Using angle-resolved photoemission spectroscopy, we investigate changes in the band dispersion of a free-electron-like surface state of $\mathrm{Si}(111)\mathrm{\text{−}}\sqrt{3}\times \sqrt{3}\mathrm{\text{−}}\mathrm{Ag}$, induced by adsorption of submonolayer Au adatoms. At room temperature, where the adatoms are in a two-dimensional adatom-gas phase, electrons are transferred from the Au adatoms to the substrate, shifting the surface band downwards and causing it to deviate from a parabolic dispersion. At 135 K where the Au adatoms are frozen at specific sites of the substrate, the band splits into two. This band splitting can be explained in terms of hybridization between the unperturbed surface band and the localized virtual bound states induced by the Au adatoms.

• Received 29 September 2005

DOI:https://doi.org/10.1103/PhysRevLett.96.036803