Plenarvorträge - DPG-Tagungen

Oberflächenphysik Freitag
going from Co to Ir reflecting the spatial extension of the 3d, 4d and 5d
orbitals. The observed trends are in good agreement with calculated d−f
hybridization matrix elements and, consequently, d − f hybridization is
governed by the spatial overlap of the respective orbitals.
O 44.6 Fr 12:30 H38
Characterisation of the chemical composition and electronic
structure of LiCoO2 thin films using photoelectron
spectroscopy — •Andreas Thissen, David Ensling, Javier
Fernandez-Madrigal, and Wolfram Jaegermann — FB
Materialwissenschaften-Oberflaechenforschung, TU Darmstadt,
Petersenstr. 23, D-64287 Darmstadt
LiCoO2 is used as cathode materials for Li intercalation batteries.
Polycrystalline thin films have been prepared by magnetron sputtering
in an UHV chamber connected to an analysis system. Chemical analysis
using (S)XPS shows that a variation of the sputtering parameters
strongly influences the stoichiometry of the films, leading to different Co
oxidation states (2+, 3+, 4+). Depending on the chemical composition
changes in the electronic structure have been studied by UPS, ResPES
and XAS. The results are in good agreement with DFT band structure
calculations reported in literature. From ResPES measurements the separation
of the O2p- and Co3d-like valence band contributions was possible,
giving the occupation numbers as well. It has been shown by AFM,
REM, XRD, Raman spectroscopy and EXAFS, that also the morphology
and crystal structure of the samples depends on preparation conditions.
(Dis)charging experiments have been carried out in an in-situ battery
device in order to perform XPS and UPS measurements of the cathode
during battery operation. Changes in the electronic structure of the Li1xCoO2
host can clearly be correlated to the charging state of the battery.
O 44.7 Fr 12:45 H38
Second Harmonic Generation on NiO — •Georgios Lefkidis,
Khompat Satitkovitchai, and Wolfgang Hübner — TU Kaiserslautern,
PO Box 3409, D-67653, Germany
The second order susceptibility tensors, which are important for Optical
Second Harmonic Generation (SHG) are studied for NiO starting
from ab-initio calculations. To represent the (001) surface of the rock-
salt NiO lattice we embed a NiO −8
5 cluster in a charge point field which
accounts for the Madelung potential due to the neighboring ions. Sim-
O 45 Zeitaufgelöste Spektroskopie II
ilarly, an embedded NiO −10
6 cluster is used to represent the bulk. The
quantum chemical calculation is performed at various levels, where the
effect of the basis set is studied, and the energy correction due to electron
correlations. A Complete Active Space Configuration Interaction is
performed, while the excited states are studied with Single Excitation
Configuration Interaction (CIS) with energy contribution from double
and triple excitations. The electric and magnetic dipole transitions are
used to calculate the χ (2ω)
eee and χ (2ω)
mee tensors.
While inversion symmetry of the bcc lattice leads to zero tensor elements
for the bulk, the breaking of the symmetry on the surface gives
non-zero elements within the electric-dipole approximation. On the other
hand some χ (2ω)
mee elements exist for both bulk and surface. The effect of
spin-orbit coupling on both tensors is discussed.
O 44.8 Fr 13:00 H38
An X-ray absorption (NEXAFS) and resonant inelastic X-ray
scattering (RIXS) study of liquid water and aqueous solutions
— •O. Fuchs 1 , L. Weinhardt 1 , F. Maier 1 , C. Heske 1 , E. Umbach
1 , Y. Zubavichus 2 , M. Grunze 2 , M. Odelius 3 , L.G.M. Pettersson
3 , A. Nilsson 4 , J.D. Denlinger 5 , Z. Hussain 5 , and R. Follath
6 — 1 Exp. Physik II, Uni Würzburg — 2 Angew. Physikal. Chemie,
Uni Heidelberg — 3 Fysikum, Stockholm University — 4 Stanford Synchrotron
Radiation Lab — 5 ALS, Berkeley — 6 BESSY, Berlin
The electronic and geometric structure of liquids and, in particular, of
water and aqueous solutions is currently strongly discussed and investigated.
