Photo-Ionization Cross Sections of the Elements Helium to Xenon

The quantity -r (V (r) calculated by Herman and Skillman, using the Hartree-Fock-Slater approach, is approximated by a series of straight lines. With such a potential the radial Schr\"odinger equation is exactly solvable with Whittaker functions. The bound-state eigenvalue equation is found and is used to adjust the parameters of the straight lines so that the model eigenvalues and those of Herman and Skillman are in reasonable agreement. With the discrete and continuum orbitals of the model, the photo-ionization cross section for all the shells of the elements helium-xenon are computed.