  We present a theory for the diffraction of large molecules or nanoparticles
at a standing light wave. Such particles can act as a genuine photon absorbers
due to their numerous internal degrees of freedom effecting fast internal
energy conversion. Our theory incorporates the interplay of three light-induced
properties: the coherent phase modulation due to the dipole interaction, a
non-unitary absorption-induced amplitude modulation described as a generalized
measurement, and a coherent recoil splitting that resembles a quantum random
walk in steps of the photon momentum. We discuss how these effects show up in
near-field and far-field interference schemes, and we confirm our effective
description by a dynamic evaluation of the grating interaction, which accounts
for the internal states.
