Observation of a Kohn Anomaly in the Surface-Phonon Dispersion Curves of Pt(111)

Surface-phonon dispersion curves have been measured by He-atom inelastic scattering for Pt(111) along the 〈11̄0〉 and 〈112̄〉 azimuths with greatly reduced scatter compared with earlier work. Along the 〈11̄0〉 direction the results reveal a number of anomalous kinks in the Rayleighmode dispersion curve. Comparison with the corresponding anomaly in the bulk suggests that the effect is due to a two-dimensional surface Kohn anomaly.