Die diesjährige SEI-Frühjahrstagung (100. Tagung!) findet vom 15. bis 17. März 2010 am DESY in Hamburg statt. Verantwortlicher ist letztmalig Herr Dr. Friedrich Wulf.

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The metal L2,3 x-ray absorption near-edge structure (XANES) of 3d transition metal (TM) compounds shows widely spread multiplet structures because of the strong interaction between the core TM-2p and 3d electrons. Therefore, one-electron calculations cannot reproduce the experimental spectra in general. The charge transfer multiplet method is the most prevalent and conventional theoretical approach for the analysis of TM-L2,3 XANES[1]. Although this approach has been successful in reproducing many experimental spectra, it cannot be used to predict multiplet structure a priori, because of the use of the adjustable parameters. An ab-initio calculation that takes multiplet effects into account is therefore strongly desirable.For this purpose, we have developed an ab-initio configuration interaction (CI) method for L2,3 XANES. In this method, fully relativistic molecular orbitals (MOs) obtained by relativistic density functional theory (DFT) are used to construct the many-electron wave-functions. The one-electron and the two-electron integrals, which determine the multiplet energies, are directly evaluated over MOs. All ligand field effects are included by using MOs. This technique has been successfully used to quantitatively predict TM-L2,3 spectra for a range of actual oxide geometries, without any adjustable parameters[2,3]. The charge transfer from ligand to TM can be included by adding more electronic configurations in the CI[4].

Water is the key compound for our existence on this planet and it is involved in many important physical, chemical, biological and geological processes. Although water is the most common molecular substance it is also most unusual with many anomalies in its thermodynamic properties such as compressibility, density variation and heat capacity. The question of the structure of the hydrogen bonding network in water has been discussed intensively for over 100 years and has not yet been resolved. This talk will describe recent x-ray spectroscopy and scattering measurements showing that the liquid can be described as fluctuations between two types of local hydrogen bonded structures driven by incommensurate requirements for minimizing enthalpy and maximizing entropy. The connection of these results to low and high density water and the 2^nd critical point model will be discussed. Ion solvation, hydrophobic interactions and bonding to metal surfaces will also be discussed.

Electrocatalytic energy conversion processes in fuel cells are expected to play a major role in the development of sustainable technologies to mitigate global warming and to lower our dependence on fossil fuels. I will demonstrate how electron and x-ray spectroscopy can be used to address fundamental questions regarding the reaction mechanism of the oxygen reduction reaction (ORR) on Pt and probe the electronic structure of Pt and adsorbed species of model system Pt catalysts. From time resolved x-ray photoelectron spectroscopy (XPS) we have determined the activation barrier for the O₂ dissociation process on Pt to be in the range around 0.25 eV. Using in-situ studies under real electrochemical conditions of single crystal surfaces using high resolution Pt L-edge spectroscopy we have identified oxide growth at certain potentials that could have some major influence on the rate of the ORR reaction. Recent dealloyed Pt-Cu catalysts have shown an enhanced activity in comparison to pure Pt by factor of 5 tested in a real fuel cell [1]. We demonstrate that this activity enhancement comes from compression strain of the Pt lattice that leads to modifications of the Pt electronic structure and subsequently weakens the surface bond of adsorbed oxygen species.

1. R. Srivats et al. Angew. Chem. Int. Ed. 46, 8988 (2007)