Electron Affinity of Thallium Measured with Infrared Laser Photodetachment Threshold Spectroscopy*

The electron affinity of thallium has been measured using tunable infrared laser photodetachment threshold spectroscopy. The relative cross section for neutral atom production following photodetachment from Tl$^{-}$ was measured with a crossed laser--ion beam apparatus over the photon energy range 0.30 -- 0.90 eV. An \textit{s}-wave threshold was observed due to the opening of 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, yielding a preliminary value for the electron affinity of thallium. No photodetachment signal was detected below this threshold and no other thresholds were observed over the present photon energy range, leading to the conclusion that the fine structure excited states of Tl$^{-}$ are not bound. The present results are compared with previous experimental [1] and theoretical [2] studies of Tl$^{-}$. \\[4pt] [1] D. L. Carpenter, A. M. Covington, and J. S. Thompson, \textit{Phys. Rev. A} \textbf{61}, 042501 (2000); [2] see for example J. Li, Z. Zhao, M. Andersson, X. Zhang, and C. Chen, \textit{J. Phys. B-At. Mol. Opt. Phys.} \textbf{45}, 165004 (2012).