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Unformatted text preview: lifetimes. Application of these may be seen in a schematic few-level atom; these are all part of the "ground state" n=0 . Only 4-5 of the possible transitions have high enough probabilities to be observed in a realistic case, satisfying the appropriate selection rules. . An electron may reach an upper level for any of a number of reasons: photoionization from a lower level collisional excitation cascade from a higher level (as in masers) recombination to an excited state two-electron processes Various of these are important for different ions. Having already discussed pure recombination, we still need to deal with collisional excitation of "forbidden" lines. These are important coolants in the energy balance of plasmas at T ~ 10 4 K because only the heavy elements have excited levels at the appropriate energies of a few eV. H and He don't....
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- Fall '10