|Aufstellung der Publikationen der Arbeitsgruppe 2010|
Aufgeführt sind Arbeiten, die im Jahr 2010 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. S. Wethekam and H. Winter
The interaction of He+ ions in front of metal surfaces is considered as a model system for the Auger neutralization process where two electrons are involved in charge transfer. For grazing scattering of keV He+ ions from clean and flat metal surfaces, small but defined fractions of ions are observed in the scattered beam which have survived the complete scattering event. This feature has opened the way to study the Auger neutralization (AN) process, i.e. the filling of the 1s hole of incident He+ ions, in detail by a new type of scattering method which has an intrinsic time resolution in the fs regime. Work based on this method provided new insights into the mechanisms of Auger neutralization in front of metal surfaces where - aside from the transition rates for the Auger process - the level shift of the He atom in front of the surface plays a key role for a detailed microscopic understanding.
2. R. Mitdank, D. Habel, O. Görke, M. Harth, H. Schubert, and H. Winter
Growth and magnetic properties of NixMn100-x single and Co/NixMn100-x bilayer films on Cu(001) have been investigated by grazing ion scattering, Auger electron spectroscopy, low energy electron diffraction, and magnetooptical Kerr effect. The increase of the coercivity field strength and the decrease of the signal strength of the hysteresis loop is used as a measure of the magnetic interface coupling in the bilayers. The strength of the antiferromagnetic/ferromagnetic interface coupling rapidly increases with increasing NiMn thickness. Above a critical NiMn thickness of about 8 ML, we observe the strongest interface coupling effects in the intermediate concentration regime with a Ni content 10≤x≤40 at a temperature of T =300 K and 5≤x≤50 for T =133 K.
C60+ and C602+ ions are scattered under grazing incidence from an atomically clean and flat Be(0001) surface at keV energies. Distances for electron transfer are deduced from shifts of angular distributions for incident C60+ and C602+ projectiles, which reflect changes in the interaction potentials at the instants of electron transfers. These distances are consistent with classical over-the-barrier model results indicating that the suppression of charge transfer observed for atomic projectiles in front of metal surfaces with a projected band gap - is absent for fullerenes.
5. J. Seifert, M. Busch, A. Schüller, D. Blauth, S. Wethekam, and H. Winter
The structure of ultrathin silica films on Mo(112) has been investigated by grazing scattering of fast He atoms from the film surface. We discuss two recent experimental techniques which allow us to study the arrangements of surface atoms of these films in detail. The methods comprise: (1) classical rainbow scattering under axial surface channeling conditions and (2) fast atom diffraction. Our work demonstrates the attractive features of grazing fast atom scattering as a powerful analytical tool in studies on the structure of surfaces. From the analysis of our data we find a clear-cut support for a structural model based on the arrangement of film atoms in terms of a 2D-network.
6. 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.
7. J. Seifert, A. Schüller, H. Winter, R. Włodarczyk, J. Sauer, and M. Sierka
He atoms and H2 molecules with energies of 2 keV are scattered under grazing angles of incidence ranging from 0.2 deg to 1.8 deg from the surface of a monolayer silica film grown on a Mo(112). We observe for scattering along low indexed atomic strings in the topmost surface layer pronounced diffraction pattern owing to diffraction effects for the elastically scattered projectiles. The diffraction patterns are analyzed in terms of semi-classical trajectory calculations making use of interaction potentials derived from density functional theory (DFT). We find good agreement with the experiments, for a 2D [Si-O-Si] network structural model for the ultrathin silica film.
Auger electron spectroscopy using excitation by grazing impact of protons and by impact of electrons was applied to determine the elemental composition of the topmost and near-surface layers of DyScO3(110). The vicinal DyScO3 crystals were annealed at 1320 K in O2 and Ar atmospheres by varying annealing time. The prepared surfaces show regularly arranged, smooth terraces with single-atomic steps. The surfaces have DyO termination after annealing in oxygen flow for 30-60 minutes. Annealing for 600 minutes of DyScO3 in oxidising or inert atmospheres resulted in a ScOx termination.
9. P. Tiwald, A. Schüller, H. Winter, K. Tökesi, F. Aigner, S. Gräfe, C. Lemell, and J. Burgdörfer
Rainbow scattering for grazing-angle incidence of atoms at surfaces along low-indexed channeling directions provides a sensitive probe of quasi-static atom-surface potentials. The dependence of the rainbow angle on the kinetic energy for the projectile motion perpendicular to the surface E⊥, varies with the electronic structure of the projectile as well as the crystallographic face of the aluminium surface. Comparison between experiment and classical Monte Carlo trajectory simulations demonstrates that the superposition of binary atom-atom potentials fails to adequately represent the equipotential surfaces. Ab-initio atom-surface potentials based on density-functional theory (DFT) are required to reach satifactory agreement with experiment.
10. 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.
11. 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.
12. H. Winter, A. Schüller, M. Busch, J. Seifert, J. Lienemann, and S. Wethekam
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.
14. 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.
15. M. S. Gravielle, A. Schüller, H. Winter, and J. E. Miraglia
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.
16. P. Tiwald, C. Lemell, A. Schüller, H. Winter, and J. Burgdörfer
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 results of 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-surfacepotentials by ab-initio methods which show fair agreement with the experiment.
17. J. Seifert, A. Schüller, H. Winter and K. Gärtner
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.
18. 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.
19. M. Busch, S. Wethekam, and H. Winter
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.
CC60+ 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.
21. A. Schüller, S. Wethekam, D. Blauth, and H. Winter, F. Aigner, N. Simonović, B. Solleder, and J. Burgdörfer, L. Wirtz
Quantum diffraction of fast atoms scattered from the topmost layer of surfaces under grazing angles of incidence can be employed for the analysis of detailed structural properties of insulator surfaces. From comparison of measured and calculated diffraction patterns we deduce the rumpling of the topmost surface layer of LiF(001) (i.e., an inward shift of Li+ ions with respect to F- ions). The effect of thermal vibrations on the measurement of rumpling is accounted for by ab initio calculations of the mean-square vibrational amplitudes of surface ions. At room temperature this leads to a reduction of the apparent rumpling by 0.008 Å. We then obtain a rumpling of (0.05±0.04) Å, which improves its accuracy achieved in previous work.
22. M. Busch, J. Lienemann, and H. Winter, M. Potthoff
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.