Aufstellung der Publikationen der Arbeitsgruppe 2006

Aufgeführt sind Arbeiten, die im Jahr 2006 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. H. Winter
"Electron emission during grazing impact of atoms on metal surfaces"
in "Photonic, Electronic, and Atomic Collisions", eds. P.D. Fainstein et al., World Scientific, Singapore 2006
, p. 641

Electron emission from metal surfaces induced by grazing impact of fast atoms is studied via the coincident detection of energy loss off scattered projectiles with the number of electrons emitted during a scattering event. We will show that this method combined with the feature of well defined trajectories located in front of the surface allows one to reveal details on the electronic excitation and emission mechanisms. It turns out that binary type of collisons between atomic projectiles and conduction electrons play the dominant role for electron emission from metal surfaces. For scattering of keV He atoms from an Al(111) surface we derive information on the momenta of fermi electrons in the selvedge of the surface and on the effective electronic surface potential. Energy loss spectra can be described in term of collisions of low energy electrons with free atoms.

 

 2. M. Busch, M. Gruyters, and H. Winter
"Spin polarization and structure of thin iron oxide layers prepared by oxidation of Fe(110)"
Surface Science 600 (2006) 4166

The structure and magnetism of thin iron oxide layers formed on a Fe(110) single crystal surface are investigated by auger electron spectroscopy, ion beam triangulation, and spin polarized secondary electron emission. The formation of a FeO(111)-like film on top of the metal substrate is observed for oxidation at elevated temperatures. Additional oxygen exposure at room temperature is suggested to lead to a gradual conversion to Fe3O4.

 

 3. M. Baron, T. Bernhard, M. Gruyters, and H. Winter
"Studies on scattering of fast H and He atoms from Fe films grown on Cu(001)"
Surface Science 600 (2006) 3924

The structure of ultrathin epitaxial Fe films grown on Cu(001) at room temperature is investigated by grazing scattering of fast H and He atoms. By making use of ion beam triangulation, direct information on the atomic structure of the film and substrate surfaces are obtained. We discuss a new variant of this method based on the detection of the number of emitted electrons. The data is analyzed via computer simulations using classical mechanics which provides a quantitative analyses with respect to projectile trajectories.

 

 4. T. Bernhard, M. Baron, M. Gruyters, and H. Winter
"Structure and magnetism of bcc(1 1 0)-like Fe films grown on Cu(001) via ion beam triangulation and spin-polarized electron capture"
Surface Science 600 (2006) 1877

The structure and magnetism of thin epitaxial Fe layers grown on Cu(001) is investigated by grazing scattering of fast H and He atoms. Information on the atomic structure of the film and substrate surfaces is obtained by making use of ion beam triangulation. The magnetic behavior is studied via the polarization of light emitted after capture of spin-polarized electrons into excited atomic terms during He scattering. For the formation of bcc(110)-like Fe films at higher coverages, we detect differences in structural and magnetic properties for room and low temperature growth. We suggest that the crystalline structure depends on the film morphology and that Cu impurities affect the magnetic properties.

 

 5. HP. Winter, H. Winter, and S. Lederer
"Fermi momentum above metal surface from electrons ejected by impact of He atoms"
Europhysics Lett. 75 (2006) 964

Kinetic emission (KE) of electrons near its threshold is studied for grazing impact of keV He atoms on flat and clean Al(111), Al(110), and Cu(111) surfaces. We observe a defined dependence of the KE threshold behaviour on the scattering conditions. The data are analyzed in the framework of a classical model for electron emission based on momentum transfer in binary encounters between atomic projectiles and conduction electrons of the metal target. From the impact velocity dependence of KE yields near threshold we obtain information on momenta of fermi electrons as function of distance from the surface and on the electronic surface potential.

