Plenarvorträge - DPG-Tagungen

Oberflächenphysik Montag
ventionelle XSW–Datenanalyse.
[1] B.W. Batterman, H. Cole, Rev. Mod. Phys. 36, 681 (1964)
[2] J. Zegenhagen, Surf. Sci. Rep. 18, 202 (1993)
[3] C.R. Wang, B.H. Müller, K.R. Hofmann, Thin Solid Films 410, 72
(2002)
O 13.2 Mo 17:15 H45
Design and Construction of a Pulsed 10 keV-ps-Electron Gun —
•A. Janzen, B. Krenzer, and M. Horn-von Hoegen — Universität
Duisburg-Essen, Institut für Laser- und Plasmaphysik, 45117 Essen
Lately, there was tremendously growing success in the application of
ultrafast electron diffraction (UED). UED is becoming the up-to-date
technique to monitor time-dependent structural changes at surfaces subsequent
to excitation with ultraintense femtosecond laser pulses.
At present, we are designing and setting up a fs-laser driven
10 keV electron gun. The goal is the production of well collimated electron
pulses with pulse lengths below 1 ps. The electrons are created by
a 20...30 fs-laser pulse via photoemission from a thin Au or Ag film
deposited onto a sapphire substrate. This talk will cover the mechanisms
responsible for the broadening of the electron pulse in time as well as
methods to minimize or compensate for these effects. Furthermore, studies
on the suitable preparation of photocathodes and the simulation of
electron trajectories in a modified LEED/RHEED electron gun will be
presented.
O 14 Postersitzung (Adsorption an Oberflächen, Epitaxie und Wachstum, Organische
Dünnschichten, Oxide und Isolatoren, Phasenübergänge, Rastersondentechniken,
Struktur und Dynamik reiner Oberflächen)
Zeit: Montag 18:00–21:00 Raum: Bereich C
O 14.1 Mo 18:00 Bereich C
Adsorption properties of Ru-dyes on TiO2 — •M. Dürr, A.
Schmid, S. Rosselli, A. Yasuda, and G. Nelles — Materials Science
Laboratories, Sony Int. (Europe) GmbH, D - 70327 Stuttgart
Photosensitization of wide band-gap semiconductor surfaces with organic
dye molecules has led to the development of high-efficiency solar
cells [1]. Within these cells, which are mainly based on nano-crystalline
TiO2, fast electron transfer from the photo-excited dye to the conduction
band of the TiO2 takes place and the dye molecule is regenerated from
a redox couple in electrolyte. Therefore the number of dye molecules adsorbed,
the type of binding to the surface as well as the evolution of the
dye at the interface are crucial for the solar cell’s operation.
For a better understanding of degradation processes of the cell based
on dye desorption into the electrolyte, we have investigated the adsorption
properties of Ru-dyes on nano-porous TiO2 by means of FTIR
spectroscopy under ambient conditions. The type of binding of the dye
molecule via its carboxylic acid groups on the TiO2 has been determined.
The influence of the number of established bonds per molecule on the
desorption properties in ethanol was studied as a model system for the
desorption process. For comparison, dye molecules with a reduced number
of COOH groups have been investigated.
[1] B. O’Regan and M. Grätzel, Nature 353, 737 (1991).
O 14.2 Mo 18:00 Bereich C
Non-dipolar contributions in XPS: X-ray standing wave experiments
on ultrathin organic films — •A. Gerlach 1 , S. Sellner 2,3 ,
F. Schreiber 1 , H. Dosch 2,3 , I.A. Vartanyants 4 , J. Zegenhagen 5 ,
T. L. Lee 5 , and B.C.C. Cowie 5 — 1 Physical Chemistry Laboratory,
Oxford University, UK — 2 MPI für Metallforschung, Stuttgart, Germany
— 3 Institut für Theoretische und Angewandte Physik, Universität
Stuttgart, Germany — 4 Department of Physics, University of Illinois,
Urbana, USA — 5 ESRF, Grenoble, France
We study the adsorption behaviour of organic molecules by means
of the X-ray Standing Wave (XSW) technique. By measuring XPS and
Auger intensities this method allows the precise and direct determination
of absorbate/substrate geometries. Our XSW experiments with the
aromatic molecules PTCDA (perylenetetracarboxylic dianhydride) and
F16CuPc (perfluorinated copper-phthalocyanine) show that higher multipole
excitations can be non-negligible. In fact, the breakdown of the
dipole approximation, which can qualitatively change the signal, is a
fundamental issue in XPS in general and a challenge for theory. Depending
on the atomic number, orbital momentum, photon and initial state
energy these non-dipole contributions can significantly influence the resulting
effective coherent position and fraction. We discuss approaches to
the analysis of these fundamental effects that can be relevant for many
XSW applications.
