Surface phonon dispersion ofc(2×2) S on Ni(100)

The dispersion of the adsorbate- and substrate-associated modes of the Ni(100) surface covered with a c(2×{}2) sulfur overlayer has been measured along the {Γ}{}--X{} direction by inelastic electron scattering. Experimental data are compared to a lattice dynamical model using an analytical Green's-function technique. The optimum fit is obtained when the sulfur atom is placed at 1.45 A{} above the nickel layer.