|Aufstellung der Publikationen der Arbeitsgruppe 2011|
Aufgeführt sind Arbeiten, die im Jahr 2011 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. R. Mitdank, D. Habel, O. Görke, M. Harth, H. Schubert, and H. Winter
2. 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 reducibility 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.
3. J. Lienemann, D. Blauth, S. Wethekam, M. Busch, H. Winter, P. Wurz, and S. A. Fuselier
Fractions of negative ions after grazing scattering of fast hydrogen and oxygen atoms and ions from diamond-like carbon (DLC) surfaces are studied as function of projectile velocity. We reveal a pronounced kinematic resonance behavior for the negative ion fraction as function of projectile velocity which provides important information on the charge transfer mechanism. The conversion of neutral atoms to negative ions during scattering from DLC surfaces is used for the detection of energetic neutral atoms onboard of spacecrafts.
4. U. Specht, M. Busch, J. Seifert, H. Winter, K. Gärtner, R. Włodarczyk, M. Sierka, and J. Sauer
Fast He atoms with energies from 200 eV up to 16 keV are scattered under grazing polar angles of incidence from a flat and clean KCl(001) surface. For scattering along low-index directions (axial surface channeling) we observe pronounced peaks in the angular distributions of scattered projectiles which can be attributed to rainbow scattering. From classical and semiclassical trajectory calculations based on individual pair and density functional theory (DFT) potentials, we obtain corresponding rainbow angles for comparison with the experimental data. The calculations were performed taking into account the rumpling of K and Cl in the topmost surface layer. Fair agreement with the experimental data is found for scattering along <100> for DFT as well as individual pair potentials calculated from Hartree-Fock wave functions.
The structure of clean and adsorbate covered surfaces as well as of ultrathin films can be investigated by grazing scattering of fast atoms from the surface. We present two recent experimental techniques which allow us to study the structure of ordered arrangements of surface atoms in detail (1) rainbow scattering under axial surface channeling conditions, and (2) fast atom diffraction. Our examples demonstrate the attractive features of grazing fast atom scattering as a powerful analytical tool in studies on the structure of surfaces. We will concentrate our discussion on the structure of ultrathin silica films on a Mo(112) surface and of adsorbed oxygen atoms on a Fe(110) surface.
C60+ molecular ions with energies of 5–45 keV are scattered under grazing angles of incidence of 1–3° from a LiF(100) surface. From the analysis of polar angular distributions, fragment size distributions, and ion fractions for scattered projectiles, information on elastic, internal excitation, and charge transfer processes are derived. The results are compared to classical molecular dynamics simulations, which reproduce the angular distributions on a quantitative level, but the internal excitation only in part. In addition to the transfer of the normal energy loss to internal degrees of freedom of the molecule, an excitation is identified and interpreted as resonant coherent excitation in the oscillating electric field in front of the surface experienced by the moving projectile. The ion fractions are in accord with a complete suppression of charge transfer between fullerene ion and surface.
7. D. Blauth and H. Winter
Negative ion fractions, projectile energy loss, and the emission of electrons is studied for grazing scattering of hydrogen and helium atoms/ions from a clean and oxidized NiAl(110) surface. Making use of translation energy spectroscopy and the coincident detection of the number of emitted electrons we have studied the electronic interaction mechanisms for the change from a clean metal target to an insulator surface via the preparation of a well defined ultrathin alumina film on top of the metal substrate. We find that already for a monolayer thick oxide film the characteristic different features of electronic processes for the surface of an insulator crystal are present.
Angular distributions of fast Ne atoms after grazing collisions with a LiF(001) surface under axial surface channeling conditions are experimentally and theoretically studied. We use the surface eikonal approximation to describe the quantum interference of scattered projectiles, while the atomsurface interaction is represented by means of a pairwise additive potential, including polarization and rumpling. Experimental data serve as a benchmark to investigate the performance of the proposed potential model, analyzing the role played by the projectile polarization.
Atom-surface potentials entering the calculations of fast atom scattering at surfaces are often taken to be purely repulsive and parameterized by variants of ZBL potential. However, such surface potentials derived from pairwise superpositions of binary potentials fail to reproduce recent rainbow scattering measurements performed for various atomic projectiles above different faces of atomically flat single crystal Al surfaces. It was found that the dependence of the rainbow angle Θrb on the projectile's kinetic energy component normal to the surface E⊥ varies with the electronic structure of the projectile as well as with the crystallographic face of the aluminum surface. We determine static atom-surface potentials by ab-initio methods which show fair agreement with the experiment.
The diffraction patterns for scattering of fast He atoms from a LiF(001) surface under a grazing angle of incidence are recorded as function of the azimuthal angle for surface channeling conditions. We observed well defined diffraction patterns which show pronounced changes with the azimuthal rotation of the target surface. Within an azimuth of about 2° the angular distribution turns from a rich pattern to a single peak of zeroth diffraction order only. The data are well reproduced by a semiclassical approach making use of individual pair potentials for the atom surface interaction.
