|Aufstellung der Publikationen der Arbeitsgruppe 2002|
Aufgeführt sind Arbeiten, die im Jahr 2002 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).
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.
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 number
of emitted electrons for grazing scattering of fast H° atoms from
a LiF(001) surface allows us to investigate 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 onsets for electron emission and electronic
excitation of the target, providing important
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.
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.
We present spin-resolved energy spectra for electrons emitted during grazing scattering of 150 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.
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.
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.
8. 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.
9. R. Pfandzelter, M. Lang, I. Urazgil´din, and
The growth and structure of ultrathin Rh films on Fe(100) are studied by grazing scattering of 50 keV He projectiles, incident along "random" and low-index surface lattice directions. Oscillations in the specular intensity for scattered ions indicate initial layer-by-layer growth, followed by multilayer growth on top of the two-monolayer film. Growth temperatures below about 400 K result in a persistent, though rough layer growth up to higher coverages. From distinct maxima in the target current as a function of the azimuthal incidence angle, we deduce that growth is epitaxial and pseudomorphic.
The energy loss of neutral Ne atoms with keV energies scattered under a glancing angle of incidence from a flat LiF(001) surface is investigated by means of a time-of-flight method. Under these scattering conditions, i.e. surface channeling, electronic excitations of the insulator surface as well as charge exchange processes play a negligible role, so that the elastic energy transfer of projectiles to atoms of the target surface can be studied. Our data are interpreted by Monte Carlo computer simulations based on quantum-chemical interaction potentials. The simulations show that thermal vibrations of lattice atoms play an important role for a quantitative description of projectile energy loss under channeling. From our simulations we derive a surface Debye temperature of (250± 50) K for the LiF(001) surface.
H. Winter, S. Lederer, K. Maass, A. Mertens, F. Aumayr, and HP. Winter
The energy loss of keV hydrogen atoms for scattering from a LiF(001) surface under a grazing angle of incidence is measured in coincidence with the number of emitted electrons. The combined time-of-flight and electron number spectra show discrete features which are attributed to specific numbers for emitted electrons and production of surface excitons. The spectra are fairly well reproduced in terms of binomial distributions. The corresponding probabilities for electron and exciton production can consistently be related to a model where the formation of negative hydrogen atoms via local electron capture from halogen lattice sites is considered as precursor for both processes.
We calculated the electric-field splitting of the Isnd 1D and 3D levels of the 3He I spectrum for n = 3, 4 and 5 and evaluated the anticrossings of singlet and triplet Stark substates. The anticrossing spectra observable after selective excitation of singlet states with asymmetric charge and current distributions are discussed. The anticrossing spectra of 3He I differ significantly fron those of 4He I due to the hyperfine interaction and, therefore, provide a valuable means for investigations on collisional excitation of He atoms.
13. R. Pfandzelter, T. Bernhard, and H. Winter
Total target currents for grazing scattering of keV protons from a crystal target are used to investigate the structure of surfaces and ultrathin films. This current shows pronounced maxima whenever the azimuthal incidence angle coincides with close-packed rows of atoms in the surface and subsurface layers. The real-space method is applied to study monolayer and bilayer films of Mn and of CoMn epitaxially grown on a Cu(001) surface.
14. R. Pfandzelter, T. Bernhard, and H. Winter
Ion beam triangulation is a real-space technique to probe structures of surfaces and ultrathin films. In-plane arrangement of surface atoms is provided from enhanced kinetic electron emission yields for the transition from planar to axial surface channeling. The technique is applied to clean (001) surfaces of fcc Cu and bcc Fe as well as ultrathin films of Mn and Co, epitaxially grown on these surfaces at different temperatures.
15. S. Wethekam, A. Mertens, and H. Winter
He+ ions as well as neutral He atoms with keV energies are scattered under a grazing angle of incidence from a clean and atomically flat Ag(111) surface. From a comparison of ion fractions observed after scattering of He+ ions and He atoms we find for projectile energies below some keV small but defined fractions of ions that have survived the complete scattering event with the surface. This feature and image charge effects on trajectories of incident ions allow us to clear up recent discussions on the microscopic interaction scenario for Auger neutralization of He+ ions in front metal surfaces. The Auger transition rates are orders of magnitude smaller than those established for He+-metal systems and are in agreement with recent calculations.
