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Re: Collisional excitations

Isn't quantization of energy during collisions one of the lessons of the
Franck-Hertz experiment?

Joel Rauber
Joel_Rauber@sdstate.edu


-----Original Message-----
From: phys-l@lists.nau.edu: Forum for Physics Educators
[mailto:PHYS-L@lists.nau.edu]
Sent: Monday, September 18, 2000 2:13 PM
To: PHYS-L@lists.nau.edu
Subject: Re: Collisional excitations



Hi all-
I agree with all that Hugh writes and add a few details.
1. At one level you can think semi-classically. It is an ordinary
inelastic collision, but the energy loss is quantized.
2. The classical force between the two atoms is the coulomb force
acting between each electron pair; all this happens in the
field of the
two nuclei.
3. There is also a distinct non-classical force resulting from the
identity of the electrons in the colliding atoms. Electrons from one
atom may exchange places with electrons from the other atom. This
"exchange force" is a major complication in calculations of atomic
interactions or interactions between electrons and atoms.
Regards,
Jack

Adam was by constitution and proclivity a scientist; I was
the same, and
we loved to call ourselves by that great name...Our first memorable
scientific discovery was the law that water and like fluids
run downhill,
not up.
Mark Twain, <Extract from Eve's
Autobiography>

On Mon, 18 Sep 2000, Hugh Haskell wrote:

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>

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