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## Saha ionization equationThe ## Additional recommended knowledgeFor a gas at a high enough temperature, the thermal collisions of the atoms will ionize some of the atoms. One or more of the electrons that are normally bound to the atom in orbits around the atomic nucleus will be ejected from the atom and will form an electron gas that co-exists with the gas of atomic ions and neutral atoms. This state of matter is called a plasma. The Saha equation describes the degree of ionization of this plasma as a function of the temperature, density, and ionization energies of the atoms. The Saha equation only holds for weakly ionized plasmas for which the Debye length is large. This means that the "screening" of the coulomb charge of ions and electrons by other ions and electrons is negligible. The subsequent lowering of the ionization potentials and the "cutoff" of the partition function is therefore also negligible. For a gas composed of a single atomic species, the Saha equation is written: where: - is the density of atoms in the
*i*-th state of ionization, that is with*i*electrons removed. - is the degeneracy of states for the
*i*-ions - is the energy required to remove
*i*electrons from a neutral atom, creating an*i*-level ion. - is the electron density
- is the thermal de Broglie wavelength of an electron
- is the mass of an electron
- is the temperature of the gas
- is the Boltzmann constant
- is Planck's constant
In the case where only one level of ionization is important, we have where ε is the energy of ionization. The Saha equation is useful for determining the ratio of particle densities for two different ionization levels. The most useful form of the Saha equation for this purpose is - ,
where This equation simply states that the potential for an atom of ionization state |

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Saha_ionization_equation". A list of authors is available in Wikipedia. |