A comparative study of phonons in polycrystalline and single‐crystalline LiF films by electron spectroscopy

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Abstract
The electron energy loss spectra of thin polycrystalline and single‐crystalline LiF films are measured by means of a high‐resolution electron spectrometer (resolution 5 to 8 meV for 30 keV electrons) in the range from 30 to 100 meV. Theoretical spectra are calculated by using the dielectric theory of inelastic electron scattering and expressing the relevant frequency‐dependent dielectric constant analytically by a dispersion formula. The over‐all agreement between experiment and theory is good. Noticeable differences are found between the spectra of polycrystalline films and of thin single crystals: an excess of energy loss intensity is observed for polycrystalline films between 60 and 80 meV. The interpretation is, that the interfaces between the microcrystals give rise to the development of additional surface modes near 72 meV, which may be related to certain normal modes of isolated cubes.