STRUCTURAL-PROPERTIES OF THE NA SI(111)2X1 SURFACE STUDIED BY PHOTOEMISSION EXTENDED X-RAY-ABSORPTION FINE-STRUCTURES/

We present a coverage dependent structural study of Na adsorption on t he Si(111)2 x 1 surface by photoemission extended x-ray-absorption fin e-structure, core-level, and valence-band photoemission spectroscopies using synchrotron radiation. In the submonolayer coverages, our resul ts indicate the existence of two Na adsorption sites having a large di fference in ionicity. Furthermore, a clear ionic to covalent transitio n is observed when completing the sodium monolayer saturation coverage , which corresponds to the alkali atom adsorbed on a threefold hollow site. These results indicate a very different behavior for the Na adso rption on Si(111)2 x 1 when compared to the Si(100)2 x 1 surface, wher e the Na-Si bond remains covalent in nature, even at low coverages.