SURFACE DIELECTRIC RESPONSE FUNCTION OF OPTICAL PHONONS IN AN IONIC-CRYSTAL FILM

The paper begins with a short review on the theory of the surface dielectric response function of optical phonons in thin films, which is an important quantity for the understanding of various surface phenomena. The dielectric response of surface phonons to infra-red excitations is analysed with some details for two test cases: a SiO2(0001) self-supported film and NaCl(001) slabs on a thick Ge substrate. The results obtained from lattice-dynamics calculations for short wave vectors are compared with the predictions of electrostatics. The latter is shown to provide a valuable description - at the level of the energy resolution achieved today in electron-energy-loss spectroscopy - of the surface dielectric response function, even for the thinnest films (11 angstrom) considered in this paper.