Anisotropic Optical Spectra of α-Al₂O₃ Single Crystals in the Vacuum Ultraviolet Region. I. Spectra of Absorption Tail and Reflectivity

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The spectra of absorption constant in the intrinsic absorption tail region of α-Al₂O₃ single crystals at 297 K, 78 K and 10 K are presented for the electric fields of incident light parallel to the optic axis ⟨c⟩ of α-Al₂O₃ and normal to it. The absorption tail spectra obey the Urbach rule for each polarization, indicating that absorptions in the tail region result from the direct interband transitions. The optical anisotropy characterized by the Urbach rule is interpreted in terms of the bond length O²⁻–Al³⁺ participating in the absorption processes. The spectra of reflectivity are presented in the range of photon energies from 4.6 eV to 120 eV for electric fields parallel to ⟨c⟩ and normal to it. Spectral positions of main maxima in the reflectivity spectra for each polarization are determined at 297 K and 10 K; the spin-orbit doublet of the Al³⁺ L_2,3 is revealed at 78.67 eV and 79.12 eV for the first time in the case of α-Al₂O₃.