Alright, so i'm looking into electron-impact ionization of Argon, and subsequently the collision ionization cross-section. While i have found articles that experimentally find these cross sections at different energies (energies of the electrons that is), such as here: http://prola.aps.org/pdf/PRA/v52/i2/p1115_1 And i can see looking at the data that it peaks at around 80-85eV, i am still unsure as to WHY this distribution of cross sections is dependant on the energy. I was hoping maybe someone here could help me and point me in the direction of an article that discusses what parameters contribute to the cross section. It is mostly the energy variation that i'm thinking about. That is why is the cross section (and how) dependant on the energy of the incoming electrons. I've been thinking about it and was thinking that it perhaps had something to do with the fact that at lower energies (velocities) the electrons were mostly hit by the argon gas via it's (the Argons that is) thermal movement, while at very high velocities the Argon might as well be standing still since it's velocity is so negligable compared to the electrons. So the optimal speed with the maximum cross section would be somewhere in between the two. However i'm far from sure that this is correct, especially considdering that even at speeds close to the ionization energy, i'd imagine that the electron is moving a LOT faster then the argon gas.