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Free-electron lasers (FELs) are opening new regimes of light–matter interaction[1-3], and the advent of spectro-temporal shaping of intense XUV to X-ray pulses[4,5] is enabling quantum-control and non-perturbative photoexcitation in a previously inaccessible spectral range[6,7]. Rapid adiabatic passage (RAP) is a particularly robust quantum control protocol, providing efficient photoexcitation that is immune to intensity fluctuations and to spatial mode inhomogeneities of the driving field, yet it has so far been restricted to the UV-to-radiofrequency range. Here we demonstrate RAP quantum control of an atomic transition in the XUV, efficiently populating the narrow-linewidth, long-lived (~1 ns) 1s2p state of helium with an ultraintense, chirped seeded-FEL pulse. Experiments were carried out at the seeded FERMI FEL (Elettra, Trieste), with the photon energy tuned to the 1s²→1s2p transition at 21.18 eV and focused into a helium gas jet. The excited-state population was probed by linear ionization with a 1 ps delayed, 40 fs, 266 nm UV pulse and the resulting 1.3 eV photoelectrons were recorded in a velocity-map-imaging spectrometer, providing a direct measure of the 1s2p population. We compared a constant-frequency (~80 fs) resonant pulse against a chirped pulse (group-delay dispersion ~−1000 fs², ~110 fs) driving RAP, while scanning the FEL intensity over three orders of magnitude (~10¹¹–10¹⁴ W/cm²). Above ~10 µJ (peak intensity ~10¹³ W/cm²), RAP provides a significantly higher yield than the compressed pulse counterpart. Furthermore, theoretical models predict an excitation probability close to 1 for a large part of the spatial FEL mode.
These results open a new route to high-fidelity control of atomic ensembles in the XUV, with applications ranging from superradiance and correlation-driven phenomena[8], to the efficient population of narrow-linewidth nuclear transitions.
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| Scientific Topics | AMO Physics |
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