 

 6. S. Lederer and H. Winter
"Energy loss of 100-keV hydrogen atoms during grazing scattering from Cu(111)"
Phys. Rev. A 73 (2006) 054901

The energy loss of 100 keV hydrogen atoms scattered under a grazing angle of incidence from a clean and flat Cu(111) surface is studied in coincidence with the number with the number of emitted electrons. From the dependence of the energy loss on the angle of incidence we deduce the position dependent stopping power which shows the same functional dependence but a factor of about two higher values than recent calculations. From the number of emitted electrons related to a specific energy loss we reveal processes related to electron emission as dominant channel for energy dissipation of scattered projectiles.

 

 7. S. Wethekam and H. Winter
"Auger neutralization of He ions at an Al(100) surface using isotope effect"
Phys. Rev. Lett. 96 (2006) 207601

He atoms and ions of the isotopes 3He and 4He are scattered with keV energies under a grazing angle of incidence from a flat and clean Al(100) surface. For the two isotopes we investigate Auger neutralization of incident He+ and He2+ ions via fractions of surviving ions. Pronounced effects for the different isotopes are observed which can be attributed to different time scales concerning the neutralization process of He ions in front of a metal surface. From the analysis of the data obtained for singly and doubly charged ions we find evidence that charge fractions for scattering of He+ ions from an Al surface result predominantly from a single (Auger) electron capture event.

 

 8. S. Wethekam, H. R. Dunham, J. C. Lancaster, and F. B. Dunning
"Charge-transfer rates for xenon Rydberg atoms at a metal surface"
Phys. Rev. A 73 (2006) 032903

The ionization of xenon Rydberg atoms excited to the lowest states in the n =17 and n =20 Stark manifolds at a flat Au(111) surface is investigated. Despite the strong perturbations in the energies and structure of the atomic states that occur as the surface is approached, it is shown that, under appropriate conditions, each incident atom can be detected as an ion and that the experimental data can be well fit by assuming that the ionization rate on average increases exponentially as the surface is approached. The ionization rates are compared to theoretical predictions.

 

 9. M. Busch, Th. Ludwig, R.Drozdowski and G. von Oppen
"Anticrossing spectra of 3He and 4He atoms excited by He+-He collisions"
J. Phys. B 39 (2006) 1929

Singlet-triplet anticrossing spectra of the helium isotope 3He have been measured for the first time. These anticrossing spectra differ strongly from those of the isotope 4He due to the hyperfine interaction. We exploited this difference for distinguishing between the excitation of target atoms and the formation of excited projectile atoms by electron capture in experimental investigations on the symmetric collision system He+-He.

 

 10. M. Busch, M. Gruyters, and H. Winter
"FeO(111) formation by exposure of Fe(110) to atomic and molecular oxygen"
Surface Science 600 (2006) 2778

The growth, structure, and morphology of ultrathin iron oxide layers formed on a Fe(110) single crystal surface are investigated by Auger electron spectroscopy, low energy electron diffraction, and grazing ion scattering. For Fe oxidation by atomic instead of molecular oxygen, the gas exposure can be reduced by almost two orders of magnitude because surface sticking and dissociation are not limiting the growth process. A well-ordered FeO(111) film with low defect density is only obtained with atomic oxygen. Compared to the bulk, the FeO lattice is laterally compressed by about 5-6% resulting in an in-plane oxygen (Fe) nearest-neighbor distance of 2.87 Å. Independent of the preparation method, long range structural order is poor if the oxide film thickness is increased to 3-5 layers. This is attributed to the relatively large lattice mismatch between FeO(111) and Fe(110).

 

11. M. Busch, M. Gruyters, and H. Winter
"Spin polarization and structure of molecular oxygen adsorbed on Fe(110)"
Surface Science 600 (2006) 4598

We investigated the structural and magnetic properties of Fe(110) for molecular oxygen adsorption at room temperature. For the c(2x2) and c(3x1) superstructures, spin-polarized secondary electron emission (SPSEE) induced by protons and electrons reveals a nearly unchanged polarization compared to clean Fe(110). An appreciable decrease in polarization is found for the disordered layer of oxygen at a coverage Θ≈1. This decrease is more pronounced for the spin polarization determined by electron capture (EC) to excited atomic He levels after grazing scattering. From a comparison of data obtained by proton-induced SPSEE and spin-polarized EC we conclude that the polarizazion at the vacuum bundary vanishes for an oxygen coverage Θ≈1 while the polarization in the underlying Fe substrate layers is hardly changed.

