Standing-Wave Optical Phonons Confined in Ultrathin Overlayers of Ionic Materials

High-resolution electron-energy-loss spectra of ultrathin ($d<3$ nm) LiBr epitaxial films on Si(100) [W. Gao et al., Solid State Commun. 87, 1013 (1993)] reveal an anomalous dependence of the frequencies of the macroscopic surface phonons on the layer thickness d. We present lattice-dynamics calculations of the spectra which prove that this anomalous dependence arises from the excitation of standing-wave optical phonons confined in the film and reflects the dispersion relation of bulk optical vibrations of LiBr in the direction normal to the surface.