Untersuchung der Dynamik von sauberen Ni(100)-Oberflächen und darauf adsorbierten (2x2)-Strukturen von C, O und S

Phonon dispersion was studied by EELS in the impact scattering regime along high symmetry directions $\overline{r} \overline{M}$ of the two dimensional Brillouin zone of Ni(100). The measurements were performed for the clean surface as well as for (2x2) phases of carbon, oxygen and sulfur adsorbed on it. In all cases data show evidence for surface phonons (S$_{4}$ and S$_{6}$ on $\overline{r} \overline{X}$, S$_{1}$, on $\overline{r} \overline{M}$) and for various resonances inside the bulk phonon band. For adsorbate phases also modes corresponding to adatom degrees of freedom were observed. Multiple scattering effects were used to enhance the phonon cross section so that also modes differing in frequency less than experimental energy resolution could be resolved. The behaviour of phonon scattering cross section with respect to scattering parameters and to impact enrgy has been compared with full dynamical scattering calculations for the $\overline{X}$ point of the clean Ni(100) surface. Theory and experiment show excellent agreement. By fitting the ratio of S$_{4}(\overline{X}$) and S$_{6}(\overline{X}$) intensities, measured under the same scattering conditions, a 2.5 % contraction of the outer interlayer distance was deduced. Surface contraction is confirmed by the analysis of surface phonon dispersion by lattice dynamical calculations. To fit the data a 20 % stiffening of the force constant describing the coupling between the atoms of first and second crystal layer has to be assumed. The phonon spectrum of c(2x2)-S on Ni(100) could be described by an analogous lattice dynamical calculation assuming the adatoms to sit in the fourfold hollow between the Ni atoms at a distance of 1.45 $\mathring{A}$ from the nickel surface. The force constants between the substrate atoms resulted to be the same as for the clean surface in contrast to what occurs after oxygen adsorption. In this last case a strong softening of surface substrate modes is observed for the c(2x2) superstructure. Randomly adsorbed oxygen has on the other hand a much weaker influence on surface lattice dynamics. These experimental facts are tentatively explained by assuming that upon adsorption stresses build up between the substrate atoms. The magnitude of these stresses depends on the adsorption distance, on the binding energy of the adatoms with the substrate and on the possibility of the surface atoms to accomodate to them. In agreement with experimental result the influence of adsorption on surface dynamics should be bigger in case of oxygen than in case of sulfur. Upon adsorption of carbon Ni(100) undergoes a (2x2) reconstruction. This is also in accord with our model as in this case the adsorption sites are practicallycoplanar with the Ni surface and the adsorption energy is also bigger than in case of oxygen and sulfur. The phonon spectrum of Ni(100) (2x2) C reflects the presence of the reconstruction. E. g. two parallel carbon modes were recorded