Dual X-ray pulses enable pump-probe X-ray Thomson scattering measurements

Pump-probe experiments provide a unique opportunity to study material response on ultrafast timescales. Typically, a high-energy optical laser heats the sample while a follow-up probe analyzes the sample a few femtoseconds later.

Kunz et al. adapted this technique by using a dual X-ray pulse and probe, thanks to the unique two-color X-ray capability at the SPring-8 Angstrom Compact Free Electron Laser (SACLA). This approach mitigates many of the downsides to using optical lasers.

“Optical-laser-driven experiments can introduce complications from pre-pulse heating, laser-plasma coupling, and spatial nonuniformity of the deposited energy,” said author Iris Kunz. “Using X-rays for both the pump and probe provides a much more localized and well-defined way to deposit energy and measure the response.”

The team’s experimental setup consisted of two X-ray pulses, one at 9.4 keV and a second at 7.0 keV, with a 20-300 fs variable delay. The pump pulse was above the K-edge — the threshold where X-ray absorption significantly increases — while the probe pulse was below it to minimize additional heating and avoid X-ray fluorescence background.

With this setup, the researchers were able to perform simultaneous X-ray Thomson scattering (XRTS), X-ray fluorescence (XRF), and X-ray diffraction (XRD) measurements on copper, nickel, and aluminum targets. Together, these diagnostic techniques provide a comprehensive picture of analyzed materials.

“This provides a new experimental capability for studying ultrafast changes in the electronic, atomic, and structural properties of materials,” said Kunz. “More broadly, the combination of XRTS, XRF, and XRD could provide a more complete picture of matter under extreme conditions by allowing electronic measurements to be cross-checked against atomic and structural diagnostics.”

Source: “A platform for two-color pump-probe X-ray Thomson scattering using the SPring-8 Angstrom Compact Free-Electron Laser,” by Iris Kunz, Willow Martin, Lex Andersen, Zain Abhari, Hannah Bellenbaum, Adrien Descamps, Siegfried Glenzer, Emma Elizabeth McBride, Tiato Osaka, Madison Singleton, Peihao Sun, Jerome Hastings, and Luke Bennett Fletcher, Review of Scientific Instruments (2026). The article can be accessed at https://doi.org/10.1063/5.0350525 .

This paper is part of the Proceedings of the 26th Topical Conference on High-Temperature Plasma Diagnostics Collection, learn more here .