1 Introduction

Hot plasmas are present in the universe in a variety of astrophysical systems, from stellar coronae to the intergalactic medium in clusters of galaxies. The good quality data obtained in the last few decades and up to date from space observatories like EINSTEIN, ROSAT, ASCA and SAX allowed to study in many details the physical condition of several astrophysical plasmas. This activity will have full bloom with the next coming X-ray space missions, like AXAF, XMM and ASTRO-E.

To determine the relevant physical parameters describing both astrophysical plasmas and laboratory plasmas, i.e. electron temperature, density distribution, ion and element abundances, we need to compare observed data with a theoretical spectral model.

Low-density high-temperature astrophysical and laboratory plasmas are generally not in a local thermodynamic equilibrium. To determine the ionization state we need to consider in great details the individual collisional and radiative ionization and recombination processes. For a detailed discussion of the ionization and recombination processes, see e.g. De Michelis & Mattioli (1981), Mewe (1988, 1990) and Raymond (1988).

In the last two decades various optically thin plasma models have been worked out (see e.g. Raymond & Smith 1977, Mewe et al. Papers I to VI 1972-1986, Landini & Monsignori Fossi 1990-1991 (hereafter LM), Sutherland & Dopita 1993). Generally speaking, the main difference among the various codes in which the most abundant astrophysical elements are considered, lies in the line emission calculations, i.e. both in the number of lines and in the atomic data considered. From the early works up to date there have been extensive improvements in the calculation of atomic parameters and, at the same time, new X-ray observatories are able to get the spectrum of a X-ray source with better and better energy resolution. It comes out the great importance of producing reliable X-ray plasma codes by using the most recent atomic data as well as by improving the algorithms for continuum and line emission processes.

The aim of this work is to compute the ionization balance for the 435 atoms and ions, from H (Z=1) to Ni (Z=28), for plasma temperatures in the range $T=0.001 \div 100$ keV using the most recent data for the ionization and recombination rates.

Here, we collected all these data making a critical review of the existing works and a detailed comparison among different sources for the available data. In a forthcoming paper (Mazzotta et al. 1998) we will use the ionization equilibrium to compute the continuum and the line radiation emissions.

The plan of the paper is the following: in Sect. 2 we describe briefly the data used for the ionization and radiative recombination rates while in Sect. 3 we discuss, in details, the data adopted for the dielectronic recombination rates that we fitted with a single analytic formula. The rate coefficients together with the ionization balance calculations are given in tables. In the last section we compare our results for the ionization equilibrium with those obtained in previous works.