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Laser Resonance Chromatography of Superheavy Metals

Periodic Reporting for period 1 - LRC (Laser Resonance Chromatography of Superheavy Metals)

Reporting period: 2019-06-01 to 2020-11-30

The LRC project aims to develop a novel optical spectroscopy method to probe the completely unexplored atomic structure of superheavy transition metals, starting with element 103, lawrencium (Lr).
Our efforts include the prediction and experimental exploration of optical transitions and spectral lines that can serve as fingerprints for multi-messenger astronomy in the search for superheavy elements in the universe.
As relativistic and many-body effects become more important with increasing proton number, and nuclear charge distribution affects optical transitions, experimental exploration of these lines is essential to advance our understanding of atomic structure and to study the effects of nuclear shells and deformations on the stability of radionuclides at the top of the Segré chart.
In the theory part of the project, IHFSCC and Dirac-Coulomb Hamiltonian-based approaches as well as multireference configuration interaction methods were used to predict the electronic structure of Lr+ (Z=103) and Rf+ (Z=104). The calculations were complemented by the calculation of the lighter element homologs, i.e. Lu+ and Hf+, with known electronic structure. The results were in good agreement with those previously published by the PI and his collaborators [E. V. Kahl et al., PRA 100 (2019) 062505, 10.1103/PhysRevA.100.062505]. In addition, we calculated the interaction potentials of Lu+-He and Lr+-He systems both in the ground state and metastable states. The interaction potential for the 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 [G. Visentin et al., FCHEM 8 (2020), 10.3389/fchem.2020.00438]. Our latest predictions will appear shortly. Moreover, the theoretical proof-of-principle for the LRC method was published in [M. Laatiaoui et al., PRL 125 (2020) 023002, 10.1103/PhysRevLett.125.023002] and [M. Laatiaoui et al., PRA 102 (2020) 013106, 10.1103/PhysRevA.102.013106] and has attracted widespread attention [D. Garisto, Physics 13 (2020) 110, physics.aps.org/articles/v13/110]. Only recently “pro-physik.de” listed it 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].
Experimentally, the main project equipment such as the laser system, cryocooler, mass spectrometer and several vacuum pumps were purchased, tested and validated for operation. Electrodynamic simulations were performed for the LRC drift tube, mass filter, and ion optics using the SIMION software package. Designs were made for the assembly frame, many vacuum chambers, and ion optics, and vacuum parts were ordered from the host institute's mechanical workshops. Only the design of the drift cell part is not yet complete, as it consists of nested vacuum chambers and needs some refinements. We ordered many electronic, optical elements and small mechanical parts as well as some consumables needed for the inauguration of the LRC apparatus. We purchased a data acquisition system and developed an FPGA-based program to control the experiment and enable data logging in list mode. The laser system and wavelength monitoring system were inaugurated and the laser beam path was installed. The team was trained in the operation of the laser system and in programming of the FPGA code. The delivered vacuum chambers and parts including the former stopping cell of the SHIPTRAP project were assembled and commissioned. Ultra-high vacuum conditions (about 10^{-8} mbar) were easily and quickly achieved in all segments of the LRC apparatus.
"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 very sensitive and are therefore not suitable for such studies. The technique used in the nobelium studies uses resonant atomic excitation and ionization followed by detection of radioactive decay. It proved to be extremely sensitive, but also susceptible to physicochemical properties of the sample atoms, which could affect its applicability to the refractory elements yet to be studied.
Our way of spectroscopy combines the advantages of high sensitivity - as in resonance ionization-based techniques - with the ""simplicity"" of optical probing as in fluorescence spectroscopy. We envisage laser probing of fusion product ions as emitted by gas traps behind in-flight separators, and exploit the ion transport properties for the detection of resonant optical pumping. The investigations in superheavy elements are unique and out of reach for other experimental installations worldwide. Starting with singly charged lawrencium, which is available with a modest production yield, we will search for the ^1S_0-^3P_1 ground state transition in ^{255}Lr+ to initiate optical pumping in this ion. By finding the right laser frequency for pumping, we will change the ratio of ions in the excited metastable state to those in the ground state. The result should be easily measurable since ions in the different states have different mobilities and therefore drift through the apparatus to the detector at different velocities, cf. [M. Laatiaoui et al., PRL 125 (2020) 023002, 10.1103/PhysRevLett.125.023002] and [M. Laatiaoui et al., PRA 102 (2020) 013106, 10.1103/PhysRevA.102.013106]. The technique is fast because the ions are probed as is, without further delays or preparation steps. In addition, broadband initial level searches as well as hyperfine spectroscopy can be performed with the same setup.
Although we focus on Lr+, the LRC approach provides unprecedented access to laser spectroscopy of many other monoatomic ions throughout the periodic table, especially transition metals including high-temperature refractory metals and elements beyond lawrencium. Other ionic species such as triply charged thorium are also expected to be within reach of the LRC approach. In addition, the method allows optimization of the signal-to-noise ratio, facilitating ion mobility spectrometry, state-selected ion chemistry, and other applications."
Setup for laser resonance chromatography during first vacuum checks. (c) M. Laatiaoui