Plenarvorträge - DPG-Tagungen

Oberflächenphysik Montag
O 14.6 Mo 18:00 Bereich C
Physisorption systems with moderate lattice mismatch: The
structure of CO2/KCl(100) — Milica Hadnadev 1 , J.-Peter
Toennies 2 , Franziska Traeger 2 , •Jochen Vogt 1 , and Helmut
Weiss 1 — 1 Chemisches Institut, Universität Magdeburg, Universitätsplatz
2, 39106 Magdeburg, Germany — 2 Max-Planck-Institut für
Strömungsforschung, Bunsenstr. 10, 37073 Göttingen, Germany
Weakly bound adsorbates with a moderate lattice mismatch between
the solid adsorptive and the substrate are examples of self-organizing
systems in which short-range forces may induce complicated long-range
order. The lattice constant of solid CO2 is 12 % smaller than that of
KCl(100). On the latter CO2 initially adsorbs in an unsaturated phase
with (2 √ 2× √ 2)R45 ◦ symmetry which is barely visible at 80 K and wellordered
at 20 K. The saturated 2D phase forms a (6 √ 2× √ 2)R45 ◦ lattice,
verified both with helium atom scattering (HAS) and low-energy electron
diffraction (LEED). Moreover, we use polarization infrared spectroscopy
(PIRS) and show that dense phase IR spectra in the region of the CO2
asymmetric stretch vibration are consistent with dynamic dipole-dipole
coupling of 12 molecules in the corresponding unit cell. The spectra reveal
an aging of the freshly prepared layer within short time. This is attributed
to the heat of adsorption (25±1 kJ/mol) being slightly lower than the
heat of sublimation of solid CO2 (27 kJ/mol). Hence the 2D layer should
be metastable and form 3D clusters. The 2D phase collapses immediately
to the latter upon exposure to acetylene, which has no lattice mismatch
to KCl(100) and a heat of adsorption of 27 kJ/mol.
O 14.7 Mo 18:00 Bereich C
Manipulation of ultrafast surface processes by means of fs-pulse
shaping — •Felix Steeb, Marlies Wessendorf, Jörg Lange,
Alexander Mönnich, Michael Bauer, and Martin Aeschlimann
— FB Physik, TU Kaiserslautern, Erwin-Schrödinger-Str. 46, 67663
Kaiserslautern
In the last years, coherent control of chemical reactions in the gas
phase [1], or in the liquid phase [2] by means of adaptive femtosecond
pulse shaping has been demonstrated.
In this paper, we present application of the pulse shaping technique to
the optimization / manipulation of ultrafast surface processes, such as
surface chemical reactions.
The apparatus used to shape 20 fs pulses consists of an all-reflective
zero-dispersion compressor in combination with a programmable 640stripe
liquid crystal spatial light modulator (SLM). A closed loop setup,
controlled by an evolutionary algorithm [3], iteratively achieves optimization
of an experimental signal which is used as feedback. We find that
the Two-Photon-Photoemission Yield, e.g. measured from an adsorbate
resonance on the system Cs/Cu(111), is a possible detection scheme that
enables us to control specific surface-related properties. The experimental
setup, preliminary results and future prospects will be discussed.
[1] A. Assion et al.: Science 282, 919 (1998)
[2] T. Brixner et al: Nature 414, 57 (2001)
[3] D. Zeidler et al.: Phys. Rev. A 64, 023420 (2001)
O 14.8 Mo 18:00 Bereich C
Growth and thermal stability of Ni adsorption layers on
the (111) Mo crystal surface — •Cezary Tomas 1,2 , Jan
Kolaczkiewicz 1 , and Herbert Pfnür 2 — 1 Institute of Experimental
Physics, University of Wroclaw, Poland — 2 Institut Für
Festkörperphysik, Universität Hannover, Germany
The low-density (111) surfaces of bcc metals have a high surface free
energy and undergo faceting at sufficiently high temperatures, when covered
with a chemisorbed layer. However, not all chemisorbed adsorbates
lead to this process. Madey and coworkers have studied the reconstruction
of the W(111) surface. They found that faceting occurs in the presence of
metals with electronegativity 2 or more on the Pauling scale. Our results
do not support this classification. We suggest that the major cause of
faceting is the adsorbate-substrate and adsorbate-adsorbate interaction
and the magnitude of the atomic radii of the adsorbate and substrate.
The goal of this study was to investigate the growth mechanism and the
thermal stability of Ni layers adsorbed on the (111) Mo surface. AES,
LEED, ∆φ and STM has been used in this work. We find that Ni does
not cause faceting. Only for coverages Θ > 3ML 3D crystallites appear
on the surface and pass into 2D forms with increasing temperature.
