Aufstellung der Publikationen der Arbeitsgruppe 1996

Aufgeführt sind Arbeiten, die im Jahr 1996 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).

 

1. T. Hecht, C. Auth, A. G. Borisov, and H. Winter
"Suppression of Auger-neutralization in the scattering of noble gas-ions from the surface of an insulator"
Physics Letters A 220 (1996) 102

We have scattered noble gas atoms and ions from a clean and flat LiF(100)-surface under a grazing angle of incidence and observed for He+- and Ne+-projectiles a substantial survival in collision. We interpret our findings in terms of a suppression of the Auger-neutralization for these ions in the interaction with the surface of the insulator. Due to the high binding energies of valence band electrons and the large energy gap for LiF Auger transition rates seem to be clearly reduced in comparison to those rates for metal targets.

 

2. H. Winter, C. Auth, and A. G. Borisov
"Formation of negative ions in grazing scattering from a LiF(100)-surface"
Nuclear Instruments and Methods in Physics Research B 115 (1996) 133

We have studied charge exchange between fast atoms/ions and the surface of an insulator. Based on investigations on the formation of negative hydrogen, oxygen and fluorine ions in the grazing scattering from a clean and flat LiF(100) surface, we discuss specific features of the interaction of fast atomic projectiles with the surface of an insulator. Under specific kinematic conditions large fractions of negative ions are observed.

 

3. A. G. Borisov and H. Winter
"Neutralization of Na+-ions in grazing scattering from the surface of thin Al-films"
Zeitschrift für Physik D 37 (1996) 263

A theoretical study on the formation of the Na I 3s-term in grazing scattering of Na+-ions from the (111)-surface of an aluminum crystal and a thin film is performed. For the crystal-surface we find quantitative agreement between experimental and theoretical results. For a thin film the quantization of the electronic states in the direction normal to the surface strongly influences the charge-exchange process. The dependence of the neutral fractions in the scattered beam on the projectile velocity is predicted to be clearly different for electron capture from the film and the semi-infinite crystal.

 

4. A. G. Borisov, H. Winter, G. Dierkes, and R. Zimny
"Detailed study on the neutralization of fast Na+-ions in grazing collisions with an Al(111) surface"
Europhysics Letters 33 (1996) 229

We performed detailed theoretical and experimental studies on the neutralization of Na+-ions with keV energies in grazing scattering from a clean and flat Al(111)-surface. Our data are described well by model calculations based on the non-perturbative CAM method. From the analysis of our experiments we deduce the effective binding

 

5. H. Winter
"Use of ion sources for highly charged ions in studies of atomic and ion-solid collisions"
Review of Scientific Instruments 67 (1996) 1420

Collisions of slow multicharged and highly charged ions, i.e., ions with kinetic energies up to some 10 keV, with atoms in the gas phase and with solids have been studied in detail over the last few years. A brief review is presented on some typical experiments and their results which make use of the specific features of highly charged ions in the fields of atomic spectroscopy, ion-atom collisions, and ion-solid interactions. In particular recent progress in the understanding of the interaction mechanisms of highly charged ions with solid surfaces will be discussed.

 

6. C. Auth and H. Winter
"Energy loss of fast protons in single collisions with noble gas atoms"
Radiation Effects and Defects in Solids 140 (1996) 29

We have measured energy loss spectra of 100 keV protons with noble gas atoms in small angle scattering under single collision conditions. We obtain from our experiments with high angular and energy resolution the mean energy loss and the energy straggling as function of the angle of scattering range from 0.3 to 6 mrad.

 

7. A. G. Borisov and H. Winter
"Formation of H- on thin aluminum films"
Nuclear Instruments and Methods in Physics Research B 115 (1996) 142

We report on a theoretical study on the formation of negative hydrogen ions in the scattering from a thin aluminum film. In contrast to a semi-infinite metal electronic states of the film show pronounced effects of quantization along the direction of the surface normal. We predict that this quantization has specific effects on the formation of H--ions in the scattering from thin films.

