Mass Spectra and Ionization Temperatures in an Argon-Nitrogen Inductively Coupled Plasma

Positive ions were extracted from the axial channel of an inductively coupled plasma (ICP) in which the outer gas flow was Ar, N₂, or a mixture of Ar and N₂. Addition of N₂ to the outer gas decreases the electron number density (n_e) in the axial channel. Ar⁺², O₂⁺, and ArH⁺ react with N-containing species in the plasma and/or during the ion extraction process. Ar⁺ remains abundant even if there is no Ar in the outer gas, which indicates the probable occurrence of charge transfer reactions between N₂⁺ and Ar. The present work corroborates two general concepts upon which several theories of the origin of suprathermal ionization in ICPs are based: (a) species are physically transported from the induction region to the axial channel; and (b) these species may react with and ionize neutral species in the axial channel. Ionization temperatures (T_ion) measured from the ratio Cd⁺/I⁺ were 5750 to 6700 K for a N₂ outer flow ICP at a forward power of 1.2 kW. This T_ion range is significantly below that obtained for an Ar outer gas ICP under otherwise similar operating parameters.