  We propose a semi-empirical formula for the cross section of ionization by
electron impact. The formula involves adjustable parameters which are
determined by comparison with measured or numerically calculated cross
sections. In the latter case, the ions are perturbed by their environment which
is a high-density plasma. As a consequence, the cross section is significantly
modified. We investigate Be-like carbon, nitrogen and oxygen as well as
aluminum ions. We also show that the formula is well-suited for interpolation
and extrapolation. Knowing the cross section, we calculate the rate coefficient
within the Boltzmann and Fermi-Dirac statistics. In the first case, the rate
can be calculated analytically. In the second one, it can be expressed in terms
of special functions, but the numerical evaluation is more convenient while
providing accurate results. Our results are compared to experiment and to other
calculations.
