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Room-temperature reflectivity measurements in the wavelength range 1 to 140 have been made on corundum and ruby. Classical oscillator analysis of the data shows that the optical lattice modes with dipole moment vibrating perpendicular to the C axis occur at 15. 7, 17. 6, 22. 6, 26. 0. Those vibrating parallel to the C axis occur at 17. 1 and 25. 0. The line-widths and strengths are reported. Many forbidden modes are observed. Two of these can have a strength which is comparable to the allowed modes but which is sensitive to surface treatment. Symmetry arguments show that these strong forbidden modes are the A₁ₔ (forbidden) phonon branches picking up dipole moment as a result of shear strain.
A. S. Barker (Fri,) studied this question.