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Plenarvorträge - DPG-Tagungen

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Oberflächenphysik Freitag<br />

going from Co to Ir reflecting the spatial extension of the 3d, 4d and 5d<br />

orbitals. The observed trends are in good agreement with calculated d−f<br />

hybridization matrix elements and, consequently, d − f hybridization is<br />

governed by the spatial overlap of the respective orbitals.<br />

O 44.6 Fr 12:30 H38<br />

Characterisation of the chemical composition and electronic<br />

structure of LiCoO2 thin films using photoelectron<br />

spectroscopy — •Andreas Thissen, David Ensling, Javier<br />

Fernandez-Madrigal, and Wolfram Jaegermann — FB<br />

Materialwissenschaften-Oberflaechenforschung, TU Darmstadt,<br />

Petersenstr. 23, D-64287 Darmstadt<br />

LiCoO2 is used as cathode materials for Li intercalation batteries.<br />

Polycrystalline thin films have been prepared by magnetron sputtering<br />

in an UHV chamber connected to an analysis system. Chemical analysis<br />

using (S)XPS shows that a variation of the sputtering parameters<br />

strongly influences the stoichiometry of the films, leading to different Co<br />

oxidation states (2+, 3+, 4+). Depending on the chemical composition<br />

changes in the electronic structure have been studied by UPS, ResPES<br />

and XAS. The results are in good agreement with DFT band structure<br />

calculations reported in literature. From ResPES measurements the separation<br />

of the O2p- and Co3d-like valence band contributions was possible,<br />

giving the occupation numbers as well. It has been shown by AFM,<br />

REM, XRD, Raman spectroscopy and EXAFS, that also the morphology<br />

and crystal structure of the samples depends on preparation conditions.<br />

(Dis)charging experiments have been carried out in an in-situ battery<br />

device in order to perform XPS and UPS measurements of the cathode<br />

during battery operation. Changes in the electronic structure of the Li1xCoO2<br />

host can clearly be correlated to the charging state of the battery.<br />

O 44.7 Fr 12:45 H38<br />

Second Harmonic Generation on NiO — •Georgios Lefkidis,<br />

Khompat Satitkovitchai, and Wolfgang Hübner — TU Kaiserslautern,<br />

PO Box 3409, D-67653, Germany<br />

The second order susceptibility tensors, which are important for Optical<br />

Second Harmonic Generation (SHG) are studied for NiO starting<br />

from ab-initio calculations. To represent the (001) surface of the rock-<br />

salt NiO lattice we embed a NiO −8<br />

5 cluster in a charge point field which<br />

accounts for the Madelung potential due to the neighboring ions. Sim-<br />

O 45 Zeitaufgelöste Spektroskopie II<br />

ilarly, an embedded NiO −10<br />

6 cluster is used to represent the bulk. The<br />

quantum chemical calculation is performed at various levels, where the<br />

effect of the basis set is studied, and the energy correction due to electron<br />

correlations. A Complete Active Space Configuration Interaction is<br />

performed, while the excited states are studied with Single Excitation<br />

Configuration Interaction (CIS) with energy contribution from double<br />

and triple excitations. The electric and magnetic dipole transitions are<br />

used to calculate the χ (2ω)<br />

eee and χ (2ω)<br />

mee tensors.<br />

While inversion symmetry of the bcc lattice leads to zero tensor elements<br />

for the bulk, the breaking of the symmetry on the surface gives<br />

non-zero elements within the electric-dipole approximation. On the other<br />

hand some χ (2ω)<br />

mee elements exist for both bulk and surface. The effect of<br />

spin-orbit coupling on both tensors is discussed.<br />

O 44.8 Fr 13:00 H38<br />

An X-ray absorption (NEXAFS) and resonant inelastic X-ray<br />

scattering (RIXS) study of liquid water and aqueous solutions<br />

— •O. Fuchs 1 , L. Weinhardt 1 , F. Maier 1 , C. Heske 1 , E. Umbach<br />

1 , Y. Zubavichus 2 , M. Grunze 2 , M. Odelius 3 , L.G.M. Pettersson<br />

3 , A. Nilsson 4 , J.D. Denlinger 5 , Z. Hussain 5 , and R. Follath<br />

6 — 1 Exp. Physik II, Uni Würzburg — 2 Angew. Physikal. Chemie,<br />

Uni Heidelberg — 3 Fysikum, Stockholm University — 4 Stanford Synchrotron<br />

Radiation Lab — 5 ALS, Berkeley — 6 BESSY, Berlin<br />

The electronic and geometric structure of liquids and, in particular, of<br />

water and aqueous solutions is currently strongly discussed and investigated.<br />

