ELECTRON NUMBER DENSITY IN A SLIGHTLY IONIZED-GAS (ARGON INDUCTIVELY-COUPLED PLASMA)

An equation defining the electron number density, n(e), in an argon inductively coupled plasma (ICP) regarded as a slightly ionized gas, is derived in terms of the thermodynamic temperature and the electronic chemical potential of the parent species in the ionization. The ICP is not regarded as being in a state of local thermodynamic equilibrium, but instead is considered to be an example of a thermodynamic steady state. The derivation requires nothing more advanced than elementary statistical thermodynamics, and the result is compared in detail with the Saha relation. It is shown that the new equation requires thermodynamic temperatures much lower than those from the Saha relation, and other areas of atomic emission spectroscopy to which the basic formulation is applicable are indicated.