|Aufstellung der Publikationen der Arbeitsgruppe 2012|
Aufgeführt sind Arbeiten, die im Jahr 2012 publiziert
und zur Veröffentlichung eingereicht oder angenommen worden sind.
Sonderdrucke oder Preprints können von den Autoren oder im Sekretariat
des Lehrstuhls angefordert werden (Postanschrift: Humboldt-Universität
zu Berlin, Institut für Physik, Physik der Grenzflächen und
dünnen Schichten, Newtonstraße 15, 12489 Berlin, Tel.: (+49
30) 20 93 - 78 91, Fax: - 78 99).
1. M. Harth, R. Mitdank, D. Habel, O.Görke, M. Tovar, H. Winter, and H. Schubert
Electrochemical methods have been applied in the catalytic system V2O5 in order to investigate the redox properties and their correlation with catalytic properties. Temperature programmed conductivity measurements using Electrochemical Impedance Spectroscopy (EIS) enabled us to determine the onset of a thermally induced reduction at about 380 °C. Rutherford backscattering (RBS) analysis provides evidence for a reduction from V+5 to V+4. Experiments under different oxygen partial pressure showed that the vanadyl oxygen is involved in the reduction process and it was possible to determine the energy of formation for an oxygen vacancy with a value of 1.23±0.03 eV. The removability of the vanadyl oxygen is assumed to be a key factor for the catalytic activity so that it can be characterized by macroscopic transport properties.
2. A. Schüller, D. Blauth, J. Seifert, M. Busch, and H. Winter, K. Gärtner, R. Włodarczyk, J. Sauer, and M. Sierka
Fast neutral atoms and molecules with energies from 0.4 up to 3 keV are scattered under a grazing angle of incidence from a clean and flat MgO(001) surface. For "axial surface channeling" conditions, we observe defined diffraction patterns in the angular intensity distributions for scattered 3He and 4He atoms as well as H2 molecules. The diffraction patterns are analyzed in terms of semiclassical trajectory calculations making use of projectile surface interaction potentials derived from density functional theory and from pair potentials calculated from Hartree-Fock wave functions. From comparison of measured and calculated diffraction patterns we deduced the rumpling of the topmost surface layer of MgO(001), i.e. an inward shift of Mg2+ ions with respect to O2- ions, of (0.03±0.03) Å.
Recently pronounced diffraction effects for grazing scattering of fast light atoms and molecules with energies up to some keV under axial surface channeling were observed. The rich diffraction patterns provide information on the interatomic spacings between axial surface channels and on the corrugation of the interaction potential. The latter effect can be used to study the structure of surfaces with fast atoms via an interferometric technique. The new method shows similarities to thermal He atom scattering (HAS), but has a number of advantages as simple tuning of the projectile energy (de Broglie wavelength) and, in particular, an orders of magnitude more efficient detection of scattered projectiles. As an example for the application of Fast Atom Diffraction (FAD) for studies on the structure of surfaces, we present results for the rumpling of the LiF(001) surface. The quantum coherence in the scattering process is preserved by specific features of surface channeling which is investigated in detail via the coincident detection of the diffraction patterns with the energy loss of scattered atoms. It turns out that the suppression of electronic excitations owing to the band gap of insulator surfaces play a key role for coherent scattering and the application of FAD in surface science.
The influence of decoherence on the diffraction during grazing scattering of fast hydrogen and helium atoms from a LiF(001) single crystal surface with projectile energies of some keV, is investigated by two-dimensional angular distributions for scattered projectiles in coincidence with their energy loss and emitted electrons from the target surface. For keV hydrogen atoms, we identify the excitations of electrons and excitons of the target surface as the dominant mechanisms for decoherence, whereas for keV helium atoms these contributions are negligibly small. The suppression of electronic excitations owing to the band gap of insulators plays an essential role for preserving quantum coherence and thus for the application of fast atom diffraction as a surface analytical tool.
The structure of the topmost layer of thin V2O3 films on a Au(111) substrate is studied via Ion Beam Triangulation. From electron emission induced by fast H atoms scattered from the film surface under a grazing angle of incidence as function of azimuthal rotation of the target, we find evidence for a reconstructed O3termination as proposed from DFT-calculations and recent experimental work using methods based on large angle impact of fast ions. From our studies we derive detailed lateral positions for the topmost O atoms.
