Plenarvorträge - DPG-Tagungen

Oberflächenphysik Mittwoch
O 28.55 Mi 16:00 Bereich C
Optical anisotropy of Cs nanostructure formation on
GaAs(110), InP(110) and InAs(110) — •Karsten Fleischer
1,2 , Gianlorenzo Bussetti 2 , Claudio Goletti 2 , Pierro
Chiaradia 2 , and Wolfgang Richter 1,2 — 1 TU-Berlin, Institut
für Festkörperphysik, Hardenbergstr. 36, 10623 Berlin, Germany
— 2 Universitá “Tor Vergata”, Dipartimento di Fisica, Via Ricerca
Scientifica 1, 00133 Roma, Italy
We present reflectance anisotropy spectra (RAS) of Cs covered GaAs,
InP and InAs cleaved (110) surfaces. In such systems Cs forms one dimensional
wirelike structures at low coverages. For the first time we studied
the formation of these wires in situ with optical and therefore nondestructive
techniques. Wire formation is accompanied with characteristic
minima in the RAS spectra between the E0 + ∆0 and E1 bulk critical
point. With growing coverages (θ > 0.3ML) and formation of the two
dimensional closed Cs layer (θ ≈ 0.5ML), changes in the Cs related minima
occur. Different wire types and also the transition into the disordered
Cs double layers at saturation coverage (θ ≈ 1ML) can be monitored
optically.
O 28.56 Mi 16:00 Bereich C
Methanol oxidation on clean and oxygen-covered copper surfaces
— •Sung Sakong and Axel Groß — Physik-Department T30,
Technische Universität München, 85747 Garching, Germany
We have studied methanol oxidation steps over clean and oxygencovered
copper surfaces by density functional theory (DFT) calculations.
The reaction pathways are determined by the nudged elastic band (NEB)
method [1] which in particularly also yields the intermediate states and
the reaction barriers. We find that on clean copper surfaces, methanol
is only weakly bound. The decomposition into methoxy and hydrogen
is activated. Upon heating, methanol rather desorbs than dissociates.
The further decomposition of adsorbed methoxy into formaldehyde and
hydrogen is hindered by large barriers (≥ 1.5eV).
The surface intermediate states in the methanol oxidation are significantly
stabilised by co-adsorbed oxygen. The DFT calculations show that
on oxygen-covered Cu(110), methanol dissociates spontaneously because
of the strong attraction between the methanol hydroxyl hydrogen and
the surface oxygen. The reaction steps are analysed in terms of the underlying
electronic structure. Our calculations confirm the importance of
co-exposed oxygen in the methanol oxidation on copper surfaces found
in experiments (see, e.g., [2]).
[1] G. Henkelman and H. Jónsson, J. Chem. Phys. 113, 9978 (2000).
[2] Ch. Ammon et al., Surf. Sci. 507, 845 (2002).
O 28.57 Mi 16:00 Bereich C
Electron Solvation during the Interaction of Na Atoms with
Polar Molecules — •A. Borodin 1 , O. Höfft 1 , U. Kahnert 1 ,
V. Kempter 1 , Y. Ferro 2 , and A. Allouche 2 — 1 Institut für
Physik und Physikalische Technologien, TU Clausthal, Leibnizstr. 4,
D–38678 Clausthal–Zellerfeld — 2 Physique des Interactions Ioniques et
Moléculaires, CNRS–UMR6633 Campus Universitaire de Saint Jérôme,
Marseille France
The interaction of Na atoms with molecular films (H2O; CH3OH; NH3)
was studied with metastable impact electron spectroscopy (MIES) under
UHV conditions. The films were grown at 90K, and exposed to Na.
The spectral feature from 3sNa ionization occurs at an energetic position
different from that found for metals or semiconductors. The results
are compared with DFT calculations. Experiment and theory agree in
the energetic positions of the main spectral features from the molecular
and sodium ionization. The calculations suggest that the 3sNa emission
observed experimentally originates from solvated 3s electrons which are
located far from the Na–core and become stabilized by solvent molecules.
The simultaneous emergence of emission from OH (or CH3O) and from
solvated 3s electrons suggests that the delocalization and, consequently,
the solvation play an important role in the Na–induced formation of
CH3O and OH species from methanol and water, respectively.
O 28.58 Mi 16:00 Bereich C
Spectral evidence for oxide free incorporation of oxygen into
Ru(0001) — •Raoul Blume 1 , Artur Böttcher 2 , Horst Conrad
3 , and Horst Niehus 1 — 1 Institut für Physik, HU-Berlin, Newtonstr.
