Plasma processing systems and methods for using pre-dissociated and/or pre-ionized tuning gases are disclosed herein. In one embodiment, a plasma processing system includes a reaction chamber, a support element in the reaction chamber, and one or more cathode discharge assemblies in the reaction chamber. The reaction chamber is configured to produce a plasma in an interior volume of the chamber. The support element positions a microelectronic workpiece in the reaction chamber, and the cathode discharge assembly supplies an at least partially dissociated and/or ionized tuning gas to the workpiece in the chamber.