Plenarvorträge - DPG-Tagungen

Oberflächenphysik Dienstag
O 25.8 Di 17:30 H39
Continuous Phase Transition of Surface Reconstructions
Investigated by X-ray Scattering: Theory and Experiment
— •Frank Grosse 1 , Bernd Jenichen 2 , Wolfgang Braun 2 ,
Vladimir M. Kaganer 2 , Dilip K. Satapathy 2 , X. Guo 2 , and
Klaus H. Ploog 2 — 1 Institut für Physik der Humboldt-Universität zu
Berlin, Newtonstr. 15, 12489 Berlin, Germany — 2 Paul-Drude-Institut
für Festkörperelektronik Hausvogteiplatz 5-7, 10117 Berlin, Germany
Atomistic simulations for the reconstruction transition on the
InAs(001) surface are performed on the basis of ab initio density functional
calculations [1]. A continuous surface phase transition of the Asrich
β2(2×4) reconstruction to the α2(2×4) reconstruction is predicted
leading to a mixture of both reconstructions. The atomic coordinates of
the simulation cells are used to calculate the X-ray diffraction intensity
employing kinematic theory. The equilibrium properties and kinetic effects
due to change in temperature as well as As-pressure are discussed.
Experiments performed at BESSY (Berlin) allow an in situ measurement
and structure analysis of the reconstruction phase transition and are in
agreement with the theoretical predictions. [1] F. Grosse, W. Barvosa-
Carter, J.J. Zinck, M. Wheeler, and M.F. Gyure. Phys. Rev. Lett. 89,
116102 (2002); F. Grosse and M. Gyure. Phys. Rev. B. 66, 075320 (2002).
O 25.9 Di 17:45 H39
Growth and morphology of ultrathin nickel films on Cu (001)
— •Leif Glaser 1 , Robert Nietubyć 1 , Alexander Föhlisch 1 ,
Julian Tobias Lau 1,2 , Michael Martins 1 , Matthias Reif 1 , and
Wilfried Wurth 1 — 1 Institut für Experimentalphysik, Universität
Hamburg, Luruper Chaussee 149, 22761 Hamburg — 2 EPFL, Lausanne,
Switzerland
We have studied the Ni L-X-ray Adsorption Fine Structure (XAFS)
for thin Nickel films that were evaporated on a Cu(001) substrate. The
measurements were made at the Beamline BW3 of the DORIS storage
O 26 Adsorption an Oberflächen II
ring at HASYLAB. The coverages ranged from 0.08 to 3.0 layers of Ni
and were calibrated by the ratio of Ni L3 to Cu L3 jump height and verified
by thermal desorption spectroscopy (TDS). We have found a clear
evolution of the XAFS with coverage. Furthermore we observed layer by
layer growth of the Nickel thin films. The changes in the x-ray adsorption
spectra are in very good agreement with the multiple scattering simulations
that were performed alongside the experiment. Surface alloying did
not take place upon annealing to 400K while at 450K clear evidence for
alloying was observed. Funding for this project by BMBF under contract
KS1 GUB/5 is gratefully acknowledged.
O 25.10 Di 18:00 H39
Growth of Cr on Ru(0001) studied by STM — M.P. Engelhardt
1 , M. Schmid 2 , A. Biedermann 2 , P. Varga 2 , •R.
Denecke 1 , and H.-P. Steinrück 1 — 1 Physikalische Chemie II,
Universität Erlangen-Nbg., Egerlandstr. 3, 91058 Erlangen, Germany
— 2 Inst. f. Allgemeine Physik, Technische Universität Wien, Wiedner
Hauptstr. 8- 10/134, 1040 Wien, Austria
We give an insight into the temperature dependent growth of Cr on
Ru(0001) using scanning tunnelling microscopy (STM). At room temperature
the growth is not purely layer-by-layer. Annealing of the resulting
Cr layers to 500 and 700 K leads to the formation of Cr islands with a
bcc(110)-like structure. A complete pseudomorphic Cr monolayer could
be prepared for evaporation at 700 K. By applying a countervoltage at
the end of the electron beam evaporator to suppress the Cr ions leaving
the evaporator smoother Cr layers are obtained. The formation of a Cr-
Ru surface alloy with a Cr:Ru ratio of 1:2 at 1000 K was observed; on this
alloy CO molecules adsorb predominately at Ru sites in on top geometry.
The conclusions are discussed in the light of Auger electron spectroscopy
and temperature- programmed desorption results [1]. Supported by the
Max-Buchner-Stiftung.
[1] M.P. Engelhardt et al., Surf. Sci. 512 (2002) 107.