15, 12489 Berlin — 2 Institut für Physikalische Chemie, Universität
Karlsruhe, Kaiserstr. 12, 76131 Karlsruhe — 3 Fritz-Haber-Institut der
Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin
The capability of a smooth and defected Ru(0001) surface towards the
oxygen accumulation has been studied by the means of thermal desorption
spectroscopy (TDS) and the high resolution scanning XPS (Elettra,
Trieste). By applying a soft Ar + -sputtering procedure individual surface
defects confined to the two outermost Ru layers were created. The oxidation
procedure applied here assures an incorporation of oxygen atoms
in the subsurface region up to 4 monolayer equivalents (MLE) without
any traces of RuO2-domains (high-pressure and low-temperature surface
oxidation). By measuring the temperature and exposure dependence of
the oxygen incorporation and analyzing the corresponding evolution of
the Ru 3d5/2 band we found characteristic electronic signatures allowing
to distinguish oxygen atoms occupying the Ru(I)-Ru(II) interlayer space
and oxygen species associated with surface defects. This assignment is
supported by mesoscopic-scale surface images as well as the quantitative
O2-TD analysis.
O 28.59 Mi 16:00 Bereich C
Investigation of Au electrodeposition processes on metal single
crystal surfaces — •Ahmed Ayyad, Jochim Stettner, and Olaf
Magnussen — Institut für Experimentelle und Angewandte Physik,
Christian-Albrechts-Universität Kiel, Leibnizstraße 19, 24098 Kiel
Metal electrodeposition processes have been a subject of substantial
basic research during the last years, motivated by current technologies
as well as future applications, such as the electrochemical formation of
ultrathin layers and nanostructures. A major, largely unsettled question
is how the electrode surface structure and adsorbates, such as anions or
organic additives, influence the growth process. We studied the homoepitaxial
electrodeposition of Au on Au(111) electrode surfaces (deposition
rates 0.05 to 0.23 ML/min.) by in-situ x-ray diffraction under real time
conditions using a hanging-meniscus transmission cell (reduced cell resistance,
unrestricted mass transport), where the Au(111) electrode surface
is completely covered by an electrolyte droplet. Parallel to the X-ray
measurements cyclic voltammograms were recorded, which confirm that
the hanging-meniscus cell allows high quality electrochemical measurements
combined with in-situ surface X-ray diffraction studies. The X-ray
diffraction data and complementary in-situ ellipsometric measurements
indicate that the deposition process is accompanied by substantial surface
roughening.
O 28.60 Mi 16:00 Bereich C
State selective detection of D2 after femtosecond-laser induced
desorption from Ru(001) — •S. Wagner 1 , M. Rutkowski 2 , R.
Dudek 3 , C. Frischkorn 1 , H. Zacharias 2 , and M. Wolf 1 — 1 Freie
Universtät, Berlin — 2 Westfälische Wilhems-Universität, Münster —
3 Fritz–Haber–Institut der MPG, Berlin
The energy transfer into different degrees of freedom in the fs-laser
induced recombinative desorption of deuterium from Ru(001) has been
investigated with (1+1’)-REMPI and time-of-flight measurements. It has
been shown that hot substrate electrons mediate this reaction within
a few hundred femtoseconds [1]. The rovibrational distributions of the
desorbing D2 molecules are detected via various B 1 Σ + u ← X 1 Σ + g Lyman
bands using tunable vacuum ultraviolet (VUV) laser radiation for
the resonant excitation step [2]. Populations of the rotational quantum
states in the vibrational ground and the first excited state are
measured. The rotational energy of D2 in the vibrational ground state
(Erot/kB = 800 ± 200K) is much lower than the rotational energy in
the excited state (Erot/kB = 1500 ± 300K). Assuming a Boltzmann-like
energy distribution, these results indicate a vibrational temperature of
Evib/kB = 1200 ± 150K. The amount of energy coupled to the translation
is Etrans/kB = 1700 ± 300K. Possible reasons for the incomplete
thermalization will be discussed.
[1] D. N. Denzler, C. Frischkorn, C. Hess, M. Wolf, and G. Ertl, Phys.
Rev. Lett., in press
[2] D. Wetzig, M. Rutkowski, H. Zacharias, and A. Groß; Phys. Rev. B
63, 205412 (2001)
O 28.61 Mi 16:00 Bereich C
Characterisation of Pd/Zn/ZnO real catalysts for methanol
steam reforming — •S. Pöllmann 1 , J. Pantförder 1 , A. Kölbl 2 ,
P. Pfeifer 2 , M. Fichtner 3 , D. Borgmann 1 , R. Denecke 1 , and
H.-P. Steinrück 1 — 1 Physikalische Chemie II, Universität Erlangen,
Egerlandstr. 3, 91058 Erlangen. — 2 FZ Karlsruhe GmbH, Institut für
Mikroverfahrenstechnik -IMVT-, Postfach 3640, 76021 Karlsruhe. — 3 FZ
Karlsruhe GmbH, Institut für Nanotechnologie -INT-, Postfach 3640,
76021 Karlsruhe.