|Aufstellung der Publikationen der Arbeitsgruppe 2001|
Aufgeführt sind Arbeiten, die im Jahr 2001 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).
The magnetic ordering of the topmost surface layer of ultrathin Cr films grown on Fe(100) is studied via spin-polarized electron emission, excited by fast protons grazingly scattered from the film surface. We find that most electrons originate from the topmost layer. Based on simple assumptions we are able to deduce the layer-dependent magnetic moments from the observed spin polarization of electrons. We demonstrate that our method has a clearly smaller probing depth than conventional spin-polarized electron spectroscopies.
Growth mode and chemical structure of Ir deposited on Fe(100) are studied by specular reflection of fast ions and ion-induced Auger electron spectroscopy. Temperatures below 500 K lead to a rough growth front, which is in agreement with a statistical growth of Ir atoms. For higher temperatures, we observe a layer-by-layer growth mode, where the Ir film is completely terminated by Fe atoms.
Chlorine ions with charge up to 11 are scattered with energies ranging from 50 to 600 keV from a clean and flat Al(111) surface under a grazing angle of incidence. We have investigated energy loss and charge fractions of scattered projectiles. We find a weak dependence of the energy loss on the projectile charge, whereas the higher charge fractions of the scattered beam show pronounced effects on the charge of incident ions.
D.M. Danailov, R. Pfandzelter, T. Igel, H. Winter, and K. Gärtner
We report on experimental observation of multimodal angular distributions of 15 keV He0-atoms scattered under glancing angles from an Fe(001) surface along a  surface direction. Based on classical molecular dynamics computer simulations of atom trajectories we deduce the He-Fe interaction potential in the eV-energy range. Our experiments visualize the transition from axial to planar surface channeling. To describe well with our dynamical row-model the multimodal azimuthal spectra, we construct mixed potential from the long-range part of the "universal" potential and for smaller distances from "individual" potential. In addition, the shift of the experimental angle spectra with respect to the calculated shows elastic horizontal bend of the real atomistic rows due to the scattering and about ten times smaller vertical sink.
Neutral Ne atoms with keV energies are scattered under channeling conditions, i.e. at a glancing angle of incidence, from a LiF(001) surface. By means of a time-of-flight method with a pulsed neutral beam we record energy distributions for scattered projectiles. For this specific system the small energy transferred to the crystal lattice (“nuclear energy loss”) during channeling via binary collisions with large impact parameters dominates the dissipation of projectile energy. All other excitations of the solid can be brought to a negligible level.
Techniques based on grazing scattering of keV light ions to study ultrathin epitaxial films are presented. Key feature of these techniques is an extreme sensitivity to morphology, elemental composition, and magnetic properties of the topmost atomic layer of the films, as demonstrated by recent experiments on Cr, Mn, Fe, and Ir films grown on magnetized Fe(100) substrates.
A. Mertens, K. Maass, S. Lederer, H. Winter, H. Eder, J. Stöckl, HP. Winter,
F. Aumayr, J. Viefhaus and U. Becker
Neutral hydrogen atoms with energies ranging from about 1 keV to 20 keV are scattered from a clean and flat LiF(001) surface under grazing angles of incidence. By detection of time-of-flight (TOF) spectra in coincidence with the signals of an electron number detector, we study electron emission and excitation phenomena. We present recent progress in an on-line processing of spectra by means of a double coincidence unit.
We report on first spin-resolved energy spectra for the emission of electrons during grazing scattering of 150 keV multicharged nitrogen ions from a magnetized Fe(001) surface. A substantial spin-polarization for KLL Auger electrons emitted in the final stage of the neutralization sequence during the interaction of multicharged ions with a metal surface is observed. We conclude from our data that the projectile L-shell is dominantly populated by electrons from the conduction band of the target. For low energy electrons we find an increase of their spin polarization with increase of the projectile charge.
We report on the scattering of 3 keV He, Ne, Ar, Kr Xe atoms and singly-charged ions from a Cu(111) surface under grazing angles of incidence. Polar angular distributions of scattered neutral projectiles show a well-defined peaked structure in the direction of specular reflection with a small FWHM of typically 0.5° at an incidence angle of 1°. The measured distributions are compared with elastic scattering computer simulations and previous studies by Harder et al. (Surf. Sci. 289 (1993) 214).
We report on spin- and energy-resolved secondary electron emission, induced by impact of fast protons and electrons on ultrathin Cr films epitaxially grown on Fe(100). Based on a simple model, we extract spin- and energy-dependent probing depths from experimental polarization spectra. For grazing impact of protons the probing depth strongly depends on the energy of secondary electrons.
11. A. Mertens, S. Lederer, K. Maass, and H. Winter, J.
Stöckl, HP. Winter, and F. Aumayr
The coincident measurement of projectile time-of-flight spectra and electron emission multiplicities for grazing scattering of fast H° atoms from a LiF(001) surface allows us to study electronic excitation and emission processes, including those without emission of electrons. Using this method, we are able to study very small electron emission yields and thus kinetic threshold behaviour for projectile induced electronic processes. We observe different thresholds for electron emission and electronic excitation of the target and derive details on the interaction mechanisms.
