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X-ray end-stations

The combination of X-ray pulses with sub-picosecond duration and perfectly synchronized and tunable UV-VIS-IR "pump" pulses is the foundation for the development of the end station for X-ray scattering and diffraction (XRD), X-ray absorption spectroscopy (XAS) and phase contrast imaging. Due to the modular design of this TREX area we will be able to switch rapidly between the main experimental modes and even to move parts of the experimental set up to other locations, e.g the Betatron beamline in E2.

XRD is used for resolving the atomic and molecular structure of a sample, such as single crystal, nanocrystalline sample, superlattice, or molecular crystal in which the periodically arranged atoms cause the incident X-ray radiation to diffract into specific directions, creating a Bragg diffraction pattern on the two-dimensional detector. By analyzing the diffraction pattern, we can resolve a three-dimensional electron density distribution within the sample. Using a pump-probe technique, we are further able to track the conformational and structural changes in the sample because of its photoexcitation or heating by a pump pulse. The samples are excited by a wavelength-tunable UV-VIS-IR pump laser beam from the optical parametric amplifier (OPA). The structure itself and the induced structural change are probed by focused monochromatic X‐ray pulses that undergo diffraction in the sample. The sample will be mounted on a Kappa goniometer or Eulerian cradle to allow precise control of individual angles. Bragg diffraction patterns are registered using a large-area imaging or photon counting X‐ray detector mounted on a robotic arm in order to cover a wide-angle diffraction.

The X-ray absorption spectroscopy measurements at the XAS end-station allow us to study the elemental composition of the sample, the oxidation state of atoms, geometry, and inter-atomic distances in the molecules, and many more atomic-level properties. With the pump-probe capability, we are able to track the changes of these parameters after photoexcitation of the samples with sub-picosecond temporal resolution. The XAS end-station uses user-specific UV-VIS-IR “pump” pulse for sample excitation and a polychromatic, pulsed X-ray emission from the PXS source as a “probe” signal. The delay of the probe pulse to the pump pulse can be set from -1 ps to a virtually unlimited number of positive values with femtosecond precision. After the probe pulse has interacted with the sample, the X-rays are spectrally dispersed by a crystal and detected by a back-illuminated X‐ray CCD camera. Owing to the ultrashort X‐ray pulse duration, measurements of structural dynamics will be possible with a temporal resolution in the order of 100 fs. The sample can be in solid, powder, or liquid form. The liquid delivery system will allow for the study of such areas as the chemical dynamics of transition metal complexes in solutions.

Petr Brůža