 

8. A. G. Borisov and R. Zimny
"Orientation of Na*atoms formed under grazing scattering at an Al surface"
Surface Science 359 (1996) 174

A study of the angular momentum orientation of excited Na* 3p atoms by grazing scattering of Na+ ions from an Al surface is presented. The coupled angular mode (CAM) method, previously used for the nonperturbativ treatment of dynamic resonant electron-transfer in grazing atom-surface collisions [1,2], is used for the study of this polarization problem. Following the dynamical approach of Brako [3,4] density matrix elements are calculated within the 3p subspace of Na. A pronounced orientation of the orbital angular momentum perpendicular to the scattering plane is obtained. For the first time, a theoretical calculation has yielded a decrease of this atomic orientation with decreasing ion-beam velocities below 0.4 Bohr. Overall agreement of the theoretical results with experimental observations is found.

 

9. E. C. Paloura, A. Mertens, K. Holldack
"The effect of ion implantation in the microstructure of Si3N4 films: an X-ray absorption study"
Nuclear Instruments and Methods in Physics Research B 113 (1996) 231

The effect of ion implantation in the microstructure of thin Si3N4 films is studied with Extended X-ray Absorption Fine Structure (EXAFS) and Near-Edge X-ray Absorption Fine Structure (NEXAFS) measurements at the N-K-edge. The stoichiometric Si3N4 films were subjected to implantation with Ar+ or N2+ ions, at 77 and 300 K. Analysis of the NEXAFS spectra indicates that ion implantation creates N-dangling bonds which introduce a characteristic resonance line (RL) and a redshift of the absorption edge, both of which depend on the projectile and the substrate temperature during implantation. Finally, the EXAFS data demonstrate that implantation modifies the microstructure down to the nearest-neighbor shell as that is indicated by the reduction of the coordination number from 3 to 2.

 

10. M. Rösler
"Comparative study of ion-induced kinetic electron emission from different nearly-free-electron metals"
Nuclear Instruments and Methods in Physics Research B 115 (1996) 278

In the particle-induced kinetic electron emission from solids the role of different excitation and scattering processes is of special interest from both the theoretical and the experimental point of view. Calculations of the emission characteristics were performed for different nearly-free-electron metals. Depending on the impact energy the relative importance of different excitation mechanisms will be discussed for Mg and Be in comparison with the results for the standard material Al. There are characteristic features related to the different strength of plasmon damping in these metals. The role of different scattering processes (elastic, inelastic ) which determine the transport of excited electrons towards the surface will be discussed in relation to the special features shown by the various excitation processes.

 

11. S. Parhofer, R. Pfandzelter, and M. Potthoff
"Energy shift of the CuL3M4,5M4,5 Auger line excited by proton impact on Cu(110): Enhanced electron correlations at the surface"
Physical Review B 53 (1996) 10377

CuL3M4,5M4,5 Auger spectra excited by impact of 1.0-MeV protons on Cu(110) were measured. In the case of grazing incidence angles, a shift of the Auger line to higher kinetic energies was observed. This is attributed to a surface effect. The observed shift is interpreted within the Cini-Sawatzky theory. Taking into account the surface core-level shift and the surface shift of the center of gravity of the valence band only is not sufficient to explain the observed shift of the Auger line. We find evidence that the shift is partly due to a change of the on-site Coulomb interaction U at the very surface. A rough estimation shows U to be increased by less than 0.6 eV at the surface when compared with the bulk.

 

12. R. Pfandzelter and S. Parhofer
"Spectroscopy of the CuL3M4,5M4,5 Auger line excited by grazingly incident MeV protons on Cu(110)"
Nuclear Instruments and Methods in Physics Research B 115 (1996) 269

We measured the fine structure of the CuL3M4,5M4,5 Auger line excited by impact of MeV protons upon a Cu(110) surface. In case of a grazing incidence, a shift of the Auger line by 0.4 eV to higher kinetic energies is observed. This is attributed to a surface effect: The grazingly incident protons undergo semin-planar channeling at the surface and mainly excite Auger electrons at topmost surface layer atoms. Computer simulations which take into account surface steps corroborate this interpretation.

 

13. R. Pfandzelter and J. Landskron
"Submonolayer growth of Ag on Cu(111) studied by proton induced Auger electron spectroscopy"
Nuclear Instruments and Methods in Physics Research B 115 (1996) 273

We studied the growth of Ag on Cu(111) by electron- and proton-induced Auger electron spectroscopy. The Auger signal versus coverage plots indicate a growth in a monolayer-by-monolayer fashion. In case of grazingly incident protons, additional information on the real mesoscopic structure in the submonolayer coverage range could be obtained by comparison of the experimental results with computer simulations. We found good agreement for Ag islands growing from Cu step edges.

