Extrapolation of scattering data to the negative-energy region
  Explicit analytic expressions are derived for the effective-range function for the case when the interaction is represented by a sum of the short-range square-well and long-range Coulomb potentials. These expressions are then transformed into forms convenient for extrapolating to the negative-energy region and obtaining the information about bound-state properties. Alternative ways of extrapolation are discussed. Analytic properties of separate terms entering these expressions for the effective-range function and the partial-wave scattering amplitude are investigated. 
L. D. Blokhintsev, A. S. Kadyrov, A. M. Mukhamedzhanov, and D. A.   Savin
nucl-th