The valence-band photoemission of the Rh(100)-c(2ϫ2)-O, Rh(100)-(2ϫ2)p4g-O, and Rh(100)-c(2ϫ2)-S surfaces has been investigated using an incident photon energy of 38 eV, along both the ⌫XЈ and ⌫M Ј directions. Local density of states calculations have also been performed for the c(2ϫ2)-O and c(2 ϫ2)-S phases using density-functional theory. For each surface overlayer the angle-resolved photoemission measurements show significant differences in the dispersion of features with binding energies between 3 and 6 eV. These peaks are assigned with reference to the density of states calculations, and the origins of the differences in dispersion are discussed in relation to the different geometric structures.

Previous work has revealed that the (2ϫ2)p4gstructure formed by adsorption of N on Ni͑100͒ 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 Ni͑100͒ surface have been monitored by means of spot-profile-analysis low-energy-electron diffraction. The chemisorption bonding of N on Ni͑100͒ and K on Ni͑100͒ 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. ͓S0163-1829͑97͒06835-5͔

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