High-resolution determination of the electron affinity of fluorine and bromine using crossed ion and laser beams

We report tunable-laser photodetachment of F^- and Br^- ions near the first threshold corresponding to the atomic ²P3/2 ground state. Using a high-resolution pulsed uv laser calibrated with molecular iodine absorption and doubly crossed laser and ion beam technique, we determine the electron affinities of fluorine and bromine with high accuracy. These determinations account for the hyperfine structure of the atomic ground states of ¹⁹F and ⁷⁹Br{,}⁸¹$Br. The threshold values obtained for the transition between the ion ground state $¹S₀$ and the atomic ground hyperfine state $²P3/2$ are 27 432.440(25) ${cm}^{{{-}}1}$ or 3.401 190(4) eV for fluorine and 27 129.170(15) ${cm}^{{{-}}1}$ or 3.363 590(3) eV for bromine, and the isotope shift of ${Br}^{{{-}}}$, $⁷⁹Br$ is found to be below our experimental resolution.