Primer on Collisions
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II) Electron-Impact Excitation out of the ground state of Atoms

D. Pressure Effects

Low & High Pressure Cross sections

Low Pressure (~ 1mTorr)
Excitation into level-i

As before, at low pressure there are only two means of populating level-i:
  • Direct:    Atom(g) + e- --> Atom(i)
  • Cascade: Atom(g) + e- --> Atom(j) --> Atom(i) + hv

Both the population density, n(i), and the intensity I(i-->l) will both be directly proportional to the ground state number density, n(g), and thus the pressure.

Graphically, if we plot these quantities as a function of pressure:

High Pressure (~ 10mTorr)

As we increase the gas pressure further, the measured intensity is no longer linear. The cross section seems to depend on the number density of target atoms.
These Pressure Effects can derive from a number of sources, such as radiation trapping and collisional transfer.

last updated: May-15-1997