Quantum defect theory VI. Extrapolations along isoelectronic sequences

Data obtained from quantum defects can be extrapolated along isoelectronic sequences and used to obtain phases for electron scattering by neutral atoms. An analytical theory is developed for the case in which polarization potentials are neglected. An alternative theory uses simple model potentials which include polarization, and which contain an adjustable parameter. Numerical methods are used to determine the parameter from quantum defect data and to make extrapolations. Phases are obtained for electron-helium scattering; the result obtained for the scattering length is λ = -1.12±0.05.