Thin flat films of cubic ionic crystals have sharp, strong infrared absorption bands at the frequencies characteristic of polar longitudinal optic modes of long wavelength in infinite crystals. The bands occur only when the incident radiation beam is not normal to the surface and only in the $p$-polarized component of the radiation. They occur not only in transmission through the films, but also in reflection when the films are deposited on thick metallic or dielectric substrates. Such a band was observed in films of LiF and was found to be in good qualitative agreement with, but somewhat stronger than, the band predicted from a dielectric dispersion curve obtained from other types of measurements by Bilz, Genzel, and Happ.

• Received 9 January 1963

DOI:https://doi.org/10.1103/PhysRev.130.2193