Until recently, most experimental results were obtained with neutron<br />

and hard X-ray scattering experiments. Recently, soft X-ray spectroscopic<br />

methods such as fluorescence-yield NEXAFS (near-edge X-ray<br />

absorption fine structure) and RIXS were applied and compared with<br />

theoretical DFT calculations and Car-Parinello molecular dynamics simulations.<br />

This combination yields complementary information especially<br />

about the dynamic hydrogen bond network and the molecular dynamics.<br />

A third-generation synchrotron, a highly efficient spectrometer for soft<br />

x-rays, and suitably designed wet cells are essential for RIXS experiments<br />

on solutions. This talk presents a new spectrometer design consisting of<br />

a spherical mirror and a plane variable line spacing grating for the sulfur<br />

L2,3, carbon K, nitrogen K, and oxygen K edges, dedicated to the study of<br />

biologically relevant molecules. Furthermore, RIXS and NEXAFS measurements<br />

for water, heavy water, NaOH solutions (pH 15), and various<br />

other aqueous solutions are discussed.<br />

Zeit: Freitag 11:15–13:00 Raum: H39<br />

O 45.1 Fr 11:15 H39<br />

Two-pulse correlation of femtosecond laser-induced diffusion of<br />

oxygen on Pt(111) — •K. Stépán, J. Güdde, and U. Höfer —<br />

Fachbereich Physik, Philipps-Universität Marburg, D-35032 Marburg<br />

We have studied the mechanism of oxygen diffusion on Pt(111) induced<br />

by femtosecond laser pulses. The dissociative adsorption of O2 on a vicinal<br />

surface was used to selectively decorate the step sites with atomic<br />

oxygen [1]. Diffusion from the step edges onto the terraces was induced<br />

at a substrate temperature of T = 80 K by 100-fs, 800-nm pump pulses<br />

with absorbed fluences of 8-16 mJ/cm 2 . It was monitored in situ with<br />

optical second-harmonic generation (SHG) by exploiting the sensitivity<br />

of SHG to surface symmetry. The diffusion mechanism was explored in<br />

the time domain by performing measurements of the diffusion efficiency<br />

as a function of the delay time between two cross-polarized pump pulses.<br />

The two-pulse correlation has a width of about 1 ps. This indicates a<br />

diffusion process induced by multiple excitations from the hot electron<br />

gas of the metal, similar to the meanwhile well-studied phenomenon of<br />

desorption induced by multiple electronic excitations (DIMET) [2].<br />

[1] P. Gambardella et al. Phys. Rev. Lett. 87, 056103 (2001)<br />

[2] J. A. Misewich et al. Phys. Rev. Lett. 68, 3737 (1992)<br />

O 45.2 Fr 11:30 H39<br />

Electron Dynamics of C6F6/Cu(111) studied by time-resolved<br />

photoelectron and resonant Auger-Raman spectroscopy —<br />

•Patrick Kirchmann 1 , Panagiotis Loukakos 1 , Uwe Bovensiepen<br />

1 , Martin Wolf 1 , Vijayalakshmi Sethuraman 2 , Franz<br />

Hennies 2 , Alexander Föhlisch 2 und Wilfried Wurth 2 — 1 Freie<br />

Universität Berlin, Fachbereich Physik, Arnimallee 14, 14195 Berlin<br />

— 2 Universität Hamburg, Institut für Experimentalphysik, Luruper<br />

Chaussee 149, 22761 Hamburg<br />

Ultrafast charge transfer times in molecular resonances at metal substrates<br />

can be obtained in the frequency domain by the ”atomic-clock”<br />

method, i.e. where the ratio between ”Raman” type and ”Auger” type<br />

decay channels of a resonant core hole excitation is evaluated [1]. On<br />

the other hand, the temporal evolution of the population of photoinjected<br />

electrons into an adsorbate resonance by a femtosecond laser pulse<br />

can be probed directly in the time-domain by two-photon-photoemission<br />

(2PPE)[2]. To compare these two approaches quantitatively, we performed<br />

systematic experiments on C6F6/Cu(111) employing resonant X-ray<br />

photoelectron spectroscopy at BESSY II and time-resolved 2PPE using<br />

sub 30 fs laser pulses in the laboratory. Respective procedures of data<br />

analysis and coverage dependent electron lifetimes will presented.<br />

[1] W. Wurth and D. Menzel, Chem. Phys. 251, 141 (2000).<br />

[2] C. Gahl, K. Ishioka, Q. Zhong, A. Hotzel, and M. Wolf; Faraday<br />

Discussion 117, 191 (2000).

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