|Aufstellung der Publikationen der Arbeitsgruppe 2009|
Aufgeführt sind Arbeiten, die im Jahr 2009 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).
Electron emission induced by H+ and He+ ions with energies up to 30 keV is studied for grazing impact on a clean and flat Cu(001) surface by making use of electron diffraction effects. Depending on the azimuthal orientation of low index directions in the crystal surface with respect to the projectile beam we observe intensity spots in the angular distributions of emitted electrons. The data are analyzed in the framework of electron diffraction at the ordered surface and reveal interesting details on the emission and coherence mechanisms for the excitation of conduction electrons by fast atomic projectiles in the surface region of a metal.
The formation of doubly exited states of He atoms during collisions of He2+ ions with energies between 60 eV and 1 keV with a Ni(110) surface is studied via Auger electron spectroscopy. We observe that the electron spectra from autoionisation of doubly excited states of 2s2, 2s2p, 2p2 configurations show a pronounced dependence on the coverage of the target surface with oxygen. For a controlled O2 adsorption on the Ni(110) surface we can explain the resulting changes in the electron spectra by the modification of the work function of the target surface. Thermal desorption and dissolution into the bulk of surface contaminations at elevated temperatures provides an alternative interpretation of recent work where the local electron spinpolarization of a Ni(110) surface was deduced from changes in the electron spectra as function of target temperature.
The formation of doubly excited states of He atoms during impact of He2+ ions with projectile energies of 60−1000 eV under near-grazing angles of incidence of 5° −20° on clean and adsorbate-covered Ni(110) surfaces is studied by means of Auger electron spectroscopy. Pronounced dependencies of electron spectra from autoionization of atoms in doubly excited 2s2, 2s2p and 2p2 configurations on the coverage of the target surface with adsorbates are observed. These are directly related to work function changes, which are studied for the controlled adsorption of oxygen. Changes of the electron spectra on the target temperature are found for adsorbate-covered surfaces only, which puts recent interpretations of similar electron spectra in terms of a high local electron spin polarization of Ni(110) into question by an alternative interpretation based on thermal desorption or dissolution into bulk of surface contaminations. The formation of doubly excited states is studied for the oxygen p(2×l) and p(3×l) superstructures on Ni(110) in order to provide well-defined experimental data for theoretical investigations.
Fast atoms with energies from 500 eV up to several keV are scattered under a grazing angle of incidence from a Fe(110) surface covered with defined superstructures of sulphur or oxygen atoms. For scattering along low index azimuthal directions of the superstructures within the surface plane under "axial surface channeling" we observe defined diffraction patterns in the angular distributions for scattered projectiles. From the analyses of those patterns we derive the widths of low indexed axial channels and the corrugation of the interaction potential across these channels. This allows us to estimate the positions of adsorbed atoms on the Fe(110) surface.
The survival of ions during grazing scattering of keV He+ ions from a clean Ni(110) surface is studied as function of target temperature. We observe ion fractions in the scattered beams of typically 10-3 which show a slight increase with temperature of the target surface. From computer simulations of projectile trajectories we attribute this enhancement for ion fractions to effects of thermal vibrations of lattice atoms on the survival of ions in their initial charge state. Based on concepts of Auger neutralization, we discuss the role of the spin polarization of target electrons on charge transfer. We do not find corresponding signatures in our data and conclude that in the present case of Ni(110) the spin polarization has to be small.
For scattering of fast atoms under axial channeling conditions from surfaces diffraction effects have been observed in the distributions for scattered projectiles. Basic features concerning this recently observed new phenomenon in ion/atom surface scattering will be demonstrated for collisions of light atoms, ions, and molecules with well ordered clean and adsorbate covered surfaces of insulators and metals. The potential of this method for the investigation of structures of surfaces is discussed.
Recent developments in studies on the structure of surfaces based on ion beam triangulation are discussed. We will outline recent experimental progress in the application of this method which is closely related to the detection of the number of emitted electrons per incident ion during scattering under surface channeling conditions. Key features are the pronounced change of electron emission for the projectile beam aligned along a low index crystallographic direction in the surface plane ("axial surface channeling") and the interpretation of data in terms of classical trajectory computer simulations. As a representative example we will discuss here studies on the structure of the low temperature c(8x2) Mn/Cu(001) phase.
Fast atoms with energies from 350 eV up to 1.5 keV are scattered under a grazing angle of incidence from a clean and flat Ni(110) surface. For scattering under "axial surface channeling" conditions, we observe – as reported recently for insulator and semiconductor surfaces – defined diffraction patterns in the angular intensity distributions for scattered fast 3He and 4He atoms. We have investigated the domain of scattering conditions where decoherence phenomena are sufficiently small in order to observe for metal targets quantum scattering of fast atomic projectiles. As a consequence, fast atom diffraction appears to be a general technique with a wide range of applicability in surface science.
We present a theoretical analysis of experimental data on ion fractions and polar angular distributions for He atoms and ions produced during grazing scattering of keV neutral He atoms from an atomically clean and flat Al(111) surface. The discussion focuses on the mechanism of Auger ionization for which we have recently presented the first quantitative treatment by an ab initio method [S. Wethekam, Diego Valdés, R. C. Monreal, and H. Winter, Phys. Rev. B 78, 033105 (2008)]. Auger ionization, the inverse process of Auger neutralization, is a dynamical process that converts kinetic energy from the projectile to electronic excitations. We calculate Auger ionization rates and perform molecular-dynamics and Monte Carlo simulations of trajectory and charge state of scattered projectiles. We achieve quantitative agreement with experimental ion fractions and angular distributions. This demonstrates that Auger ionization is an efficient mechanism of ionization. We also discuss the sensitivity of the results on the theoretical input used in the simulations and give an estimate on the contribution of resonant processes.
