Spectrum of La IX : an interesting example of configuration interaction

The spectrum of lanthanum was photographed in the 80–1220-Å region. The 5s5p4f configuration in La ix was established for what is to the authors’ knowledge the first time in the In i isoelectronic sequence. Four levels belonging to the odd-parity system 5s2(5p+4f ) and twenty-seven levels belonging to the even-parity system 5s2(5d+6s+5g)+5s5p2+5s5p4f have been established. Two of the four odd-parity levels and twenty-five of the thirty-two even-parity levels are new. We carried out least-squares-fitting and relativistic Hartree–Fock calculations to show the strong configuration interaction effects in the even-configuration system. Forty-seven lines were classified in La ix, of which thirty-seven are new lines.