 

12. S. Lederer, H. Winter, HP. Winter, and F. Aumayr
Electronic excitations during grazing scattering of hydrogen atoms on KI(001) and LiF(001) surfaces"
Europhys. J. D, im Druck

The energy loss of hydrogen atoms with energies of 400 eV and 1 keV is studied in coincidence with the number of emitted electrons during grazing scattering from atomically clean and flat KI(001) and LiF(001) surfaces. The energy loss spectra for specific numbers of emitted electrons are analyzed in terms of a binary interaction model based ob the formation of transient negative ions via local capture of valence band electrons from anion sites. Based on computer simulations we derive for this interaction scenario probabilities for the production of surface excitons, for electron loss to the conduction band of KI, for emission of electrons, and for formation of negative hydrogen ions. The pronounced differences of data obtained for the two surfaces are attributed to the different electronic structures of KL and LiF.

 

 13. V. K. Valev, M. Gruyters, A. Kirilyuk and Th. Rasing
"Temperature dependence of magnetization-induced second harmonic generation at buried exchange biased interfaces"
Phys. Rev. B 73 (2006) 233101

We have investigated the temperature dependence of magnetization-induced optical second harmonic generation (MSHG) in the exchange-biased CoO/Cu/Fe multilayer. Below the blocking temperature, there is a strong MSHG response from the CoO/Cu interface which, for large exchange bias values, equals and even dominates the contribution of the ferromagnetic Fe interfaces. In a previous publication we showed that there is a correlation between exchange bias and the MSHG asymmetry. Here we demonstrate the relationship between the second harmonic Kerr rotation and exchange bias and, based on that, we offer additional evidence that pinned uncompensated spins are present beyond the range of Cu spacer thickness where a hysteresis loop shift is observed.

 

 14. V. K. Valev, M. Gruyters, A. Kirilyuk and Th. Rasing
"Direct observation of exchange bias related uncompensated spins at the CoO/Cu interface"
Phys. Rev. Lett. 96 (2006) 067206

Magnetization-induced optical second harmonic generation (MSHG) from the exchange-biased CoO/Cu-( X )/Fe multilayer shows the presence of pinned uncompensated spins at the CoO/Cu interface. For increasing Cu spacer thickness, the exchange bias measured via the hysteresis loop shift diminishes and disappears at X =3.5 nm, while the MSHG signal still shows a strong magnetic contribution from the CoO interface. This indicates that the magnetic interaction between Fe and CoO layers is sufficiently strong to induce order in the antiferromagnetic layer even at a spacer thickness for which there is no observable hysteresis loop shift.

 

15. A. Schüller and H. Winter
"Interaction potentials for Na, Al, and Ar atoms from rainbow scattering under axial surface channeling from Al(111) and Al(001) surfaces"
Nucl. Instr. Meth. B, im Druck

Na, Al, and Ar atoms with energies ranging from 3 to 80 keV are scattered from Al(111) and Al(001) surfaces under axial surface channeling conditions. Well defined peaks in the angular distributions for scattered projectiles are attributed to "rainbow-scattering" and are analyzed in terms of the effective scattering potential. Computer trajectory simulation of the rainbow angles with established pair-potentials leads to good agreement for potential energies above about 30 eV. At smaller potential energies we observe pronounced deviations for the three different projectile atoms which are attributed to an additional contribution to the overall potential due to the effect of embedding the atoms into the electron gas in front of the metal surface. Taking into account this additional potential leads to a consistent description of data.

