Parameter-free first-principles phonon calculations are used in conjunction with multiple-scattering electron-energy-loss calculations to reveal ne_w_k>calized_modes on Cu(l 11) and Ag(l 11). The discovery of these z-polarized localized modes along TM and VK provides for the first time good agreement between theory and all the existing helium-atom-scattering and electron-energy-loss experimental results. The new results indicate that it is not necessary to evoke exotic models such as anomalous surface dynamical effects on these densely packed surfaces.

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We show that shear horizontal modes Sl and S5 may have measurable inelastic electron scattering cross sections if a c(2&2) overlayer is adsorbed on a (001) surface. The nonzero cross sections arise from preferential shadowing of substrate atoms by the overlayer. Whether Sl or S5 has nonzero cross section is an indicator of the adsorption site. Also, the number of nonzerocross-section Rayleigh (S4) modes depends on the adsorption site.