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 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.

We have correlated the experimentally measured intensities of the solution absorption spectra of Pr3+, Nd3+, Er3+, Tm3+, and Yb3+ in three different solvents with a theoretical expression derived by Judd. The expression, P=Σ lim λTλσ(fNψJ‖∪(λ)‖fNψJ′′)2λ=2,4,6is derived from a theory in which the ligand-field interactions with the central 3+ ion cause a mixing of higher configurations of opposite parity into the fN configuration and gives rise to induced electric-dipole transitions. A prerequisite for the intensity calculations is the computation of complete intermediate coupling eigenvectors for the energy levels investigated. Other possible mechanisms such as magnetic-dipole and electric-quadrupole transitions are considered. The oscillator strengths of important magnetic-dipole transitions are tabulated. The results give the first experimental verification of the ability of the theory to account for large changes in certain bands of the same lanthanide observed in different solvents. Conclusions are drawn respecting the Tλ's obtained in a given medium for all the lanthanides studied, and also for those obtained for a given lanthanide in different media. The J dependence of the Tλ's is removed to facilitate their intercomparison, and the resulting parameters are tabulated. The Tλ's for Yb3+ are obtained by an extrapolation of the values computed for Tm3+ and Er3+. The nature of the model required by the theory is discussed.