 Critical density of an atomic transition (source code)

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= Critical density of an atomic transition
{title2=$n_{\rm crit}=A_{ul}/q_{ul}$}

The critical density of an atomic transition is the electron density at which its collisional de-excitation rate equals its spontaneous radiative decay rate. A multilevel atom requires summed competing rates rather than a single-transition ratio. This atomic density scale is distinct from cosmological critical density.

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