Use of laser-excited atomic fluorescence spectroscopy for characterization of an argon inductively coupled plasma

Abstract

Laser-excited atomic fluorescence spectroscopy (LEAFS) is investigated and employed as a diagnostic technique for study of the argon inductively coupled plasma (ICP). Computer simulations are used to describe the behavior of non-steady state laser-excited fluorescence (LEF) for multi-level atomic systems under conditions expected to be encountered in the ICP and an atmospheric pressure flame. These simulations are then compared to experimental data collected under similar conditions in the ICP and a flame. These studies show that LEAFS should be a useful tool for characterization of an ICP, with certain limitations. Relatively small changes in saturated LEF signals under changing quenching and mixing conditions are both predicted theoretically and observed experimentally for several atomic systems. This independence from quenching and mixing effects allows one to relate saturated LEF signals directly to relative number densities of species when spatially scanning over an inhomogenous medium, such as an ICP discharge, where significant changes in quenching can be encountered in a single scan. SSI values are also found to be useful as indicators of relative collisional quenching rates and relative degree of LTE establishment in the ICP, as well as ease of saturation for a given transition. Comparative studies, using LEAFS, of distributions of argon metastable atoms and various analyte atoms and ions in the ICP lead to the conclusion that Penning ionization is not a significant contributor to the populations of excited and ionic analyte levels in the ICP. Further comparative studies lead to the support of a charge transfer mechanism as a possible explanation of suprathermal excitation and ionization conditions previously observed in the ICP.