Calculation of multiphoton-ionization Greens functions using the Wentzel-Kramers-Brillouin approximation. II

Recently, a technique [M. Edwards, Phys. Rev. A 45, 409 (1992)] was presented for calculating multiphoton-ionization Greens functions based on the Wentzel-Kramers-Brillouin (WKB) approximation. In that paper it was shown that the ability of this method to produce accurate cross sections and angular distributions depended critically on the single-particle potential for the valence electron that was used in the calculation. In this work we present a supporting technique for determining the optimal potential for use with this method in low-Z alkali-metal atoms. The technique produces the parameter values for a parametric model potential whose WKB Greens function has poles that coincide with the measured energy levels of the atom. We have determined the parameter values for a specific model potential in Li and Na. For these cases, only two parameters are needed to produce a potential whose WKB energy levels match all of the measured ones to within 165 cm-1. We have calculated cross sections for two-photon ionization of Li and Na for a range of laser-photon energies using this model potential and present comparisons with previous work.