Scientists in Austria and the United States have exceeded a long-accepted limit in laser physics, generating X-rays with higher energies than conventional theory says should be possible, by coaxing two electrons in a helium atom to release their energy at the same moment.
The work, published in Nature Photonics, comes from a collaboration between TU Wien and the University of California San Diego and builds on attosecond-physics techniques that contributed to the 2023 Nobel Prize in Physics. For decades, researchers have known that firing laser light at certain atoms knocks an electron loose, accelerates it in the laser's electric field, and releases a burst of high-frequency light — including X-rays — when the electron collides back into its parent atom. But the energy of that light has always had a hard ceiling, known as the cutoff, set by the properties of the incoming laser.
Two Electrons, Not One
"The mechanism behind short X-ray pulses has been well known for a long time," said Prof. Tenio Popmintchev of TU Wien's Institute of Photonics. "The laser tears a single electron away from the atom. The electron is then accelerated in the laser's electric field until it eventually collides with the atom again. The energy it loses in the process is emitted in the form of light."
The new experiment instead removed both of helium's electrons in succession, using carefully controlled ultraviolet pulses to arrange for the two to return to the atom at exactly the same moment. "The energy of two electrons is then released all at once," said Dimitar Popmintchev, a postdoctoral researcher on the team. "And when more energy is available, a single higher-energy X-ray photon with higher frequency can also be generated." Because the two electrons remain linked by quantum correlations even after being freed at different instants, their combined return produces a second, weaker plateau of X-ray emission at energies well beyond where the single-electron process cuts off.
Why It Matters Beyond the Lab
The finding reopens a boundary that attosecond physicists have treated as essentially fixed since the field's founding experiments in the 1990s, conducted at the same TU Wien institute where the current researchers now work. Pushing X-ray generation to higher energies without larger, more expensive lasers could eventually aid research fields that depend on ultrafast X-ray pulses, from probing chemical reactions as they happen to imaging structures too fast for conventional X-ray sources to capture. The team's next step is examining whether even more electrons, coordinated in the same way, can push the boundary further still.