Probing the first stage of dust formation in argon/acetylene plasmas using microwave cavity resonance spectroscopy and laser-induced photodetachment

The crucial first step in the formation mechanism of hydrocarbon nanoparticles in low pressure Ar/C2H2 RF discharges is the creation of negative hydrocarbon ions and their consequent growth due to polymerization reactions. In order to enhance the understanding of this first step, we probe the negative ion density by means of combining non-intrusive microwave cavity resonance (MCR) spectroscopy with laser-induced photodetachment. With the MCR technique we measure, with sub-microsecond time resolution, the space-averaged electron density just after plasma ignition. When a high intensity 355nm laser pulse is sent through the discharge, electrons will be detached from the smallest (C2H-) negative ions within the volume of the laser beam and, hence, the electron density experiences a sudden increase that can be detected with the MCR technique. The detected amount of electrons additionally caused by detachment processes is a measure for the negative ion density just before the laser pulse. Changing the timedelay between plasma ignition and the moment on which the laser is fired through the discharge consequently gives important information about the time-evolution of the ion density. First measurements show that it is possible to probe with high precision the negative ion density and that the low pressure Ar/C2H2 RF discharge under investigation is significantly electronegative.