Plenarvorträge - DPG-Tagungen

Oberflächenphysik Donnerstag
is identified as the lowest energy configuration over a broad range of oxygen
pressures. The stabilization of the Fe3O4(001)-surface goes together
with significant changes in the electronic and magnetic properties, e.g. a
halfmetal-to-metal transition. [1] K. Reuter and M. Scheffler, Phys. Rev.
O 30 Hauptvortrag Denecke
B 65, 035406, (2002). [2] R. Pentcheva et al., Phys. Rev. Lett. 90, 076101
(2003). (in collab. with M. Scheffler and W. Moritz; DFG support, PE
883)
Zeit: Donnerstag 10:15–11:00 Raum: H36
Hauptvortrag O 30.1 Do 10:15 H36
Surface chemistry studied by in-situ x-ray photoelectron spectroscopy
— •Reinhard Denecke — Physikalische Chemie II, Universität
Erlangen-Nürnberg, Egerlandstr. 3, 91058 Erlangen
Important aspects of surface science are the dynamics of adsorption
and reaction processes. Using high-resolution x-ray photoelectron spectroscopy
together with high flux synchrotron radiation and in combination
with a molecular beam, such time-dependent processes can be
followed on a time scale of seconds. Spectroscopic information about the
O 31 Oberflächenreaktionen II
species involved can be obtained from the binding energy shifts of adsorbate
and substrate core levels. In the case of hydrocarbons, the vibrational
fine structure can additionally be used to identify surface species.
From quantitative information obtained by intensity analysis, kinetic parameters
can be derived. Examples are discussed for bimolecular reactions
(CO oxidation), activated adsorption (dissociative adsorption of methane
and ethane) and thermal dehydrogenation of hydrocarbons.
Supported by the DFG (Ste 620/4-2).
Zeit: Donnerstag 11:15–13:00 Raum: H36
O 31.1 Do 11:15 H36
Rastertunnelmikroskopische Untersuchung elektroneninduzierter
Prozesse an D2O-Molekülen und -Clustern auf fcc(111)-
Metalloberflächen — •Heiko Gawronski, K. Morgenstern, K.-
F. Braun und K.-H. Rieder — Freie Universität Berlin, Institut für
Experimentalphysik, Arnimallee 14, 14195 Berlin
Diffusion von D2O-Molekülen und kleinen Clustern auf Ag(111)
und Au(111) wird bei 5,5K mittels des Elektronenstroms eines
Tieftemperatur-Rastertunnelmikroskops(RTM) induziert. Hierbei wird
die Abhängigkeit der Diffusion von der Anregungsenergie und der Anregungsdauer
untersucht. Dies erlaubt es, Aussagen über die Abhängigkeit
der Oberflächenreaktionen vom Adsorptionsplatz, der Clustergröße sowie
der angeregten molekularen Schwingung zu treffen. Zunächst werden
die Adsorbate über ihre scheinbaren Höhen identifiziert. Anschließend
induziert man eine diffusive Bewegung durch Injektion von Tunnelelektronen
verschiedener Energie und bestimmt so die Anregungsenergien
der D2O-Moleküle auf den beiden Substraten. Für einzelne Moleküle liegen
diese bei (400±10)mV auf Ag(111) bzw. (440±10)mV auf Au(111)
wobei die Diffusion nach durchschnittlich 0,5ms erfolgt. Die Anregung
der Diffusion eines D2O-Clusters mit der RTM-Spitze direkt über einem
Cluster benötigt auf Au(111) eine Anregungsenergie von (250±10)mV,
während eine Anregung mit der Spitze im Abstand einiger nm erst bei
(480±10)mV stattfindet. Diese Ergebnisse werden mit Ergebnissen von
H2O auf Cu(111) und auf Ag(111) verglichen.
