Cavity ring down detection of CH3 in a remote Ar/CH4-plasma

The Cavity Ring Down (CRD) technique has been applied for the absolute determination of the methyl radical density in a remote Ar/CH4-plasma generated by a cascaded arc. The CRD absorption spectrum of CH3 has been measured on the B 2A’1 E- X 2A’~ transition around 216 nm for various settings of the cascaded arc current and the CH4-flow. A clear influence of these settings on the shape of the spectrum has been observed. This could be due to the difference in the plasma temperature between these settings, but evidence has been found that for this type of plasma interferences of acetylene absoiptions in this wavelength region are more important. A claim from literature that CH3 absorptions can be best measured at 213.9 nm has been investigated and found wrong for this type of plasma. Cli, densities in the range 1-5 x 10’~ m3 have been observed, confinning the expectation that CH, is the most abundant radical in the expanding Ar/CH4-plasma