The electronic structure of thorium monoxide: Ligand field assignment of states in the range 0-5 eV

Configuration interaction ligand field theory (CI LFT) calculations of the electronic energy levels of ThO were performed by treating the molecular electronic states as Th ²⁺ free-ion levels perturbed by the ligand field of O^2- . Twenty nine experimentally characterized ThO v = 0 energy levels, together with the energy difference between the v = 0 levels of the Y and W states were fitted using a CI LFT model that included Th ²⁺ 7s ² , 6d7s, 6d² , 7s7p, 6d7p, 5f7s, and 7p² configurations. Predictions from these calculations were used to provide tentative assignments for 171 out of 250 ThO band heads listed by Gatterer et al. ["Molecular Spectra of Metallic Oxides", Specola Vaticana (1957)]. Term energies for 30 electronic states have been determined based on these assignments. Subsequently, the CI LFT model was refined by fitting to a set of 59 electronic term energies. The inclusion of CI effects together with integer valence, atomic-in-molecule, ionic bonding ideas reveals atomic energy level patterns that are multiply replicated in the molecular energy level patterns of six Th ²⁺ O^2- atomic ion configurations (6d7s, 6d² , 7s7p, 6d7p, 5f7s, and 7p² ) revealing the underlying atomic ion structure that gives rise to the complex and seemingly erratic unassigned bands reported in the Vatican Atlas. © 2018 Wiley Periodicals, Inc.