Removal of the clock reconstruction of Ni(100)-(2x2) p4g-N by coadsorption of K: A spot-profile-analysis low-energy-electron-diffraction and angle-resolved ultraviolet-photoemission-spectroscopy study

Previous work has revealed that the (2x2)p4gstructure formed by adsorption of N on Ni(100) is a clock reconstruction. A coverage of 0.5 ML of N atoms located in alternate fourfold hollow sites on the surface results in an expansion of each hollow by means of rotation of the Ni atoms in alternate clockwise and counterclockwise directions. Here we report on the removal of this substrate reconstruction by coadsorption of K. The structural changes induced by the alkali metal on the N precovered Ni(100) surface have been monitored by means of spot-profile-analysis low-energy-electron diffraction. The chemisorption bonding of N on Ni(100) and K on Ni(100) has been investigated by means of angle-resolved ultraviolet-photoemission spectroscopy. The band structure of the adsorbate-induced states is presented. The changes in the dispersive behavior of the N-derived bands upon coadsorption of K have been related to the structural changes. Our data suggest a mechanism for the removal of the reconstruction based on the K-induced restoration of the screening of the repulsive forces between the topmost Ni atoms.