11:00 AM - *SF02.01.01
Direct Measurement of 5f Delocalization with U XES
University of Wisconsin-Oshkosh1
Delocalization in the actinide 5f states is a very important phenomenon, but not very well understood. The effect of 5f delocalization can be seen in one of the most fundamental of elemental parameters, atomic size. In the early actinides, the Wigner-Seitz radii change with filling in a manner consistent with the addition of delocalized electrons. [1-3] The observation of this effect was so striking that it lead temporarily to the incorrect hypothesis that the Actinides were a 6d, not 5f, filling series.  This misconception was subsequently corrected, as it was shown that 5f filling could account for the observed behavior.  Nevertheless, the measurement of 5f dispersions with angle-resolve photoelectron spectroscopy has been something of a disappointment. While metallic U valence states can show energy variation with crystal momentum [6,7], the dispersion of the 5f derived states is very weak , on the order of 0.1 eV, with little or no exhibition of connection to the high symmetry points or lines in the Brillouin Zone, both of which are easily seen in strongly dispersing systems. [9-13] Moreover, it has been known for decades [14,15] that the X-ray Absorption Spectroscopy (XAS) Branching Ratio (BR) is the same for localized n= 2 systems (UF4 and UO2) and the delocalized, n = 3 system, metallic U. Thus, the situation is problematic: there should be strong 5f dispersion in the early actinides, but spectroscopically no manifestation can be found.
Here, that situation is rectified. It will be shown that X-Ray Emission Spectroscopy (XES) of the M4,5 levels can provide the needed information, with a distinct difference between the delocalized U metal and localized uranium dioxide and uranium tetrafluoride cases. A Peak Ratio (PR) model, built upon electric dipole selection rules, has been developed and utilized, with quantitative agreement between experiment and theory. Possible expansion to other types of 5f mixing systems will be discussed.
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Collaborators: S. Nowak, R. Alonso-Mori, T. Kroll, D. Nordlund, T.-C. Weng, and D. Sokaras, SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA; S.-W. Yu, Lawrence Livermore National Laboratory, Livermore, CA, 94550, USA