  We study double ionization of Mg by electron impact through the vantage point
of classical mechanics. We consider all electron-electron correlations in a
Coulomb four-body problem, where three electrons belong to the atom and the
fourth electron causes the impact ionization. From our model we compute the
double-ionization probability of Mg for impact energies from 15, to 125 eV.
Double ionization occurs through eight double-ionization mechanisms, which we
classify into four categories: inner shell capture, direct, delay and ionized
inner shell mechanisms. We show that delay and ionized inner shell mechanisms
require electron-electron correlations among the four electrons, and are
responsible for the second increase in the double-ionization probability.
Furthermore, we show that our theoretical prediction about the relative
prominence of certain double ionization mechanisms is in agreement with
experimental results on the relative prominence of non-first- over first-order
mechanisms.

Atomic Physics