Coherent Anti-Stokes Raman Scattering Applied to Hydrocarbons in a Microwave Excited Process Plasma

Coherent anti-Stokes Raman scattering (CARS) carried out using the BOXCARS technique is applied to quantify the density and temperature of hydrogen, methane, and methyl in a process plasma generated by a surfatron in the pressure range from 30 to 3000 Pa. Below approximately 500 Pa all species and their temperatures have a flat distribution dominated by diffusive processes. At higher pressure the plasma localizes near the inner surface of the surfatron tube. A strong depletion of the methane ground state density is observed, where the gas temperature remains at 400 K on a moderate level. On the other side the methyl density is high at the position of high methane depletion. The measured methyl density is a factor of 200 lower than the methane density, in contrast to the factor 10–20 often reported in the literature. The measured data can be interpreted by proper modeling by means of the Boltzmann equation of electrons, balancing of chemical reactions and diffusive processes. The calculated methane and methyl densities agree with the measured ones only if the electron temperature is assumed to be near 1 eV and thus much lower than predicted in previous papers.