Periodic Reporting for period 4 - LRC (Laser Resonance Chromatography of Superheavy Metals)
Reporting period: 2023-12-01 to 2024-12-31
The LRC project aims at developing a novel optical spectroscopy method to study the completely unexplored atomic structure of the superheavy transition metals, starting with element 103, lawrencium (Lr). Our efforts include the prediction and experimental exploration of spectral lines that can serve as fingerprints for multi-messenger astronomy in the search for superheavy elements in the universe. Since relativistic and many-body effects become more important with increasing proton number and the nuclear charge distribution affects the optical transitions, the experimental study of these lines is essential to advance our understanding of atomic structure and to investigate the effects of nuclear shells and deformations on the stability of radionuclides at the upper end of the Segré chart.
Currently, optical spectroscopy ends at element 102, nobelium. Beyond that, the atomic structure is experimentally unknown, and experiments become extremely challenging. Contemporary methods based on the detection of fluorescent atoms are not sensitive enough and are therefore not suitable for such studies. Our novel way of spectroscopy is called laser resonance chromatography and combines the advantages of high sensitivity - as in resonance ionization-based techniques - with the "simplicity" of optical probing as in fluorescence spectroscopy. The technique is fast and efficient and thus best suited for laser probing of fusion product ions as emitted by gas traps behind in-flight separators. Until the end of the project no optical spectroscopy of Lr+ ions could be performed, but now that we know the spectral regions in which to search for these lines, and with our meanwhile established spectroscopy method, the time is ripe to catch up and tackle optical spectroscopy of this elusive element behind the S3 separator of the GANIL/SPIRAL2 facility.
Currently, optical spectroscopy ends at element 102, nobelium. Beyond that, the atomic structure is experimentally unknown, and experiments become extremely challenging. Contemporary methods based on the detection of fluorescent atoms are not sensitive enough and are therefore not suitable for such studies. Our novel way of spectroscopy is called laser resonance chromatography and combines the advantages of high sensitivity - as in resonance ionization-based techniques - with the "simplicity" of optical probing as in fluorescence spectroscopy. The technique is fast and efficient and thus best suited for laser probing of fusion product ions as emitted by gas traps behind in-flight separators. Until the end of the project no optical spectroscopy of Lr+ ions could be performed, but now that we know the spectral regions in which to search for these lines, and with our meanwhile established spectroscopy method, the time is ripe to catch up and tackle optical spectroscopy of this elusive element behind the S3 separator of the GANIL/SPIRAL2 facility.
In the theory part of the project, IHFSCC and Dirac-Coulomb Hamiltonian-based approaches as well as MRCI methods were used to predict the electronic structure of Lr+ and Rf+. The calculations were complemented by the calculation of the lighter element homologs, i.e. Lu+ and Hf+, with known electronic structure [Atoms 10 (2022) 48]. The results were in good agreement with those previously published by the PI and his collaborators [PRA 100 (2019) 062505]. The rigorous electronic structure calculations for the Rf+ ion were the first of their kind and have been published in [PRA 104 (2021) 022813]. In addition, we calculated the interaction potentials of Lu+/He and Lr+/He systems both in the ground and metastable states. The interaction potential for Lr+ in the metastable state could not be studied previously by the PI and collaborators because open-shell systems challenge scaler relativistic calculations performed in [FCHEM 8 (2020)]. Meanwhile we reported both distinct mobilities of Lr+ in He as function of gas temperature and reduced electric fields [PRA, 108, (2023) 012802]. Similar theory works for Rf+/He and Th3+/He systems were conducted and the results were reported in two separate papers [PRA 110 (2024) 012805; PRA 110 (2024) 023105]. Moreover, the theoretical proof-of-principle for the LRC method was published in [PRL 125 (2020) 023002] and [PRA 102 (2020) 013106] and has attracted widespread attention [Physics 13 (2020) 110; physics.aps.org/articles/v13/110]. In 2021 “pro-physik.de” listed our work among the few most important achievements in 2020 in the field of atomic and quantum physics [D. Eidemüller, Jahresrückblick Atom- und Quantenphysik 2020, pro-physik.de]. Currently, additional theory work is undertaken to reveal hyperfine parameters for Ac+ excited states and to predict the transport properties of this cation in He gas. Ac+ is one of the best suited cations for LRC experiments under online conditions as it is electronically similar to Lr+ but can be produced with relatively higher rates. Our preliminary results look very promising and the results will be published soon.
