SODIUM-INDUCED 2 X 1-] 1 X-1 SURFACE STRUCTURAL TRANSITION ON SI(111)

Sodium deposited on the cleaved Si(111)2 X 1 surface induces a 2 X 1 --> 1 X 1 surface structural transition at about 1/2 monolayer coverage. Angle-resolved direct and inverse photoemission reveal the surface to be semiconducting. The measured energy dispersion of the empty Na-induced surface state is only consistent with calculations that favor the threefold-hollow site for the Na adsorption position. Its bonding character (ionic versus covalent) will be discussed.