Untersuchung dynamischer Oberflächeneigenschaften und zweidimensionaler Phasenübergänge an einer reinen und adsorbatbedeckten Pt(111)-Oberfläche mittels hochaufgelöster He-Streuung

The surface dynamics and the 2D-phases of clean and adsorbate covered Pt(111) has been investigated by high resolution He-scattering. The He-beam technique offeres a unique possibility to characterize 2D-phases and to monitor their mutual transitions, irrespective of the nature of the substrate and without any damaging.The dispersion of the vibrational modes of the Pt(111)-surface in the absence of impurities as well as in the presence of oxygen and xenon has been measured by inelastic helium scattering. A significant asymmetry between the two crystal directions $\overline{\Gamma} \overline{M}$ and $\overline{\Gamma} \overline{K}$ of the clean surface is observe. The measured surface phonon dispersions are compared with theoretical calculations based on a nearest neighbor central force model. The agreement between theory and experiment is good, if the intralayer force constant of the Ptatoms in the first layer is reduced to forty percent of its value in thebulk. The analysis shows that we have been able to observe a mode which has its maximum amplitude in the second layer. In the case of the ordered p(2x2) superstructure, the additional dispersion branch appearing as a result of the Brillouin zone folding was also measured. The experimental results show the first direct evidence for the appearance of a surface Rayleigh phonon gap at the new zone boundary. The gap energy was determined to be $\sim$ 0.6 meV at the $\overline{M}_{o}$-point. Lattice dynamical calculations show that this relatively large gap is due to bond stretching interactions between adsorbate and substrate atoms. Previous measurements have shown that the presence of disordered oxygen on metal surfaces (0/Ni(100)) has a negligible influence on the Rayleigh dispersion curve. We have measured the dispersion curve on a Pt(111) surface in the presence of disordered oxygen, which according to ion scattering data islocated subsurface. The frequency of the Rayleigh phonons at the $\overline{M}$-point is 12% below the clean surface value. This corresponds to a reduction of theinterlayer force constant k$_{12}$ by about 30% and thus supports the subsurface location of the oxygen atoms. The surface phonon dispersion of 1-, 2-, 3-, and 25-layer Xe-films has been measured along the $\overline{\Gamma} \overline{M}$ and $\overline{\Gamma} \overline{K}$-azimuth. The monolayer shows a dispersionless mode, which is typical for independent vibrating atoms (Einstein oszillator). On the other hand, the 25-layer Xe-film shows a distinct Rayleighwave. The substantial differences between the surface phonon dispersion curves of 1-, 2-, 3- and 25-layer Xe films gives the possibility to measure phase equilibria between the different layers. The thermodynamic properties of the 2D-Xe gas and of monolayer, bilayer and bulk Xe films were investigated under quasi-equilibrium conditions with the 3D-Xe gas. The phase diagram, the lattice spacings, the isosteric and latent heats of adsorption were determined. The structural analysis shows that Xe films up to one full monolayer exhibit on Pt(111) a variety of structural phases: commensurate, incommensurate and incommensurate rotated. This is the first time that all these phase are observed for a noble gas/metal system. Moreover the existence of a commensurate - incommensurate transition, induced by varying the surface temperature at constant coverage, is experimentally demonstrated (T$_{IC} \approx$ 57-60K) for the first time. The detailed monitoring of various C-I transitions (vs. Xe coverage, surface temperature and 3D-Xe pressure) shows marked deviations from the behavior observed so far for Kr/Graphite. It seems that the slightly compressed domains and not the domain walls are the first to rotate in the incipient stage of the transition. The results call for a more detailed theoretical consideration. The growth of multilayer Xe-films on Pt(111) has also been studied. The analysis of the data shows that in the studied temperature range (25-50 K)the adsorbed Xe behaves as type 1 film; i. e. complete wetting occurs, characterized by a layer-by-layer growth. All Xe-films (1,2,3,5 and 25-Xe layers) are build of large domains ($\sim$ 300 $\mathring{A}$) which are rotated by ± 3° with respect to the R30° commensurate structure (Novaco-McTague orientational instability); i. e. the monolayer structure effectively determines the structure of the multilayer films