Dispersion of Adsorbate Vibrational Modes—thec(2×2)Oxygen Overlayer on Ni(100)

The dispersion of surface vibrational modes associated with the $c(2\mathrm{}\times 2)$ adsorbate layer of oxygen on Ni(100) is measured throughout the two-dimensional Brillouin zone along the [110] direction. Two oxygen modes and two nickel surface phonons are observed. A lattice-dynamical analysis gives good agreement with the data when the oxygen atom sits at ∼0.9Å above the nickel surface at the fourfold hollow site.