A multi‐MHz single‐shot data acquisition scheme with high dynamic range: pump–probe X‐ray experiments at synchrotrons

The technical implementation of a multi‐MHz data acquisition scheme for laser–X‐ray pump–probe experiments with pulse limited temporal resolution (100 ps) is presented. Such techniques are very attractive to benefit from the high‐repetition rates of X‐ray pulses delivered from advanced synchrotron radiation sources. Exploiting a synchronized 3.9 MHz laser excitation source, experiments in 60‐bunch mode (7.8 MHz) at beamline P01 of the PETRA III storage ring are performed. Hereby molecular systems in liquid solutions are excited by the pulsed laser source and the total X‐ray fluorescence yield (TFY) from the sample is recorded using silicon avalanche photodiode detectors (APDs). The subsequent digitizer card samples the APD signal traces in 0.5 ns steps with 12‐bit resolution. These traces are then processed to deliver an integrated value for each recorded single X‐ray pulse intensity and sorted into bins according to whether the laser excited the sample or not. For each subgroup the recorded single‐shot values are averaged over ～10⁷ pulses to deliver a mean TFY value with its standard error for each data point, e.g. at a given X‐ray probe energy. The sensitivity reaches down to the shot‐noise limit, and signal‐to‐noise ratios approaching 1000 are achievable in only a few seconds collection time per data point. The dynamic range covers 100 photons pulse^?1 and is only technically limited by the utilized APD.     

Optimized spatial overlap in optical pump–X‐ray probe experiments with high repetition rate using laser‐induced surface distortions

Ultrafast X‐ray diffraction experiments require careful adjustment of the spatial overlap between the optical excitation and the X‐ray probe pulse. This is especially challenging at high laser repetition rates. Sample distortions caused by the large heat load on the sample and the relatively low optical energy per pulse lead to only tiny signal changes. In consequence, this results in small footprints of the optical excitation on the sample, which turns the adjustment of the overlap difficult. Here a method for reliable overlap adjustment based on reciprocal space mapping of a laser excited thin film is presented.     

Development of picosecond time‐resolved X‐ray absorption spectroscopy by high‐repetition‐rate laser pump/X‐ray probe at Beijing Synchrotron Radiation Facility

A new setup and commissioning of transient X‐ray absorption spectroscopy are described, based on the high‐repetition‐rate laser pump/X‐ray probe method, at the 1W2B wiggler beamline at the Beijing Synchrotron Radiation Facility. A high‐repetition‐rate and high‐power laser is incorporated into the setup with in‐house‐built avalanche photodiodes as detectors. A simple acquisition scheme was applied to obtain laser‐on and laser‐off signals simultaneously. The capability of picosecond transient X‐ray absorption spectroscopy measurement was demonstrated for a photo‐induced spin‐crossover iron complex in 6 mM solution with 155 kHz repetition rate.     

Alternative difference analysis scheme combining R‐space EXAFS fit with global optimization XANES fit for X‐ray transient absorption spectroscopy

Time‐resolved X‐ray absorption spectroscopy (TR‐XAS), based on the laser‐pump/X‐ray‐probe method, is powerful in capturing the change of the geometrical and electronic structure of the absorbing atom upon excitation. TR‐XAS data analysis is generally performed on the laser‐on minus laser‐off difference spectrum. Here, a new analysis scheme is presented for the TR‐XAS difference fitting in both the extended X‐ray absorption fine‐structure (EXAFS) and the X‐ray absorption near‐edge structure (XANES) regions. R‐space EXAFS difference fitting could quickly provide the main quantitative structure change of the first shell. The XANES fitting part introduces a global non‐derivative optimization algorithm and optimizes the local structure change in a flexible way where both the core XAS calculation package and the search method in the fitting shell are changeable. The scheme was applied to the TR‐XAS difference analysis of Fe(phen)₃ spin crossover complex and yielded reliable distance change and excitation population.     

Undulator beamline optimization with integrated chicanes for X‐ray free‐electron‐laser facilities

An optimization of the undulator layout of X‐ray free‐electron‐laser (FEL) facilities based on placing small chicanes between the undulator modules is presented. The installation of magnetic chicanes offers the following benefits with respect to state‐of‐the‐art FEL facilities: reduction of the required undulator length to achieve FEL saturation, improvement of the longitudinal coherence of the FEL pulses, and the ability to produce shorter FEL pulses with higher power levels. Numerical simulations performed for the soft X‐ray beamline of the SwissFEL facility show that optimizing the advantages of the layout requires shorter undulator modules than the standard ones. This proposal allows a very compact undulator beamline that produces fully coherent FEL pulses and it makes possible new kinds of experiments that require very short and high‐power FEL pulses.     

