Abstract. Plasma ionization composition and level population calculations require, in particular, the cross sections of direct ionization from each quantum state into each state which may be generated by means of removal of any electron. We analysed published data and propose here an empirical formula for cross sections of direct electron-impact ionization of positive atomic ions. The cross sections given by this formula are in satisfactory agreement with those calculated in the distorted-wave (DW) approximation; therefore, we believe that for any direct state-tostate ionization channel this formula provides a reasonably accurate prediction of the DW result. Comparisons with published data and with the Lotz formula are reported as well.

In a recombining plasma the metastable states are known to accumulate population thereby slowing down the recombination process. We show that an account of the doubly-excited autoionizing states, formed due to collisional recombination of metastable ions, results in a significant acceleration of recombination. A fully timedependent collisional-radiative (CR) modeling for stripped ions of carbon recombining in a cold dense plasma demonstrates an order of magnitude faster recombination of He-like ions. The CR model used in calculations is discussed in detail.

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Calculations of line shapes of highly excited (Rydberg) atoms and ions are important for many topics in plasma physics and astrophysics. However, the Stark broadening of the radiative transitions originating from high-n levels of hydrogen or hydrogen-like ions is rather complex, making the detailed calculations of their spectral structure very cumbersome. Here, we suggest a simple analytical method for an approximate calculation of such line shapes. The utility of the method is demonstrated in application to the line broadening in plasma, where a very good accuracy is achieved over a range of transitions, species and plasma parameters. Although the method is especially suitable for transitions with n 1, it describes rather well even first members of the spectroscopic series with n as low as 2. Accurate computer simulations are used to verify the validity of the method.