Plenarvorträge - DPG-Tagungen

Oberflächenphysik Dienstag
der the static conditions of a UHV experiment.
O 26.5 Di 16:45 H45
Photodesorption of small molecules adsorbed on ultrathin Ag
— •Olaf Weisse, Claudia Wesenberg, and Eckart Hasselbrink
— FB Chemie, Universitaet Duisburg-Essen, Universitaetstr. 5, 45117
Essen, Germany
Since the early 1990, Ag is known to grow on the Si(100) surface in a
layer-by-layer fashion. The resulting surface forms an epitaxial Ag(111)
layer. These ultrathin Ag films(20-100ML) are ideally suited to study the
elektron dynamics in such films and the influence of the film thickness
on charge transfer to the adsorbate and subsequent photodesorption. In
our experiments, NO2 is chosen as the probe molecule. Its adsorbtion is
performed at 95 K on the Ag-covered Si(100) surface. Photodesorption
is induced using a Nd:YAG laser (2.3eV/3.5eV/4.6eV) and the desorbed
molecules are detected by QMS. The film thickness is varied in order
to obtain information about the charge transfer dynamics between the
ultrathin Ag film and the NO2 adsorbate. The photodesorption crosssection
of NO2 is expected to decrease with increasing film thickness.
We belive that the photodesorption cross-section will increase due to hot
electrons generated in the semiconductor.
O 26.6 Di 17:00 H45
Surface plasmons of metal islands and their influence on the infrared
absorption of adsorbates — •A. Priebe, G. Fahsold, and
A. Pucci — Kirchhoff-Institut für Physik, Universität Heidelberg,
The surface enhanced infrared absorption (SEIRA) of adsorbates on
nano-structured metal films are widely used in chemistry and biology. For
the first monolayer the typical asymmetric SEIRA line shape can be explained
by an interaction of the adsorbate with electron-hole pairs of the
metal film. But asymmetric SEIRA lines are observed also for vibrations
in multilayers. From experimental results it is known that the asymmetry
and the enhancement of SEIRA lines show a strong dependency on
the filling factor of the metal in the island film. In this work we show
how both, the asymmetry and the enhancement could be explained with
respect to their change with filling factor. Experimental SEIRA spectra
from adsorbates on nanoparticle arrays with ordered facets can be
described by a theory that only considers the interaction of adsorbate
vibrations with surface plasmons of the metal film.
A. Priebe, M. Sinther, G. Fahsold, and A. Pucci J. Chem. Phys. 119
(2003), 4887
O 26.7 Di 17:15 H45
Adsorption of CO on the (10¯10) surface of Rhenium — •C.
Pauls, D. Przyrembel, and K. Christmann — Inst. f. Chemie, FU
Berlin, Takustr. 3, 14195 Berlin
We have performed LEED, thermal desorption (TD), and work function
change (∆Φ) measurements of CO interacting with the ’open’
Re(10¯10) surface. Upon exposure at 200 K, negatively polarised (∆Φ ≈
780 meV) molecular α states are populated. TD spectra reveal these
states between 250 and 400 K (Edes= 95...118 kJ/Mol) and, in addition,
pronounced β states due to dissociated CO between 600 and 800 K. Direct
CO exposure at 500 K exclusively yields dissociated CO as evidenced by
i) 2 nd order desorption kinetics at low coverages (Edes= 207 kJ/Mol) and
O 27 Gaedepreisvortrag Sander
ii) the formation of a p(1x2) LEED phase at a CO coverage of Θ= 0.25.
carbon]. Between 0.125 and 0.16 ML an additional c(2x4) phase appears.
During desorption, both phases undergo an order-disorder transition. In
the coverage range of the p(1x2) phase the respective TD spectra show a
common leading edge (zero-order kinetics) until the critical temperatur
of ∼ 660 K is reached.
