QUASI-STATIONARY STATE DECAY THEORY OF NONRESONANT MULTIPHOTON IONIZATION

A non-perturbative theory which is based on the three-dimensional gaug e-invariant quasistationary state decay approach is developed to descr ibe the non-resonant multiphoton ionization of real atoms. Using this approach, we are able to (a) solve a time-dependent Schrodinger equati on for an atom-field system in a controllable approximation. Thus this leads us to (b) the possibility of describing non-resonant multiphoto n processes which are straightforwardly treated in our formalism with none of the gauge difficulties sometimes encountered in previous appro aches. Low-order low-frequency moderate-field approximations of the th eory eventually reduce to results of earlier efforts, namely, the pion eering non-perturbative description of multiphoton ionization by Keldy sh and refinements to it through partial consideration of atom-field i nteraction in initial state (Faisal, Reiss), Besides, (c) we can analy se the exceptional stabilization effect in a high-frequency superinten se laser field. Our analytic high-frequency results provide a simple y et instructive extension to the weak-bound regime of ionization proces ses. To illustrate the main features of the stabilization phenomenon, simple atomic systems consisting of an electron bounded initially by C oulomb and delta-well binding potentials are considered. The calculate d ionization probabilities show that there is no stabilization effect in the case of negative ions, but on the contrary, for neutral atoms a nd positively charged ions, the stabilization takes place.