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 10.07.2008   Карта сайта     Language По-русски По-английски
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10.07.2008

Nature 454, 196-199 (10 July 2008) | doi:10.1038/nature07099; Received 19 January 2008; Accepted 8 May 2008



Quadrupole transitions revealed by Borrmann spectroscopy


Robert F. Pettifer1,4, Stephen P. Collins2,1 & David Laundy3




  1. Department of Physics, University of Warwick, Coventry CV4 7AL, UK
  2. Diamond Light Source Ltd, Diamond House, Harwell Science and Innovation Campus, Didcot O11 0DE, UK
  3. STFC Daresbury Laboratory, Daresbury, Warrington WA4 4AD, UK
  4. Deceased.


Correspondence to: Stephen P. Collins2,1 Correspondence and requests for materials should be addressed to S.P.C. (Email: steve.collins@diamond.ac.uk).





The Borrmann effect1, 2—a dramatic increase in transparency to X-ray beams—is observed when X-rays satisfying Bragg's law diffract through a perfect crystal. The minimization of absorption seen in the Borrmann effect has been explained by noting that the electric field of the X-ray beam approaches zero amplitude at the crystal planes, thus avoiding the atoms. Here we show experimentally that under conditions of absorption suppression, the weaker electric quadrupole absorption transitions are effectively enhanced to such a degree that they can dominate the absorption spectrum. This effect can be exploited as an atomic spectroscopy technique; we show that quadrupole transitions give rise to additional structure at the L1, L2 and L3 absorption edges of gadolinium in gadolinium gallium garnet, which mark the onset of excitations from 2s, 2p 1/2 and 2p 3/2 atomic core levels, respectively. Although the Borrmann effect served to underpin the development of the theory of X-ray diffraction3, 4, 5, 6, this is potentially the most important experimental application of the phenomenon since its first observation seven decades ago. Identifying quadrupole features in X-ray absorption spectroscopy is central to the interpretation of 'pre-edge' spectra, which are often taken to be indicators of local symmetry, valence and atomic environment7. Quadrupolar absorption isolates states of different symmetries to that of the dominant dipole spectrum, and typically reveals orbitals that dominate the electronic ground-state properties of lanthanides and 3d transition metals, including magnetism. Results from our Borrmann spectroscopy technique feed into contemporary discussions regarding resonant X-ray diffraction8 and the nature of pre-edge lines identified by inelastic X-ray scattering7. Furthermore, because the Borrmann effect has been observed in photonic materials, it seems likely that the quadrupole enhancement reported here will play an important role in modern optics.



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