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 ${}^{232}$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.