Fast helium atoms and hydrogen molecules with energies from 400 eV up to several keV are grazingly scattered from a Fe(110) surface covered by oxygen and sulphur atoms forming c(2×2) and c(1×3) superstructures, respectively. For scattering along low index azimuthal directions we observe defined diffraction patterns in the angular distributions for scattered projectiles. From the evaluation of those diffraction patterns we derive the widths of low indexed axial channels, the corrugation of the interaction potential across these channels, and the normal positions of adsorbed atoms above the Fe lattice. Our analysis is based on semiclassical models using hard wall approximation as well as individual potentials for the interaction of projectiles with the surface. By comparing the results of different models, we discuss the robustness of the information on the geometrical structure of the surfaces.
12. H. Winter, J. Seifert, D. Blauth, M. Busch, A. Schüller, and S. Wethekam
The structure of ultrathin oxide layers grown on metal substrates is investigated by grazing scattering of fast atoms from the film surface. We present three recent experimental techniques which allow us to study the structure of ordered oxide films on metal substrates in detail. (1) A new variant of a triangulation method with fast atoms based on the detection of emitted electrons, (2) rainbow scattering under axial surface channeling conditions, and (3) fast atoms diffraction for studies on the structure of oxide films. Our examples demonstrate the attractive features of grazing fast atom scattering as a powerful analytical tool in surface physics.
Auger electron spectroscopy using excitation via grazing impact of protons was applied to determine the elemental composition of the topmost and near-surface layers of a NdGaO3(110) substrate. The preparation conditions of vicinal NdGaO3 substrates were optimized by varying the annealing temperature, time, and gas atmosphere. Well prepared surfaces show regularly arranged, atomically smooth terraces with single-atomic steps. The surfaces were always NdO terminated with a small amount of Ga (2 - 4 %) atoms on the surface. A Ga and O depletion layer with a thickness of about 4 nm has been detected at optimized preparation conditions.
The structure of a monolayer silica film on a Mo(112) surface is investigated by grazing scattering of 25 keV H0 atoms. By detection of the number of projectile induced emitted electrons as function of azimuthal angle of rotation of the target surface, the geometrical structure of atoms forming the topmost layer of the silica film is determined via ion beam triangulation. From our data we find evidence for the arrangement of surface atoms in terms of a two-dimensional Si-O-Si network model.
15. 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.
16. R. Mitdank, D. Habel, O. Görke, M. Harth, H. Schubert, and H. Winter
The structure of a monolayer crystalline silicia film grown on a Mo(112) substrate is investigated via grazing scattering of fast atoms. For scattering along low indexed directions in the surface plane ("axial surface channeling") the corrugation of the interaction potential leads to an azimuthal out off plane scattering with an intensity enhancement for the maximum deflection angle, the so called "rainbow". From the comparison of the experimental angular distributions for scattered projectiles with classical trajectory simulations we obtain information on the arrangement of atoms in the topmost surface layer. Our work provides evidence for the structural model of a two-dimensional network for the silica film.
Recent progress of studies on the interaction of keV atomic and fullerene ions during grazing scattering from solid surfaces is reported. We focus on electron capture processes for two systems that have been investigated in detail during the last years, the neutralization of He and C60 ions at metal surfaces. Both species can be considered as model systems for Auger neutralization and for resonant neutralization of a large molecule, respectively. Basic experimental concepts and results obtained for scattering from an Al(100) surface are discussed.
Diffraction patterns produced by grazing scattering of fast atoms from insulator surfaces are used to examine the atom-surface interaction. The method is applied to He atoms colliding with a LiF(001) surface along axial cristallographic channels. The projectile-surface potential is obtained from an accurate DFT calculation, which includes polarization and surface relaxation. For the description of the collision process we employ the surface eikonal approximation, which takes into account quantum interference between different projectile paths. The dependence of projectile spectra on the parallel and perpendictular incident energies is experimentally and theoretically analyzed, determining the range of applicability of the proposed model.
20. M. Reinhardt, J. Seifert, M. Busch, 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.
21. S. Wethekam, H. Zettergren, Ch. Linsmeier, H. Cederquist, and H. Winter
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 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.
22. Ahmad Kamal Ariffin, Christoph Janowitz, Beate Müller, Lenart Dudy, Philipp Sippel, Rüdiger Mitdank, Helmut Dwelk, Alica Krapf, and Recardo Manzke
The exact shape of the superconducting dome in the Bi-cuprates depends on the proper determination of the hole content in the CuO2 layers. Single crystals of Bi(Pb)-2201 were studied over a wide doping range from the overdoped to the heavily underdoped regime by x-ray absorption spectroscopy (XAS) in order to determine the hole content of the compounds. By analysing the Cu-LIII edge this technique proved to be successful for polycrystalline oxide-based high-Tc superconductors, while for single crystals an unexpected variation of the absorption intensity within the ab-plane on a scale of 10-15% with respect to the angle of the incoming linearly polarized light was observed. Due to it, the procedure of hole content determination on polycrystals cannot be adopted straight forward to single crystals. We will show that this technique also works for single crystals, but is in need of higher experimental effort due to the polarization effects.