O 14.9 Mo 18:00 Bereich C
CO adsorption and desorption processes on Pt(355) investigated
by in-situ high resolution XPS — •B. Tränkenschuh, T.
Fuhrmann, C. Papp, J. F. Zhu, R. Denecke, and H.-P. Steinrück
— Physikalische Chemie II, Universität Erlangen-Nürnberg, Egerlandstr.
3, 91058 Erlangen
The adsorption and thermal desorption of CO on a Pt(355) surface
was studied by the combination of a supersonic molecular beam and
in-situ high resolution XPS. This surface contains five atom wide (111)
terraces separated by monatomic steps with (111) orientation. By using
synchrotron radiation we are able to clearly distinguish between adsorption
sites at steps and at terraces in O 1s and C 1s spectra, even at
fast measuring times (about 4 s per spectrum). The terrace adsorption
sites are comparable to those on a Pt(111) surface, namely bridge and
on-top bound species which are well known in literature [1]. Our measurements
show that first the step sites and than the on-top and bridge sites
on the terraces are occupied. The desorption is studied by temperatureprogrammed
XPS and TPD. Supported by the DFG (STE 620/4-2).
[1] M. Kinne et al., J. Chem. Phys. 117, 23 (2002).
O 14.10 Mo 18:00 Bereich C
Influence of Surface Roughness on the Wetting Behaviour of
Liquid Helium on Alkali Metal Surfaces — •Martin Zech, Armin
Fubel, Jürgen Klier, and Paul Leiderer — University of Konstanz,
Department of Physics, 78457 Konstanz, Germany
For many times wetting transitions have been observed for helium and
hydrogen on certain alkali metal surfaces. However, there exist discrepancies
in the measured contact angles and accompanying hysteresis, in
the movement of the helium contact lines, and sometimes even in the
wetting temperature. Surface roughness is expected to have a significant
influence on the wetting properties, but the working mechanism is not
clearly understood yet. For this reason we have developed a special low
temperature setup. This allows simultaneously an in situ evaporation of
the alkali metals, an investigation of the substrate surface using a scanning
tunnelling microscope (STM), and an investigation of the wetting
behaviour of liquid helium on those surfaces. A coherence between different
surface nanostructuring and emerging wetting phenomena is aimed
to be obtained.
O 14.11 Mo 18:00 Bereich C
Adsorption and decompostion of prenal on Pt(111) — •Jan
Haubrich, Alexander Krupski, Conrad Becker, and Klaus
Wandelt — Institut für Physikalische und Theoretische Chemie, Wegelerstrasse
12, D-53115 Bonn, Germany
The adsorption of prenal on Pt(111) is investigated with HREELS,
TPD and LEED. After adsorption of prenal on Pt(111) at 100K the
desorption of fragments between 1 and 100 amu has been studied with
TPD for series of a increasing exposures. Only signals of the masses 2,
28 and 84 (prenal) have been detected. The TPD results indicate that
molecular prenal desorbs from a monolayer, a 2nd adsorption state and
multilayer at 199K, 177K and 160 K, respectively. For the mass 28, a signal
is observed around 420K, saturating below the monolayer exposure
and pointing towards the decarbonylation of an irreversibly adsorbed
submonolayer species. Several signals are detected for molecular hydrogen
desorption between 285K and 480K. HREELS experiments carried
out between 100K and 500K show only small shifts for the reversibly adsorbed
species, while the species remainig above 199K shows some shifts
and sizeable changes in scattering intensities. Above 420K only traces of
hydrocarbon species can be observed by HREELS.
O 14.12 Mo 18:00 Bereich C
HR-XPS study of furan and pyrrole on Ni(111): reactions of
unsaturated hetero ring systems — •C. Papp, R. Denecke, and
H.-P. Steinrück — Physikalische Chemie II, Universität Erlangen-
Nürnberg, Egerlandstr. 3, 91058 Erlangen
The examination of the two hetero-cycles, furan (C4H4O) and pyrrole
(C4H5N) on the Ni(111) surface with temperature programmed XPS
(TP-XPS) and temperature programmed desorption (TPD) was conducted
in a larger effort to understand the adsorption and reaction of
aromatic molecules on single crystal surfaces. The XP spectra of C 1s,
O 1s and N 1s core levels were recorded with the high resolution and
high flux of the third generation synchrotron source MAX II. In both
hetero-cycles the inequivalent C atoms can be clearly resolved. The two
molecules show significant differences in their reaction behavior. Furan
decomposes at 200 K by forming CO, which is desorbing at 430 K, and