Authors: Liang-Wen Pi and Anthony F. Starace
Affiliation: Department of Physics and Astronomy, The University of Nebraska, Lincoln, NE 68588-0299
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 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," Handbuch
der Physik Vol. 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 in the soft X-ray region. 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. A233, 70 (1955)).