Plenarvorträge - DPG-Tagungen

Oberflächenphysik Dienstag
tum mechanically while taking the feedback between nuclei and electrons
is into account self-consistently. The computational efficiency of
this method allows a more realistic multi-dimensional treatment of the
desorption processes. We have applied this method to the laser-induced
desorption of NO from NiO(100) using a two-state two-dimensional potential
energy surface derived from ab initio quantum chemistry calculations;
we have extended this potential energy surface to seven dimensions
employing a physically reasonable model potential. We focus on
the velocity, rotational and vibrational distributions of the desorbing NO
molecules and compare our results to the experiment and wave packet
simulations. Furthermore, we added a surface oscillator to model the energy
transfer to the substrate. Including recoil processes in the simulation
has a decisive influence on the desorption dynamics, as far as the velocity
and rotational distribution is concerned. In particular, the bimodality in
the velocity distribution observed in low dimensions and in the experiment
disappears in a high-dimensional treatment.
O 20.8 Di 13:00 H45
Influence of Mn and Co on the Properties of Active Sites in
Molybdena Based Catalysts; DFT Cluster Model Studies —
•Renata Tokarz-Sobieraj and Malgorzata Witko — Institute of
Catalysis and Surface Chemistry, Polish Academy of Sciences, ul. Niezapominajek
8, 30-239 Krakow, Poland
O 21 Hauptvortrag Moresco
Molybdenum oxide (VI) in combination with other elements such as
Bi, V, Co, or Mn forms a large group of materials (molybdates, molybdenum
bronzes, heteropolyacids) that are used as catalysts in many reactions
of very different types. In the present work the local electronic
and geometric structure of the clean and reduced manganese molybdate
(MnMoO4) as well as cobalt molybdate (CoMoO4) is discussed and compare
with pure MoO3 to examine the influence of the additional atoms
(Mn, Co) on chemical properties of active sites. In all studied systems
the nucleophilicity of surface oxygen (measured by its charge) scales with
the coordination number. Replacement of Mo atom by Mn or Co atoms
decreases the charge (nucleophilicity) of oxygen sites. The surface Mn
and Co ions form Lewis centers, which are less strong than the Mo ions;
in molybdates the presence of Mn and Co atoms increase the acidity of
Mo centers. In all studied systems metal-oxygen bonds are characterized
by mixed ionic-covalent character, where the Mn/Co-O bonds are more
covalent than Mo-O bonds.
Zeit: Dienstag 14:00–14:45 Raum: H36
Hauptvortrag O 21.1 Di 14:00 H36
Manipulation of large molecules by low temperature STM:
model systems for molecular electronics — •Francesca
Moresco — Institut für Experimentalphysik, FU Berlin, Arnimallee
14, 14195 Berlin
The ability of Low Temperature Scanning Tunneling Microscope to
manipulate atoms and to build nanostructures with atomic precision can
be extended to the manipulation of larger molecules and to selectively
modify their internal degrees of freedom. Manipulation experiments on
individual molecules show an exciting diversity of physical, chemical,
O 22 Hauptvortrag Hövel
and electronic phenomena. They permit a deeper insight into the quantum
electronics of molecular systems and provide important information
on the conformational and mechanical properties of single complex
molecules.
In this talk, some examples of recent manipulation experiments performed
on single molecules will be presented: The principle of a conformational
molecular switch have been realized under the action of the
STM-tip, the small intramolecular changes inside a complex molecule
have be recorded giving information on the internal mechanics of the
molecule, and the electronic contact between a molecular wire and a
metallic nanoelectrode have been investigated with atomic precision.
Zeit: Dienstag 14:45–15:30 Raum: H36
Hauptvortrag O 22.1 Di 14:45 H36
Electronic structure and morphology of supported clusters as
observed by photoemission and STM/STS — •Heinz Hövel —
Universität Dortmund, Experimentelle Physik I, D-44221 Dortmund
The investigation of clusters, i.e. small particles with nanometer dimensions,
in contact with surfaces is a presently very active new direction of
research after many years of successful work on free clusters in vacuum.
Understanding their electronic structure and answering the question of
how it is changed by the interaction with a surface is not only of fundamental
interest but has also several important applications, for example
in the fields of nano-electronics and catalysis. In general, the contact with
the surface also influences the morphology of the clusters. With scanning
tunneling microscopy (STM) one can measure the cluster height and the
O 23 Rastersondentechniken II
shape of facets on top of them. Scanning tunneling spectroscopy (STS)
is able to probe the electronic structure of individual clusters, while photoemission
averages over different sizes, isomers and orientations on the
surface. Metal clusters grown in preformed nanometer sized pits on a
graphite surface proved to be well suited for a combination of these different
techniques on one and the same sample in ultrahigh vacuum [1].
Recently, the combination of STM/STS and photoemission, both measured
at low temperatures, was used to reveal the existence of confined
Shockley surface states on the (111) facets on top of gold clusters with
about 10 4 atoms [2].
[1] H. Hövel, Appl. Phys. A 72, 295 (2001).
[2] I. Barke, H. Hövel, Phys. Rev. Lett. 90, 166801 (2003).
Zeit: Dienstag 15:45–18:15 Raum: H36
O 23.1 Di 15:45 H36
Near-field optical imaging of single molecules by means of
a triangular aperture probe — •D. Molenda 1 , G. Colas
des Francs 1 , U. C. Fischer 1 , H. Fuchs 1 , and A. Naber 2 —
1 Westfälische Wilhelms-Universität, 48149 Münster — 2 Institut für
Angewandte Physik, 76131 Karlsruhe
Recently we have introduced a triangular aperture probe for scanning
near-field optical microscopy (SNOM) that combines a high optical resolution
capability with a high transmission [1]. It turned out that the field
pattern of such a probe is highly confined to only one side of the trian-
gular aperture which is in strong contrast to a circular aperture. In order
to further investigate the field pattern, we performed measurements of
single dye molecules (TDI) embedded in a thin PMMA film. The aim was
to map the electrical field components of the aperture in 3 dimensions
by imaging a large number of randomly oriented molecular dipoles. The
measured fluorescence patterns are compared with theoretical calculations
which are based on a field-susceptibility technique. The calculated
field patterns are in good agreement with the experiments and thus allow
us to infer the dipole orientation of a molecule from the measured image.
Due to the highly confined field of the triangular aperture probe we are