  We present new results on the ionization by electron impacts in a dense
plasma. We are interested in the density effect known as the ionization
potential depression and in its role in atomic structure. Rather than using the
well-known Stewart-Pyatt or Ecker-Kr\"oll formulas for the ionization potential
depression, we consider a distribution function of the ionization energy, which
involves the plasma fluctuations due to ion dynamics. This distribution is
calculated within classical molecular dynamics. The removal of the noise yields
a new distribution which is composed of a small set of Gaussian peaks among
which one peak is selected by considering the signal-to-noise ratio. This
approach provides an ionization potential depression in good agreement with
experimental results obtained at the Linac Coherent Light Source facility. Our
results are also compared to other calculations. In a second part, we
investigate the effects of the ionization potential depression and the
fluctuations on ionization by electron impacts. We propose a new expression of
the cross section, based on an average over the ionization energy distribution.
This cross section can be calculated analytically. The main strength of our
work is to account for the fluctuations due to ion dynamics.
