Plenarvorträge - DPG-Tagungen
Plenarvorträge - DPG-Tagungen
Plenarvorträge - DPG-Tagungen
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Oberflächenphysik Freitag<br />
O 45.3 Fr 11:45 H39<br />
Reflexions-Flugzeitspektrometer für Elektronen-<br />
Koinzidenzspektroskopie — •Carsten Winkler, Gwilherm<br />
Kerherve und Jürgen Kirschner — Max-Planck-Institut für<br />
Mikrostrukturphysik, Weinberg 2, 06120 Halle<br />
Ein Reflexions-Flugzeitspektrometer für die Elektronen-<br />
Koinzidenzspektroskopie (e2e) an Oberflächen wird vorgestellt.<br />
Das Gerät zeichnet sich durch seinen kompakten Aufbau und die<br />
einfache Betriebsweise aus. Abhängig von dem Abstand zwischen Probe<br />
und Spektrometer können entweder die Flugzeitauflösung oder der<br />
erfassbare Raumwinkel optimiert werden.<br />
Für die folgenden Experimente wurde das Spektrometer mit einer<br />
Misch-Optimierung betrieben: die von der Oberfläche gestreuten Elektronen<br />
driften zunächst über eine Strecke von ca. 190 mm in Richtung Spektrometer<br />
(Raumwinkel: 0.3 sterad), wo sie dann eine rotationssymmetrische<br />
Beschleunigungs- (ca. 190 mm) sowie die Reflexionseinheit (20 mm)<br />
durchlaufen. Der Nachweis der reflektierten Elektronen erfolgt mittels eines<br />
Multichannelplates (∅ 20 mm). Mit der verwendeten Elektronenquelle<br />
können so typischerweise Flugzeitauflösungen von 2.7 ns und 1.9 ns bei<br />
kinetischen Energien von 40 eV bzw. 10 eV erreicht werden. Dies entspricht<br />
Energieauflösungen von 1.8 und 0.6 eV. Die Spektrometergeometrie,<br />
die optimalen Betriebsparameter und die charakteristischen Grössen<br />
wie Zeit- und Energieauflösung werden diskutiert. Darüberhinaus werden<br />
erste Flugzeitspektren von Koinzidenzmessungen an Cu(110) und<br />
Co/Cu(110) vorgestellt.<br />
O 45.4 Fr 12:00 H39<br />
Modification of adsorbate substrate coupling by means<br />
of metallic-thin-film overlayers — •Marlies Wessendorf,<br />
Carsten Wiemann, Michael Bauer, and Martin Aeschlimann<br />
— Department of Physics, University of Kaiserslautern, D-67663<br />
Kaiserslautern<br />
We investigated the lifetime of Caesium atoms adsorbed on thin Ag<br />
film overlayers on Cu(111) by means of time-resolved two-photon photoemission<br />
spectroscopy (TR-2PPE). Direct and indirect interaction with<br />
the silver overlayer gives rise to significant changes in the lifetime of the<br />
adsorbate excitation. The electronic band structure of the Ag / Cu(111)<br />
substrate can be controlled by variation of the Ag interface layer thickness<br />
in a distinct way. Tuning the layer thickness allows to adjust the<br />
excited states lifetimes of the adsorbate and, hence, gives the possibility<br />
to control specific properties of chemical surface reactions such as efficiency<br />
and reaction channel. In this paper we report our recent results<br />
for Cs adsorption on Ag / Cu(111) overlayer systems. The experimental<br />
results contain information about the dynamics of the decay of the investigated<br />
intermediate adsorbed state as well as of the motion of the alkali<br />
atoms perpendicular to the surface after excitation.<br />
O 45.5 Fr 12:15 H39<br />
Time-Resolved Laser-Synchrotron 2 Photon Photoemission —<br />
•Annette Pietzsch, Franz Hennies, Vijayalakshmi Sethuraman,<br />
Alexander Föhlisch, and Wilfried Wurth — Institut für<br />
Experimentalphysik, Universität Hamburg, Luruper Chaussee 149, 22761<br />
Hamburg<br />
Time-resolved two-Photon Photoemission (2PPE) allows the investigation<br />
of unoccupied electronic states by populating them with a pump<br />
pulse and probing them with a delayed probe pulse. There, the intensity<br />
from a certain intermediate state is measured as a function of the variable<br />
delay time between the two pulses. Since optical lasers have a limited<br />
energy range, excitation with a laser pulse followed by subsequent probing<br />
of a synchrotron pulse offers the great advantage of being tunable<br />
O 46 Grenzfläche fest-flüssig<br />
in energy. This technique of time-resolved laser-synchrotron 2PPE has<br />
been performed in a proof-of-principle experiment at the MBI-Beamline<br />
at BESSY.<br />
The ns-electron dynamics at the BaF2/Si(001) interface has been investigated<br />
revealing the relaxation time scale of silicon band bending at<br />
the interface. The excitation and decay processes of excited states in C60<br />
films have been studied leading to an estimation of the life time of the<br />
laser excited states.<br />