 

16. HP. Winter, F. Aumayr, C. Lemell, J. Burgdörfer, S. Lederer, and H. Winter
"Kinetic electron emission by grazing atom scattering from clean flat metal surfaces"
Nucl. Instr. Meth. B, im Druck

Kinetic electron emission (KE) induced by grazing scattering of relatively slow (typically keV/amu) ground state atoms on atomically clean monocrystalline flat metal surfaces has been investigated by coincident measurements of the projectile energy loss with the number of ejected electrons (electron number statistics). For this grazing scattering geometry the projectiles remain at well defined trajectories above the first atomic layer of the surface, and the coincident electron detection permits precise measurements of even very small KE yields (<10-4 electrons per projectile). This is of importance for studying KE near and below its "classical" impact velocity threshold, which corresponds to the minimum momentum transfer from a projectile to quasi-free metal electrons necessary for exciting them from the Fermi edge into vacuum. The projectile energy is dissipated in weak excitations of a large number of conduction electrons while only a small fraction of the dissipated energy will result in ejection of electrons into vacuum. The distinctly different KE threshold behaviour for impact of He atoms on Al(111), Al(110), and Cu(111) surfaces provides information on the Fermi electron momenta as a function of distance from the surface, from which the respective electronic surface potentials can be derived. For impact of heavier Ne and Ar atoms small but definite "subthreshold KE" yields are observed below the "classical" KE threshold, which can be explained by higher momentum components in the local spectral density of electronic states above the surface.
Change in surface scattering from planar to axial channeling exhibits a clear change of the electron number statistics, which can be applied in a novel way for surface structure determination ("surface triangulation").

 

17. HP. Winter, F. Aumayr, H. Winter, and S. Lederer
"Recent advances in slow heavy particle induced electron emission"
Mat. Fys. Medd. Dan. Vid. Selsk. 52 (2006) 525

We present recent developments for electron emission induced by impact of slow (projectile velocity ≤ 1 a.u. = 2,18 x 106 m/s) atoms, molecules, and singly and multiply charged ions on atomically clean monocrystalline metal and insulator surfaces. We show, in particular, that with grazing incident projectiles on monocrystalline flat surfaces the coincident measurement of projectile energy loss with the number of emitted electrons electron yields caused by potential and kinetic emission can be distinguished. Furthermore, for grazing impact of neutral ground state atoms on monocrystalline flat metal surfaces a very precise determination of the small total electron yield near the kinetic emission threshold can be achieved, and the measured yields are in good agreement with a classical model for electron emission from binary collisions of projectiles with quasi-free metal electrons above the target surface. We also present some results on slow molecular projectile non-proportionaliy effects in kinetic emission. Finally, we mention as two novel applications of KE a surface structure determination based on KE by grazing-incident ions or atoms, and a method for evaluations of mixed ion beam fraction for different ion species with nearly equal charge-to-mass ratios.

 

18. D. Blauth, K. Maass, and H. Winter
"Compact detector for studies on electron number distributions"
Nucl. Instr. Meth B., zur Veröffentlichung eingereicht

The design and operation of a compact detector for recording the number of electrons emitted during collisions of atomic projectiles with solid targets is discussed. The electrons are detected by means of a surface barrier detector (SBD) biased to a voltage of typically 20 to 30 kV, where the pileup of signals from individual electrons results in pulse heights proportional to the number of emitted electrons. The detector can be operated in ultra high vacuum for studies on electron emission phenomena during impact of fast atomic projectiles or ions on metals, semiconductors, or insulators. As an example we discuss work on ion beam triangulation of ultrathin Al2O3 films formed on a NiAl(110) surface.

 

19. H. Winter
"Probing surface magnetism with ion beams"
Nucl. Instr. Meth. B, im Druck

Ion beams can be used to probe magnetic properties of surfaces by a variety of different methods. Important features of these methods are related to trajectories of atomic projectiles scattered from the surface of a solid target and to the electronic interaction mechanisms in the surface region. Both items provide under specific conditions a high sensitivity for the detection of magnetic properties in the region at the topmost layer of surface atoms. This holds in particular for scattering under planar surface channeling conditions, where under grazing impact atoms or ions are reflected specularly from the surface without penetration into the subsurface region. Two different types of methods are employed based on the detection of the spin polarization of emitted or captured electrons and on spin blocking effects for capture into atomic terms. These techniques allow one to probe the long range and short range magnetic order in surface region.

