A tale of two lines: Searching for the 5s - 5p resonance lines in Pm-like ion spectra

Highly charged ions in the promethium sequence have been suggested to show spectral features resembling the alkali sequence ions. Guided by calculations, the 5s - 5p resonance lines have been sought in variety of experiments. In the light of the most extensive calculations of Pm-like ions yet, applying relativistic multi-reference Moeller-Plesset second-order perturbation theory, the experimental evidence is reviewed and the line identification problem assessed.