Nonhydrogenic photoabsorption cross sections near threshold are well approximated by radial dipole-matrix elements between phase-shift-modulated regular and irregular Coulomb continuum and quantum-defect bound-state wave functions. Results based upon numerical quadratures are inaccurate for high-Rydberg transitions, while previous analytical formulas are difficult to evaluate near threshold. A simple analytic algorithm for the near-threshold region based on the analytic properties of the Coulomb functions in the complex energy plane is presented.

• Received 23 May 1988

DOI:https://doi.org/10.1103/PhysRevA.38.4633