SIMBAD references

Many energy levels of singly ionized iron (Fe II, $Z=26$) remain uncertain or experimentally unknown. Their identification and spectral line data are required in reliable astrophysical spectral analyses. In motivation for improving the atomic data of Fe II, we analysed emission spectra of a Fe-Ne plasma produced by a Penning discharge lamp recorded by high-resolution Fourier transform spectroscopy in the region 9000-27 000 cm$^{-1}$ (11 111-3704 Å). Semi-empirical transition probability calculations and stellar spectra of Fe II were used to guide the analysis. In total, 24 energy levels of the 3d$^6$4f and 3d$^6$5d configurations of Fe II lying between 122 351 and 127 881 cm$^{-1}$ were confirmed in the laboratory for the first time, in agreement with their identities proposed by previous investigations involving only stellar spectra. Uncertainties in level energies and transition wavelengths of the 24 levels are improved by up to an order of magnitude compared to previously published values. These results will enable more reliable application of Fe II in astrophysical spectroscopic analyses and support further investigations of the spectrum and energy levels of Fe II.