Saha ionization equation

Developed by the Indian astrophysicist Meghnad Saha in 1920, this formula describes the degree of ionization of a gas as a function of the temperature T, density, and ionization energy.

Let $X = n e / n$ be the ionization fraction, i.e., the electron density relative to the total density of atoms and ions. Then the Saha equation is

$\frac{X^2}{1-X} = \frac{1}{nh^3} \left(2\pi m_ek_BT\right)^{3/2} e^{-I/k_BT}$