The Saha equation (Saha ionization equation
or Saha-Langmuir equation) yields information about the
ionization of a sufficiently-thin plasma in
thermodynamic equilibrium.
It yields the ratio of number densities
of ions at two successive states of ionization, i.e., those
with ielectrons
missing verses those with i+1 electrons missing.
It is used in stellar models. One form:
Ni+1 Zi+1 2
———— = ———— ———— (2π mekT)3/2e-χi/kT
Ni Zi neh²
Ni+1, Ni - number density of two successive ionization states i and i+1.
Zi+1, Zi - partition function for the same (which differs for each state of ionization).