Surface vibrations and (2×1) superstructures on fcc (110) metal surfaces

Electron-energy-loss spectra are presented for a number of adsorbate overlayers which form (2×1) superstructures on the fcc (110) surfaces of Ni and Cu. These include (2×1) structures of H, ${\mathrm{N}}_2$, CO, and O. Lattice-dynamical slab calculations are used to analyze the vibrational properties observed in the loss spectra which result from symmetry selection rules for dipole activity of the surface phonons. The calculated spectral densities, together with the symmetry selection rules, are used to analyze proposed models for the adsorbate overlayer structures.