 

14. S. Gelfort, H. Kerkow, R. Stolle, V. P. Petukhov, and E. A. Romanowski
"Angular dependence of the electronic energy loss for low energy heavy ions under channeling conditions"
Nuclear Instruments and Methods in Physics Research B 115 (1996) 315

The electronic stopping of heavy ions with 2 £ Z1 £ 36 in á 110ñ direction of a 350 nm thick silicon crystal was measured for velocities below Bohr velocity. Large oscillations in the electronic stopping power are observed for best-channeled ions of the same velocity. whereas for ions emerging from the foil some degrees to the channel axis the oscillations are strongly reduced.

 

15. A. G. Borisov, U. Wille
"Resonant electron transfer between highly charged ions and metal surfaces: First-order vs nonperturbative transition rates"
Nuclear Instruments and Methods in Physics Research B 115 (1996) 137

Transition rates for resonant electron transfer between highly charged ions and metal surfaces are calculated within a simple first-order model and within the nonperturbative coupled-angular-mode (CAM) method. In the first-order model, image potentials are disregarded and hybridization effects are simulated by describing the ionic orbitals in terms of hydrogenic wave functions in the parabolic (Stark) representation. The CAM method employs a fairly realistic electronic potential that includes the classical image potentials. For typical cases, the deviations between first-order and CAM transition rates are such that the associated transition distances agree, on average, within one atomic unit. The reason for this agreement is indicated.

 

16. T. Hecht, C. Auth, and H. Winter
"Scattering of fast He-atoms from single terraces of an Al(111)-surface"
Nuclear Instruments and Methods in Physics Research B, im Druck

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.

 

17. H. Winter, A. Mertens, C. Auth, and A. G. Borisov
"Complete negative-ion conversion of halogen atoms and positive ions in surface scattering from KI(100)"
Physical Review A 54 (1996) 2486

Fast positive ions and atoms with energies ranging from some 100 eV to 100 keV are scattered under a grazing angle of incidence from a clean and flat (100)surface of a monocrystalline KI sample. We observe for halogen projectiles an almost complete negativ-ion conversion, i.e., negative-ion fractions up to 98.5 %. Our data show a characteristic dependence on the projectile velocity, which provides important additional information on the mechanisms of formation of negative ions in the scattering from the surface of an insulator.

 

18. T. Igel, R. Pfandzelter, and H. Winter
"Intensity oscillations in grazing scattering of fast He+-ions during heteroepitaxial growth of Cr on Fe(100)"
Europhysics Letters 35 (1996) 67

We report on grazing scattering of 25 keV He+-ions during epitaxial growth of Cr on Fe(100). The angular distribution and intensity of the scattered particles depend on the coverage in an oscillatory way, which is due to the periodical change of the surface morphology in two-dimensional growth. At high growth temperatures (600 K), pronounced oscillations which persist with little decay in the amplitude indicate (almost) ideal layer-by-layer growth; at low temperatures (350 K), strongly damped oscillations and low intensities of scattered projectiles are due to transient layer-by-layer growth. The angular distributions are well described by Monte-Carlo simulations using a classical description for the trajectories. This enables one to derive detailed information on the morphology of the growth front, like, e.g., the mean island distance.

 

19. M. Rösler
"Secondary electron emission from simple metals: Comparative studies for Al, Mg, and Be"
Scanning Microscopy, im Druck

In the secondary electron emission (SEE) from solids the role of different excitation processes is now as ever of special interest from both the theoretical and the experimental points of view. Depending on the primary energy the relative importance of different excitation mechanisms related to conduction as well as core electrons will be discussed for different simple metals. So far first principles results are available only for Al for primary energies up to 10 keV. Starting from a microscopic description of the SEE based on the transport equation formalism, calculations were performed for other nearly-free-electron metals (Mg, Be) up to primary energies used in scanning electron microscopy. In this way it is possible to obtain more general statements about the role of different excitation processes responsible for SEE. Special attention is devoted to the contribution of emitted electrons stemming from the excitation of conduction electrons by decay of bulk plasmons generated by the primary electron. The different strength of this excitation mechanism in Al, Mg, and Be is related to the different magnitude of the plasmon damping in these metals.

