SURFACE PHONONS OF NA-INDUCED SUPERSTRUCTURES ON AL(111)

We report the surface phonon measurement by high-resolution electron-e nergy-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 properti es of the system as a first step. The lattice dynamical calculation fo r the Al(111)-(root 3 x root 3)R30 degrees-Na surface with a substitut ional adsorption geometry reproduces the observed strongly dispersing surface: resonant mode, while the calculated dispersion curves far a t hreefold adsorption geometry differ largely from the observed ones. Th is is explained in terms of the structural difference which causes dif ferent restoring forces at Na atoms. The observed dipole active surfac e resonant mode at the <(Gamma)over bar> point is assigned to a perpen dicular motion of Na against the almost fixed substrate which is simil ar to the one of which frequency was determined by some nb initio calc ulations. For the Al(111)-(2 X 2)-Na surface, the measured surface pho non dispersion curves locale very close to Rayleigh mode of the clean Al(111) and its backfold at (M) over bar(t) point.