= Electron fugacity in the Saha equation
{title2=$z_e=e^{\mu_e/kT}$}
For classical nondegenerate <Electrons>, the free-particle Boltzmann occupancy has the form $z_e\exp[-p^2/(2mkT)]$, with chemical potential measured relative to its rest energy. Counting the two spin states in phase space gives $n_e=2z_e(2\pi mkT/h^2)^{3/2}$. A bound level of energy $-I_n$ has relative occupancy $(g_n/g_+)z_e e^{I_n/kT}$ against its parent ion. Eliminating $z_e$ gives the level-specific <Saha ionization equation>.
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