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 O2- exhibits 29 resolved vibronic transitions to the three low-lying electronic states of neutral O2 (X 3Σg-, a 1Δg, b 1Σg+) from the X 2ΠJ (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 EA0(O2) = 0.448 ± 0.006 eV. The derived bond dissociation energy is D0(O2-) = 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 O2 and of O2- 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 O2- X 2Πg ground state is adiabatically stab...