Re: Collisional excitations

[Hugh Haskell <hhaskell@MINDSPRING.COM>]

>      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

Since there is no annihilation/creation process going on in
collisional excitation, there are no restrictions on the energy of
the exciting particle, except that it must meet the minimum energy
requirement and, as always, momentum must be conserved in the
collision (which actually means that the energy of the colliding
particle--or, more properly, the internal kinetic energy of the
system in the center of mass reference frame--must be a bit more than
just the excitation energy of the target particle. Of course, it
can't be too large, either or instead of causing the desired
excitation, it may do something else.

This is a bit oversimplified, and the details depend on whether the
particles are of like or opposite charge, whether
particle-antiparticle annihilation is a possibility, whether it is
possible for the two to merge into a single particle (for instance,
if an electron and a hydrogen atom collide under just the right
conditions, they can form a H-minus ion, a phenomenon seen commonly
in the solar atmosphere). But basically, the only requirements for
collisional excitation is that there be enough energy present to
cause the excitation, and that both total energy and momentum be
conserved by the process. The quantum mechanical calculation of these
events is not trivial.

Hugh
--

Hugh Haskell
<mailto://haskell@ncssm.edu>
<mailto://hhaskell@mindspring.com>

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