Far Infrared Spectra of ${\mathrm{Al}}_2$${\mathrm{O}}_3$:${\mathrm{Cr}}^{3+}$ and ${\mathrm{Al}}_2$${\mathrm{O}}_3$:${\mathrm{Ti}}^{3+}$

The absorption sepctrum of ruby containing 1% ${\mathrm{Cr}}_2$ ${\mathrm{O}}_3$ has been observed at 77°K and below, over the range from 10 to 160 ${\mathrm{cm}}^{-1\mathrm{}}$, using a large grating spectrometer. Although chromium ion pairs are known to have energy-level intervals in this region, no absorption lines were observed, even in a sample 15-cm long. Thus, these spin-changing transitions are, as might be expected, quite forbidden. In ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$:${\mathrm{Ti}}^{3+}$, levels were observed at 37.8 and 107.5 ${\mathrm{cm}}^{-1\mathrm{}}$ with strong transitions connecting them. A broad lattice absorption was also observed. The position of the 37.8-${\mathrm{cm}}^{-1\mathrm{}}$ level is in reasonable agreement with predictions from earlier microwave relaxation measurements in the ground state. The infrared linewidths are large, indicating a rapid relaxation without spin flip. The level spacings are smaller than predicted by crystal-field theory, indicating that the effective trigonal field and spin-orbit coupling are reduced by a dynamic Jahn-Teller effect.