The spectrum of Zr XII was observed with a low-inductance spark and a laser-produced plasma in the region from 70 to 630 Å on the 10.7-m grazing incidence spectrograph at NBS. From the identification of 36 lines, a system of 28 energy levels was determined. The level system (Cu I isoelectronic sequence, 3<i>d</i><sup>10</sup><i>nl</i>) includes the series <i>np</i> (<i>n</i> = 4-6), <i>nd</i> (<i>n</i> = 4-6), <i>n</i>ƒ (<i>n</i> = 4-8), and <i>ng</i> (<i>n</i> = 5-7). The 4ƒ <sup>2</sup>F term has an anomalously small fine-structure interval (1 ±3 cm<sup>-1</sup>). The ionization energy is determined from the ng series (<i>n</i> = 5-7) to be 1 905 500 ± 200 cm<sup>-1</sup> (236.25 ± 0.03 eV).
Joseph Reader and Nicolo Acquista, "Spectrum and energy levels of eleven-times ionized zirconium (Zr XII)," J. Opt. Soc. Am. 69, 1659-1662 (1979)