Plenarvorträge - DPG-Tagungen

Oberflächenphysik Dienstag
ments and spectroscopic images (dI/dV maps) taken at T = 3.9 K on
this superlattice showing site-dependent characteristic features very different
from the clean Ag(111) surface. Calculations within a nearly free
electron model and tight binding simulations show good agreement with
our data. Furthermore, spectra obtained on compressed and slightly disordered
lattices were successfully simulated and show the dependence of
the LDOS on the local environment. Specifically, we identify the observed
spectroscopic structures with singularities at the high-symmetry points
of the mini-Brillouin zone created by the hexagonal Ce adatom superlattice.
The stability of this superlattice results from the energy gain of
the system manifesting itself by the opening of gaps at the mini-Brillouin
zone boundery.
[1] F. Silly, M. Pivetta, M. Ternes, F. Patthey, J. Pelz, and W.-D.
Schneider, Phys. Rev. Lett., in press.
O 17.8 Di 13:00 H36
Probing of bulk band edges with STM: An ab initio analysis
— •A. Dick 1,2 , M. Hansmann 2 , J.I. Pascual 2,3 , G. Geballos 2,4 ,
H.-P. Rust 2 , K. Horn 2 , and J. Neugebauer 1 — 1 Universität Paderborn,
Dekanat Physik, Warburger Straße 100, D-33098 Paderborn —
2 Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-
14195 Berlin — 3 Institut de Ciència de Materials de Barcelona - CSIC,
08193 Bellaterra, Spain — 4 Laboratorio Nazionale TASC-INFM, 34012
Trieste, Italy
O 18 Adsorption an Oberflächen I
The analysis of standing electronic waves at noble metal surfaces by
scanning tunneling microscopy/spectroscopy (STM/STS) has given valuable
insight into different surface properties such as, e.g., surface state
lifetimes on free surfaces as well as artificial atomic structures. These
studies, however, were mainly limited to surface states. We have therefore
studied the electronic structure of the Ag(110) surface employing
low temperature STS and density-functional theory. In the calculations
atomic relaxation has been fully taken into account and the convergence
of the slab thickness has been carefully checked. Based on these results
a generalized Tersoff-Hamann theory [1] has been used to simulate STS
spectra. A comparison of the experimental and theoretical spectra showed
excellent agreement and clearly demonstrate that bulk states can be
probed in STS.
[1] A.Selloni et al., Phys. Rev. B 31, 2602 (1985)
Zeit: Dienstag 11:15–13:15 Raum: H38
O 18.1 Di 11:15 H38
Composition of the surface oxide on Pd(100) in an O2 and CO
environment — •Jutta Rogal, Karsten Reuter, and Matthias
Scheffler — Fritz-Haber-Institut der MPG, Berlin,Germany
The oxidation of the Pd(100) surface leads to a ( √ 5 × √ 5)R27 ◦ surface
oxide structure, which was recently identified to be essentially a
PdO(101) overlayer on Pd(100)[1]. In thermodynamic equilibrium with
an O2 gas phase this surface oxide represents the most stable phase over
a wide range of environmental conditions, suggesting that the surface oxide
may play some role in the catalytic oxidation of CO on the Pd(100)
surface.
To get a first insight into this catalytic reaction we investigate the
structure and composition of the surface oxide in equilibrium with an
arbitrary O2 and CO gas environment using density-functional theory
and ”constrained” atomistic thermodynamics[2]. Studying the simultaneous
adsorption of CO and O we arrive at a (T, p)-phase diagram of the
stable structures, and discuss possible kinetic effects due to the ongoing
reaction.
