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The interpretation of K pre-edges, RIXS-MCD and distortions in fluorescence yield spectra

LCLS-SSRL Seminar
by LCLS Announcement
LCLS-SSRL seminar "The interpretation of K pre-edges, RIXS-MCD and
distortions in fluorescence yield spectra", Frank de Groot (Utrecht University), January 20, 2010, 3:30 PM, SSRL Conference Room, B137, Room 322.

A brief introduction is given of x-ray absorption spectroscopy, including the multiplet interpretation of XAS spectral shapes [1]. The user-friendly CTM4XAS program can be downloaded for free [2]. It includes the atomic multiplet, crystal field and charge transfer effects and allows a detailed simulation of XAS, XPS and XES spectra of transition metal systems.
The main part of the talk deals with the role of Resonant Inelastic X-ray Scattering (RXES) to unravel the nature of the states that are visible in the pre-edge region of the 3d metal K edges [3]. 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. LDA+U calculations prove that the screening of the core hole is strongly orientation dependent [4].
X-ray magnetic circular dichroism (XMCD) is a powerful tool for the element-specific study of the magnetic structure of complex systems. The magnetic moments of 3d transition metals (TM) are generally studied at the L absorption edge using soft X-rays whose short penetration depth limits the number of possible applications. It will be shown that XMCD combined with RIXS at the K absorption pre-edge of Fe in magnetite yields a dichroic signal that is of the same order of magnitude as L-edge XMCD. This allows for truly bulk sensitive, element- and site-selective measurements of 3d TM magnetic moments and their ordering under demanding sample environments [5].
At the end, three examples are given where intrinsic differences arise between fluorescence and electron yield:
(1) energy dependent fluorescence yield cross sections
(2) broken coherence effects in fluorescence yield [2].
(3) additional structure in the 2p4f quadrupole L3 pre-edge region of La systems.

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[1] Core Level Spectroscopy of Solids
Frank de Groot and Akio Kotani (Taylor & Francis CRC press, 2008)
[2] Email to f.m.f.degroot@uu.nl to download.
[3] F. de Groot, G. Vankó and P. Glatzel, J. Phys. Cond. Matt. 21, 104207 (2009)
[4] A. Juhin, F de Groot et al (submitted)
[5] M.Sikora, A. Juhin, F de Groot, P. Glatzel et al (submitted)
[6] E. Suljoti, F. M. F. de Groot, et al. Phys. Rev. Lett. 103, 137401 (2009)
Questions? Comments? You can contact the author at communications@slac.stanford.edu.