Spectra of ionized Y, Zr, Nb, and Mo have been observed in sliding-spark and triggered-spark discharges on 10.7-m normal- and grazing-incidence spectrographs at the National Bureau of Standards in Washington, D.C. From these observations the 4s24p3-4s4p4 transitions in Y VII, Zr VIII, Nb IX, and Mo x have been identified. The 4s24p3-4s24p25s transitions in Y VII-Mo X, previously identified by Rahimullah et al. [Phys. Scr. 14, 221–223 (1976); 18,96–106 (1978)], have been confirmed. InY VII the 4s24p3-4s24p26s and 4s4p4-4p5 transitions also have been found. The parameters obtained from least-squares fits to the energy levels are compared with Hartree—Fock calculations. Preliminary values of the ionization energies have been determined as 110.02 ± 0.15 eV for Y VII, 133.7 ± 0.5 eV for Zr VIII, 159.2 ± 0.7 eV for Nb IX, and 186.4 ± 1.2 eV for Mo X.
Joseph Reader and Nicolo Acquista, "4s24p3-4s4p4 and 4s24p3-4s24p25s transitions in Y VII, Zr VIII, Nb Ix, and Mo x," J. Opt. Soc. Am. 71, 434-441 (1981)