Electron-ion collisional ionization cross sections and rates for the Na isoelectronic sequence

In this paper, we present the electron-impact ionization cross sections for eight highly charged Na-like ions in the range 18 less than or equal to Z less than or equal to 39 calculated by means of a distorted-wave Born excha nge approximation method including relativistic corrections. Direct ionizat ion from the ground and excited states (n = 3-5) is calculated. For the gro und state, contributions due to excitation-autoionization are also calculat ed, taking account of branching ratios and configuration interaction. A sys tematic study of the dependence of the cross sections on impact energy and nuclear charge is carried out for the direct ionization of all states and f or excitation-autoionization. The results of the calculations are fitted by empirical formulas to facilitate use in practical applications; the fitted values are found to be in good agreement with the calculated results. An a ccurate empirical formula for calculating ionization rates is also given. ( C) 2001 Academic Press.