Methyl Reactivity in Methane-Containing Low-Temperature Plasma studied by VUV Absorption of Synchrotron Radiation at 150nm

Highly reactive species often are the culprits governing the behavior of plasma. Herein, they pose an enormous challenge as their high reactivity leads to low densities, difficult to measure, and in turn lack of fundamental data, like reaction rates etc., that are crucial for modeling. In low-temperature plasma-driven dry reforming of methane, CH₃ has been postulated to react with O(1D) with a very high rate to get an agreement between experiments and kinetic modeling [1]. Here, CH₃ spectra are detected by Fourier Transform VUV absorption spectroscopy using synchrotron radiation at the SOLEIL facility. The δ1(0-0) Rydberg band located at 150nm is used, being approximately four times stronger than the conventional β1 band at 216nm [2]. Thanks to the higher sensitivity achieved with this band, CH₃ absolute density could be determined in a CH₄ glow discharge with varying oxygen admixture, either through O₂ or CO₂, and dilution with noble gases. Scans of pressure, current, residence time and dilution factor grant insights into underlying mechanisms. The CH₃ density seems to exhibit a maximum for intermediate oxygen admixtures, pointing towards more intricate processes than simple destruction of CH₃ by more available oxygen. The VUV data is complemented by OES and FTIR measurements.