Surface Phonons of NaF and LiF Films

Abstract

Low energy electron loss spectra of evaporated films of NaF and LiF have been measured for various primary energies, incident and scattering angles, film thicknesses and substrates. Multiple energy loss peaks were observed in every spectrum, and the loss energies were about 66 meV for LiF and 44 meV for NaF. These values are slightly below the limiting frequencies, ω_s, of LiF and NiF. The energy loss is attributed to the excitations and absorption of high-frequency surface phonons of NaF and LiF films, but the loss energies are almost constant regardless of experimental conditions. This constancy of the loss energies seems to be caused by the fact that the scattering probability of low energy electrons does not fall off as q⁻³, where q is the momentum parallel to the surface of the excited or absorbed surface phonons.