The low lying electronic states of O₃^-

The energies of the three lowest lying excited states of the ozonide anion (1 ²B₂, 1₂A₁, 1 ²A ₂) at the optimized geometry of the X ²B₁ ground state are theoretically predicted at the MRCI-SD level of theory using large atomic natural basis sets. The calculated vertical excitation energy T_ν=2.85 eV for the 1 ²A₂←-X ²B₁ transition, which has a large transition moment, is in good agreement with the experimental results for the isolated O ₃^- anion in host matrices and solution between 2.69-2.81 eV. The state symmetry forbidden transition 1 ²B₂←X ₂B₁ is calculated with T_ν=2.26 eV. The excitation energy for the 1 ²A₁←X ²B ₁ transition is theoretically predicted with T_ν=2.28 eV and a very low transition moment. The theoretical results are also discussed in comparison with recently observed low lying absorption bands of O ₃^-.