Electron affinity of arsenic and the fine structure of As⁻ measured using infrared photodetachment threshold spectroscopy

The binding energy and fine-structure splittings of the arsenic negative ion (As⁻) 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 630–810 meV. An s-wave threshold was observed due to the opening of the As⁻(4p^4 3P₂) to As(4p^3 4S_3/2) ground-state to ground-state transition, which yields the electron affinity of As to be 804.8(2) meV. s-wave thresholds were also observed for detachment from the J=1 and J=0 excited levels of As⁻, permitting accurate determination of the fine-structure splittings of 127.6(2) meV for ³P₁−³P₂ and 164.3(10) meV for ³P₀−³P₂. The present values are consistent with previous measurements and substantially reduce the uncertainties.