Re: Collisional excitations

[Philip Zell <zell@ACT.ORG>]

     I found it in an intro text by Beiser. The intro modern physics text I
     used when I took the course was Ford's, and he didn't discuss Franck
     Hertz; nor is it discussed in the intro text by Serway, Moses, and
     Moyer.  Seems like an important piece of work and straightforward
     enough that all intro texts could discuss it.

     I wonder how, during the inelastic collision between and electron and
     an atom, resulting in electronic excitation of the atom, energy is
     transferred from the free electron to the atom.  It's an
     electromagnetic interaction.  The free electron and the orbital
     electrons in the atom repulse each other (I believe one of you
     mentioned that yesterday); will the electron, while it is
     decelerating, emit a photon?  Is it this photon that excites the
     atomic transition?

     Let's see... the rate of deceleration determines the photon's energy,
     and there's no guarantee that the photon's energy will match the
     relevant transition energy in the atom. But Franck Hertz showed that
     the only requirement for the free electron to excite a transition in
     the atom during the collision is that the free electron's kinetic
     energy be greater than the energy for the transition in the atom.


     So, how is the free electron's kinetic energy converted into the
     electronic energy of the atom?

     Philip


______________________________ Reply Separator _________________________________
Subject: Re: Collisional excitations
Author:  "phys-l@lists.nau.edu: Forum for Physics Educators"
<PHYS-L@lists.nau.edu> at internet
Date:    9/19/00 8:59 AM


The Franck-Hertz experiment is well known ("by those who know it well", as
one of my old profs used to say) and is featured quite often in a standard
physics curriculum, typically in a 3rd semester introductory course (at my
school this semester is call "introduction to modern physics" and actually
isn't sequentially numbered relative to the standard 1st year course.  The
experiment is also oftened mentioned in an upper division modern physics or
quantum mechanics course.  (Of course, curricula vary, as does individual
mileage)

A few intermediate to elementary references.

McGervey's "Introduction to Modern Physics", page 87 in my older edition.
Kranes's "Modern Physics", 2nd ed., page 195.
Rohlf's "Modern Physics from alpha to Z_nought", page 83.



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: Tuesday, September 19, 2000 8:29 AM
> To: PHYS-L@lists.nau.edu
> Subject: Re: Collisional excitations
>
>
>
>      Franck-Hertz, eh?  I'd heard of it but never read the experiment.
>      Thank you all for your comments.
>
>      Philip Zell
>
>
> ______________________________ Reply Separator
> _________________________________
> Subject: Re: Collisional excitations
> Author:  "phys-l@lists.nau.edu: Forum for Physics Educators"
> <PHYS-L@lists.nau.edu> at internet
> Date:    9/19/00 5:03 AM
>
>
> That is right. Positive energy values as those of the
> exciting particle
> are not quantised. They are continuous.
>
> regards,
>
> Sarma.
> They are continuous.
> At 04:20 PM 9/18/00 -0400, you wrote:
> > > -----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.")
> >
> >
> > --
> > John E. Gastineau                john@gastineau.org
> > 140 Tenderfoot Road              (301) 387-8494
> > Oakland  MD  21550-6957          (301) 387-8495 fax
> > USA                              http://gastineau.org