Plenarvorträge - DPG-Tagungen

Oberflächenphysik Montag
hydrocarbon fragments. Pyrrole dissociates at 290 K, but shows no desorption
of any nitrogen containing species up to 600 K, and even at 1000
K some nitrogen is left on the surface. For both molecules the thermal
evolution of the hydrocarbon fragments is analysed. This work was supported
by a European Community ARI Program (HPRI-CT-1999-00058).
O 14.13 Mo 18:00 Bereich C
DFT study of Li adsorption on TiSe2 (0001) — •C. Ramírez 1 ,
W. Schattke 1 , and R. Adelung 2 — 1 Institut für Theoretische Physik
und Astrophysik, CAU, Kiel — 2 Technische Fakultät der CAU, Kiel
Alkali adsorption on a transition metal dichalcogenide substrate has
attracted many investigations, however a theoretical description of its
coverage dependence is still missing. In a first attempt we considered Li
as an alkali prototype at a specified selection of coverages, Θ = 0.06, 0.11,
0.25, 0.33, 0.5, and 1 ML, adsorbed on TiSe2 (0001).
By means of density-functional theory we obtained through structural
optimisation the coverage dependence of the physical properties adsorption
energy, work function, and valence electron distribution. A preference
of the hcp adsorption sites combines with an optimum coverage of
0.33 ML for the most stable structure. The work function curve shows
a behaviour similar to that found for alkali adsorption on metals. The
redistribution of the charge density at adsorption and the partial density
of states for the constituents of the compound are presented.
This work was supported by the Deutsche Forschungsgemeinschaft
(DFG), Forschergruppe FOR 353.
O 14.14 Mo 18:00 Bereich C
Electronic and optical properties of Au(111) and C60 on Au(111)
— •Elizabeta Ćavar, Marie-Christine Blüm, Marina Pivetta,
François Patthey, and Wolf-Dieter Schneider — Institut de
Physique des Nanostructures, Ecole Polytechniqe Fédérale de Lausanne
(EPFL), CH-1015 Lausanne, Switzerland
We have performed scanning probe investigations of the system C60
on Au(111) with a UHV STM operating at 50K using Pt/Ir tips. Topographic
images with molecular orbital resolution reveal different orientations
of C60 molecules in monolayer islands of C60 on Au(111). STS
measurements show energy resolved LDOS of monolayer islands of C60
on Au(111) exhibiting states derived from HOMO, LUMO and LUMO+1
as well as the surface state of Au(111). We employed spatially resolved
STM induced light emission spectroscopy on clean Au(111) surface areas
and C60 monolayer islands on Au(111) on the same sample. On the
clean Au(111) areas we observed the decay of excited localized surface
plasmons at ≈ 710 nm. For the C60 islands photon emission appears at
similar wavelengths, however, at strongly reduced photon intensity.[1]
[1] R. Berndt, R. Gaisch, J. K. Gimzewski, B. Reihl, R.R. Schlittler,
W. D. Schneider and M. Tschudy, Science 262, 1425-1427 (1993).
O 14.15 Mo 18:00 Bereich C
Adsorption and Dissociation of Methanol on Pd(111) — •A.
Bayer, J. Pantförder, S. Pöllmann, D. Borgmann, R. Denecke,
and H.-P. Steinrück — Physikalische Chemie II, Universität
Erlangen-Nürnberg, Egerlandstr. 3, D-91058 Erlangen
Methanol adsorbed on a Pd(111) surface at 100 K dissociates upon
annealing mainly to CO and H, with methoxy as an intermediate. The
investigation of this reaction by high resolution X-ray photoelectron spectroscopy
(XPS) with monochromized Al Kα radiation is complicated because
of two reasons: First, the C 1s binding energy differences between
methanol, methoxy, and CO are too small to discriminate between different
species [1]. Second, the O 1s binding energy region is superposed
by the broad substrate Pd 3p3/2 peak. To overcome the latter problem,
the Pd 3p3/2 peak of the clean surface was subtracted from the measured
spectra after intensity correction via the Pd 3p1/2 peak. In addition, the
surface sensitivity was enhanced by choosing a grazing emission angle.
The resulting difference spectra show discrete O 1s peaks with binding
energy differences of 0.2-1.4 eV for the different species. Additional measurements
by ultraviolet photoelectron spectroscopy (UPS) and temperature
programmed desorption (TPD) allow the assignment to methanol,
methoxy, and CO.
Supported by the DFG (Schwerpunktsprogramm 1091, Ste620/3-3).
[1] M. Rebholz and N. Kruse, J. Chem. Physics 95 (1991) 7745.
