Potential Barrier Features in Two-Photon Ionization Processes in Atoms

The development of novel, intense, and tunable X-ray sources, such as the various FELs and SLAC's LCLS, opens a new regime in which nonlinear processes in the soft X-ray region can be investigated. As was the case in single-photon ionization processes, in which many prominent features in photoionization cross sections and in photoelectron angular distributions were understood by means of a model potential approach (see, e.g., A.F. Starace, Theory of Atomic Photoionization, \emph{Handbuch der Physik} {\bf 31}, ed. W. Mehlhorn (Springer-Verlag, Berlin, 1982), pp. 1-121), such an approach may be expected to provide similar understanding for multiphoton processes. We report here model potential results on the frequency dependence of two-photon ionization cross sections from inner subshells of rare gas and other closed-shell atoms. Our initial investigations focus on potential barrier effects. We use second order perturbation theory in the X-ray field and sum intermediate states using the well-known Dalgarno-Lewis method (A. Dalgarno and J.T. Lewis, Proc. R. Soc. A {\bf 233}, 70 (1955)).