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{{AfC topic|stem}}
Time-Resolved X-ray Solution Scattering (TR-XSS), also known as
Hwan Kim, Kyung, et al. "Topical Review: Molecular reaction and solvation visualized by time-resolved X-ray solution scattering: Structure, dynamics, and their solvent dependence." Structural Dynamics 1.1 (2014).
</ref>
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 confusion with the term diffuse scattering known from imperfections in a crystal (a weak side effect compared to Bragg scattering peaks) the term is no longer in use.
== Pump-Probe Technique ==
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.
The intensity distribution of scattered light contains information on all molecules in the liquid sample, including the target molecule for the investigation (solute) as well as the solvent molecules surrounding it ([[Solvation_shell|solvation shell]]) and all solvent molecules. To highlight the information gathered on the excited solute molecules, detector images with scattering patterns I(θ) from the sample in the [[Ground_state|ground state]] (I<sub>''laser off''</sub>) are subtracted from scattering pattern after [[Excited_state|exciting]] the solute with a pump laser (I<sub>''laser on''</sub>). The difference intensity ΔI(θ,Δt) = I<sub>''laser off''</sub> (θ) – I<sub>''laser on''</sub>(θ,Δt)
only contains intensity contributions from the excited solute and solvation shell molecules, which reacted to the chemical reaction triggered by the laser pulse.
== Application ==
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