Theoretical study of the vertical electronic spectra of O⁻₂ and the superoxides LiO₂ and NaO₂

Abstract

The vertical electronic spectra of the ion O⁻ ₂ and superoxides LiO₂ and NaO₂ are studied at the multireference configuration interaction (MRCI) level. For isolated O⁻ ₂ (with a ground state X²Π_g(2σ² _g1π⁴ _u1π³ _g)), the present calculations place 1²Π_u(1π_u → 1π_g) at 5.20eV, 1²Σ⁺ _g(2σ_g → 1π_g) at 6.32 eV and 2²Σ⁺ _u(1σ_u → 1π_g) at 15.62eV, confirming the assignments made on the basis of electron scattering experiments on O₂(X³Σ⁻ _g). An absorption band at 6.41 eV observed for O⁻ ₂-doped KC1 crystals is reassigned to 1π_g → 2σ_u states of O⁻ ₂, instead of a charge-transfer excitation O⁻ ₂ → K⁺. A strong absorption band near 5.5eV for NaO₂ crystals is confirmed to correspond to a π → π∗ (²B₁ ← X²A₂) excitation localized on the O⁻ ₂ moiety (f value ≍ 0.078 and vertical ΔE = 5.28 eV). Another band at 3.8 eV for NaO₂ crystals, previously assigned to 1² Δ_u X²Π_g of O⁻ ₂, is reassigned to charge-transfer states 1π_g(O⁻ _g) → 3s, 3p_x, 3p _y (Na⁺) with vertical ΔE's from 3.95 to 4.45eV and f values from 0.008 to 0.023. No experimental data are available for LiO₂. Our calculations assign to this radical a strong π → π∗ absorption, with ΔE = 4.71 eV and f ≍ 0.1. Next in intensity one finds the charge-transfer state occupying 2p _y (Li⁺), with ΔE = 5.18eV and f ≍ 0.031. For LiO₂ and NaO₂, the transition 1²B₂ ← X²A₂ is of very low intensity (f ≤ 10⁻³) as it involves the components of the 1π_g MO of O⁻ ₂ (i.e. both the transition moment and ΔE are small).