Study of thorium in hypersonic gas jets: Ionization potentials of Th and Th+
Abstract
Laser ionization spectroscopy was performed on both neutral and singly ionized 232Th with the aim of identifying the nuclear-clock isomer in the singly charged ionic state of 229Th. A search for an efficient laser ionization scheme of 232Th+ 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 232Th 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 233U recoil sources was used to study 229Th+ but no photo-ionization signal could be observed.
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