Study of the low-lying states of NiO− and NIO using anion photoelectron spectroscopy

The 2.33 and 3.49 eV photoelectron spectra of NiO− obtained with a new apparatus using field-free electron energy analysis are reported. The electron affinity of NiO is determined to be 1.470(3) eV. A spin–orbit splitting of 260(40) cm−1 for the ground Π2 anion state is observed. A bond length of 1.668(4) Å and vibrational frequency of 660(40) cm−1 are determined from spectral simulations of the neutral Σ-3 ground state←anion Π3/22 state transition. A transition from an excited Σ-4 anion state [Te=900(40) cm−1, ω=760(40) cm−1] to the ground neutral state is assigned. New spectroscopic constants are obtained for the Φ3 state [Te=0.65(1) eV], the Δ1 state [Te=0.94(1) eV, ω=615(15) cm−1, and re=1.600(6) Å] and the second Π3 state [Te=1.194(10) eV, 2A=411(30) cm−1]. These findings are in general agreement with those recently reported by Wu and Wang [J. Chem. Phys. 107, 16 (1997)], although several reassignments of the photoelectron spectra (PES) are made based on comparison with calculations and newly resolved fine structure.