Gerda Neyens

The ionization potential (IP) of radium monofluoride (RaF) was measured to be 4.969(2)[10] eV, revealing a relativistic enhancement in the series of alkaline earth monofluorides. The results are in agreement with a relativistic coupled-cluster prediction of 4.981(7) eV, incorporating up to quantum electrodynamics corrections. Using the same computational methodology, an improved calculation for the dissociation energy ( D 0 ) of 5.54(5) eV is presented. This confirms that RaF joins the group of diatomic molecules for which D 0 > IP , paving the way for precision control and interrogation of its Rydberg states.

The apparent invariance of the strong nuclear force under combined charge conjugation and parity (CP) remains an open question in modern physics^1,2. Precision experiments with heavy atoms and molecules can provide stringent constraints on CP violation via searches for effects due to permanent electric dipole moments and other CP-odd properties in leptons, hadrons and nuclei³. Radioactive molecules have been proposed as highly sensitive probes for such searches⁴, but experiments with most such molecules have so far been beyond technical reach. Here we report the production and spectroscopic study of a gas-phase actinium molecule, ²²⁷AcF. We observe the predicted strongest electronic transition from the ground state, which is necessary for efficient readout in searches of symmetry-violating interactions. Furthermore, we perform electronic- and nuclear-structure calculations for ²²⁷AcF to determine its sensitivity to various CP-violating parameters, and find that a realistic, near-term experiment with a precision of 1 mHz would improve current constraints on the CP-violating parameter hyperspace by 3 orders of magnitude. Our results thus highlight the potential of ²²⁷AcF for exceptionally sensitive searches of CP violation.

Precise experimental control and interrogation of molecules and calculations of their structure are enriching the investigation of nuclear and particle physics phenomena. Molecules containing heavy, octupole-deformed nuclei, such as radium, are of particular interest. Here, we report precision laser spectroscopy measurements and theoretical calculations of the structure of the radioactive radium monofluoride molecule ²²⁵ Ra ¹⁹ F. Our results reveal fine details of the short-range electron-nucleus interaction, indicating the high sensitivity of this molecule to the distribution of magnetization, within the radium nucleus. These results provide a stringent test of the description of the electronic wave function inside the nuclear volume, highlighting the suitability of these molecules for investigating subatomic phenomena.

We present two newly constructed experimental setups—REBEL (Resonant Excitation of Beams with Electromagnetic fields and Lasers) and STRIPE (Stopping and Trapping of Radioactive Isotopes for Precision Experiments)—integrated into a single offline beamline at KU Leuven. REBEL is designed for collinear laser spectroscopy of ion bunches, including isobaric separation with a multi-reflection time-of-flight mass spectrometer, enabling high-sensitivity measurements of mass-selected fast-ion beams. In contrast, STRIPE focuses on the deceleration, trapping, and laser cooling of ions in a segmented linear Paul trap, optimized for long interrogation times and precision spectroscopy. The shared infrastructure features stable high-voltage operation (<10 ppm), modular vacuum sections, and a fast-beam switchyard to route ions to either experiment. Initial results include a mass-resolving power of R ≈ 12 900 in REBEL and successful ion trapping and laser cooling of ions with a kinetic energy of 10 keV in STRIPE, with improved performance achieved using a frequency-modulated cooling laser. This dual-system platform enables the development and benchmarking of advanced spectroscopy and trapping techniques and is compatible with future operation at radioactive ion beam facilities.

The apparent invariance of the strong nuclear force under combined charge conjugation and parity (CP) remains an open question in modern physics. Precision experiments with heavy atoms and molecules can provide stringent constraints on CP violation via searches for effects due to permanent electric dipole moments and other CP-odd properties in leptons, hadrons, and nuclei. Radioactive molecules have been proposed as highly sensitive probes for such searches, but experiments with most such molecules have so far been beyond technical reach. Here we report the first production and spectroscopic study of a gas-phase actinium molecule, $^{227}$AcF. We observe the predicted strongest electronic transition from the ground state, which is necessary for efficient readout in searches of symmetry-violating interactions. Furthermore, we perform electronic- and nuclear-structure calculations for $^{227}$AcF to determine its sensitivity to various CP-violating parameters, and find that a realistic, near-term experiment with a precision of 1 mHz would improve current constraints on the CP-violating parameter hyperspace by three orders of magnitude. Our results thus highlight the potential of $^{227}$AcF for exceptionally sensitive searches of CP violation.

Measurements of the 5 p P 3 / 2 2 → 9 s S 1 / 2 2 and 5 p P 1 / 2 2 → 8 s S 1 / 2 2 transitions in the indium atom, combined with new atomic physics calculations, were used to extract the changes in mean-square nuclear charge radii, δ 〈 r 2 〉 , of the indium ( Z = 49 ) isotopes In 101 – 111 , In 113 – 123 , and In 125 – 131 . With a proton hole in the closed nuclear shell of Z = 50 , indium provides a detailed study of the effect of unpaired nucleons adjacent to the proton-shell closure, allowing investigation into the charge radii for isotopes between the two major neutron-shell closures at N = 50 and N = 82 . A study of the variations in charge radii between neighboring isotopes with neutron number (the ‘odd-even staggering') is presented and provides further insight and challenges for the theoretical description of the size of proton-hole nuclei. Two nuclear theories, density functional theory and the valence-space in-medium similarity renormalization group method, were employed to interpret the data. The new information obtained in this work provides valuable insights into the successes and shortcomings of the theoretical approaches employed.

We employed laser spectroscopy of atomic transitions to measure the nuclear charge radii and electromagnetic properties of the high-spin isomeric states in neutron-rich indium isotopes (Z = 49) near the closed proton and neutron shells at Z = 50 and N = 82. Our data reveal a reduction in the nuclear charge radius and intrinsic quadrupole moment when protons and neutrons are fully aligned in 129In(N = 80), to form the high spin isomer. Such a reduction is not observed in 127In(N = 78), where more complex configurations can be formed by the existence of four neutron-holes. These observations are not consistently described by nuclear theory.

Nuclear charge radii of neutron-rich Sc 47 – 49 isotopes were measured using collinear laser spectroscopy at CERN-ISOLDE. The new data reveal that the charge radii of scandium isotopes exhibit a distinct trend between N = 20 and N = 28 , with Sc 41 and Sc 49 isotopes having similar values, mirroring the closeness of the charge radii of Ca 40 and Ca 48 . Theoretical models that successfully interpret the radii of calcium isotopes could not account for the observed behavior in scandium radii, in particular the reduced odd-even staggering. Remarkably, the inclusion of the new Sc 49 radius data has unveiled a similar trend in the charge radii of N = 28 isotones and Z = 20 isotopes when adding the protons atop the Ca 48 core and the neutrons atop the Ca 40 core, respectively. We demonstrate that this trend is consistent with the prediction of the seniority model.