Adsorbate-Induced Rayleigh-Phonon Gap ofp(2×2)OPt(111)

The dispersion of the vibrational modes of the $\frac{p(2\mathrm{}\times 2)\mathrm{O}}{\mathrm{Pt}\mathrm{}\left(111\right)\mathrm{}}$ system has been measured by inelastic He scattering along the $\overline{\Gamma }\overline{M}$ azimuth. The experimental results show the first direct evidence for the appearance of an energy gap in the Rayleigh curve at the $p(2\mathrm{}\times 2)$ reduced-zone boundary. Both the size of the gap (∼ 0.85 meV) and the appearance of a new mode near the $\overline{\Gamma }$ point are reproduced by a lattice-dynamical analysis, if three-body interactions of the bond-stretching type are included in the calculations.