X‐ray beam‐position feedback system with easy‐to‐use beam‐position monitor

X‐ray beam‐position stability is indispensable in cutting‐edge experiments using synchrotron radiation. Here, for the first time, a beam‐position feedback system is presented that utilizes an easy‐to‐use X‐ray beam‐position monitor incorporating a diamond‐fluorescence screen. The acceptable range of the monitor is above 500 µm and the feedback system maintains the beam position within 3 µm. In addition to being inexpensive, the system has two key advantages: it works without a scale factor for position calibration, and it has no dependence on X‐ray energy, X‐ray intensity, beam size or beam shape.     

Picosecond time‐resolved laser pump/X‐ray probe experiments using a gated single‐photon‐counting area detector

The recent developments in X‐ray detectors have opened new possibilities in the area of time‐resolved pump/probe X‐ray experiments; this article presents the novel use of a PILATUS detector to achieve X‐ray pulse duration limited time‐resolution at the Advanced Photon Source (APS), USA. The capability of the gated PILATUS detector to selectively detect the signal from a given X‐ray pulse in 24 bunch mode at the APS storage ring is demonstrated. A test experiment performed on polycrystalline organic thin films of α‐perylene illustrates the possibility of reaching an X‐ray pulse duration limited time‐resolution of 60 ps using the gated PILATUS detector. This is the first demonstration of X‐ray pulse duration limited data recorded using an area detector without the use of a mechanical chopper array at the beamline.     

Analysis of the halo background in femtosecond slicing experiments

The slicing facility FemtoSpeX at BESSY II offers unique opportunities to study photo‐induced dynamics on femtosecond time scales by means of X‐ray magnetic circular dichroism, resonant and non‐resonant X‐ray diffraction, and X‐ray absorption spectroscopy experiments in the soft X‐ray regime. Besides femtosecond X‐ray pulses, slicing sources inherently also produce a so‐called `halo' background with a different time structure, polarization and pointing. Here a detailed experimental characterization of the halo radiation is presented, and a method is demonstrated for its correct and unambiguous removal from femtosecond time‐resolved data using a special laser triggering scheme as well as analytical models. Examples are given for time‐resolved measurements with corresponding halo correction, and errors of the relevant physical quantities caused by either neglecting or by applying a simplified model to describe this background are estimated.     

An in situ atomic force microscope for normal‐incidence nanofocus X‐ray experiments

A compact high‐speed X‐ray atomic force microscope has been developed for in situ use in normal‐incidence X‐ray experiments on synchrotron beamlines, allowing for simultaneous characterization of samples in direct space with nanometric lateral resolution while employing nanofocused X‐ray beams. In the present work the instrument is used to observe radiation damage effects produced by an intense X‐ray nanobeam on a semiconducting organic thin film. The formation of micrometric holes induced by the beam occurring on a timescale of seconds is characterized.     

Characteristics of a tapered undulator for the X‐ray absorption fine‐structure technique at PLS‐II

An in‐vacuum undulator (IVU) with a tapered configuration was installed in the 8C nanoprobe/XAFS beamlime (BL8C) of the Pohang Light Source in Korea for hard X‐ray nanoprobe and X‐ray absorption fine‐structure (XAFS) experiments. It has been operated in planar mode for the nanoprobe experiments, while gap‐scan and tapered modes have been used alternatively for XAFS experiments. To examine the features of the BL8C IVU for XAFS experiments, spectral distributions were obtained theoretically and experimentally as functions of the gap and gap taper. Beam profiles at a cross section of the X‐ray beam were acquired using a slit to visualize the intensity distributions which depend on the gap, degree of tapering and harmonic energies. To demonstrate the effect of tapering around the lower limit of the third‐harmonic energy, V K‐edge XAFS spectra were obtained in each mode. Owing to the large X‐ray intensity variation around this energy, XAFS spectra of the planar and gap‐scan modes show considerable spectral distortions in comparison with the tapered mode. This indicates that the tapered mode, owing to the smooth X‐ray intensity profile at the expense of the highest and most stable intensity, can be an alternative for XAFS experiments where the gap‐scan mode gives a considerable intensity variation; it is also suitable for quick‐XAFS scanning.     

Multimodal hard X‐ray imaging of a mammography phantom at a compact synchrotron light source

The Compact Light Source is a miniature synchrotron producing X‐rays at the interaction point of a counter‐propagating laser pulse and electron bunch through the process of inverse Compton scattering. The small transverse size of the luminous region yields a highly coherent beam with an angular divergence of a few milliradians. The intrinsic monochromaticity and coherence of the produced X‐rays can be exploited in high‐sensitivity differential phase‐contrast imaging with a grating‐based interferometer. Here, the first multimodal X‐ray imaging experiments at the Compact Light Source at a clinically compatible X‐ray energy of 21 keV are reported. Dose‐compatible measurements of a mammography phantom clearly demonstrate an increase in contrast attainable through differential phase and dark‐field imaging over conventional attenuation‐based projections.     