O 26.8 Di 17:30 H45
Electronic properties and hydrogen absorption of Pd and Ni:O
nano-clusters — •Andreas Borgschulte, Ruud J. Westerwal,
Sven de Man, Bernard Dam, Jan H. Rector, and Ronald
Griessen — Vrije Universiteit, FEW - Vaste-stoffysica, De Boelelaan
1081, 1081 HV Amsterdam
We studied by scanning tunneling microscopy the electronic and geometric
structure of Pd and Ni nano-clusters grown on oxidized yttrium
surfaces. The catalytic activity of Pd and Ni nano-clusters is estimated
from the optically monitored switching kinetics of the underlying yttrium
layer. The electronic structure depends strongly on the cluster
size and changes after hydrogenation. Small Pd clusters are insulating as
evidenced by tunneling spectroscopy. The critical thickness of the metalinsulator
transition coincidences with the minimal thickness for a significant
catalytic Pd activity. Below the critical thickness nano-clusters have
lost the special catalytic properties of (bulk) Pd. In contrast to Pd, Nisurfaces
oxidize in normal environment (air). In that case the interplay
between hydrogen and oxygen chemisorbed on Ni clusters as well on crystalline
surfaces is studied. Surfaces with a partially reduced oxide show
the catalytically highest activity. The results are compared with X-ray
diffraction studies on the interaction of hydrogen with NiO. We discuss
the relevance of the results for a search of novel catalysts for hydrogen
storage and fuel cells, in particular the role of oxides as catalysts.
O 26.9 Di 17:45 H45
Coadsorption of H and CO on Pd(210) — •Christian Mosch,
Markus Lischka, and Axel Groß — Physics-Department, Technical
University of Munich, D-85747 Garching, Germany
The study of the interaction of molecules with stepped surfaces is of
importance in order to close the structure gap between surface science
and heterogeneous catalysis. We have focused on the adsorption of hydrogen
[1,2] and CO on Pd(210) by performing density functional theory
(DFT) calculations within the generalized gradient approximation. The
(210) surface can be regarded as being built up by small (100) terraces.
CO is known to adsorb on Pd(100) in an upright fashion at the bridge
position [3,4]. On Pd(210), there are two inequivalent bridge positions.
And indeed, we find a relatively small corrugation in the CO adsorption
energies with the two bridge sites being energetically almost degenerate.
CO is furthermore known as a strong poison in heterogeneous catalysis.
We have therefore also addressed the coadsorption of CO with atomic
hydrogen. There is a strong inhibition of the hydrogen adsorption due to
the presence of CO which is analyzed in terms of the electronic structure
of the adsorbate system.
[1] P.K. Schmidt et al., Phys. Rev. Lett. 87, 096103 (2001).
[2] M. Lischka and A. Groß, Phys. Rev. B 65, 075420 (2002)
[3] A. M. Bradshaw and F. M. Hoffmann, Surf. Sci. 72, 513 (1978).
[4] A. Eichler and J. Hafner, Phys. Rev. B 57, 10110 (1998).
Zeit: Mittwoch 15:15–16:00 Raum: H1
Hauptvortrag O 27.1 Mi 15:15 H1
Stress on the atomic scale: From surface reconstruction to stress
oscillations and magnetostriction of atomic layers — •Dirk
Sander — Max-Planck-Institut für Mikrostrukturphysik, Weinberg 2,
D-06120 Halle
Mechanical stress is an important factor, which governs a number
of fundamental aspects in surface science, film growth, and magnetic
anisotropy. Examples are e.g. the surface reconstruction of clean and
adsorbate-covered surfaces, oscillatory stress relaxation in nanosize islands,
and the magnetoelastic coupling in ferromagnetic monolayers.
Stress measurements by the cantilever bending technique are presented
for each example. Film stress in the GPa range, in excess of the tensile
strength of high-strength materials, is measured for epitaxially strained
layers. Structural relaxation due to island growth and coalescence, and
surface alloy formation are comparatively subtle effects, which are detected
by the respective stress signature. The in situ combination of
stress measurements during film growth with magnetoelastic stress measurements
upon magnetization reversal identifies a strain-induced change
of the magnetoelastic coupling constants from the respective bulk value.
The impact on the peculiar magnetic anisotropy of ferromagnetic layers
is discussed.