 

20. H. R. Dunham, S. Wethekam, J.C. Lancaster, and F. B. Dunning
"Ionization of Xenon Rydberg Atoms at Si(100) Surfaces"
Nucl. Instr. Meth. B, im Druck


The ionization of xenon Rydberg atoms excited to the lowest states in the n = 17 and n = 20 Stark manifolds at Si(100) surfaces is investigated.  It is shown that, under appropriate conditions, a sizable fraction of the incident atoms can be detected as ions.  Although the onset in the ion signal is perturbed by stray fields present at the surface, the data are consistent with ionization rates similar to those measured earlier at metal surfaces.

 

21. F.B. Dunning, S. Wethekam, H.R. Dunham, and J.C. Lancaster
"Charge Transfer Rates for Xenon Rydberg Atoms at Metal and Semiconductor Surfaces"
Nucl. Instr. Meth. B, im Druck


Recent progress in the study of  charge exchange between xenon Rydberg atoms and surfaces is reviewed. Experiments using Au(111) surfaces show that under appropriate conditions each incident atom can be detected as an ion and that the data can be well fit by assuming that, on average, the ionization rate increases exponentially as the surface is approached. The ionization dynamics, however, are strongly influenced by the perturbations in the energies and structure of the atomic states that occur as the ion collection field is applied and as the atom approaches the surface. These lead to avoided crossings between different atomic levels causing the atom to successively assume the character of a number of different states and lose much of its initial identity. The effects of this mixing are disscussed. Efficient surface ionization is alos observed at Si(100) surfaces although the ion signal is influenced by stray fields present at the surface.

 

22. S. Wethekam, A. Schüller, and H. Winter
"Grazing scattering of fast C60+ ions from an Al(001) surface under axial surface channeling"
Nucl. Instr. Meth. B, im Druck


10 keV C60+ clusters are scattered from a clean and flat Al(001) surface under a grazing angle of incidence. The angular distributions of clusters are observed as function of the azimuthal angle of rotation of the target close to the <110> direction in the surface plane. For sufficiently small impact energies with respect to the motion along the surface normal, fractions of fragmented clusters are small, and projectiles are specularly reflected from the topmost layer of surface atoms. For a slight detuning of the azimuthal angle of incidence with respect to the <110> direction, scattered C60 clusters show a characteristic azimuthal deflection in the opposite direction to the sense of rotation of the target surface. The data are analyzed in terms of computer simulations based on classical trajectories calculated from an effective interaction potential for C60+ clusters in front of the surface.

 

23. S. Wethekam and H. Winter
Study on interactions of He atoms and ions with an Al(111) surface
Nucl. Instr. Meth. B, im Druck


He+ ions and He0 atoms with keV energies are scattered under a grazing angle of incidence from an Al(111) surface. Fractions of surviving ions and energy gains of ions prior to neutralization are derived from charge state distributions for scattered projectiles and shifts of angular distributions for impact of charged and neutral projectiles. By comparing the experimental data with theoretical predictions for the interaction potential, the neutralization rate, and the He 1s level shift we reveal deficiencies of the present theoretical understanding for the interaction of He atoms and ions with Al(111), a "model system" for the study of atom-surface interactions. We demonstrate that our data is very sensitive on interaction potentials, the (Auger) neutralization rate, and the He 1s level shift. Our work provides important experimental input for in-depth tests of theoretical models for the interaction of He atoms and ions with Al(111).

