Degenerate Four Wave Mixing Spectroscopy of Free Radicals in Low-Pressure Plasmas

Four wave mixing spectroscopies offer new ways to measure chemical species in extremely hostile environments. Among the most demanding of these applications is the measurement of radical chemistry within plasmas. Most plasmas are highly luminous and are characterized by steep gradients in temperature and concentration. These hostile properties thwart fluorescence and absorption diagnostics. In addition, in many useful plasma reactors the electrodes and associated electronics impose cumbersome optical geometries. We are using degenerate four-wave mixing (DFWM) spectroscopy to sidestep these impediments and to provide sensitive and spatially resolved measurements of concentration and temperature. In conjunction with computer simulations this data will lead to a better understanding of fundamental chemistry within plasma environments. In this paper we describe proof-of-principal experiments that use NH radical spectra to profile the evolution of gas mixtures containing ammonia, hydrogen (or fluorine) and argon passed through a microwave discharge. We will also present data from similar experiments in which hydrocarbon radicals are the important reaction intermediates produced by the plasma chemistry.