|Publikationen der Arbeitsgruppe in Physical Review Letters|
In a recent paper [A. J. Window et al., Phys. Rev. Lett. 107, 016105 (2011)], it was proposed that V2O3(0001) is terminated by the so-called O3 termination, a reconstruction with a terminating distorted hexagonal oxygen layer. We show that the surface is terminated by vanadyl (V=O) groups instead. This conclusion is based on quantitative low-energy electron diffraction combined with scanning tunneling microscopy, fast atom scattering, and density functional theory employing the Heyd-Scuseria-Ernzerhof functional. New insights into the subsurface sensitivity of ion beam triangulation show that results previously interpreted in favor of the O3 termination are reconcilable with vanadyl termination as well.
We demonstrate that quantum scattering of fast atoms and molecules under grazing angles of incidence can be exploited to study the structure of organic molecules on metal surfaces. Making use of keV H and He atoms as well as H2 molecules, the surface structures of the chiral amino acid alanine adsorbed on a Cu(110) surface is studied. We present a detailed investigation on the (3×2) phase of a monolayer of enantiopure and racemic alanine on Cu(110), revealing the formation of an elongated surface unit cell of c(n × 2) symmetry with n = 3.16 ± 0.04 for the sticking out methyl groups of the alanine molecules.
In studies on two structures of oxygen adsorbates on Mo(112), we demonstrate the potential of Fast Atom Diffraction (FAD) to derive the surface unit cell size and its symmetry. Helium atoms with energies of 1-2 keV are scattered from an adsorbate covered Mo(112) surface along low indexed surface directions under grazing angles of incidence. From the observed diffraction patterns, the lateral periodicity of the surface structures is derived. In addition to the periodic lattice, information on the structure within the unit cell can be obtained from double slit type of interference.
30. J. Lienemann, A. Schüller, D. Blauth, J. Seifert, S. Wethekam, M. Busch, K. Maass, and H. Winter
The coherence for diffraction effects during grazing scattering of fast hydrogen and helium atoms from a LiF(001) surface with energies up to some keV is investigated via the coincident detection of two-dimensional angular distributions for scattered projectiles with the projectile energiy loss. From the analysis of data we identify electronic excitations of the target surface as an important mechanism for decoherence and for the transition from quantum to classical scattering. The suppression of electronic excitations owing to the band gap of an insulator plays an essential role for preserving quantum coherence and thus for the application of fast atom diffraction as surface analytical tool.
The structure of a monolayer silica film on a Mo(112) surface is investigated by grazing scattering of 25 keV H0 atoms. By detection of the number of projectile induced emitted electrons as function of azimuthal angle of rotation of the target surface, the geometrical structure of atoms forming the topmost layer of the silica film is determined via ion beam triangulation. From our data we find evidence for the arrangement of surface atoms in terms of a two-dimensional Si-O-Si network model.
Fast atoms with energies from 500 eV up to several keV are scattered under a grazing angle of incidence from a Fe(110) surface covered with defined superstructures of sulphur or oxygen atoms. For scattering along low index azimuthal directions of the superstructures within the surface plane under "axial surface channeling" we observe defined diffraction patterns in the angular distributions for scattered projectiles. From the analyses of those patterns we derive the widths of low indexed axial channels and the corrugation of the interaction potential across these channels. This allows us to estimate the positions of adsorbed atoms on the Fe(110) surface.
27. 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 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.
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.
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.
The structure and magnetism of ultrathin Fe films expitaxially grown on a Cu(001) surface are investigated by grazing scattering of fast H and He atoms or ions. By making use of a new variant of ion beam triangulation based on the detection of the number of emittede electrons we obtain direct information on the structure of the film surface. We observe for room temperature growth a dominant and defined fcc-like structure. Complex surface reconstructions as reported in recent STM and LEED studies are only observed for cooling and H2 dosing of the film.
