Two-electron atoms in strong fields

Multiphoton excitation and ionization of two-electron atoms under strong laser pulses, with emphasis on double excitation, is reviewed. After an introduction to the basic concepts of traditional single-photon spectroscopy and methodology, the main phenomena and experimental observations under multiphoton excitation in the perturbative regime are summarized. That is followed by the discussion of the main ideas underlying various non-perturbative approaches, especially as they pertain to the two-electron problem. The present state of understanding non-perturbative time-dependent behavior, in that context and in relation to issues such as correlation, double excitation, sequential and direct double ionization, is then formulated and discussed. The article ends with a summary of effects relating to autoionizing states in strong fields and an outlook for future developments. (C) 1998 Elsevier Science B.V. All rights reserved.