Abstract
Surface phonon spectra in ultrathin α- Al 2 O 3 films prepared on metallic substrates show excitation of modes in the frequency range of the surface dipole forbidden ω_-branches according to the dielectric theory of EELS. We compare calculations of the z-polarisation of the FuchsKliewer modes based on lattice dynamics with dielectric theory and semi-microscopic results for a simple NaCl lattice type thin film in the limit of k→0 appropriate to the dipolar-scattering regime. The form of the primary-beam-energy-dependence of the ω_-branch in thin α- Al 2 O 3 films is similar to the lattice dynamics prediction but is of much greater intensity. While the strong k-dependence may be attributed in part to significant z-polarisation of the ω_-branch, mixing with microscopic surface modes seems likely.
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