CH 3 detection in a low-density supersonic arcjet plasma during diamond synthesis

We report on the measurement of methyl radical (CH3) densities in a low-density supersonic arcjet plasma used in the synthesis of diamond films. Single-pass, high-sensitivity ultraviolet (UV) absorption spectroscopy has been employed to study the X(2A2″)→B(2A1′) transition of the methyl radical near 216 nm. The minimum detectable CH3 density is found to be ∼4×1013 cm−3, which corresponds to a fractional absorption of 2×10−3 at a gas temperature of 1200 K. The dependence of the measured methyl column density on pressure and CH4/H2 flow ratio has been studied. The results are used to revise our previous estimates of the reactive “sticking” coefficient for CH3, and we now find that it is of order 10−2 under conditions where we have previously documented the growth of high quality diamond films.