Plenarvorträge - DPG-Tagungen

Oberflächenphysik Donnerstag
The growth morphology of CoO films deposited in the temperature
range of 300-500 K has been studied by Spot Profile Analysis of Low
Energy Electron Diffraction (SPA-LEED). At a coverage of 3 ML the
growth is pseudomorphic to the Ag(100) substrate with a compression of
4% in the CoO film. Relaxation to the bulk lattice of CoO occurs at a
O 35 Hauptvortrag Bauer
coverage of 6 ML. At 10 ML, splitting of the (00) spot indicates the presence
of terraces which we relate to the carpet growth mode of the film.
Upon annealing in O2 at 500 K, both CoO(100) and CoO(111) structures
were found, whereby the (111) structure shows a ( √ 3 × √ 3)R30 ◦
reconstruction. The corresponding structural model will be presented.
Zeit: Donnerstag 14:00–14:45 Raum: H36
Hauptvortrag O 35.1 Do 14:00 H36
Femtosecond ultraviolet photoelectron spectroscopy for the
study of ultrafast surface processes — •Michael Bauer — Fachbereich
Physik, TU Kaiserslautern, 67663 Kaiserslautern, Deutschland
The technique of ultraviolet photoelectron spectroscopy allows detailed
insights into static properties of molecular adsorption such as bond character,
adsorption geometry or intermolecular interaction.
In combination with an optical pump-probe scheme this technique can
in principle also be used to monitor changes in the adsorbate state on a
femtosecond time-scale and, in consequence, the evolution of the chemical
O 36 Hauptvortrag Weinelt
surface state during the course of a chemical reaction. This has become
possible by the development of laser-driven short-pulse EUV sources delivering
sub-10 fs pulses at photon energies of up to 500 eV.
Recent experimental results will be presented that show the potential
of time-resolved UPS for such studies. In particular it is possible to
identify and follow different steps within a surface chemical reaction at
a time-resolution < 50 fs. This includes the electronic excitation of the
system under investigation, the consequent change in the chemical state
of an adsorbed molecule and the real-time observation of a vibrational
excitation of the adsorbate.
Zeit: Donnerstag 14:45–15:30 Raum: H36
Hauptvortrag O 36.1 Do 14:45 H36
Dynamics of electron relaxation and exciton formation on
Si(001) — •Martin Weinelt — Lehrstuhl für Festkörperphysik,
Universität Erlangen-Nürnberg, Staudtstr. 7, 91058 Erlangen, Germany
Carrier dynamics in silicon is of both fundamental and technological
importance. Equally relevant as the knowledge of the bulk electronic
properties is the understanding of electron dynamics at interfaces. Coupling
of bulk electrons to surface or interface states can dominate electron
recombination and affects device performance. The underlying electrontransfer
processes occur on (sub-)picosecond timescale and can be studied
O 37 Zeitaufgelöste Spektroskopie I
following the electron dynamics after femtosecond-pulse laser excitation.
Identification of the individual processes of carrier scattering, trapping
and recombination requires a detailed knowledge of the surface electronic
structure. The necessary comprehensive information of momentum, energy
and lifetime of excited electrons is obtained by means of angle-,
energy-, and time-resolved two-photon photoelectron spectroscopy. Combining
this experimental approach with many-body perturbation theory
the dynamics of excited electronic states at the Si(100) surface is elucidated.
The recombination of hot carriers at the surface is ruled by
picosecond relaxation of excited electron-hole pairs, resulting in the formation
of an exciton which lives for nanoseconds.
Zeit: Donnerstag 15:45–17:30 Raum: H36
O 37.1 Do 15:45 H36
Electron-phonon coupling for surface states on Pd(111) —
•Andrea Melzer, Martin Weinelt, and Thomas Fauster —
Lehrstuhl für Festkörperphysik, Staudtstraße 7, D-91058 Erlangen
Electrons in surface states can be scattered by phonons to bulk states
or to states within the surface band at different parallel momentum. The
first process usually requires phonons with large momentum compared
to the second process. On the Pd(111) surface an unoccupied sp-like
surface state exists and can be studied by time-resolved two-photon photoemission.
Scattering to bulk states reduces the population in contrast
to scattering within the surface band. The first process shows up in a
reduced lifetime while both processes increase the linewidth. We have
measured the change of lifetime and linewidth for the surface state on
Pd(111) for variable temperatures up to 900 K. For the electron-phonon
mass enhancement parameter a value of 0.36 is found which is significantly
larger than for the occupied surface states on other fcc(111) surfaces.
The time-resolved data indicate large contributions from intraband
scattering in contrast to theoretical expectations.
O 37.2 Do 16:00 H36
Break junctions under femtosecond laser illumination:
steps towards time-resolved photocurrent spectroscopy on
the nanoscale — •W. Pfeiffer 1 , S. Dantscher 1 , C. Kennerknecht
1 , S. Schramm 1 , H.B. Weber 2 , and J.U. Würfel 2 —
1 Physikalisches Institut EP1, Universität Würzburg, 97074 Würzburg
— 2 Forschungszentrum Karlsruhe, Institut für Nanotechnologie,
D-76021 Karlsruhe
Microscopic break junctions provide fascinating possibilities to investigate
microscopic charge transport phenomena. Up to now the studies
are restricted to DC current measurements. Consequently, the illumination
of the contact and the investigation of the resulting photocurrents
opens a new field of research. Especially, ultrashort laser pulses combined
with time-resolved spectroscopy could then reveal details of the charge
transfer dynamics that are of utmost importance for the understanding
of the conductivity in nanoscale contacts. We present first experiments
on the illumination of tunnel junctions and single molecule contacts with
ultrashort laser pulses (800 nm and 400 nm, 50 fs). The junctions are
stable up to intensities of 10 8 Wcm −2 and thus allow the investigation of
microscopic transport in intense laser fields. The mechanisms leading to
a light induced modulation of the conductance are discussed for tunnel
junctions and single molecule contacts.
O 37.3 Do 16:15 H36
Lifetimes of quasiparticle excitations in 4d transition metals
Mo and Rh — •Alexander Mönnich, Daniela Bayer, Michael
Bauer, and Martin Aeschlimann — Dep. of Physics, University of
Kaiserslautern, D-67663 Kaiserslautern
The dynamics of excited electrons in metals are crucial for a detailed
understanding of various chemical and physical phenomena on metal surfaces.
It is already known that noble metals show a longer lifetime of
quasiparticle excitations than transition metals due to the higher densityof-states
around the Fermi level. But also the electron dynamics between
different transition metals can exhibit significant divergences.
With the time resolved two-photon photoelectron spectroscopy (TR-
2PPE) method we investigated the electron dynamics of Mo and Rh in an
energy range up to 3eV above the Fermi level. LMTO-RPG-GW calculations
predict a surprising large difference between the averaged lifetimes
of electron quasiparticles in the 4d transition metals Mo and Rh [1] that
were confirmed in our experiment. The characteristics of electronic structure
responsible for energy dissipation processes of hot electrons will be
discussed.
[1] V. P. Zhukov, F. Aryasetiawan, E. V. Chulkov, P. M. Enchenique
PRB 65 11511 (2002)