Throwing electrons on target atoms at a speed of about one million meters per second causes the atoms to multiple ionize, i.e., during the collision, one or several outer shell electrons leave the atom. As in bowling and billiards, the mechanisms involve some kind of instability, which makes the game interesting and the outcome not necessarily predictable: Depending on the initial conditions, the impact electron can "knock down" zero, one, two or more electrons from the target by communicating part of its kinetic energy to the outer shell electrons strongly bound by the Coulomb interaction. 

We have considered a Hamiltonian dynamical system to model the electron impact of magnesium with two effective outer shell electrons. We have compared the experimentally measured cross-sections with the ones computed from this model, and identified the mechanisms responsible for the single and double ionization of magnesium. In essence, these mechanisms are chaotic, involving some kind of strong dependence to initial conditions. For double ionization mechanisms, we provide evidence that the Two-Step 2 in which the impact electron "knocks down" the two outer shell electrons sequentially, is dominant over the Two-Step 1 in which the target electron "knocks down" one outer shell electron, and then this outer shell electron "knocks down" the second outer shell electron. 

Credit: Written by Cristel Chandre