Untersuchung von Struktur und Dynamik der Platinum(100)-Oberfläche mit hochauflösender Heliumstreuung

A helium surface scattering device with enhanced beam intensity and detection efficiency has been built and tested. The machine allows the total scattering angle $\chi$ to be continuously varied from 60° to 120$^\circ$ under measuring conditions. A concept for a further monochromatization of the helium nozzle beam is presented. The structure and dynamics of the Pt(100) surface have been investigated with the new apparatus by means of diffraction measurements and energy-loss spectroscopy. Three well defined structural phases have been characterized. In the investigation, new features in the diffraction from Pt(100) have been found,revealing very large unit cells. The investigation of the growth kinetics of the reconstructed phase indicate an interesting behaviour of the kinetic exponent as afunction of temperature and time. The phonon dispersions of the different phases of Pt(l00) have been measured for the first time. The metastable unreconstructed Pt(100)-(1x1) surface was prepared by NO adsorption, following a receipe developed by Bonzel et al.. The receipe has been improved by using specular helium scattering for sensitively monitoring the adsorbate coverage. With the improved procedure we were able to prepare a (1x1) surface with a hydrogen coverageof $\theta_{H} \le$ 1%, a coverage substantially lower than the one obtained so far. The superior cleanliness of the surface is reflected in the fact that reconstruction begins already at temperatures below T= 390K. The surface atom density of the reconstructed surface is 20% larger than the one on the unreconstructed Pt(100)-(1x1) surface. Upon lifting the reconstruction the excess of Pt atoms have to accommodate onto the surface. From diffraction measurements we have inferred that the islands formed are separated by an average distance of $\simeq$ 110$\mathring{A}$. Annealing the sample at $\simeq$ 1000K creates the metastable Pt(100)-hex phase which forms a quasi-hexagonal adlayer on the square substrate. The structure has been determined to be c(26x150). Annealing the sample above 1100K creates the Pt(100)-hex-rot phase which is the stable phase below 1000K. It forms a quasi-hexagonal adlayer with atomic rows slightly rotated with regard to the substrate. The superstructure observed can be described by (13-1 / 18 43) in the matrix notation. Compared to the hexagonal Pt(111) surface the adlayer is uniformly compressed by 3.8% in good agreement with a recent X-ray study. With helium scattering we have found for the reconstructed phases a peak splitting in the fifth order spots that have not been observed in LEED. Therefore, the supercells we obtain for the reconstructed phases are much larger than the ones deduced from previous LEED studies. The area of the unit cell of the hexagonal phase is approximately six times the area of the unit cell of the hexagonal rotated phase. The lower symmetry of the hexagonal (unrotated) phase can be regarded as the reason for its transformation into the hexagonal rotated phase at high temperatures. The reconstructed surface emerging from heating the unreconstructed phase to a temperature between 400K and 500K does not correspond to a genuine c(26x150) phase. Measurements during the phase transition support a more complex picture: Depending on temperature and time, the transition seems to pass through at least three stages in which different processes prevail. The islands which are present on the surface can beexpected to play an important role in the transformation. In measurements of the phonon dispersion of the unreconstructed surface only the Rayleigh mode is observed. Its dispersion can be well described by slab calculations using only bulk force constants. After the adsorption of about one monolayer of hydrogen the energy of the Rayleigh mode is reduced by 5% at the $\overline{X}$ and $\overline{M}$ points. The phonon dispersion of the reconstructed phases also displays the Rayleigh mode. Additional branches that are observed, can be explained by backfolding the Rayleigh mode. The change in the surface dynamics from the unreconstructed to the reconstructed surfaces is reflected in a 20% decrease of the Rayleigh phonon energy in the $\overline{X}$ and $\overline{M}$ points. This result compares well with the behaviour of Ir(100) in a recent EELS study