The only stable state of O2- is the X 2Πg ground state and it (still!) has an adiabatic electron detachment energy of 0.45 eV

The ultraviolet photoelectron spectrum Of O-2(-) exhibits 29 resolved vibronic transitions to the three low-lying electronic states of neutral O-2 (X 3Sigma(g)(-), a (1)Delta(g), b (1)Sigma(g)(+)) from the X (2)Pi(j) (J = 3/2 and 1/2) spin-orbit states of the anion. A Franck-Condon simulation, using the established molecular constants of the neutral oxygen states, matches every observed feature in the spectrum. The 0-0 origin transition is unambiguously assigned, yielding the electron affinity EA(0)(O-2) = 0.448 +/- 0.006 eV. The derived bond dissociation energy is D-0(O-2(-)) = 395.9 +/- 0.6 kJ/mol. Coupled-cluster theory at the CCSD(T)/aug-cc-pVTZ level is used to determine the potential energy curves Of O-2 and Of O-2(-) in its ground state and two excited states, in both the electronically bound and unbound regions. Stabilization methods are employed to characterize the anion curves at bond lengths where their electronic energies lie above that of the ground-state neutral. The calculations confirm that the O-2(-) X (2)Pi(g) ground state is adiabatically stable, but the lowest electronically excited states Of O-2(-) (a (4)Sigma(u)(-) and A (2)Pi(u)) are adiabatically unbound with respect to electron detachment. The calculations predict the anionic doublet-quartet splitting to be T-e(a (4)Sigma(u)(-)) - T-e (X (2)Pi(g)) = 2.40 eV and the first excited doublet at an energy of T-e(A (2)Pi(u)) - Te(X (2)Pi(g)) = 3.39 eV. at an energy of T-e(A (2)Pi(u)) - Te(X (2)Pi(g)) = 3.39 eV. These observations are consistent with electron scattering on O-2 and other experimental data, and they sharply refute recent interpretations of electron-capture detector experiments that EA(O-2) approximate to 1 eV, that O-2(-) has multiple excited states below the neutral ground-state minimum, and that the doublet-quartet splitting is 0.12 eV [Chen, E. S.; Chen, E. C. M. J. Phys. Chem. A 2003, 107, 169.].