Two Electrons Firing as One Break the X-Ray Energy Limit in Helium Atoms

Physicists have produced X-rays with higher energies than the standard theory of laser-driven X-ray generation allows, by getting two electrons in a helium atom to dump their energy at the same moment. The result, from researchers at TU Wien in Vienna and the University of California San Diego, was published in Nature Photonics.

The process they exceeded is called high-harmonic generation. A strong laser pulse tears an electron from an atom, the laser's electric field drives it away and then swings it back, and when it collides with the atom again it releases the energy it gained as a short burst of much higher-frequency light, including X-rays. "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. Ferenc Krausz used the mechanism in pioneering experiments at the same institute in the 1990s, and attosecond physics built on it was recognized with the 2023 Nobel Prize in Physics.

For a single electron, theory sets a firm ceiling. "It can be shown that when the electron returns to its atom, it generates a whole range of different frequencies with roughly equal intensity," said Dimitar Popmintchev, a postdoctoral researcher on the team. "But there is an upper limit. Frequencies above this limit are much weaker; it is simply not possible to exceed a certain maximum frequency." Beyond that cutoff, X-ray production drops off sharply.

The team wanted to know what would happen if two electrons took part. They used intense ultraviolet laser pulses on helium, which has two electrons, and arranged for both to be removed one after the other. Though freed at different moments, the electrons stayed linked by quantum correlations, so their motions could not be treated as independent. By tuning the pulses, the researchers made both return to the atom at the same time. "The energy of two electrons is then released all at once," Dimitar Popmintchev said. "And when more energy is available, a single higher-energy X-ray photon with higher frequency can also be generated."

The evidence appeared in the spectrum of the emitted light. Beyond the usual cutoff, the researchers found a second region of sustained emission, known as a plateau, at much higher photon energies than the standard theory predicts. It was weaker than the main emission but extended well past the established range.

A control experiment supports the explanation. The extra signal showed up with helium but not when the team ran similar experiments on argon and neon, whose outer electrons did not show the same behavior under those conditions. Helium's two electrons are especially strongly correlated, which the researchers say is what makes the simultaneous release possible.

The finding may matter beyond making more energetic X-rays. The team suggests the emission could serve as a very sensitive "sensor" of how electrons influence each other. The shape of the spectrum, its maximum energy and how it changes with the polarization of the laser light would carry information about interacting electrons on attosecond timescales, where one attosecond is a billionth of a billionth of a second.

The researchers say the idea could eventually extend from single helium atoms to molecules and solid materials in which electrons interact strongly. That could feed into work on quantum computing and the design of nanomaterials whose properties are tightly controlled, and it turns a technique built to watch single electrons into one that can watch them in groups.