The spectra of Cs iii, Ba iv, and La v have been observed in a sliding spark discharge on the 10.7 m normal-incidence vacuum spectrograph at NBS. Analysis of the observations of Cs iii and La v has yielded the energy levels of the 5s 25p5 and 5s5p6 configurations and nearly all levels of the 5s 25p 45d and 5s 25p 46s configurations that can combine with the 5s 25p5 2P° ground term. The observations for Ba iv have yielded the levels of the 5s 25p5 and 5s5p6 configurations. The 5s 25p 45d + 5s 25p 46s + 5s5p6 levels of Cs iii and La v have been theoretically interpreted, with configuration interaction included. The energy parameters determined from a least-squares fit to the observed level values are compared with Hartree-Fock calculations. The ionization energies are found to be 33.38 ± 0.25 eV for Cs iii, 47.1 ± 0.6 eV for Ba iv, and 61.6 ± 0.6 eV for La v. By extrapolating these values the ionization energy of Ce vi is estimated as 77.6 ± 1.2 eV.
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