IONIZATION OF MANY-ELECTRON ATOMS BY A QUASI-STATIC ELECTRIC-FIELD

We present a general expression for the field ionization probability o f atoms and ions under a quasistatic external electric field. While th e treatments available as yet are only for the case of a single electr on in the outer nl subshell, the expression obtained here is applicabl e 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 a ccurate method to determine the prefactor in the expression for the fi eld ionization probability. Ionization probabilities are calculated us ing the WKB approximation; therefore, the results become exact in the weak-field limit. It is shown that in certain cases the field ionizati on probability depends significantly on the projection of the total or bital 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 purp ose are discussed. The general expression obtained is used to calculat e 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 Pa II 6p level as an examp le. For the Li I 2p level, the present calculations disagree with prev iously published calculations based on the use of Hartree-Fock wave fu nctions; possible reasons for the discrepancy are discussed. For this level and the 3d level of Li I, our results are compared with experime ntal data available in literature and are found to be in agreement.