|Aufstellung der Publikationen der Arbeitsgruppe 1998|
Aufgeführt sind Arbeiten, die im Jahr 1998 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).
Substantial fractions of fast atoms or ions are converted to negative ions during grazing scattering from a clean and flat monocrystalline surface of alkali-metal halides. We interpret the experimental data by a model of local electron capture from the halogen sites of the crystal in binary-type collision events. Due to the band gap of the insulator, the probability for subsequent electron loss is low, resulting in large fractions of negative ions that survive from the collisional formation.
From studies on the energy loss of fast atomic projectiles (v = 0.5 vo) ranging from protons to potassium ions in the scattering from a clean and flat Al(111)-surface under a grazing angle of incidence we derive stopping powers as function of distance from the surface plane. We observe that oscillations for the stopping powers with the atomic number Z1 show a pronounced dependence on the electron density as predicted by nonlinear density-functional calculations. For sufficiently low electron densities those oscillations follow the intuitive picture in showing minima of stopping for projectiles with noble gas configurations.
We report on the results of a joint experimental and theoretical study on the formation of O--ions in grazing scattering from an Al(111) surface. The O-fractions in the scattered beam are studied over a wide range of collision energies. This allows us to observe a resonance structure for the O- formation probability as a function of the collision velocity component parallel to the surface (vp). Such a characteristic vp-dependence is the signature of a kinematically induced charge transfer process between the O--ion and the Al surface. The theoretical treatment of charge transfer is based on the Coupled Angular Mode (CAM) method. The multistate aspect of the problem originating from the open-shell structure of the O- ion is taken into account.
Studies on grazing scattering of 25 keV H+ and He+ ions from clean Fe(001) and submonolayer films of Cr and Mn on Fe(001) are reported. We find that angular distributions of reflected ions directly depend on the defect structure of the topmost surface layer. Surface defects of different dimensions (thermal displacements, surface steps, islands) can be separated due to characteristic effects on the scattering process. Computer simulations based on the binary collision approximation permit a quantitative analysis of data.
He-atoms with keV-energies as well as a laser beam are scattered under a grazing angle of incidence from a flat and clean Al(111)-surface. From the analysis of angular distributions for scattered projectiles we deduce that the projectiles are scattered with respect to the planes of single terraces formed by topmost layer atoms. With our method we are able to resolve terraces of crystallographic domains of the target showing different orientations with respect to the macroscopic surface plane.
We have scattered ions with energies <10 keV/amu and atomic numbers Z1 <20 from an Al(111)-surface under grazing angles of incidence. From a study of the projectile energy loss as a function of angle of incidence we derive position dependent stopping powers which show pronounced oscillations with Z1. Comparison of the stopping powers at different distances from the surface plane, i.e. for different electron densities, reveals characteristic shifts in those oscillatory structures as predicted by nonlinear density functional theory.
H. Winter, G. Siekmann, H. W. Ortjohann, J. C. Poizat, and J. Remillieux
He+-ions with energies from 3 keV to 1.4 MeV are scattered from a clean and flat Al(111)-surface under a grazing angle of incidence, and the scattered projectiles are analyzed with respect to charge states. We observe defined kinematic thresholds for the formation of He+- as well as of He2+-ions which are in agreement with the onset of a kinematically induced Auger-ionization process. Comparison of our data with foil transmission experiments shows a clearly different dependence of the charge state fractions on the projectile energy (velocity).
In a detailed analysis of image charge effects on the trajectories of He+-ions with keV-energies scattered from a clean Al(111)-surface under a grazing angle of incidence we find evidence for contributions from two different Auger processes. From our data we derive Auger neutralization rates for the filling of the 1s-hole and the Auger deexcitation rate for the 2s 3S-level as function of the distance from the surface. The rates are obtained from the experiments in a straigthforward manner and can be considered as a profound basis for the comparison with theoretical calculations.
The magnetic ordering of the topmost surface layer of Mn films grown on Fe(100) at different temperatures is investigated via the polarization of fluorescence light emitted after capture of spin polarized electrons into excited atomic terms during scattering of 25 keV He+-ions. The observed spin polarization for submonolayer films is in accordance with an in-plane ferromagnetic order of Mn moments with antiparallel coupling to moments of the Fe substrate. For monolayer and multilayer films the detected spin polarization vanishes. This may serve as an indication for in-plane antiferromagnetic or ferrimagnetic order.
Ultrathin Mn films are grown on Fe(100) and studied by Auger electrons induced by 25 keV protons scattered under grazing incidence from the film surface. The electron spectra indicate that significant interfacial alloying occurs during growth of the first monolayer. Using model calculations we conclude that about 60 % of the Mn atoms deposited have undergone place exchanges with Fe atoms from the substrate interface layer. Subsequent monolayers grow as pure Mn films. These findings are supported by conventional electron-induced Auger electron spectroscopy.