O 14.16 Mo 18:00 Bereich C
Excited potential energy surface from spin-restricted densityfunctional
calculations: H/Al(111) — •Michael Lindenblatt
and Eckhard Pehlke — Institut für Theoretische Physik und Astrophysik,
Universität Kiel
Excited-state potential-energy surfaces are needed for the description
of non-adiabatic processes during chemisorption, such as the loss of spinpolarization
of an atom which is approaching a metal surface. A process
of this type has been discussed as a possible explanation for the chemicurrents
observed experimentally by H. Nienhaus et al.[1],[2].
To compute potential-energy surfaces of spin-polarized configurations
away from the Born-Oppenheimer ground state we have carried out spinrestricted
(fixed moment) density-functional calculations with the code
fhi96spin (from M. Bockstedte et al.[3]). To prevent the spin-polarization
from spreading over the whole metal slab at small atom-surface separation,
we apply an additional potential. As an example, we present
potential-energy surfaces for H/Al(111).
[1] H. Nienhaus, Surf. Sci. Rep. 45, 1 (2002).
[2] J. R. Trail et al. , J. Chem. Phys. 119, 4539 (2003).
[3] M. Bockstedte, A. Kley, J. Neugebauer, M. Scheffler, Comp. Phys.
Commun. 107, 187 (1997).
O 14.17 Mo 18:00 Bereich C
STM-Untersuchungen von C64-Picoröhren auf Au(111) —
•Belinda Baisch 1 , Jens Walther 2 , Rainer Herges 2 und Olaf
Magnussen 1 — 1 Institut für experimentelle und Angewandte Physik,
Christian-Albrechts-Universität, Leibnizstraße 19, 24118 Kiel —
2 Institut für Organische Chemie der Christian-Albrechts-Universität
Kiel, Otto-Hahn-Platz 3, 24098 Kiel
Die Bildung ultradünner Filme und Monoschichten von fullerenartigen
Molekülen auf Oberflächen ist von hoher Aktualität. Hier
wurde das Adsorptionsverhalten und die molekulare Ordnung von
5,24:6,11:12,17:18,23-Tetra[1,2]benzenotetrabenzo[a,e,i,m]cyclohexadecen
(”C64-Picoröhren”) auf Au(111) mittels Rastertunnelmikroskopie (STM)
untersucht. Bei den untersuchten Adsorbaten handelt es sich um
einen vollständig konjugierten röhrenförmigen Kohlenwasserstoff, bestehend
aus vier Anthracen-Einheiten. Durch Adsorption aus 1 mM
Lösungen in Benzol oder Toluol wurden auf flammengetemperten
Au(111)-Einkristallen molekulare Adschichten mit Bedeckungen im
(Sub-)Monolagenbereich präpariert. STM-Untersuchungen dieser Adschichten
zeigen, ähnlich wie für C60-Adschichten, eine hexagonale Ordnung
und Abstände zwischen nächsten Nachbarn von 10 ˚A.
O 14.18 Mo 18:00 Bereich C
Step Decoration of Gold Single Crystal Electrode Surfaces with
Pd — •Fernando Hernandez Ramirez and Michael Nielinger
— Institut fuer physikalische u. theor. Chemie, Universitaet Bonn, Roemerstrasse
164, 53117 Bonn, Germany
Surfaces of regularly stepped single crystals can be decorated with foreign
metals in order to change their physicochemical properties.
Step decoration of vicinally stepped platinum single crystals was
achieved for many metals and can be easily monitored in cyclic voltammetry
due to the suppression of the corresponding hydrogen adsorption
at step sites. By using the system Au(hkl)/Pd reactive bonding sites shall
be introduced at regular spacings. Here step decoration is more difficult
to prove. Cyclic voltammetry shows two Pd-oxidation peaks, the first one
being almost independent of Pd coverage. Hydrogen adsorption on the
modified stepped surfaces, but not on Au(111)/Pd, takes place only when
more than a critical amount of Pd, corresponding to a monoatomic row of
Pd at step sites, has been deposited. These facts support the assumption
of step decoration and show that hydrogen only adsorbs at 3-fold hollow
sites of Pd.
O 14.19 Mo 18:00 Bereich C
Influence of adsorbed rare gas on the Au(111) surface state:
combined UPS and STS studies — •T. Andreev, I. Barke, and
H. Hövel — Universität Dortmund, Experimentelle Physik I, D-44221
Dortmund
The Au(111) Shockley surface state is compared before and after adsorbing
different rare gas layers (Ar, Kr and Xe). We used ultraviolet
photoelectron spectroscopy (UPS) and scanning tunneling spectroscopy
(STS) in combination to get comprehensive information by using the advantages
of both methods. The energetic position of the surface state in
UPS is determined using an analytic model, which takes the finite angular
resolution of the electron energy analyzer into account. STS was mea-