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Re: Collisional excitations



-----Original Message-----
From: phys-l@lists.nau.edu: Forum for Physics Educators
[mailto:PHYS-L@lists.nau.edu]On Behalf Of Joel Rauber
Sent: Monday, September 18, 2000 3:23 PM
To: PHYS-L@lists.nau.edu
Subject: Re: Collisional excitations


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

Joel Rauber

Yes--Franck_Hertz is evidence of the quantization for the energy _absorbed_
by the target atoms, but does not show that the initial energy of the
incident electron is quantized. As long as the incident electron has
sufficient energy to cause the target atom to make the transition, there is
a non-zero probability for the interaction to take place. The probability is
measured using the concept of cross section.

The plot of cross section vs. incident electron energy is known as an
excitation function. That function always starts at zero, rises fairly
quickly, and then drops off at various rates, typically between 1/E and
1/E^3 at high energies. The slope of the high energy end depends on the
particular transition being induced in the atom.

(My thesis title was something like "The electron impact excitation of
krypton and xenon.")


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John E. Gastineau john@gastineau.org
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