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

TL;DR

This study measures the ionization potentials of thorium and thorium ions using laser spectroscopy in gas jets, combining experimental and theoretical methods to improve understanding of thorium's atomic properties relevant to nuclear-clock research.

Contribution

It provides new high-precision measurements of thorium ionization potentials and demonstrates the effectiveness of hypersonic gas jets for high-resolution spectroscopy, validated by advanced theoretical calculations.

Findings

Ionization potential of Th: 6.306879(14) eV

Ionization potential of Th$^+$: 12.300(9) eV

High-resolution spectroscopy achieved with 240 MHz resolution

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.