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 4ƒ6<i>d</i>, 4ƒ7<i>s</i>, and 5<i>d</i>6<i>p</i>. Radial parameters were fitted to 4ƒ5ƒ and 4ƒ6<i>d</i>. A value for the ionization limit of 314 200±200 cm<sup>-l</sup> was deduced from the 4ƒ6<i>s</i>, 7<i>s</i> series.
JACK SUGAR, "New Energy Levels of Triply Ionized Praseodymium," J. Opt. Soc. Am. 61, 727-732 (1971)