II) Electron-Impact Excitation out of the ground state of Atoms
D. Pressure Effects
Low & High Pressure Cross sections
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:
 |
 |
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.
 |
 |
 |
 |
 |