The spectrum of Y xi was observed with a low-inductance vacuum 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 40 lines, a system of 29 energy levels was determined. The level system (Cu II isoelectronic sequence, 3d10nl) includes the series ns (n = 4–7), np (n = 4–6), nd (n = 4–6), nf(n = 4–7), and ng (n = 5–8). The 4ƒ 2F term is inverted. The observed energy levels are compared with Hartree-Fock calculations. The ionization energy is determined from the ng series (n =5–8) to be 1 660 000 ± 200 cm−1 (205.82 ± 0.03 eV).
Joseph Reader and Nicolo Acquista, "Spectrum and energy levels of ten-times ionized yttrium (Y XI)," J. Opt. Soc. Am. 69, 1285-1288 (1979)