In ruby a Cr3+ ion and its six nearest neighbouring O2- ions form a heavily distorted octahedron. Consequently, the magnitude of odd crystal field of the higher order is comparable with the first-order odd field. Because of the site group of Cr3+ ion in ruby approximately retains C3v symmetry, the higher-order crystal field component of T2u type is very small and can be neglected, but those of T1u type and A2u type must be taken into account. The higher-order odd field components make appreciable contribution to Y band and U⊥ band, but their contribution to U‖ band is negligible.The intensities of U‖ band and Y band are discussed with proper attention paid to the odd vibrations of ions. It has been concluded that the U band absorption is mainly produced through the coupling of odd vibrations with electrons. With this point of view, the oscillator strength, the dichroism, the band shape, etc., of the absorption bands of ruby are analysed. The results are in good agreement with experimental data.
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