Electron Number Density Measurements in Ar and Ar-N 2 Inductively Coupled Plasmas

The emission spectrum of the series limit of Al was used to estimate the electron number densities (n _e ) in inductively coupled Ar or Ar-N₂ plasmas (ICP). The spectra of the series limit were recorded either with a silicon-intensified target vidicon detector or by a computer-controlled, scanning spectrometer. Background stripping was performed on the spectra recorded by both instruments to enhance signal recovery. Under comparable experimental conditions, the n _e values in the Ar-supported ICP were higher than those observed in Ar-N₂ plasma, when pure N₂ was used in the outer flow. However, the n _e value of the Ar ICP under the conditions commonly used in analytical laboratories (15 mm observation height and 1200 W forward power) was comparable to that found in pure N₂ outer flow, Ar-N₂ plasmas operated under conditions (5 mm observation height and 3000 W forward power) suitable for exciting spectral lines of high excitation energies. Whereas the use of higher, carrier gas flows tended to reduce the n _e values of the Ar ICP substantially, this trend was not observed for the pure N₂ outer flow Ar-N₂ plasma.