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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:

N_{i+1}Z_{i+1}2 ———— = ———— ———— (2π m_{e}kT)^{3/2}e^{-χi/kT}N_{i}Z_{i}n_{e}h^{3}

- 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) at ionization state i. - m
_{e}- mass of an electron. - n
_{e}- number density of free electrons. - T - temperature.
- h - Planck constant.
- k - Boltzmann constant.

http://en.wikipedia.org/wiki/Saha-Langmuir equation

http://personal.psu.edu/rbc3/A501/saha.pdf

Boltzmann equation

electron pressure

number density (n)

state of ionization

temperature