 

20. A. Dubus and M. Rösler
"Microscopic considerations concerning the relation between electron yield and stopping power: calculations for aluminium and carbon"
Nuclear Instruments and Methods in Physics Research B 115 (1996) 251

It has been evidenced experimentally that, for proton impact on a thick solid target, the ratio L of the backward electron yield g B and the electronic stopping power (dE /dx)e is approximately constant over a large energy range (a few keV to several MeV). The macroscopic theoretical models (Sternglass 1957; Schou 1980; ...) assume constancy of this ratio. However, from a microscopic point of view, there is no obvious reason for that. We present in this paper a microscopic calculation (microscopic cross sections and Monte Carlo simulation method for the transport) for proton induced electron emission from aluminium and amorphous carbon. The electron yields and the factors L from thick and thin (forward and backward emissions) targets are calculated and compared to experimental results.

 

21. C. Auth and H. Winter
"Image charge attraction of multicharged Xe-ions in front of insulator surfaces"
Physics Letters A 217 (1996) 119

Multicharged Xeq+-ions (q < 20) are scattered with keV energies under a grazing angle of incidence from the surface of two different insulators: LiF(100) and Kl(100). From the analysis of angular distributions for scattered projectiles we deduce energy gains caused by the dielectric response of the target material due to the presence of the external charge ("image charge interaction"). A direct comparison of the interaction energies gained by the projectiles on the incident path with the two different targets is consistent with the assumption of a dominant population of Radberg levels ("hollow atom") in front of insulators for grazing ion-surface scattering.

 

22 A. G. Borisov, V. Sidis, and H. Winter
"Diabatic Energy Level Confluence: The Mechanism of Negative Ion Conversion of Neutral Atoms in Grazing Scattering from Insulator Surfaces"
Phys. Rev. Lett. 77 (1996) 1893

The mechanism mediating electron transfer from an alkali halide surface to an atom by bringing diabatically the relevant atomic and surface energy levels into near resonance is elucidated. The mechanism is supported by parameter free calculations on a model F/LiF(100) system where all sites of the crystal lattice but one, the active site, are represented by (polarizable) point charges. The electron transfer interaction between the atom and the active F site of the surface is computed and used in dynamics calculations of negative ion formation in a sequence of binary collisions.

 

23. R. Pfandzelter, T. Igel, and H. Winter
"Intermixing during growth of Cr on Fe(100) studied by proton- und electron- induced Auger-electron spectroscopy"
Physical Review B 54 (1996) 4496

1- and 2-ML Cr films have been grown on Fe(100) under layer-by-layer conditions and studied using Auger electron spectroscopy induced by electrons and grazingly incident protons. Exploiting the different sampling depths for proton and electron excitation, we could quantitatively evaluate the layer-dependent concentration profiles of Cr and Fe near the film surface and their evolution in time. For a coverage of 1 ML, more than half of the deposited Cr atoms have undergone place exchange with Fe substrate atoms, which results in a mixed first and second layer. For 2 ML, the first and second layer consist predominantly of Cr. Long-time measurements show that as-grown 1-ML films are not stable.

 

24. A. G. Borisov, R. Zimny, D. Teillet-Billy, and J. P. Gauyacq
"Energy and lifetime of one-electron multicharged-ion states in front of an Al  surface"
Physical Review A 53 (1996) 2457

The energy and width of one-electron multicharged-ion states interacting with an Al surface are determined using the nonperturbative coupled angular mode method. The case of the He+, Li2+, and 07+ states is studied. The various states within the n manifolds mix to form hybrids with very different energies and widths. The hybridization within the manifolds can be analyzed in terms of Stark mixing.

 

25. A. Mertens, C. Auth, H. Winter, and A. G. Borisov
"Scattering of alkali-atoms and -ions from alkali-halide surfaces: do surface states within the band gap affect charge exchange?"
Physical Review A, im Druck

Fast alkali-atoms and -ions are scattered with keV-energies under grazing incidence from the surface of the alkali-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.

 

26. A. G. Borisov, D. Teillet-Billy, J. P. Gauyacq, H. Winter, and G. Dierkes
"Resonant charge transfer in grazing scattering of alkali-metal ions from an Al(111) surface"
Physical Review B 54 (1996) 17166

The nonperturbative coupled-angular-mode (CAM) method is applied to the treatment of the parallel velocity-assisted charge-transfer process in grazing scattering of alkali-metal ions at Al(111) surface. The neutralization of Na+, K+, and Li+ projectiles is studied. A multistate treatment of the problem including projectile ground and excited states is performed. Calculated neutral fractions in the scattered beam agree on a quantitative level with available experimental results over a wide range of scattering conditions. The role of the population of excited states is discussed.

