Austauschwechselwirkungen,Intrabandschwingungen und Ti3+-Kristallfeldaufspaltung in dotiertem Al2O3

The absorption spectra of ruby with various Cr concentrations have been studied in the wavelength region between 50 μ and 1 mm at liquid helium and nitrogen temperatures. At liquid helium temperatures two strong absorptions have been found at 37.7 and 106 cm^?1 and weak absorptions at 18.8, 30.7, 33.2 and 43.0 cm^?1. These lines depend on the Cr concentration and vanish at N₂ temperatures. The two strong absorptions have also been observed in Al₂O₃ crystals doped with Ti or V. Sapphire did not show any of these absorptions. The origin of the two strong absorptions in Al₂O₃ crystals doped with Ti, V or Cr are discussed by consideration of magnetic dipole transitions of exchange coupled Cr³⁺ - pairs, the crystal field splitting of Ti³⁺ impurities and impurity — induced lattice modes. The weak absorption lines, on the other hand, may be assigned to transitions between the exchange levels of the second-nearest and fourth-nearest Cr³⁺ pairs if one makes use of the data obtained from the EPR and fluorescence spectra. For the second-nearest Cr³⁺ pairs group theory of the exchange interaction predicts transitions withΔS=2, whereas for the fourth-nearest neighbours no restriction forΔS exists.