Fig. 3: Schematic level diagram.
From: Detection of the 5p – 4f orbital crossing and its optical clock transition in Pr9+

Shown are the ground and lowest two fine-structure states in Pr\({}^{9+}\) between which the clock transitions take place, energy splittings not to scale. Levels with the same total angular momentum \(F=I+J\) are admixed by the magnetic hyperfine interaction, allowing the \({}^{3}\)G\({}_{3}\) clock states to decay via E2 transition rather than M3, which would take of order 10 million years to decay.