Recently, extensive ab initio calculations predicted a very unusual ordered-surface alloy structure for the adsorbate system Na on Al(111) at a coverage of H = 0.5. For this surface structure a surprisingly strong intermixing of Na and Al atoms has been reported. Subsequently, this complicated geometry was conÐrmed in great detail by lowenergy electron di †raction and surface-extended x-ray adsorption Ðne structure experiments. However, a scanning tunnelling microscopy (STM) study arrived at a signiÐcantly di †erent interpretation for the surface geometry.
Using the full-potential linearized-augmented plane-wave method we have evaluated the surface electronic structure and valence densities for the theoretically predicted geometry and we simulated STM images by employing the Terso †-Hamann approach. The calculated STM images exhibit only one corrugation per surface unit cell (for a bias up to Ô1 eV), which is in accord with the experimental STM results. The charge transfer from alkali atoms to the substrate makes Na atoms in this imaging regime nearly invisible in STM. Thus, the Terso †-Hamann interpretation scheme starting from correct surface geometry is consistent with the STM data, and the difficulty in identifying structures in measured STM images with atoms is solved. The interpretation of the STM image is in agreement with the above-mentioned experiments and calculations. We also compare these results to those of additional calculations for H = 1/3, i.e. the structure of Na on Al(111). Copyright 1999 John Wiley (Õ3 Â Õ3) ( & Sons, Ltd.