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 $〈1\overline{1}0〉$ and $〈11\overline{2}〉$ azimuths with greatly reduced scatter compared with earlier work. Along the $〈1\overline{1}0〉$ 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.