This project is funded by the DFG in the framework of the priority<br />
program ”Dynamics of Electron Transfer Processes at Interfaces”.<br />
O 45.6 Fr 12:30 H39<br />
Electron relaxation dynamics in Ag nanoparticles on Graphite<br />
— •C. Kennerknecht, M. Merschdorf, and W. Pfeiffer —<br />
Physikalisches Institut, Universität Würzburg, Am Hubland, 97074<br />
Würzburg, Germany<br />
The properties of supported nanoparticles differ substantially from homogeneous<br />
films or bulk material. This makes them interesting for applications,<br />
like for example in catalysis [1] or for single electron tunneling<br />
devices [2]. We show that 2-photon-photoemission spectroscopy can be<br />
used as local time-resolved probe for both relaxation dynamics in the<br />
nanoparticles as well as local transient potential shifts that are directly<br />
related to the charge transfer between particle and substrat.<br />
Resonant multiphoton photoemission spectra map the transient electron<br />
energy distribution in the nanoparticles and reveal the internal thermalization<br />
and cooling of the electron gas. A phenomenological model<br />
based on the Boltzmann equations that includes the charge transfer between<br />
substrate and nanoparticle is used to simulate the transient electron<br />
energy distribution. Optimization of the model parameters shows<br />
that the injection of excited electrons into the nanoparticle accounts for<br />
almost half of the total deposited energy in the nanoparticle strongly<br />
influencing the transient electron distribution in the nanoparticles. In<br />
agreement with results from tunneling spectroscopy [2] our measurements<br />
reveal a rather weak electronic coupling between nanoparticle and substrate.<br />
[1] M. Valden, et al., Science 281, 1647 (1998)<br />
[2] T. Ohgi, et al., Appl. Phys. Lett. 79, 2453 (2001)<br />
O 45.7 Fr 12:45 H39<br />
Scattering by Cu adatoms between image-potential bands<br />
on Cu(001) — •Klaus Boger, Martin Weinelt, and Thomas<br />
Fauster — Lehrstuhl für Festkörperphysik, Universität Erlangen-<br />
Nürnberg, Staudtstr. 7, 91058 Erlangen<br />
With increasing resolution and sensitivity of photoelectron spectroscopy<br />
the influence of defects is becoming more and more obvious.<br />
The scattering processes induced by adsorbate atoms can be studied by<br />
time- and angle-resolved two-photon photoemission. We have examined<br />
the dynamics of electrons in image-potential states on the Cu(001)<br />
surface for different coverages of statistically distributed Cu adatoms.<br />
The following scattering mechanisms were distinguished [1]: Scattering<br />
into the bulk, elastic and inelastic intraband scattering, resonant and<br />
inelastic interband scattering. The rates for these different processes<br />
were estimated by modeling the time-resolved measurements with<br />
optical Bloch equations. By varying the defect density the source of the<br />
various processes can be identified: The Cu adatoms mainly provoke<br />
elastic scattering. Inelastic scattering is due to interaction with electrons<br />
in the bulk.<br />
[1] K. Boger, M. Weinelt, J. Wang, Th. Fauster, Appl. Phys. A 78, 161<br />
(2004)<br />
Zeit: Freitag 11:15–12:45 Raum: H45<br />
O 46.1 Fr 11:15 H45<br />
In-situ Electrochemical Corrosion studied with Synchrotron<br />
Radiation — •Frank Uwe Renner 1,2 , Andreas Stierle 2 , Helmut<br />
Dosch 2 , and Jörg Zegenhagen 1 — 1 ESRF, BP 220, 38043<br />
Grenoble Cedex, France — 2 Max-Planck-Institut für Metallforschung,<br />
70569 Stuttgart, Germany<br />
In our humid atmosphere, most corrosion processes are electrochemical<br />
in nature, driven by contact potentials, e.g., forming between dissimilar<br />
metals. For the basic understanding of corrosion and similar technical<br />
processes, in-situ structural methods capable of atomic resolution, such<br />
as scanning probe or hard X-ray techniques are necessary. Binary metal<br />
alloys serve as model systems for more complicated technically used metal<br />
alloys. We used in-situ methods, like X-ray diffraction and anomalous<br />
X-ray scattering and ex-situ AFM, to study Cu3Au(111) single crystal<br />
surfaces in 0.1MH2SO4 electrolyte as a function of electrode potential.<br />
During the initial electrochemical corrosion Cu atoms are dissolved and<br />
a passivating layer is formed. The experiments show the formation of an<br />
epitaxial ultra-thin CuxAu1−x(111) phase on the surface at a potential
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