Experimentally, the Laser Resonance Chromatography setup is operational, see attached image (LRC_apparatus.jpg). The chromatographic performance of the apparatus was evaluated by analyzing the arrival time distributions (ATDs) of laser ablated Hf+ ions and the ATD peak separations when comparing Lu+ with Yb+ ions in their ground states. For the first time, a metastable ATD peak was observed in the Lu+ arrival time distributions. Also for the first time, laser resonance chromatography was successfully demonstrated by initiating the resonant 1S0-3P1 optical transition in this ion, allowing optical pumping to the 3D1 metastable state, see attached image (LRC_signal.png). Systematic studies were then performed to elucidate the effect of drift pressure on collision-induced quenching and the effect of sideband cooling on the broadening of the arrival time distributions. For the first time, we measured the hyperfine parameters of the 3P1 state in 176Lu+ and determined the isotope shift of the spectral line relative to the line of 175Lu+. To measure the extraction and transmission efficiencies, we used a 223Ra source, which provides 219Rn+ recoil ions. In a typical bunching mode operation, the overall efficiency of the apparatus is found to be 0.6 %, similar to efficiencies reported for laser resonance ionization-based techniques. The results of the inaugural experiments, including the efficiency measurements, have recently been reported in [NIMB 555 (2024) 165461]. Another manuscript is in preparation in which we report laser resonance chromatography on the two stable Lu+ isotopes. We expect these experimental results to be published by the end of this year.
Experimentally, the Laser Resonance Chromatography setup is operational, see attached image (LRC_apparatus.jpg). The chromatographic performance of the apparatus was evaluated by analyzing the arrival time distributions (ATDs) of laser ablated Hf+ ions and the ATD peak separations when comparing Lu+ with Yb+ ions in their ground states. For the first time, a metastable ATD peak was observed in the Lu+ arrival time distributions. Also for the first time, laser resonance chromatography was successfully demonstrated by initiating the resonant 1S0-3P1 optical transition in this ion, allowing optical pumping to the 3D1 metastable state, see attached image (LRC_signal.png). Systematic studies were then performed to elucidate the effect of drift pressure on collision-induced quenching and the effect of sideband cooling on the broadening of the arrival time distributions. For the first time, we measured the hyperfine parameters of the 3P1 state in 176Lu+ and determined the isotope shift of the spectral line relative to the line of 175Lu+. To measure the extraction and transmission efficiencies, we used a 223Ra source, which provides 219Rn+ recoil ions. In a typical bunching mode operation, the overall efficiency of the apparatus is found to be 0.6 %, similar to efficiencies reported for laser resonance ionization-based techniques. The results of the inaugural experiments, including the efficiency measurements, have recently been reported in [NIMB 555 (2024) 165461]. Another manuscript is in preparation in which we report laser resonance chromatography on the two stable Lu+ isotopes. We expect these experimental results to be published by the end of this year.
The laser resonance chromatography technique has now evolved from a theoretically possible to an experimentally proven and promising method for optical spectroscopy of transition metal cations and is being adapted to the circumstances of future online beams at the S3 separator of the GANIL/SPIRAL2 facility. In the coming years, online commissioning experiments will be performed on 214Ac+, which can be produced at a sufficient rate for optimization purposes prior to level searches in 255Lr+. Although the focus here is on Lr+, our spectroscopic approach provides unprecedented access to laser spectroscopy of many other monoatomic ions throughout the periodic table, especially the transition metals, including the high temperature refractory metals and elements beyond lawrencium.