Until recently, most experimental results were obtained with neutron
and hard X-ray scattering experiments. Recently, soft X-ray spectroscopic
methods such as fluorescence-yield NEXAFS (near-edge X-ray
absorption fine structure) and RIXS were applied and compared with
theoretical DFT calculations and Car-Parinello molecular dynamics simulations.
This combination yields complementary information especially
about the dynamic hydrogen bond network and the molecular dynamics.
A third-generation synchrotron, a highly efficient spectrometer for soft
x-rays, and suitably designed wet cells are essential for RIXS experiments
on solutions. This talk presents a new spectrometer design consisting of
a spherical mirror and a plane variable line spacing grating for the sulfur
L2,3, carbon K, nitrogen K, and oxygen K edges, dedicated to the study of
biologically relevant molecules. Furthermore, RIXS and NEXAFS measurements
for water, heavy water, NaOH solutions (pH 15), and various
other aqueous solutions are discussed.
Zeit: Freitag 11:15–13:00 Raum: H39
O 45.1 Fr 11:15 H39
Two-pulse correlation of femtosecond laser-induced diffusion of
oxygen on Pt(111) — •K. Stépán, J. Güdde, and U. Höfer —
Fachbereich Physik, Philipps-Universität Marburg, D-35032 Marburg
We have studied the mechanism of oxygen diffusion on Pt(111) induced
by femtosecond laser pulses. The dissociative adsorption of O2 on a vicinal
surface was used to selectively decorate the step sites with atomic
oxygen [1]. Diffusion from the step edges onto the terraces was induced
at a substrate temperature of T = 80 K by 100-fs, 800-nm pump pulses
with absorbed fluences of 8-16 mJ/cm 2 . It was monitored in situ with
optical second-harmonic generation (SHG) by exploiting the sensitivity
of SHG to surface symmetry. The diffusion mechanism was explored in
the time domain by performing measurements of the diffusion efficiency
as a function of the delay time between two cross-polarized pump pulses.
The two-pulse correlation has a width of about 1 ps. This indicates a
diffusion process induced by multiple excitations from the hot electron
gas of the metal, similar to the meanwhile well-studied phenomenon of
desorption induced by multiple electronic excitations (DIMET) [2].
[1] P. Gambardella et al. Phys. Rev. Lett. 87, 056103 (2001)
[2] J. A. Misewich et al. Phys. Rev. Lett. 68, 3737 (1992)
O 45.2 Fr 11:30 H39
Electron Dynamics of C6F6/Cu(111) studied by time-resolved
photoelectron and resonant Auger-Raman spectroscopy —
•Patrick Kirchmann 1 , Panagiotis Loukakos 1 , Uwe Bovensiepen
1 , Martin Wolf 1 , Vijayalakshmi Sethuraman 2 , Franz
Hennies 2 , Alexander Föhlisch 2 und Wilfried Wurth 2 — 1 Freie
Universität Berlin, Fachbereich Physik, Arnimallee 14, 14195 Berlin
— 2 Universität Hamburg, Institut für Experimentalphysik, Luruper
Chaussee 149, 22761 Hamburg
Ultrafast charge transfer times in molecular resonances at metal substrates
can be obtained in the frequency domain by the ”atomic-clock”
method, i.e. where the ratio between ”Raman” type and ”Auger” type
decay channels of a resonant core hole excitation is evaluated [1]. On
the other hand, the temporal evolution of the population of photoinjected
electrons into an adsorbate resonance by a femtosecond laser pulse
can be probed directly in the time-domain by two-photon-photoemission
(2PPE)[2]. To compare these two approaches quantitatively, we performed
systematic experiments on C6F6/Cu(111) employing resonant X-ray
photoelectron spectroscopy at BESSY II and time-resolved 2PPE using
sub 30 fs laser pulses in the laboratory. Respective procedures of data
analysis and coverage dependent electron lifetimes will presented.
[1] W. Wurth and D. Menzel, Chem. Phys. 251, 141 (2000).
[2] C. Gahl, K. Ishioka, Q. Zhong, A. Hotzel, and M. Wolf; Faraday
Discussion 117, 191 (2000).