  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.
