Binding energies of the fine-structure levels of the indium negative ion In${}^{-}$ are measured using infrared photodetachment threshold spectroscopy. The relative cross section for neutral atom production is measured with a crossed ion-beam–laser-beam apparatus over selected photon energy ranges between 300 and 700 meV. An $s$-wave threshold is observed due to the opening of the In${}^{-}$ ($5p^2$${}^3{P}_0$) to In($5p$${}^2{P}_{1/2}$) ground-state-to-ground-state transition, which determines the electron affinity of In to be 383.92(6) meV. The present result is in good agreement with previous theoretical calculations, but it differs substantially from the previously measured electron affinity and reduces the uncertainty by a factor of 150. $s$-wave thresholds are also observed for detachment from the excited fine-structure levels of In${}^{-}$, permitting accurate determination of the fine-structure intervals of 76.06(7) meV for $J=0–1$ and 170.6(6) meV for $J=0–2$, which are in good agreement with the previous measurements and substantially reduce the uncertainties.

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