Zeit: Dienstag 15:45–18:00 Raum: H45
O 26.1 Di 15:45 H45
STM and STS study of Cu(111)-Cs — •Thomas von Hofe, Jörg
Kröger, and Richard Berndt — Institut für Experimentelle und
Angewandte Physik, Universität Kiel, D-24098 Kiel, Germany
Alkali-covered noble metal surfaces have been of theoretical and experimental
interest for a long time since these are the simplest of metal-metal
systems. Here we present first results of Cs-covered Cu(111) obtained by
STM and STS experiments at 9K for coverages between approximately
0.3 and 0.9 ML. Cs does not grow in homogeneous films as one would
expect, but a kind of island growth takes place. These islands reveal a
p(2x2) LEED pattern and show a quantum well state whose lifetime has
been measured.
O 26.2 Di 16:00 H45
Rastertunnelmikroskopie und -spektroskopie an einzelnen
Kobalt-Carbonylen auf Cu(100) — •Peter Wahl 1 , Ari P.
Seitsonen 2 , Lars Diekhöner 1 , M. Alexander Schneider 1 ,
Gero Wittich 1 und Klaus Kern 1 — 1 MPI für Festkörperforschung,
Heisenbergstr. 1, 70569 Stuttgart — 2 Phys. Chem. Institut, Universität
Zürich, Schweiz
Der Einfluß von Adsorbaten auf die magnetischen Eigenschaften von
einzelnen Adatomen auf der Cu(100) Oberfläche wird untersucht. Als
magnetische Sonde benutzen wir die Kondo-Resonanz in der Nähe der
Fermi-Energie. Die Moleküle werden durch Reaktion von Kobalt mit
Kohlenmonoxid im UHV auf der Oberfläche präpariert und dann im
Rastertunnelmikroskop (STM) bei 6K untersucht. Die Moleküle können
durch kontrollierte Dissoziation analysiert und identifiziert werden. Die
auf den Molekülen gemessenen Spektren werden mit denen von einzelnen
Kobaltadatomen verglichen. Die Ergebnisse werden mit Hilfe von
DFT-Rechnungen interpretiert.
O 26.3 Di 16:15 H45
Non-adiabatic spin effects in O2 dissociation at Al(111) — •Jörg
Behler 1 , Sönke Lorenz 1 , Bernard Delley 2 , Karsten Reuter 1 ,
and Matthias Scheffler 1 — 1 Fritz-Haber-Institut, Faradayweg 4-6,
D-14195 Berlin — 2 Paul Scherrer Institut, CH-5232 Villigen PSI
The experimentally well established low initial sticking coefficient of
thermal oxygen molecules impinging on an Al(111) surface is still lacking
a physical explanation. The calculation of the six-dimensional potential
energy surface using density functional theory (DFT) followed by
extensive molecular dynamics (MD) simulations results in a far too high
sticking probability due to the absence of energy barriers in this adiabatic
description. By including a constrained DFT method we extend
this standard approach to describe the spin triplet state of the oxygen
molecule and the singlet state of the metal surface. This is a generalization
of the fixed spin moment approach, and it enables us to investigate
the effect of non-adiabatic processes in the dissociative adsorption. For
the total energy surface of the spin triplet O2 we find energy barriers up
to 0.5 eV for most molecular orientations agreeing well with the experimental
data. MD simulations on this surface yield a significantly lower
sticking probability than obtained in the adiabatic case.
O 26.4 Di 16:30 H45
CO-H interaction and H-dissolution probed by SFG and TDS:
Specific differences between Pd nanoparticles and Pd(111) —
•Günther Rupprechter, Matthias Morkel, and Hans-Joachim
Freund — Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg
4-6, 14195 Berlin, Germany
CO hydrogenation and H bulk dissolution on Pd model catalysts were
studied by sum frequency generation (SFG) vibrational spectroscopy and
thermal desorption spectroscopy (TDS) at pressures ranging from ultrahigh
vacuum (UHV) to 1 bar. Well-facetted Pd nanoparticles (ca. 5
nm), grown on Al2O3/NiAl(110), as well as Pd(111) were employed as
model surfaces and examined using a UHV-compatible high-pressure cell.
Although the Pd nanoparticles mostly exhibt (111) faces, their adsorption/reaction
behaviour is quite different from Pd(111).
Strong differences with respect to the adsorption of the individual
molecules were observed, e.g. an easier formation of a hydride phase on
Pd nanoparticles. Even more pronounced differences occurred for CO-H
coadsorbate structures under UHV and mbar pressure which are strongly
related to the formation of bulk H. It is demonstrated that the dynamic
adsorbate structure of a high-pressure reaction can not be mimicked un-