The free-ion energy-level schemes of the Pr3+, Nd3+, Pm3+, Sm3+, Dy3+, Ho3+, Er3+, and Tm3+ aquo ions have been determined from their absorption spectra in dilute acid solution at 25°. Energy-level assignments were made by comparison with crystal spectra, and on the basis of correlations between calculated and observed band intensities. For most of the ions, it was possible to identify several transitions giving rise to bands at energies as high as 45 000–50 000 cm−1. Sufficient numbers of assignments were made to justify inclusion of the effects of configuration interaction in the calculation of the energy-level parameters. Variation of the electrostatic, spin–orbit coupling, and configuration-interaction parameters across the lanthanide series is examined.

The optical spectra of the lanthanides doped into single crystal LaF 3 have been interpreted in terms oftransitions within 4fN configurations. Energy matrices combining free-ion terms with a crystal field for an approximate model which assumes C 2v instead of the actual C 2 site symmetry were diagonalized. Excellent correlations were obtained between experimental transition energies and the computed level structures. We also report the results of previously unpublished experimental spectroscopic investigations ofNd 3 + and Sm 3 +:LaF 3 , as well as predicted energy levels for Pm3+:LaF 3 . The spectroscopic data for each ion were independently interpreted using an effective-operator model, then the model parameters were intercompared. Systematic trends have been identified, and a comprehensive energy level diagram is presented.

The free-ion energy level scheme of Eu3+ (aquo) to ∼40 000 cm−1 has been determined from the absorption spectrum of Eu3+ in dilute acid solution at 25°. Energy-level assignments were made on the basis of correlations between calculated and observed intensities. A least-squares fit of 26 J levels, including the effects of configuration interaction, gave an rms deviation of 40 cm−1. The parameter values were E1 = 5573.0, E2 = 26.708, E3 = 557.39, ζ4f = 1326.0, α = 25.336, β = − 580.25, and γ = 1155.7.

A total of 23 experimental “free-ion” energy levels of Tb3+ (aquo) have been fit with an rms deviation of 56 cm−1. The resulting parameters are: E1 = 6021.5, E2 = 29.030, E3 = 608.54, ζ4f = 1709.5, α = 20.131, β = − 370.21, and γ = 1255.9. The latter three parameters arise from including the effects of configuration interaction in the calculation. Assignments were based upon a correlation between calculated and observed band intensities at energies up to ≈40 000 cm−1.

We have correlated the experimentally determined band intensities in the solution absorption spectra of the trivalent lanthanides with a theoretical expression derived by Judd. The spectra were measured in a single medium, dilute acid solution, and, in most cases, in the range ≈6000–50 000 cm−1. In general, the correlation between calculated and observed intensities was very good, even at higher energies. The variation of the intensity parameters over the series is discussed as is the somewhat unexpected degree of correlation obtained in the ultraviolet region.