Surface phonons on the Pt(111) surface: A comparison of He-scattering experiments with lattice-dynamical calculations

Angular distributions and surface-phonon dispersion curves have been measured by He-atom scattering for clean Pt between T=160 and 373 K and for Pt-H(1×{}1) at 160 K along the 〈110〉 and 〈112〉 azimuths. Inelastic-scattering intensities over a wide range of wave vectors are also reported for the clean surface. The results are analyzed with use of the distorted-wave Born approximation for a model potential describing the He-atom--Pt-surface potential. A set of 14 force constants adjusted to the bulk-phonon dispersion data cannot explain the measured time-of-flight spectra and the observed first-layer relaxation. However, by modifying at the surface the nearest-neighbor tangential force constants (α₁''=1.7α₁ᵇ, and α₁'=1.3α₁ᵇ), the radial force constants (β₁''=0.39β₁ᵇ), and the three-body force constant (γ₁''=0.037γ₁ᵇ)--- where the superscripts {∥} and {⊥} denote forces within the surface plane and between the first and second planes, respectively---a good fit to all the data is achieved. The new surface force constants are discussed in terms of electronic-charge redistribution within the first layer.