A b i n i t i o study of the electronic states of O−2 i n v a c u o and in simulated ionic solids

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Abstract
The ground electronic state and several low-lying excited states of the superoxide ion O−2 have been studied by ab initio computations. For comparison the ground state of O2 is also included. Parallel computations are carried out for the species in vacuo and in a simulated KCl crystal lattice. The in vacuo results indicate that even though the X 2Πg and A 2Πu states of O−2 are resonance states whose energies are above that of O2, they are electronically stable within the domain of the calculation. The a 4Σ−u state exhibits an energy minimum lying below O2(X) where it should be a stable species. All other states with energies less than 4 eV above the O−2 ground state are found to be electronically unstable in the region of internuclear distances studied (1–2 Å). By contrast, in the ionic crystal lattice all low-lying electronic states are rendered electronically stable by the Madelung potential of the lattice. Computed spectroscopic parameters are in good agreement with experiment for the X and A states of O−2 in vacuo. There is also substantial agreement between the computed energy curves for both the X and A states in the point-charge lattice and those measured in alkali halide lattices, including the prediction of appreciable crystal-field splitting in the 〈110〉 orientation. Further, the spectroscopic parameters of the electron-scattering resonance states in vacuo agree well with those of the analogous lattice-stabilized excited electronic states in the solid.

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