Atom Scattering from Disordered Surfaces in the Sudden Approximation:   Double Collisions Effects and Quantum Liquids
  The Sudden Approximation (SA) for scattering of atoms from surfaces is generalized to allow for double collision events and scattering from time-dependent quantum liquid surfaces. The resulting new schemes retain the simplicity of the original SA, while requiring little extra computational effort. The results suggest that inert atom (and in particular He) scattering can be used profitably to study hitherto unexplored forms of complex surface disorder. 
Daniel A. Lidar