We study the dynamic diffraction of atoms by a standing light wave in the limit where the recoil shift is much larger than the atomic linewidth. We demonstrate the appearance of a damped Pendellösung as well as anomalous transmission, which is equivalent to optical pumping into dark superpositions of the same internal atomic state with different external momenta. As an example, we discuss the case of metastable He coupled to the ground state via an anti-Stokes - Raman transition. We show that besides leading to a very narrow velocity selection for the deflected atoms, this effect could, in principle, be used to build a large-angle coherent beamsplitter and a large-area interferometer for metastable helium. We calculate the signal contrast and flux including initial velocity spread and decoherence through spontaneous emission for such an interferometer.