Low density and soft matter Figure 1. (a) Schematic drawing of the energy level scheme. The intense XUV pulses produced by FERMI drive Rabi oscillations between the 1s and 2p state of helium. This leads to a splitting of the energy levels while the XUV pulse acts on the atoms. This energy splitting is mapped to the continuum upon photoionization, where it appears as the known Autler-Townes (AT) doublet in the kinetic energy distribution of emitted electrons. (b) Measured kinetic energy distributions of emitted electrons for different chirp applied to the XUV pulses. The chirp is given as group delay dispersion (GDD). It can be clearly seen, that for a down-chirp (GDD < 0 fs2) the higher energy state is mostly occupied, whereas for an up-chirp (GDD > 0 fs2) the situation is reversed. F. Richter1, U. Saalmann2, E. Allaria3, M. Wollenhaupt4, B. Ardini5, A. Brynes3, C. Callegari3, G. Cerullo5, M. Danailov3, A. Demidovich3, K. Dulitz6, R. Feifel7, M. Di Fraia3,8, S. D. Ganeshamandiram1, L. Giannessi3,9, N. Gölz1, S. Hartweg1, B. von Issendorff1, T. Laarmann10,11, F. Landmesser1, Y. Li1, M. Manfredda3, C. Manzoni12, M. Michelbach1, A. Morlok1, M. Mudrich13, A. Ngai1, I. Nikolov3, N. Pal3, F. Pannek14, G. Penco3, O. Plekan3, K. C. Prince3, G. Sansone1, A. Simoncig3, F. Stienkemeier1, R. J. Squibb7, P. Susnjar3, M. Trovo3, D. Uhl1, B. Wouterlood1, M. Zangrando3,8, L. Bruder1 1 Institute of Physics, University of Freiburg, Germany 2 Max-Planck-Institut für Physik komplexer Systeme, Dresden, Germany 3 Elettra-Sincrotrone Trieste S.C.p.A., Italy 4 Institute of Physics, University of Oldenburg, Germany 5 IFN-CNR, Dipartimento di Fisica, Italy 6 Institut für Ionenphysik und Angewandte Physik, Universität Innsbruck, Austria 7 Department of Physics, University of Gothenburg, Sweden 8 Istituto Officina dei Materiali - CNR (CNR-IOM), Trieste, Italy 9 Istituto Nazionale di Fisica Nucleare - Laboratori Nazionali di Frascati, Roma, Italy 10 Deutsches Elektronen-Synchrotron DESY, Hamburg, Germany 11 The Hamburg Centre for Ultrafast Imaging CUI,Hamburg, Germany 12 IFN-CNR, Milan, Italy 13 Department of Physics and Astronomy, Aarhus University, Denmark 14 Institute for Experimental Physics, University of Hamburg, Germany e-mail: lukas.bruder@physik.uni-freiburg.de Acknowledgments We acknowledged the funding from the Bundesministerium für Bildung und Forschung (BMBF) LoKo-FEL (05K16VFB) and STAR (05K19VF3); the European Research Council (ERC) Starting Grant MULTIPLEX (101078689); the Deutsche Forschungsgemeinschaft (DFG) RTG 2717 and grant 429805582 (project SA 3470/4-1) and project STI 125/24-1; the BadenWürttemberg Stiftung Eliteprogram for Postdocs; the Swedish Research Council and Knut and Alice Wallenberg Foundation, Sweden; the Danish Agency for Science, Technology, and Innovation for funding through the instrument centre DanScatt. The research leading to this result has been supported by the COST Action CA21101 Confined Molecular Systems: From a New Generation of Materials to the Stars (COSY). Original paper F. Richter et al., Nature 636, 337 (2024); DOI: 10.1038/s41586-024-08209-y. Fig. 1 t E Ekin 2 1 Rabi ioniza�on AT doublet XUV pulse 17 18 19 20 Electron kinetic energy (eV) 0 1 2 3 Electron yield (arb.u.) GDD: 695fs 2 GDD: 334fs 2 GDD: 135fs 2 GDD: -591fs 2 GDD: -1127fs 2 Elettra Highlights 2024–2025 51
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