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

Atmospheric microwave-induced argon plasmas with and without analyte inject ion have been exposed to power interruption experiments in order to study t ransport processes and to reveal dominant excitation balances. From the tim e-dependent behaviour of line intensities due to electron cooling and quenc hing during the power interruption, it is found that electron loss channels , such as diffusion, convection and the dissociative recombination of molec ular ions, are much larger than for inductively coupled plasmas. It is foun d 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 mol ecular compounds are injected (>0.5%), probably due to a decrease of the el ectron density. Furthermore, it is found that in the recombination zone dow nstream in the plasma an electron-independent excitation mechanism, probabl y thermal excitation, is responsible for the population of radiative levels of analytes with relatively low excitation energies. From the downstream p ropagation of a disturbance created in the ionizing part of the plasma the local axial gas velocity has been determined. In the analyte excitation zon e of the plasma typical velocities are around 25 m s(-1), whereas in the re combining zone velocities of 12-18 m s(-1) are obtained.