Ionization of many-electron atoms by a quasistatic electric field

We present a general expression for the field ionization probability of atoms and ions under a quasistatic external electric field. While the treatments available as yet are only for the case of a single electron in the outer nl subshell, the expression obtained here is applicable to atoms and ions with any number of electrons in the outer subshell and with partially filled inner subshell~s!. We also present a more accurate method to determine the prefactor in the expression for the field ionization probability. Ionization probabilities are calculated using the WKB approximation; therefore, the results become exact in the weak-field limit. It is shown that in certain cases the field ionization probability depends significantly on the projection of the total orbital angular momentum in the direction of the field. A proper way to normalize the optical-electron wave-function asymptotics is explained and the difficulties in the use of Hartree-Fock routines for that purpose are discussed. The general expression obtained is used to calculate field ionization probabilities for several states of some atoms and ions that are of special interest for spectroscopic diagnostics: C I, Li I, and Ba II. We also discuss the case of a significant spin-orbit interaction, treating the ionization of the Ba II 6p level as an example. For the Li I 2p level, the present calculations disagree with previously published calculations based on the use of Hartree-Fock wave functions; possible reasons for the discrepancy are discussed. For this level and the 3d level of Li I, our results are compared with experimental data available in literature and are found to be in agreement. @S1050-2947~98!08208-0#