= Statistical equilibrium of atomic level populations
{title2=$\sum_{j\ne i}(n_jR_{ji}-n_iR_{ij})=0$}
= Statistical equilibrium
{synonym}
An atomic population is in <statistical equilibrium> when the total rate into each level equals the total rate out, subject to conservation of the total number of atoms or ions. The rates can include electron-impact transitions and <spontaneous emission>. This is weaker than thermodynamic equilibrium: a collisionally excited, radiatively decaying dilute gas generally does not have Boltzmann level populations.
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