In-situ quantitative measurements of methyl radicals in oxygen-containing reactive environments by VUV absorption spectroscopy using synchrotron radiation

Absolute methyl radical number densities are measured in-situ in a low-temperature discharge in methane and oxygen diluted in argon/helium, using vacuum ultraviolet synchrotron radiation absorption spectroscopy. Instead of the methyl absorption band around 216 nm, the exploitation of the more sensitive band at 150 nm is demonstrated. Densities as low as (1.6 ± 0.2) × 10¹³ cm^-2 are detected. With oxygen admixture, the expected monotonous decrease of CH₃ density is not observed, but a distinct maximum at a few percent of oxygen, separating a regime dominated by the electron kinetics for low percentages from an oxidation-dominated regime at higher admixtures.