# Ionization cross-section energy dependency?

## Main Question or Discussion Point

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 [Broken] 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.

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The cross section of an electron with Ar if I remember correctly peaks at about 10 eV,... but back to your question...

The easiest explaination would be:
The wavelength of the electron depends on its velocity/energy. If an object is much smaller (the Ar) than the incoming wave (the e) then the wave has a tendency to pass over the object with little disturbance (i.e. little probablity of being hit). If the wavelength of the waves is much much smaller than the atom the wave (the e) can pass inbetween (through the empty interior space of the atom) with greater likelyhood than hitting the small composite particles (quarks, and e). However if the electrons wavelength is on the same order of the size of the atom (the electron shell) then in that range maximum probablity of hitting the atom occurs.

Ahh ok that makes sense, you wouldn't happen to have a link to any specific articles discussing this in more detail would you?

Hello,

This could help you....Reviews of Modern Physics. Volume 38 Number 1 January 1996.
I have the PDF file, if you want it I could mail you. [edited out e-mail address]

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But still that does not tell you why it is energy dependant. I am also looking for such articles. I will let you know when I get a good one.

I could give you a simple explanation.
The ionisation energy is discrete. It is a quantum phenomenon. Only electrons possessing the ionising energy will result in ionization. Electrons having both less and more energy will fail to ionize the molecules/atoms.
Therefore you get a Maxwell-Boltzman like distribution.