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 at the early stage of adsorption show that Na adsorption initially induces buckling of Si dimers to produce the local c(4 x 2) structure. A prominent loss peak observed in the HREELS spectra shows the metallic nature of the surface and is interpreted as a surface plasmon excitation of the Na 3s electrons partially occupying the low-lying substrate empty 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 surface plasmon peak disappears completely. We propose a structural model for the (4 x 1) surface.