The spectrum of Pr<sup>3+</sup> produced by a sliding spark discharge was photographed in the wavelength range of 800 to 3100 Å with a 35-ft Eagle vacuum spectrograph. An analysis of the resulting line list led to the discovery of all the levels of the 4ƒ<sup>2</sup> (except the <sup>1</sup>S<sub>0</sub>), 4ƒ5<i>d</i>, 4ƒ6<i>s</i>, and 4ƒ6<i>p</i> configurations. The interaction parameters for each configuration were derived from a least-squares fit of the energy formulas to the observed levels.
JACK SUGAR, "Analysis of the Spectrum of Triply Ionized Praseodymium (Pr IV)," J. Opt. Soc. Am. 55, 1058-1059 (1965)