Surface phonons of Na-induced superstructures on Al(111)

We report the surface phonon measurement by high-resolution electron-energy-loss spectroscopy of the Na-induced superstructures on Al(111). Semiempirical simple lattice dynamical analysis using several central forces is applied to understand qualitatively the vibrational properties of the system as a first step. The lattice dynamical calculation for the Al(111)-(√3x √3)R30°-Na surface with a substitutional adsorption geometry reproduces the observed strongly dispersing surface resonant mode, while the calculated dispersion curves for a threefold adsorption geometry differ largely from the observed ones. This is explained in terms of the structural difference which causes different restoring forces at Na atoms. The observed dipole active surface resonant mode at the Γ̄ point is assigned to a perpendicular motion of Na against the almost fixed substrate which is similar to the one of which frequency was determined by some ab initio calculations. For the Al(111)-(2X2)-Na surface, the measured surface phonon dispersion curves locate very close to Rayleigh mode of the clean Al(111) and its backfold at M̄′ point.