Researchers from the Vienna University of Technology and the University of California, San Diego, have produced X-rays with energies beyond the long-established cutoff predicted by conventional laser physics. By directing intense ultraviolet laser pulses at helium atoms, they showed that two quantum-correlated electrons can return to the atom together and release their energy in a single, unusually high-energy X-ray photon.
The finding, reported in Nature Photonics, builds on work that helped establish attosecond physics. Laser technology developed at the Vienna University of Technology in the 1990s contributed to advances recognized by the 2023 Nobel Prize in Physics.
How lasers produce powerful X-ray pulses
Scientists have known for decades that laser light can cause certain atoms to emit extremely short pulses of much higher-frequency radiation, including X-rays. Established theory predicts the maximum energy of this radiation from the properties of the incoming laser. Beyond this boundary, known as the energy cutoff, X-ray production becomes very weak.
“The mechanism behind short X-ray pulses has been well known for a long time,” says Professor Tenio Popmintchev from the Institute of Photonics at the Vienna University of Technology. “The laser rips a single electron from the atom. The electron is then accelerated in the laser’s electric field and eventually collides with the atom again. The energy lost in the process is released in the form of light.”
The laser’s electric field first drives the electron away from its parent atom and then sends it back. When the electron returns and releases its energy, it produces light at a much higher frequency than the original laser beam. This process is known as high-harmonic generation.
Nobel laureate Ferenc Krausz used the mechanism in pioneering experiments in the 1990s. Tenio Popmintchev and his colleagues now work in the same Vienna University of Technology laboratory where Krausz conducted earlier research, but their latest experiments have pushed the process beyond its previously understood limits.
“We see that when the electron returns to the atom, different frequencies are generated with approximately the same intensity,” says Dimitar Popmintchev, a postdoctoral researcher in Tenio Popmintchev’s team at the Vienna University of Technology. “But there is an upper limit. Frequencies above this upper limit become much weaker. It is simply impossible to exceed a certain maximum frequency.”
That prediction describes a classical process involving a single electron. The researchers wanted to determine whether the outcome would change if two electrons took part.
Two electrons exceed the expected energy limit
The team used intense ultraviolet laser pulses to interact with helium atoms. In contrast to the usual one-electron process, the experiment removed both of helium’s electrons in succession.
The first electron was emitted and accelerated by the laser field, followed by the second. Although the electrons were released at different times, their behavior remained linked through quantum correlations. Their motions therefore could not be treated as completely independent.
By carefully controlling the ultraviolet pulses, the researchers arranged for both electrons to return to the atom at the same time.
“Using a UV-driven pulse, we can arrange for both electrons to return to the atom exactly at the same time,” Dimitar Popmintchev says. “The energy of two electrons is then released at once. And when more energy is available, a single high-energy X-ray photon with a higher frequency may also be produced.”
This coordinated release makes it possible to generate X-ray photons that exceed the traditional energy cutoff. Instead of relying on the energy of one returning electron, the process draws on the binding energy of two interacting electrons.
An unexpected X-ray signal
The researchers found experimental evidence that the two-electron process produces X-rays at unusually high energies.
When they analyzed the spectrum of the emitted coherent radiation, they identified a second region of sustained emission at photon energies far above those predicted by standard theory. This additional region, known as a plateau, was weaker than the primary radiation but extended well beyond the previously established energy range.
The unexpected signal appeared only when the researchers used helium. Similar experiments with argon and neon did not produce the same behavior in their outer electrons.
This difference supports the proposed explanation. Helium has a particularly strong correlation between its two electrons, allowing them to interact in a way that releases energy simultaneously. Under the experimental conditions, the outer electrons of argon and neon did not produce the same effect.
A new way to study quantum interactions
Beyond producing higher-energy X-rays, the discovery could provide a new method for investigating how electrons interact inside atoms and other materials.
Anomalous coherent X-ray emission could act as a sensitive spectroscopic sensor for electron correlations. By analyzing the shape of the emission spectrum, its maximum energy and its response to the polarization of the laser light, researchers may be able to study interacting electrons over extremely short time scales.
These interactions occur over attoseconds, with one attosecond equal to one billionth of a second. Observing matter at such speeds is important for understanding how electrons move and interact in quantum systems.
The same principle could eventually be extended beyond individual helium atoms to molecules and solid materials containing strongly interacting electrons.
The harmonic-generation technique that helped establish attosecond physics is now also revealing the behavior of multiple interacting quantum particles. The resulting insights could contribute to research in quantum computing and to efforts to understand and design nanomaterials with precisely controlled properties.
Source: www.sciencedaily.com