Soft X-ray free-electron lasers (FELs) have gained significant attention as a research tool in X-ray ultrafast spectroscopy due to their ultra-high pulse brightness and ultra-short duration. Combined with an independent optical laser to perform pump-probe experiments with time resolution has wide-ranging application value and can have great impact on ultrafast dynamics research in fields such as energy catalysis, solid state physics, materials science, and biology. However, the inherent temporal and spatial jitter of soft X-ray FEL pulses significantly limits the time resolution in these experiments due to the low level of synchronization between the two independent light sources. Here, we present a spatiotemporal coupling device suitable for soft X-ray FELs. The device uses a self-designed four-blade slit device which is suitable for ultra-high vacuum environments to complete the spatial coupling between the two foci of both the soft X-ray FEL and optical laser, reducing the negative effects caused by spatial jitter of soft X-ray FEL beam spots. Based on this, a wavefront-splitting scheme is used to reflect and separate approximately 30% of the soft X-ray FEL beam for arrival time diagnosis. Based on the principle of transient decrease in the reflectivity of semiconductor material surfaces induced by X-rays, precise time measurement is achieved on a shot-by-shot basis through spectral encoding. After experiments, the data is rearranged according to the arrival time delay between the two pulses, effectively increasing the time resolution of the pump-probe experiment to the femtosecond scale.
Spherically Bent Crystal Analyzers (SBCAs) are the core optical components of x-ray spectrometers. They have been widely used in many x-ray spectroscopy end-stations at synchrotron radiation and x-ray free electron laser facilities around the world. Owing to the monochromatic and focal properties of SBCAs, x-ray spectrometers with high efficiency and high energy resolution can be well applied to the study of x-ray absorption spectroscopy (XAS) and emission spectroscopy (XES). Hence, the quality of SBCAs is the key factor in determining the performance of x-ray spectrometers. Previously, we have investigated the focal properties of Si(444) SBCAs by using a laboratory’s Rowland circle device. However, the original device is limited by movement distance of motors and the space between detector and x-ray source. It is only applicable to SBCAs with a radius of curvature of 500 mm, and the maximum Bragg angle is 86°. Here, we present a new simple near backscattering detection device, which is based on a long linear guideway, to inspect the surface morphology, crystal face morphology, and focal performance of SBCAs at Bragg angle of 88°. By simply adjusting the distance between the source, SBCAs, and detector, focal performance at the focal point can be detected, while crystal face can be imaged off the focal point. By switching the x-ray source to the LED light source, surface morphology is able to be imaged as well. Furthermore, SBCAs with different radius of curvatures within 1000 mm are all measurable by this new device.
Transient structural information of matter can be obtained by time-resolved X-ray measurement, such as ultrafast X-ray diffraction (UXRD) and ultrafast X-ray absorption Spectroscopy (UXAS). A time-resolved ultrafast X-ray source is necessary for ultrafast X-ray spectroscopy measurements, such as XFEL or synchrotron radiation source. Because of the high cost of X-ray free electron laser (XFEL) and synchrotron radiation source, we designed a laboratory ultrafast plasma X-ray source driven by 800Hz high-energy laser. The X-ray pulse duration is shorter than 100fs. Copper was chosen as the target material of the source, and the expected photon flux can reach 107 photons/s. The target material can also be replaced by other common target materials.
In many single-pulse experiments of X-ray free electron lasers, the spectrum and intensity distribution of the self-amplifying spontaneous radiation beams fluctuate significantly. It is necessary to perform accurate spectral characterization of each pulse. In this paper, we present an in-line spectrometer that can observe the distribution of energy and incident intensity of single pulse X-ray photons in real time. The X-ray diffraction is achieved by using a high precision transmission crystal bending to fixed pressure bending, and the spectrum is recorded by a spatial resolution detector. At the same time, most of the incident flux is transmitted to the downstream experiment. In this paper, based on the X-ray crystal diffraction theory, geometric optical path designs were carried out, high-precision transmission curved-crystal with fixed bending was developed, the optical path of in-line transmission spectrometer was built, and the copper Kα1 and Kα2 obtained by the test were used for spectral calibration. The experimental results show that each pixel on the detector corresponds to 0.43eV, and the half-width of Kα1 is 3.44 eV. The Single-pulse spectrometer can be used for hard x-ray free electron lasers spectroscopy experiments.
X-ray spectroscopy is an important technique for studying the material electronic structure, oxidation state and coordination, which have wide applications in energy catalysis, environmental science fields. The crystals diffract X-rays because their internal atoms are spatially ordered and the lattice spacing is on the nanometer scale, which is similar to the X-ray wavelength. In this paper, a technique based on bending and epoxy adhesive is proposed to fabricate a bent crystal analyzer. The radius of convex surface is 1‰ smaller than the concave one. The wafers and spherical substrates were cleaned with acetone and ethanol in an ultra-clean room. To remove residual organic compounds, UV ozone cleaning procedure should be used. The results show that the measured curvature radius of the bent crystal analyzer is 1000.550 mm; the surface RMS of the surface is 1.34λ and the energy resolution is better than 5 eV, which can distinguish Cu Kα1 and Kα2 fluorescence lines.
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