[1] M. Todorova et al., Surf. Sci. 541, 101-112 (2003)
[2] K. Reuter and M. Scheffler, Phys. Rev. B 68, 045407 (2003)
O 18.2 Di 11:30 H38
In-situ XPS studies of CO adsorption on Pd(111) in the pressure
range from 10 −8 to 1 mbar — •J. Pantförder, S. Pöllmann,
D. Borgmann, R. Denecke, and H.-P. Steinrück — Physikalische
Chemie II, Universitaet Erlangen-Nuernberg, Egerlandstr. 3, 91058 Erlangen
In an effort to extend the usable pressure range for electron based spectroscopies
from UHV into the so-called pressure gap region, we have built
a new experimental setup for in-situ XPS up to 1 mbar. The principle of
the high pressure cell is based on several stages of differentially pumping
between sample and electron detection, similar to earlier experimental
setups [1]. In our apparatus the measurements can be performed either
by using an ambient pressure in the cell or by application of a directed
gas beam yielding a considerably lower background pressure. The performance
of the high pressure setup will be discussed by the pressure
dependence of CO adsorption on Pd(111), which can be compared to
SFG and XP measurements [2].
Supported by the DFG (Schwerpunktsprogramm 1091, Ste620/3-3).
[1] H.J. Ruppender et al., Surf. Interface Anal. 15 (1990) 245; R.W.
Joyner et al., Surf. Sci. 87 (1979) 501.
[2] V. V. Kaichev et al., J. Phys. Chem. B 107 (2003) 3522.
O 18.3 Di 11:45 H38
Adsorption of water on Pd(111) — •Ari P Seitsonen 1 , Bako
Imre 2 , Gabor Palinkas 2 , and Jürg Hutter 1 — 1 Physikalisch
Chemisches Institut der Universität Zürich — 2 Chemical Research
Centre of the Hungarian Academy
The adsoprtion of water on transition metal surfaces has recently received
wealth of interest. Our studies are motivated by the STM experiments
by Salmeron and co-workers [Mitsui et al, Science 297 (2002)
1850]. They were able to follow the initial adsorption and diffusion of
small water molecules on Pd(111). We have performed density functional
theory (DFT) calculations on water monomer and dimer on the same surface,
using periodic plane wave/pseudo potential method. We find that
the water dimer adsorbs with only one of the molecules adsorbed directly
on the surface, the second one being bound mainly by the hydrogen bond
between the molecules. Our diffusion barrier for the monomer agrees excellently
with the experimental one, however our diffusion barrier for
the dimer is higher than for the monomer, contradicting the conclusions
achieved in the experiments.
O 18.4 Di 12:00 H38
Angular dependence of H2 dissociation at DB steps of Si(001)
— •C. Stanciu and U. Höfer — Fachbereich Physik, Philipps-
Universität Marburg, D-35032 Marburg
The dynamics of dissociative adsorption of molecular hydrogen at the
DB steps of Si(001) was investigated by performing angular dependent
measurements of the initial sticking coefficient for different kinetic energies
of H2 from a supersonic molecular beam. Optical second-harmonic
generation (SHG) was used as a sensitive monitor of hydrogen coverage
at the steps sites. For Ekin = 350 meV, a kinetic energy that considerably
exceeds the mean barrier height of Ea = 80 meV, the polar distribution
is strongly forward-peaked (∼ cos 7.8 θ) whereas for Ekin < Ea,
H2-momentum parallel to the step edges was found to enhance dissociation,
giving rise, e.g., to a broad polar distribution (∼ cos 1.2 θ) for
Ekin = 30 meV. This behaviour contrasts the one previously observed for
H2 adsorption on the terraces [1]. A sharp angular distribution, directed
along the direction of the dangling bonds, was observed in that case for
Ekin < Ea, indicating a strong lateral corrugation of the barrier height
due to the high directionality of the dangling bonds. The behaviour at
the steps, however, suggests the dominant influence of a geometric corrugation.
Possible origins of such a lateral variation of the barrier position
parallel to the step edges, and effects the steering of low energetic
molecules will be discussed.
[1] M. Dürr and U. Höfer, Phys. Rev. Lett. 88, 076107 (2002)