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

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Abstract
Atmospheric microwave-induced argon plasmas with and without analyte injection have been exposed to power interruption experiments in order to study transport processes and to reveal dominant excitation balances. From the time-dependent behaviour of line intensities due to electron cooling and quenching during the power interruption, it is found that electron loss channels, such as diffusion, convection and the dissociative recombination of molecular ions, are much larger than for inductively coupled plasmas. It is found that in the ionizing part of the plasma electron dominated mechanisms are responsible for the population of radiative levels. Significant changes in the responses to power interruption are observed when small amounts of molecular compounds are injected (>0.5%), probably due to a decrease of the electron density. Furthermore, it is found that in the recombination zone downstream in the plasma an electron-independent excitation mechanism, probably thermal excitation, is responsible for the population of radiative levels of analytes with relatively low excitation energies. From the downstream propagation of a disturbance created in the ionizing part of the plasma the local axial gas velocity has been determined. In the analyte excitation zone of the plasma typical velocities are around 25 m s-1, whereas in the recombining zone velocities of 12-18 m s-1 are obtained.

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