Evidence of quantum motion of hydrogen on Pd(111) in helium-diffraction data

We investigate the behavior of hydrogen on the Pd(111) surface by elastic scattering of a monoenergetic thermal He beam. Dramatic changes in the intensities of the diffraction peaks are observed as a function of sample temperature in the range of 140--350 K, and are attributed to changes in H coverage. A consistent explanation of the diffraction data is achieved when we consider the H system as a correlated quantum fluid where particle delocalization occurs at medium and low coverages.