Predicting quasibound states of negative ions: ${\mathrm{La}}^{-}$ as a test case

We demonstrated the accurate prediction of a quasibound spectrum of a negative ion using a high-precision theoretical approach. We used ${\mathrm{La}}^{-}$ as a test case due to a recent experiment that measured energies of 11 resonances in its photodetachment spectrum attributed to transitions to quasibound states [Phys. Rev. A 102, 042812 (2020)]. We identified all of the observed resonances and predicted one more peak just outside the range of the prior experiment. Following the theoretical prediction, the peak was observed at the predicted wavelength, validating the identification. The same approach is applicable to a wide range of negative ions.

Figure 1

Partial energy-level diagram of relevant bound states of ${\mathrm{La}}^{-}$ (black), neutral La (blue), and quasibound excited states of ${\mathrm{La}}^{-}$ (red) in the (a) ${}^{5}G$ and (b) ${}^{5}F$ manifolds. Numbered arrows indicate resonance transitions observed previously by our group [28] (Peaks 13–23) and in the present study (Peak 24) that have been identified in the present study.

Figure 2

Measured ${\mathrm{La}}^{-}$ photodetachment spectrum above the ground-state threshold energy of 557.546 meV. Data below 920 meV are from our previous work [20, 28]; data above 920 meV are from the present study. The numbered peaks are due to resonant detachment via excitation of quasibound negative ion states; the Peak 24, which was predicted and verified in the present study, is indicated in red.

Figure 3

Measured photodetachment spectrum in the vicinity of the Peak 24, which is due to the ${\mathrm{La}}^{-}{}^{3}F_2\to {}^{5}F_3$ transition. The solid line is a fit to the data of a Fano profile with a linear background. The inset shows the remaining peak after the linear background has been subtracted from the measured neutral signal.