Laser Resonance Ionization Spectroscopy of Thorium

High-lying Rydberg and autoionizing (AI) states of thorium (Th) have been studied via resonance laser ionization spectroscopy at both TRIUMF Canada’s particle accelerator centre and Oak Ridge National Lab (ORNL). Multiple Rydberg series converging to the ionization potential (IP) were observed via different stepwise laser excitation schemes and were assigned to be the $6d^{2}7s{(}^4{F}_{3\mathrm{/}2})$ $np$, $nd$, and $nf$ series. Analysis of these series enabled the determination of the IP to be 50868.735(54) cm${}^{-1}$, which improved the precision by two orders of magnitude over the current adopted NIST value of 50867(2) cm${}^{-1}$. Additionally, four AI Rydberg series were identified and assigned to $nf$ and $nd$ series converging to the $6d^{2}7s$ ${}^4{F}_{5\mathrm{/}2}$ and $6d^{2}7s$ ${}^2{D}_{3\mathrm{/}2}$ metastable states of Th${}^{+}$. The measured energies of the Rydberg and AI Rydberg states are reported, and observed perturbations within the series are discussed.