Researchers from the Vienna University of Technology and the University of California, San Diego, have generated X-rays with energies beyond the long-established cutoff predicted by conventional laser physics. Using intense ultraviolet laser pulses on helium atoms, they caused two quantum-correlated electrons to return to the atom simultaneously and release their energy together. The resulting X-ray photons reached energies that cannot be explained by the usual single-electron model, revealing a new way to study interactions between electrons on attosecond timescales.
The work builds on research 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 generate X-ray pulses
Scientists have known for decades that exposing certain atoms to intense laser light can produce 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. Above this boundary, known as the energy cutoff, X-ray emission 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 accelerates it back toward the atom. When the electron returns and releases its energy, it produces light at a much higher frequency than the original laser beam.
Nobel laureate Ferenc Kraus used this mechanism in pioneering experiments in the 1990s. Tenio Popmintchev and his colleagues now work in the same Vienna University of Technology laboratory where Kraus conducted earlier research. Their latest experiments, however, 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.”
This prediction describes a classical process involving one electron. The researchers wanted to determine whether the result would change if two electrons participated.
Two electrons exceed the expected energy cutoff
To investigate, the team directed intense ultraviolet laser pulses at helium atoms. Unlike 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 left the atom at different times, their behavior remained linked by 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,” says Dimitar Popmintchev. “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 produce X-ray photons above 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 with 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.
The difference supports the proposed explanation. Helium has a particularly strong correlation between its two electrons, allowing them to interact in a way that releases their energy simultaneously. Under the experimental conditions, the outer electrons of argon and neon did not produce the same effect.
The findings are reported in Nature Photonics.
A new tool for studying quantum interactions
Beyond generating higher-energy X-rays, the discovery could provide a new way to investigate how electrons interact inside atoms and other materials.
Anomalous coherent X-ray emission may act as a sensitive spectroscopic “sensor” of electron correlations. By examining the shape of the emission spectrum, its maximum energy and how it changes with the polarization of the laser light, researchers can gather information about interacting electrons over extremely short timescales.
These interactions occur over attoseconds. One attosecond is equal to one billionth of a billionth of a second. Observing processes at this speed is essential for understanding how electrons move and interact in quantum systems.
The principle could eventually be extended beyond individual helium atoms to molecules and solid materials containing strongly interacting electrons.
The technique used in the work, known as harmonic generation, originally helped make attosecond physics possible. The new results show that it can also expose the complex behavior of multiple interacting quantum particles.
Such insights could contribute to research in quantum computing and help scientists understand and design advanced nanomaterials with carefully controlled properties.
Source: www.sciencedaily.com
