Measurement of the methyl radical concentration profile in a hot‐filament reactor

The spatial profile of methyl radical concentration in a hot‐filament reactor has been measured using cavity ring‐down spectroscopy (CRDS) at a wavelength of 213.9 nm for which the CH3 absorption cross section has been shown to be nearly independent of temperature. Methyl radicals are generated with a 25 mm long tungsten filament heated to 2400 K in a slowly flowing mixture of 0.6% CH4 in H2 (20 Torr total pressure). CRDS is employed to measure CH3 absorbance as a function of a distance perpendicular to the axis of the filament. The CH3 absorbance profiles do not change when the direction of the process gas flow through the reactor is reversed, which indicates cylindrical symmetry of the CH3 distribution about the filament. Consequently, the radial CH3 concentration in the reactor is determined by Abel inversion of the CH3 absorbance profile. The CH3 concentration peaks ∼4 mm from the filament (1.04×1014 molecules/cm3). Methyl radicals decay rapidly as a function of a distance from the filament and disapp...