Polarization dependence of Si(111)-2×1 surface-phonon and surface-state excitations

Electron-energy-loss spectroscopy measurements for the Si(111)-2×1 surface have been performed to investigate the polarization of the dipole-excited surface-phonon and surface-state excitations. We have studied the dependence of the dipole scattering cross section on the azimuthal orientation of the crystal with respect to the scattering plane. Azimuthal asymmetry for dipole cross sections are reported and are in agreement with recent theoretical predictions on the origin and dynamics of the strong dipole-excited surface-phonon loss. Considerations of symmetry and measurements of finite-$q$ vibrational linewidths and defect-dependent effects provide further insight into the vibrational and electronic structure of this surface.