Structural study of the Si–alkali-metal interface with x-ray standing waves

X-ray standing-wave measurements on Cs and Rb adlayers on the Si(111)7×7 surface are presented. Low [≊0.06 monolayer (ML)] and medium (≊0.15 ML) coverages have been analyzed. The (111) and the (1¯11) reflections have been used. Since the experimental results clearly indicate that multisite occupancy takes place, the threefold hollow (${\mathit{H}}_3$), the threefold filled (${\mathit{T}}_4$), and all the dangling-bond sites have been considered as possible adsorption sites. Distinction has been made between the faulted and the unfaulted half of the 7×7 cell. The Si–alkali-metal bond length has been assumed to be the sum of the covalent radii of the two elements. Our results indicate that both Rb and Cs adsorb preferentially on the ${\mathit{H}}_3$ and the ${\mathit{T}}_4$ sites. The remaining atoms adsorb most at the atop sites for Rb and at the adatom sites (i.e., on top of the silicon adatom) for Cs.