Surface x-ray diffraction onK/Si(001)(2×1)andCs/Si(001)(2×1)

The geometric structures of $\mathrm{K}/\mathrm{S}\mathrm{i}\mathrm{}\left(001\right)(2\mathrm{}\times 1)$ and $\mathrm{C}\mathrm{s}/\mathrm{S}\mathrm{i}\mathrm{}\left(001\right)(2\mathrm{}\times 1)$ formed at room temperature have been investigated using surface x-ray diffraction. The analysis of the intensity distribution along both half-order and integer-order reciprocal-lattice rods (135 and 113 independent reflections in total for K and Cs, respectively) indicates that at saturation coverage in both cases the alkali metal $\mathrm{}\left(A\right)\mathrm{}$ adsorbs in two different sites. The average adsorption sites on the clean Si surface are at the dimer bridge site on top of the dimer rows, and at the valley bridge site in the groove between the Si dimers. Actually, the dimerization is (at least partially) lifted upon $A$ adsorption. There is no evidence for a large $z$ disorder of the top-layer Si atoms, which would be characteristic of an asymmetry of the Si-Si dimer. We derive K-Si bonding lengths in the range between 2.90 and 3.58 Å, and Cs-Si bonding lengths between 3.33 and 3.50 Å. The minimum bond lengths are at most 11% lower than values found in bulk KSi and CsSi. This can be interpreted by the formation of a weak covalent bond. In order to obtain good fits to the data, up to four layers of the Si substrate have to be included into the refinement. The Si-Si distances do not deviate by more than 5–10 % from the bulk Si-Si bond length (2.35 Å). The only significant difference between the K and Cs adsorption structures is found for the alkali saturation coverage. For K we find a coverage of 1.2 ML, whereas only 0.3 ML is found for Cs.