The Third Spectrum of Praseodymium (Pr iii) in the Vacuum Ultraviolet.

Measurements of the spectrum of doubly ionized praseodymium from 821 to 2103 Å are given. One hundred fifty-three energy levels deduced from these wavelengths and an earlier line-list of longer wavelengths are presented. These levels are identified with the configurations 4f ² 6s, 4f ² 7s, 4f ² 8s, 4f ² 6p, 4f ² 6d, 4f ² 5f, 5d ² 4f, and 4f5d6s and are given term designations. Radial energy integrals belonging to these configurations are parametrically deduced from the known levels. A value of 21.625 eV (174420 cm ^-1 ) for the ionization energy of Pr iii, with an estimated uncertainty of 0.016 eV (130 cm ^-1 ), is derived from the 4f ² ns series (n = 6, 7, 8).