4s^24p^3–4s4p^4 and 4s^24p^3–4s^24p^25s transitions in Y vii, Zr viii, Nb ix, and Mo x

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); Phys. Scr.18, 96– 106 ( 1978)], have been confirmed. In Y 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.