  We show that matter-wave diffraction off a single standing laser wave can be
used as an accurate measurement scheme for photophysical molecular parameters.
These include state-dependent optical polarizabilities and photon-absorption
cross sections, the relaxation rates for fluorescence, internal conversion, and
intersystem crossing, as well as ionization or cleavage probabilities. We
discuss how the different photophysical processes manifest as features of the
interference pattern, and determine the accuracy of molecular parameters
estimated from a realistic measurement with finite particle numbers. The
analysis is based on an analytic calculation in Wigner representation, which
accounts for the laser-induced coherent and incoherent dynamics, for the finite
longitudinal and transverse coherence in the matter-wave beam, the
gravitational and Coriolis acceleration, and an imperfect standing laser wave.
