|Aufstellung der Publikationen der Arbeitsgruppe 2013|
Aufgeführt sind Arbeiten, die im Jahr 2013 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. 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.
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.
4. 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.
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.
We demonstrate that quantum scattering of fast atoms and molecules under grazing angles of incidence can be exploited to study the structure of organic molecules on metal surfaces. Making use of keV H and He atoms as well as H2 molecules, the surface structures of the chiral amino acid alanine adsorbed on a Cu(110) surface is studied. We present a detailed investigation on the (3×2) phase of a monolayer of enantiopure and racemic alanine on Cu(110), revealing the formation of an elongated surface unit cell of c(n × 2) symmetry with n = 3.16 ± 0.04 for the sticking out methyl groups of the alanine molecules.
7. H. Winter
Recently, quantum effects were observed for the scattering of fast atoms from surfaces under a grazing angle of incidence. We discuss basic features of Fast Atom Diffraction (FAD) which adds a further powerful method to the established tools in ion beam analysis. Attractive features of FAD in studies on the structure of surfaces comprise negligible radiation damage, no charging effects, and an extreme sensitivity to the topmost layer of surface atoms. Scanning the quantum wavelength associated with the motion of a massive particle by the variation of its kinetic energy allows one to apply interferometic concepts to surface analysis based on the scattering of fast atoms.
8. J. Seifert, M. Busch, E. Meyer, and H. Winter
We have studied structures of the chiral amino acid alanine adsorbed on Cu(110) via low energy electron diffraction (LEED) as well as scattering of fast light atoms and molecules. The adsorption process was controlled in-situ by the intensity of specularly reflected 2keV He atoms. For projectile energies less than 1 keV we applied the method of fast atom diffraction (FAD) for studies on the structure of adsorbed alanine molecules on atomically flat Cu(110) surface with focus on a p(3 x 2) adsorbate phase. The results are consistent with LEED and explain distortions in LEED patterns via an elongated surface unit cell with incommensurate c(3.16 x 2) symmetry of parts of the adsorbate. From triangulation using fast atoms via the azimuthal rotation of the target surface, the positions of sticking out methyl groups are derived.
9. N. Targosz-Sleczka, K. Cerski, A. Huke, G. Ruprecht, D. Weissbach, L. Martin, A. I. Kilic, M. Kaczmarski, and H. Winter
The enhanced electron screening effect in nuclear reactions taking place in dense astrophysical plasmas is extremely important for determination of stellar reaction rates in terrestrial laboratories as well as in prediction of cross sections enhancement in interiors of stars such as White and Brown Dwarfs or Giant Planets. This effect resulting in reduction of the nuclear Coulomb potential by the atomic electrons has been confirmed in many laboratory experiments. Unfortunately, experimental screening energies are much higher than the theoretical predictions and the reason for that remains unknown. Here, we present absorbing results of the experiment studying d + d nuclear reactions in different deuterized metallic targets under ultra high vacuum conditions. The total cross sections and angular distributions of the 2H(d,p)3H and 2H(d,n)3He reactions have been measured using a deuteron beam of energies between 8 and 30 keV provided by the electron cyclotron ion source. The atomic cleanness of the target surface has been secured by combining Ar sputtering of the target and Auger electrons spectroscopy. Due to application of an on-line analysis method, the homogeneity of the implanted deuteron densities could be continuously monitored. We will discuss probable causes of the large discrepancy between theoretical and experimental data.
10. 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.