Optical Spectrum of Cr3+ Ions in Spinels

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 $D_{\text{3d}}$ site. The parameters for a best fit for both crystals were: Δ = 18 250 cm⁻¹, $\text{B\hspace{0.17em}=\hspace{0.17em}700}{\mathrm{cm}}^{\text{−1}}\text{, C\hspace{0.17em}=\hspace{0.17em}3200}{\mathrm{cm}}^{\text{−1}}\text{, ζ\hspace{0.17em}=\hspace{0.17em}250}{\mathrm{cm}}^{\text{−1}}\text{, ν\hspace{0.17em}=\hspace{0.17em}−200}{\mathrm{cm}}^{\text{−1}}\text{, ν′\hspace{0.17em}=\hspace{0.17em}−1700}{\mathrm{cm}}^{\text{−1}}$ , and $\text{k\hspace{0.17em}=\hspace{0.17em}0.7.}$ The ${}^{2}E$ splitting was directly observed optically for both spinels, and in the case of the ZnAl₂O₄:Cr³⁺, the ${}^4{A}_2$ 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.