Theoretical and experimental study of the Na/Si(100)2 x 1 surface structure

The structure of clean and sodium-covered Si(100)2 x 1 interface is studied theoretically by ab-initio all-electron total-energy calculations (DMol) using very large clusters (up to 77 atoms) and experimentally by polarization-dependent photoemission extended X-ray absorption fine structure (PEXAFS). We have determined the Si-Si dimer bond length and also found that the Si-Si dimer is relaxed upon Na deposition. We find that the Na-Si bond length is equal to the sum of Si and Na covalent radii. We also use the unique ability of PEXAFS to measure the distances between the nearest neighbors of both adsorbate and substrate atoms. The distances obtained for clean Si(100)2 x 1 surface and Na/Si(100)2 x 1 surface from the DMol calculations and PEXAFS experiments are in excellent agreement. Our results allow us to conclude clearly that the Na atom is adsorbed on a single site, the cave, which is also shown to have the lowest adsorption energy.