|Aufstellung der Publikationen der Arbeitsgruppe 2000|
Aufgeführt sind Arbeiten, die im Jahr 2000 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).
Secondary electron emission is a useful tool to observe electronic interactions
of ions penetrating matter. For an 8 nm carbon foil we have measured the
well-known Z1-oscillations in the electron yield in backward
direction with respect to the ion beam. In forward direction we observe
an excess yield relative to the backward direction which reflects the
shell structure of the projectiles in a similar manner as the mean charge
of the emerging ions. This enhanced electron yield in forward direction
is probably caused by the capability of the projectiles to accumulate
secondary electrons around the moving core. The assumption is supported
by the results obtained for the ratio of the forward and backward yields
as a function of the projectile energy which is nearly constant in the
range below Bohr velocity.
2. J.P. Gauyacq, A.G. Borisov, and H. Winter
In the theoretical description of resonant charge transfer between atoms and metal surfaces, electronic transition rates are generally obtained from one-electron treatments. We discuss how these transition rates relate to problems where several electrons can participate in the resonant charge transfer process. Based on two limiting cases of a closed shell of equivalent electrons and of an open shell, we outline recent progress concerning the theoretical description of the formation of negative ions in scattering from metal surfaces. These results confirm the qualitative differences in the electronic structure of various negative ions
The energy loss of Ne+ ions with keV energies scattered under grazing incidence from a LiF(001) surface is studied with a time-of-flight (TOF) technique. Since charge exchange in front of the wide-band-gap insulator is suppressed, the energy loss of slow ions moving in front of the solid can be investigated under defined interaction conditions. From our theoretical analysis of data we find clear evidence for an energy loss mechanism based on the excitations of optical phonons in the insulator.
Real-time studies of submonolayer epitaxy via scattering of fast ions are applicable over a wide range of growth temperatures and deposition rates. Computer simulations of ion trajectories and nucleation theory yield quantitative information on atomistic growth processes. For homoepitaxy of Fe on Fe(100) we deduce island densities, monomer diffusion barrier, cluster binding energies, and postdeposition island ripening. Detailed information on transitions in critical cluster size are obtained.
Electronic processes, during the scattering of fast atoms and ions from the surface of an insulator, are characterized by a variety of interesting features. Recent studies show that the electronic structure of an insulator target, comprising of a wide electronic band gap, affects electronic transfer, emission and excitation, as well as projectile energy loss, in specific ways. Concepts for the description of atomic collisions in the gas phase can be partly applied to interaction of atomic projectiles with solid surfaces. We cases focus on where the projectiles impinge on the surface at glancing angles of incidence.
Protons with energies ranging from 300 eV to 25 keV are scattered under glancing angles of incidence from a flat and clean LiF(001) surface. We have recorded energy spectra for specularly reflected projectiles and analyze the data in terms of stopping cross sections. From our analysis we deduce a threshold behaviour for electronic stopping at the low energy end. We find pronounced effects on energy loss owing to charge exchange and excitation of valence band electrons. At energies below 1 keV energy dissipation proceeds predominantly via excitation of optical phonons.
We measured angular distributions in grazing scattering of 10 keV Heo0-atoms incident along the  direction of an Fe(001) surface. Depending on the incidence angle different types of multimodal distributions are observed. Classical molecular dynamics computer simulations relate the angular characteristics to rainbow singularities and focusing effects.
J.I. Juaristi, C. Auth, H. Winter, A. Arnau, K. Eder, D. Semrad, P. Bauer,
F. Aumayr, and P.M. Echenique
The energy loss of slow ions during grazing scattering from a LiF(100) surface as a function of the projectile number Z1 is observed to show oscillations similar to those occuring in metals. A model of stopping of ions in an elctron gas where screening is calculated from denstity functional theory reproduces well the experimental data. The same model gives good agreement with the energy loss obtained in transmission experiments performed with H and He projectiles. Analysis of these results allows us to gain new insights in the stopping of slow ions in ionic crystals.
T. Hecht, H. Winter, A.G. Borisov, J.P. Gauyacq, and A.K. Kazansky
Electron capture by Li+ and H projectiles in grazing scattering from Cu(111) and Cu(110) surfaces is studied experimentally and theoretically. Whereas data for Cu(110) can be described by established theoretical methods treating resonant charge transfer with a free-electron metal, data for Cu(111) show pronounced deviations from this approach. We interpret our observations by the effect of the projected L-band gap of the Cu(111) surface. In particular, the quantum states of reduced dimension (2D surface state continuum) play a dominant role in electron transfer.
Protons with energies in the keV domain are scattered from a clean and flat LiF(001) surface under a grazing angle of incidence. For specific projectile energies the periodic Coulomb field owing to the point charge lattice of the ionic crystal leads to the "Resonant Coherent Excitation" (RCE) of the n = 1 to the n = 2 states. This excitation is probed in our experiments via the intensity of Lyman-a light, emitted in the radiative decay of the excited states, 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
Electron emission from H0 projectiles (1 – 20 keV) grazingly scattered off a LiF(001) single crystal surface has been studied for impact angles from 0.5° to 1.8°. We observe that the threshold energy for electron emission increases with smaller projectile impact angle, whereas at higher impact energies smaller angles are more efficient for producing electrons. Our data are analyzed in terms of position- and velocity-dependent electron production rates which indicate that an electron promotion mechanism is responsible for electron emission.
Neutral Ne atoms with keV energies are scattered under channeling conditions, i.e. under a glancing angle of incidence, from a LiF(001) surface. By means of a time-of-flight method with a pulsed neutral beam energy distributions for scattered projectiles are obtained. We find for this specific system that the small energy transferred to the crystal lattice during channeling via binary collisions with large impact parameters dominates the dissipation of projectile energy, whereas all other excitations of the solid can be brought to a negligible level.
T. Hecht, H. Winter, A.G. Borisov, J.P. Gauyacq, and A.K. Kazansky
The formation of negative ions H-, O-, S-, F-, Cl-) is studied for grazing scattering of fast ions from Cu(110) and Cu(111) surfaces. In a detailed experimental and theoretical investigation we reveal that the projected L-band gap of the Cu metal affects charge transfer in a specific manner. From the analysis of the negative ion fractions as functions of projectile velocity we conclude that, for the Cu(111) surface the electronic 2D surface state continuum plays an essential role in the projectile-surface electron transfer.
14. R. Pfandzelter, M. Ostwald, and H. Winter
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.
15. R. Pfandzelter and H. Winter
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.
16. C. Auth and H. Winter
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.
17. 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.
Growth and chemical composition of V films on Fe(100) are studied for temperatures between 370 and 620 K. Whereas the first atomic layer grows smoothly, growth of subsequent layers is rougher and favored at high temperatures. We observe a pronounced temperature-dependent alloying, which is confined to both interface layers.
19. H. Winter and A. Mertens
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.
20. R. Pfandzelter and H. Winter
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.
21. 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.
22. R. Pfandzelter, T. Bernhard, and H. Winter
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.
23. R. Pfandzelter, T. Hecht, and H. Winter
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).
24. R. Pfandzelter, M. Ostwald, and H. Winter
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.
25. 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.