Electrons move so quickly in atoms and molecules that studying their behavior requires flashes of light that last only a fraction of a billionth of a billionth of a second.
Now scientists have found a way to concentrate these ultra-short X-ray flashes into a tiny spot and make them stronger where they hit a sample.
In an experiment at the European X-ray Free Electron Laser (European XFEL), researchers achieved record intensity and fluence for attosecond pulses in hard X-ray light. Their peak intensity exceeded 1020 Watts per square centimeter.
“This is a world record in intensity and fluence for an attosecond pulse in the hard X-ray region,” said Ichiro Inoue, lead researcher and scientist at the University of Tokyo.
This advance could help scientists study how electrons respond to intense light and eventually find ways to control them during chemical reactions.
How precision mirrors concentrate X-ray energy
The experiment, conducted in April 2026, used X-ray flashes estimated to last 300 attoseconds. An attosecond is a billionth of a billionth of a second, making these pulses useful for studying some of the fastest processes involving electrons.
The researchers passed the pulses through an advanced Kirkpatrick-Baez (AKB) optical system, which consists of two precision mirrors. Each flash was reflected from the mirrors a total of four times. One mirror focused the X-rays vertically, while the other focused them horizontally, concentrating the beam into a spot just 180 × 330 nanometers.
In short, the researchers somehow focused the X-rays into an area about 100,000 times smaller than the cross-section of a typical human hair.
The success builds on previous work at the European XFEL, where researchers demonstrated attosecond pulses in the hard X-ray regime with terawatt-scale power. However, generating strong impulses is only part of the problem.
The energy must also reach the sample in a sufficiently concentrated form to produce the desired effects – a challenge also faced by other X-ray laser technologies that are used to study matter at tiny scales.
“A few years ago, we demonstrated terawatt-scale hard X-ray attosecond pulses at the European
Copper atoms help verify recording intensity
To independently check the intensity, the team aimed the X-ray pulses at a copper foil and examined the light emitted by its atoms.
The incoming X-rays knocked electrons out of the deep inner shells of the copper atoms, leaving vacancies or holes behind. Then electrons from the outer shells fell into these gaps, releasing energy in the form of X-rays.
Given sufficiently intense illumination, this emission can stimulate other excited copper atoms to emit X-rays that travel in the same direction. The resulting amplification creates a stronger, more focused signal. This process is called enhanced spontaneous emission (ASE), a phenomenon related to the emission processes that make X-ray lasers possible.
“It’s almost magical to observe this. Normal fluorescence spreads in all directions, regardless of the direction of the incident beam. But when the ASE threshold is exceeded, you see how the light cone begins to condense in the forward direction,” said Anders Madsen, who was not directly involved in the study but is project manager of the facility where the experiment was carried out.
Initially, the ASE signal grew rapidly as the researchers increased the pulse energy. However, at higher energies the signal began to flatten, a behavior called saturation.
This response was consistent with the extremely high intensities estimated from the pulse energy, duration, and focused spot size, and allowed independent verification of the calculations.
The next step in electron capture in action
The achievement could help researchers study the behavior of electrons in atoms and molecules, including how matter responds to intense X-rays. In the longer term, these studies could contribute to efforts to control electrons in chemical reactions.
Researchers are already exploring ways to capture such rapid changes, including experiments that image electrons and atoms during chemical reactions. However, the experiment demonstrated the generation and verification of intense X-ray pulses rather than the direct control of electrons during a reaction.
Initially, the researchers used an AKB mirror system brought from Japan. European
The study is published in the journal optics.
