  Exciting experiments in the field of atom and molecule optics have lately
drawn much attention to the effects involved in the coherent diffraction of
particle beams. We review the influence of the finite size of the particles and
of their energy level spectrum on the diffraction pattern. In turn, we
demonstrate how experimental diffraction measurements allow to determine these
quantities of weakly bound molecules by considering the diffraction of the
dimer and trimer of helium, and the deuterium molecule dimer.
