Fig. 1: The \(5p-4f\) orbital crossing and level structures of selected ions along the isoelectronic sequence.
From: Detection of the 5p – 4f orbital crossing and its optical clock transition in Pr9+
a Configuration-averaged binding energies for relevant configurations of the Sn-like (50 electron) isoelectronic sequence, as a function of atomic number \(Z\). b–d Grotrian diagrams for low-lying levels of the \(5{p}^{2}\), \(5p4f\), and \(4{f}^{2}\) configurations for \(Z=56\), 59, and 62, respectively. Pr\({}^{9+}\) is situated close to the configuration crossing point, and the corresponding diagram shows that inter-configuration optical transitions are allowed. Fig. 1c shows lines that were measured (blue) in the electron beam ion trap (EBIT). Strongly mixed \(J=2\) levels are indicated with multiple colors. We identified potential clock transitions (dashed magenta lines), which are not observable in the EBIT.
