 Ionization equilibrium

Ionization equilibrium means that the rates populating and depopulating ionization stages balance under a specified physical model. For a neutral species photoionized at rate $\Gamma$ per atom and singly charged ions recombining with coefficient $\alpha$, a simple balance is $n_0\Gamma =n_e{n}_{+}\alpha$.

This kinetic balance need not be local thermodynamic equilibrium. Collisional ionization and radiative recombination can balance in a thin plasma that does not obey a Saha population law. Moreover, a rapidly cooling or flowing gas may retain a nonequilibrium ionization state if its ionization or recombination times exceed the evolution time. A temperature alone is therefore insufficient to declare ionization equilibrium.