High‐resolution spectroscopy on an X‐ray absorption beamline

A bent‐crystal spectrometer based on the Rowland circle geometry has been installed and tested on the BM30b/FAME beamline at the European Synchrotron Radiation Facility to improve its performances. The energy resolution of the spectrometer allows different kinds of measurements to be performed, including X‐ray absorption spectroscopy, resonant inelastic X‐ray scattering and X‐ray Raman scattering experiments. The simplicity of the experimental device makes it easily implemented on a classical X‐ray absorption beamline. This improvement in the fluorescence detection is of particular importance when the probed element is embedded in a complex and/or heavy matrix, for example in environmental sciences.     

Ionization dynamics of dense matter generated by intense ultrashort X‐ray pulses

We investigate a pump‐probe X‐ray Thomson scattering (XRTS) experiment that might be carried out at a free electron laser facility to study warm‐to‐hot states of dense matter. Ultrashort and intense X‐ray pulses with different energies (1,560–1,830 eV) heat a 1 µm thick Al target isochorically and create homogeneous and uncompressed warm‐to‐hot states of dense matter. A second pulse with variable delay probes this heated state via XRTS. The X‐ray laser–target interaction is modelled within radiation‐hydrodynamic simulations applying the HELIOS‐CR code. The HELIOS‐CR results qualitatively agree with Monte‐Carlo simulations, where the laser pulse absorption is simulated based on a uniform random sequence of events. The electron feature in the simultaneously observed X‐ray scattering spectrum is a function of the degree of ionization and the target temperature. Therefore, the temporal evolution of the plasmon peak measures the ionization dynamics on ultra‐short time scales. The XRTS spectrum is calculated based on the Chihara formula utilizing the Born‐Mermin approximation for the free electron dynamic structure factor. The proposed experiment will reveal important details of the ionization dynamics on ultra‐short time scales as well as of the relaxation on ps time scales.     

The multi‐purpose hard X‐ray beamline BL10 at the DELTA storage ring

The layout and the characteristics of the hard X‐ray beamline BL10 at the superconducting asymmetric wiggler at the 1.5 GeV Dortmund Electron Accelerator DELTA are described. This beamline is equipped with a Si(111) channel‐cut monochromator and is dedicated to X‐ray studies in the spectral range from ～4 keV to ～16 keV photon energy. There are two different endstations available. While X‐ray absorption studies in different detection modes (transmission, fluorescence, reflectivity) can be performed on a designated table, a six‐axis kappa diffractometer is installed for X‐ray scattering and reflectivity experiments. Different detector set‐ups are integrated into the beamline control software, i.e. gas‐filled ionization chambers, different photodiodes, as well as a Pilatus 2D‐detector are permanently available. The performance of the beamline is illustrated by high‐quality X‐ray absorption spectra from several reference compounds. First applications include temperature‐dependent EXAFS experiments from liquid‐nitrogen temperature in a bath cryostat up to ～660 K by using a dedicated furnace. Besides transmission measurements, fluorescence detection for dilute sample systems as well as surface‐sensitive reflection‐mode experiments are presented.     

Implementation of ultrafast X‐ray diffraction at the 1W2B wiggler beamline of Beijing Synchrotron Radiation Facility

The implementation of a laser pump/X‐ray probe scheme for performing picosecond‐resolution X‐ray diffraction at the 1W2B wiggler beamline at Beijing Synchrotron Radiation Facility is reported. With the hybrid fill pattern in top‐up mode, a pixel array X‐ray detector was optimized to gate out the signal from the singlet bunch with interval 85 ns from the bunch train. The singlet pulse intensity is ～2.5 × 10⁶ photons pulse^?1 at 10 keV. The laser pulse is synchronized to this singlet bunch at a 1 kHz repetition rate. A polycapillary X‐ray lens was used for secondary focusing to obtain a 72 µm (FWHM) X‐ray spot. Transient photo‐induced strain in BiFeO₃ film was observed at a ～150 ps time resolution for demonstration.     

