Plenarvorträge - DPG-Tagungen

Oberflächenphysik Montag
O 14.47 Mo 18:00 Bereich C
Desorption of Atoms from Optically Excited Alkali Halides —
•M. Rohlfing 1 , N.-P. Wang 2 , P. Krüger 2 , and J. Pollmann 2 —
1 School of Engineering and Science, International University Bremen,
P.O. Box 750761, 28725 Bremen — 2 Institut für Festkörpertheorie, Universität
Münster, Wilhelm-Klemm-Str. 10, 48149 Münster
We investigate the emission of halogen atoms from optically excited
alkali halides (e.g., KI). To this end we employ an ab-initio approach
to the excited states and the corresponding total-energy surfaces. Based
on density-functional theory, the electronic correlation effects are treated
within many-body perturbation theory (i.e., the GW method and the
Bethe-Salpeter equation). This yields the total energy of the system,
both in its ground state and in the excited state, as a function of the
geometry. As prototype materials KI and KBr are discussed. Our results
indicate that at the KI(001) surface, a surface exciton can be excited leading
to direct emission of a neutral iodine atom without any desorption
barrier. The emission occurs on a sub-picosecond time scale and leads to
kinetic energies of the iodine atoms of several 100 meV, in accordance
with recent time-of-flight experiments.
O 14.48 Mo 18:00 Bereich C
Surface dynamics of water-glycerol mixtures investigated
by XPCS — •Michael Sprung 1 , Simone Streit 1 , Christian
Gutt 1 , Anders Madsen 2 , Tilo Seydel 3 , and Metin Tolan 1 —
1 Experimentelle Physik I, Universität Dortmund, 44221 Dortmund —
2 ESRF, Grenoble, France — 3 ILL, Grenoble, France
The structure and dynamics of the surface of water / glycerol-mixtures
has been investigated by X-ray photon correlation spectroscopy (XPCS)
and by means of specular x-ray reflectivity.
The aim of the experiments was to investigate (a) the dynamics of the
crossover from propagating to overdamped capillary waves by XPCS, (b)
the temperature dependence of the capillary wave induced surface roughness
and (c) the detailed electron density profile of the surface structure.
Experiments were performed in a broad temperature range and with different
water/glycerol mixtures. We could for the first time observe the
transition from overdamped to propagating waves on the surface. Of special
interest was also the surface dynamics and structure of supercooled
mixtures.
O 14.49 Mo 18:00 Bereich C
Scanning Probe Microscopy of Antimony Single Crystals: Periodic
and Non-periodic Features of the (0001) Cleavage Surface
— •B. Stegemann 1,2 , C. Ritter 1 , B. Kaiser 1 , and K. Rademann 1
— 1 Institut für Chemie, Humboldt-Universität zu Berlin — 2 Institut für
Experimentalphysik, Freie Universität Berlin
Morphology, atomic structure and irregular features of the Sb(0001)
cleavage plane of antimony single crystals are investigated in detail by
STM and AFM. Atomically resolved STM images of the hexagonal structure
of surface atoms on Sb(0001) are presented, extending the base of
STM data available on elemental surfaces. Lateral and vertical lattice
parameters are determined. As the observed lattice spacing of 4.31 ˚A on
Sb(0001) agrees well with the known bulk data, surface reconstruction
can be excluded. Cleavage is found to occur always between adjacent
double layers yielding at least diatomic cleavage steps of 3.75 ˚A height.
Crystal imperfections such as vacancies (0D defects) and step edges (1D
defects) are elucidated and imaged with atomic resolution. Moreover,
twinned interlayers (2D defects) formed upon cleavage of Sb at room
temperature are revealed, confirming the twinning angle of 2.45 degrees,
which has been predicted by the model of twinning in Sb crystals. Results
are discussed in comparison with other layered materials and with regard
to their relevance for the use of Sb(0001) as support for nanostructures.
O 14.50 Mo 18:00 Bereich C
Surface dynamics of gold clusters on thin polymer films investigated
by XPCS — •Virginie Chamard 1 , Markus Krämer 1 ,
Christian Gutt 1 , Anders Madsen 2 , and Metin Tolan 1 —
1 Experimentelle Physik I, Universität Dortmund, 44221 Dortmund —
2 ESRF, Grenbole, France
Using the technique of X-Ray Photon Correlation Spectroscopy
(XPCS) it is possible to observe dynamics at low frequencies (from 10 6
s −1 to 10 −3 s −1 ) and small lengthscales of micro- to nanometers, which
are not or hardly accessible to other techniques.