In studies on two structures of oxygen adsorbates on Mo(112), we demonstrate the potential of Fast Atom Diffraction (FAD) to derive the surface unit cell size and its symmetry. Helium atoms with energies of 1-2 keV are scattered from an adsorbate covered Mo(112) surface along low indexed surface directions under grazing angles of incidence. From the observed diffraction patterns, the lateral periodicity of the surface structures is derived. In addition to the periodic lattice, information on the structure within the unit cell can be obtained from double slit type of interference.
Angular distributions for grazing scattering of keV H atoms from an Al2O3(1120) surface were recorded. These distributions reveal defined diffraction patterns which can be understood in terms of quantum scattering from well-ordered surfaces. From the observation of so called "Laue circles" we conclude a high degree of longitudinal coherence for Fast Atom Diffraction (FAD) at surfaces which allows one to resolve periodicity intervals of several 100 Å. We demonstrate this feature in scattering experiments from the reconstructed (12x4) phase of an Al2O3(1120) surface obtained after annealing at temperatures of about 2000 K.
8. M. S. Gravielle, J. E. Miraglia, A. Schüller, and H. Winter
Pairwise interaction potentials for multi-electron atoms moving in front of a LiF(001) surface are investigated theoretically and experimentally. From angular distributions of fast He, N, S, Cl and Kr atoms grazingly scattered under axial surface channeling conditions, rainbow angles are experimentally determined for a wide range of energies for the motion normal to the surface plane. These angles are used as a benchmark to probe the pairwise potential model. In the simulations the scattering process is described by means of the surface eikonal approximation, while the atom-surface interaction is derived by adding binary interatomic potentials that include the proper asymptotic limit.
H atoms with energies up to 1 keV were scattered under a grazing angle of incidence from an Al2O3(1120) surface. After preparation of the target by annealing at a temperature of 1700°C well defined angular distributions for scattered projectiles are observed showing for scattering in the regime of axial channeling rich diffraction patterns. For scattering along the low indexed  direction diffraction spots are arranged on a circle which is attributed to full coherence preserved during the collision with the surface. By an azimuthal rotation of the target surface by some degrees Laue circles of higher orders can be observed which is accompanied with a substantial enhancement of resolution for periodicity intervals of the target surface. This is demonstrated by results for the (12x4) phase of the Al2O3(1120) surface which indicate an overall transfer width for fast atom diffraction of several 100 Å.
10. J. Seifert and H. Winter
The adsorption of oxygen on a Mo(112) surface is studied via scattering of fast He atoms under a grazing angle of incidence. We show that, in addition to the intensity for reflected atoms, diffraction effects for fast atoms can be used to monitor adsorption and to obtain also information on the unit cell of adsorbed atoms. The new method is demonstrated by studies on oxygen exposure where as a function of O2 dose and target temperature a variety of superstructures can be found. Our conclusions are supported by work performed with LEED.
11. J. Lienemann, D. Hülsewede, J. Seifert, K. Maass, M. Busch, and H. Winter
Angular distributions for scattering of 12 keV He atoms from a LiF(001) surface under a grazing angle of incidence were recorded in coincidence with the projectile energy loss and the number of electrons emitted from the target surface during the collision. For scattering along the low indexed <110> and <100> directions of the crystal surface collisional rainbow peaks were observed. For scattering along a <110> direction the resulting rainbow peaks can be attributed to scattering from strings of anions which form active sites for charge exchange and emission of electrons. The data can be interpreted by trajectory computer simulations where charge transfer takes place from F- sites.
12. J. Seifert and H. Winter
In studies on titanium oxide thin films we demonstrate the potential of Fast Atom Diffraction (FAD) and triangulation methods to derive the surface unit cell with enhanced surface sensitivity. Helium atoms with energies of 1-2 keV are scattered from the surface along low indexed surface directions under grazing angles of incidence. From the observed diffraction patterns, the lateral periodicity of the surface structures is derived. For low TiOx coverages a well-ordered c(2 x 4) superstructure and for higher coverage a p(8 x 2) film is observed. Based on FAD and triangulation methods for azimuthal rotation of the target the arrangement of topmost atoms in smaller sub-unit cells is revealed.