Connecting excitation and ionization cross sections in electron-atom scattering

In the convergent close-coupling method for calculating electron-atom scattering, excitation and ionization are associated with cross sections of negative- and positive-energy target states, respectively. We show that the zero-energy boundary can be continuously accessed from either the negative- or positive-energy side, thereby enabling the estimation, via interpolation, of cross sections of arbitrarily high-level Rydberg target states for the target orbital angular momentum included in the calculations.