Quadrupole transitions revealed by Borrmann spectroscopy

Borrmann spectroscpy: Invisible touch The Borrmann effect is one of the most remarkable phenomena in X-ray diffraction physics, causing the sudden invisibility of a thick, opaque crystal to X-rays. At a precisely judged angle of incidence and wavelength, X-rays can pass through a crystal with hardly any absorption, seemingly avoiding interactions with the atomic planes. This is due to a large reduction of the interaction with the electric dipoles produced by the atoms. Now Pettifer et al. show that while electric dipole transitions at the atoms are reduced in the Borrmann effect, the more exotic electric quadrupole transitions become stronger. Exploiting this effect leads to a new type of X-ray absorption spectroscopy that probes electronic states that are difficult to measure with other approaches. In this first study, analysis of the X-ray absorption spectra from gadolinium gallium garnet shows clear quadrupole features associated with absorption events, some previously unknown.