A surface anomalous diffraction study of the Ni(100) (3x3)-(Cs + O) system.

Alkali metal coadsorption systems represent a step along the pathway from simple model adsorbate overlayers to more technologically relevant real systems. However, such is their complexity that very few systems have been structurally determined. Here we present a surface X-ray diffraction investigation of one of these systems, Ni(100)-(3×3)-(Cs+O). Here a structural determination is particularly challenging due to the presence of three species in the surface layers and by the size of the unit cell. As a first step, anomalous scattering has been used to determine whether there is a contribution of the nickel substrate to the fractional order diffraction intensity. Measurements of the fractional order rods at 10 eV and 200 V below the nickel K edge (8333 eV) were used to probe the nickel contribution to the fractional order rods. It was found that the intensity of the scattering was unchanged, indicating that the fractional order peaks are caused by scattering from the coadsorbates only. This shows that the nickel surface layers are not changed by the adsorption and thus sets a useful constraint on the number of possible structures.