In recent times, novel techniques and innovations of equipment have lead to a substantial improvement of laser spectroscopic diagnostics on low-temperature plasmas. This applies especially to the detection of charged species (electrons, positive and negative ions) and neutral radicals (atoms and molecules) as well as the measurement of electric fields. Spatially and temporally resolved measurements with high spectral resolution allow the non-invasive determination of a whole list of important parameters, e.g. temperature, density, velocity distribution, degree of dissociation, diffusion and reaction rates, sticking coefficients, voltages, currents etc..
Available techniques are based generally either on scattering, absorption, fluorescence or four-wave mixing. Within this general frame a large variety of alternative schemes has been developed, especially in the case of multi-photon interaction. This sometimes confusing multitude of alternatives is necessary since as a matter of fact all schemes and techniques are limited to a certain parameter range and their applicability is determined strongly by the experimental conditions. In this context, microplasmas at atmospheric pressures are a special challenge. Small spatial dimensions, high pressure and related quenching, and often bright emission background are all contributing to an environment unfavourable for most standard diagnostics. Therefore, a strong demand exists for further development and novel ideas.
The talk intends to give an overview on recent developments in diagnostics of low-temperature plasmas in general by introducing selected examples. The requirements and limits will be analyzed and opportunities for diagnostics will then be discussed on the basis of this background.
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