The surface phonon structure of UO 2 single crystals investigated by high resolution electron energy loss spectroscopy

Abstract

UO 2 single crystals oriented along the (111) surface were cleaned in ultra high vacuum by brief ion bombardment followed by annealing at 600 K. The state of the surfaces was monitored by LEED and Auger electron spectroscopy. High resolution (45 cm −1) electron energy loss (HREEL) spectra of 10 eV electrons specularly reflected from the clean samples show characteristic energy losses and gains equally spaced from the elastically backscattered electron beam. These features correspond to the excitation or annihilation of one or several optical surface phonons (Fuchs-Kliewer modes). These surface phonons are known to be completely described by the bulk IR optical properties of the material. The HREEL spectra were processed by use of the dielectric theory applied to the interaction of electrons with surface vibrations. Data analysis yields the real and imaginary parts of the dielectric function in the IR frequency range. A single oscillator was found, characterized by the following parameters: resonance frequency, ω TO = 278 ± 2 cm −1; oscillator strength, Δϵ = 16.5 ± 0.5; damping, γ = 42 ± 2 cm −1. These values are critically compared with other results obtained by inelastic neutron scattering and by IR reflectivity.