Measurement of absolute CH3 concentration in a hot-filament reactor using cavity ring-down spectroscopy

Methyl radicals were generated in a hot-filament diamond synthesis reactor using a resistively heated tungsten filament (length, 20 mm) in a slowly flowing mixture of 0.5% CH4 in H2. The UV absorbance of CH3 was measured during deposition using a new line-of-sight optical technique called cavity ring-down spectroscopy (CRDS). Measurements were carried out at 213.9 nm, a wavelength at which the CH3 absorption cross-section has been shown by others to be independent of the temperature over a large range. We observed that the CH3 absolute concentration varied strongly as a function of the position between the substrate and the filament, and its value was strongly influenced by the substrate temperature.