We have studied the scattering of 1-10 keV He-atoms from Al(111) under grazing angles of incidence. The scattered particle polar angular distributions, measured in the plane defined by the incident beam direction and the surface normal, peak around the direction of specular reflection and exhibit very small widths (about 0.1° FWHM for 0.4° incidence angle). Computer simulations reproduce the experiments, showing that angular scattering is determined by multiple scattering at (1) thermally elongated atoms and (2) electrons, which seems to be the dominant mechanism at the smallest incidence angles. For larger beam energies kinematically induced Auger ionisation should become possible, contributing to the angular straggling due to image attraction.
12. R. Pfandzelter, T. Igel, and H. Winter
We have investigated the emission of polarized light after capture of spin polarized electrons in glancing-angle collisions of 25 keV He+-ions with the surface of clean Fe(100) and ultrathin Mn films grown on Fe(100) at different temperatures. The intensity of fluorescence light emitted in the HeI 23S - 33P, l = 388.9 nm transition shows oscillations with Mn coverage, as observed in the intensity of reflected projectiles. The spin polarization of captured electrons vanishes for a deposition of Mn > 0.5 ML, in accordance with an in-plane ferromagnetic ordering of Mn moments in the submonolayer regime and an in-plane antiferromagnetic or ferrimagnetic ordering of the topmost layser for monolayer and multilayer films.
We report on a new kind of rainbow scattering observed for 25 keV He+-ions scattered under glancing angles of incidence from an irregularly stepped Fe(001)-surface. Polar angular distributions of scattered projectiles show an additional peak at smaller angles, which is ascribed to scattering at isolated downward steps on the surface. Based on classical trajectory calculations we identify the observed feature as a rainbow and deduce the interaction potential for fast ions at large distances.
We have scattered protons with energies ranging from 2 keV to 9 keV from a clean and flat LiF(001)-surface under a grazing angle of incidence and studied the positive ion fractions of scattered projectiles. From a combined variation of projectile energy and azimuthal angle of the target we conclude a pronounced effect on the ion fractions by resonant coherent excitation and ionization of hydrogen atoms owing to the oscillating electric fields acting on fast projectiles in front of the insulator surface.
We have measured the energy loss of slow protons scattered with energies from 300 eV to 28 keV from a clean and flat LiF(001)-surface under a grazing angle of incidence. Our data reveal a threshold behaviour of stopping at low projectile energies. The effect on the outgoing charge state indicates that electron capture and loss are dominant mechanisms for the stopping of slow protons in a wide-band-gap insulator. Our data allows one also to deduce information on charge transfer in front of the surface of an insulator.
16. A. G. Borisov, D. Teillet-Billy, J. G. Gauyacq, A.
Mertens, C. Auth, H.Winter, and J. A. M. C. Silva
Results of a joint experimental and theoretical study on the formation of F-, Cl-, and I-ions in grazing scattering from an Al(111) surface are presented. The negative ion fractions in the scattered beam are studied over a wide range of collision energies. Negative ion fractions show a pronounced dependence on the collision velocity component parallel to the surface, which is a signature of the kinematically induced projectile-surface charge transfer process. The theoretical treatment is based on the coupled angular mode method. We demonstrate the importance of the multielectron/multistate aspect of the charge transfer problem originating from the presence of the six equivalent electrons on the outer p-shell of negative ions. In the case of the I-ion formation, the Spin-Orbit interaction in the I-atom has to be taken into account.
The contribution to the kinetic electron emission coming from the excitation of conduction band electrons in binary collisions with slowly moving ions is analyzed. The scattering suffered by conduction band electrons, which are described within the free electron model, on the effective potential created by the projectile is studied, taking into account the shift of the Fermi sphere due to the nonzero velocity of the ion. The effective potential is calculated self-consistently within the density functional theory for a static impurity embedded in an electron gas. Using the excitation rate of conduction electrons obtained from this nonlinear approach, the energy distribution of emitted electrons is calculated within a transport equation formalism.
18. T. Igel, R. Pfandzelter, and H. Winter
We report on first experiments on the emission of polarized light after capture of spin polarized electrons during grazing scattering of ions from the surface of ultrathin magnetic films. The spin polarization observed during initial growth of Mn or Cr on Fe(100) shows a pronounced dependence on the coverage. A qualitative analysis taking into account growth morphology and interkfacial alloying reveals a complex evolution with coverage of the long-range magnetic order of the topmost film layer, comprising ferromagnetic and antiferromagnetic alignments.
19. R. Pfandzelter, T. Igel, M. Ostwald, and H. Winter
Grazing scattering of 25 keV He+-ions during homoepitaxial growth of Fe on Fe(100) is reported. The angular distribution of scattered projectiles depends on the coverage in a characteristic way owing to the periodic change in surface morphology. An analysis based on a classical description for ion trajectories enables one to derive quantitative information on growth mode and growth kinetics. At 550 K pronounced oscillations show near perfect layer-by-layer growth persisting for many layers.
20. J.I. Juaristi, A. Arnau, P.M. Echenique, C. Auth,
and H. Winter
In a theoretical study on the stopping of slow multicharged ions in an electron gas we find an intricate dependence of the stopping power on the inner shell configurations of the projectiles. In a detailed treatment for nitrogen ions we obtain opposite effects of stopping on the occupations of the K- and L-shells. Experiments on energy loss of multicharged nitrogen ions during grazing scattering from an Al(111)-surface are consistently explained by our calculations.
