Correlation between Plasmon Excitations and Wavefields under Reflection High-Energy Electron Diffraction Condition

The energy-loss spectra of the specular reflection electron beam from the Si(111)7 × 7 surface were measured under one beam condition and 〈112〉 incident azimuth using a novel energy-filtered reflection high-energy electron diffraction apparatus with high spatial and glancing angle resolutions. All measured spectra were precisely peak resolved based on the Poisson distribution, and the mean number of excitations for the main surface plasmon and weak bulk plasmon was obtained. The mean number of surface plasmon excitation was proportional to 1/sin θ with respect to the glancing angle θ, ~70% of Lucas’ theoretical value. The surface plasmon excitation showed a moderate increase under Bragg reflection conditions, correlated with the integrated intensity of the wavefield in vacuum including the subsurface. Conversely, the mean number of bulk plasmon excitation showed an inverse behavior to the intensity of the specular reflection. The wavefield in the crystal was localized on the vertical bonds between the Si bilayers when the bulk plasmon increased. For 〈112〉 incidence, large anomalous increase was observed both in surface and bulk plasmon excitations at a glancing angle of θ = 4.2°. At this angle, the specular spot was located quite close to the intersection of several Kikuchi lines, with very weak specular reflection intensity, suggesting that the Kikuchi electrons had a strong influence on the energy-loss spectrum. The Kikuchi lines were formed by inelastic scattering inside the crystal, consistent with their contribution to the increase in the bulk plasmon excitation, but they did not explain the anomalous enhancement of surface plasmons as well. The relationship between Kikuchi lines and surface plasmon excitation enhancement is still a work in progress.