The spectrum of the copperlike ion Sr x was observed with a low-inductance spark in the region 70–630 Å on the 10.7-m grazing-incidence spectrograph at the National Bureau of Standards. From the identification of 30 lines, a system of 23 energy levels of the type 3d10nl was determined. The level system includes the configurations ns (n = 4–7), np (n = 4–6), nd (n = 4–6), nƒ (n = –6), and ng (n = 5). The 4ƒ 2F term is inverted. Also identified were 12 transitions of the type 3d104s–3d94s4p and 3d104p–3d94p2, permitting the determination of several 3d94s4p and 3d94p2 levels. The observed 3d10nl energy levels and parameters are compared with Hartree–Fock calculations. The ionization energy is determined from the 3d10ns and nƒ series to be 1 430 000 ± 500 cm-1 (177.30 ± 0.06 eV). Data for 3d—4p transitions in Sr IX and Sr XI and 3p–3d transitions in Sr XII are also presented.
Nicolo Acquista and Joseph Reader, "Spectrum and energy levels of nine-times ionized strontium (Sr x)," J. Opt. Soc. Am. 71, 569-573 (1981)