Mass Spectrometric Evidence for Suprathermal Ionization in an Inductively Coupled Argon Plasma

Abstract

Mass spectra have been obtained of species in the axial channel of an inductively coupled argon plasma by extracting ions from the inductively coupled plasma into a vacuum system housing a quadrupole mass spectrometer. Ionization temperatures ( Tion) are obtained from relative count rates of m/z-resolved ions according to two general types of ionization equilibrium considerations: (a) the ratio of doubly/singly charged ions of the same element, and (b) the ratio of singly charged ions from two elements of different ionization energy. The Tion values derived from measurement of Ar+2/Ar+, Ba+2/Ba+, Sr+2/Sr+, and Cd+/I+ are all greater than those expected from excitation temperatures measured by other workers. The latter three values for Tion are in reasonable agreement with values obtained by optical spectrometry for a variety of argon inductively coupled plasmas.