REFINEMENT OF THE${\rm{Si}}(111) \mbox{-} (\surd{3} \times \surd{3}){\rm{R30}}^\circ - {\rm{Ag}}$STRUCTURE BY LOW-ENERGY ELECTRON DIFFRACTION

We have reinvestigated the bond geometry of the${\rm{Si}}(111) \mbox{-} (\surd{3} \times \surd{3}){\rm{R30}}^\circ - {\rm{Ag}}$surface by means of low-energy electron diffraction using a much larger experimental data set than that previously used. The${\rm{Si}}(111) \mbox{-} (\surd{3}\times \surd{3}){\rm{R30}}^\circ - {\rm{Ag}}$surface consists of a$(\surd{3}\times \surd{3}){\rm{R30}}^\circ$lattice ofAgatoms which replaces the topmostSiatoms, and forces the remainingSiatoms to form trimers. TheAg-Agbond length turned out to be3.47±0.12 Å. TheAgatoms are laterally displaced from the bulk positions of theSiatoms which they have replaced by 0.53 Å resulting in aAg-Sibond length of2.36±0.17 Å. The missing topSilayer and the formation ofSitrimers lead to strong distortions in deeperSilayers, most notably a buckling in the third and fourthSilayer with a magnitude of about 0.35 Å and 0.2 Å, respectively. Applying the concept of ‘split positions’, the low Debye temperature ofAghas been interpreted as being caused by strong in-plane (either static or dynamic) movements of the Ag atoms perpendicular to theAg-Sibonding.