Laser ionization spectroscopy was performed on both neutral and singly ionized ${}^{232}\mathrm{Th}$ with the aim of identifying the nuclear-clock isomer in the singly charged ionic state of ${}^{229}\mathrm{Th}$. A search for an efficient laser ionization scheme of ${}^{232}\mathrm{Th}^{+}$ was conducted in an argon-filled gas cell. This revealed a congested spectrum due to collisional quenching effects and the presence of several autoionizing 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 ${}^{232}\mathrm{Th}$ 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 6.306 879(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 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 $\mathrm{Th}$ and ${\mathrm{Th}}^{+}$, respectively. Further calculations using a pseudorelativistic Hartree-Fock method reveal strong mixing in the used intermediate state at $26113.27{\mathrm{cm}}^{-1}$ of $\mathrm{Th}$. A dedicated fast-extraction gas cell with ${}^{233}\mathrm{U}$ recoil sources was used to study ${}^{229}\mathrm{Th}^{+}$, but no photoionization signal could be observed.

Close

PrevNext

View figure in article

• Atomic spectra
• Electronic excitation & ionization
• Fine & hyperfine structure
• Laser spectroscopy
• Laser ablation
• Radioactive beams