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Abstract: The phase-locked extreme ultraviolet (XUV) harmonic comb generated by the UV-seeded free-electron laser (FEL) FERMI provides intense attosecond pulse trains (APTs) with controllable temporal waveforms [1]. The high degree of coherence, along with the tunability in wavelength and harmonic intensity of a seeded FEL, makes these APTs ideally suited for scanning across spectral features such as resonances and Cooper minima, enabling the extraction of atomic phases. In this work, we aim to measure the variation of the atomic phase across the argon (Ar) 3s Amusia-Cooper minimum (ACM) using the Reconstruction of Attosecond Beating by Interference of Two-photon Transitions (RABBITT) technique. We demonstrate the first implementation of a commensurate visible laser as a probe field in combination with the XUV APTs. To overcome timing jitter between the XUV and the probe pulses, we employ a timing tool based on single-shot correlation analysis of the sideband signal [2]. This approach allows us to retrieve the relative phase between the XUV harmonics and the visible probe pulse. By reordering the acquired spectra, we can get the phase variation of the Ar 3s channel relative to the 3p reference channel, observing the theoretically predicted phase dip in the vicinity of the 3s ACM [3]. Furthermore, we demonstrate that the controlled harmonics in the comb allow us to avoid contamination from shake-up processes. These results demonstrate the capability of seeded FEL-based RABBITT measurements to access subtle weak channel and multi-photon atomic phase variations and highlight their potential for time-resolved studies of electron dynamics in complex systems and future tests of causality on the attosecond time scale [4].
[1] P. K. Maroju et al., Nature, 386–391, 2020
[2] P. K. Maroju et al., Nat. Photon., 200–207, 2023
[3] M. Li et al., Phys. Rev. Lett., 183202, 2025
[4] R. Tahouri and J. M. Dahlström New J. Phys. 28 063205, 2026
Keywords: Attosecond; RABBITT; FEL; XUV.
| Scientific Topics | AMO Physics |
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