Spectrum of first-nearest-neighborCr3+pairs in ruby

The optical absorption, emission, and excitation spectrum and the exchange splittings of the first-neighbor ${\mathrm{Cr}}^{3+}$ pairs in ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ have been studied. The earlier experimental level assignment is shown to be inconsistent with the theory of Pryce and Naito. The new ground-state level scheme is based on the $\sigma$-polarized fluorescence line at 7301.5 Å which terminates on the $S=2\mathrm{}$ level, which is ∼280 ${\mathrm{cm}}^{-1\mathrm{}}$ above the $S=0\mathrm{}$ level, and the $\pi$-polarized transition to the $S=3\mathrm{}$ level at 7367.5 Å. The new parameters of the ground-state exchange $J\left({\overrightarrow{\mathrm{S}}}_1·{\overrightarrow{\mathrm{S}}}_2\right)+j{\left({\overrightarrow{\mathrm{S}}}_1·{\overrightarrow{\mathrm{S}}}_2\right)}^2$ are $J=54\mathrm{}$ and $j=-8.7\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$. The infrared pair transitions obtain their strength mainly from the single-ion mechanism. The weakness of the exchange-induced dipole moment, which usually dominates in pair spectra, is attributed to a near cancellation of the single-electron components. A comparison of the relative $\sigma$ and $\pi$ intensities suggests that the odd-parity crystal field is somewhat different than for single ions. Transitions where both ions of the pair are excited to the ${}_{}{}^2{}_{}{}^{}E$ states have been observed at nearly twice the $R$-line energy. The $\sigma$ polarization of these transitions agrees with the exchange-induced dipole-moment mechanism. The temperature dependence of the absorption of the $S=0\mathrm{} \mathrm{and} 1$ levels is in good agreement with the level populations derived from the ground-state exchange splittings.