Electron affinity of thallium measured with threshold spectroscopy

Abstract

The electron affinity of thallium has been precisely measured using laser photodetachment threshold spectroscopy. The relative photodetachment cross section from the negative ion $^{205}\mathrm{Tl}^{\ensuremath{-}}$ was measured using a tunable infrared laser over the photon energy range 300--900 meV (4130--1380 nm). A pair of closely spaced $s$-wave thresholds were observed due to transitions from the negative ion ground state $\mathrm{T}{\mathrm{l}}^{\ensuremath{-}}$ $(6{p}^{2}^{3}P_{0})$ to the two hyperfine levels of the neutral atom ground state Tl $(6p^{2}P_{1/2},$ $F=0,1)$, which determines the electron affinity of $^{205}\mathrm{Tl}$ to be 320.053(19) meV. 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.