The 3p6 3d8–3p5 3d9 transitions in Sr xiii, Y xiv, Zr xv, Nb xvi, and Mo xvii have been newly measured by means of a low-inductance vacuum spark and a 10.7-m grazing-incidence spectrograph. The measurements have led to an improved analysis of this complex transition group in these ions. All levels of the combining configurations have been established. The energy parameters determined from least-squares fits to the observed levels are compared with Hartree–Fock calculations. The effective interaction αL(L + 1) for the 3p6 3d8 configuration decreases markedly with increasing ionization. The effective electrostatic interactions D1(3p3d) and X2(3p3d) for the 3p5 3d9 configuration are practically constant through the sequence.
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