Resonance energy shifts during nuclear Bragg diffraction of x rays

We have observed dramatic changes in the time distribution of synchrotron x rays resonantly scattered from ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ nuclei in a crystal of yttrium iron garnet, which depend on the deviation angle of the incident radiation from the Bragg angle. These changes are caused by small shifts in the effective energies of the hyperfine-split nuclear resonances, an effect of dynamical diffraction for the coherently excited nuclei in the crystal. The very high brightness of the synchro- tron x-ray source allows this effect to be observed in a 15-min measurement.