Draft:Time-resolved X-ray solution scattering: Difference between revisions

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Time-Resolved X-ray Solution Scattering (TR-XSS), also known as Liquidography<ref>Kim, Tae Kyu, et al. "Spatiotemporal Kinetics in Solution Studied by Time‐Resolved X‐Ray Liquidography (Solution Scattering)." ChemPhysChem 10.12 (2009): 1958-1980.</ref> is a measurement technique ([[X-ray scattering techniques]]) capable of observing the structural dynamics of molecules dissolved in a liquid on a femtosecond (10^-15 s) timescale [[femtochemistry]] with better than [[angstrom]] spatial resolution. As such, it can be used to create movies of chemical reactions as they unfold, in real-time, and help to understand fundamental processes in nature. <ref>Wang, Qianci, Longteng Yun, and Jie Yang. "Ultrafast molecular movies: Probing chemical dynamics with femtosecond electron and X-ray diffraction." CCS Chemistry 6.5 (2024): 1092-1109.</ref><ref>

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).

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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 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_{''laser off''}) are subtracted from scattering pattern after [[Excited_state|exciting]] the solute with a pump laser (I_{''laser on''}). The difference intensity

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_{''laser off''}) are subtracted from scattering pattern after [[Excited_state|exciting]] the solute with a pump laser (I_{''laser on''}). The difference intensity

ΔI(θ,Δt) = I_{''laser off''} (θ) – I_{''laser on''}(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. By changing the delay time Δt between pump and probe pulse, snapshots of different stages of the structural dynamic of the excitation progress can be captured. In the end, those snapshots are combined to construct a [[The_Horse_in_Motion|movie of the observed]] chemical reaction.

== Application ==

• [[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>

• [[Molecular_switch|Molecular switching]] materials.<ref>Hansen, Bianca L., et al. "Excited-state structural characterisation of a series of nanosecond-lived [Fe (terpy) 2] 2+ derivatives using X-ray Solution Scattering." (2024).</ref>

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• Light-induced molecular [[Dissociation_(chemistry)|dissociation reactions]] or complex formations.<ref>Nimmrich, Amke, et al. "Solvent-dependent structural dynamics in the ultrafast photodissociation reaction of triiodide observed with time-resolved x-ray solution scattering." Journal of the American Chemical Society 145.29 (2023): 15754-15765.</ref><ref>Reinhard, Marco, et al. "Ferricyanide photo-aquation pathway revealed by combined femtosecond Kβ main line and valence-to-core x-ray emission spectroscopy." Nature Communications 14.1 (2023): 2443.</ref>

• [[Metal-metal_bond|Metal-metal bond]] dynamics.<ref>Haldrup, Kristoffer, et al. "Ultrafast X-ray scattering measurements of coherent structural dynamics on the ground-state potential energy surface of a diplatinum molecule." Physical review letters 122.6 (2019): 063001.</ref><ref>Kim, Kyung Hwan, et al. "Direct observation of bond formation in solution with femtosecond X-ray scattering." Nature 518.7539 (2015): 385-389.</ref>

== Complementary techniques ==

TR-XSS can deliver real-time information on [[Molecular_geometry|molecular structures]]. Additional information on electronic structure can be obtained by combining the technique with X-ray Absorption Spectroscopy ([[X-ray_absorption_spectroscopy|XAS]]) or X-ray Emission Spectroscopy ([[X-ray_emission_spectroscopy|XES]]) recorded at the same time.<ref name="source1" /><ref>Haldrup, K., et al. "Guest–host Interactions investigated by time-resolved X-ray spectroscopies and scattering at MHz rates: solvation dynamics and photoinduced spin transition in aqueous Fe (bipy) 32+." The Journal of Physical Chemistry A 116.40 (2012): 9878-9887.</ref> Together, those techniques provide a strong tool set for investigating kinetics and dynamics of the structural and electronic degree of freedom in solution phase samples.

== Prerequisites/Limitations ==

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.