Magnetic and electric dipole transitions between levels of the 4fx configuration perturbed by a static crystalline field are treated. The expression obtained for the pure‐electronic electric‐dipole transition probability involves matrix elements of an even‐order unit tensor between the two 4fx states involved in the transition. The contributions to the transition probability from interactions, via the crystalline field, with the nd 94fx−1, 4fx−1 nd, 4fx−1 ng configurations are shown to add linearly, in such a manner as to multiply each odd k crystal‐field parameter Ak q by a constant. If ``J mixing'' in the 4fx configuration is neglected ΔJ between the upper and lower 4fx levels is restricted to six units or less. If ``L mixing'' is neglected then ΔL is also restricted to six units or less. Application is made to the fluorescence spectra of PrCl3 and EuCl3. Many of the missing and weak transitions are explained.

Intensities of Crystal Spectra of Rare‐Earth Ions

The present status of our knowledge of the structure of the spectra of the doubly and triply ionized spectra of the rare earths is derived partly from experimental data of the emission spectra of the free ions which provide the energy level scheme in great detail but are difficult and laborious to analyze. For the lower levels knowledge of the structure comes from the crystal absorption and fluoresence spectra. In all cases approximate theoretical calculations of the energies are essential.

The Spectra of the Doubly and Triply Ionized Rare Earths

Free‐ion energy‐level schemes for the 4f6 and 4f8 configurations of Eu3+, Sm2+, and Tb3+ are given, which have been calculated from energy matrices including interactions among the three highest multiplicities. The values obtained for the parameters are F2 = 401 cm—1, ζ = 1320 cm—1 for Eu3+; F2 = 330 cm—1, ζ = 1050 cm—1 for Sm2+; and F2 = 434 cm—1, ζ = 1705 cm—1 for Tb3+. The wavefunctions and g values for levels to 40 000 cm—1 are tabulated for Eu3+ and Tb3+. Application of the free‐ion wavefunctions to the Stark splitting of EuCl3 and TbCl3 is discussed.

Structure of the f6 Configuration with Application to Rare‐Earth Ions

The rare‐earth ions may exist in the divalent state in suitable host crystals such as CaF2. All of the trivalent ions from Ce to Yb (and probably also La) are reduced in situ to the divalent state in CaF2 by gamma irradiation. The spectra of most of these ions show that the ground and first few excited states derive from fn configurations, but the weak absorption due to these is masked at higher energies by strong broad bands of the parity‐permitted fn→fn—1d transitions. The excitation energy of these spectra have been calculated in a first approximation as the energy difference between the ``Hund rule,'' single‐determinant states of the configurations fn—1d and fn. This procedure satisfactorily accounts for the remarkable variations in the excitation energy in passing from one ion to the next in the series with the exception of Gd++, Ce++, and Tb++. Gd++ probably has f7d for its ground configuration, while Ce++ and Tb++ are borderline cases. The spectral structure probably arises chiefly from the crystal...

Survey of the Spectra of the Divalent Rare‐Earth Ions in Cubic Crystals

50th anniversary of the Judd–Ofelt theory: An experimentalist's view of the formalism and its application

The spectral of the rare earths have been photographed under controlled excitation so that either the spectra of the doubly or triply ionized elements are brought out with maximum intensity. A mild excitation is used in addition to give the first and second spectra for comparison. Some regularities are immediately apparent and vary very gradually through the rare-earth group. The general features of the spectra are discussed in this paper.

B. Dunn

Papers

Emission Spectra of the Doubly and Triply Ionized Rare Earths