Adsorption geometry of (2×1) Na on Si(001)

We have carried out a fully dynamical low-energy electron-diffraction intensity spectra analysis of the Si(001) (2×1)-Na system. We found that at coverage ${\mathrm{\Theta }}_{\mathit{c}}$=0.5 (i.e., defined as one Na atom per Si dimer), the Na atoms adsorbed as ordered one-dimensional overlayer chains at quasihexagonal sites proposed by Levine. The vertical distance of the Na atoms above the substrate was 1.85 Å, which gave a nearest-neighbor Na-Si bond length of 2.975 Å. The ‘‘effective’’ Na radius at this configuration was 1.8 Å, which was only 3% smaller than the bcc metallic Na radius. The Si-Si ‘‘dimer’’ distance was relaxed from the 2.45 Å value of the clean (2×1) Si(001) surface to 2.64 Å. The effective Na radius of 1.8 Å determined in this work was ∼0.35 Å larger than that determined by a recent total-energy-minimization calculation.