Elastic and Inelastic Helium Beam Scattering from Clean and Hydrogen Covered PALLADIUM(111) Surfaces.

The structural and dynamical properties of clean and hydrogen covered Pd(111) surfaces using elastic and inelastic scattering of monoenergetic thermal He beams were studied. The measurements were performed under ultra high vacuum conditions. The experimental work can be divided into three parts:. 1. Elastic He scattering from clean and H saturated Pd(111) surfaces. The surface atomic arrangements of these two well-ordered surfaces are determined by fitting the experimental data with the calculated scattering intensities using the Eikonal approximation and the hard corrugated wall model. 2. The temperature and coverage dependence of the elastically scattered He beam intensities from the H covered Pd(111) surface. Diffraction intensities were measured as a function of surface temperature in the range 140K-350K. The measurements revealed two remarkable features: (I) A structural transformation from a phase with C _{3V} symmetry at saturation coverage (140K) to another phase with C_{6V } at lower coverages (270K). (II) An anomalous attenuation of the specular He beam accompanying this transformation. The changes of the diffraction intensities provide strong evidence of a fundamental change in the surface charge density corrugation. 3. Inelastic He atom scattering from clean and H saturated Pd(111) surfaces. Surface phonon dispersion curves along the <112 > direction were measured. Several models have been proposed to interpret the observed changes of the surface charge density corrugation in the sample temperature range 140K-350K. Classical models of the motion of hydrogen on Pd(111) either fail to reproduce the measured attenuation or lead to contradictory and unphysical conclusions regarding the H-metal bond length or surface thermal equilibrium. In contrast, a quantum mechanical model proposed here provides consistent explanations of the observed anomalies, and when incorporated in the sudden approximation formalism produced an excellent fit to the measured diffraction intensity over the whole range of temperature and coverage studied. Furthermore, a lattice dynamics slab calculation was carried out to identify the nature and polarization of the measured surface phonon branches. In order to describe the influence of the H atoms on the Pd surface vibrational modes, the H atoms were treated within the slab model in an adiabatic fashion similar to that used in treating the electronic bond charges in the well-known bond charge model.