A recent breakthrough in the analysis of Nd IV resulted in the establishment of 37 energy levels of the ground configuration 4f3. We report here the completion to all 41 levels of this configuration. Wavelength measurements extended to 2800 Å and in higher excitation conditions lead to the classification of 1426 lines involving the excited configurations 4f25d, 4f26p, 4f26s. Owing to configuration mixing between 5p64f26p and the core-excited 5p54f4, numerous 5p64f25d–5p54f4 transitions, normally dipole-forbidden, are observed. Altogether 111 odd parity levels and 121 even parity ones have been established. Their parametric least-squares fits have rms deviations of 91 cm−1 and 37 cm−1, respectively. The interaction 5p64f25d–5p54f35d has an impact on the 4f3–4f25d transition probabilities computed by means of the Cowan codes. Theoretical values of lifetimes are reported for all the even parity levels.

The spectrum of neodymium excited by a sliding spark source was photographed on two vacuum ultraviolet normal-incidence spectrographs. About 250 lines attributed to Nd V, hitherto unknown, have been identified. The analysis of this spectrum established all the energy levels of the configurations 4f2, 4f5d, 4f6s and 4f6p (except for 4f2 1S0). Altogether, 48 known levels classify about 160 lines. Their theoretical calculation includes a least-squares fit with an rms error of 28 cm−1 for the even-parity levels and 26 cm−1 for the odd-parity ones, as well as the best values for relevant radial interaction parameters. In particular, interactions with the core-excited configurations 5p54f3 and 5p54f25d are discussed. Intensities derived from phosphor image plates are used to estimate an effective temperature in the spark of Teff=3.6 eV.

The spectrum of Yb IV has been reinvestigated and its analysis has been extended. The present work is supported by the comparison of line intensities with transition probabilities derived from the Cowan (1981) codes. Starting from the ¢rst analysis by Sugar, Kaufman and Spector (1978), the number of established levels has been increased from 111 to 193, including high excitation levels of the new 4f 12 7s+4f 12 6d even con¢gurations. Of the 1023 classi-¢ed lines about one half are new. Computed transition probabilities are given for selected lines.

The emission spectrum of neodymium produced by vacuum spark sources was observed in the vacuum ultraviolet on two normal-incidence spectrographs. In an initial result, more than 550 lines have been identified as transitions from 85 4f25d levels to 37 levels of the 4f3 ground configuration in the free ion Nd3+. The levels 4f34F3/2 and 4I11/2, responsible for the well-known 1064 nm laser line, have respective positions of 11 698.57 ±0.1 cm−1 and 1897.07 ±0.1 cm−1 above the ground level 4I9/2. The newly found levels of 4f3 constitute the first isolated 4fN configuration (N > 2) and therefore enable checks of effective parameters that represent far configuration interaction. Slater parameters Fk(4f, 4f) derived from Nd3+:LaCl3 are 3% to 5% smaller than in the free ion.

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Although the trivalent lanthanide ions embedded in compounds have a wealth of applications in laser physics and the lighting industry, the theoretical interpretation of their ground-state configurations 4fN in the free ion spectra was restricted by the lack of experimental data. In the first part of this paper, the present status of their group-theoretical description and prospects for improvement are given. In the second part, this description is applied for the first time to a truly isolated configuration 4f3 in Nd IV by performing a parametric fit of level energies, taking into account Coulomb and spin-dependent interactions beyond the first order of perturbation. A root mean square (rms) deviation 5.39 cm−1 (i.e. 1/13 000 of the energy spread of the configuration) is obtained with 23 free parameters and 41 newly determined levels. The contributions of all parameters to the level energies are detailed for the first time. It is shown that energy shifts caused by the effective electrostatic parameters related to the third order of perturbation amount to about 10−3 of the total energy spread of the configuration 4f3. As concerns spin-dependent operators, the relative contributions of the spin–orbit interaction (SO), of the combined effects of spin–other-orbit and configuration interaction (SOO + CI) and of the spin–spin interaction (SS) are in the ratios 571:12:1. The Marvin integrals derived from the experimental energies are in a very good agreement with Hartree–Fock estimates. The ground-state configurations 4f2 of Pr IV and of the newly analysed spectrum Nd V are studied with the same methods.

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