X-ray spectroscopy under the Bormann transmission condition

When X-rays satisfy Bragg's law for a perfect crystal, a significant transparency to X-ray beams is observed. This is the so-called Bormann effect, and is caused because the X-ray electric field approaches zero amplitude at the crystal planes, corresponding to almost no scattering by atoms. Recently, Dr. S. P. Collins (Diamond Light Source, United Kingdom) and his colleagues attempted several very interesting experiments - X-ray spectroscopy under the Bormann transmission condition. The main idea is that the electric quadrupole absorption transitions could be effectively enhanced under conditions of absorption suppression. The measured sample is gadolinium gallium garnet (Gd₃Ga₅0₁₂) cut parallel to the (100) planes, and some new spectral features were observed in the L_I (8,376 eV), L_II (7,930 eV) and L_III (7,243 eV) edges for gadolinium, at different temperatures. They are basically additional peaks on the low energy side, and correspond to an electric quadrupole transition from 2s, 2p_1/2 and 2p_2/3 to the narrow, half-filled 4f states, respectively. For more information, see the paper, "Quadrupole transitions revealed by Borrmann spectroscopy", R. F. Pettifer et al., Nature, 454, 196-199 (2008).