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arXiv · 2507.21946

Study of thorium in hypersonic gas jets: Ionization potentials of Th and Th$^+$

A. Claessens·F. Ivandikov·M. Brasseur·A. Dragoun·Ch. E. Düllmann·R. Ferrer·Yu. Kudryavtsev·P. Palmeri·P. Quinet·S. Raeder·D. Renisch·P. Van den Bergh·P. Van Duppen
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Abstract

Laser ionization spectroscopy was performed on both neutral and singly ionized $^{232}$Th with the aim of identifying the nuclear-clock isomer in the singly charged ionic state of $^{229}$Th. A search for an efficient laser ionization scheme of $^{232}$Th$^+$ was conducted in an argon-filled gas cell. This revealed a congested spectrum due to collisional quenching effects and the presence of several auto-ionizing states, one of which has a laser ionization efficiency of at least $1.2 \%$. Using a threshold approach, the second ionization potential was determined to be $12.300(9)\,$eV. The subsequent study on atomic $^{232}$Th validated the threshold approach. Conducting spectroscopy in a hypersonic gas jet, suppressed the gas-collision-induced quenching, revealing a Rydberg series that converges to the first ionization potential, determined to be $6.306879(14)\,$eV. The gas jet also cools down the thorium, allowing for high-resolution laser spectroscopy with a resolution of $240(30)\,$MHz. Using the Multiconfigurational Dirac-Hartree-Fock (MCDHF) method, the ionization potentials were computed, showing a relative difference of 0.06\% and 0.19\% between theory and our experimental values for the ionization potentials of Th and Th$^+$ respectively. Further calculations using a pseudo-relativistic Hartree-Fock method reveal strong mixing in the used intermediate state at $26113.27\,$cm$^{-1}$ of Th. A dedicated fast-extraction gas cell with $^{233}$U recoil sources was used to study $^{229}$Th$^+$ but no photo-ionization signal could be observed.

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A. Claessens, F. Ivandikov, M. Brasseur, A. Dragoun, Ch. E. Düllmann, R. Ferrer, Yu. Kudryavtsev, P. Palmeri, P. Quinet, S. Raeder, D. Renisch, P. Van den Bergh, P. Van Duppen. 2025-07-29. Study of thorium in hypersonic gas jets: Ionization potentials of Th and Th$^+$. https://arxiv.org/abs/2507.21946

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