Effect of Repulsive Lateral Interaction on the Adsorption, Desorption and Growth of Adsorbates.

Research project P 13841 Effect of Repulsive Lateral Interaction on Adsorption Dieter SEMRAD 11.10.1999 This project aims at a better understanding of the adsorption properties and the kinetics of particles (atoms or simple molecules) adsorbed on single crystal surfaces as 2-dimensional gases or solids. The gas phase may turn out to be stable within a wide range of temperature and coverage, as a result of repulsive interactions between the adsorbed particles. It is asumed that the ad-particles are polarized by the substrate and are aligned with their dipole axis perpendicular to the surface leading to a repulsive dipole-dipole interaction. We will investigate the nature of this alignment with respect to the metallic substrate. In addition, these systems provide a very attractive opportunity to study `isolated` ad-particles at high concentration, yielding good signal to noise ratio. Yet still open questions concern the sites on the substrate lattice where the particles are adsorbed and the vibration dynamics of isolated paricles and its evolution with coverage. From today`s point of view promising candidates are Xe on Pd (111) and CO on Cu (110) for the physisorption and chemisorption case, respectively. At larger coverage preliminary He- diffraction measurements reveal hitherto unknown low temperature phases, e.g., for CO on Cu (110). After annealing, these phases irreversibly transform into high temperature states whose nature appears to be quite different from the low temperature phases. The corresponding phase transitions will be further investigated. Two complementary experimental methods will be applied: Thermal Energy He Atom Scattering (TEAS) and Temperature Programmed Desorption (TPD). TEAS in its various operational modes (specular He scattering, He diffraction from ad-layers, and inelastic He scattering) together with TPD provide full information , on the thermodynamic, structural and vibrational properties of the ad-layer.