Quasistationary state decay theory of non-resonant multiphoton ionization

Abstract

A non-perturbative theory which is based on the three-dimensional gauge-invariant quasistationary state decay approach is developed to describe the non-resonant multiphoton ionization of real atoms. Using this approach, we are able to (a) solve a time-dependent Schrodinger equation for an atom-field system in a controllable approximation. Thus this leads us to (b) the possibility of describing non-resonant multiphoton processes which are straightforwardly treated in our formalism with none of the gauge difficulties sometimes encountered in previous approaches. Low-order low-frequency moderate-field approximations of the theory eventually reduce to results of earlier efforts, namely, the pioneering non-perturbative description of multiphoton ionization by Keldysh and refinements to it through partial consideration of atom-field interaction in initial state (Faisal, Reiss). Besides, (c) we can analyse the exceptional stabilization effect in a high-frequency superintense laser field. Our analytic high-frequency results provide a simple yet instructive extension to the weak-bound regime of ionization processes.