|Aufstellung der Publikationen der Arbeitsgruppe 1997|
Aufgeführt sind Arbeiten, die im Jahr 1997 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).
A. Mertens, C. Auth, H. Winter, and A. G. Borisov
Fast alkali-metal-atoms and ions are scattered with keV energies under grazing incidence from the surface of the alkali-metal halides LiF(100), KCl(100), Kl(100), and the scattered beams are analyzed with respect to their charge fractions. From our experiments we find no evidence for occupied or unoccupied electronic surface states within the band gap of the insulator.
The chemical composition of ultrathin Cr films grown on Fe(100) under layer-by-layer conditions has been studied by proton- and electron-induced Auger electron spectroscopy. Exploiting the largely different probing depths for proton and electron excitation we can quantitatively evaluate the layer-dependent concentration profiles of Cr and Fe in the film. The main results are: (1) for a coverage of 1 ML about half of the deposited Cr atoms exchange places with Fe atoms; (2) place exchange processes are essentially confined to the topmost and second layer; (3) for coverages larger than 3 ML the topmost layer consists almost exclusively of Cr atoms; (4) the as-grown 1 ML films are not stable.
Growth of ultrathin films of Cr on Fe(100) has been studied by grazing scattering of 25 keV H+ and He+ ions from the film surface. We find that growth is epitaxial and pseudomorphic. The first monolayer grows two-dimensionally in a wide range of growth temperatures (300 - 600 K). For subsequent layers growth proceeds in a layer-by-layer mode at high temperatures; at low temperatures the reduced mobility leads to kinetic roughening. The nucleation length can be manipulated within a factor of 5 by changing growth temperature or deposition rate. This can be exploited to grow almost perfect layers. It is shown that a manipulation of the nucleation length at the beginning of growth has a persisting effect on the growth of further layers.
We have studied charge exchange of Na+-ions with energies ranging from 1 keV to 100 keV in the scattering from LiF(100)-, Kl(100)-, and KCl(100)-surfaces under a grazing angle of incidence. In the experiments we observe generally small fractions of neutral atoms and conclude that the density of occupied surface states within the band gap of the insulator is negligible.
5. C. Auth, A. Mertens, H. Winter, A. G. Borisov, and
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.
Protons with energies ranging from 5 keV to 710 keV are scattered from an Al(111)-surface under a grazing angle of incidence. The scattered beams are analyzed with high angular and energy resolution with respect to the energy loss of the projectiles. We observed a linear increase of the energy loss with velocity up to u » 1.5 a.u. (about 50 keV) and a subsequent constant loss of about 3 keV for higher velocities (energies). The data show a very weak dependence on angles relevant for the scattering event.
J. Leuker, H. W. Ortjohann, R. Zimny, and H. Winter
Fast He+-, Li+- and Na+-ions with energies ranging from about 5 keV to 360 keV are scattered under a grazing angle of incidence from a magnetized Fe(110) surface. The spin polarization of electrons captured into excited atomic terms is investigated via the polarization of the fluorescence light. We present data for this capture as a function of projectile velocity and of target temperature. For the Fe(110) surface we observe a pronounced dependence of the spin polarization on the projectile velocity and discuss our results in the framework of some simple theoretical concepts
By making use of effects of the image charge on projectile trajectories in scattering of a fast beam of metastable He-atoms with keV energies form an Al(111) surface under a glancing angle of incidence, we have developed a new method to analyze fast atomic beams with respect to relative populations of excited metastable states. Base on our experimental studies we propose the construction of a corresponding detector.
9. H. Winter, C. Auth, A. Mertens, A. Kirste, and M. J.
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 show that scattering of 25 keV He+-ions under the condition of planar surface channeling offers a sensitive technique to study the thermal behavior of metal surfaces. The angular distributions of scattered projectiles directly depend on thermal displacements of surface layer atoms and reveal characteristic features caused by scattering at surface steps. Based on computer simulations we deduce an anomalous increase of the mean square displacement normal to the surface for temperatures above about 750 K. An increase in the density of surface steps can be excluded.
11. A. G. Borisov, B. Bahrim, D. Teillet-Billy, J. P.
Gauyacq, C. Auth, and H. Winter
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.
The spin polarisation of electrons captured into excited atomic levels of atoms during the scattering of fast ions from a magnetized Ni(110) surface under a grazing angle of incidence is deduced from the polarization of the fluorescence light. In our studies we have investigated the dependence of the spin polarization on projectile velocity and angle of incidence and observed generally small polarizations for captured electrons.
The secondary electron yield induced by slowly moving (v = 0.6 vo, vo - Bohr velocity) heavy ions was researched on a thin carbon foil in forward and backward direction in respect to the ion beam. Only for the backwards electron emission an oscillatory behaviour of the yield was observed in dependence of the atomic number Z1 of the projectiles while in forward direction the electron yield increases nearly monotonically with Z1. For the forward electron emission the yield is somewhat higher. The excess of electrons between forward and backward electron emission seems to be proportional to the mean charge of the emerging ions which indicates a surface effect for the forward electron emission. The Z1-oscillation in the backward electron emission corresponds to oscillations as was found by energy loss measurements on an argon gas target. These oscillations correlate with the number of charge exchange events in gases. From the common oscillatory behaviour it was concluded that charge exchanges during single collision events are everywhere dominant for the energy loss of slowly moving heavy ions in gases as well in solids.
We have scattered fast He-atoms with keV-energies under a grazing angle of incidence from a clean and flat Al(111)-surface. From the analysis of the angular distributions for scattered projectiles and for scattered light of a He-Ne-laser we find evidence for a reflection of the fast atoms from single terraces of the crystal surface. From our experiments we deduce in a simple manner an averaged width for terraces forming a stepped surface.
15. R. Pfandzelter
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.
16. T. Hecht, C. Auth, and H. Winter
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.
17. C. Auth, A. Mertens, and H. Winter
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.
18. 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).
We have scattered protons and hydrogen atoms with energies of some keV from a LiF(001) surface under a grazing angle of incidence. From the intensity of Lyman-a radiation (transition from n = 2 to n = 1, l = 121.6 nm) as a function of projectile energy for different azimuthal orientations of the crystal surface, we find clear evidence for a resonant coherent excitation of n = 2 states of hydrogen atoms in the oscillating electric field in front of the insulator surface.
Growth of ultra-thin films of Mn on Fe(001) has been studied by grazing ion-surface scattering and Auger electron spectroscopy. We find that growth is epitaxial and pseudomorph and starts in a layer-by-layer mode. Favorable growth temperature is about 570 K; lower temperatures lead to kinetic roughening. After four layers, the growth mode changes from layer (two-dimensional) to island (three-dimensional) growth for temperatures above about 420 K. Below this temperature, a (metastable) layer-by-layer growth is observed. A quantitative analysis of the data yields the nucleation length in two-dimensional growth and the three-dimensional island density for the different growth temperatures.
21. T. Hecht, H. Winter, and A. G. Borisov
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.
22. T. Igel, R. Pfandzelter, and H. Winter
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.
23. T. Igel, R. Pfandzelter, and H. Winter
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.
24. R. Pfandzelter, T. Hecht, and H. Winter
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.
25. 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.
26. R. Pfandzelter, T. Kowalski, T. Igel, and H. Winter
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.
27. T. Hecht and H. Winter
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.