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

Alkali metal coadsorption systems represent a step along the pathway from s imple model adsorbate overlayers to more technologically relevant real syst ems. However, such is their complexity that very few systems have been stru cturally determined. Here we present a surface X-ray diffraction investigat ion of one of these systems, Ni(100)-(3 x 3)-(Cs+O). Here a structural dete rmination 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, an omalous scattering has been used to determine whether there is a contributi on of the nickel substrate to the fractional order diffraction intensity. M easurements of the fractional order rods at 10 eV and 200 eV below the nick el K edge (8333 eV) were used to probe the nickel contribution to the fract ional order rods. It was found that the intensity of the scattering was unc hanged, 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 numb er of possible structures.