The magnetization reversal of ultrathin Co films on Cu(001) has been investigated by grazing ion scattering and magneto-optical Kerr effect. Differences in the behavior of surface and bulk magnetization are found and attributed to the reduced coordination and site symmetry at the surface. The reversal behavior of the surface magnetization depends on the chemical surface composition. For pure Co films, the reversal of the bulk magnetization is preceded by a complete reversal of the surface magnetization. A particular magnetic state of the surface is suggested as precursor for magnetization reversal.
R. Pentcheva, K. A. Fichthorn, M. Scheffler, T. Bernhard, R. Pfandzelter,
and H. Winter
We present a combined theoretical and experimental study of island nucleation and growth in the deposition of Co on Cu(001) - a prototype for understanding heteroepitaxial growth involving intermixing. Experimentally ion scattering is employed. Using density-functional theory, we obtain energy barriers for the various elementary processes and incorporate these into a kinetic Monte Carlo program to simulate the heteroepitaxial growth. Both the simulations and the experiments show a unique N-shape dependence of the island density on temperature that stems from the interplay and competition of the different processes involved.
He+ ions as well as neutral He atoms with keV energies are scattered under a grazing angle of incidence from a clean and atomically flat Ag(111) surface. From a comparison of ion fractions observed after scattering of He+ ions and He atoms we find for energies below some keV small but defined fractions of ions that have survived the complete scattering event with the surface. This feature allows us to clear up the microscopic interaction scenario for Auger neutralization of He+ ions at a Ag(111) surface. The Auger neutralization rates are 2 to 3 orders of magnitude smaller than conventional rates derived from experiments for He+-metal systems and agree with recent calculations.
Total target currents for grazing scattering of keV protons from a crystal target are used to investigate the structure of surfaces and ultrathin films. This current shows pronounced maxima whenever the azimuthal incidence angle coincides with close-packed rows of atoms in the surface and subsurface layers. The real-space method is applied to study monolayer and bilayer films of Mn and of CoMn epitaxially grown on a Cu(001) surface.
P. Roncin, A.G. Borisov, H. Khemliche, A. Momeni, A. Mertens, and H. Winter
For slow F+ ions (v < 0.1 a.u.) scattered from a clean and flat LiF(001) surface under a grazing angle of incidence large fractions of negative F- ions in the reflected beams are observed, whereas no negative ions are found for neutral F0 projectiles. From detailed studies on projectile energy loss and charge transfer, we find evidence for a correlated double-electron capture process in the formation of the F- ions.
We report on first spin-resolved energy spectra for the emission of electrons during grazing scattering of 150 keV multicharged nitrogen ions from a magnetized Fe(001) surface. A substantial spin-polarization for KLL Auger electrons emitted in the final stage of the neutralization sequence during the interaction of multicharged ions with a metal surface is observed. We conclude from our data that the projectile L-shell is dominantly populated by electrons from the conduction band of the target. For low energy electrons we find an increase of their spin polarization with increase of the projectile charge.
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.
T. Hecht, H. Winter, A.G. Borisov, J.P. Gauyacq, and A.K. Kazansky
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.
J.I. Juaristi, C. Auth, H. Winter, A. Arnau, K. Eder, D. Semrad, P. Bauer,
F. Aumayr, and P.M. Echenique
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.
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.
J. I. Juaristi, A. Arnau, P. M. Echenique, C. Auth, and H. Winter
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.
We have measured the energy loss of slow protons scattered with energies from 300 eV to 28 keV from a clean and flat LiF(001)-surface under a grazing angle of incidence. Our data reveal a threshold behaviour of stopping at low projectile energies. The effect on the outgoing charge state indicates that electron capture and loss are dominant mechanisms for the stopping of slow protons in a wide-band-gap insulator. Our data allows one also to deduce information on charge transfer in front of the surface of an insulator.
We have scattered protons and hydrogen atoms with energies of some keV from a LiF(001) surface under a grazing angle of incidence. From the intensity of Lyman-a radiation (transition from n = 2 to n = 1, l = 121.6 nm) as a function of projectile energy for different azimuthal orientations of the crystal surface, we find clear evidence for a resonant coherent excitation of n = 2 states of hydrogen atoms in the oscillating electric field in front of the insulator surface.
