Surface phonon spectroscopy of Ni(111) studied by inelastic electron scattering

We report the experimental determination and theoretical analysis of the surface phonon dispersion relations for clean Ni(111) along the {Γ}{}M{} symmetry direction. The surface phonon spectra were obtained with a high-resolution electron energy loss spectrometer operating in the off-specular impact scattering regime. Kinematic conditions were varied in order to selectively examine the Rayleigh mode and ``gap'' mode, as well as contributions from bulk phonons. Comparison of the experimental surface phonon dispersion relations and inelastic scattering cross sections with lattice dynamical and quantum multiple scattering calculations demonstrate that tensile surface stress is present at the level of 1.6±{}0.2 N/m, and that the intraplanar surface force constant is (11±{}3)% softer than in bulk nickel.