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

Sorry, I didn't notice the last sentence of your question. I think a
quick and dirty answer is that *any* electromagnetic interaction
between two particles involves the exchange of virtual photons. This
would include collisions between two neutral atoms, since their
interaction at these energy levels would be primarily between their
outer electrons.

I suspect that, for the purposes of your intro. astronomy class, this
is an issue best not raised, unless a student asks about it. I would
think that what was in my last post would probably be about all you
need to say about the topic.

Hugh
--

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

Let's face it. People use a Mac because they want to, Windows because they
have to..
******************************************************