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 3d<sup>10</sup>nl 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 <sup>2</sup>F term is inverted. Also identified were 12 transitions of the type 3d<sup>10</sup>4s–3d<sup>9</sup>4s4p and 3d<sup>10</sup>4p–3d<sup>9</sup>4p<sup>2</sup>, permitting the determination of several 3d<sup>9</sup>4s4p and 3d<sup>9</sup>4p<sup>2</sup> levels. The observed 3d<sup>10</sup>nl energy levels and parameters are compared with Hartree–Fock calculations. The ionization energy is determined from the 3d<sup>10</sup>ns and nƒ series to be 1 430 000 ± 500 cm<sup>-1</sup> (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)