9. A. G. Borisov, V. Sidis, and H. Winter
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.
Oxygen atoms and ions with energies ranging from 2 to 100 keV are scattered at grazing angles of incidence from a LiF(100)surface. We analyze the charge states of the scattered beams as a function of the projectile energy. The most striking result of our study is the observation of negative ion fractions of up to about 70 %. We interpret these large fractions of O- ions to capture in binary-type collisions of fluorine 2p electrons from the target and the subsequent suppression of loss for these captured electrons due to the energy gap of the insulator.
Multicharged xenon ions are scattered with keV energies from a LiF(100) surface under a glancing incidence. From the angular distributions of scattered projectiles, we deduce the existence of an attractive force acting on the projectiles on the incident path. We interpret this as the dielectric response of the insulator due to the presence of an ion. The interaction energies gained on the incident trajectory are reproduced by an "overbarrier" model of stepwise capture of electrons from the fluorine 2p band of LiF into Rydberg levels of the projectile.
The energy loss of 50 to 250 keV proton scattered under single-collision conditions from He atoms is investigated in terms of its dependence on the angle of scattering. At the higher projectile energies we observe an enhanced energy loss at scattering angles around 0.5 mrad. Such a behavior cannot be understood on the basis of two-body scattering models. Based on our theoretical studies we show that the combined effects of the screened target potential and of electronic transitions have to be considered for the energy loss of proton scattering in light gases.
Xeq+ ions with charge up to q=33 and energies 3.7q keV are scattered under a grazing angle of incidence from a clean and flat Al(111) surface. Because of the image charge interaction the ions are accelerated on the incident path towards the surface plane which results in increased effective angles of incidence for the scattered projectiles. From the angular distributions for reflected neutralized projectiles we deduce the image charge interaction energies gained by the incident ions in front of the surface. Our data are in fair agreement with a q3/2 dependence for the image energies as predicted from a simple classical overbarrier model.
We report on a new method to perform rf-resonance spectroscopy of stable terms in atoms and ions. Fast 14N+ ions are scattered under grazing incidence from a flat surface, where stable atomic terms are populated preferentially and polarized with respect to their orbital angular momenta and nuclear spins. By a modification of this polarization via magnetic dipole transitions induced by a resonant rf field, we have measured the hf structures of most stable terms in 14N i,ii,iii and deduce from our data the nuclear electric quadrupole moment of 14N with improved precision: Q(14N)=20.0±0.2 mb.
3. H. Winter, H. Hagedorn, H. Nienhaus, R. Zimny, and J. Kirschner
We have observed the capture of polarized electrons into excited terms of atoms after the interaction of fast ions with a magnetized Fe(110) surface at grazing incidence. The spin polarization of captured electrons results in a modified circular polarization fraction of fluorescence light. This experiment has considerable potential as a new analytical tool for investigating surface magnetism with extreme surface sensitivity, as a method for detailed studies of ion-surface interaction, and as a means to produce nuclear spin-polarized beams.
2. H. Winter, M. Langheim, A. Schirmacher, R. Zimny, and H.J. Andrä
The orientation of angular momenta in the ground terms of 14N atoms after the interaction of 350-keV 14N+ ions with a solid surface at grazing incidence is investigated by a Zeeman quantum-beat technique. After the ion-solid interaction, a term-selective and highly polarized fast beam of nitrogen atoms is observed. The phenomenon is interpreted in terms of a Pauli-principle-induced selective population.
In order to observe energy-level splittings in free ions, we used a new technique based upon the superposition of a fixed-frequency c.w. laser beam and of a post-accelerated (or -decelerated) fast ion beam. Narrow spectral features observed in fluorescence under these conditions are easily identified within a simple hole-burning model. Implications of our results for high-resolution spectroscopy of free ions are discussed and an application is given in the case of hyperfine-structure measurements.