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

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.

Electronic Energy Levels of the Trivalent Lanthanide Aquo Ions. III. Tb3

Results of luminescence studies on chelates of La3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, Tm3+, Yb3+, and Lu3+ are reported It is shown that the intramolecular energy transfer efficiency is a sensitive function of the relative positions of the resonance energy levels of the ions and the metastable triplet states of the complexes In the case of Eu3+ it is demonstrated that, by a proper choice of ligands, it is possible to selectively excite only the lowest resonance level of the two existing resonance levels of this ion Coupling of the rare earth ions to the ligands is discussed, and a comparison is made between the optical properties of rare earth ions in chelates and in inorganic salts

Intramolecular Energy Transfer in Rare Earth Chelates. Role of the Triplet State

Potential-energy curves for molecular hydrogen and its ions

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.

Emission Spectra of the Doubly and Triply Ionized Rare Earths

A study of the Zeeman effect of absorption lines of GdCl3·6H2O and its dependence on the orientation of the crystal has given the constants for the magnetic ellipsoids for the ground state and 11 excited states. Evidence is presented for identifying the first three absorption groups as transitions from 8S to 6P7/2, 6P5/2, and 6I7/2. The magnetic observations were made at 77°K, 4.2°K, and 1.7°K and field-free spectra also taken at 195°K and 273°K. Very strong fluorescence is observed from the lowest excited state to the ground state.The magnetic axes are approximately but not exactly the same for all observed states. The direction of maximum susceptibility is not that of the twofold crystal axis but perpendicular to it. There are small but definite deviations from isotropic magnetic behavior for the ground state.

Absorption, Fluorescence, and Magnetic Properties of Gadolinium Chloride (GdCl 3 ·6H 2 O)†

A sealed-off hollow cathode discharge tube with iron electrodes and rare gas filling is described which will give an iron spectrum with very sharp lines and intensities of high reproducibility. Extensive data are given of the dependence of the iron intensities on the character of the rare gas, its pressure, and the discharge current. For most purposes a neon filling at about 3 mm pressure is recommended. The lines of such a tube are recommended for wavelength standards. They may serve as intensity standards with the proper precautions.

Hollow iron cathode discharge as source for wavelength and intensity standards

The absorption and fluorescence spectra of DyCl3·6H2O have been observed at 4.2°K and 77°K. This makes an analysis of the low-lying levels possible including their Stark splitting. The strong lines have been interpreted as transitions between the 6H, 6P, and 6F multiplets. The Zeeman effect gives a splitting of s1=6.73 if the acute bisectrix is parallel to the field.

Absorption, Fluorescence, and Energy Levels of the Dysprosium Ion*

The absorption spectrum of YbCl3·6H2O has two sharp lines near 9700 A which are interpreted as the transitions from the M=5/2 Stark component of the 2F7/2 level of the Yb+++ ion to the M=3/2 and 5/2 levels of the other doublet component 2F5/2. The Zeeman splittings of the two lines were investigated at 4°K as a function of the orientation of the crystal. They can be interpreted by assuming that the crystal field has a pronounced magnetic symmetry axis (probably sixfold) perpendicular to the twofold symmetry axis of the crystal. The magnetic ellipsoid is degenerate into a pair of planes perpendicular to this axis. The direction of the magnetic axis is slightly different for the three observed states and bears no relation to the optic axes of the crystal.

G. H. Dieke

Papers

Emission Spectra of the Doubly and Triply Ionized Rare Earths

Absorption, Fluorescence, and Magnetic Properties of Gadolinium Chloride (GdCl 3 ·6H 2 O)†

Hollow iron cathode discharge as source for wavelength and intensity standards

Absorption, Fluorescence, and Energy Levels of the Dysprosium Ion*

Absorption Spectrum and Magnetic Properties of Ytterbium Chloride