions are thought to proceed through an intermediate excited-state minimum (the so-called pericyclic minimum), which leads to isomerization via nonadiabatic relaxation to the ground state of the photoproduct. Here, we used femtosecond (fs) soft x-ray spectroscopy near the carbon K-edge (~284 electron volts) on a table-top apparatus to directly reveal the valence electronic structure of this transient intermediate state. The core-to-valence spectroscopic signature of the pericyclic minimum observed in the experiment was characterized, in combination with time-dependent density functional theory calculations, to reveal overlap and mixing of the frontier valence orbital energy levels. We show that this transient valence electronic structure arises within 60 ± 20 fs after ultraviolet photoexcitation and decays with a time constant of 110 ± 60 fs.

The efficiency with which renewable fuels and feedstocks are synthesized from electrical sources is limited at present by the sluggish oxygen evolution reaction (OER) in pH-neutral media. We took the view that generating transition-metal sites with high valence at low applied bias should improve the activity of neutral OER catalysts. Here, using density functional theory, we find that the formation energy of desired Ni4+ sites is systematically modulated by incorporating judicious combinations of Co, Fe and non-metal P. We therefore synthesized NiCoFeP oxyhydroxides and probed their oxidation kinetics with in situ soft X-ray absorption spectroscopy (sXAS). In situ sXAS studies of neutral-pH OER catalysts indicate ready promotion of Ni4+ under low overpotential conditions. The NiCoFeP catalyst outperforms IrO2 and retains its performance following 100 h of operation. We showcase NiCoFeP in a membrane-free CO2 electroreduction system that achieves a 1.99 V cell voltage at 10 mA cm-2 , reducing CO2 into CO and oxidizing H2 O to O2 with a 64% electricity-to-chemical-fuel efficiency.

The ability to determine the electronic structure of catalysts during electrochemical reactions is highly important for identification of the active sites and the reaction mechanism. Here we successfully applied soft X-ray spectroscopy to follow in operando the valence and spin state of the Co ions in Li 2 Co 2 O 4 under oxygen evolution reaction (OER) conditions. We have observed that a substantial fraction of the Co ions undergo a voltage-dependent and time-dependent valence state transition from Co 3+ to Co 4+ accompanied by spontaneous delithiation, whereas the edge-shared Co–O network and spin state of the Co ions remain unchanged. Density functional theory calculations indicate that the highly oxidized Co 4+ site, rather than the Co 3+ site or the oxygen vacancy site, is mainly responsible for the high OER activity. Determining catalyst electronic structures during electrochemical reactions is crucial to understand mechanisms. Here authors perform in operando soft X-ray spectroscopy on a cobalt oxide catalyst during O 2 evolution and observe voltage and time-dependent valence state transitions.