The usual approximations concering the Stark broadening of the Lyman lines emitted by hydrogen or hydrogenic ions are discussed. It is shown by a simple comparison of the relevant time scales associated with the fluctuations of the slowly varying ion microfield and with the damping of the radiating dipole, that the static approximation (valid at high densities) breaks down in the line center at low densities. In this case the perturbing ions and electrons can be described in the same way, i.e., in terms of collisions. Additional effects occuring at low densities such as fine structure, Lamb shift and Doppler effects are also discussed. It is pointed out that a similar discussion may also be followed in the derivation of the rate equations. © 1990.
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