 

27. H. Winter
"Image charge effects in ion surface scattering"
Journal of Physics: Condensed Matter 8 (1996) 10149

Image charge effects on the trajectories of fast ions during the grazing scattering from a solid surface are discussed. It is shown that these effects can be made use of in detailed studies on charge exchange phenomena in front of a surface. In some detail we shall outline recent progress in studies on 'skipping motion' and on the neutralization of multicharged ions.

 

28. R. Pfandzelter, T. Igel, and H. Winter
"Interfacial mixing of ultrathin Cr films on Fe(100) studied by Auger electron spectroscopy"
Surface Science, im Druck

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.

 

29. R. Pfandzelter, T. Igel, and H. Winter
"Initial growth of ultrathin Cr films on Fe(100)"

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 pseudomorph. 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.

 

30. H. Kerkow, V. P. Petukhov, E. A. Romanovski, R. Stolle, and S. Gelfort
"Energy dependence of the proton-induced convoy electron yield"
Vacuum 47 (1996) 433

In this work the convoy electron yield of protons penetrating a 60 nm carbon foil with energies in the range from 40 keV up to 400 keV was measured. A maximum yield at about 100 keV proton energy was found. The decrease of the yield to higher proton energies follows a power law with an exponent of n = 2.2. The convoy electron yield also decreases, while the fraction of neutralized projectiles increases. The results for energies above the maximum are discussed on the basis of electron capture by the protons on the backside of the foil.

 

31. M. Rösler, J. F. Garcia
"Contribution of Charge Transfer Processes to Ion-Induced Electron Emission"
Physical Review B 54 (1996) 17158

Charge changing events of ions moving inside metals are shown to contribute significantly to electron emission in the intermediate velocity regime via electrons coming from projectile ionization. Inclusion of equilibrium charge state fractions, together with two-electron Auger processes and resonant-coherent electron loss from the projectile, results in reasonable agreement with previous calculations for frozen protons, though a significant part of the emission is now interpreted in terms of charge exchange. The quantal character of the surface barrier transmission is shown to play an important role. The theory compares well with experimental observations for H projectiles.

 

32. M. Rösler, J. F. Garcia
"Ion-induced electron emission from simple metals: Charge state effects"
Nuclear Instruments and Methods in Physics Research B, im Druck

Charge changing events of ions moving inside metals contribute significantly to the stopping power as well as to the electron emission at intermediate velocities v » vo (vo, Bohr velocity). Their effect on the backward electron yield from Al bombarded by He ions is investigated here within the transport equation formalism. The excitation of target electrons by the moving ions and the generation of excited electrons by electrons loss from different charged species (He+, He0) are both taken into account. Excitation of projectile electrons by Auger processes and resonant-coherent electron loss from the projectile have been separately considered.

 

33. A. Dubus, M. Rösler
"Theoretical Study of Electron Emission Induced by Neutral Particles"
Nuclear Instruments and Methods in Physics Research B, im Druck

Charge state effects are known to be important for particle induced electron emission. Indeed, the charge state of the incident beam has a strong effect on the emission yields. Incident neutral hydrogen atoms can induce secondary electrons after an electron loss process. At high impact energies (above 200 keV) target electrons are excited by both the proton and the electron which can be treated in a first approximation as independent "primary particles". A more precise description should include different microscopic processes: (i) target electron excitation by the undissociated neutral H0, (ii) excitation of the projectile electron by the loss process accompanied by target electron excitation, (iii) excitation of target electrons by the proton and the electron. At high impact energies it is justified to neglect the capture processes. It is the aim of this paper to calculate the electron emission characteristics of aluminum including all these processes. The various microscopic processes are incorporated in a Monte-Carlo simulation code.

 

34. A. Mertens, C. Auth , and H. Winter
"Scattering of fast Na+-ions from alkali-halide surfaces under a grazing angle of incidence"
Nuclear Instruments and Methods in Physics Research B, im Druck

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.

 

35. H. Winter, M. Wilke, and M. Bergomaz
"Energy loss of fast protons in grazing scattering from an Al(111)-surface"
Nuclear Instruments and Methods in Physics Research B, im Druck

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 respection to the energy loss of the projectiles. We observe a linear increase of the energy loss with velocity up to v » 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.

