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 5<i>s</i><sup>2</sup>(5<i>p</i> + 4<i>f</i>) and twenty-seven levels belonging to the even-parity system 5<i>s</i><sup>2</sup>(5<i>d</i> + 6<i>s</i> + 5<i>g</i>) + 5<i>s</i>5<i>p</i><sup>2</sup> +5<i>s</i>5<i>p</i>4<i>f</i> 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.
© 1998 Optical Society of America
R. Gayasov and Y. N. Joshi, "Spectrum of La IX : an interesting example of configuration interaction," J. Opt. Soc. Am. B 15, 2614-2619 (1998)