[Phys-L] colors when electrons scatter from a gas

[Brian Whatcott <betwys1@sbcglobal.net>]

Browsing for insights, I found the master's thesis by  J. Hi on the effects observed and also modeled using Monte Carlo sampling, on an electron beam as used in a scanning electron microscope somewhat helpful for background.
https://trace.tennessee.edu/cgi/viewcontent.cgi?article=3329&context=utk_gradthes

BrianW   
   -    

   - Carl Mungan via Phys-l <phys-l@mail.phys-l.org>UnsubscribeTo:PHYS-LCc:Carl MunganThu, Mar 10 at 9:16 AMSomeone asked me what determines the colors when an electron beam scatters from gas atoms (an example would be the bluish light seen as the beam track in the introductory e/m experiment)?   
   
The question specifically is why one sees only the 492 nm line (see https://physics.stackexchange.com/questions/79441/which-cyan-colored-line-is-produced-in-the-thomson-e-m-apparatus ;<https://physics.stackexchange.com/questions/79441/which-cyan-colored-line-is-produced-in-the-thomson-e-m-apparatus>) and not the full visible spectrum of helium that one sees from a helium discharge tube (done in a different introductory lab experiment)?   
   
We suppose it has something to do with a trade-off involving gas density, state lifetimes, and the electron beam current.   
   
-----   
Carl E. Mungan, Professor of Physics  410-293-6680 (O) -3729 (F)   
Naval Academy Stop 9c, 572C Holloway Rd, Annapolis MD 21402-1363   
mailto:mungan@usna.edu ;   http://usna.edu/Users/physics/mungan/   
   
_______________________________________________   
Forum for Physics Educators   
Phys-l@mail.phys-l.org   
https://www.phys-l.org/mailman/listinfo/phys-l