Optical spectrum of Cr³⁺ ions in spinels

The optical absorption and fluorescence spectra of two spinels, ZnAlsO^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 DM site. The parameters for a best fit for both crystals were: A=18250 cm^-1, 5=700 cm^-1, C=3200 cm^-1, ζ=250 cm^-1, v= -200 cm^-1, v′= -1700 cm^-1, and B = 0.7. The ²E splitting was directly observed optically for both spinels, and in the case of the ZnAl ₂O₄:Cr^3+-1, the ⁴A ₂ splitting was also observed. In addition to fitting the energies and zero-field splittings, the theory gave values of absolute A-line intensities and lifetimes which agree well with observed values. Vibrational sidebands and pair lines observed in fluorescence were identified.