D.M. Danailov, R. Pfandzelter, T. Igel, H. Winter, and K. Gärtner
In glancing-angle scattering of keV-ions from a crystal surface, the ion reflection takes place in the eV-part of the interaction potentials. The elastic interactions are determined by the energy transverse to atomic rows, which can be of the order of 10 eV. A row-model using averaged potentials according to the Lindhard cylindrical potential has been developed using step-by-step integration of Newton's equations of motion. Preiously [D. Danailov, K. Gärtner, A. Caro, Nucl. Instr. and Meth. B 153 (1999) 191; presented on COSIRES, Okayama, 1998] we reported that zig-zag trajectories within surface channels and the corresponding multimode azimuthal angular distributions of reflected ions are very sensitive to the interaction potential used in the simulation. Here we simulate the scattering of 15 keV He-atoms from Fe(100) surfaces at different angles of incidence comparable with previously published experimental results [D. Danailov, T. Igel, R. Pfandzelter, H. Winter, Nucl. Instr. and Meth. B 164-165 (2000) 583]. Our results show that for interaction energies below about 4 eV the well-known "universal" potential works well. However, for energies between 4 and 13 eV the "individual" He-Fe potential (D. Danailov, K. Gärtner, A. Caro, Nucl. Instr. and Meth. B 153 (1999) 191; presented on COSIRES, Okayama, 1998) gives a better agreement with the experimental data. For interaction energies above 13 eV both potentials are similar. We have constructed a mixed He-Fe potential, which describes the experimental observations well. The row-model enables us to deduce the He-Fe interaction potential in the eV-range. In addition, a shift in the experimental angular spectra compared with the calculated spectra indicates that the atomistic rows undergo an elastic horizontal bend due to the scattering and an order of magnitude smaller vertical displacement.
13. H. Winter
A detailed discussion of the scattering of fast atoms and ions from solid surfaces under a grazing angle of incidence is presented. Theoretical and experimental results are used to demonstrate that collisions employing this scattering geometry provide interesting new phenomena and insights into atom-surface interactions.
The energy loss for Ne atoms scattered with keV energies from a LiF(001) surface under channeling conditions, i.e., glancing angles of incidence, is studied via computer simulations using classical trajectories. Our simulations show that the elastic energy transfer to the crystal lattice depends sensitively on lattice vibrations at the crystal surface. From comparison with recent experimental data we derive surface Debye temperatures of about 215 K for the LiF(001) surface.
The temperature dependence of the magnetization near the surface of a band ferromagnet is measured with monolayer resolution. The simultaneous application of highly surface-sensitive techniques enables one to deduce the layer-dependent magnetization curves at a Fe(100) surface. Analysis of data is based on a simple mean-field approach. Implications for modern theories of itinerant-electron ferromagnetism are discussed.
16. J. Stöckl, HP. Winter, F. Aumayr, A. Mertens, K. Maass,
S. Lederer, and H. Winter
Neutral hydrogen atoms with energies ranging from 300 eV to 1.5 keV are scattered from a clean and flat LiF(001) surface under grazing angles of incidence. By detection of time-of-flight (TOF) spectra in coincidence with the number of emitted electrons, we study electron emission and excitation phenomena near their respective thresholds. We find in this energy regime population of surface excitons as dominant inelastic interaction channel. The data are consistently interpreted in the framework of a binary collision model where the formation of negative hydrogen ions is the common precursor of electronic excitation of the target and electron emission.
17. R. Pfandzelter and H. Winter
We present spin-polarized energy spectra for electrons emitted during grazing scattering of 140 keV multicharged nitrogen ions from magnetized Fe(100) and nonmagnetic, polycrystalline Ta surfaces. For scattering from Fe we observe a substantial spin polarization parallel to the average spin polarization of conduction band electrons. Experimental procedure, evaluation of data, and implications on concepts concerning the interaction of multicharged ions with surfaces are discussed.
18. H. Winter, C. Auth, and T. Hecht
Concepts and experiments are presented for studies on "Resonant Coherent Excitation" (RCE) of hydrogen atoms during surface channeling from a LiF(001) surface. This excitation is probed in our experiments via the intensity of Lyman-a light, emitted in the radiative decay of the excited states, and the charge fractions of scattered projectiles as function of projectile energy and azimuthal orientation of the crystal surface. We find clear evidence for RCE with a good signal to background ratio that allows us to perform detailed studies on position, width, and lineshape of the resonance.
19. P. Roncin, A.G. Borisov, H. Khemliche, A. Momeni,
A. Mertens, and H. Winter
For slow F+ ions (v < 0.1 a.u.) scattered from a clean and flat LiF(001) surface under a grazing angle of incidence large fractions of negative F- ions in the reflected beams are observed, whereas no negative ions are found for neutral F0 projectiles. From detailed studies on projectile energy loss and charge transfer, we find evidence for a correlated double-electron capture process in the formation of the F- ions.
20. H. Winter, J.I. Juaristi, I. Nagy, A. Arnau, and P.M.
The energy loss of slow ions scattered in front of a Al(111) surface is explained using a theoretical description based on scattering theory. The explicit inclusion of gradient corrections to account for a non-uniform electron density at the surface provides good agreement with the measured data over a wide range of distances (electron densities) for different ions with atomic number Z1 <= 20.