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 Θc=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 {}, which gave a nearest-neighbor Na-Si bond length of 2.975 {}. The ``effective'' Na radius at this configuration was 1.8 {}, which was only 3% smaller than the bcc metallic Na radius. The Si-Si ``dimer'' distance was relaxed from the 2.45 {} value of the clean (2×{}1) Si(001) surface to 2.64 {}. The effective Na radius of 1.8 {} determined in this work was {~}0.35 {} larger than that determined by a recent total-energy-minimization calculation.