A comparison of theory and experiment for photo-ionization cross-sections II. Sodium and the alkali metals

A summary is given of the results of experimental determinations of the atomic photo-ionization cross-sections of the alkali metals. Detailed calculations on sodium are described, using Hartree-Fock wave functions, and it is shown that the theory is capable of reproducing the main features of the experimental results. A comparison with line-strength calculations is made, and the applicability of sum-rule tests is discussed. The finite minimum which is found experimentally in the cross-sections of certain of the alkali metals is attributed to spin-orbit perturbations of the free-electron wave functions. For caesium and rubidium the theoretical calculation based on this effect is shown to give minima of a magnitude comparable to those of the experimental results. The accuracy of previous calculations on the cross-sections of Ca ll is discussed, use being made of sum-rule tests.