Novel Approaches to Unravel the Nature of X-Ray Absorption Spectra

 

Frank de Groot

 

Inorganic Chemistry and Catalysis

Utrecht University, Netherlands

 

New experimental techniques and developments in the theoretical analysis of X-ray absorption will be discussed. Topic (c) discusses new IR experiments using a FEL source:

(a)    An introduction to X-ray absorption spectroscopy

(b)   X-ray spectromicroscopy under in-situ conditions

(c)    Multiple Photon Dissociation IR of cluster-adsorbate complexes

(d)   The consequences of the improved resolution in metal K edges.

 

(a)    In the first part of the talk, an introduction is given of x-ray absorption spectroscopy, including the multiplet interpretation of XAS spectral shapes. The atomic multiplet, crystal field and charge transfer effects will, in short, be explained. Some recent developments include the determination of the differential-orbital covalence and the analysis of the charge transfer multiplet ground state into single-particle components.

(b)   With the recent developments in nanoreactors, Scanning Transmission X-ray Microscopy (STXM) experiments make it possible to follow metal L edges at 1 bar and 500 C, with a spatial resolution of 25 nm. This allows the in-situ spectroscopic study of a single nanoparticle (in action). Recent STXM results show the spatial distribution of the degree of reduction of single iron-oxide nanoparticles.

(c)    Multi-photon dissociation (MPD) IR is measured using the Free Electron Laser for Infrared eXperiments (FELIX) in Rijnhuizen. We have studied Sc, V, Fe, Co, Ni and Rh nanoparticles in interaction with CO and/or H2 molecule. This research has provided a number of remarkable nanoparticle-adsorbate structures, for example the highly symmetric V6H8CO+ nanoparticle-adsorbate complex. In case of rhodium, it is shown that the saturated cationic carbonyl complexes Rhn(CO)m do not contain the bridge-bonded CO that occurs for the neutral Rh2(CO)8 and Rh4(CO)12 molecules. 

(d)   The last part of the talk deals with the role of Resonant X-ray Emission Spectroscopy (RXES) to unravel the nature of the states that are visible in the pre-edge region of the 3d metal K edges. Using the 1s2p RXES of LiCoO2 as example, the presence of a 1s4p dipole pre-edge next to the traditional 1s3d quadrupole pre-edge is explained. The non-local nature of this dipole pre-edge peak is proven from its absence in systems with isolated Co atoms and trends as a function of Co-O-Co angle. In the outlook, a number of additional experimental routes is suggested, which shows that the use of HERFD and selective XAS techniques is only just starting.

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The book Core Level Spectroscopy of Solids has appeared in February 2008.
See the website (also contains reprints of most papers) for details:
http://www.anorg.chem.uu.nl/people/staff/FrankdeGroot/index.htm
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