Precise Measurement of the Electron Affinity of Thallium using Laser Photodetachment Threshold Spectroscopy

The electron affinity of thallium has been precisely measured using laser photodetachment threshold spectroscopy. The relative photodetachment cross section from the negative ion Tl$^{-}$ was measured using a narrowband tunable infrared laser over the photon energy range 300 -- 900 meV (4130 -- 1380 nm). A single s-wave threshold was observed in this range due to the Tl$^{-}$ (6\textit{p}$^{2}$ $^{3}$\textit{P}$_{0}$) to Tl (6\textit{p} $^{2}$\textit{P}$_{1/2}$) ground state-to-ground state transition, which determines the electron affinity of Tl. These results substantially improve the precision of the Tl electron affinity and resolve long-standing discrepancies in the literature between previous experimental and theoretical values.