BioCARS: a synchrotron resource for time‐resolved X‐ray science

BioCARS, a NIH‐supported national user facility for macromolecular time‐resolved X‐ray crystallography at the Advanced Photon Source (APS), has recently completed commissioning of an upgraded undulator‐based beamline optimized for single‐shot laser‐pump X‐ray‐probe measurements with time resolution as short as 100 ps. The source consists of two in‐line undulators with periods of 23 and 27 mm that together provide high‐flux pink‐beam capability at 12 keV as well as first‐harmonic coverage from 6.8 to 19 keV. A high‐heat‐load chopper reduces the average power load on downstream components, thereby preserving the surface figure of a Kirkpatrick–Baez mirror system capable of focusing the X‐ray beam to a spot size of 90 µm horizontal by 20 µm vertical. A high‐speed chopper isolates single X‐ray pulses at 1 kHz in both hybrid and 24‐bunch modes of the APS storage ring. In hybrid mode each isolated X‐ray pulse delivers up to ～4 × 10¹⁰ photons to the sample, thereby achieving a time‐averaged flux approaching that of fourth‐generation X‐FEL sources. A new high‐power picosecond laser system delivers pulses tunable over the wavelength range 450–2000 nm. These pulses are synchronized to the storage‐ring RF clock with long‐term stability better than 10 ps RMS. Monochromatic experimental capability with Biosafety Level 3 certification has been retained.     

Characterization of X‐ray gas attenuator plasmas by optical emission and tunable laser absorption spectroscopies

X‐ray gas attenuators are used in high‐energy synchrotron beamlines as high‐pass filters to reduce the incident power on downstream optical elements. The absorption of the X‐ray beam ionizes and heats up the gas, creating plasma around the beam path and hence temperature and density gradients between the center and the walls of the attenuator vessel. The objective of this work is to demonstrate experimentally the generation of plasma by the X‐ray beam and to investigate its spatial distribution by measuring some of its parameters, simultaneously with the X‐ray power absorption. The gases used in this study were argon and krypton between 13 and 530 mbar. The distribution of the 2p excited states of both gases was measured using optical emission spectroscopy, and the density of argon metastable atoms in the 1s₅ state was deduced using tunable laser absorption spectroscopy. The amount of power absorbed was measured using calorimetry and X‐ray transmission. The results showed a plasma confined around the X‐ray beam path, its size determined mainly by the spatial dimensions of the X‐ray beam and not by the absorbed power or the gas pressure. In addition, the X‐ray absorption showed a hot central region at a temperature varying between 400 and 1100 K, depending on the incident beam power and on the gas used. The results show that the plasma generated by the X‐ray beam plays an essential role in the X‐ray absorption. Therefore, plasma processes must be taken into account in the design and modeling of gas attenuators.     

The small‐angle and wide‐angle X‐ray scattering set‐up at beamline BL9 of DELTA

The multi‐purpose experimental endstation of beamline BL9 at the Dortmund Electron Accelerator (DELTA) is dedicated to diffraction experiments in grazing‐incidence geometry, reflectivity and powder diffraction measurements. Moreover, fluorescence analysis and inelastic X‐ray scattering experiments can be performed. Recently, a new set‐up for small‐angle and wide‐angle X‐ray scattering utilizing detection by means of an image‐plate scanner was installed and is described in detail here. First small‐angle X‐ray scattering experiments on aqueous solutions of lysozyme with different cosolvents and of staphylococcal nuclease are discussed. The application of the set‐up for texture analysis is emphasized and a study of the crystallographic texture of natural bio‐nanocomposites, using lobster and crab cuticles as model materials, is presented.     

Characterization of spatial coherence of synchrotron radiation with non‐redundant arrays of apertures

A method to characterize the spatial coherence of soft X‐ray radiation from a single diffraction pattern is presented. The technique is based on scattering from non‐redundant arrays (NRAs) of slits and records the degree of spatial coherence at several relative separations from 1 to 15 µm, simultaneously. Using NRAs the spatial coherence of the X‐ray beam at the XUV X‐ray beamline P04 of the PETRA III synchrotron storage ring was measured as a function of different beam parameters. To verify the results obtained with the NRAs, additional Young's double‐pinhole experiments were conducted and showed good agreement.     

The Munich Compact Light Source: initial performance measures

While large‐scale synchrotron sources provide a highly brilliant monochromatic X‐ray beam, these X‐ray sources are expensive in terms of installation and maintenance, and require large amounts of space due to the size of storage rings for GeV electrons. On the other hand, laboratory X‐ray tube sources can easily be implemented in laboratories or hospitals with comparatively little cost, but their performance features a lower brilliance and a polychromatic spectrum creates problems with beam hardening artifacts for imaging experiments. Over the last decade, compact synchrotron sources based on inverse Compton scattering have evolved as one of the most promising types of laboratory‐scale X‐ray sources: they provide a performance and brilliance that lie in between those of large‐scale synchrotron sources and X‐ray tube sources, with significantly reduced financial and spatial requirements. These sources produce X‐rays through the collision of relativistic electrons with infrared laser photons. In this study, an analysis of the performance, such as X‐ray flux, source size and spectra, of the first commercially sold compact light source, the Munich Compact Light Source, is presented.