Measurements of the spectrum of triply ionized praseodymium produced by a 500-A sliding-spark discharge revealed new high-lying energy levels of both even and odd parity. Nearly all levels of the 4ƒ5ƒ configuration were found as well as portions of 4ƒ6d, 4ƒ7s, and 5d6p. Radial parameters were fitted to 4ƒ5ƒ and 4ƒ6d. A value for the ionization limit of 314 200±200 cm-l was deduced from the 4ƒ6s, 7s series.
JACK SUGAR, "New Energy Levels of Triply Ionized Praseodymium," J. Opt. Soc. Am. 61, 727-732 (1971)