COMBINED THEORETICAL AND EXPERIMENTAL-DETERMINATION OF THE ATOMIC-STRUCTURE AND ADSORPTION SITE OF NA ON THE SI(100)2X1 SURFACE

The Na/Si(100)2 x 1 interface is studied by both ab initio local density functional total energy DMol molecular calculations using very large cluster models and photoemission EXAFS which provides the unique feature of probing both Na adsorbate and Si substrate environments. Theoretical and experimental bond lengths are found to be in very good agreement and enable a definite assignment of the adsorption site: Na is adsorbed on a single site, the cave, with no Na-Na distance consistent with any ''double layer'' models. The growth and existence of a second Na layer are shown to occur only in presence of very low level impurities.