Abstract
Positive ions were extracted from the axial channel of an inductively coupled plasma (ICP) in which the outer gas flow was Ar, N<sub>2</sub>, or a mixture of Ar and N<sub>2</sub>. Addition of N<sub>2</sub> to the outer gas decreases the electron number density (<i>n<sub>e</sub></i>) in the axial channel. Ar<sup>+2</sup>, O<sub>2</sub><sup>+</sup>, and ArH<sup>+</sup> react with N-containing species in the plasma and/or during the ion extraction process. Ar<sup>+</sup> remains abundant even if there is no Ar in the outer gas, which indicates the probable occurrence of charge transfer reactions between N<sub>2</sub><sup>+</sup> 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 (<i>T</i><sub>ion</sub>) measured from the ratio Cd<sup>+</sup>/I<sup>+</sup> were 5750 to 6700 K for a N<sub>2</sub> outer flow ICP at a forward power of 1.2 kW. This <i>T</i><sub>ion</sub> range is significantly below that obtained for an Ar outer gas ICP under otherwise similar operating parameters.
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