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

Abstract

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 ( ne) 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 ( Tion) 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 Tion range is significantly below that obtained for an Ar outer gas ICP under otherwise similar operating parameters.