11. J. Lienemann, A. Schüller, D. Blauth, J. Seifert, S. Wethekam, M. Busch, K. Maass, and H. Winter
The coherence for diffraction effects during grazing scattering of fast hydrogen and helium atoms from a LiF(001) surface with energies up to some keV is investigated via the coincident detection of two-dimensional angular distributions for scattered projectiles with the projectile energiy loss. From the analysis of data we identify electronic excitations of the target surface as an important mechanism for decoherence and for the transition from quantum to classical scattering. The suppression of electronic excitations owing to the band gap of an insulator plays an essential role for preserving quantum coherence and thus for the application of fast atom diffraction as surface analytical tool.
The formation of doubly excited states of He atoms during collisions of He2+ ions with projectile energies between 74 eV and 124 eV with a Fe(110) and a Ni(110) surface is studied via Auger electron spectroscopy. We observe that the electron spectra from autoionization of doubly excited states of 2s2, 2s2p, 2p2 configurations show a pronounced dependence on the coverage with oxygen for both surfaces. For a controlled O2 adsorption on the Fe(110) and Ni(110) surface we can explain the resulting changes in the electron spectra by the modification of the measured work functions of the target surfaces. In terms of thermal desorption and dissolution into the bulk of surface contaminations at elevated temperatures, we present an alternative interpretation of similar previous studies by another group, where the local electron spin polarization of Fe(110) and Ni(110) surfaces was deduced from changes in the electron spectra as function of target temperature.
C60+ and C70+ fullerenes with keV energies are scattered under grazing polar angles of incidence from an atomically clean and at KCl(001) surface. For this model system of molecule surface interactions, the elastic properties of the fullerenes in front of the surface are studied by polar angular distributions. From the analysis of fragment spectra, the internal excitations of scattered molecules are deduced and excitation mechanisms are identified. Charge fractions indicate a kinematically induced neutralization of the fullerenes. Via an analysis of negatively charged fragments, the transition from a "soft" scattering event with intact outgoing fullerenes to post-collision multifragmentation is analyzed. The data is compared to 3D molecular dynamics simulations based on empirical bond-order potentials.
The orientation of the magnetization of a Ni(110) surface was investigated using techniques with different probing depths. By making use of electron capture into excited states of fast He atoms, we found that the magnetization of the topmost surface layer is not aligned along the easy axes of Ni. However, for a 50 ML film Fe on Ni(110) we observed the magnetization of the topmost Fe surface layer is along the easy axes of Fe.
C60+ fullerenes with keV energies are scattered at grazing angles of incidence from atomically clean and flat LiF(001), KCl(001), Al(001), Be(0001), Ni(110) surfaces as well as p(2×1) and p(3×1) oxygen superstructures on Ni(110). The elastic properties of C60 are derived from a comparison of experimental data with 3D molecular dynamics simulations for different interaction potentials. In terms of a simple model for the hybridization of C60 with the surface, we find evidence for a close relation between electronic structure of the surface and elasticity of C60.
16. U. Specht, M. Busch, J. Seifert, A. Schüller, H. Winter, K. Gärtner, R. Włodarczyk, M. Sierka, and J. Sauer
Fast He, Ne, Ar and N atoms with projectile energies from 1 up to 60 keV are scattered under grazing polar angles of incidence from a flat and clean KCl(001) surface. For the scattering along low- index directions (axial surface channeling) we observe pronounced peaks in the angular distributions of scattered projectiles which can be attributed to rainbow scattering. From classical trajectory calculations based on universal and individual pair as well as density functional theory (DFT) potentials, we obtained corresponding rainbow angles for comparison with the experimental data. Fair agreement was found for DFT and individual pair potentials calculated from Hartree-Fock wave functions.
The recently discovered effects of quantum scattering for impact of keV atoms and molecules on well ordered surfaces at glancing angles of incidence has demonstrated substantial potential as surface analytical tool. We will discuss basic features of the new method and outline recent developments in studies on the real-space structure of insulator, semi-conductor, and metal surfaces as well as ordered adsorbate covered metal surfaces and ultra-thin films.
18. 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.
19. H. Winter, A. Schüller, J. Seifert, J. Lienemann, S. Wethekam and M. Busch
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.
20. M. Busch, J. Lienemann, J. Seifert, A. Schüller, and H. Winter
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.
21. J. Seifert, E. Meyer, and H. Winter
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.
22. J. Seifert and H. Winter
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.
Study of the deuteron fusion reactions at very low energies in metallic environments enables us to determine the strength of the astrophysical electron screening effect in the terrestrial laboratories. Experiments performed under high and ultra high vacuum conditions showed that the experimentally determined screening energies corresponding to the reduction of the Coulomb barrier were significantly larger than the theoretical values calculated in terms of the dielectric function theory. As the origin of the so-called enhanced screening effect observed in nuclear reactions taking place in metals is unexplained we discuss here the interplay between a strong plasma screening and a narrow resonance placed close to the reaction threshold, which leads to the target material dependence of the reaction cross-section.