The electronic structure of the LaO molecule is studied using frozen-core four-component multiconfigurational quasidegenerate perturbation theory. The ground state and nine experimentally observed excited states are examined. The ground state is 2σ21/2+ and its gross atomic orbital population is La (5 p5.76 6s0.83 6 p0.14 p *0.21 d *1.17 f *0.26) O (2 p4.63), where p*, d*, and f* are the polarization functions of La that form molecular spinors with O 2ps. We found that it is not necessary to consider the excitation from the O 2p electrons when analyzing the experimental spectra. This validates the foundation of the ligand field theory on diatomic molecules, including the La atom where only one electron is considered. The spectroscopic constants Re, ωe, and T0 calculated for the ground state and low-lying excited states A′ (Δ23/2), A′ (Δ2 5/2) A (Π2 1/2), and A (Π2 3/2) are in good agreement with the experimental values.
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