Applicability of a nonrelativistic asymptotic description of high-energy photoionization

Abstract

We show that while high-energy photoionization cross sections converge to their asymptotic forms only at relativistic energies, the leading deviation from the asymptotic energy dependence of nonrelativistic photoionization cross sections at high energy is the same for all cross sections. This factor can be obtained explicitly, and only asymptotic forms are needed for the remainder at high nonrelativistic energies. Since the factor is common, ratios of photoionization cross sections reach their asymptotic values at much lower energies than the cross sections themselves. Results are presented both in independent-particle approximation as well as including correlation effects, which modify the asymptotic behavior, especially for light elements or outer shells.