Spectrum and energy levels of doubly ionized scandium (Sc iii)*

The spectrum of Sc iii has been observed by using a vacuum sliding spark at 250-A peak current. Excellent separation of the Sc i, ii, and iii spectra was achieved by careful control of the excitation-circuit parameters. In all, 93 lines in the region 550–9400 Å are listed, which give rise to 23 new levels. The ionization energy of Sc iii is revised to 199 677.37 ±0.1 cm−1, based on the ng series, and series formulas are presented which were used to predict some of the newly found levels to within 0.25 cm−1. An isoelectronic comparison of Sc iii with K i and Ca ii is made.