|Aufstellung der Publikationen der Arbeitsgruppe 2007|
Aufgeführt sind Arbeiten, die im Jahr 2007 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).
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.
2. A. Schüller and H. Winter
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.
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.
4. D. Blauth, K. Maass, and H. Winter
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.
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.
6. H. R. Dunham, S. Wethekam, J.C. Lancaster, and F. B. Dunning
F.B. Dunning, S. Wethekam, H.R. Dunham, and J.C. Lancaster
8. S. Wethekam, A. Schüller, and H. Winter
9. S. Wethekam and H. Winter
10. S. Wethekam and H. Winter
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.
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.
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.
14. M. S. Gravielle, I. Aldazabal, A. Arnau, V. H. Ponce, J. E. Miraglia, F. Aumayr, S. Lederer, and H. Winter
Electron emission from LiF, KCl and KI crystal surfaces during grazing collisions of swift protons is studied both theoretically and experimentally. Owing to the localized character of the electronic structure of these surfaces we propose a model that allows us to describe the process as a sequence of atomic transitions from different target ions. Our calculations show good agreement with experimental data. The energy loss of projectiles and the role played by the charge of the incident particle are also investigated.
Angular distributions, fragmentation and charge fractions are studied for grazing scattering of C60+ fullerenes with keV energies from a clean and flat Al(001) surface. At low energies for the motion along the surface normal, C60+ ions are scattered nearly elastically, whereas for larger normal energies energie loss is substantial. We compare our experimental results with classical trajectory simulations exploiting the Tersoff potential between atoms in the cluster and different types of interaction potentials for the cluster with the surface. The internal energy of scattered clusters is deduced from the analysis of fragments. We observe that the loss of kinetic energy for the motion along the surface normal is almost completely transfered to internal excitations of the cluster, whereas the energy transfer to the metal surface is negligible. The charge state distributions for scattered projectiles can be understood by a full neutralization of incident ions at the surface and subsequent delayed electron emission.
Electron emission induced by impact of atomic particles on solid surfaces is of substantial interest in fundamental research and technological applications. As examples we mention particle detectors, surface analytical tools, display technology, or plasma wall interactions.
The structural and magnetic properties of FexMn single and Co/FexMn bilayers have been investigated by grazing ion scattering, Auger electron spectroscopy, low energy electron diffraction, and magneto-optical Kerr effect. The increase in coercivity of the Co hysteresis loops is used as a measure of the magnetic interface coupling in the bilayers. For FeMn films of 4 monolayers (ML), the onset of coupling is still detectable for temperatures below 160 K. The strength of the coupling rapidly increases with increasing FeMn thickness. At 8 ML FeMn appreciable interaction is observed at room temperature. For further increasing FeMn thickness, the Co films reveal a significant difference in the behavior of surface and bulk magnetization.
18. A. Schüller und H. Winter
Fast atoms with keV energies are scattered under a grazing angle of incidence from a clean and flat LiF(001) surface. For scattering along low index azimuthal directions within the surface plane ("axial surface channeling") we observe pronounced peak structures in the angular distributions for scattered projectiles which are attributed to "supernumerary rainbows". This phenomenon can be understood in framework of quantum scattering only and is observed here up to projectile energies of 20 keV. We demonstrate that the interaction potential and, in particular, its corrugation for fast atomic projectiles at surfaces can be derived with high accuracy.
Angular distributions, fragmentation, and charge fractions are studied for grazing scattering of C60+ fullerenes with keV energies from a clean and flat Al(001) surface. At low energies for the motion along the surface normal, C60+ ions are scattered nearly elastically, whereas for larger normal energies energy loss is substantial. We compare our experimental results with classical trajectory simulations exploiting the Tersoff potential between atoms in the cluster and different types of interaction potentials for the cluster with the surface. The internal energy of scattered clusters is deduced from the analysis of fragments. We observe that the loss of kinetic energy for the motion along the surface normal is transfered to internal excitations of the cluster, whereas the energy transfer to the metal surface is negligible. The charge state distributions for scattered projectiles can be understood by a full neutralization of incident ions at the surface and subsequent delayed electron emission.
20. K. Czerski, A. Huke, L. Martin, N. Targosz, D. Blauth, A. Gorska, P. Heide, and H. Winter
Observation of the enhanced electron screening in metallic environments is of fundamental importance for the understanding of strongly coupled astrophysical plasmas. Experimental screening energies determined by different groups for many metals are much larger than the theoretical predictions. However, a comparison between experimental and theoretical data is rather ambiguous because of contributions of systematic errors in the experiments. One of the most important problems is the uncertainty resulting from oxidation of the target surface during the measurements. Here, we present results of the first ultra-high vacuum (UHV) experiment s studying d+d nuclear reactions in a deuterized Zr target for which the experimental discrepancies are especially large. The total cross sections and angular distributions of the 2H(d,p)3H and 2H(d,n)3He reactions have been measured using a deuteron beam of energies between 8 and 30 keV provided by an electron cyclotron resonance (ECR) ion source with excellent long term stability. The cleanness of the target surface has been assured by combining Ar sputtering of the target and Auger spectroscopy. In an on-line analysis method, the homogeneity of the implanted deuteron densities could be monitored also . The resulting screening energy for Zr confirms the large value obtained in a previous experiment at poorer vacuum conditions.
A. Schüller, K. Gärtner, and H. Winter
Fast Ne atoms with energies from 3 to 70 keV are scattered under a grazing angle of incidence from a flat and clean LiF(001) surface. For scattering along low index directions (axial surface channeling) we observe pronounced peaks in the angular distributions of scattered projectiles which can be attributed to rainbow scattering. On the basis of classical trajectory calculations we derive from the rainbow angles the effective interaction potential of Ne atoms in front of LiF(001) and the underlying interatomic pair potentials in the eV energy range. Fair agreement with the data is found for individual pair potentials calculated from Hartree-Fock wave functions where the charge state of Li and F in the ionic crystal is taken into account.
23. D. D. Neufeld, H. R. Dunham, S. Wethekam, J. C. Lancaster, F. B. Dunning
The ionization of xenon Rydberg atoms excited to the lowest states in the n=20 Stark manifold at Si(100) surfaces possessing a robust (~ 10Å) native oxide layer is investigated. The data show that a sizeable fraction of the incident atoms are ionized relatively far from the surface through enhanced tunneling due to the presence of localized stray fields at the surface associated with surface charging or with surface inhomogeneities. A simple model is presented to justify this assertion.