 

24. S. Wethekam and H. Winter
"Effect of thermal vibrations on grazing scattering of atoms from crystal surfaces"
Nucl. Instr. Meth. B, im Druck


The influence of thermal vibrations of target atoms and their correlations on grazing scattering of keV atoms from crystal surfaces is investigated. Classical 2D and 3D trajectory calculations are performed and thermal vibrations are included using the Debye model as well as its high-temperature limit and uncorrelated thermal vibrations. Ion beam triangulation, rainbow structures for scattering along low-indexed directions of the surface plane, and polar angular distributions for impact along high-indexed directions are studied. We demonstrate that ion beam triangulation can be analyzed using uncorrelated thermal vibrations, while a realistic description of angular distributions requires more detailed modeling. For the peak positions of rainbow structures thermal vibrations play a minor role and they are well described by 2D simulations based on continuum potentials.

 

 25. I. Konovalov, L. Makhova, R. Mitdank, D. Wett, K.-H. Hallmeier, and R. Szargan
"X-ray photoconductivity due to trap-sensitive relaxation of hot carriers"
Appl. Phys. Lett. 88 (2006) 222106

We observed the temperature-dependent modulation of the electrical conductivity in ZnO thin films under periodic illumination by soft x rays. At specific temperatures, small variations of the excitation energy near the x-ray absorption edges resulted in large element-specific variations of the conductivity modulation. The emission rate of electrons at traps E1/E2 and E3/E4 in ZnO at these specific temperatures roughly equals the excitation frequency. We conclude that relaxation of electrons, excited from localized core levels into the conduction band, predominantly happens into trap states with the same localization. The experimental results were explained using symmetry selection rules and local transition probabilities.

 

26. A. Schüller and H. Winter
"Interatomic potentials between noble gas and Ag atom from axial surface channeling"
Nucl. Instr. Meth. B, im Druck

He, Ne, and Ar atoms with energies from 3 to 60 keV are scattered from a Ag(111) surface under axial surface channeling conditions. From peaked structures in the angular distributions owing to "rainbow-scattering" we derive effective scattering potentials. We compare the observed rainbow angles with results from computer simulations using different types of established interatomic potentials and focus on a detailed discussion for potential energies larger than 10 eV. It turns out that an approximation proposed by O'Connor and Biersack using a Moliere potential with an adjustment of the screening length (NIM B 15 (1986) 14) is in good agreement with the experimental data. In our simulations we investigate the sensitivity of the rainbow angles on a variation of the interatomic potential.

 

27. S. Lederer, H. Winter, and HP. Winter
"Energy loss and electron emission during grazing scattering of fast noble gas atoms from an Al(111) surface"
Nucl. Instr. Meth. B, im Druck

Electron loss and electron emission for grazing impact of noble gas atoms with energies in the keV domain are investigated via time of flight spectra recorded in coincidence with the number of emitted electrons. The data is analyzed in terms of computer simulations concerning the interaction of the fast atoms with the electron gas in the selvedge of the Al(111) surface. The interaction is approximated by binary collisions of the fast atoms with Fermi electrons of the conduction band and differential cross sections obtained for electron scattering from free atoms. For an effective number of collisions of about 50 the energy loss spectra are fairly well reproduced by our calculations. We show that for our condotions the shift of the energy spectra for the emission of an additional electron from the surface is close to the work function of the target.

 

28. A. Schüller, S. Wethekam, and H. Winter
"Diffraction of fast atomic projectiles during grazing scattering from a LiF(001) surface"
Phys. Rev. Lett., im Druck

Light atoms and molecules with energies from 300 eV to 25 keV are scattered under a grazing angle of incidence from a LiF(001) surface. For impact of neutral projectiles along low index directions for strings of atoms in the surface plane we observe a defined pattern of intensity spots in the angular distribution of reflected particles which is attributed to diffraction effects at the ordered crystal surface. The data is consistently described using concepts of diffraction theory and specific features of grazing scattering of atoms from insulator surfaces. Experimental results for scattering of H, D, 3He and 4He atoms as well H2 and D2 molecules can be unequivocally referred to atom diffraction with de Broglie wavelengths as low as about 0.001 Å. For ions showing a considerable energy loss no diffraction effects are detected.

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