Researchers exceed predicted X-ray energy limit in helium

Researchers at TU Wien and the University of California San Diego have generated X-rays from helium at energies above the limit predicted by conventional laser theory. The team reported in Nature Photonics that an interaction involving two electrons enabled the higher-energy emissions.

In the conventional process, a laser pulls an electron from an atom, accelerates it and drives it back. Its return releases energy as light. This process produces a range of frequencies, but emissions become much weaker above a predicted energy cutoff.

For the helium experiment, the researchers used intense ultraviolet laser pulses to free the atom’s two electrons at different moments. By controlling the pulses, they arranged for the electrons to return together. According to the team, quantum correlations connected their behavior, allowing their combined energy to produce a single X-ray photon above the conventional cutoff.

Measurements revealed a second band of sustained X-ray emission at higher photon energies. This band was weaker than the main emission but reached beyond the expected range. Comparable tests with argon and neon did not show the same effect in their outer electrons, supporting the researchers’ explanation involving helium’s strongly correlated electrons.

The researchers say the technique could help probe electron interactions over attosecond timescales. They suggest that examining the emitted spectrum and its response to laser polarization could reveal details of those interactions, with possible future extensions to molecules and solid materials.