Resonance lines of Cs II, Ba III, and La IV

The resonance line spectra of ions in the Xe I isoelectronic sequence, consisting of the five transitions to the 5p6 150 ground state from levels with J = 1 in the 5p5 5d and 6s configurations, have been observed for Cs II, Ba III, and La IV. The observations were made with a sliding spark on the 10.7-m normal-incidence vacuum spectrograph at NBS. The resonance transitions from the 5p5 6d and 7s configurations were also observed for these ions, except for that from 5p5 6d 3P1 of Ba III. Several resonance transitions from higher nd and ns levels were also observed. Estimated values for the J = 1 levels of the 5p5 7s configuration of Ce V were obtained by extrapolation. The derived ionization energies in eV are Cs II 23.17(4), Ba III 35.79(6), La IV 45.95(6), Ce V 65.55(25).