"Forbidden" reflections in resonant diffraction of synchrotron radiation in yttrium aluminum garnet [Y.sub.3][Al.sub.5][O.sub.12]

The purely resonant Bragg reflections (13, 13, 0) and (14, 0, 0) in yttrium aluminum garnet [Y.sub.3][Al.sub.5][O.sub.12] at energies near the K absorption edge of yttrium have been studied experimentally and theoretically. The anisotropic tensor atomic factor of yttrium corresponding to dipole-dipole resonance transitions depends on three independent parameters changing with energy. The intensities of the reflections (14, 0, 0) and (13, 13, 0) are shown to depend on the parameter difference [f.sub.1](E) - [f.sub.2](E) and the parameter [f.sub.3](E), respectively, which are attributable to distortions of the wave functions of the excited atoms and change greatly with photon energy E. Studying various reflections has allowed one to determine the various components of the tensor atomic factor and to compare them with the results of numerical calculations.