We experimentally settle the three-decades-old question of whether or not promethiumlike heavy ions have strong resonance lines in hot plasmas. In 1980, Curtis and Ellis [Phys. Rev. Lett. 45, 2099 (1980)] predicted that promethiumlike heavy ions should have an alkali-metal structure with a ground-state configuration of $4f^{14}5s$, and should exhibit strong $5s\text{−}5p$ resonance lines in hot plasmas. However, after many experimental efforts, no clear indication of the predicted resonance lines was found. The work presented herein resolves this question by clearly showing that the predicted resonance lines for bismuth are negligibly weak because of the presence of the ${\left[4f^{13}5s^2\right]}_{7/2}$ metastable state, even though the ground-state configuration is $4f^{14}5s$, as predicted. To obtain these results, we used an electron-beam ion trap that made it possible to exploit a fine-tuned charge-state distribution, and analyzed the experimental spectra with collisional-radiative model calculations.

• Received 17 October 2013

DOI:https://doi.org/10.1103/PhysRevA.89.010501