Phasenübergänge und Dynamik von Festkörperoberflächen und physisorbierten Schichten

Zeppenfeld, P.

Book/Report

FZJ-2018-03152

Phasenübergänge und Dynamik von Festkörperoberflächen und physisorbierten Schichten

Zeppenfeld, P. (Corresponding author)FZJ*

1989
Kernforschungsanlage Jülich, Verlag Jülich

Jülich : Kernforschungsanlage Jülich, Verlag, Berichte der Kernforschungsanlage Jülich 2305, 166 p. (1989)

Please use a persistent id in citations: http://hdl.handle.net/2128/18687

Report No.: Juel-2305

Abstract: Structural and dynamical properties of surfaces and of thin physisorbed films have been investigated: thermal He-scattering has been used to study the clean Cu(110)surface and rare gas adlayers adsorbed on Pt(111), while quasielastic neutron scattering has been used to investigate surface diffusion and melting of thin methanefilms on graphite. The measurement of the surface phonon dispersion on Cu(110) reveals a significant change of the force field in the vicinity of the surface. The experimental data have been analyzed by comparison with a simple lattice dynamical calculation. Good overall quantitative agreement is obtained by taking into account the large lattice relaxation of the Cu(110) surface and including an anisotropic surface stress within the topmost atomic layer. The thermal behaviour of the Cu(110) surface is characterized by a strong decrease of the coherently scattered He-diffraction intensities above T $\thicksim$ 550 K. An analysis of the diffraction peak profiles and the diffuse elastically scattered intensities, however, shows that this thermal anomaly cannot be explained by a structural change at the surface ; in particular, a surface roughening transition below 900 K can be ruled out. Supported by inelastic He-scattering experiments, the thermal behaviour can instead be explained by an enhanced surface anharmonicity leading to an enlarged mean square displacement of the surface atoms. This result is confirmed by theoretical calculations of the dynamical He-scattering cross section; in addition, estimates of the magnitude of the surface anharmonicity are obtained. Rare gas layers physisorbed on the Pt(111) surface in the submonolayer coverage regime exhibit a variety of structural phases and corresponding phase transitions. For the first time high order commensurate (HOC) phases were observed in Kr and Ar adlayers. The thermal expansion of the adlayer as determined from thermal Hediffraction patterns has been used as an experimental criterium to distinguish between HOC and incommensurate (I) phases. In addition, from the superstructure of the He-diffraction pattern a (5 x 5)RO° HOC unit cell with a 12-atomic basis is deduced for Kr and a (4 x 4)RO°-structure with 9 atoms per unit cell for Ar. In both cases the HOC-phases are formed in a first order structural phase transition through the compression of the adlayer when increasing the coverage above a critical value (0.85 and 0.75 of a monolayer for Kr and Ar, respectively). While for Kr the structure at lower coverage is incommensurate, there is experimental evidence for further HOC-phases also at lower coverage in Ar adlayers. Small amounts of preadsorbed impurity atoms (H, CO, Xe) strongly influence the structure of the Kr and Ar adlayers. Concentrations as low as 0.2% of a monolayer lead to a rotation of the azimuthal orientation of the adlayer with respect to the Pt substrate by 30°. It could be demonstrated that the influence of impurities on the nucleation and final orientation of the adlayer does not depend on the chemical nature of the impurity but originates from the decoration of the surface steps on the Pt(111) substrate. The lattice dynamics of thin physisorbed rare gas films (Ar, Kr and Xe) on Pt(111) has been studied as a function of film thickness (1 to 25 monolayers). The dispersion of the mainly vertically polarized surface phonons accessible to He-scattering exhibit a discrete evolution from a nearly dispersionless adlayer mode for the monolayer towards a Rayleigh wave dispersion for the 25-layer films. This behaviour can be explained by the layer-by-layer growth mode of the rare gases on Pt(111). A strong vibrational coupling is observed between the rare gas adlayers and the Pt substrate, leading to a hybridization of the adlayer mode with the Pt Rayleigh wave and a linewidth broadening due to the coupling of the adlayer phonons to the Pt bulk phonons. In addition, the influence of the van Hove singularity of the density of states at the longitudinal bulk band edge of the substrate on the Ar monolayer phonons could be observed experimentally. Treating the Pt substrate as an elastic continuum and including anharmonic coupling between adlayer atoms an overall quantitative agreement with the experimental data is obtained. Binary mixtures of the rare gases on Pt(111) have been studied in the submonolayer coverage regime. Above a critical temperature all three systems investigated (Ar-Kr, Kr-Xe and Ar-Xe) form completely mixed, twodimensional stochastic alloys. While Kr-Xe and Ar-Kr mixtures retain a quasi-cristalline order with correlation lengths $\geq$ 40 $\mathring{A}$, the Ar-Xe alloy has a disordered, amorphous structure. This is explained by the ratio of the atomic radii (0.93 and 0.92 for Ar-Kr and Kr-Xe, respectively and 0.85 for Ar-Xe). The quasi-cristalline structures are characterized by a "mean lattice" parameter which linearly depends on the molar fraction of the constituents of the mixture (Vegard's law). Although positional order is reduced, all mixtures are orientationally well ordered and correlated with the high symmetry directions of the substrate. Finally the melting transition of thin CH,(111) films on graphite has been investigated using quasielastic neutron scattering. It is shown that the CH,(111)-films do surface melt below the bulk melting point. In the temperature regime between 76 and 89 K a quasi-liquid surface film (diffusion constant D - 10-5 cm2/s) of 1-2 layers is observed. The experimental spectra can be described by jump diffusion on a twodimensional hexagonal lattice. Approaching the bulk melting temperature (T$_{M}$ = 90.66 K) the number of mobile quasi-liquid methane layers rapidly increases and the diffusive motionbecomes continuous and isotropic.

Contributing Institute(s):