Diatomic Molecule Electronic Structure beyond Simple Molecular Constants

Molecules with partly filled 3d or 4f shells have a surprisingly large number of low lying electronic states, resulting in incredibly complex spectra. This paper attempts to show that, although the spectra are complicated, the explanation of the global electronic structure is quite simple. Ligand Field Theory displays all of the important relationship between the d N or f N structures in the free‐atomic‐ion and the molecule. The splitting of the core into numerous substates causes a gratuitous repetition and intermingling of the usual ϱ, π‐valence structures which are so familiar for molecules with filled‐shell cores. The 4f electron is shown to be a useful probe of the unknown structures of more diffuse valence orbitals. A semi‐empirical theory is described which allows important features of the molecular level diagram to be predicted using atomic energy level tables and a scrap of paper. The same theory provides a basis for detailed numerical parametrization and quantitative predictions of energy levels, perturbation matrix elements, hfs splittings, and transition intensities.