The Keldysh Theory of Strong Field Ionization and its Extensions

Abstract

This paper starts with a concise history of the development of the Keldysh approximation and an overview of its more important general properties, and then a number of new developments are reported as well. These developments include the examination of an exhaustive list of possible S-matrix theories that may be applied to photoionization, and the identification of those among them that are suitable for the strong-field case. Emphasis is placed on the general distinctions between phenomena in different intensity regimes. Intensity limits are given for: the convergence of perturbation theory; the applicability of the electric dipole approximation; the utility of one-dimensional models; the validity of non-relativistic theories; and the separability of the equations of motion. A relativistic version of the Keldysh theory is introduced.