How is an atom ionized by electron impact? Can someone walk me through in detail what happens when an atom is ionized by colliding with an electron?
I would prefer a solid example so I can understand it more concretely.
What I think:


*

*The electron collides with the atom giving it energy, but then goes away after that.

*This energy excites an electron inside the atom, which receives enough energy to leave the atom.

*The atom is ionized.
However, I also thought of another way:


*

*The electron collides with the atom and gets bound up in the atom.

*The atom is now ionized (negative ion, since extra  electron)
Are any of these descriptions correct?
 A: To combine some of the comments that have already been stated, plus some extra remarks, into something more formal:


*

*The second process described in the OP is not usually called ionization (even though the end result is an ion), but rather electron capture. 

*In general, though, this cannot happen exactly as stated for an isolated atom $A$ capturing a free electron to form its negative ion $A^-$, because the state $A+e^-$ with a free electron has more energy than the bound state $A^-$. This energy must then be emitted in some way, which can be the emission of a photon, excitation of internal degrees of freedom (specifically, the nuclear motion in a molecule), or delivering a kick to some third body in the neighbourhood.

*Ionization by electron impact usually denotes processes of the type $$ A + e^-\to A^+ + 2e^-,$$ where the initial electron has enough energy to 'tear' out one of the electrons of $A$ to create a positive ion $A^+$ plus a second free electron.

*However, the sequential picture described in the OP (the first electron deposits energy and goes away, and only afterwards is the deposited energy used to eject the second electron) is in general unlikely to be a good model compared to a single-step model where the second electron is ionized immediately during the collision.
Telling apart these two processes experimentally, on the other hand, can be rather tricky, and the distinction between them is not even that easy to draw theoretically. To pick an example close to my neck of the woods (with a surprisingly detailed and accurate Wikipedia entry), non-sequential double ionization of noble gas atoms enables you to distinguish relatively clearly between collisional ionization and collisional-excitation-plus-detachment, but this need not always be the case.

*As pointed out in the comments, atoms and molecules are intrinsically quantum mechanical objects, so any sort of understanding of collisional ionization that tries to make sense of it using a 'billiard balls' picture of electrons will at best be an approximate picture. In certain regimes (like the NSDI experiments linked to above) this can be a surprisingly accurate approximation, but in general there will be lots of quantum mechanical effects which completely escape explanation via classical mechanics.
