Low density and soft matter Figure 1. (a) Experimental set up. Linearly polarized undulator radiation is monochromatized (1) and focused on HCIs (2) which are produced and trapped by means of an electron beam (not shown) tightly focused by the magnetic field of EBIT. X-rays following resonant excitation are scattered by the ions and recorded by two SDDs (3), mounted one perpendicular and the other parallel to the polarization plane. Insets (4) show schematic level diagram (right) and angular distribution of fluorescence emission (left). (b) Exemplary absorption spectra recorded by two detectors placed at 90 degree and 0 degree relative to polarization plane. Left-axis (blue): Fluorescence yield recorded within the polarization plane; Right-axis (pink): Fluorescence yield perpendicular to the polarization plane. (c) Ratio of cross sections for the resonant scattering of light parallel and perpendicular to the polarization vector of the incident light against the principal quantum number n of the excited electron. For increasing n, emission becomes more isotropic. (d) Extracted lifetimes for 1s np 1P1 states. Moto Togawa1,2,3, Jan Richter4,5, Chintan Shah6,1,7, Marc Botz1,3, Joshua Nenninger1, Jonas Danisch1, Joschka Goes1, Steffen Kühn1, Pedro Amaro8, Awad E.A. Mohamed9,10, Yuki Amano11, Stefano Orlando9, Roberta Totani9,12, Monica de Simone¹³, Stephan Fritzsche14,15,16, Thomas Pfeifer1, Marcello Coreno9, Andrey Surzhykov4,17, José R. Crespo, López-Urrutia1 1 Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany 2 European XFEL, Holzkoppel 4, 22869 Schenefeld, Germany 3 Heidelberg Graduate School for Physics, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 226, 69120 Heidelberg, Germany 4 Physikalisch-Technische Bundesanstalt, D-38116 Braunschweig, Germany 5 Institut für Theoretische Physik, Leibniz UniversitätHannover, Appelstraße 2, 30167 Hannover, Germany 6 NASA/Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA 7 Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA 8 Laboratory of Instrumentation, Biomedical Engineering and Radiation Physics (LIBPhys-UNL), Department of Physics, NOVA School of Science and Technology, NOVA University Lisbon, 2829-516 Caparica, Portugal 9 CNR-ISM, Istituto Struttura della Materia, Trieste branch, 34149 Trieste, Italy 10 Physics Division, School of Science and Technological Innovation, University of Camerino, Via Madonna delle Carceri 9, Camerino, MC, Italy 11 Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa, 252-5210, Japan 12 Elettra – Sincrotrone Trieste S.C.p.A., Strada Statale 14, km 163.5, 34149 Trieste, Italy 13 CNR-IOM, Istituto Officina dei Materiali, 34149 Trieste, Italy 14 Helmholtz-Institut Jena, Fröbelstieg 3, D-07743 Jena, Germany 15 GSI Helmholtzzentrum für Schwerionenforschung GmbH, Planckstraße 1, D-64291 Darmstadt, Germany 16 Theoretisch-Physikalisches Institut, Friedrich-Schiller-Universität Jena, D-07745 Jena, Germany¹⁷ Institut für Mathematische Physik, Technische Universität Braunschweig, Mendelssohnstrasse 3, D-38106 Braunschweig, Germany Acknowledgments Financial support was provided by the MaxPlanck-Gesellschaft (MPG), Bundesministerium für Bildung und Forschung (BMBF) through project 05K13SJ2, and by the AHEAD-2020 Project grant agreement 871158 of the European Union’s Horizon 2020 Programme. C.S. acknowledges support from NASA under award number 80GSFC21M0002 and MaxPlanck-Gesellschaft (MPG). P.A. acknowledges support from Fundação para a Ciência e Tecnologia, Portugal, under grant No. UID/ FIS/04559/2020 (LIBPhys). We acknowledge Elettra Sincrotrone Trieste for providing access to its synchrotron radiation facilities (long-term proposals nr. 20205206). Fabio Zuccaro is acknowledged for technical support during beamtime preparation and realization. J.R. and A.S. acknowledge funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy – EXC-2123 QuantumFrontiers – 390837967. Original paper M. Togawa et al. Phys. Rev. Lett. 133, 163202, https://doi.org/10.1103/PhysRevLett.133.163202. (b) (c) (d) Fig. 1 Elettra Highlights 2024–2025 45
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