Adatom vibrations on Si(111) reconstructed surfaces

The vibrational electron energy-loss spectrum of the Si(111)(7×{}7) surface exhibits a well defined loss at 570 cm^-1 and a structure at 200--270 cm^-1. A similar spectrum is displayed by a Si surface with a pseudo (1×{}1) structure. Ab initio calculations of the dynamical matrix show that these features are due to localized vibrations involving Si adatoms and the surface atoms lying underneath. The latter are fivefold coordinated Si atoms exhibiting the electronic structure of an acceptor impurity localized at the surface.