Excitation balances and transport properties in atmospheric microwave-inducedplasmas studied by power interruption experiments

Abstract

Atmospheric microwave-induced argon plasmas with and without analyte injectionhave been exposed to power interruption experiments in order to studytransport processes and to reveal dominant excitation balances. From thetime-dependent behaviour of line intensities due to electron cooling andquenching during the power interruption, it is found that electron losschannels, such as diffusion, convection and the dissociative recombination ofmolecular ions, are much larger than for inductively coupled plasmas. It isfound that in the ionizing part of the plasma electron dominated mechanismsare responsible for the population of radiative levels. Significant changes inthe responses to power interruption are observed when small amounts ofmolecular compounds are injected (>0.5%), probably due to a decrease of theelectron density. Furthermore, it is found that in the recombination zonedownstream in the plasma an electron-independent excitation mechanism,probably thermal excitation, is responsible for the population of radiativelevels of analytes with relatively low excitation energies. From thedownstream propagation of a disturbance created in the ionizing part of theplasma the local axial gas velocity has been determined. In the analyteexcitation zone of the plasma typical velocities are around25 m s-1, whereasin the recombining zone velocities of 12-18 m s-1 are obtained.