The spectrum of doubly ionized praseodymium was recorded and analyzed, with the result that 118 energy levels of odd parity and 130 levels of even parity were derived. These include nearly all the levels of the 4ƒ3, 4ƒ25d, 4ƒ26s, and 4ƒ26p configurations as well as some low levels of the 4ƒ25d2 and 4ƒ26d configurations. Electrostatic and spin-orbit parameters were determined for the 4ƒ26s configuration. From the hfs of the 4ƒ26s levels, a magnetic dipole interaction constant of 0.46 cm-1 was obtained. An ionization potential of 23.2 V was calculated from levels of the 4ƒ25d and 4ƒ26d series.
JACK SUGAR, "Analysis of the Third Spectrum of Praseodymium," J. Opt. Soc. Am. 53, 831-839 (1963)