 

36. C. Auth, A. Mertens, H. Winter, A. G. Borisov, and V. Sidis
"Formation of negative ions in grazing scattering from insulator surfaces"
Physical Review A, zur Veröffentlichung eingereicht

Substantial fractions of fast atoms or ions are converted to negative ions during grazing scattering from a clean and flat monocrystalline surface of alkali halides. We interpret the experimental data by a model of local electron capture from the halogen sites of the crystal in binary type of collision events. Due to the band gab of the insulator the probability for subsequent electron loss is low, resulting in large fractions of negative ions that survive from the collisional formation.

 

37. H. Winter and A. G. Borisov
"Formation of hydrogen atoms in grazing scattering from an aluminum surface"
Nuclear Instruments and Methods in Physics Research B 115 (1996) 211

Fast protons with keV-energies are scattered from an Al(111) surface under a grazing angle of incidence. We have observed angular distributions of scattered projectiles separated with respect to the final charge state of the particles. From our data we deduce an effect of the image charge interaction on the projectiles on the outgoing path which is clearly smaller than expected from previous theoretical treatments on charge exchange for hydrogen 1s in front of an aluminum surface. We have calculated with the non-perturbative CAM method resonant tunneling rates and atomic level shifts for different choices for the atom-surface potential and conclude that the neutralization has to be dominated by the Auger process in order to reproduce the experimental data.

 

38. J. Leuker, H.W. Ortjohann, R. Zimny, and H. Winter
"Capture of polarized electrons into excited terms of fast atoms during grazing scattering from a magnetized Fe(110)-surface"
Surf. Sci., zur Veröffentlichung eingereicht

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 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 framwork of some simple theoretical concepts.

 

39. T. Hecht, H. Winter, and B. McCullough
"New method to analyze fast metastable atomic beams"
Rev. Sci. Instr., zur Veröffentlichung eingereicht

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 from 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. Based on our experimental studies we propose the construction of a corresponding detector.

 

40. H. Winter, C. Auth, A. Mertens, A. Kirste, and M.J. Steiner
"Nonlinear effects in the stopping of slow ions in an electron gas"
Phys. Rev. Lett., zur Veröffentlichung eingereicht

From studies on the energy loss of fast atomic projectiles 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 position dependent stopping powers. For a constant projectile velocity v = 0.5 vo (energy » 6 keV/amu) we observe Z1-oscillations for the stopping powers which show a pronounced dependence on the distance from the surface plane as predicted by nonlinear density-functional calculations.

 

41. H. Winter
"Use of ion sources for highly charged ions in studies on atomic and ion-solid collisions" (Review)
Review of Sci. Instruments 67 (1996) 1674

Collisions of slow multicharged and highly charged ions, i.e., ions with kinetic energies up to some 10 keV, with atoms in the gas phase and with solids have been studied in detail over the last few years. A brief review is presented on some typical experiments and their results which make use of the specific features of highly charged ions in the fields of atomic spectroscopy, ion-atom collisions, and ion-solid interactions. In particular recent progress in the understanding of the interaction mechanisms of highly charged ions with solid surfaces will be discussed.

 

42. R. Pfandzelter, T. Igel, and H. Winter
"Anomalous thermal behavior of the Fe(100)-surface observed by grazing ion- surface scattering"
Phys. Rev. Lett., zur Veröffentlichung eingereicht

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.

 

43. A.G. Borisov, B. Bahrim, D. Teillet-Billy, J.P. Gauyacq, C. Auth, and H. Winter
"O--formation in grazing scattering from an Al(111)-surface"
Physical Review A, zur Veröffentlichung eingereicht

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 Oion is taken into account.

 

44. T. Igel, R. Pfandzelter, and H. Winter
"Growth and structure of ultrathin Mn films on Fe(001)"
Surf. Sci., zur Veröffentlichung eingereicht

Growth of ultrathin 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 five layers, the growth mode changes from layer (2D) to island (3D) growth. A quantitative analysis of the data yields the nucleation length in 2D growth and the 3D island density for the different growth temperatures.

 

45. H. Winter and J. Leuker
"Capture of polarized electrons into excited atomic states in grazing scattering of fast ions from a magnetized Ni(110)-surface"
Phys. Letters A, zur Veröffentlichung eingereicht

The spin polarization 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 a small polarization for the captured electrons.

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