Coadsorption of potassium and nitrogen on the Ni(100) surface

R-factor analysis carried out over a wide area of parameter space has yielded structural parameters as deep as the third Ni layer. Simulations of data obtained for Ni(100)-(K+N) were not attempted due to the similarity of the PD curves with those of the Ni/N system. From the point of view of PD, there are no structural differences between the pure N and the K coadsorbed systems, in contrast to the SPA-LEED and ARUPS results. A scanning tunnelling microscopy (STM) study has been performed on the Ni(100)-(K+N) system. By gradually dosing the Ni/N surface with K, it was possible to observe the K atoms initially adsorbing preferentially along [001] step edges, before forming protrusions approx 10 A in diameter on the terraces at higher coverages. The disturbance of the substrate underlying these protrusions has been imaged and suggests a local interaction of K with the surface. As the K coverage increased to 0.5 ML, the protrusions appear to coalesce. The K atoms have also been shown to be immobile on the surface throughout the range of coverages studied. It was not possible to observe the lifting of the reconstruction. The Ni(100)-(K+N) coadsorption system has been studied by spot profile analysis low energy electron diffraction (SPA-LEED). It was found that as K was incrementally dosed onto the Ni/N surface, a change in the observed LEED pattern from (2x2)p4g to c(2x2) occured at a K coverage of theta = 0.16 ML. This transformation was accompanied by an increase in the background intensity, attributed to the formation of a disordered K layer. Angle resolved ultraviolet photoelectron spectroscopy (ARUPS) measurements have been made on the Ni(100)-N and Ni(100)-(K+N) systems. States previously thought characteristic of the clock reconstruction have been shown to survive the phase transition on K coadsorption. The results also show that K coadsorption induces a change in the dispersion of N derived even states from pi, py character to become more p sub z -like (i.e. more like O 2p sub z states of Ni(100)c(2x2)-O). This has been interpreted as an indication that the N atoms move upwards to occupy higher positions above the surface in the coadsorption system. A possible mechanism behind the removal of the clock reconstruction based on the K-induced restoration of screening between Ni atoms at the surface, so allowing them to rotate back to their clean crystal positions and hence squeeze the N atoms out of their almost coplanar sites, has been proposed. Photoelectron diffraction (PD) data for Ni(100)-N was modelled by single scattering cluster - spherical wave calculations. A systemati