ADSORPTION OF SUBMONOLAYER NA ON THE SI(001) SURFACE

We have investigated the initial stage of Na adsorption on the Si(001) surface using scanning tunneling microscopy (STM) and high-resolution election energy loss spectroscopy (HREELS). The STM images obtained a t the early stage of adsorption show that Na adsorption initially indu ces buckling of Si dimers to produce the local c(4 x 2) structure. A p rominent loss peak observed in the HREELS spectra shows the metallic n ature of the surface and is interpreted as a surface plasmon excitatio n of the Na 3s electrons partially occupying the low-lying substrate e mpty band. Further increase of Na coverage does not expand the c(4 x 2 ) structure, but forms a new ordered structure (4 x 1) when the surfac e plasmon peak disappears completely. We propose a structural model fo r the (4 x 1) surface.