Plasma electron temperatures and electron energy distributions measured by trace rare gases optical emission spectroscopy

Abstract

This article reviews a spectroscopic method for extracting plasma electron temperatures and electron energy distributions: trace rare gases optical emission spectroscopy. Specifically, traces of Ne, Ar, Kr, and Xe are added to the plasma and the intensities of emissions from the Paschen 2p levels are recorded. Intensities are also computed from a model that includes direct excitation from the ground state, as well as two-step excitation through the 3P2, and 3P0 metastable levels. A Maxwellian electron energy distribution function (EEDF), described by an electron temperature (Te), is assumed, and Te is extracted from the best match between the observed and calculated relative emission intensities. By choosing emission from specific sets of levels, the range of electron energies effective in exciting emission can be selected and various portions of the EEDF can be investigated. Accurate measurement of Te depends critically on accurate cross sections for electron impact excitation, and hence a large portion of this article is devoted to a critical review of this subject. Improving on previous treatments, the model for computing emission intensities and electron temperatures includes a complete analysis of the complex excitation and de-excitation of the metastable levels. Previous measurements of Te and EEDFs in chlorine and oxygen inductively coupled plasmas are re-evaluated with the current model. In general, the current version of the model yields similar results; specific differences are discussed.