Aufstellung der Publikationen der Arbeitsgruppe 1999

Aufgeführt sind Arbeiten, die im Jahr 1999 publiziert und zur Verööffentlichung 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. R. Pfandzelter, T. Igel, and H. Winter
"Capture of polarized electrons during scattering of He+-ions from clean and Mn-covered Fe(100)-surfaces"
J. Magn. Magn. Mat. 192 (1999) 43

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.


 2. A. G. Borisov, D. Teillet-Billy, J. G. Gauyacq, A. Mertens, C. Auth, H. Winter, and J. A. M. C. Silva
"Formation of negative halogen ions in grazing scattering from an A(111) surface"
Phys. Rev. B59 (1999) 8218

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.


 3. T. Igel, R. Pfandzelter, and H. Winter
"Surface Magnetism of Ultrathin Mn- and Cr-Films on Fe(100) studied via capture of polarized electrons by He+-ions"
Appl. Surf. Sci. 142 (1999) 532

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.


 4. R. Pfandzelter, T. Igel, M. Ostwald, and H. Winter
"Fe(100) homoepitaxy studied by grazing scattering of fast ions"
Appl. Surf. Sci. 142 (1999) 470

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.


 5. J. I. Juaristi, A. Arnau, P. M. Echenique, C. Auth, and H. Winter
"Charge state dependence of the energy loss of slow highly charged ions in the electron gas of a metal"
Phys. Rev. Lett. 82 (1999) 1048

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.


 6. T. Igel, R. Pfandzelter, and H. Winter
"Spin polarized electron capture during ion-impact on a ferromagnetic surface"
Phys. Rev. B59 (1999) 3318

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.


 7. R. Pfandzelter
"Origin of  intensity oscillations in grazing ion scattering during epitaxial growth: a computational approach"
Surf. Sci. 421 (1999) 263

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.


8. H. Kerkow, R. Stolle, V. P. Petukhov, and E. A. Romanovskii
"Electron emission from thin foils induced by slowly moving heavy ions"
Nucl. Instr. Meth. B, zur Veröffentlichung eingereicht

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.


9. H. Kerkow, R. Stolle, V. P. Petukhov, and E. A. Romanovskii
"Secondary electron emission from thin carbon foils during atomic and molecular ion bombardment"
Phys. stat. solid., zur Veröffentlichung eingereicht

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 [1]. 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.


 10. T. Igel, R. Pfandzelter, and H. Winter
"Surface magnetism of ultrathin Cr, Mn, and Fe films on Fe(100) studied via electron capture in grazing ion surface collisions"
Nucl. Instr. Meth. B157 (1999) 286

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 is estimated.


 11. R. Pfandzelter
"Submonolayer homoepitaxy on Fe(100) studied by grazing ion-surface scattering:experiments and computer simulations"
Nucl. Instr. Meth. B157 (1999) 291

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 density.


 12. J. I. Juaristi, A. Arnau, P. M. Echenique, C. Auth, and H. Winter
"Charge state dependence of the energy loss of slow nitrogen ions reflected from an aluminum surface under grazing incidence"
Nucl. Instr. Meth. B157 (1999) 87

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.


13. J. P. Gauyacq, A. G. Borisov, and H. Winter
"Negative ion formation in collisions on metal surfaces: Equivalent versus non equivalent electron description of the negative ions"
Comments At. Mol. Phys., im Druck

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.


 14. T. Hecht, R. Pfandzelter, H. Winter, and A. G. Borisov
"Charge exchange of fast metastable He-atoms in front of an Al(111)-surface"
Nucl. Instr. Meth. B157 (1999) 82

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 reference.


15. H. Kerkow, R. Stolle, V. P. Petukhov, and E. A. Romanovskii
"Thickness dependence of the secondary electron yield form thin carbon foils during heavy ion bombardment"
Phys. stat. solid., zur Veröffentlichung eingereicht

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.


 16. H. Winter, C. Auth, T. Hecht, and A. Mertens
"Charge exchange and excitation phenomena in front of insulator surfaces"
Nucl. Instr. Meth. B157 (1999) 32

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.


17. A. G. Borisov, A. Mertens, H. Winter, and A. K. Kazansky
"Evidence for the stopping of slow ions by excitations of optical phonons in insulator"
Phys. Rev. Lett., im Druck

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.


18. R. Pfandzelter, T. Igel,and H. Winter
"Real-time study of nucleation, growth, and ripening during Fe/Fe(100) homoepitaxy via ion scattering"
Phys. Rev. Lett., zur Veröffentlichung eingereicht

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.


19. H. Winter
"Scattering of atoms and ions from insulator surfaces"
Progress in Surface Science, im Druck

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.


