Room-temperature reflectivity measurements in the wavelength range 1 to 140 μ 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 μ. Those vibrating parallel to the $C$ axis occur at 17.1 and 25.0 μ. 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 ${\mathrm{A}}_{1u}$ (forbidden) phonon branches picking up dipole moment as a result of shear strain.

• Received 1 July 1963

DOI:https://doi.org/10.1103/PhysRev.132.1474