Surface plasmon excitation in ultrathin Mg films on Si(111)

We investigated the dispersion of the surface plasmon in ultrathin Mg(0001) films, grown on a Si(111)-7$\times $7 surface, as a function of film thickness and parallel momentum ($q_{\vert \vert })$, using angle-resolved high-resolution electron-energy-loss spectroscopy (HREELS). In Mg films thicker than $\sim $ 3 ML, surface plasmon excitations exhibit negative dispersions for small $q_{\vert \vert }$(long wavelength limit). In contrast, the surface plasmons of ultrathin Al(111) films are known to exhibit positive dispersions near $q_{\vert \vert }\sim $ 0. The surface plasmon energies of the Mg films increase as the film thickness decreases. The plasmon line widths reveal similar trends, namely, for a given film thickness the line width decreases initially with increasing $q_{\vert \vert }$ while it increases with film thickness. Possible explanations for the observed thickness dependence of the surface plasmon dispersion and damping will be discussed.