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Historically, TR-XSS was also called X-ray Diffuse Scattering (XDS)<ref name="source1">Haldrup, Kristoffer, et al. "Observing solvation dynamics with simultaneous femtosecond X-ray emission spectroscopy and X-ray scattering." The journal of physical chemistry B 120.6 (2016): 1158-1168.</ref> to differentiate scattering of an unordered system (e.g. liquid) from solid state scattering. However, due to
== Pump-Probe Technique ==
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TR-XSS is a pump-probe technique, in which the liquid sample is first excited with a short laser pulse and subsequently probed with a short X-ray pulse. The liquid sample contains low concentrations (~1-100 mM) of a solute molecule in solution, which is delivered to the beam interaction region either by a liquid jet or through a capillary. A continuous supply of new sample through the delivery system avoids radiation damage by the X-ray and laser pulses. The intensity I(θ) of the [[X-ray_diffraction|X-rays]] scattered on the sample is recorded as a function of the scattering angle θ with a two dimensional X-ray detector. To capture small structural changes in real space (like sub-angstrom bong elongations in a molecule), large scattering angles (0°-60°) in the reciprocal space are detected. Thus, TR-XSS experiments are recorded in wide-angle X-ray scattering (WAXS) geometry.
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== Application ==
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• [[Dye-sensitized_solar_cell|Dye-sensitised solar cells]]. Fundamental research on structural processes in ruthenium-based and iron-based solar cells<ref>Kunnus, Kristjan, et al. "Vibrational wavepacket dynamics in Fe carbene photosensitizer determined with femtosecond X-ray emission and scattering." Nature communications 11.1 (2020): 634.</ref><ref>Bressler, Christian, et al. "Solvation dynamics monitored by combined X-ray spectroscopies and scattering: photoinduced spin transition in aqueous [Fe (bpy) 3] 2+." Faraday discussions 171 (2014): 169-178.</ref><ref>Gaffney, Kelly J. "Capturing photochemical and photophysical transformations in iron complexes with ultrafast X-ray spectroscopy and scattering." Chemical Science 12.23 (2021): 8010-8025.</ref>
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== Prerequisites/Limitations ==
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TR-XSS is a pump-probe technique. It requires a pulsed laser to excite chemical reactions in a solution as well as a pulsed X-ray source of high peak brilliance for the probe. The time-resolution depends on the pulse with of the X-rays and the laser, as well as on the thickness of the probed sample. Examples of experimental setups capable of the technique can be found at [[Synchrotron|synchrotrons]] (ID09 at [[European Synchrotron Radiation Facility|ESRF]], [[Advanced Photon Source|APS]]), laboratory sources<ref>Gaffney, Kelly J. "Capturing photochemical and photophysical transformations in iron complexes with ultrafast X-ray spectroscopy and scattering." Chemical Science 12.23 (2021): 8010-8025.</ref><ref>https://chemistry.brown.edu/people/peter-m-weber. Accessed 05.12.2024</ref> or [[Free electron laser|free-electron lasers]] (e.g.: FXE at [[European_XFEL|EUXFEL]], XPP at LCLS, BL3 at [[SACLA]], Alvra at [[SwissFEL]], [[Pohang University of Science and Technology|PAL-XFEL]]). Typical time-resolutions are 100 ps (due to the X-ray pulse of of 100 ps [[FWHM]] for synchrotrons and 100 fs at free-electron lasers. Beside the time-resolution, the experimental setups strongly depend on the repetition rate of the X-ray pulses. The above mentioned X-ray sources vary from tens of Hz to 3.5 MHz repetition rate, and thus the amount of information that can be gathered during experimental time can differ by orders of magnitude.
For a solution to be investigated with TR-XSS, the dissolved molecules need to absorb light at the wavelength of the pump laser, while the surrounding solvent ideally does not absorb at the same laser wavelength. Additionally, molecules including heavy atoms (e.g. metals) scatter more X-ray photons than light elements, thus the recorded signals become larger and easier to analyze.
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