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Collisional excitations



I am about to discuss some basic atomic physics with my intro
astronomy students. I'll have no problem explaining radiative
transitions and how they are quantized, but it occurred to me that a
student might ask if collisional excitations happen and if they are
quantized; I don't know about the physics of collisional excitation.

Radiative excitation of an electron from n = a to n = b in an atom
requires that the incoming photon have energy Eb - Ea. What are the
requirements for excitation of the electron from n = a to n = b if the
source of the energy for the excitation is collisional? Can the
particle that is colliding with the atom have any energy in excess of
Eb - Ea ? Is the collisional excitation the result of the exchange of
a photon between the colliding particle (say an electron) and the
atom? What if the collision is between 2 neutral atoms?

Thanks.

Philip Zell