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Timing X ray flashes to follow electron motion

A new way to measure and synchronize extremely short X ray flashes could help researchers follow the electronic changes that begin chemical reactions.

Published November 8, 2026Intelligence / Frontier Tech / Experimental Systems

A chemical reaction begins before atoms have time to rearrange. Electrons respond first, redistributing the charge that holds molecules together. To follow those opening moments, scientists need flashes of light so brief that the motion stays sharp, and a reliable way to know when each flash arrives.

A study published in Nature on November 4 supplies a new part of that toolkit. Researchers working at the Linac Coherent Light Source at SLAC National Accelerator Laboratory measured hard X-ray pulses lasting an average of 170 attoseconds. One attosecond is a billionth of a billionth of a second.

The team also demonstrated synchronization between pairs of pulses. That matters because many ultrafast experiments use one flash to disturb a sample and a second to examine its response. Adjusting the interval builds a sequence of measurements, revealing how matter changes over time.

Hard X-rays add another advantage: their short wavelengths and high energy let researchers investigate atomic-scale structure and penetrate dense materials. Combining that reach with dependable timing could make it easier to connect electronic changes to the physical behavior of a material.

Measuring the flash with another flash

The apparatus divides an X-ray pulse into two paths, allowing one copy to arrive slightly later than the other. In an August conference presentation, lead author Yanwen Sun described a mirror-pair arrangement that changes the path length while keeping the outgoing beam stable. The reported delay scans had precision finer than 20 attoseconds.

To measure pulse duration, the researchers used a process in which a material absorbs two photons together. Its response depends strongly on the overlap between the two flashes. Scanning the delay produces an intensity autocorrelation: a measurement of how a pulse overlaps with a delayed copy of itself. This yielded the average duration of 170 attoseconds directly in the time domain.

The Nature study also measured the pulses’ coherence time, which describes how long their light waves maintain a predictable relationship. Duration, coherence and synchronization all affect how reliably scientists can interpret an experiment. A short flash becomes more useful when its timing and wave properties can be characterized.

Building on pulses already in use

The advance follows a related paper published in Nature Photonics on October 20. That team, led by Ichiro Inoue, confirmed hard X-ray pulses lasting roughly 100 to 400 attoseconds at LCLS. They inferred pulse duration from the fluorescence produced when intense X-rays struck copper, combined with measurements of the beam. Their results showed considerable variation from one pulse to the next.

In SLAC’s October account, scientist Agostino Marinelli said that “measuring pulses this short is really, really difficult.” The November work brings another measurement method and a precisely controlled pair of flashes to this growing experimental toolkit.

Researchers are already pursuing questions that require the extra penetration of hard X-rays. SLAC’s October report described experiments probing fleeting electronic states in dense plasma. Such states can change before a slower measurement captures them, making their interpretation dependent on theoretical assumptions. Faster measurements offer a way to test those assumptions against observations.

What better timing can reveal

Earlier experiments show why that test matters. In a separate study described by SLAC in August, researchers used two timed X-ray flashes to follow a molecule immediately after one of its electrons was removed. They recorded ten timestamps within the first ten femtoseconds, observing electronic changes before chemical bonds began to rearrange. A femtosecond contains 1,000 attoseconds.

The measured behavior proved more complicated than leading simulations predicted. Theorists had to account for additional motion and interactions to bring their models closer to the experiment. That is a practical scientific return from faster observation: it can expose the missing physics in a calculation that otherwise looks convincing.

Work on liquid water offers another example. In 2024, an international team used attosecond X-rays to examine the immediate electronic response to radiation, before movement of the heavier atomic nuclei blurred the signal. The measurements helped resolve an argument over whether earlier signals reflected two distinct structural arrangements in water. They instead traced the disputed feature to atomic motion.

The November result is a laboratory measurement advance, with its strongest implications still ahead in experiments that use the new timing system. The authors identify X-ray spectroscopy and diffraction as applications. Together, those methods could help researchers follow how electronic states and atomic-scale structure change during the same ultrafast event.