16. S. Lederer, K. Maass, D. Blauth, H. Winter, HP. Winter,
and F. Aumayr
Coincident measurements of projectile energy loss and kinetic electron emission yield for grazing scattering of 150 eV/amu to some keV/amu neutral hydrogen and helium atoms from an atomically clean and flat Al(111) surface allow us to correlate electron emission and inelastic interaction mechanisms at a metal surface. Our data show evidence for a threshold behavior of kinetic electron emission which is interpreted by energy transfer in binary encounters of projectiles in the electron selvage of a quasi-free electron gas. Contributions of electron emission to projectile energy loss are found to be negligibly small.
17. S. Lederer, A. Mertens, H. Winter, F. Aumayr, HP.
Winter, and V. Staemmler
In coincident studies on projectile energy loss and number distributions of emitted electrons for scattering of hydrogen atoms from an atomically clean and flat LiF(001) surface we derive probabilities for emission of electrons and production of surface excitons near their respective kinematic thresholds. We analyze our data in terms of electron transfer in binary atomic collisions with one collision partner being imbedded at the anion site of an ionic crystal and derive information on the energy defects in these collisions.
18. S. Wethekam, A. Mertens, and H. Winter
He atoms and He+ ions are scattered with keV energies under a grazing angle of incidence from an atomically flat and clean Ag(111). We have measured charge fractions of specularly reflected beams and studied the threshold behaviour for ionization of projectiles in terms of kinematically induced Auger ionization. From comparison of data for neutral and ionized projectiles we could show that precise studies on the kinematic onset of ionization can only be performed with neutral projectiles. Small but defined fractions of ions survive the scattering event with the surface and blur data close to the threshold owing to a background for the fractions of ions.
19. T. Bernhard, R. Pfandzelter, and H. Winter
Grazing scattering of 25 keV He atoms is applied to study island nucleation and growth of Co on Cu(001) in real time and real space. With decreasing growth temperature, we observe, starting at 470 K, step-flow growth, layer growth, bilayer growth, and, below 210 K, reentrant layer growth. The density of nucleated islands shows a complex "N-shaped" dependence on temperature, which is at variance with standard nucleation theory. Results are explained on the basis of recent density-functional calculations.
20. R. Pfandzelter, H. Winter, I. Urazgil'din, and M.
We report on the emission of spin-polarized electrons during grazing and oblique impact of 2 - 100 keV H+, He+, Ne+, and Ar+ ions upon a magnetized Fe(100) surface. A combined analysis of spin state and energy of emitted electrons elucidates processes occuring in inelastic ion-surface scattering like electron cascading or plasmon-assisted electron emission. We find evidence for a significant enhancement of the spin moment at the topmost layer of Fe(100).
21. R. Pentcheva, M. Scheffler, K. A. Fichthorn,
T. Bernhard, R. Pfandzelter, and H. Winter
We present a combined theoretical and experimental study of island nucleation and growth in the deposition of Co on Cu(001) - a prototype for understanding heteroepitaxial growth. Experimentally atom scattering is employed. Using density-functional theory, we obtain energy barriers for the various elementary processes and incorporate these into a kinetic Monte Carlo program to simulate the heteroepitaxial growth. Both the simulations and the experiments show a unique "N-shape"-dependence of the island density on temperature that stems from the interplay and competition of the different processes involved in heteroepitaxy with intermixing.
22. A. G. Borisov, V. Sidis, P. Roncin, A. Mertens,
and H. Winter
For slow F+ ions (v < 0.05 a.u.) scattered from a clean and flat LiF(001) surface under a grazing angle of incidence large fractions of negative F- ions are observed in the reflected beam. For neutral F0 projectiles, however, we find a threshold behaviour of negative ion formation with no negative F- ions being produced for slow neutral projectiles. From studies on projectile energy loss and charge fractions, we find evidence for a correlated double-electron capture process in the formation of F- ions during scattering of F+ projectiles. Key feature of this process is its quasi-resonant character, because in the final states the energy defect of the reaction is reduced due to Coulomb attraction between a negative projectile and the two holes left at the surface. Charge transfer is described theoretically using quantum chemistry calculations and close coupling solution of the time-dependent Schrödinger equation, in fair agreement with the experimental observations.
