Resonant X-ray emission with a standing wave excitation

The Borrmann effect is the anomalous transmission of X-rays in perfect crystals under diffraction conditions. It arises from the interference of the incident and diffracted waves, which creates a standing wave with nodes at strongly absorbing atoms. Dipolar absorption of X-rays is thus diminished, which makes the crystal nearly transparent for certain X-ray wave vectors. Indeed, a relative enhancement of electric quadrupole absorption via the Borrmann effect has been demonstrated recently. Here we show that the Borrmann effect has a significantly larger impact on resonant X-ray emission than is observable in X-ray absorption. Emission from a dipole forbidden intermediate state may even dominate the corresponding X-ray spectra. Our work extends the domain of X-ray standing wave methods to resonant X-ray emission spectroscopy and provides means for novel spectroscopic experiments in d-and f-electron systems.