Low-lying states of FeO and FeO^- by slow photoelectron spectroscopy

High-resolution anion photoelectron spectra of FeO^- were acquired by slow electron velocity-map imaging of trapped and cooled ions. Ions were cooled to different temperatures by controlling the conditions in the trap, allowing us to disentangle contributions in the spectra from two different anion states. The spectra show that photodetachment of the two anion states accesses three low-lying neutral FeO states with one state in common, allowing us to derive term energies of all five states. The ground anion state is confirmed to be the X ⁴Δ state, and FeO is found to have an electron affinity of 1.4950 eV. We assign the anion a ⁶Σ⁺ state, and obtain a term energy of 0.117 eV. The A ⁵Σ⁺ and a ⁷Σ⁺ neutral states are reassigned, and have term energies of 0.258 and 0.616 eV, respectively, relative to the neutral X ⁵Δ₄ ground state.