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.
Cornelius H. H. Van Deurzen, John G. Conway, and Sumner P. Davis, "Spectrum and energy levels of doubly ionized scandium (Sc III)," J. Opt. Soc. Am. 63, 158-163 (1973)