23. H. Winter
The coincident detection of projectile energy loss with the number of emitted electrons for the scattering of atoms from atomically clean and flat insulator and metal surfaces under grazing angles of incidence allows one to identify the relevant electron excitation and emission processes. From detailed studies for the scattering of H and He atoms from a LiF(001) as well as from an Al(111) surface we find evidence for clearly different interaction mechanisms. For not too high projectile velocities with respect to kinematic thresholds, electron emission for the insulator target is understood by an electronic promotion mechanism with the formation of H- ions as dominant precursor, whereas for the metal target electronic excitations are governed by direct energy transfer in binary collisions with the atomic projectiles. Based on these features we reveal a microscopic understanding for the well established, but so far poorly understood property that electron emission induced by atom or ion impact on insulator targets is more efficient than for metals
24. H. Winter and HP. Winter
We present a simple classical free electron gas model for the threshold behaviour of the kinetic electron emission caused by collisions of atomic projectiles with a metal. In our model we assume that electron emission close to the threshold is dominated by an energy transfer in binary collisions between atomic projectiles and a free electron gas. From conservation laws and phase space considerations we derive the threshold behaviour for the kinetic electron emission and the mean electron energy transfer, which compare well with recent experimental studies on electron emission from the selvage of atomically clean and flat Al(111) under grazing impact of light neutral atoms.
25. H. Winter, A. Mertens, S. Lederer, C. Auth,
F. Aumayr, and HP. Winter
We present an analysis of data obtained in coincident studies on projectile energy loss and number distributions of emitted electrons for scattering of hydrogen atoms from an atomically clean and flat LiF(001) surface. The emission of electrons, production of surface excitons, and formation of negative ions is explained by a model of electron capture by projectiles at fluorine lattice sites where the formation of negative hydrogen ions is the common precursor for the electronic processes of relevance here. We describe the interactions in terms of concepts for gas phase collisions and derive probabilities for the detachment of negative ions during the collision with the insulator surface.
26. S. Wethekam, A. Mertens, and H. Winter
He+ ions and He atoms with keV energies are scattered under a grazing angle of incidence from an atomically flat and clean Cu(111) surface. From the comparison of measured charge fractions for specularly reflected charged and neutral projectiles we find evidence for the survival of incident ions from neutralization during the complete scattering event. The survival of ions can only be understood, if the neutralization proceeds around the distance of closest approach, i.e. at the jellium edge. Thus neutralization of ions proceeds closer to the surface than derived in most former evaluations of experimental data where response phenomena are approximated by the concept of classical image charge interactions. From the analysis of our data we derive Auger neutralization rates which compare well with recent theoretical work.
27. A.G. Borisov, A. Mertens, S. Wethekam, and H. Winter
Cs ions with energies ranging from 10 keV to 1.8 MeV are scattered under a grazing angle of incidence from a flat and clean Cu(111) surface. The observed fractions of Cs atoms in the scattered beams and their dependence on projectile velocity are well described by a model of kinematically assisted resonant charge transfer between projectile and 2D surface state continuum of the target surface. Comparison with calculations for a target represented by the electronic structure of a free electron metal shows neutral fractions which are enhanced for the Cu(111) by more than one order of magnitude. This is the strongest effect of the projected band-gap of a metal surface on charge transfer observed so far.
28. H. Winter, K. Maass, S. Lederer, HP. Winter, and F.
Fast He+ ions and He0 atoms are scattered under grazing incidence from a clean and flat Al(111) surface. The target current recorded as function of azimuthal rotation with respect to the surface normal is enhanced for scattering along low-index crystallographic directions in the surface plane. From coincident studies on the number of electrons emitted per ion impact we discovered the interaction mechanism. In contrast to earlier interpretations our data reveal that the increase of apparent total yield for surface channeling along low-index directions is due to penetration of the surface layer by a small fraction of projectiles which produce a substantially larger number of electrons.