Determination of atomic scattering lengths from measurements of   molecular binding energies near Feshbach resonances
  We present an analytic model to calculate the atomic scattering length near a Feshbach resonance from data on the molecular binding energy. Our approach considers finite-range square-well potentials and can be applied near broad, narrow, or even overlapping Feshbach resonances. We test our model on Cs$_2$ Feshbach molecules. We measure the binding energy using magnetic-field modulation spectroscopy in a range where one broad and two narrow Feshbach resonances overlap. From the data we accurately determine the Cs atomic scattering length and the positions and widths of two particular resonances. 
A. D. Lange, K. Pilch, A. Prantner, F. Ferlaino, B. Engeser, H.-C.   Naegerl, R. Grimm, and C. Chin