21. T. Igel, R. Pfandzelter, and H. Winter
25 keV He+-ions are scattered from a clean and Mn-covered (1ML) Fe(100)-surface. The spin polarization of electrons captured into excited 33P states of He-atoms is probed via an analysis of polarized fluorescence light. For projectiles impinging from grazing to normal incidence we observe the same spin polarization of captured electrons. By a coverage of 1 ML of Mn-atoms we demonstrate the surface sensitivity of the method and discuss its potential for spin-sensitive microscopy and sputter depth profiling.
22. R. Pfandzelter
Grazing ion-surface scattering is a real-space technique to obtain statistical information on the morphology of disordered surfaces, particularly suited to study growth processes. Based on computer simulations of ion scattering from submonolayer films modeled by Markov chains, we address the question which morphological quantity causes oscillations in the intensity of reflected ions during layer-by-layer growth. We find that the relevant quantity is the pair correlation between atoms separated by a characteristic distance given by the experimental parameters, rather than surface roughness or step density, as assumed in simple models.
23. H. Kerkow, R. Stolle, V.P. Petukhov, and E.A. Romanovskii
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.
24. H. Kerkow, R. Stolle, V.P. Petukhov, and E.A. Romanovskii
Secondary electron yield and the mean charge of the emerging ions are
measured on thin carbon foils for nitrogen atoms and molecules at particle
velocities between 0.3 and 0.7 vo (vo - Bohr velocity).
As expected the mean charge of the emerging ions was two times higher
for the molecules than for the atoms while secondary electron yield were
always lower than 2 for all velocities as is established by other authors
. By energy loss measurements on thin carbon foil (3.5 nm) the Coulomb
explosion of the nitrogen molecules could be established which indicates
a beam spreading on the first layer of the target. The results are interpreted
by the assumption of a disturbed charge equilibrium in the solid by the
secondary electron production which reduces the electron yield.
25. T. Igel, R. Pfandzelter, and H. Winter
The long-range magnetic ordering at the surface of
ultrathin epitaxial films of Cr, Mn, and Fe on Fe(100) has been studied by electron
capture spectroscopy. The observed spin polarization indicates different magnetic
orderings comprising parallel and antiparallel coupling of moments. Based on simple
assumptions for electron capture, the layer- and coverage-dependent magnetization profile
26. R. Pfandzelter
Grazing ion-surface scattering is used to study in real space and real
time epitaxial growth processes. By example of 25 keV He+-ions
scattered during submonolayer homoepitaxy on Fe(100), we show that an
interpretation of experiments is straightforward by means of classical
mechanics computer simulations. A fit of intensity and angular profile
of the specular beam allow one to deduce basic quantities like the island
27. J.I. Juaristi, A. Arnau, P.M. Echenique, C. Auth,
and H. Winter
The experimentally observed monotonic increase of the energy loss with
charge q for Nq+ ions impinging on an Al(111) surface under
grazing angle of incidence is explained by a model that accounts for
the effect of K and L shell vacancies in the stopping power. Our model
allows us to estimate the characteristic time scales (and distances
from the surface) for the neutralization and relaxation of multicharged
ions. We use a transport cross section formulation of the electronic
stopping of ions in an electron gas, as well as a self-consistent calculation
of the scattering potential within density functional theory.
28. 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
29. T. Hecht, R. Pfandzelter, H. Winter, and A.G. Borisov
A beam of keV metastable He-atoms is produced by the
passage of He*-ions through an alkali vapour target and scattered under a grazing angle of
incidence from a clean and flat Al(111)-surface. Charge exchange between atoms and surface
is studied via the effect of image charge deflection on trajectories of scattered
projectiles. The angular distributions for scattered atoms are analyzed in terms of
electron transition rates which form the basis for a Monte-Carlo computer simulation.
Resonant charge transfer plays a dominant role leading to an ionization of the metastable
projectiles. The transition rates for this process are calculated by making use of the
Coupled Angular Mode (CAM) method and dynamic properties of the interactions are taken
into account by translational factors connecting the projectile and metal frames of
30. H. Kerkow, R. Stolle, V.P. Petukhov, and E.A. Romanovskii
The thickness dependence of the secondary electron yield was researched
with 10 keV/u chlorine ions on thin carbon foils in forward and
backward direction with respect to the ion beam. A best fit to the experimental
results was attained by the assumption of a fixed contribution from
the uppermost layers and secondly, from an amount of each deeper layer
which exponentially diminished in dependence on the distance to the
surface. An escape depth of 11 nm was found on both sides of the foils.
31. H. Winter, C. Auth, T. Hecht, and A. Mertens
We discuss charge exchange and excitation phenomena during the scattering
of fast atoms and ions from the surface of an insulator under a grazing
angle of incidence. We show that the electronic structure of an insulator
target, comprising an electronic band gap, affects charge exchange, excitation,
and projectile energy loss in a characteristic manner.