  A few years ago, diffraction of atoms by double slits and gratings was
achieved for the first time, and standard optical wave-theory provided an
excellent description of the experiments. More recently, diffraction of weakly
bound molecules and even clusters has been observed. Due to their size and to
possible breakup processes optical wave-theory is no longer adequate and a more
sophisticated approach is needed. Moreover, surface effects, which can modify
the diffraction pattern of atoms and molecules, give rise to further
complications. In this article a fully quantum mechanical approach to these
questions is discussed.
