Dynamics of the clean and hydrogen covered Pd(111) surface

We report a study of the dynamics of the clean and hydrogen-covered Pd(111) surface using elastic and inelastic scattering of a monoenergetic thermal He beam. We present surface phonon dispersion curves along the ▪ direction measured at 140 K for saturated H(1 × 1) coverage, and at 320 K for ⋍ 10% coverage. We discuss these results in the light of recent evidence that H behaves like a correlated quantum fluid on this surface of Pd and completely delocalizes at medium and low coverages. We present possible explanations and suggest further experiments.