O 31.2 Do 11:30 H36
Interaction of He, Ne and Ar metastable atom beams
with multilayer tunnel systems — •Domokos Kovacs 1 , Johannes
Berndt 1 , Jörg Winter 1 , and Detlef Diesing 2 —
1 Experimentalphysik 2, Ruhr-Universität Bochum — 2 Institut für
Schichten und Grenzflächen 3, Forschungszentrum Jülich
The collision of metastable rare gas atoms He, Ne and Ar with a metal
surface is a well investigated process in surface science. The process leads
with an efficient rate to a relaxation of the atoms and a simultaneous electron
emission from the metal surface. The energy spectrum of the emitted
electrons is a wide distribution with a maximal energy of 4 eV. Due to the
significant width of the emitted electron spectrum one can think about
a distribution of excited defect electrons in the electron emitting metal
surface. This kind of electronic excitation is difficult to detect in a bulk
metal. In a 15 nm thick silver film however the defect electrons may reach
the opposite interface of the metal film. In a multilayer tunnel system,
separating the thin silver film from an aluminium film by a 2 nm thick
oxide spacer one can detect an electron current from the base electrode
to the silver electrode which is exposed to the metastable beam. Competing
processes with deexcitation are discussed. By the application of
a grid filter in the transport tube between the discharge and the vacuum
chamber one can seperate between ion neutralization reactions and
metastable deexcitation. By an asymmetric chopping unit which allows
the exposition of the samples to alternating particle-photon and photon
fluxes we discuss the contribution of the photon induced tunnel current
to the total tunnel current.
O 31.3 Do 11:45 H36
Molecular scattering and adsorption at metallic surfaces studied
by ab initio molecular dynamics simulations — •Axel Groß —
Physik-Department T30, Technische Universität München, 85747 Garching
Molecular dynamics simulations based on density functional theory
(DFT) calculations have been performed to study the interaction of simple
molecules with metallic surfaces. In the simulations, the surface atoms
have been treated dynamically thus allowing a realistic description of the
energy transfer from the impinging molecules to the substrate. In particular,
we focus on the systems O2/Pt(111) [1] and H2/Pd(100). The
energy transfer and dissociation process of O2 which can adsorb both
molecularly as well as dissociatively on Pt(111) is analysed in detail. In
the simulation of H2 adsorption on metallic surfaces, the substrate atoms
are usually kept fixed because of the large mass mismatch between H2
and the metal atoms [2]. This approximation will be critically questioned
by examining the influence of the recoil of the metal atoms on the H2
adsorption dynamics.
[1] A. Groß, A. Eichler, J. Hafner, M.J. Mehl, and D.A. Papaconstantopoulos,
Surf. Sci. 539, L542 (2003).
[2] A. Groß, Surf. Sci. Rep. 32, 291 (1998).
O 31.4 Do 12:00 H36
First-principles Investigation of Structural and Chemical
Properties of Nanoporous Carbon — •Suljo Linic, Johan M.
Carlsson, and Matthias Scheffler — Fritz-Haber-Institut der
Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin
Production of styrene by dehydrogenation of ethylbenzene(ET) is one
of the most important processes in chemical industry. Iron oxide is used
as catalyst, but it has been observed that ET decomposes during an induction
period leaving nanoporous carbon (NPC) on the oxide support.
NPC are curved, defective, graphitic sheets and experiments have shown
that such materials are active catalysts for oxidative dehydrogenation of
ET.[1] Recently, it was therefore suggested that the actual catalyst under
reaction conditions is NPC.[2] This motivates a theoretical study of
NPC. Our results indicate on the one hand that flat and curved basal
planes of graphitic sheets are chemically inert. Vacancies on the other
hand leave dangling bonds and increase the density of state close to the
Fermi level. This gives the vacancies a much higher reactivity, which is
examplified by a large exothermal energy for dissociative adsorption of
O2. We have then studied oxidation of the defects as function of pressure
and temperature. The resulting oxygenated vacancies show interesting
properties as active sites in the dehydrogenation process. [1] M. S. Kane
et al., Ind. Eng. Chem. Res. 35, 3319 (1996). [2] G. Mestl et al., Angew.
Chem. Int. Ed. 40, 2066 (2001).