Identification of double ionization mechanisms, results bearing on electron correlation measurements

Electron impact double ionization of atoms and molecules offers a way to directly measure two-electron correlation, provided a number of conditions are satisfied. The ionizing collisions should take place in the binary, impulsive regime so that all of the energy and momentum lost by the projectile electron in each of the collisions is fully transferred to the target electrons. It is also necessary that the target electrons acquire sufficient kinetic energy from the ionizing collisions to leave the target atom or molecule in times short compared to an electron orbital period. Finally, the double ionization mechanisms must be well understood and separated from interfering processes. Measurements of the double ionization cross-sections for the two 3s electrons of magnesium have led to the identification and separation of two double ionization mechanisms. From the measurements the means by which two-electron momentum distributions can be obtained are clear.