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The optical absorption and fluorescence spectra of two spinels, ZnAl2O4:Cr3+ and MgAl2O4:Cr3+ have been measured at various temperatures between 2°K and room temperature. The data have been fitted to a crystal-field theory which includes spin-orbit coupling and the trigonal field of the D3d site. The parameters for a best fit for both crystals were: Δ = 18 250 cm−1, B = 700 cm−1, C = 3200 cm−1, ζ = 250 cm−1, ν = −200 cm−1, ν′ = −1700 cm−1, and k = 0.7. The E2 splitting was directly observed optically for both spinels, and in the case of the ZnAl2O4:Cr3+, the A24 splitting was also observed. In addition to fitting the energies and zero-field splittings, the theory gave values of absolute R-line intensities and lifetimes which agree well with observed values. Vibrational sidebands and pair lines observed in fluorescence were identified.

The spectrum of Er3+ in single crystals of Y2O3 has been examined from 0.225 to 2.4 μ in absorption and fluorescence, and energy levels have been assigned. The Slater integrals and spin—orbit coupling parameter have been determined, which give the best over-all agreement between the centers of gravity of the observed groups of levels and the 4f11 SLJ energy levels of the ``free'' Er3+ ion; they are F2=429.583 cm—1, F4=65.012 cm—1, F6=7.136 cm—1, ζ=—2383.17 cm—1. Of the two inequivalent cation sites, C2 and C3i, one can with consistency demonstrate that the site without inversion symmetry (C2) gives rise to the entire observed spectrum.

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