Quasi-three body systems - properties and scattering
  We investigate systems of three mutually interacting particles with masses of which the inner is much bigger than the intermediate and the latter is much bigger than the outer. Then the three-body problem reduces to the two-body scattering or structure of the light one in the field of the pseudo-nucleus formed by two others. We calculate analytically the properties of considered systems, such as the scattering cross-sections, hyperfine splitting, Auger decay of exited states and Lamb shits, presenting them as expansions in powers of the ratio of light to intermediate particle masses. This ratio is the small parameter of the studied problems. 
M. Ya. Amusia
physics.atom-ph