The Electron Affinity of Indium and the Fine Structure of In$^{-}$ Measured using Infrared Photodetachment Threshold Spectroscopy*

The binding energies of the fine structure levels of the indium negative ion (In$^{-})$ have been measured using infrared photodetachment threshold spectroscopy. The relative cross section for neutral atom production was measured with a crossed ion-beam-laser-beam apparatus over selected photon energy ranges between 300 -- 670 meV. An $s$-wave threshold was observed due to the opening of the In$^{-}$ (5$p^{2} \quad ^{3}P_{0})$ to In (5$p$ $^{2}P_{1/2})$ ground-state to ground-state transition, yielding an accurate value for the electron affinity of In. Thresholds were also observed for detachment from the J=1 and J=2 excited fine structure levels of In$^{-}$, permitting accurate determination of the fine structure splittings of the negative ion.