Electronic states of CuO

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Abstract
A b initio SCF and CI calculations are reported for the ground state and low-lying excited electronic states of CuO (i.e., within ∼20×103 cm−1 of the X̃ 2Π ground state), employing an ab initio effective potential for the Cu atomic core (1s–3p), and including comparisons of several atomic orbital basis sets. The calculated bond lengths, energy separation, and population analyses for the lowest two electronic states, X̃ 2Π and Y 2Σ+ [corresponding roughly to Cu+ (3d10) and O− (2p5), where the 2p oxygen hole is, repectively, 2pπ and 2pσ], do not vary appreciably among the basis sets tested (minimal and split valence bases on Cu, and a split valence basis on oxygen, with and without d-polarization functions and diffuse p functions on oxygen) and are in reasonable agreement with previous experimental and theoretical estimates. Most of the remaining excited states of CuO below ∼20×103 cm−1 may be rationalized in terms of atomic-like excitations originating from the X̃ 2Π ground state: 3d(Cu) → 4s(Cu) and 3d(Cu) → 2 pπ (O) (the latter formally corresponds to Cu2+O2−, though covalent 2p‘gs–4s bonding reduces the charge to ∼Cu+O−). Close agreement (rms deviation of ∼103 cm−1) is obtained between calculated (CI) and observed adiabatic transition energies for ten Σ, Π, and Δ doublet excited states (relative to the X̃ 2Π state), after the calculated excitation energies are reduced by ∼5×103 cm−1, a term which corrects primarily for limitations in the Cu atom basis set, and whose magnitude can be obtained both in terms of calculated and observed atomic quantities and by direct least-squares fitting of calculated and observed CuO transition energies. The calculations yield the first comprehensive assignment of Kronig symmetry (±) for the observed 2Σ states (δ 2Σ−, A 2Σ−, A′ 2Σ+, and G 2Σ+) and one which is consistent with recent analyses of experimental data. The extent of specific molecular correlation effects is analyzed, and found to be important in the assignment of several of the observed transitions. The CI mixing of different principal configurations may be significant for states involving the ‘‘Cu2+O2−’’ configuration (as also suggested in previous studies), and the assignments based on the present CI results for these states are only tentative.