K adsorption on Ag(110): effect on surface structure and surface electronic excitations

The growth of K adlayers on Ag(110) was studied by energy loss spectroscopy-low energy electron diffraction (ELS-LEED) which allows the analysis of both elastically and inelastically scattered electrons. Ordered (1 x n) missing row reconstructions and disordered adsorption were investigated. We find that extended surface reconstruction takes place already at 3% K coverage generating a (1 x 3) missing row structure. Elastic spot profile analysis shows that a long range correlation between K adatoms is present along the [<1(1)over bar>0] direction on the reconstructed surface, implying the stabilization of the reconstruction also at long distances from the K adatoms. With increasing coverage [(1 x 2) and (1 x 3) structures] the distance between the K adatoms along [<1(1)over bar>0] decreases and the adsorption sites become correlated also along [001] in contrast with the behaviour of K on Ni, Cu and Pd. For disordered K adsorption we observe that a weak correlation between the adatoms along both directions is already present in the background. The inelastic intensity shows only one marked loss at the surface plasmon frequency, indicating that in the investigated coverage range both K-induced s and p levels are empty. Ag surface plasmon dispersion and damping are affected by the presence of K in accord with the electronic mechanism generating the reconstruction. For the ordered (1 x n) reconstructions the structural anisotropy is enhanced, while the anisotropy of Ag surface plasmon dispersion is reduced. K adsorption strongly affects in particular the quadratic term of the dispersion along [001], which drops by 60% already at 3% K coverage while the linear term remains initially unaffected independently of surface reconstruction. Above 30% K coverage also the linear term decreases and the anisotropy present for the surface plasmon dispersion of the bare Ag(110) is removed.