Saha equation

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 i electrons 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

• N_i+1, N_i - number density of two successive ionization states i and i+1.
• Z_i+1, Z_i - partition function for the same (which differs for each state of ionization).
• χ_i - ionization potential (energy needed to ionize from neutral to ionization state i).
• m_e - mass of an electron.
• n_e - number density of free electrons.
• T - temperature.
• h - Planck constant.
• k - Boltzmann constant.