Two-Electron Effects Extend High-Harmonic Generation into the keV Regime

Isobel McSweeney; Andres Marchisio; Javier Rivera-Dean; Philipp Stammer; Paraskevas Tzallas; Marcelo F. Ciappina; Maciej Lewenstein

arXiv · 2606.24765

Two-Electron Effects Extend High-Harmonic Generation into the keV Regime

Isobel McSweeney·Andres Marchisio·Javier Rivera-Dean·Philipp Stammer·Paraskevas Tzallas·Marcelo F. Ciappina·Maciej Lewenstein

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Abstract

Two-electron processes can generate high harmonics beyond the conventional single-active-electron cutoff. Motivated by recent experimental evidence of an extended secondary plateau in the helium high-harmonic spectrum [S. Wang et al, Optica, (2023); S. Wang et al, In Print in Nature Photon., (2026)], we present a two-electron generalisation of the strong-field approximation. We analyse the resulting expressions using the saddle-point method and determine the extended cutoff. We find good agreement with classical predictions of cutoff scalings of $4.7$ and $5.5$ times the ponderomotive energy, which significantly exceed the established single-electron scaling of 3.17. We calculate high-harmonic spectra generated via a two-electron process in helium atoms driven by an intense few-cycle infrared laser pulse. Our results demonstrate that the harmonic spectrum extends far beyond the water window, reaching photon energies up to $\approx 1.2\,\mathrm{keV}$ in the soft x-ray region. The large spectral bandwidth can support the generation of sub-attosecond soft x-ray pulses, which are of particular interest for probing ultrafast dynamics across matter, including applications in core-level spectroscopy and biological imaging.

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Isobel McSweeney, Andres Marchisio, Javier Rivera-Dean, Philipp Stammer, Paraskevas Tzallas, Marcelo F. Ciappina, Maciej Lewenstein. 2026-06-23. Two-Electron Effects Extend High-Harmonic Generation into the keV Regime. https://arxiv.org/abs/2606.24765

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