Abstract
The dispersion of the vibrational modes of the Pt(111) surface in the absence of impurities as well as in the presence of oxygen has been measured by inelastic helium scattering along the \\ ̄ gG $ ̄ M azimuth. In the case of the ordered p(2×2)O superstructure, the additional dispersion branch appearing as a result of the Brillouin zone folding was also measured. The experimental results show the first direct evidence for the appearance of a surface Rayleigh phonon gap at the new zone boundary. The gap was determined to be ~ 0.85 meV at the M̄ 0 point. Lattice-dynamical calculations show that this relatively large gap is due to bond stretching interactions between the adsorbate and substrate atoms. The dispersion curve in the presence of disordered oxygen, obtained by annealing (1200 K) the Pt surface in an oxygen atmosphere and assumed to be located subsurface on the basis of ion scattering data has been measured. The frequency of the Rayleigh phonon at the M̄ point is 12% below the clean surface value. This is at variance with the case of disordered oxygen on metal surfaces (O/Ni(100)) which has been shown to have a negligible influence on the Rayleigh dispersion curve. According to the lattice dynamical calculations this 12% frequency lowering corresponds to a reduction of the interlayer force constant k 12 by about 30% and thus supports the subsurface location of the oxygen atoms.
Published Version
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