Eh. Wahl et al., MEASUREMENT OF ABSOLUTE CH3 CONCENTRATION IN A HOT-FILAMENT REACTOR USING CAVITY RING-DOWN SPECTROSCOPY, DIAMOND AND RELATED MATERIALS, 5(3-5), 1996, pp. 373-377
Methyl radicals were generated in a hot-filament diamond synthesis rea
ctor using a resistively heated tungsten filament (length, 20 mm) in a
slowly flowing mixture of 0.5% CH4 in H-2. The UV absorbance of CH3 w
as measured during deposition using a new line-of-sight optical techni
que called cavity ring-down spectroscopy (CRDS). Measurements were car
ried out at 213.9 mm, a wavelength at which the CH3 absorption cross-s
ection has been shown by others to be independent of the temperature o
ver a large range. We observed that the CH3 absolute concentration var
ied strongly as a function of the position between the substrate and t
he filament, and its value was strongly influenced by the substrate te
mperature.