O 14.3 Mo 18:00 Bereich C
Quantum dynamics of the H2 interaction with metal surfaces
— •Arezoo Dianat, Sung Sakong und Axel Groß — Physik-
Department T30, Technische Universität München, 85747 Garching, Germany
The adsorption and scattering of H2 on metal surfaces have been studied
by high-dimensional quantum dynamical simulations. The potential
energy surfaces (PES) of the H2-metal interaction were derived from ab
initio total-energy calculations. The open structure of the Pd(110) surface
leads to a strongly corrugated and anisotropic PES for H2/Pd(110).
This has significant consequences on the interaction dynamics as a function
of the angle of incidence and the rotational state. As one of the
consequences we predict rotational heating in desorption which has not
been observed before in H2 desorption from metal surfaces. In addition,
we obtain high intensities in the off-specular and rotationally inelastic
diffraction peaks. Furthermore, we will address the adsorption of H2 on
Rh(111) where experimentally the opening up of an additional adsorption
channel at higher kinetic energies has been predicted [1].
[1] M. Beutl, J. Lesnik, and K.D. Rendulic, Surf. Sci. 429, 71 (1999).
O 14.4 Mo 18:00 Bereich C
Far Infrared study of ultrathin metal films on MgO — •A.
Priebe, G. Fahsold, M. Lust, O. Skibbe, and A. Pucci —
Kirchhoff-Institut für Physik, Universität Heidelberg,
With far infrared spectroscopy it is possible to determine the dynamic
conductivity of ultrathin metal films and their change due to adsorbates.
Metal deposition strongly changed the reflectivity at the TO phonon frequency
of MgO. This effect leads to an enhanced sensitivity with respect
to the metal film charge carriers. Exposure of CO lead to further spectral
changes which are clearly visible at the TO frequency of MgO, too. From
the spectral structures we calculate adsorbate induced changes of the relaxation
rate of the free charge carriers in the metal. Also, we discuss
the influence of the polarization and of the incident angle of the infrared
beam on this spectral feature.
O 14.5 Mo 18:00 Bereich C
Adsorption energies on bimetallic overlayer systems at the
solid-vacuum and solid-liquid interface — •Ataollah Roudgar
and Axel Groß — Physik-Department T30g, Technische Universität
München, James-Franck-Strasse 1, 85747 Garching, Germany
We have determined the adsorption energies of atomic hydrogen and
of CO on bimetallic surfaces as a microscopic probe of the reactivity. The
calculations have been performed using density functional theory (DFT)
calculations within the generalized gradient approximation. For PdCu
bimetallic surfaces, we find a rather strong interaction between the Pd
and Cu atoms. Consequently, both Pd/Cu and Cu/Pd overlayer systems
exhibit an intermediate behavior between pure Cu and pure Pd. This is
in contrast to the sustem Pd/Au where both the expansion of the pseudomorphic
Pd overlayers and the relatively weak interaction between Pd
and Au lead to hydrogen and CO adsorption energies that are even larger
than on clean Pd surfaces.
Furthermore, we have studied the local reactivity of one layer pseudomorphic
Pd overlayers on Au(111) surface in the presence of a water
layer. We find a two-dimensional ice-like hexagonal structure of water on
the metal surface to be energetically most favorable. We also find that
the water-metal interaction is rather weak. The hydrogen and CO adsorption
energies are changed by less than 5% and 10%, respectively, by
the presence of water. This indicates that theoretical adsorption studies
at the solid-vacuum interface might also be relevant for the solid-liquid
interface.