Ethynyl anion dynamics in the first 10 milliseconds of an argon-acetylene radio-frequency plasma

The time evolution of the ethynyl anion (C2H-) density, just after plasma ignition, has been studied by means of microwave cavity resonance (MCR) spectroscopy in concert with laser-induced photodetachment under varying gas pressure and temperature in an argon-acetylene radio-frequency plasma. C2H- acts as initiator for spontaneous dust particle formation in these plasmas. With an intense 355-nm Nd:YAG laser pulse directed through the discharge, electrons are detached from C2H- present in the laser path only. This results in a sudden increase in the electron density in the plasma, which can accurately and with sub-microsecond time resolution be measured with the MCR technique. By adjusting the time after plasma ignition at which the laser is fired through the discharge, the time evolution of the C2H- density can be studied. We have operated in the linear regime: the photo-detachment signal is proportional to the laser intensity. This allowed us to study the trends of the C2H- signal as a function of the operational parameters of the plasma. The C2H- density steadily increases in the first few milliseconds after plasma ignition, after which it reaches a steady state. While keeping the gas density constant, increasing the gas temperature limits the amount of C2H- and saturates at a temperature of about 90 °C. A zero-dimensional model for the time evolution of the C2H- density fits the experimental data well. Furthermore, two reaction pathways are proposed to explain the observed trends.