Spectrum and energy levels of the high-lying singly excited configurations of Nd III: New Nd III experimental energy levels and wavelengths, with transition probability and ionisation energy calculations

Aims. We aim to accurately determine bound-to-bound transition wavelengths and energy levels of the high-lying open-shell configurations 4f³7s, 4f³6d, and 4f³5f of doubly ionised neodymium (Nd III, Z = 60) through high-resolution spectroscopy and semi-empirical calculations. This study is motivated by lanthanide atomic data requirements in astronomy, such those involved in investigations of kilonova spectra. Methods. Fourier transform spectra of Nd Penning and hollow cathode discharge lamps were recorded within the region 32500–54000 cm^‑1 (3077–1852 Å) and grating spectra of Nd vacuum sliding sparks were recorded within the regions 820–1159 Å and 1600–3250 Å. New energy levels were found using the observed wavelengths measured accurate to a few parts in 10⁸ in Fourier transform spectra and to a few parts in 10⁷ in grating spectra. Atomic structure and transition probability calculations of Nd III were carried out using the Cowan codes, where energy parameters were adjusted to fit all known Nd III levels. Finally, Nd-rich stellar spectra were also used to aid in the analysis. Results. In total, 355 transitions were classified from observed spectra involving 116 previously experimentally unknown energy levels of the 4f³7s, 4f³6d, and 4f³5f configurations of Nd III, all reported here for the first time. Three newly identified levels of the 4f³5d configuration and one newly identified 4f⁴ level are also reported. Typical level energy uncertainties are 0.01 cm^‑1 for the 4f³7s and 4f³6d levels and 0.3 cm^‑1 for the 4f³5f levels. In addition, calculated energy levels up to 130 936 cm^‑1 are presented, including eigenvector composition and calculated level lifetimes. Calculated transition probabilities and wavelengths between 1900–50 000 Å are also given. Using newly established levels of the 4f³7s configuration and the recently established levels of the 4f³6s configuration, the ionisation energy of Nd III has been estimated at 178 090 ± 330 cm^‑1. This result offers up to twice the accuracy of the most recently published value.