In the present work we examined the dynamics of thin polymer films
covered with nanometer sized gold clusters. We performed intensity-
intensity autocorrelation measurements of the samples at different temperatures.
If the sample is heated to temperatures higher than the glass
transition temperature of the polymer, the clusters sink inside to a position
a few ˚A below the surface. There they act as markers for the capillary
wave movement of the surrounding polystyrene. Furthermore the clusters
perform a Brownian Motion inside the polymer which can, as well as the
capillary waves, be characterized by correlation measurements. The resulting
viscosity is several orders of magnitude higher than the viscosity
of pure polymer systems without gold. In addition it has been found that
the viscosity of the gold/polymer system is less temperature dependent
than that of the polymer.
O 14.51 Mo 18:00 Bereich C
Kinetische Prozesse an Oberflächen in metallischen Gläsern
unter Einfluß von Teilchendeposition — •Sebastian Vauth
und Stefan G. Mayr — I. Physikalisches Institut, Georg-August-
Universität Göttingen, Tammannstr.1, 37077 Göttingen
In MD-Simulationen wird das Diffusionsverhalten von amorphen Cu-
Ti Schichten unterhalb der Glastemperatur untersucht. Um Oberflächen
mit einzubeziehen, werden in einer Richtung die sonst geltenden periodischen
Randbedingungen aufgehoben. Es werden die Diffusionskonstanten
der Elemente diskutiert in Abhängigkeit von der Entfernung zur Oberfläche,
der Atomsorte und der Zusammensetzung der Probe. Verteilungsfunktionen
von Sprungweiten sowie das Verhalten einzelner Atome liefern
Informationen über den zugrundeliegenden Diffusionsmechanismus.
Weiterhin wird der Einfluß der Deposition von Atomen mit thermischer
Energie und auch von Ionenbestrahlung der Oberfläche auf die atomare
Kinetik untersucht - auch im Hinblick auf Strukturbildung. Entsprechende
experimentelle Untersuchungen an dünnen Schichten metallischer
Gläser werden im Vergleich diskutiert. Diese Arbeit wird vom SFB 602
TP B3 der DFG gefördert.
O 14.52 Mo 18:00 Bereich C
Ultrafast dynamics of the Si(001) surface after electronic excitation
— •Jan van Heys and Eckhard Pehlke — Institut für
Theoretische Physik und Astrophysik, Universität Kiel
Due to the strong Jahn-Teller coupling between the buckling angle and
the electronic structure of the dimers on the Si(001) surface, this surface
represents a promising model system for studying ultrafast surface
atomic motion in response to electronic excitation. Depending on excitation
density, due to the creation of electron-hole pairs in the danglingbond
surface-states the energy preference for buckled dimers as opposed
to symmetric dimers can be nearly lost.
We present first results from ab initio molecular dynamics simulations
of the non-adiabatic motion of the electrons and ions at the Si(001) surface
after strong electronic excitation. Electrons are described within the
time-dependent density functional theory while the motion of the ions
follows from classical Ehrenfest dynamics.
We find that, provided the excitation exceeds order of 1/2 electron per
dimer, the buckling angle flips on a timescale of about 150 fs.
O 14.53 Mo 18:00 Bereich C
Chiral adenine rows on Cu(110) – a theoretical contribution
— •Martin Preuß, Wolf G. Schmidt, and Friedhelm Bechstedt
— Institut für Festkörpertheorie und Theoretische Optik, Friedrich-
Schiller-Universität, Max-Wien-Platz 1, 07743 Jena, Germany
The adsorption of adenine on copper allows for studying the concepts of
self-assembly, adsorbate-induced chirality and low-dimensionality. Both
LEED and STM [1] reveal a long-range ordering of adenine on Cu(110)
in one-dimensional chains in the ±�a1 +�a2 direction. The sign depends on
the way of adsorption: mediated by the substrate, the formerly prochiral
adenine molecules lose their mirror-plane symmetry and, after dimerization,
form chiral chains. We study the process of adenine adsorption
on Cu(110) in detail by means of a first principles approach. The calculations
are carried out in the framework of density-functional theory
using PAW potentials in the generalized gradient approximation. Possible
molecular dimer chain models are presented and interpreted in terms of
stability and chemistry. We especially address the problem of discrimination
between different geometries and the bonding mechanism of adenine
to the substrate.
[1] Q. Chen, D. J. Frankel, N. V. Richardson, Langmuir 2002, 18, 3219