 20. J. Schmidt, D.H. Rapoport, and H. J. Fröhlich 
"Microwave-frequency alternating current scanning tunneling microscopy by difference frequency detection: atomic resolution imaging on graphite"
Rev. Sci. Instr. 70 (1999) 3377

We present a detailed description of an experimental setup for alternating current tunneling microscopy (ACSTM), in which two slightly detuned high frequency signals are mixed at the tunneling junction and the resulting difference frequency signal is amplified using conventional STM electronics. This signal is used to control the distance between the microscope tip and the sample. With graphite as a model surface atomic resolution images have been obtained. It is demonstrated that the origin of the generated signal on graphite is the nonlinearity of the static I-V characteristics.


 21. M.R.C. Hunt, J. Schmidt, and R.E. Palmer
"Electron-beam induced fragmentation in ultra-thin C60 films on Si(100)-2x1-H: Mechanisms of cage destruction"
Phys. Rev. B60 (1999) 5927

We demonstrate that irradiation of ultra-thin (1-4 monolayer) films of C60 grown on Si(100)-2x1-H with 3.3 and 0.5 keV electron beams leads to substantial modification of the fullerene molecules and their ultimate destruction.
The fragmented fullerenes coalesce under continued electron irradiation to form an amorphous material withgraphitic local structure. No measurable desorption of fullerene or derived fragments is observed under the irradiation conditions used. At electron doses below those at which complete fullerene destruction occurs we observe evidence indicating polymerisation, due to electronic excitation of the molecules, and fullerene hydrogenation by atomic hydrogen liberated by the incident electron beam. An examination of possible fragmentation mechanisms resulting from larger electron doses leads to the conclusion that decay of electronic excitations into vibrational modes of the molecule is the most likely route through which cage destruction occurs.


22. H. Winter, C. Auth, and A. Mertens
"Stopping of low energy protons during grazing scattering from a LiF(001) surface"
Nucl. Instr. Meth. B, im Druck

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.


23. D.M. Danailov, R. Pfandzelter, T. Igel, and H. Winter
"Multimodal angular distributions in grazing scattering of fast He atoms from Fe(001) surfaces"
Nucl. Instr. Meth. B, im Druck

We measured angular distributions in grazing scattering of 10 keV Heo0-atoms incident along the [100] 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.


24. J.I. Juaristi, C. Auth, H. Winter, A. Arnau, K. Eder, D. Semrad, P. Bauer, F. Aumayr, and P.M. Echenique
"Unexpected behaviour of the stopping of slow ions in ionic crystals"
Phys. Rev. Lett., im Druck

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.


 25. J. Schmidt, D.H. Rapoport, G. Behme, and H.J. Fröhlich
"Microwave-mixing scanning capacitance microscopy on pn-junctions"
J. Appl. Phys. 86 (1999) 7094

We describe a new approach to scanning capacitance microscopy.  A mixing technique is employed for imaging local capacitance variations simultaneously with the sample topography using an atomic force microscope (AFM) with a conductive tip.  A SiO2/Si sample with lateral pn-junctions formed by ion-implantation has been investigated. Microwave signals incident on the metal-oxide-semiconductor (MOS) structure formed by the AFM tip and the sample give rise to mixing signals due to the non-linear voltage dependence of the space charge capacitance in the Si. In our experiments two microwave input signals with frequencies f1 and f2 and a variable DC bias voltage were applied to the tip-sample MOS structure. The dependence of the generated sumfrequency and  third harmonic signals on the DC sample voltage shows that the f1+f and 3f signals are proportional to dC/dV and $d^2 C/d V^2, respectively. Images of the sumfrequency and third harmonic signals delineating the pn-junctions on our model sample are presented and the DC bias voltage dependence of the images is discussed.


26. T. Hecht, H. Winter, A.G. Borisov, J.P. Gauyacq, and A.K. Kazansky
"Role of band gap and 2D surface state continuum in charge transfer in front of a Cu(111) surface"
Phys. Rev. Lett., im Druck

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.


27. C. Auth and H. Winter
Resonant coherent excitation of hydrogen atoms during grazing scattering from a LiF(001) surface"
Phys. Rev. A, zur Veröffentlichung eingereicht

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 lineshape of the resonance.


28. H. Eder, A. Mertens, K. Maass,  H. Winter, HP. Winter, and F. Aumayr,
"Electron emission during grazing Ho - LiF(001) collisions"
Phys. Rev. Lett., zur Veröffentlichung eingereicht

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.


29. A. Mertens and H. Winter
"Energy transfer of fast atomic projectiles to a crystal lattice under channeling"
Phys. Rev. Lett., zur Veröffentlichung eingereicht

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.