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Aiming at understanding and controlling the major mechanisms and processes in microwave-generated plasmas, the ground-state densities of the most relevant atomic species and their spatial distributions were determined in these plasmas. They can be measured by optical emission and absorption spectroscopy. Similar to the determination of the silicon ground-state density from the analysis of the self-absorbed spectral lines of the silicon ground-state multiplet 3p 23 P-3p4s 3 P 0 in the ultraviolet range at about 251 nm, the neutral concentrations of other species like carbon and titanium were evaluated from further spectroscopic analyses. Furthermore, the spatial distributions were explored in detail. New developments of microwave-generated plasma sources and their optical emission spectroscopy to investigate the kind of species in the plasmas, as well as the gas and excitation temperatures, are also presented.